COLUMN ENGINEER’S NOTEBOOK This article was published in ASHRAE Journal, June 2014. Copyright 2014 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org. Steven T. Taylor How to Design & Control Waterside Economizers BY STEVEN T. TAYLOR, P.E., FELLOW ASHRAE ASHRAE/IES Standard 90.11 requires either airside economizers or waterside econo- mizers for most cooling system applications. Air economizers are more common and almost always more energy efficient. But there are some applications where water economizers are preferred, such as:

•• High-rise office buildings with floor-by-floor air load on the chillers even when the chillers must run to handlers where access to outdoor air is architecturally deliver the desired chilled water temperature. The first limited. costs are basically the same as a nonintegrated econo- •• Large data centers, where air economizers are not mizer, but the number of hours the economizer can be always physically practical because airflow rates are so operational is vastly extended, significantly improving high, or air economizers are precluded by concerns (val- energy performance and cost effectiveness. Integrated id or not) that they can increase particle contamination economizers are also easier to control, as described in risk and cause low humidity levels. Water economizers more detail later. There are few, if any, advantages to can also provide energy savings on par with air econo- nonintegrated economizers. mizers on data centers that have hot aisle containment •• As shown in Figure 2, the same cooling towers and and warm supply air temperatures, e.g., “cold” aisle sup- condenser water pumps should be used to serve both ply air temperatures over 80°F (27°C) and hot aisles over the economizer heat exchanger and the chiller condens- 100°F (38°C). These warm air temperatures, in turn, al- ers. Some designers provide separate towers and pumps, low chilled water supply temperatures over 65°F (18°C), at considerable expense because of concerns about low readily attainable by cooling towers for most of the year chiller head pressure due to low condenser water sup- in most climates. ply temperatures. But head pressure control is easily and •• Small fan coil systems where airside economizers are inexpensively addressed by making the condenser isola- physically impractical or overly expensive. tion valves modulating and controlling them off the head •• Chilled beams, radiant cooling, and other hydronic pressure control signal output that is standard on most systems for which airside economizers do not apply. chiller controllers. The valves throttle flow through the Here are some tips for designing cost effective and condenser as needed to maintain chiller minimum lift energy efficient waterside economizers: regardless of how cold the condenser water supply tem- perature is in economizer mode. System Design & Equipment Selection •• The capacity and quantity of cooling towers and con- •• To maximize performance (and also to meet ASHRAE denser water pumps remains the same as they would be Standard 90.1 prescriptive requirements), the economiz- without the economizer. For office building applications, er must be “integrated” with the chillers, meaning the this is intuitively clear: we know that when the economizer economizer has to be able to reduce the load on the chill- is on, weather will be cold so loads will be well below design ers even if it cannot handle the entire load. This means loads; hence only one of the two chillers (in the example the heat exchanger must be in series with, and upstream shown in Figure 2) will be needed, freeing the other to sup- of, the chillers as shown in Figure 2, rather than in parallel with the chillers Figure 1. In the series position the econo- Steven T. Taylor, P.E., is a principal of Taylor Engineering in Alameda, Calif. He is a mem- mizer can pre-cool the return chilled water to reduce the ber of SSPC 90.1 and chair of TC 4.3, Ventilation Requirements and Infiltration.

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FIGURE 1 Nonintegrated economizer. FIGURE 2 Integrated economizer.

Cooling Tower 1 Cooling Tower 2 Cooling Tower 1 Cooling Tower 2

Chiller 1 Chiller 1

Chiller 2 Chiller 2

Bypass WSE-Only Modulating Plate and Frame Valve Heat Exchanger Minimum Chiller Minimum Chiller Flow Bypass Flow Bypass

