1、OR-05-1 1-1 Cost-Effective Design of Dual Heat and Energy Recovery Exchangers for 100% Ventilation Air in HVAC Cabinet Units Yaw Asiedu, PhD Robert W. Besant, PEng Fellow ASHRAE Associate Member ASHRAE Carey J. Simonson, PhD, PEng ABSTRACT This paper shows how a combined air-to-air heat and energy r
2、ecovery system design problem can be formulated for HVAC cabinet units and solved for the least life-cycle cost system whilestill retaining a small timeperiodpayback. Math- ematical expressions are presented to address the complicat- inginteraction between the components of the unit to facilitate th
3、e design process. The design process is illustratedfor the city of Chicago where both large heating and cooling loads occur in HVAC applications. The example design problem presented shows that payback periods of a little over a year are often achievedfor retrofitted units, and the life-cycle cost s
4、avings for auxiliary heating and cooling ventilation air far exceeds the capital cost even when only a 1 O-year life cycle is considered. INTRODUCTION The sizing of heat exchangers and energy wheels for ventilation heat and moisture exchange in HVAC applications has not been done accurately and cost
5、-effectively because it is a complex design problem requiring many pieces of informa- tion. Information required includes not only mass flow rate of ventilation air and capital cost of the energy recovery device but also the cost of auxiliary heating and cooling equipment and their operating costs,
6、operating setpoints for temperature and humidity, and the ambient air temperature and humidiy properties for an entire typical weather year. The calculations needed to evaluate the operating energy cost involve functions ofthese parameters integrated over time and are quite complex (ASHRAE 2000). Th
7、is is further complicated when the unit utilizes two components to recover both heat and moisture. The issue of developing systematic approaches to the sizing of single-component units for different HVAC config- urations has received some attention in the past five years. Besant and Simonson (2000)
8、present a discussion of various configurations of heat and energy recovery devices and guide- lines on how the operating energy cost may be evaluated, and Asiedu et al. (2003) developed mathematical expressions and design tables for a simple HVAC configuration using the city of Chicago as the illust
9、rative example. The sizing of two heat and energy recovery units is more complicated because of the interdependence of the components. It is the purpose of this paper to show how this dual air-to-air heat and energy exchanger system design problem can be formulated and solved for the least life-cycl
10、e cost system while still retaining a small time period payback. The design process is illustrated for the city of Chicago, where both large heating and cooling loads occur in HVAC applications. Detailed mathematical expressions are presented to calculate the operating energy demands to facilitate t
11、he design process. Finally, an example design problem is presented showing that payback periods of a little over a year are often achieved and the life-cycle cost savings for auxiliary heating and cooling ventilation air far exceeds the capital cost of the heat exchanger or energy wheel even when on
12、ly a 10-year life cycle is considered. ENERGYWHEELANDHEATEXCHANGER SELECTION PROBLEM The energy recovery unit discussed in this paper is shown schematically in Figure 1. This system, which is typical of some new cabinet units, features 100% ventilation air with no recirculation. This schematic featu
13、res a rotary energy wheel with inlet supply air from outside ambient air and inlet exhaust air from the outlet of a sensible heat exchanger. This sensible heat exchanger could be a heat wheel or another type of heat exchanger, such as a cross-flow plate exchanger or a heat pipe Yaw Asiedu is an oper
14、ational research analyst at the Department ofNational Defense, Ottawa, Ontario, Canada. Robert W. Besant is professor emeritus and Carey J. Simonson is an associate professor, Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, SK, Canada. 02005 ASHRAE. 857 heat exchanger, i
15、f they are more cost-effective and convenient to install. This analysis would not change if such a substitution were made. The heat exchanger has an inlet supply air from a cooling coil or, when the coil is not used, from the supply air outlet of the energy wheel. The exhaust air inlet to the heat e
16、xchanger is directly from the conditioned space. Unlike the case of a system with only one exchanger (e.g., a heat wheel or energy wheel), this problem is complicated by the fact that when ali the heat transfer units are operating, the inlet and outlet conditions to a downstream unit depend on the p
17、erfor- mance of an upstream unit, which, in turn, depends on the performance of the downstream unit. Such a system would have to be running for several minutes before steady-state operating conditions are reached. In this paper, it is assumed that the system is operating at steady-state conditions.
