Refrigeration System Performance using Liquid-Suction Heat Exchangers
精读与泛读
S. A. Klein, D. T. Reindl, and K. BroWnell
College of Engineering
University of Wisconsin - Madison
Abstract
Heat transfer devices are provided in many refrigeration systems to exchange energy betWeen the cool gaous refrigerant leaving the evaporator and Warm liquid refrigerant exiting the condenr. The liquid-suction or suction-line heat exchangers can, in some cas, yield improved system performance While in other cas they degrade system performance. Although previous rearchers have investigated performance of liquid-suction heat exchangers, this study can be distinguished from the previous studies in three Ways. First, this paper identifies a neW dimensionless group to correlate performance impacts attributable to liquid-suction heat exchangers. Second, the paper extends previous analys to include neW refrigerants. Third, the analysis includes the impact of pressure drops through the liquid-suction heat exchanger on system performance. It is shoWn that reliance on si
mplified analysis techniques can lead to inaccurate conclusions regarding the impact of liquid-suction heat exchangers on refrigeration system performance. From detailed analys, it can be concluded that liquid-suction heat exchangers that have a minimal pressure loss on the loW pressure side are uful for systems using R507A, R134a, R12, R404A, R290, R407C, R600, and R410A. The liquid-suction heat exchanger is detrimental to system performance in systems using R22, R32, and R717.
Introduction
Liquid-suction heat exchangers are commonly installed in refrigeration systems With the intent of ensuring proper system operation and increasing system performance.Specifically, ASHRAE(1998) states that liquid-suction heat exchangers are effective in:
英语四级考试报名入口2012年7月28日1) increasing the system performance
2) subcooling liquid refrigerant to prevent flash gas formation at inlets to expansion devices
3) fully evaporating any residual liquid that may remain in the liquid-suction prior to reaching the compressor(s)
教堂英文>take me to your heart歌词Figure 1 illustrates a simple direct-expansion vapor compression refrigeration system utilizing a liquid-suction heat exchanger. In this configuration, high temperature liquid leaving the heat rejection device (an evaporative condenr in this ca) is subcooled prior to being throttled to the evaporator pressure by an expansion device such as a thermostatic expansion valve. The sink for subcoolinggo forth
the liquid is loW temperature refrigerant vapor leaving the evaporator. Thus, the liquid-suction heat exchanger is an indirect liquid-to-vapor heat transfer device. The vapor-side of the heat exchanger (betWeen the evaporator outlet and the compressor suction) is often configured to rve as an accumulator thereby further minimizing the risk of liquid refrigerant carrying-over to the compressor suction. In cas Where the evaporator alloWs liquid carry-over, the accumulator portion of the heat exchanger Will trap and, over time, vaporize the liquid carryover by absorbing heat during the process of subcooling high-side liquid.
Background
Stoecker and Walukas (1981) focud on the influence of liquid-suction heat exchangers in both single temperature evaporator and dual temperature evaporator systems utilizing refrigerant mixtures. Their analysis indicated that liquid-suction heat exchangers yielded greater performance i
mprovements When nonazeotropic mixtures Were ud compared With systems utilizing single component refrigerants or azeoptropic mixtures. McLinden (1990) ud the principle of corresponding states to evaluate the anticipated effects of neW refrigerants. He shoWed that the performance of a system using a liquid-suction heat exchanger increas as the ideal gas specific heat (related to the molecular complexity of the refrigerant) increas. Domanski and Didion (1993) evaluated the performance of nine alternatives to R22 including the impact of liquid-suction heat exchangers. Domanski et al. (1994) later extended the analysis by evaluating the influence of liquid-suction heat exchangers installed in vapor compression refrigeration systems considering 29 different refrigerants in a theoretical analysis. Bivens et al. (1994) evaluated a propod mixture to substitute for R22 in air conditioners and heat pumps. Their analysis indicated a 6-7% improvement for the alternative refrigerant system When system modifications included a liquid-suction heat exchanger and counterfloW system heat exchangers (evaporator and condenr). Bittle et al. (1995a) conducted an experimental evaluation of a liquid-suction heat exchanger applied in a domestic refrigerator using R152a. The authors compared the system performance With that of a traditional R12-bad system. Bittle et al. (1995b) also compared the ASHRAE method for predicting capillary tube performance (including the effects of liquid-suction heat exchangers) With experimental data. Predicted capillary tube mass floW rates Were Within 10% of predicted values and subcooling levels Were Within 1.7 C (3F) of actual measurements.
This paper analyzes the liquid-suction heat exchanger to quantify its impact on system capacity and performance (expresd in terms of a system coefficient of performance, COP). The influence of liquid-suction heat exchanger size over a range of operating conditions (evaporating and condensing) is illustrated and quantified using a number of alternative refrigerants. Refrigerants included in the prent analysis are R507A, R404A, R600, R290,
R134a, R407C, R410A, R12, R22, R32, and R717. This paper extends the results prented in previous studies in that it considers neW refrigerants, it specifically considers the effects of the pressure drops,and it prents general relations for estimating the effect of liquid-suction heat exchangers for any refrigerant.
