Dening Quality. Building Comfort.

AAONAIRE ENERGY RECOVERY

Table of Content

4

. Executive Summary 5 Energy Savings...... 5 WhyHumidity AAONAIR ControlE air-t . .o .-air . . ene . . rgy . . r. e .c o. v . e .ry? ...... 5 Air Conditioning Load Reduction ...... 6 8 Heat Recovery Ventilators ...... 9 APPLEnergyICATI RecoveryONS Ventilators ...... 9 9 ASHRAE Standard 84 ...... 9 PERAHRIFORMA StandardNCE RATI 1060N . G . S ...... 9 Guideline V ...... 10 Recovery Energy Efficiency ...... 10 Combined Energy Efficiency...... 12

...... 13

ECONOMIC CONSIDERATIONS 14 Payback Analysis ...... 16 Payback Analysis ...... 19 Heat Recovery Ventilators ...... 20 RATEnergyE OF ENE RecoveryRGY TRA VentilatorsNSFER ...... 22 26 AAONAIRE Rotary air-to-air energy exchangers ...... 26 DESEngineeredIGN CONS IPolymerDERATI vsONS . Aluminum Wheel Material...... 26 Silica gel ...... 27 Sensible Heat Exchangers (HRVs) vs . Total Energy Exchangers (ERVs) ...... 28 33 Air Leakage...... 33 TECHCondensationNICAL CONS andIDE FrostRATI FormationONS ...... 35 Frost Threshold Limits ...... 36 Frost Control and Prevention...... 36 Controls ...... 39 Pressure Drop ...... 39 Fan Power ...... 39 Cleaning ...... 40 41

. SUMMARY 42 . APPENDIX: 47 . References: Executive Summary

AAONAIRE energy recovery systems complete information to allow designers provide energy savings by recycling energy to fully characterize thermal and airflow instead losing energy through exhaust air performance. In addition to separate streams. AAONAIRE systems also enhance sensible, latent, and total effectiveness at indoor air quality by allowing larger two airflows for both summer and winter amounts of outside air to be provided to test conditions, the standard requires the space and through improved humidity information on pressure loss as well as air control. AAONAIRE systems save money leakage. through both an initial HVAC equipment reduction and ongoing lifecycle operating In many climates, the AAONAIRE system savings. For much of the country, the eliminates three to four tons of air payback for the AAONAIRE system is less conditioning load for every 1000 cfm of than one year. After the payback period, the ventilation air. When this benefit is claimed system will continue to provide savings for in the form of air conditioning equipment the life of the product. rightsizing, the initial cost of the system is typically on par with a system that AAONAIRE units have been certified to does not utilize energy recovery. Payback perform according to AHRI Standard is often immediate for the southeastern 1060. The certified ratings program US and much of the midwest because the requires testing, rating and independent AAONAIRE energy recovery system actually verification of component performance at has a lower initial cost than a system not standard conditions and rated flow. Testing using energy recovery. When payback is is in accordance with ASHRAE Standard not immediate, most payback periods are 84. AHRI certified ratings include very less than one year.

4 Dening Quality. Building Comfort. hy AAONAIRE air-to-air energy recovery? W Humidity Control Building systems that do more with less are increasingly recognized not only as Humidity directly affects the comfort level an ethical priority but as good business. and health in the conditioned space as Market demand for environmentally shown in Figure 2. The application of the responsible heating and air conditioning AAONAIRE energy recovery device can (HVAC) systems and engineering design affect the relative humidity and result in continues to grow rapidly. In addition, more comfortable conditions. Humidity building code requirements for increased that reaches excessive levels, for even short outdoor air ventilation rates have placed periods of time, can create an environment new demands on HVAC equipment and on that promotes the growth of fungi and building operating budgets. bacteria. Human exposure to fungi and bacteria can cause serious health issues. Air-to-air energy recovery is the process of recovering energy from an airstream at a SUMMER CONDITIONS high temperature or humidity to an airstream at a low temperature SUPPLY AIR OUTDOOR AIR or humidity. This process is 79˚F DB 95˚F DB important in maintaining 66˚F WB 78˚F WB acceptable indoor air quality (IAQ) while maintaining low energy costs and reducing overall energy consumption.

Energy can be recovered either RETURN AIR EXHAUST AIR in its sensible (temperature 75˚F DB 91˚F DB only) or latent (moisture) 63˚F WB 75˚F WB form, or a combination of both. Units that recover energy from temperature only are called WINTER CONDITIONS sensible heat exchange devices or heat recovery ventilators SUPPLY AIR OUTDOOR AIR (HRVs). Units that transfer both 59˚F DB 7˚F DB heat and moisture are known 47˚F WB 6˚F WB as energy or enthalpy devices or energy recovery ventilators (ERVs). Energy Savings

% RETURN AIR EXHAUST AIR By recovering up to 80 of the 72˚F DB 20˚F DB energy of the exhaust air, as 54˚F WB 18˚F WB shown in Figure 1, far less energy is spent cooling and heating Figure 1. Sample AAONAIRE Operating Conditions the outside air supplied to the building. This energy savings can typically reduce the operating cost by thousands of dollars per year for a single unit.

5 Dening Quality. Building Comfort. Healthy Humidity AAONAIRE energy recovery ventilators are Bacteria perfectly suited to help control humidity. Viruses In the summer, when outside humidity is high, the energy wheel dehumidifies the Fungi outside air. This greatly reduces the latent Mites load on the air conditioning equipment. Respiratory Infections It also eliminates the problems with high Allegenic Rhinitis indoor moisture levels that can occur and Asthma in hot, humid climates. Air conditioners Chemical Interactions use much energy to dehumidify moist Ozone airstreams. Excessive moisture in the air Production 0 20 40 60 80 100 of a building can result in mold, allergies, and bacterial growth. ERVs can enhance Relative Humidity (%) dehumidification with packaged unitary Adapted from Sterling, Arundel & Sterling 1985 air conditioners. Additionally during the winter, when outside air is dry, the energy Figure 2. Humidity Level Effect wheel humidifies the incoming outside air. This result is increased comfort and reduced dehumidification and Air Conditioning Load Reduction humidification requirements year-round.

The ERV effectively reduces the outside air design conditions. For example,% if outside design is 95°F and 78°F wb and the return are is 75°F and 63°F wb, an 80 effective energy wheel would precondition the outside air to 79°F and 66°F wb. This effectively changes the design conditions of the air conditioning equipment to 79°F and 66°F wb, which results in a load reduction of 3.7 tons for each 1000 cfm of outside air. This can be seen in Figure 3a and 3b. Operating Conditions Outdoor Air = 95°F Return Air = 75°F No Return Air in Supply Outdoor Air equal 78°F wb 63°F wb Air; Supply Air flow; EATR = 0 OACF ≈ 1

80

70

60

50 Total E ective E ective Total % 40

30 Energy Recovery Energy 20

10 Tons Saved with 80 Saved Tons

2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000 Airow (cfm)

Figure 3a. Energy Savings vs. Airflow with AAONAIRE Energy Recovery

6 Dening Quality. Building Comfort. Point Airflow Dry Bulb Wet Bulb Enthalpy Density Name (cfm) (°F) (°F) (Btu/lb) (lb/cu.ft.) Outside Air 1000 95 78 41.4 .0696 Return Air 1000 75 63 28.5 .0730 Air Entering 1000 79 66 31.1 .0723 Cooling Coil

The enthalpy difference between the outside air and the air entering the cooling coil is 10.3 Btu/lb. The average density of the air streams is 0.071 lb/cu.ft. At 1000 cu.ft./min the energy saved is: ⎛ Btu⎞ ⎛ lb ⎞ ⎛ ft 3 ⎞ ⎛ min⎞ Btu 10.3 ⎜ 0.071 ⎟ ⎜ 1000 ⎟ 60 = 43,878 = 3.7 tons ⎝ lb ⎠ ⎝ ft 3 ⎠ ⎝ min⎠ ⎝ hr ⎠ hr

€

Dry Bulb Temperature - ˚F

Figure 3b. Psychrometric Chart for 80% Effective Energy Recovery

7 Dening Quality. Building Comfort. The psychrometric chart of Figure 4 shows the temperature and humidity moderating affect of the% AAONAIRE energy recovery system. This Figure assumes a 75 effective total energy wheel.

The center point of the chart represents the return air conditions entering the AAONAIRE energy recovery system. The differing outside air conditions, represented by circles, are shown at various points on the psychrometric

F chart. The resultant air properties entering the ˚ - p m HVAC equipment is represented by diamonds. In e T n o all cases, heating and cooling requirements are ti ra u at significantly reduced with the addition of the, S nt oi AAONAIRE energy recovery system. P ew D b, ul B et W

Dry Bulb Temperature - ˚F

Figure 4. Energy Savings with AAONAIRE Energy Recovery

APPLICATIONS

Typical comfort air In most comfort air conditioning conditioning applications applications, the AAONAIRE energy K-12 Schools Swimming pools recovery ventilator lowers the Universities Locker rooms enthalpy of the building supply air Hospitals Residential during warm weather and raises it Assisted Care Facilities Lodging during cold weather by transferring Medical Office Buildings Hotel/Motel energy between the ventilation air Strip Malls Dormitories supply and exhaust airstreams. Department Stores Manufacturing Restaurants Religious & Worship The AAONAIRE energy recovery Animal ventilation Federal Buildings Plant ventilation Libraries devices for comfort air conditioning Convenience Stores Office Properties applications may be sensible heat exchange devices (HRVs - transferring Table 1. Typical comfort air conditioning applications sensible energy only) or energy exchange devices (ERVs - transferring both sensible energy and moisture).

