CHAPTER 26 AIR-TO-AIR ENERGY RECOVERY EQUIPMENT Applications ............................................................................. 26.1 Use of Air-to-Air Heat or Heat and Mass Exchangers Basic Heat or Heat and Water Vapor Transfer Relations....... 26.2 in Systems ........................................................................... 26.27 Types of Air-to-Air Heat Exchangers ...................................... 26.5 Economic Considerations ...................................................... 26.35 Performance Ratings ............................................................. 26.19 Additional Technical Considerations .................................... 26.20 Energy and/or Mass Recovery Calculation Comparison of Air-to-Air Heat or Heat and Mass Exchanger Procedure ........................................................................... 26.36 Characteristics ................................................................... 26.26 Symbols .................................................................................. 26.41 IR-TO-AIR energy recovery is the process of recovering heat Table 1 Typical Applications for Air-to-Air Energy Recovery and/or moisture between two airstreams at different tempera- A Method Application tures and humidities. This process is important in maintaining accept- able indoor air quality (IAQ) while keeping energy costs low and Comfort-to-comfort Residences reducing overall energy consumption and carbon dioxide emission. Offices This chapter describes various technologies for air-to-air energy Classrooms Retail recovery. Thermal and economic performance, maintenance, and Bars and restaurants related operational issues are presented, with emphasis on energy Swimming pools recovery for ventilation. Locker rooms Air-to-air energy recovery should be considered for every build- Operating rooms ing in which energy is used to condition the air. This is consistent with Nursing homes ASHRAE’s strategic plan to support a sustainable built environment, Animal ventilation and aligns with the United Nations International Panel for Climate Plant ventilation Smoking exhaust Change’s call for action to reduce the emissions of carbon dioxide Process-to-process Dryers related to energy use. and Ovens Energy can be recovered either in its sensible (temperature only) or Process-to-comfort Flue stacks latent (moisture) form, or a combination of both from multiple Burners sources. Sensible energy can be extracted, for example, from outgoing Furnaces airstreams in dryers, ovens, furnaces, combustion chambers, and gas Incinerators turbine exhaust gases to heat supply air. Units used for this purpose are Paint exhaust Welding exhaust called sensible heat exchange devices or heat recovery ventilators (HRVs). Devices that transfer both heat and moisture are known as volume flow rate and pressure drop. Economic factors such as cost energy or enthalpy devices or energy recovery ventilators (ERVs). of energy recovered and capital and maintenance cost (including HRVs and ERVs are available for commercial and industrial applica- pumping power cost) play a vital role in determining the economic tions as well as for residential and small-scale commercial uses. feasibility of recovery ventilators for a given application. Air conditioners use significant energy to dehumidify moist air- streams. Excessive moisture in the air of a building can result in mold, 1. APPLICATIONS allergies, and bacterial growth. ERVs can enhance dehumidification with packaged unitary air conditioners. Introducing outdoor or venti- Air-to-air energy recovery systems may be categorized according lation air is the primary means of diluting air contaminants to achieve to their application as (1) process-to-process, (2) process-to-comfort, acceptable indoor air quality. ERVs can cost-effectively provide large or (3) comfort-to-comfort. Some typical air-to-air energy recovery amounts of outdoor air to meet a building’s minimum ventilation applications are listed in Table 1. requirements as prescribed in ASHRAE Standards 62.1 and 62.2. In comfort-to-comfort applications, the energy recovery device Types of ERVs include fixed-plate heat exchangers, rotary wheels, lowers the enthalpy of the building supply air during warm weather heat pipes, runaround loops, thermosiphons, and twin-tower enthalpy and raises it during cold weather by transferring energy between the recovery loops. Performance is typically characterized by effective- ventilation air supply and exhaust airstreams. ness; pressure drop, pumping, or fan power of fluids; cross flow (i.e., Air-to-air energy recovery devices for comfort-to-comfort appli- amount of air leakage from one stream to the other); and frost control cations may be sensible heat exchange devices (i.e., transferring sen- (used to prevent frosting on the heat exchanger). Recovery efficiency, sible energy only) or energy exchange devices (i.e., transferring both the ratio of output of a device to its input, is also often considered. In sensible energy and moisture). These devices are discussed further energy recovery ventilators, effectiveness refers to the ratio of actual in the section on Additional Technical Considerations. energy or moisture recovered to the maximum possible amount of When outdoor air humidity is low and the building space has an energy and/or moisture that can be recovered. appreciable latent load, an ERV can recover sensible energy while possibly slightly increasing the latent space load because of water Fluid stream pressure drops because of the friction between the vapor transfer within the ERV. It is therefore important to determine fluid and solid surface, and because of the geometrical complexity of whether the given application calls for HRV or ERV. the flow passages. Pumping or fan power is the product of the fluid HRVs are suitable when outdoor air humidity is low and latent space loads are high for most of the year, and also for use with swim- The preparation of this chapter is assigned to TC 5.5, Air-to-Air Energy ming pools, chemical exhaust, paint booths, and indirect evaporative Recovery. coolers. 26.1 26.2 2020 ASHRAE Handbook—HVAC Systems and Equipment ERVs are suitable for applications in schools, offices, residences Effectiveness and other applications that require year-round economical preheat- When mass flow rates, temperatures, and humidities of the inlets ing and/or precooling of outdoor supply air. and outlets are known, the sensible, latent, or total effectiveness of In process-to-process applications, heat is captured from the the exchanger for those operating conditions can be determined. process exhaust stream and transferred to the process supply air- Conversely, if the respective effectiveness of the exchanger at stream. Equipment is available to handle process exhaust tempera- specific mass flow rates is known or rated, and the temperatures and tures as high as 1600°F. humidities of the inlets are specified, it is possible to estimate con- Process-to-process recovery devices generally recover only sen- ditions at the outlets, as long as confounding variables such as sible heat and do not transfer latent heat, because moisture transfer occurrence of condensation or internal leakages are insignificant. is usually detrimental to the process. In cases involving condensable The general definition of transfer effectiveness , on which gases, less recovery may be desired to prevent condensation and ASHRAE Standard 84 bases its definitions of effectiveness, is possible corrosion. Actual transfer of moisture and/or energy In process-to-comfort applications, waste heat captured from = ----------------------------------------------------------------------------------------------------------------- (1) Maximum possible transfer between airstreams process exhaust heats building makeup air during winter. Typical applications include foundries, strip-coating plants, can plants, plat- Referring to Figure 1, the gross sensible effectiveness s or ing operations, pulp and paper plants, and other processing areas sensible of an energy recovery ventilator is given as with heated process exhaust and large makeup air volume require- · ments. m2cp 1T1 – cp 2T2 sensible = -------------------------------------------------------· (2a) Although full recovery is usually desired in process-to-process mmincp 1T1 – cp 3T3 applications, recovery for process-to-comfort applications must be modulated during warm weather to prevent overheating the makeup Referring to Figure 1, the gross latent effectiveness L or latent of an air. During summer, no recovery is required. Because energy is energy recovery ventilator is given as saved only in the winter and recovery is modulated during moderate m· h W – h W weather, process-to-comfort applications save less energy annually 2 fg 1 1 fg 2 2 latent = --------------------------------------------------------------· - (2b) than do process-to-process applications. mminhfg 1W1 – hfg 3W3 Process-to-comfort recovery devices generally recover sensible Referring to Figure 1, the gross total effectiveness T or total of an heat only and do not transfer moisture between airstreams. energy recovery ventilator is given as · 2. BASIC HEAT OR HEAT AND WATER VAPOR m2h1 – h2 ---------------------------------- TRANSFER RELATIONS total = · (2c) mminh1 – h3 The second law of thermodynamics states that heat energy always where transfers from a region of high temperature to one of low temperature. · mn = mass flow rate at station n, lbm/h This law can be extended to say