18、It is further assumed that the cooling coil is selected after the energy wheel and heat exchanger have been selected such that Figure 1 Schematic of a HVAC system with air-to-air heat/ energy recovery. Area proportional to the annual sensible cooling energy recovered by the air-to-air heat exchanger
19、 Area proportional to the annual auxiliary sensible energy needed Area proportional to the annual auxiliary sensible energy needed to heat the ventdation air proportional to the annual sensible heating y recovered by the air-to-air heat exchanger it is able to meet the peak load demand imposed on it
20、 by the exchangers at the set design operating condition. The exact operation of both exchangers depends on the ambient air conditions and the supply and exhaust air condi- tions for the space. While the energy wheel is likely operated throughout the year, the cooling coil and heat exchanger are onl
21、y operated when they can improve the overall energy recovery of the system. In the winter months, the heat exchanger is operational only during the very cold periods when the chiller is not used and auxiliary heating may be needed. In the summer, the heat exchanger may be used continuously. If the h
22、ourly weather data, .e., temperatures and enthal- pies, in a location are plotted as monotonically increasing functions with the time (number of hours in the year) as the abscissa and enthalpy or temperature as the ordinate, then the outside air enthalpy may be assumed to be described by (1) h = F(t
23、) =y* + a* sinh(b*t + z*) and the outside air temperature by T= G(t) =y + a sinh(bt + 2) . (2) where y*, y, u*, a, b*, b, z*, and z are constants to be deter- mined through regression analysis and t is the time in hours of the year. Time zero corresponds to the lowest ambient hourly temperature or e
24、nthalpy and hour 8760 corresponds to the highest ones. The superscripts * and are used here and in subsequent discussions to denote the energy wheel and heat exchanger parameters, respectively, in instances where a distinction between the two is required. More details of these functions are shown in
25、 Appendix A. Arranging the ambient temperature and enthalpy as shown in Figure 2 is very useful (Asiedu et al. 2003) because Area representing the total annual cooling energy recovered by the air-to-air energy exchanger Area representing the annual auxiliary energy needed to cool rea representing th
26、e annual auxiliary energy needed to heat and humidify the ventilation air ea representing the total annual heating energy recovered by the air-to-air energy exchanger Figure 2 Distribution of (a) ambient air temperature and (b) enthalpy, showing the areas that are proportional to the (a) sensible an
27、d (6) total energy recovered by un air-to-air energy exchanger und the annual auxiliary energy needed to condition the ventilation air to the supply conditions to the space. 858 ASHRAE Transactions: Symposia the area between these lines and supply conditions to the space (Tdes and hdes) represent th
28、e annual heating and cooling energy required to condition the outdoor ventilation air to the supply conditions when no air-to-air energy exchanger is in the and c,i will be equal to the indoor conditions. only important effectiveness value, and it is written as For the heat exchanger, the sensible e
29、ffectiveness is the -_ - HVAC system. Without an energy wheel or heat exchanger when the supply air humidity is controlled, ventilation air would be heated by an auxiliary heater and moisture may be (7) added, ifrequired,from the ambient enthalpy curve h(t) UP to hdes for h (l-,) (5) and the supply
30、outlet temperature as Ts, = G(t) = E,T,+ TJ1 = c,Te,+ G(t)(l - ss , (6) where F(t) and G(t) are from Equations 1 and 2, and he,i and q,i depend on the performance of the heat exchanger when it is operating. When the heat exchanger is not operating, he,i Ti,o = partial humid- ification or no humidifi
31、cation ofthe supply air would give very different results. In region A, the form of Equation 5 used is more complicated and derived as follows. From Equation 7, TL, = EH(Ti,i - Ti,i + .;,i (10) Since Te,o = Te,i (i.e., the exhaust outlet from the heat exchanger equals the exhaust inlet to the energy
32、 wheel) and the supply outlet from the energy wheel), when the chiller is shut off, Equation 1 O may be rewritten as T .= T ( i.e., the supply inlet to the heat exchanger equals s,1 s,u or EH = 60% ( Tc) EH = 0% ( hc) E, = 70% ( Tc) i E, = 70% ( hc) 26.1 U/kg Figure 4 Critical outdoor temperature (T
33、i) and enthalpy Fc) calculated with Equations 25 and 9, respectively, for the shown supply air design conditions and energy exchanger eflectivenesses. For hs,i A * fc r=o * c rC r=o IC r=o * IC * fC * ,* (I -E)T,I + 250iW,f -cf F(r)dt - Hei - I- E -EHES Hs r=o r=o (19) The apount of energy recovered