8 Dening Quality. Building Comfort. Heat Recovery Ventilators

air correction factor (OACF), which is the AAONAIRE HRVs are suitable when outside ratio of supply inlet to outlet air flow. air humidity is low and latent space loads AHRI Standard 1060 are high for most of the year, and also for use with swimming pools, locker rooms and indirect evaporative coolers. AHRI Standard 1060, Rating Air-to-Air Energy Recovery Ventilators Energy Recovery Ventilation Equipment, is an industry-established standard for rating air-to-air heat/energy exchanger AAONAIRE ERVs are suitable for performance for use in energy recovery applications in schools, offices, residences ventilation equipment. This standard, and other applications that require year- based on ASHRAE Standard 84, establishes round economical preheating and/or definitions, requirements for marking precooling of outside supply air. and nameplate data, and conformance PERFORMANCE RATINGS conditions intended for the industry, including manufacturers, engineers, in- stallers, contractors, and users. Standard Standard laboratory rating tests and temperature and humidity conditions at predictive computer models give which equipment tests are to be conducted AAONAIRE exchanger performance values are specified for summer and winter for (1) heat transfer, (2) moisture transfer, conditions. Published ratings must be (3) cross-stream air transfer, (4) average reported for each of the seven performance exhaust mass airflow, and (5) supply mass factors specified in ASHRAE Standard 84. airflow leaving the exchanger. Effectiveness The AHRI certification program using ratios for heat and mass water vapor Standard 1060 is used to verify ratings transfer have been separately determined published by manufacturers . by rating tests in a laboratory that is staffed and instrumented to meet requirements of AHRI Standard 1060 requires balanced ASHRAE Standard 84 and AHRI Standard airflow rates (see Figure 5) and the 1060. Winter:following conditions: ASHRAE Standard 84 t tw t Outside air at 1 = 35°F and 1 = 33°F tw p – p ASHRAE Standard 84, Method of Testing Inside (room) air at 3 = 67°F 3 2 3 Air-to-Air Heat Exchangers, (1) establishes Summer: = 58°F and = 0 a uniform method of testing for obtaining t d tw performance data; (2) specifies the data t required, calculations to be used, and Outside air at 1 = 95°F an 1 = 78°F tw p – p reporting procedures for testing each of Inside (room) air at 3 = 75°F 3 2 3 seven independent performance factors STATE 1 = 63°F and = 0 STATE 2 and their uncertainty limits; and (3) OUTSIDE AIR specifies the types of test equipment. The independent performance factors specified Q Q s AIR-TO-AIR by Standard 84 are sensible (ε ), latent RECOVERY WHEEL CROSS-FLOW AIRSTREAM (εL), and total (εt) effectivenesses; supply (ΔPs) and exhaust (ΔPe) air pressure drops; INSIDE AIR

exhaust air transfer ratio (EATR), which STATE 4 STATE 3 characterizes the fraction of exhaust air transferred to the supply air; and outside Figure 5. Air Leakage in Energy Recovery Units

9 Dening Quality. Building Comfort. Balanced mass airflows, as required for recovery ventilation components and the some ASHRAE and AHRI standard test energy efficiency of packaged heating, methods, are rarely achieved in field ventilating, and/or air conditioning operation for air handling systems. Fans systems utilizing such components. are nearly constant-volume devices Guideline V uses the results of tests usually designed to run at a preset rpm. based on ASHRAE Standard 84 and AHRI Significantly more mass airflow will be Standard 1060. Guideline V provides a transported in cold (winter) conditions method of calculating the effect of energy than in hot (summer) conditions. recovery ventilation on the cooling and heating system overall efficiency. For estimating changes in exchanger Recovery Energy Efficiency performance factors at each operating condition, ASHRAE Standard 84 specifies Ps Pe s L t knowledge of seven performance factors AHRI Guideline V provides a procedure for (i.e., Δ , Δ , ε , ε , ε , EATR, and OACF), but calculating the efficiency of energy recovery AHRI Standard 1060 certifies performance ventilation and its effect on efficiency at only a few standard operating conditions. and sizing of building HVAC systems for a At other operating conditions, these given operating condition. Some additional performance factors must be extrapolated information will be needed to find the using accepted correlations. Recovery Energy Efficiency (RER) and the Combined Energy Efficiency (CEF) of the Variables that can affect these performance complete AAONAIRE Energy Recovery Unit. Net conditioning delivered factors for total energy transfer or sensible RER = heat transfer devices include (1) water Total electric power consumed vapor partial-pressure differences; (2) heat transfer area; (3) air velocity or mass flow rates through the heat exchangers; (4) airflow arrangement or geometric€ Where the net space conditioning energy configuration, or characteristic dimension can be heating, humidification, cooling, of the flow passage through the recovery dehumidification or a combination thereof ventilator; and (6) method of frost control. and the total electric power consumed includes the power required to move air Current testing standards do not validate through both sides of the heat exchanger exchanger performance for testing as well as any additional power, such as the conditions that require freezing or ERV or HRV drive motor. condensing temperatures, unbalanced airflow ratios, high pressure differentials, The power required to move air through or air leakage rates based on varying the the heat exchanger is a function of both the inputs. pressure drop through the heat exchanger, Guideline V as well as the motor and fan efficiency of the air-moving device. The power required to rotate an AAONAIRE heat exchanger can Guideline V, Calculating the efficiency be measured directly from the small drive of energy recovery ventilation and its motor. Note that this energy is generally so effect on building HVAC system efficiency, small relative to the other energies as to be exists to establish a method of calculating inconsequential. the energy efficiency of applied energy

10 Dening Quality. Building Comfort. The RER is calculated from the effectiveness and pressure drop for the heat exchanger and the air moving efficiency of the fan/motor combination according to the following expression: εnet m˙ min (h1 − h2) RERTotal = Pwrblwr + Pwrcomp

Where

net : € εm˙ = Net effectiveness (sensible, latent, or total, as applicable), as defined in AHRI s Standard 1060 and determined in accordance with AHRI Standard 1060. = mass flow rate of supply air (at measurement station 2 as defined in ASHRAE m˙ Standard 84 and as illustrated in Figure 5), in units of mass of dry air per unit € e time. = mass flow rate of exhaust air (at measurement station 3 as defined in ASHRAE m Standard 84 andm as illustratedm in Figure 5), in units of mass of dry air per unit ˙ min ˙ ˙ � time. s e h 1 = The lesser of and = Total enthalpy of outdoor air entering the AAHX (measurement Station 1 as h2 defined� in A� SHRAE Standard 84 and as illustrated in Figure 5). Pwrblwr = Total enthalpy of return air entering the AAHX (measurement Station 3 as Pwrcompdefined in ASHRAE Standard 84 and as illustrated in Figure 5). = P = Blower power Where = Direct power input to the AAHX component /635 : P = QΔp η f

Note: When the net effectiveness and pressure drops are determined according to AHRI-1060, it is for specific supply and exhaust air flow rates . In any air-to-air heat exchanger, the effectiveness decreases and the pressure drop€ increases as the air flow rates increase . Therefore, the preceding expressions for the RER are valid only when the effectiveness has been determined at the supply and exhaust air flow rates of interest .

Information Needed For Information for sample Calculations calculation

% % % Energy Recovery Effectiveness (εcooling) Sensible = 71 , Latent = 64 , Total=68 Supply Air Flow 4000 ft3/min Exhaust Air Flow 4000 ft3/min ∆P across Energy Recovery Component 1.25 in. w.c. (5.40 lb/ft2) Fan/Motor Efficiency 0.60

Supply Air Enthalpy (h1) 92°F, 74°F wb = 37.9 Btu/lb

Exhaust Air Enthalpy (h3) 75°F, 63°F wb = 28.7 Btu/lb

Pwrcomp 50 W

Table 2. Combined Energy EfficiencyD ata

11 Dening Quality. Building Comfort. εnet m˙ min (h1 − h3) RERTotal = Pwrblwr + Pwrcomp

ε (Q⋅ ρ )(h − h ) RER = net air 1 3 € Total   QsΔps + QeΔpe   + Pwrcomp  C ⋅η f 

 3  0.684000 ft 0.075 lb  37.9 28.7 Btu  60 min   • 3  ( − )  €   min ft  lb  hr   RER = Total      3 3  4000 ft 5.4 lb  4000 ft 5.4 lb    • 2  +  • 2    min ft   min ft    + 50 W ft − lb   44.25 f     min   (0.60)   W          

Btu 112,608 hr Btu € RER = = 67.14 Total 1677W W ⋅ hr

€ Combined Energy Efficiency

When an air-to-air heat exchanger is combined with a unitary air conditioner, (i.e. AAONAIRE) the air-to-air heat exchanger provides a portion of the total cooling capacity and the vapor compression cycle of the unitary air conditioner provides the rest. Consistent with the basic Net Cooling Capacity principle, EER = Total Power Consumed

€ the cooling system EER of a unitary air conditioner with an Air-to-Air Heat eXchanger (AAHX) cooling component can be defined as: AAHX Net Cooling Capacity + Unitary Net Cooling Capacity SysEER = AAHX Electric Power Consumption + Unitary Electric Power Consumption

€

12 Dening Quality. Building Comfort.

The combined cooling system efficiency (CEFcooling) is calculated from the efficiency of the air-to-air heat exchanger (RER) and the efficiency of the packaged equipment (EERunitary) 1 according to the following CEFexpression:= Y (1− Y) + RERAAHX EERunitary

Where: € Net Cooling Capacity Y = AAHX Net Cooling CapacityAAHX + Net Cooling Capacityunitary

Btu w h € ⋅ and RER is expressed in Note: RER can be calculated on the basis of total energy recovery, latent recovery or sensible recovery effectiveness . The � selection of the RER basis will depend on the analysis being conducted: use total for cooling and dehumidification, latent for dehumidification only and sensible for cooling without dehumidification .

For an AAONAIRE unit with a direct% expansion system of EER=12 and where the ERV component (AAHX) is handling 30 of the system load at design conditions, the CEFcooling is given by 1 CEF = cooling Y (1− Y) + With only that addition of RHR EER AAHX unitary an energy recovery wheel, the AAONAIRE unit is 33% 1 more efficient than the CEF = € cooling 0.3 (1− 0.3) standalone DX system. + 67.14 12

€ Btu CEF = 15.9 cooling w ⋅ h

ECONOMI€ C CONSIDERATIONS

Initial installed HVAC system cost is often lower for AAONAIRE air-to-air energy recovery devices because mechanical refrigeration and fuel-fired heating equipment can be reduced in size. Thus, a more efficient HVAC system may also have a lower installed total cost. The installed cost of heat recovery systems becomes lower per unit of flow as the amount of outside air used for ventilation increases.