Plate and Frame Heat Exchanger

ply tower water to the economizer heat exchanger. But this gpm/hp (9.3 L[s·kW]) for 24/7 applications such as data is also true for data centers where the load may require all centers. These efficiencies are 10% above those shown to chillers to run even in cold weather. The reason is that the be cost effective for non-economizer applications.2 load on the towers is actually reduced by the economizer •• Cooling towers should be selected so that as many since compressor heat is reduced, and condenser water tower cells as possible can be enabled when the econo- flow to the chiller condensers may be less than design be- mizer is enabled to maximize efficiency and capacity cause it is colder (and in fact may be reduced by the throt- while maintaining minimum flow rates required by the tling of the head pressure control valves discussed earlier), tower manufacturer to prevent scaling. Low minimum making water available to the economizer heat exchanger flow rates can be achieved using weir dams and special without the need to add pumps. nozzles in the hot water distribution pans. •• While cooling tower capacity is not affected by the •• Chilled water pump head increases due to the pres- economizer, it may be necessary to reduce the design sure drop of the heat exchanger when in economizer approach temperature to meet Standard 90.1 Section mode. However, in applications such as offices where 6.5.1.2.1 waterside economizer requirements, particu- the loads are low when the economizer is on, pump head larly for plants with high loads in cold weather. This is may not need to increase above design head when the discussed further in the sidebar, “Example Design Pro- economizer is off; excess head may be available for the cedure” (Page 34). heat exchanger when the economizer is active due to the •• It is critical that cooling towers be very efficient since reduced chilled water flow to coils. they will be running at full speed many hours of the year. •• The heat exchanger should be a plate & frame type and A minimum of 90 gpm/hp (7.6 L[s·kW]) at Standard 90.1 selected for an approach of about 3°F (1.7°C) (i.e., the tem- conditions (95°F to 85°F at 75°F [35°C to 29°C at 24°C] wet perature of the chilled water leaving the heat exchanger is bulb) is recommended for office type applications and 110 equal to 3°F (1.7°C) above the temperature of the condenser

JUNE 2014 ashrae.org ASHRAE JOURNAL 31 Advertisement formerly in this space. water entering the heat exchanger). Waterside Economizer Controls Heat exchanger cost increases expo- Control systems for waterside nentially with approach temperature economizer are generally the same so very close approaches should be as for conventional plants except tested for cost effectiveness. The heat they require one added sensor that exchanger pressure drop on the con- is optional on conventional plants: a denser water side should be similar to wet-bulb temperature sensor. This is that of the condensers so the flow rate usually a combination temperature will be similar when serving either and relative humidity sensor with the condensers or heat exchanger. software or electronics that converts On the chilled water side, pressure the two signals to wet-bulb tem- drop is typically limited to about 5 or perature. Unfortunately, humidity 6 psi (34 or 41 kPa) to limit the chilled sensors are notoriously unreliable water pump energy impact. The heat and require frequent recalibration. exchanger performance must be cer- To improve reliability it is essential tified per AHRI Standard 4003 as re- to specify a high quality sensor (see quired by Standard 90.1. Reference 8 for recommendations) Advertisement formerly in this space. •• To maximize economizer perfor- and also provide a quality check by mance, and also performance of the having the control system compare system even when not on economizer, local dew-point temperature to the the chilled water system must be de- dew-point temperature data mea- signed for a very high temperature sured at the nearest National Oceanic rise (ΔT). This maximizes the chilled and Atmospheric Administration water return temperature which al- (NOAA) site via the Internet. If the lows the economizer to operate more local dew-point temperature (calcu- hours. The design procedure is sim- lated from the wet-bulb temperature ple: all cooling coils should be the larg- using psychrometric routines stan- est they can be within the cleanability dard in most digital control systems) limitations of Standard 62.1,4 which substantially differs from the NOAA requires that dry coil pressure drop at dew-point temperature, alarms can 500 fpm (2.5 m/s) face velocity must be generated indicating a need for not exceed 0.75 in. w.c. (188 Pa).* This humidity sensor recalibration. will typically be an eight row coil with Recommended control sequences about 12 fins per inch (5 fins per cm). for waterside economizers:‡ Using high ΔT coils also reduces first •• Reset chilled water supply tem- costs, energy demand, and annual perature setpoint based on valve de- energy costs and should be used for mand, i.e., raise the water tempera- all designs5 but especially those with ture until one chilled water control waterside economizers. valve is full wide open.§

*Here is a simple way to test a coil for Standard 62.1 compliance with a manufacturer’s coil selection program: Start with the desired coil including desired rows, fin type, and fin density; adjust the airflow rate up or down until the face velocity is 500 fpm (9 km/h); reduce the entering drybulb temperature to 60ºF (15ºC) to ensure a dry coil; then run the selection. To comply, the pressure drop under these conditions must be 0.75 in. w.c. (188 Pa) or less. ‡Time delays required to prevent rapid mode changes are required but not included here for clarity. §Many engineers have concerns about the impact of CHWST reset on humidity control. When the waterside economizer is active, outdoor air is cool so humidity should not be a concern. But if the reset logic is based on chilled water valve position, CHWST reset will never have a significant impact on space humidity control regardless of weather. This is because space humidity is a function of the supply air humidity ratio which in turn is a function of the coil leaving dry-bulb temperature setpoint. Regardless of CHWST, the air leaving a wet cooling coil is nearly saturated; lowering CHWST only slightly reduces supply air humidity ratio. So as long as the supply air temperature can be maintained at the desired setpoint, which resetting off valve position ensures, resetting CHWST will not impact space humidity.