34、 in the region denoted as B, where the energy transfer rate must be reduced or the energy wheel to meet the design enthalpy hdes, is given by EE( 1 - ec,/tn is the area of region B in Figure 5, which is the Equation 15 expresses h, as a function of known parameters (E, E, and E, from certified manuf
35、acturer data, G(t) and F(t) from weather data, and and from the indoor condi- tions of the space) and is plotted as the solid line in Figure 5. Total Heating Requirements Considering only the shaded energy areas in Figure 5, without an energy wheel, the total heating requirement is where Atot,h is t
36、he area between the outdoor enthalpy line (h,J and enthalpy of the air supplied to the building space (hdes). Atot,h can be expressed as area between the hs,i line and hdes line for t tc (h hc). Math- ematically, des * Arec,h, = (hdes-hs,j)dt I = 1, tdes des = j hdesdt- 1 F(t)dt (21) t = 1, t = f, T
37、hus, the total energy recovered is given by * * qrec,h = msArec,h with * * * - Arec,h - rec,hA + Arec,hB (23) 861 ASHRAE Transactions: Symposia Recovered Sensible Heat in the Heat Exchanger The heat exchanger is only used during cold periods (i.e., h,; t*, and substituting Equation 24 into Equation
38、8 gives Auxiliary Heating and Outdoor Conditions Humidification Heat Exchanger Energy Wheel Ts,o Tc and h,O wdes) then condi- tioning ventilation air to design conditions would require first subcooling and condensing supply water vapor and then reheating this air to Tdes. With the arrangement in Fig
39、ure 1, the cooling coil is used to subcool the supply air from Ts,o, Ws,o to T, Ws,i. The energy for the other processes is derived from recovered energy from the heat and energy wheels and the auxiliary heating equipment in situations where the heat exchanger effectiveness is low. The psychrometric
40、 chart for this air-conditioning process is shown in Figure 7(a). Figure 7(b) shows the psychrometric chart process for the case where the humidity ratio of the ambient air is less than that of the design humidity ratio (i.e., very dry and hot weather condi- tions). In this case, there is no need to
41、 subcool (for dehumid- ification) and reheat the ventilation air, and the heat exchanger will not be operational. Figure 8 depicts the energy recovered from the heat exchanger and the auxiliary energy required to bring the supply air to the design conditions once it leaves the heat exchanger. The re
42、gion below the Tdes line represents the energy requirements during the cooling period where dehu- midification is required. In this situation, the supply inlet temperature for the heat exchanger is Tscool. It can be assumed that the cooling coils are controlled such that the supply inlet temperature
43、 of the heat exchanger is always equal to a specific Tscoolvalue dependent on wdes. The regions above the Tdes line represent energy recovery in the energy wheel and require 3 2 E G I O .I .c1 P .- a Temperature - T Tdcs i Figure 7 Psychrometric chart process showing subcooling and reheating of supp
44、ly air during warm weather periods using chillers and heat exchangers. ASHRAE Transactions: Symposia 863 Sensible Annual Recovered Cooling Energy In Energy Wheel Sensible Annual Auxiliary Heating Energy During Periods Requiring ehuniidification T, 10, Sensible Annual Recovered Heating Energy During
45、Periods Requiring Dehumidification Sensible Annual Auxiliary Energy During Periods Not Requiring Dehumidification only a 3% difference between the value of R from Equation 3 1 (0.936) and that derived from a direct examination of the weather data (0.967). The complete derivation of Equation3 1 is gi
46、ven in Appendix B. Cooling with Dehumidification As indicated earlier, when the supply air has a humidity ratio greater than the design humidity ratio ( Wdes), condition- ing ventilation air to design conditions would require first subcooling and condensing supply water vapor to a tempera- ture equa
47、l to Tscool and then reheating this air to Tdes. The heat- ing energy recovered by the heat exchanger during this period is a function of q, Tscool, and the effectiveness of the exchanger. Ifthis effectiveness is too high, then there is the risk of heating the air above the design temperature. In su
48、ch a case, the heat transfer process would have to be controlled. The crit- ical effectiveness, E, the exchanger effectiveness beyond which this would be necessary, is given by Figure 8 Temperature versus time showing the energy regions of an air-to-air heat exchanger operating in series with an ene
49、rgy wheel and a cooling coil in an HVACsystern for a typicalperiod of the year requiring cooling. Iscool- I des Tscooi - C,i EcH = Thus, from Equation 8, the heat exchanger supply out temper- ature is given as auxiliary energy during periods requiring cooling but not dehumidification (the heat exchanger is not operated during this period). The chiller outlet temperature will be equal to the design temperature Tdes in this instance. Each separate region (above or below Tdes line) depicted in Figure 8 represents the total amount
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