13 Dening Quality. Building Comfort. AAONAIRE energy recovery cost benefits Selecting total energy recovery equipment are best evaluated considering all capital, results in the transfer of moisture from the installation, operating, and energy-saving airstream with the greater humidity ratio costs over the equipment life under normal to the airstream with the lesser humidity operating conditions in terms of the life- ratio. This is desirable in many situations cycle cost. As a rule, neither the most because humidification costs are reduced efficient nor the least expensive energy in cold weather and dehumidification loads recovery device will be most economical. are reduced in warm weather. The absolute cost of energy and relative Payback Analysis costs of various energy forms are major economic factors. High energy costs favor high levels of energy recovery. In regions The annual energy savings and payback where electrical costs are high relative to of the AAONAIRE energy recovery device fuel prices, heat recovery devices with low can be demonstrated with a few simple pressure drops are preferable. calculations. Annual energy savings are based on climatic conditions. The analysis Heat recovery equipment may reduce that will be done uses the format of the size requirements for primary utility “Bin and Degree Hour Weather Data for equipment such as boilers, chillers, and Simplified Energy Calculations”, Dry Bulb burners, as well as the size of piping and Temperature and Mean Coincident Wet electrical services to them. Larger fans Bulb (MCWB) from ASHRAE. and fan motors (and hence fan energy) are generally required to overcome increased The recovered energy savings can be hand static pressure loss caused by the energy calculated, using equations discussed in recovery devices. Auxiliary heaters may be this document, or selection software can required for frost control. be used to provide the recovered energy savings.

Information Needed For Information for sample Calculations calculation Location Chicago, IL AAONAIRE Operating Schedule 4am – 8pm, 5 days per week Net Outdoor Airflow Rate 4000 cfm Net Indoor Airflow Rate 4000 cfm Electric Utility Rate $0.10 per kW-h Gas Utility Rate $10 per MMBtu Electric Heating Efficiency COP = 1 Gas Heating Efficiency 82% efficiency Packaged Equipment Cooling Efficiency COP = 2.9 (EER = 10) % % % Energy Recovery Effectiveness Sensible = 71 , Latent = 64 , Total (εcooling ) = 68 ∆P across Energy Recovery Component 1.25 in. w.c.

Table 3. AAONAIRE Sample Payback Analysis

14 Dening Quality. Building Comfort. Ventilation AAONAIRE Net Annual Dry Bin MCWB Enthalpy Load w/o Recovered Ventilation Cooling Bulb Occurrence AAONAIRE Load Load Savings F F Hours Btu/lb MBtu MBtu MBtu Dollars Annual Cooling Energy Savings 97 73 1 36.87 99 69 30 1 92 75 24 38.83 4,075 2,779 1,297 28 87 71.21 95 35.45 11,027 7,525 3,502 75 82 68.26 234 32.9 16,918 11,474 5,444 115 77 65.55 345 30.72 12,629 8,456 4,173 85 699 44,748 30,303 14,446 $303

Table 4. Cooling Savings for Chicago, IL

Ventilation AAONAIRE Net Annual Dry Bin MCWB Enthalpy Load w/o Recovered Ventilation Heating Bulb Occurrence AAONAIRE Load Load Savings

F F Hours Btu/lb MBtu MBtu MBtu Dollars Annual Heating Energy Savings 67 60.54 364 27.08 157 114 43 1 62 55.26 356 23.59 2,621 2,059 561 25 57 51.08 256 21.07 7,618 5,843 1,775 71 52 46.41 262 18.52 18,814 13,694 5,119 167 47 41.87 211 16.23 23,397 16,690 6,707 204 42 38.7 268 14.74 36,586 26,047 10,539 318 37 33.76 426 12.57 73,970 52,284 21,686 638 32 29.03 377 10.58 78,345 55,176 23,169 673 27 24.87 215 8.94 50,711 35,686 15,025 435 22 20.88 192 7.47 50,180 35,320 14,860 431 17 15.85 122 5.73 35,541 24,988 10,553 305 12 10.37 60 3.95 19,282 13,542 5,740 165 7 5.33 32 2.42 11,176 7,849 3,327 96 2 0.93 21 1.13 7,924 5,570 2,354 68 -2 -2.89 14 0.04 5,272 3,712 1,561 45 -7 -8.00 14 -1.36 5,598 3,942 1,656 48 3190 427,192 302,516 124,675 $3,689

Table 5. Heating Savings for Chicago, IL

15 Dening Quality. Building Comfort. AAONAIRE TCoolinghe annual savings in each table areMBtu found to kW-h using the following equations: 1 = Recovered x x Utility Rate x = $$$ Savings conversion Load Efficiency

Cooling kW − h $0.10 per 1 = 30,303 MBtu x x x = $303 Savings 0.2931 MBtu kW-h€ 2.9

AAONAIRE 1 Heating MMBtu to MBtu = Recovered x x Utility Rate x = $$$ Savings € conversion € Efficiency Load

1 Heating MMBtu $10 per = 302,516 MBtu x x x = $3,689 Savings 1000 MBtu MMBtu€ 0.82

Table 6. Annual Energy Savings for Chicago, IL

€

$3992 For$ this$ example in Chicago, IL, the AAONAIRE would provide an annual savings of 303 + 3689 = . If electric heat had been used instead of gas heat, the savings for electric heat could be found utilizing the same calculation method. Heating 302,516 kW − h $0.10 per 1 = x 0.2931 x x = $8,864 Savings MBtu MBtu kW-h 1.0

The payback for the Chicago, IL example can be shown in the following worksheet . € € Payback Analysis

Cooling Load Without AAONAIRE With AAONAIRE Peak Ventilation Cooling Load (tons) 14.1 4.5 Space Cooling Load (tons) 50.0 50.0 Total Cooling Load (tons) 64.1 54.5 Cooling Equipment Reduction (tons) 9.6

Table 7. AAONAIRE Cooling Load Reduction

Heating Load Without AAONAIRE With AAONAIRE Peak Ventilation Heating Load (MBtu/hr) 400 118 Space Heating Load (MBtu/hr) 480 480 Total Heating Load (MBtu/hr) 880 598 Heating Equipment Reduction (MBtu/hr) 282

Table 8. AAONAIRE Heating Load Reduction

16 Dening Quality. Building Comfort. Payback Analysis Energy recovery unit Cooling cost ($/ton installed) = $1,000 installed costs vary by Fossil fuel heating cost ($/MBtu/hr installed) = $4.50 application$ with ranges $ from$ 0.25/cfm to AAONAIRE ($/cfm installed) = 2.25 5.00/cfm. Because the AAONAIRE is factory Avoided Cooling Equipment Cost = +$9,600 installed as an integral Avoided Heating Equipment Cost = +$1,269 part of the rooftop unit it AAONAIRE First Cost = - $9000 is always a cost effective Net AAONAIRE First Cost = $1869 alternative to separate, standaloneThe AAONAIRE ERV units energy. Annual Savings = $3992 recovery device would have a payback of $1869 less than one year Payback Period = $ = .47 years and continue to save 3992 thousands of dollars per year for the life of Table 9. AAONAIRE Payback Analysis for Chicago, IL the unit. €

Figure 6 shows the energy savings for the building with and without the AAONAIRE energy recovery ventilator. Both figures show a monthly energy savings throughout the year.

Figure 6. AAONAIRE Heating and Cooling Energy Savings for Chicago, IL

17 Dening Quality. Building Comfort. Using the same AAONAIRE unit and the following operating conditions, heating and cooling savings can be calculated for cities with differing climatic conditions. Information Needed For Information for sample Calculations calculation

AAONAIRE Operating Schedule 4am – 8pm, 5 days per week Net Outdoor Airflow Rate 4000 cfm Net Indoor Airflow Rate 4000 cfm Electric Utility Rate $0.10 per kW-h Fossil Fuel Utility Rate $10 per MMBtu Packaged Equipment Cooling Efficiency COP = 2.9 (EER = 10.0) Heat Pump Heating Efficiency COP = 4.0 (EER = 13.6) Fossil Fuel Heating Efficiency 82% efficiency Electric Resistance Heating Efficiency COP = 1

Table 10. AAONAIRE Heating and Cooling Savings Inputs

Annual Cooling Annual Heating Operation Operation Electric Cooling Heat Pump Fossil Fuel Resistance Savings Savings Savings Savings Chicago, IL $303 $2,216 $3,689 $8,864 New York, NY $310 $1,856 $3,091 $7,425 Miami, FL $2,686 $80 $134 $322 Atlanta, GA $682 $1,061 $1,767 $4,245 Minneapolis, MN $286 $2,723 $4,534 $10,894 Dallas, TX $1,190 $776 $1,292 $3,104 Houston, TX $1,851 $499 $830 $1,995 Denver, CO $231 $2,068 $3,444 $8,273 Los Angeles, CA $88 $313 $521 $1,252 Seattle, WA $29 $1,508 $2,512 $6,032

Table 11. AAONAIRE Annual Savings for Select Cities

18 Dening Quality. Building Comfort. The following chart shows an estimated payback period for the continental United States. The • following assumptions were made in the creation of the chart. • • Office building with HVAC system$ operating 16 hr/day, 5 days/week • AAONAIRE installed cost of 3.60 per cfm $ • Air conditioning$ equipment installed$ cost of 1,000 per ton Energy costs of 0.10 per kW-hr and% 10 per MMBtu Energy recovery effectiveness of 68

Payback - Instant

Payback <_1 year Payback 1-2 year Payback 2- 4 year

Figure 7.

AAONAIRE Payback Map

RATE OF ENERGY TRANSFER

The second law of thermodynamics states The rate of energy transfer depends on that heat energy always transfers from a the operating conditions and the intrinsic region of high temperature to one of low characteristics of the energy exchanger. As temperature. This law can be extended to in a conventional heat exchanger, energy say that mass transfer always occurs from transfer between the airstreams is driven a region of high vapor pressure to one of by cross-stream dry-bulb temperature low vapor pressure. The AAONAIRE ERV differences. Energy is also transported facilitates this transfer with a polymer piggyback-style between the streams by energy transfer media that conducts cross-stream mass transfer, which may heat and permanently bonded silica gel include air, gases, and water vapor. In desiccant that is permeable to water vapor. another mode of energy transfer, water Moisture is transferred when there is a vapor condenses into liquid in one of difference in vapor pressure between the the two airstreams of the exchanger. The two airstreams. condensation process liberates the latent