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6,7 then cooling towers (and/or heat exchang- Example Design Procedure ers) will need to be reselected for closer approach temperatures. 1. Calculate the chilled water load at 4. Determine the heat exchanger condenser 50°F (10ºC) dry-bulb temperature and water supply temperature equal to the re- Example. Assume the plant in Figure 2 45°F (7.2ºC) wet-bulb temperature. quired reset chilled water supply temperature served an office building with floor-by- This is the performance test condition floor air handlers. The design conditions of prescribed by Standard 90.1 for build- the two chillers at the design cooling peak TABLE 1 ings other than data centers.† This can Chiller design conditions (each). are shown in Table 1. The pump flow rates be done using standard load calculation Chilled Condenser match the chiller design rates and cooling software by selecting a spring or fall Water Side Water Side towers were selected at 68ºF (20ºC) wet- month and overriding design outdoor air CAPACITY, TONS 300 345 bulb temperature, a 7ºF (3.9ºC) approach. temperatures. All other load assumptions The cooling loads were then recalculated FLOW, GPM remain the same. Loads should be re- 290 550 at 50ºF/45ºF (10ºC/7.2ºC) outdoor air duced from design loads due to reduced ENTERING WATER, °F 69 75 temperature conditions; the load drops from 600 tons to 350 tons (2110 kW to conduction and outdoor air conditioning LEAVING WATER, °F 44 90 loads. 1231 kW). Coil selection programs were DP, PSI 6.5 6.7 then used to determine the chilled water 2. Use coil selection software or other conditions required at the air handlers, coil models to determine the warmest and condenser water conditions were chilled water supply temperature that TABLE 2 Waterside economizer heat exchanger determined using the steps above. The can meet 100% of the chilled water load design conditions. resulting heat exchanger design condi- determined above for all coils. To do this, tions are shown in Table 2. first determine the warmest supply water Chilled Water Condenser Water Then, cooling tower selection soft- temperature that can meet the load at the (Hot) Side (Cold) Side ware was used to see if the selected design flow for each coil, then select the TOTAL LOAD, TONS 350 350 cooling towers could cool 550 gpm (35 coldest of these and use it to determine FLOW, GPM L/s) (275 gpm (17.5 L/s) across each the chilled water flow through all the other 560 550 tower) from 62.3ºF (16.8ºC) to 47ºF coils. The coil software will also determine ENTERING WATER, °F 65 47.0 (8.3ºC), a 2ºF (1.1ºC) approach to the the chilled water return temperature from LEAVING WATER, °F 50 62.3 45ºF (7.2ºC) wet bulb temperature. But, each coil. Typically, the chilled water sup- the software indicated that the towers ply temperature can be reset 5ºF (3ºC) or DP, PSI 5 4.8 were only able to deliver 48ºF (8.9ºC). more above the design chilled water sup- So, the towers had to be reselected for a ply temperature. This can be true even for determined above less the 3ºF (1.7ºC) ap- 73ºF (22.8ºC) leaving water temperature data centers despite the consistently high proach temperature. (5ºF/2.8ºC approach to design wet-bulb load by taking advantage of redundant air 5. Calculate the condenser water return temperature) to achieve a 2ºF (1.1ºC) handlers to effectively increase available temperature to match the chilled water load approach at the conditions above. Note coil area. based on the condenser water flow and sup- that heat exchanger approach also could ply temperature determined above. 3. Select the condenser water flow rate have been reduced to deliver the desired equal to a multiple of design condenser 6. Use cooling tower selection software to chilled water temperature, but it is usu- water pump flow rates as required to verify that the cooling towers can provide the ally more cost effective to invest in larger closely match the chilled water flow rate required condenser water supply temperature cooling towers since they also improve determined above. at 45ºF (7ºC) wet-bulb temperature. If not, efficiency when the economizer is off.

†Data center performance test conditions vary by climate zone. See Standard 90.1, Table 6.5.1.2.1 for details.