Heat Exchanger Effectiveness
The ability of a liquid-suction heat exchanger to transfer energy from the Warm liquid to the cool vapor at steady-state conditions is dependent on the size and configuration of the heat transfer device. The liquid-suction heat exchanger performance, expresd in terms of an effectiveness, is a parameter in the analysis. The effectiveness of the liquid-suction heat exchanger is defined in equation (1):
英语词典软件Where the numeric subscripted temperature (T) values correspond to locations depicted in Figure 1. The effectiveness is the ratio of the actual to maximum possible heat transfer rates. It is related to the surface area of the heat exchanger. A zero surface area reprents a system Without a liquid-suction heat exchanger Whereas a system having an infinite heat exchanger area corresponds to an effectiveness of unity.
The liquid-suction heat exchanger effects the performance of a refrigeration system by in fluencing both the high and loW pressure sides of a system. Figure 2 shoWs the key state points for a vapor compression cycle utilizing an idealized liquid-suction heat exchanger on a pressure-enthalpy diagram. The enthalpy of the refrigerant leaving the condenr (state 3) is decread prior to entering the expansion device (state 4) by rejecting energy to the vapor refrigerant leaving the evaporator (state 1) prior to entering the compressor (state 2). Pressure loss are not shoWn. The cooling of the condensate that occurs on the high pressure side rves to increa the refrigeration capacity and reduce the likelihood of liquid refrigerant flashing prior to reaching the expansion device. On the loW pressure side, the liquid-suction heat exchanger increas the temperature of the vapor entering the compressor and reduces the refrigerant pressure, both of Which increa the specific volume of the refr igerant and thereby decrea the mass floW rate and capacity. A major be
nefit of the liquid-suction heat exchanger is that it reduces the possibility of liquid carry-over from the evaporator Which could harm the compressor. Liquid carryover can be readily caud by a number of factors that may include Wide fluctuations in evaporator load and poorly maintained expansion
devices (especially problematic for thermostatic expansion valves ud in ammonia rvice).
（翻译）冷却系统利用流体吸热交换器
克来因教授，布兰顿教授, , 布朗教授
威斯康辛州的大学–麦迪逊
摘录
加热装置在许多冷却系统中被用到，用以制冷时遗留在蒸发器中的冷却气体和离开冷凝器发热流体之间的能量的热交换.这些流体吸收或吸收热交换器，在一些情形中，他们降低了系统性能, 然而系统的某些地方却得到了改善. 虽然以前研究员已经调查了流体吸热交换器的性能, 但是这项研究可能从早先研究的三种方式被加以区别. 首先，这份研究开辟了一个无限的崭新的与流体吸热交换器有关联的群体.其次，这份研究拓宽了早先的分析包括新型制冷剂。第三, 研究包括压力的冲击降低了流体吸热交换器的系统性能. 在简单的技术信息分析中表明流体吸热交换器对冷却系统性能的冲击可能导致错误
的结论.从详细说明分析里，它能得出一个结论，那就是液体- 吸加热交换器在低压区域上的临界压力使用 R507A ， R134a ， R12 ， R404A ， R290 ，R407C ， R600 和 R410A这些制冷剂，对系统是有用的。而使用 R22 ， R32 和 R717对系统的性能是有害的.verification
介绍
流体吸热交换器被普遍的安装在正确合适的系统操作和提高系统性能的制冷系统中。很明显， ASHRAE(1998) 液体- 吸加热交换器的确是有效的他表现在:
1)增加系统性能
会计就业前景
2)液体制冷剂防止散发气体进入扩充装置。
一些剩余的液体在到达之前被完全蒸发了。图 1 列举了一个简单的指示。压缩物 (s) 可能利用流体吸热交换器保持的液体扩充蒸汽压缩的性能.
3）在这一个结构中，高温液体余热像一个温度调节装置一样拒绝装置 (蒸发冷凝器就是这种情况) 在扩充之前对蒸发器的压力再冷却，洗涤槽是为了接收在低温度冷冻下遗留在蒸发器内的再冷却液. 因此，流体吸热交换器是一种从液体到蒸汽热交换的间接装置. 热交换器 (在蒸发器出口和压缩物吸收之间) 的蒸汽边界经常承担积聚压缩物吸的液体，藉此将对滞留的液体制冷剂的危险性减到更少. 在蒸发
器允许液体滞留的情形中, 在热交换器中积聚部分会困住而且，超过一定的时间后，在液体再冷却的过程中，滞留的液体被吸收热量而蒸发.
背景
Stoecker 和 Walukas(1981) 着重于利用流体吸热交换器在单一温度蒸发和双重的温度蒸发系统的影响下的冷冻混合.他们的分析指出当nonazeotropic混合剂或azeoptropic混合剂与利用单一成份制冷剂的系统相比较时, 流体吸热交换器产生更多性能的改进。McLinden(1990) 用了相关的原则评价新的制冷剂被预期的效果. 他指出作为理想的特殊性气体使用在流体吸热交换器中增加这项系统的性能(谈到制冷剂的复杂的分子结构)。 Domanski 和 Didion(1993) 评估了包括流体吸热交换器的替代品
R22 的九个性能. Domanski et al. (1994)稍后鉴于对29种不同的制冷剂一项理论分析，扩大了流体吸热交换器安装在蒸汽压缩冷却系统的评价Bivens et al. (1994)评估了一种被提议的混合物来替代为空调和热泵中使用的 R22。他们的分析指出当系统修正心理疾病的自我治疗