19 Dening Quality. Building Comfort. heat of condensation, which is transferred Using the airstream numbering convention to the other stream as sensible heat; this shown in Figure 8, on a typical summer x ms two-step process is also called latent heat day, supply air at temperature, humidity, or transfer. enthalpy of 1 and mass flow rate enters xthe AAONmAIRE ERV, while exhaust airx from Latent energy transfer between airstreams the conditioned space entersx at conditions occurs only when moisture is transferred 3 and 3. Because conditions at 3 are from one airstream to another without lower than conditions at 1, heat and mass condensation, thereby maintaining the transfer from the supply airstream to the latent heat of condensation. Once moisture exhaust airstream because of differences has crossed from one airstream to the other, in temperature and vapor pressures across it may either remain in the vapor state or the separating wall. Consequently, the condense in the second stream, depending supply air exit properties decrease, while on the temperature of that stream. those of the exhaust air increase. Exit properties of these two streams can be AAONAIRE rotating energy recovery calculated, knowing the flow rates and the units are used because of their moisture effectiveness of the heat exchanger. exchange function. Passage of air or other gases across the exchanger is a negative ASHRAE Standard 84 defines effectiveness consequence. As well, some cross-stream Actual Transfer of Moisture or Energy ε =as mass transfer may occur through leakage Maxium Possible Transfer Between Airstreams even when such transfer is unintended. This may alter exchanger performance from its design value, but for most HVAC applications with exhaust air from occupied€ Heat Recovery Ventilators spaces, small transfers to the supply air are s not important. From Figure 8, the sensible effectiveness ε Heat transfer differs in principle from of a AAONAIRE heat recovery ventilator is mass transfer. Heat transfer only occurs given asq m c (t2 − t1) m c (t3 − t4 ) ε = s = s ps = s pe when there is a temperature difference. s qs, max Cmin (t3 − t1) Cmin (t3 − t1) In the case of air-to-air exchange between the supply and exhaust airflow, heat

transfer by conduction and convection qs only occurs when there is a temperature€ difference between these airstreams. The where is the actual sensible heat transfer following facts about heat/mass exchanger rate given by qs = εsqs, max performance should be recognized:

The effectiveness for moisture transfer qs,max may not equal the effectiveness for heat € transfer. The total energy effectiveness where is the maximum sensible heat may not equal either the sensible or latent transfer rate given by q = 60C (t3 − t1) effectiveness. s, max min

Net total energy transfer and effectiveness need careful examination when the direction of sensible (temperature-driven) € transfer is opposite to that of latent (moisture or water vapor) transfer.

20 Dening Quality. Building Comfort. where

s 2: SUPPLY AIR LEAVING 1: SUPPLY AIR ENTERING t ε = sensible effectiveness 1 x x w = dry-bulb temperature at 2 1, 2 ms location 1 in Figure 8, °F ENERGY RECOVERY = supply dry air mass flow rate, DEVICE me lb/min w x x = exhaust dry air mass flow rate, 2, 3 4 Cmin cpsms cpeme lb/min cps = smaller of and 3: EXHAUST AIR ENTERING 4: EXHAUST AIR LEAVING = supply moist air specific heat at constant pressure, Figure 8. Airstream Numbering Convention cpe Btu/lb·°F = exhaust moist air specific heat at constant pressure, qs Btu/lb·°F The sensible heat energy transfer from Assuming no water vapor condensation in wherethe AAONAIRE heat recovery ventilator can the AAONAIRE HRV, the leaving supply air be estimated from condition is C min q = 60m c (t − t ) = 60Q ρ c (t − t ) t2 = t1 − εs (t1 − t3) s s ps 2 1 s s ps 2 1 msc ps 60 60 qs = mec pe (t4 − t3) = Qeρec pe (t4 − t3) € € and the leaving exhaustC air condition is 60 min qs = εsmminc p (t1 − t3) t4 = t3 − εs (t1 − t3) mec pe €

Qs

€Q e € = volume flow rate of supply air, cfm The preceding equations assume steady- = volume flow rate of exhaust air, s state operating conditions; no heat or cfm 3 e moisture transfer between the heat ρ = density of dry supply air, lb/ft 3 t1 t2 t3 t4 exchanger and its surroundings; no cross- ρ = density of dry exhaust air, lb/ft leakage, and no energy gains from motors, , , , = inlet and exit temperatures fans, or frost control devices. Furthermore, of supply and exhaust airstreams, mmin ms me condensation or frosting does not occur or is respectively negligible. These assumptions are generally = smaller of and true for larger commercial AAONAIRE HRV applications. Note that the HRV only allows transfer of sensible heat energy associated with heat transfer because of a temperature difference between the airstreams. These equations apply even in winter, if there is no condensation in the HRV.

21 Dening Quality. Building Comfort. t

Sensible heat exchangers (HRVs) can be The total effectiveness ε of an AAONAIRE − − used in virtually all cases, especially for energyq recoverym ventilator(h2 h1) is givenm (h as3 h4 ) ε = t = s = e swimming pool, paint booth, and reheat t − − q ,max mmin (h3 h1) mmin (h3 h1) applications. t Energy Recovery Ventilators

qt � The AAONAIRE ERV allows the transfer of where is the actual total heat transfer ε both sensible and latent heat, the latter due rate given by qt = tqt,max to the difference in water vapor pressures between the airstreams. (For a full q explanation of the following latent transfer t,max equations see Appendix.) where� is the maximum total heat transfer rate given60 by From Figure 8, assuming no condensation qt,max = mmin (h1 − h3 ) L in the AAONAIRE ERV, the latent effect- iveness ε of an energy recovery ventilator is given as where q m h (w1 − w2 ) m h (w4 − w3) L s fg e fg � εL = = = t qL, max mmin h fg (w1 − w3) mmin h fg (w1 − w3 ) h ε = total effectiveness m = enthalpy at locations indicated in q s L Figure 8, Btu/lb € m = supply dry air mass flow rate, lb/ where is the actual latent heat transfer e min rate given by q = ε q L L L,max = exhaust dry air mass flow rate, lb/ mmin ms me min q L,max = smaller of and where� is the maximum heat transfer The leaving supply mair enthalpy condition rate given by 60 min qL,max = mmin h fg (w1 − w3 ) is h = h − ε w − w s 1 t m ( 1 3 ) s

where

� L h � fg and the leaving exhaust air enthalpy εw = latent effectiveness m conditionh is= h − ε min h − h = enthalpy of vaporization, Btu/lb 4 3 t m ( 1 3) = humidity ratios at locations e ms indicated in Figure 8 m = supply dry air mass flow rate, e q lb/min L � Assuming the stream at state 1 is of higher m = exhaust drym air massm flow rate, min s e humidity, the latent heat recovery from lb/min the ERV can be60 estimatedε from− = smaller of and qL = L mmin h fg (w1 w3)

�

22 Dening Quality. Building Comfort. where

hfg w w w w = enthalpy of vaporization or heat of vaporization of water vapor, Btu/lb 1, 2, 3, 4 = inlet and exit humidity ratios of supply and exhaust airstreams, respectively qt The total energy60 εtransfer −between the streams is given by qt = t mmin (h1s h3e ) where

� h s

h e 1 = enthalpy of supply air at inlet, Btu/lb h s 3 = enthalpy of exhaust air at inlet, Btu/lb h e 2 = enthalpy of supply air at outlet, Btu/lb 4 = enthalpy of exhaust air at outlet, Btu/lb

AAONAIRE ERVs can be used where there is an opportunity to transfer heat and mass (water vapor) (e.g., humid areas, schools, offices with large occupancies). Latent energy transfer qs can be positive or negative depending on the direction of decreasing vapor pressure. An qL airstream flowing through an ERV may gain heat energy (+ ) from the adjoining stream, but qs qL will lose the latent energy (– ) if it transfers the water vapor to the adjoining stream, because of transfer of moisture. The total net energy gain is the difference between and , as shown in Example 1.

Example 1. Total Heat Recovery in Summer Return air at 75°F and 63°F wb (ρ = 0.073 lb/ft3) with a flow rate of 10,600 cfm is used to precool 8500 cfm of supply outdoor air at 95°F and 81°F wb (ρ = 0.069 lb/ft3) using an AAONAIRE total energy exchanger. The sensible and total effectiveness for this heat exchanger are 70% and 56.7%, respectively. Assuming EATR = 0 and OACF ≈ 1, determine the leaving supply air conditions and energy recovered.

Dry Bulb Temperature - ˚F

23 Dening Quality. Building Comfort. Point Dry Wet Relative Density Humidity Enthalpy Name Bulb Bulb Humidity (lb/cu.ft.) Ratio (Btu/lb) (°F) (°F) (%) (lb/lb) Return Air 75 63 52 0.073 0.0096 28.5 Outdoor Air 95 81 55 0.069 0.0198 44.6 Supply Air 81 71.6 64 0.072 0.0145 35.5

Solution:

1. Calculate the theoretical maximum heat transfer. The outdoor airstream is a lesser or limiting airstream for energy and moisture transfer. Determine entering airstream Outside air (95°F db, 81°F wb) h = 44 6. Btu/lb w = 0 0198. lb/lb enthalpies and humidity ratio from psychrometric1 chart1 .

Return air (75°F db, 63°F wb) h3 = 28 5. Btu/lb w3 = 0 0096. lb/lb

The theoretical maximum sensible and total heat transfer rates can be obtained as q (sensible) = (60)(0 .069 lb/ft3)(8500 ft3/min)(0 .24 Btu/lb·°F)×(95 – 75) follows:max = 169,000 Btu/h

3 3 qmax (total energy) = (60)(0 .069 lb/ft )(8500 ft /min)(44 6. –28 .5) = 567,000 Btu/h

2. Determine supply sensible and total effectiveness. ε = 70% The datas given for the design conditions provide the following effectiveness ratios: % εt = 56 .7

q = (0 .567)(567,000 Btu/h) = 321,000 Btu/h total recovered 3. Calculatet energy transfer at design conditions.

qs = – (0 .7)(169,000 Btu/h) = -118,000 Btu/h sensible recovered

qt = qs + qlat

q = 203,000 Btu/h latent recovered Therefore,lat

24 Dening Quality. Building Comfort. 4. SupplyCalculate air leaving conditions air conditions. Btu −118,000 hr t2 = 95°F + = 81°F  lb   ft 3   Btu  60 0.069 3   8500  0.24  ft   min  lb⋅°F 

Btu 321,000 Btu − Btu € h = 44.6 + hr = 35.5 2 lb  lb   ft 3  lb 60 0.069 3   8500   ft   min

€ w tw From the psychrometric chart, the supply air humidity ratio and wet-bulb temperature 2 2 areExhaust found air to beconditions = 0.0145 and = 71.6°F. Btu +118,000 hr t4 = 75°F + = 86.2°F  lb   ft 3   Btu  60 0.073 3   10,600  0.24  ft   min  lb⋅°F

Btu 321,000 Btu + Btu € h = 28.5 + hr = 35.4 4 lb  lb   ft 3  lb 60 0.073 3   10,600   ft   min

w tw From€ the psychrometric chart, the exhaust humidity ratio and wet bulb temperature are found to be 4 = 0.0134 and 4 = 71.6°F.