•• Enable the economizer if the chilled water return PAHX = predicted heat exchanger approach temperature is greater than the predicted heat exchang- PACT = predicted cooling tower approach er leaving water temperature (PHXLWT) plus 2°F (1.1°C). DAHX = design heat exchange approach The 2°F (1.1°C) differential is needed to avoid expending PLRHX = predicted heat exchanger part-load ratio a lot of cooling tower fan energy for only minimal econo- (current chilled water flow rate divided by mizer load reduction. PHXLWT is estimated using the design HX chilled water flow rate) equation: DTWB = design wet-bulb temperature DA = design cooling tower approach PHXLWT = TWB + PAHX + PACT CT PA = DA × PLR m = slope developed from the manufacturer’s HX HX HX cooling tower selection program or empiri- PA = m × (DT – T ) + DA CT WB WB CT cally after the plant is operational. Typical where values are 0.2 to 0.5 for near constant load

TWB = current wet-bulb temperature applications like data centers. For office

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type applications, m is typically in the range measured directly with a flow meter (full bore magnetic –0.2 to 0. or ultrasonic type are recommended to prevent fouling) •• Disable the waterside economizer if it is not reduc- or deduced from heat exchanger pressure drop. ing the chilled water return temperature by at least 1°F •• Tower speed control: (0.6°C). 1. When waterside economizer is disabled: Control •• Disable chillers when HXLWT is at or below the de- speed to maintain normal condenser water tempera- sired chilled water supply temperature setpoint. tures which should be reset from load or wet-bulb tem- •• Enable chillers when chilled water supply tempera- perature.9 ture is greater than desired setpoint. Note that multiple 2. When waterside economizer and chillers are en- chillers may need to be enabled if the current chilled wa- abled: Run tower fans at 100% speed. ter flow is well above the design flow of a single chiller. 3. When chillers are disabled: Control speed to •• Run as many tower cells as tower minimum flow maintain HXLWT at desired chilled water supply tem- limits will allow. perature setpoint. •• Control condenser water flow to roughly match the The only complex sequence above is predicting when current chilled water flow but reduce flow (within tow- the economizer should be enabled. Fortunately, if the er minimum flow constraints) as needed to maintain a prediction calculation is off and the economizer is minimum 5°F (2.8°C) range. The lower flow and higher enabled prematurely, it will shortly be disabled and range improves tower efficiency and reduces pump pow- the plant will see no disruptions in chilled water flow er. Flow can be controlled by staging pumps, modulat- or supply temperature. This contrasts with noninte- ing speed on variable speed pumps, and/or modulating grated economizers where switching from economizer isolation valves on the heat exchanger. Flow rate can be to chillers can be disruptive and guessing wrong about

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economizer performance can result in chiller short less energy efficient. When waterside economizers cycling and temporary loss of chilled water supply tem- are used, they usually require only the addition of perature control. a heat exchanger between the condenser water and chilled water; other components remain the same, Conclusions although cooling tower approach sometimes must Waterside economizers are preferred to airside be reduced to meet Standard 90.1 performance economizers in some applications despite being requirements. Economizers should always be piped in an integrated (series) arrange- ment. Controls are straightfor- ward except predicting when the economizer should be enabled which requires knowledge of wet- bulb temperature and predicted heat exchanger and cooling tower approaches. But with integrated economizer design, this predic- tion is not critical to reliable plant performance.

References 1. ASHRAE/IES Standard 90.1-2013, Energy Standard for Buildings Except Low-Rise Residential Advertisement formerly in this space. Buildings. 2. Taylor, S. 2012. “Optimizing the design and control of chilled water plants, part 4: chiller & cooling tower selection.” ASHRAE Journal, March. 3. AHRI 400-2001, Liquid-to-Liquid Heat Exchangers. 4. ASHRAE Standard 62.1-2010, Ventilation for Acceptable Indoor Air Quality. 5. Taylor, S. 2011. “Optimizing the design and control of chilled water plants part 3: pipe sizing and optimizing ΔT.” ASHRAE Jour- nal December. 6. Stein, J. 2009. “Waterside economizing in data centers: design and control consider- ations.” ASHRAE Transactions 115(2). 7. Stein, J., S. Taylor. 2013. “Seminar 21: Water Side Economizers, a.k.a. Free Cooling, Integrated Water Side Economizer.” ASHRAE Annual Conference. 8. NBCIP. 2005. “Supplement Product Testing Report: Duct-Mounted Relative Humidity Transmitters.” http://tinyurl.com/ of6leku. 9. Taylor, S. 2012. “Optimizing the design and control of chilled water plants, part 5: optimized control sequences.” ASHRAE Journal June.

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