ASHRAE 4725-2004 Minimizing TEWI in a Compact Chiller for Unitary Applications《最小TEWI在一个紧凑式制冷机中的整体应用》.pdf
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1、4725 Minimizing TEWI in a Compact Chiller for Unitary Applications Philip R. Barnes ABSTRACT A simulation model was developed to investigate strate- gies for reducing total equivalent warming impact (TEWI) in compact water chillers. The focus was on minimizing refrig- erant charge while increasing e
2、jciency, using R-41 OA as an example. Compactflat plate heat exchangers with refrigerant channels similar in scale to microchannels appear capable of reducing total system charge by about 80% compared to conventional air-air split systems. Results are also compared to those obtainedfor highly ejcien
3、t air-to-air unitary systems, in which minimum-TEWIdesign strategies require larger heat exchangers having greater chaTe. Overall, the two approaches achieve comparable reductions in global warming impacts; the chiller depends more on reducing direct emis- sions, compared to unitary systems dependen
4、ce on reducing indirect emissions through use of jlat multiport tubes with foldedjns. INTRODUCTION The phaseout of hydrochlorofluorocarbons (HCFCs) has led to their replacement by hydrofluorocarbons (HFCs), which are greenhouse gases. To characterize the global warm- ing effects of new systems, incl
5、uding those using natural refrigerants, Sand et al. i proposed the total equivalent warming impact (TEWI) to account for the release of refrig- erant into the atmosphere (direct effects) and the release of carbon dioxide from electricity generation (indirect effects). Many alternatives to HCFCs are
6、not greenhouse gases but may be either toxic or hazardous (e.g., hydrocarbons, ammo- nia), making it necessary to minimize charge when those refngerants are used. Charge-minimizing heat exchangers and systems differ substantially from conventional designs, and Clark W. Bullard, Ph.D. Fellow ASHRAE p
7、rototypes are costly to fabricate. Therefore, it is economical to employ simulation modeling to conduct initial explorations of the parameter space, quantifying trade-offs between two ways for minimizing TEWI-increasing efficiency and reduc- ing charge. Previous analyses have provided rough comparis
8、ons of TEWI for residential space conditioning (heating and air conditioning combined) systems, including the option of using a secondary (e.g., chilled water or glycol) loop to keep flammable or toxic refrigerants outdoors i. While establish- ing that the direct contribution to TEWI is a small frac
9、tion of the indirect, they were not focused on the geometric trade-offs involved in designing low-charge heat exchangers, for either air-to-air systems or chillers. This paper addresses those geometric trade-offs explicitly and examines their effect on the directhndirect fraction of TEWI. The next s
10、ection describes two residential-scale “base- line” systems from which improvements can be measured. One is a conventional North-American-style split system; the other is a hermetic chiller utilizing compact brazed plate heat exchangers. The residential scale was chosen only to provide a familiar st
11、arting point for the analyses, which are normalized per unit cooling capacity, in the interest of generalizing the resulting insights across a broader range of unitary air-condi- tioning system and chiller sizes. The next sections briefly describe the optimization process and explain the results of
12、the TEWI minimization and then describe the sensitivity analysis performed near the opti- mum with respect to several model assumptions. Descriptions of the baseline systems and the methods for modeling the brazed plate heat exchangers were detailed by Barnes and Bullard 2. Philip R. Barnes is a mec
13、hanical design engineer at Florida Power 4%/yr refrigerant leak rate 5; and 900 h/yr runtime (typical of Washington, D.C., Indianapolis, Los Angeles) 6. For the conventional system with -258 g/kW refrigerant charge i, this corresponds to a 90/10 indirecvdirect TEWI fraction, which, of course, can va
14、ry greatly with assumptions about system runtime (climate, insulation, and shading) and refrigerant leakage. While the following analyses illustrate the methodology for the most common case of HFC refrigerant R-410A where flammability and toxicity are generally not at issue, they also show how the p
15、resence of charge constraints could influence strategies for minimizing TEWI by limiting heat exchanger size. Using the same model, Kirkwood and Bullard 7 explored the extent to which TEWI could be reduced in air-to- air split systems by using microchannel heat exchangers. They examined microchannel
16、s because of their compactness for a given heat transfer capacity and low pressure drop, compared to traditional round-tube/plate-fin heat exchangers. Their simulations suggested that TEWI could be reduced by approx- imately 13% compared to a conventional R-410A system, at the AR1 210/240-B standard
17、 rating condition (26.7“C dry bulb/19.4“C wet bulb indoor; 27.8“C dry bulb outdoor found in DOE 6). This improvement was achieved by increasing system COP (3.8 to 43, which required large heat exchang- ers, thus limiting charge reduction (258 to 235 gkW). There remains some potential for improvement
18、 in the microchannel design either by extruding smaller microchannel ports or decreasing liquid line length (10.8 m in Kirkwoods simula- tions) to reduce total charge. However the overall result (diffi- culty of reducing charge while increasing efficiency) is not surprising; due to the dominance of
19、air-side heat transfer resis- tance, large heat exchangers are needed and refrigerant charge increases proportionally. Compact Hermetic Chiller Another way to minimize the direct TEWI effect would be to minimize charge by building a small chiller to take advantage of the compactness obtainable with
20、refrigerant-to- water heat exchange instead of refrigerant-to-air. Commer- cially available compact brazed plate heat exchangers (CBEs) are used in a wide variety of applications (food processing, chemical processes, and pharmaceutical industries). Due to their very compact nature, high surface-volu
21、me ratios, rel- tively low pressure drops, and their ability to utilize chevrons and bumps imprinted on the plates, they rely more on heat transfer coefficient and less on area to transfer heat. Many studies have examined liquid-liquid heat transfer and pressure drop in CBEs 8-18. However, only a fe
22、w have examined evaporation and condensation in CBEs 19-23, with only Yan 22,23 providing correlations. Those correla- tions for two-phase heat transfer and pressure drop were used in a simulation model with correlations from Shah and Focke I51 for single-phase heat transfer and Focke et al. i i for
23、 single-phase pressure drop. Criteria for choice were described by Barnes and Bullard 2. The compact chiller using CBEs was simulated for the ARI 550/590 standard rating condition for chillers (0.054 L/s per kW at 29.4“C inlet condenser water, 0.043 LIS per kW at 6.7“C outlet evaporator water) 24. I
24、t was assumed that refrig- erant lines could be quite short, as indicated in Table 1. To Table 1. Hermetic Chiller Model Inputs 336 ASHRAE Transactions: Research Conventional CBE Chiller provide chilled water to every room, total pipe lengths were conservatively assumed to bel 50 m and 50 m for the
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