25 Dening Quality. Building Comfort. DESIGN CONSIDERATIONS Engineered Polymer vs. Aluminum Wheel Material OA #1

EA #4 Aluminum as a material for energy recovery wheels has been used since the early 1900’s. SA #2 It is accepted as a rugged material with good heat transfer characteristics, however, aluminum wheels have a very heavy weight RA #3 OACF EATR and associated cost, problematic corrosion in salt air applications and problems with Figure 9. AAONAIRE Rotary Air-to-Air maintaining the desiccant coating over Energy Exchanger time due to loss of the bond between the aluminum and the epoxy material used to AAONAIRE rotary air-to-air energy hold the desiccant. exchangers AAONAIRE technology, in contrast to the older aluminum technology, makes use of an engineered polymer with desiccant The AAONAIRE is a rotary air-to-air energy permanently imbedded. The AAONAIRE exchanger which has a revolving cylinder energy transfer matrix design overcomes filled with an air-permeable medium the problems associated with traditional having a large internal surface area. aluminum wheels by providing lower cost, Adjacent supply and exhaust airstreams lighter weight, cleanable, permanently each flow through half the exchanger in bonded desiccant that is completely a counterflow pattern (Figure 9). Heat corrosion resistant in seacoast applications. transfer media may be selected to recover To further the maintainability, AAONAIRE sensible heat only, HRVs, or total (sensible wheels are made in segments which can be plus latent) heat, ERVs. easily removed from the unit for cleaning Sensible heat is transferred as the medium outside the unit. picks up and stores heat from the warmer airstream and releases it to the cooler one. Latent heat is transferred as the medium adsorbs water vapor from the higher- humidity airstream and desorbs moisture into the lower humidity airstream, driven in each case by the vapor pressure difference between the airstream and energy exchange medium. Thus, the moist air is dried while the drier air is humidified. In AAONAIRE ERVs, both sensible and latent heat transfer Figure 10. AAONAIRE Segmented Wheel occur simultaneously. AAONAIRE HRVs (not coated with desiccant) can also transfer water via a mechanism of condensation and re-evaporation driven by dew point and vapor pressure; the effectiveness This feature eliminates costly and varies strongly with conditions. Because troublesome drain pans under the wheel, a AAONAIRE rotary exchangers have a requirement of the aluminum application. counterflow configuration and normally Because of the robust design and the easily use small-diameter flow passages, they cleanable segments, a life of 20-30 years are quite compact and can achieve high with original factory performance can be transfer effectiveness. achieved.

26 Dening Quality. Building Comfort. 40

Silica Gel Silica gel

30 Alumina (Spherical) Silica gel is an inert, highly porous solid adsorbent material that structurally Molecular Sieves 20 resembles a rigid sponge. It has a very 0 Per 100 lb. Adsorbent 100 lb. 0 Per 2 large internal surface composed of myriad microscopic cavities and a vast system of Alumina (Granular) capillary channels that provide pathways 10 connecting the internal microscopic cavities Adsorption - lb. H - lb. Adsorption to the outside surface of the “sponge”.

0 The characteristic curve for adsorption 0 20 40 60 80 100 Relative Humidity (%) of water on silica gel is shown in Figure 11 as % weight adsorbed versus relative Figure 11. Desiccant comparison (after W.R. Grace, humidity of the airstream in contact with Davision Chemical Division, “Silica Gels”) the silica gel. The amount of water adsorbed rises almost linearly with increasing relative humidity until relative humidity reaches about 60%. It then plateaus out at In addition to being non-corrosive, polymer about 40% adsorbed as relative humidity is also flexible. If, during maintenance, a tool approaches 100%. The curve for molecular would impact the media, it can pass through sieves, by contrast, rises rapidly to plateau the media wraps without damaging the at about 20% adsorbed at 20% relative heat transfer matrix. This same robustness humidity. This helps to explain why the is not found in the aluminum wheel design. molecular sieve is an excellent choice for The face of an aluminum wheel is similar regenerated applications such as desiccant to the face of a aluminum heat exchange cooling and dehumidification systems coil; the fluted design is very delicate. If which are designed to reduce processed impacted with a tool of any sort, the flutes airstreams to very low relative humidities. bend and block air from passing through On the other hand, silica gel has superior the wheel and reducing the efficiency and characteristics for the recovery of space effectiveness of the wheel. comfort conditioning energy.

Even though many aluminum wheel designs The use of silica gel on AAONAIRE energy are very heavy, few aluminum wheels recovery ventilation applications involves are segmented. Therefore the entire hub a process cycle where the silica gel is assembly must be taken apart to remove alternately exposed to airstreams having the wheel from the cassette. On larger nearly equal relative humidity somewhere units, the wheel cannot be removed from in the mid range of this curve (typically the unit at all. By contrast the AAONAIRE between 40 and 60% for comfort cooling cassette containing the polymer wheel applications). When the airstream with slides out of the air handling or rooftop the higher relative humidity passes over unit for easy service. Polymer wheels the silica gel coated wheel, moisture is larger than 30 inches are segmented, with adsorbed from the airstream into the silica the weight of each segment ranging from gel. Then when the airstream with the 4-23 pounds. The segments can easily be lower relative humidity contacts the silica removed without the use of tools. gel, moisture is desorbed (removed) from the silica gel and put into the airstream.

27 Dening Quality. Building Comfort. Adsorption is different than absorption. transfer is by examining cases for summer Absorption occurs when one substance has operation. Example 2 is at summer full fully entered or permeated into the other. load conditions; at these conditions the Adsorption is a surface phenomenon and outdoor temperature and humidity require occurs when one substance sticks to the maximum cooling. Example 3 is at summer surface of the other and does not change part load conditions; at these conditions the the matrix property of the substrate. cooling requirement is below maximum, Adsorption would be analogous to static but outdoor humidity is still high. cling. When a fabric sticks to a person, Both examples will use the person does not absorb the fabric, the an energy recovery person adsorbs the fabric. effectiveness% of Sensible Heat Exchangers (HRVs) vs. 75 for both

Total Energy Exchangers (ERVs) the ERV ENTHALPY and the HRV. Both heat recovery ventilators and energy recovery ventilators transfer sensible heat. HUMIDITY RATIO

HRVs transfer sensible heat only. ERVs LATENT transfer latent energy, or moisture, as well. SENSIBLE The transfer of moisture is important because the latent portion % of the outdoor air load exceeds 60 of DRY-BULB TEMPERATURE the total load for many climates. The best way to illustrate Figure 12. Sensible and Latent Energy the impact of latent energy on Psychrometric Chart

Example 2: Sensible and Total Energy Recovery Analysis of energy recovery at full load determines the amount of air conditioning reduction that can be realized . For this example, outdoor air at full load is 95°F and 78°F wb . The room air is 75°F and 63°F wb .

Dry Bulb Temperature - ˚F

28 Dening Quality. Building Comfort. Relative Dry Bulb Wet Bulb Enthalpy Point Name (°F) (°F) Humidity (Btu/lb) (%) Outdoor Air 95 78 47 41.4 Return Air 75 63 50 28.2 Leaving Air - ERV 80 67 51 31.5 Leaving Air - 80 74 76 37.7 HRV

Solution:

The psychrometric chart shows the leaving air conditions for both devices. Notice that the process line for the HRV is horizontal (no moisture transfer), while the process line for the ERV is directly% toward room air conditions. While both products have a sensible effectiveness of 75 , the total effectiveness (sensible & latent) of each is determined by the transfer of enthalpy, not sensible temperature only. The maximum possible enthalpy transfer in this case is 13.2 Btu/lb of air, which is the difference between outdoor air and room air (41.4-28.2). The% ERV transferred 9.9 Btu/lb (41.4-31.5), which translates into a total effectiveness of 75 . The HRV,% on the other hand, only transferred 3.7 Btu/lb (41.4- 37.7), for a total effectiveness of 28 .

The total effectiveness of the HRV is low% because it transfers% only sensible heat% . In this example, the total outdoor air %load is 37 sensible and 63 latent. The HRV is 75 effective on the sensible portion %and 0 effective% on the% latent portion. As a result, the HRV’s total effectiveness is only 28 [(.37 x 75 )+(.62 x 0 )]. The equipment reduction will be only 1.3 tons for every 1000 cfm of outdoor air. % % % The total effectiveness of the ERV is 75 [(.37 x 75 )+(.62 x 75 )] because the wheel transfers both sensible and latent energy with virtually identical efficiencies. As a result, the wheel will reduce the air conditioning load by 3.5 tons for every 1000 cfm of out- door air.

29 Dening Quality. Building Comfort. Example 3: Part Load Sensible and Total Energy Recovery Consider the part load condition of 75°F and 72°F wb .

Dry Bulb Temperature - ˚F

Dry Bulb Wet Bulb Relative Enthalpy Point Name (°F) (°F) Humidity (Btu/lb) (%) Outdoor Air 75 72 87 35.8 Return Air 75 63 50 28.2 Leaving Air - 75 65 59 30.0 ERV Leaving Air - 75 72 87 35.8 HRV

Solution:

% As the psychrometric chart illustrates, the 75 effective ERV will create leaving air conditions that are three quarters of the distance between outdoor% air and return air conditions. Even though the dry bulb temperature% did not %change, 75 of the total energy was transferred (the% outdoor load is 0 sensible and 100 latent). The HRV would have an effectiveness of 0 in this case.

Transferring latent energy at part load is extremely important for maintaining proper indoor humidity levels. Most systems operate at part load conditions much of the time. If the outdoor air is not pre-conditioned% by a total energy wheel, the indoor humidity levels would likely climb above 60 rh. This is a real problem building owners and consulting engineers face today; humidity control is far more difficult than temperature control.

30 Dening Quality. Building Comfort. Example 2 & 3 Summary:

The ability of ERVs to transfer latent energy translates into two key benefits that HRVs do not offer: 1. At full load, the ERV will reduce the outdoor air load by 2 1/2 times more than a HRV. For comparison, an ERV will reduce the load by 3.5 tons per 1000 cfm of outdoor air, where a HRV will reduce the load by 1.3 tons. 2. At part load conditions, the ERV always pre-conditions the outdoor air to near room air conditions. This function is very important to maintaining indoor humidity at desired levels. HRVs, on the other hand, have little effect if the outdoor dry bulb temperature is near (within 7°F) room air dry bulb temperature.

Humidity control is important to control in humid environments. However, humidity transfer is not always desired. In some applications, such as indirect evaporative cooling, a HRV is much better suited. Example 4. Indirect Evaporative Cooling Recovery Return air at 86°F and 63°F wb (ρ = 0.072 lb/ft3) with a flow rate of 32,000 cfm is used to precool 32,000 cfm of supply outdoor air at 102°F and 68°F wb (ρ = 0.070 lb/ft3) using an heat recovery ventilator and indirect evaporative cooling. The evaporative cooler increases the exhaust air to 90% rh before it enters the AAONAIRE HRV. The sensible effectiveness is given as 78%. Assuming EATR = 0 and OACF ≈ 1, determine the leaving supply air conditions and energy recovered.

Dry Bulb Temperature - ˚F

31 Dening Quality. Building Comfort. Point Dry Bulb Wet Relative Enthalpy Density (°F) Bulb Humidity (Btu/lb) (lb/cu.ft.) Name (°F) (%) Return Air 86.0 62.8 26.1 28.2 0.072 Outdoor Air 102.0 68.0 15.9 32.1 0.070 Return Air - After Evap 64.0 63.0 94.8 28.6 0.074 Cooler Exhaust Air 92.0 71.8 37.6 35.4 0.070 Supply Air - 72.4 57.8 40.8 24.9 0.074 HRV Supply Air - 72.4 64.1 64.3 29.3 0.073 ERV

Solution: First, determine the return air condition entering the exchanger (i.e., after it is adiabatically cooled). Air at 86°F and 63°F wb cools to 64°F and 63°F wb as shown by the process line from ‘return air’ point to the ‘return air – after evap cooled’ point in the psychrometric chart for example 4. In this problem the volumetric flows are equal, but the mass flows are not.

1. Calculate the theoretical maximum heat transfer. This would be the maximum temperature difference between the outside air entering the wheel and the air entering the wheel after leaving 3the indirect3 evaporative cooler. qmax (sensible) = lb/ft ft /min Btu/lb·°F °F = Btu/h (60)(0.070 )(32,000 )(0.24 ) × (102 – 64 ) 1,226,000

2. Calculate actual energy transfer at the design conditions. qactual = Btu/h Btu/h

(0.78)(1,226,000 ) = 956,000 recovered a. Leaving supply air temperature is 3. Calculate leaving air conditions. Btu −956,000 hr t2 = 102°F + = 72.4°F  lb   ft 3   Btu  60 0.070 3   32,000  0.24  ft   min  lb⋅°F

€

32 Dening Quality. Building Comfort. b. Leaving exhaust air temperature is

Btu +956,000 hr t4 = 64°F + = 92°F  lb   ft 3   Btu  60 0.074 3   32,000  0.24  ft   min  lb⋅°F

It can be seen from the chart and corresponding table that the HRV provides a lower relative humidity€ to the air conditioning unit than the ERV and therefore provides an energy saving benefit to the building. The air entering the cooling coil will have a relative humidity of 40.8% with the HRV, versus 64.3% with the ERV, the air conditioning unit will be required to perform little or no latent cooling (dehumidification) and will save energy using the HRV instead of an ERV.

Both ERVs and HRVs save energy. HRVs are ideally suited to applications where problems may be created by extreme differences in interior and exterior moisture levels and in which heat transfer is desired but humidity transfer is not (e.g., swimming pools, kitchens, indirect evaporative cooling). ERVs are suitable for applications in schools, offices, residences and other applications that require year-round economical preheating and/or precooling of outside supply air.

TECHNICAL CONSIDERATIONS

Air Leakage recovery system leaks into (or exits) either or both airstreams. Cross-flow air Air leakage refers to any air that enters or leakage results from inadequate sealing leaves them supply or m exhaustm airstreams. construction between ambient and cross- Zerom air leakage in either airstream would stream seal interfaces. Internal air leakage require 1 to equal 2 and 3 to equal occurs when holes or passages are open to 4. External air leakage occurs when the the other airstream. Internal air leakage ambient air surrounding the energy occurs when heat or energy exchanger OA #1 design allows (1) tangential air movement in the wheel’s rotational direction and (2) EA #4 air movement through holes in the barrier between airstreams. Under some pressure SA #2 differentials, air leaks in and out of each airstream in nearly equal amounts, giving the illusion that there is no air leakage. Heat RA #3 OACF EATR and water vapor transfer could appear to be greater than it actually is. Air leakage is Figure 13. AAONAIRE Rotary Air-to-Air seldom zero because external and internal Energy Exchanger air pressures are usually different, causing air to leak from higher-pressure regions to lower-pressure regions.

33 Dening Quality. Building Comfort. Supply Air Fan Outside Air Cross-flow air leakage is Supply usually caused by pressure Purge Section differentials between Purge Section airstreams. Carryover Purged Flow Exhaust Air air leakage (specific to Exhaust Rotation wheels) is caused by Exhaust Air Exhaust continuous rotation Fan Wheel of trapped exhaust air Front Cross Section with Side Cross Section with Purge in cavities in the heat Upstreram Exhaust Section transfer surface, which reverses airflow direction Figure 14. Cross sections (front and side) of enthalpy as the wheel rotates and wheel system with purge section spills this exhaust air into the supply airstream

Cross-flow leakage occurs because the In many applications, recirculating some differential static pressure across the two air is not a concern. However, critical airstreams drives air from a higher to applications such as hospital operating a lower static pressure region. Leakage rooms, laboratories, and clean rooms primarily occurs due to the difference in require stringent control of carryover%. static pressures between states 2 and 3 Carryover can be reduced to less than 0.1 and/or between states 1 and 4, as shown of the exhaust airflow with a purge section in Figure 13. This is a major cause of but cannot be completely eliminated. cross-flow leakage, and underscores the importance of fan locations that circulate The quantitative estimate of the air the airstreams. Cross-flow can also be leakage is expressed by two dimensionless caused by factors such as provisions for parameters: the Exhaust Air Transfer Ratio surging, geometrical irregularities, and (EATR) and Outside Air Correction Factor local velocity distribution of the airstreams. (OACF). c c EATR 2 − 1 The AAONAIRE design uses an optimum = c c 3 − 1 draw-thru, draw-thru design where the supply fan draws the first air stream c c c through the wheel and the exhaust fan 1 2 3 draws second air stream through the wheel where� , , and are the concentrations as shown in Figure 13. This configuration of inert gas at states 1, 2 and 3, respectively allows balanced pressures between the as shown in Figure 15. Note that EATR two air streams . represents an exhaust air leakage based on observed relative concentration of inert Carryover occurs as air is entrained with- m gas in supply airflowOACF. 1 in the rotation medium and is carried into = m the other airstream. Carryover occurs 2 each time a portion of the matrix passes m m the seals dividing the supply and exhaust 1 2 airstreams. Because carryover from where� and are the mass flow rates of exhaust to supply may be undesirable, a the incoming fresh airstream at state 1 and purge section can be installed on the heat 2, respectively as shown in Figure 15. OACF exchanger to reduce cross-contamination helps estimate the extra quantity of outside as shown in Figure 14. air required at the inlet to compensate for the air that leaks into or out of the

34 Dening Quality. Building Comfort. exchanger, and to meet the required net neglected, four distinct air/moisture supply airflow to the building space. Ideal regimes may occur as the warm airstream airflow conditions exist when there is no cools from its inlet condition to its outlet air leakage between the streams; EATR condition. First, there is a dry region is close to zero, and OACF approaches 1. with no condensate. Once the warm Deviations from ideal conditions indicate airstream cools below its dew point, air leakage between the airstreams, which there is a condensing region, which wets complicates the determination of accurate the heat exchange surfaces. If the heat values for pressure drop and effectiveness. exchange surfaces fall below freezing, the condensation freezes. Finally, if the warm The theoretical carryover of a wheel without airstream temperature falls below its a purge section is directly proportional to dewpoint, sublimation causes frost to form. the speed of the wheel and% the void volume The locations of these regions and rates of of the medium (75 to 95 void, depending condensation and frosting depend on the on type and configuration). For example, %a duration of frosting conditions; airflow 6 ft diameter, 2 in. deep wheel with a 90 rates; inlet air temperature and humidity; void volume operating at 14 rpm has a heat exchanger core temperature; heat carryover2 volumetric flow of exchanger effectiveness; the geometry, ⎛ 6 ⎞ ⎛ 2 ⎞ ⎛ rev ⎞ π ft ft (0.9) 14 = 60 cfm configuration, and orientation; and heat ⎝ 2 ⎠ ⎝ 12 ⎠ ⎝ min⎠ transfer coefficients.

Sensible heat exchangers, which are ideally suited to applications in which heat transfer € If the wheel is handling a 10,000 cfm is desired but humidity transfer is not balanced flow,60 the percentage carryover is (e.g., swimming pools, kitchens, indirect •100 = 0.6% 10,000 evaporative cooling), can benefit from the latent heat released by the exhaust gas when condensation occurs. One pound of moisture condensed transfers about € 1050 Btu to the incoming air at room The exhaust fan, which is usually located at temperature. the exit of the exchanger, should be sized to

STATEinclude 1 leakage and purge air flowsSTATE. 2 Condensation increases the heat transfer OUTSIDE rate and thus the sensible effectiveness; AIR it can also significantly increase pressure Q Q AIR-TO-AIR drops in heat exchangers with narrow RECOVERY WHEEL CROSS-FLOW AIRSTREAM airflow passage spacing. Frosting fouls the heat exchanger surfaces, which initially INSIDE AIR improves energy transfer but subsequently STATE 4 STATE 3 restricts the exhaust airflow, which in turn Figure 15. Air Leakage in Energy Recovery Units reduces the energy transfer rate. In extreme cases, the exhaust airflow (and supply) can become blocked. Defrosting a fully blocked heat exchanger requires that the unit be Condensation and Frost Formation shut down for an extended period.

For frosting or icing to occur, an airstream Condensation, ice formation, and/or must be cooled to a dew point below 32°F. frosting may occur on heat exchange Total heat exchangers transfer moisture surfaces. If entrance and exit effects are from the airstream with higher moisture

35 Dening Quality. Building Comfort. content (usually the warmer airstream) (through desorption, re-evaporation and/ to the less humid one. As a result, frosting or sublimation) by the entering outdoor or icing occurs at lower supply air supply air. temperatures in enthalpy exchangers than in sensible heat exchangers. In comfort air Thus for areas with winter design tem- conditioning applications, condensation peratures between -5°F and 22°F (depend- may occur in the supply side in summer ing on the indoor RH), the enthalpy wheel and in the exhaust side in winter. component enjoys a significant advantage Frost Threshold Limits over sensible units in that frost control is generally not required. Many regions, 70 where sensible HRVs require frost control,

60 will not need frost protection for an ERV Region of Region of no application. Even in the northern United 50 condensate/frost condensate/frost build-up build-up States, applications such as schools and of-

40 fice buildings can be designed without frost control because most of the frosting hours 30 are at night when the building is unoc- cupied. Bin data, such as that provided by 20 ERV Frost HRV Frost Indoor Relative Humidity (%) Indoor Relative Threshold Threshold ASHRAE, may be consulted to qualify day- 10 time applications in cold climates for frost- free operation. -20 0 20 40

Outdoor Temperature (˚F) Table 12 lists frost threshold temperatures for AAONAIRE rotary energy recovery Figure 16. Comparison of Frosting Thresholds ventilation wheels over a range of indoor for ERVs and HRVs air temperatures and relative humidities. Note that the total energy AAONAIRE wheel will tolerate limited hours of operation below the frost threshold without damage Only in climates experiencing severe or significant reduction of airflow. Frost winter design conditions will energy control is not required until entering air recovery ventilation systems require temperatures are below the threshold. frost protection or defrost means. The Frost Control and Prevention temperature below which frost will begin to accumulate on heat exchanger surfaces is referred to as the frost threshold Enthalpy wheels can provide continuous temperature. It is a function of outdoor frost-free operationIndoor at severe Air D ry winter Bulb Indoor temperature and indoor relative humidity. Temperature Air RH (%) Figure 16 compares the frost threshold 70°F 72°F 75°F 80°F of a typical sensible heat exchanger with 20 -14 -13 -11 -8 that of a typical enthalpy wheel. Note that while frost forms between 22 and 30°F in a 30 -3 -2 -1 3 sensible heat exchanger, frost thresholds for 40 5 7 9 11 total energy exchangers are generally 20 to 50 12 13 15 18 30 degrees lower. This is due to the removal 60 18 19 21 26 of water from the exhaust airstream by the total energy exchangers, effectively Table 12. Frost Threshold Temperature (°F) lowering the dewpoint of the exhaust. The water removed is subsequently picked up

36 Dening Quality. Building Comfort. conditions if provided with controls to outside air condition ‘preheat (min)’ moves avoid the frosting threshold. Two methods away from the saturation curve. Now the commonly employed with the AAONAIRE entering wheel air condition of ‘preheat design: The first method, variable step (min)’ allows the wheel to operate at full control reduces the effectiveness of heat energy recovery effectiveness without transfer and allows the return air to encountering saturation, avoiding the remove any frost formation on the portion frost threshold. The exhaust air condition of the wheel in the return air stream. The with preheat, is represented as ‘exhaust second method, preheating of the outdoor (min)’. In any climate where frost control is air, is the generally preferred strategy required for longer than a few hours due to from an energy savings and design load severity of climate and/or humidification reduction standpoint. Example 5 illustrates of the space, the preheat strategy will the psychrometric processes associated represent significant operating savings with preheat for avoiding frost formation when compared to variable step capacity in enthalpy wheels. control. Unless the number of hours below the frost control threshold is so small as to The preheat strategy has two process lines: be insignificant, preheat is the preferred first, the incoming air is preheated (see the frost control strategy. In addition, preheat lower left hand side of the psychrometric offers the greater reduction in heating chart). Note that the new preheated plant design loads.

Example 5: Preheat with Total Energy Recovery Indoor air at 70°F and 55.8°F wb (ρ = 0.074 lb/ft3) with a flow rate of 1000 cfm and supply outdoor air at -15°F and -15°F wb (ρ = 0.089 lb/ft3) with a flow rate of 1000 cfm enter a rooftop unit. The sensible and latent effectiveness is given as 75%. Assuming EATR = 0 and OACF ≈ 1, determine the amount of preheat required to prevent frost formation.

Dry Bulb Temperature - ˚F

37 Dening Quality. Building Comfort. Solution:

The frosting point of an application can be found on a psychrometric chart by drawing a line between the indoor% conditions and outdoor conditions. The point at which the line crosses the 100 rh value on the psychrometric chart is the frosting point of the application. The minimum amount of preheat required is the amount to move the outdoor conditions horizontally right on the psychrometric chart so that the % line between the indoor conditions and outdoor conditions no longer intersects the 100 rh curve. % In this example with% an indoor air condition of 70°F and 40 rh and an outdoor air condition of -15°F and° 100 rh enough preheat must be applied to raise the outside air temperature, entering the energy recovery device, by 13.3°F or more. (Preheat temperature entering wheel -1.7 F or greater.) Outdoor Return Air (Indoor) Conditions Winter 70°F and 20% rh 70°F and 30% rh 70°F and 40% rh Design (Frost Threshold -14°F) (Frost Threshold -3°F) (Frost Threshold 5°F) Temp Preheat Temp Required Preheat Temp Required Preheat Temp Required at Design Capacity ∆T at Design Capacity ∆T at Design Capacity ∆T 5 ------0 - - - - 2.5 2.5 -5 - - -4.3 0.7 0.8 5.8 -10 - - -6.3 3.7 -0.6 9.4 -15 -14.7 0.3 -7.9 7.1 -1.7 13.3 -20 -16.7 4.3 -9.1 10.3 -2.5 17.5 -25 -18.3 6.7 -10.0 15.0 -3.1 21.9 -30 -19.4 10.6 -10.7 19.3 -3.6 26.4 -35 -20.3 14.7 -11.3 23.7 -3.9 31.1 -40 -21.0 19.0 -11.7 28.3 -4.2 35.8

Table 13. Preheat Frost Control Temperatures and Capacity Requirements at Selected Indoor and Outdoor Conditions

The amount of heat requiredPreheater for preheatSize can= bem˙ foundc ΔT using Table 13 and the equation: For this example, the minimum preheat heater sizep is

 x ft 3   60 min  y lb of air  0.24 Btu Preheater Size = (ΔT)    1   3  €  min   hr   ft   lb⋅°R 

1000 3 60 0.088 0.24  ft  min lb of air  Btu 1.7 15 Preheater Size =      3  (− − (− )) €  min   1 hr   ft   lb⋅°R  Btu Preheater Size = 16,900 = 5kW hr €

By applying a 7 kW heater to the outside air the exhaust air relative humidity is reduced to 81% and the energy€ recovery device can operate without risk of frost formation .

38 Dening Quality. Building Comfort. Controls

directly proportional to the pressure drop, Heat exchanger controls may control the pressure drop through the energy frost formation or regulate the amount of recovery unit should be known. The energy transferred between airstreams pressure drop may be used with the fan at specified operating conditions. For efficiency to characterize the energy used example, ventilation systems designed by the exchanger and in turn the efficiency to maintain specific indoor conditions at (not effectiveness) of an application. extreme outdoor design conditions may Fan Power require energy recovery modulation to Ps provide an economizer function, to prevent overheating ventilation supply air during The fan power required by the supply air /6356 cool to moderate weather or to prevent is estimatedP sfrom= QsΔps η f over humidification. Modulation methods include changing rotational speeds of (or Pe stopping) heat wheels or bypassing part of one airstream around the heat exchanger T� he fan power required by the exhaust /6356 using dampers. air is estimatedPe = Q fromeΔpe η f

In supply air bypass control, the amount of where supply air allowed to pass through the wheel Ps establishes the supply air temperature. An � Pe air bypass damper, controlled by a wheel = fan power for supply air, hp ps supply air discharge temperature sensor, = fan power for exhaust air, hp regulates the proportion of supply air Δ = pressure drop of supply air permitted to bypass the exchanger. caused by fluid friction, in. of pe water A dead band control, which stops or limits Δ = pressure drop of exhaust air the exchanger, may be necessary when no caused by fluid friction, in. of f recovery is desired (e.g., when outside air water temperature is higher than the required η = overall efficiency of fan and supply air temperature but below the motor or product of fan and exhaust air temperature). When outside motor efficienciesP air temperature is above the exhaust air temperature, the equipment operates at The total pumping power of the ERV can full capacity to cool incoming air. During P P P be given as = s + e very cold weather, it may be necessary to heat the supply air, stop the wheel, or, in Cleaning small systems, use a defrost cycle for frost control. � Pressure Drop The method used to clean a heat exchanger depends on the transfer medium or mechanism used in the energy recovery Pressure drop for each airstream through unit and on the nature of the material to an energy recovery unit depends on many be removed. AAONAIRE energy wheels are factors, including exchanger design, mass self cleaning with respect to dry particles. flow rate, temperature, moisture, and inlet Smaller particles pass through the wheel. and outlet air connections. The pressure Larger particles land on the surface and drop must be overcome by fans or blowers. are blown clear as the wheel turns into Because the power required for circulating the opposite airflow path. For this reason, airstreams through the recovery unit is

39 Dening Quality. Building Comfort. SUMMARY

the primary cleaning need for AAONAIRE energy wheels is to remove oil-based aerosol AAONAIRE energy recovery systems films that have collected on the desiccant provide energy savings by recycling energy surfaces. Such films can close off micron- instead of losing energy through exhaust air sized pores at the surface of the desiccant streams. AAONAIRE systems also enhance material, reducing its ability to adsorb indoor air quality by allowing larger and desorb moisture. Grease build-up amounts of outside air to be provided to from kitchen exhaust, for example, is often the space and through improved humidity removed with an automatic water-wash control. AAONAIRE systems save money system. Other kinds of dirt may be removed through both an initial HVAC equipment by vacuuming, blowing compressed air reduction and ongoing lifecycle operating through the passages, soaking the units in savings. The payback for the AAONAIRE soapy water or solvents, steam cleaning or system for much of the country will pay manual spray cleaning. for itself in less than one year and provide savings for the life of the product. AAONAIRE energy wheels use a silica gel desiccant permanently bonded without AAONAIRE systems can be optimized for adhesives to the heat exchange surface; sensible or total energy transfer. Heat therefore, the desiccant will not be washed recovery ventilators are suitable when off in the cleaning process. AAONAIRE outside air humidity is low and latent space energy wheels larger than 30 inches loads are high for most of the year, and also in diameter are made with removable for use with swimming pools, locker rooms, segments for easy cleaning. Proper cleaning kitchens and indirect evaporative coolers. of the energy wheel should restore latent Energy recovery ventilators are suitable for effectiveness to near original performance. applications in schools, offices, residences and other applications that require year- Cleaning frequency depends on the quality round economical preheating or/and of the exhaust airstream. Comfort air precooling of outside supply air. conditioning systems generally require only infrequent cleaning. In a reasonably AAONAIRE units have been certified to clean indoor environment, such as a school perform according to AHRI Standard or office building, experience shows that 1060. The certified ratings program reductions of airflow or loss of sensible requires testing, rating and independent effectiveness may not occur for ten or verification of component performance at more years. Where there is moderate standard conditions and rated flow. Testing tobacco smoke, or within cooking facilities, is in accordance with ASHRAE Standard reduction in effectiveness can occur 84. AHRI certified ratings include very much faster. In applications experiencing complete information, to allow designers high levels of tobacco smoke, such as to fully characterize thermal and airflow smoking lounges, nightclubs, bars, and performance. In addition to separate restaurants, the energy transfer surfaces sensible, latent, and total effectiveness at may require cleaning as often as every six two airflows for both summer and winter months. Similar washing cycles may also test conditions, the standard requires be appropriate for industrial applications, information on pressure loss as well as air such as welding or machining operations, leakage. involving ventilation of high levels of smoke or oil-based aerosols.

40 Dening Quality. Building Comfort. In many climates, the energy wheel AAONAIRE energy transfer matrix design eliminates three to four tons of air overcomes the problems associated with conditioning load for every 1000 cfm of traditional aluminum wheels by providing ventilation air. When this benefit is claimed lower cost, lighter weight, cleanable, in the form of air conditioning equipment permanently bonded desiccant that is rightsizing, the initial cost of the system completely corrosion resistant in seacoast is typically on par with a system that applications. To further the maintainability, does not utilize energy recovery. Payback AAONAIRE wheels are made in segments is often immediate for the southeastern which can be easily removed from the US and much of the midwest because the unit for cleaning outside the unit. The AAONAIRE energy recovery system actually AAONAIRE energy transfer method reduces has a lower initial cost than the system not required HVAC system capacity more than using energy recovery. When payback is fixed plate devices because of the ability to not immediate, most payback periods are transfer both sensible and latent heat. less than two years.

The polymer construction of the AAONAIRE wheel yields a highly efficient, easily cleanable, light weight, corrosion resistant and flexible energy recovery wheel. The Contact your local AAON representative to learn more about the AAONAIRE system and other ways that AAON can help you to conserve energy, improve indoor air quality and save money.

41 Dening Quality. Building Comfort. APPENDIX:

ERV Latent Energy Transfer Calculations

L

Assuming no condensation in the AAONAIRE ERV, the latent effectiveness ε of an energy recovery ventilator is given as − − q m h (w1 w2) m h (w4 w3 ) ε = L = s fg = e fg L − − (1a) qL,max mmin h fg (w1 w3 ) mmin h fg (w1 w3)

qL � where is the actual latent heat transferε rate given by qL = LqL,max (1b)

qL,max � where is the maximum heat transfer rate given by qL,max = 60mmin h fg (w1 − w3 ) (1c)

where

L � hfg εw = latent effectiveness = enthalpy of vaporization, Btu/lb ms = humidity ratios me = supply dry air mass flow rate, lb/min mmin ms me = exhaust dry air mass flow rate, lb/min = smaller of and

Because the enthalpy of vaporization from Equation (1a) can be dropped out from numerator and denominator, Equation (1a)m w can− wbe rewrittenm w as− w m ( 1 2 ) ( 4 3) ε = w = s = e m m m w − w m w − w (1d) w,max min ( 1 3) min ( 1 3 )

m � m L w where ε is moisture effectiveness, numerically equal to latent effectiveness ε , and is actual moisture transfer ratem givenε bym w = m w,max (1e)

ms,max � where is the maximumm moisturem transferw w rate given by s,max = w,min ( 1 − 3) (1f)

�

42 Dening Quality. Building Comfort. Assuming no water vapor condensation in the ERV, the leaving humidity ratios can be given m as follows. The supplyw air= leavingw − ε humidityw,min w ratio− w is 2 1 L m ( 1 3 ) (2a) s

� m and the leaving exhaustw =airw humidity+ ε w ,ratiomin w is− w 4 3 L m ( 1 3 ) (2b) s

t

The total effectiveness� ε of an AAON− AIRE energy recovery− ventilator is given as q m (h2 h1) m (h3 h4 ) ε = t = s = e t − − (3a) qt,max mmin (h3 h1) mmin (h3 h1)

qt

where� is the actual total heat transferε rate given by qt = tqt,max (3b)

qt,max � where is the maximum total heat transfer rate given by qt,max = 60mmin (h1 − h3 ) (3c)

where

t � h ε = total effectiveness ms = enthalpy me = supply dry air mass flow rate, lb/min mmin ms me = exhaust dry air mass flow rate, lb/min = smaller of and

The leaving supply air condition ism h = h − ε min w − w s 1 t m ( 1 3 ) (4a) s

� and the leaving exhaust air conditionm is h = h − ε min h − h 4 3 t m ( 1 3) (4b) e

�

43 Dening Quality. Building Comfort. qL

Assuming the stream at state 1 is of higher humidity, the latent heat recovery from the ERV can be estimated from60 60 qL = msh fg (w1 − w2 ) = Qsρsh fg (w1 − w2 ) (5a)

60 60 qL = meh fg (w4 − w3 ) = Qeρeh fg (w4 − w3 ) (5b) � 60ε − qL = L mmin h fg (w1 w3) (5c) �

hfg w w w� w = enthalpy of vaporization or heat of vaporization of water vapor, Btu/lb 1, 2, 3, 4 = inlet and exit humidity ratios of supply and exhaust airstreams, respectively qt

The total energy60 transfer between60 the streams is given60 by qt = qs + qL = ms(h1s − h2s) = Qsρs(h1s − h2s) = [msc ps(t1 − t2) + msh fg (w1 − w2 )] (6)

60 60 60 qt = qs + qL = me (h4e − h3e ) = Qeρe (h4e − h3e ) = [mec pe (t4 − t3) + meh fg (w4 − w3 )] (7a) �

60ε − qt = t mmin (h1s h3e ) (7b) �

where

h s �

1 = enthalpy of supply air at inlet, Btu/lb h s h3e = enthalpy of exhaust air at inlet, Btu/lb h e 2 = enthalpy of supply air at outlet, Btu/lb 4 = enthalpy of exhaust air at outlet, Btu/lb

44 Dening Quality. Building Comfort. Example A. Negative Latent Energy Transfer Inlet supply air enters an AAONAIRE ERV with a flow rate of 9350 cfm at 95°F and 20% rh . Inlet exhaust air enters with a flow rate of 9050 cfm at 75°F and 50% rh . Assume that the energy exchanger was tested under ASHRAE Standard 84, which rated the sensible heat transfer effectiveness at 50% and the latent (water vapor) transfer effectiveness at 50% . Assuming the specific heat of air is 0 .24 Btu/lb·°F and the latent heat of vaporization to be 1100 Btu/lb, determine the sensible, latent, and net energy gained by the exhaust air .

Dry Bulb Temperature - ˚F

Airflow Dry Relative Humidity Specific Enthalpy Point Name (lb/min) Bulb Humidity Ratio Volume (Btu/lb) (°F) (%) (lb/lb) (cu.ft./lb) Inlet Supply Air Flow (1) 660 95 20 0.0071 14.14 30.6 Inlet Exhaust Air Flow (3) 660 75 50 0.0093 13.68 28.1 Exit Supply & Exhaust Air 660 85 32 0.0082 13.91 29.4 Flow (2,4)

Solution:

% 14.14 3 h 30.6Btu lb 0.0071 FromV1 = the psychrometricft lb chart,1 the= properties of air at 95°Fw1 = and 20lb rhlb areof dry air

% � 13.68 3 � h 28.1Btu� lb 0.0093 andV 3the= propertiesft lb of air at 75°F3 and= 50 rh are w3 = lb lb of dry air

9350 3 min � The mass flow rate at stateQ 11 is obtainedft from� � m1 = = 3 = 660 lb/min V1 14.14 ft lb

�

45 Dening Quality. Building Comfort. 9350 3 min Similarly, the mass flow rateQ1 at state 3 ftis obtained from m1 = = 3 = 660 lb/min V1 14.14 ft lb

These equal� mass flow rates conform with ASHRAE Standard 84. Exit temperatures of the(660 airstreamslb/min)(0.24 can beBtu obtained/lb⋅°F) as follows: t2 = 95°F − 0.5 (95°F − 75°F) = 85°F (660 lb/min)(0.24 Btu/lb⋅°F)

(660 lb/min)(0.24 Btu/lb⋅°F) t4 = 75°F + 0.5 (95°F − 75°F) = 85°F � (660 lb/min)(0.24 Btu/lb⋅°F)

� The exit humidity of (the660 airstreamslb/min) is found as follows: w2 = 0.0071− 0.5 (0.0071− 0.0093) = 0.0082 lb/lb of dry air (660 lb/min)

(660 lb/min) w4 = 0.0093 + 0.5 (0.0071− 0.0093) = 0.0082 lb/lb of dry air � (660 lb/min)

� 660 /min 0.24 / 85 75 1584 /min The sensibleq2 = ( heatlb gained)( by theBtu exhaustlb⋅° Fstream)( °F is− found°F as) = Btu

660 /min 1100 / 0.0082 0.0093 799 /min � The latentq2 = heat( gainedlb by)( the exhaustBtu lb stream)( is −found as) = − Btu

1584 799 785 /min � The net heat energy qgained= qs + byqL the= exhaust− airstream= Btu is therefore

% If the incoming� outdoor air conditions had been at 95°F and 14 relative humidity, the net energy gained by the exhaust airstream would have been zero. The outlet exhaust airstream enthalpy at 85°F and 0.0082 lb/lb of dry air is given in the psychrometric chart as 29.4 Btu/lb. The net heat gained by the exhaust airstream is approximately 945 Btu/ min.

46 Dening Quality. Building Comfort. References: Air Conditioning Heating Refrigeration Institure (AHRI), Arlington, VA. www.ahrinet.org AirXchange, Rockland, MA. www.airxchange.com ASHRAE, Atlanta, GA. www.ashrae.org ASHRAE. HVAC Systems and Equipment Handbook. Atlanta: ASHRAE, 2008.

47 Dening Quality. Building Comfort. Dening Quality. Building Comfort.

2425 S. Yukon Ave. • Tulsa, OK 74107 • (918) 583-2266 AAONAIRE • R85590 • 110103

It is the intent of AAON to provide accurate and current product information. However, in the interest of product improvement, AAON reserves the right to change pricing, specifications, and/or design of its product without notice, obligation or liability. Copyright © AAON, all rights reserved throughout the world. AAON® and AAONAIRE® are registered trademarks of AAON, Inc., Tulsa, OK.