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Article Excerpt
INTRODUCTION
Widely used unitary air conditioners operate on a vapor compression refrigeration (VCR) cycle, with a capillary as the expansion device. The growing awareness of the need to sustain the ecology of the planet has resulted in the phase-out of the harmful refrigerants containing chlorine atoms, such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Although a replacement for CFCs has been found, the search for good alternatives for HCFCs--especially R-22-is still on. According to the literature, there is no single compound refrigerant that can directly replace R-22 (Calm and Domanski 2004). However, Domanski and Didion (1993) identified a few mixtures of hydroflourocarbon (HFC) refrigerants using a semitheoretical model, CYCLE-11. Devotta et al. (2001) carried out a steady-state cycle simulation of seven selected alternatives using the NIST CYCLE_D program. From the comparative thermodynamic analysis, they concluded that a binary mixture of R-410A (e.g., R32/R125, 50%/50%) is the next popular choice for a new system with a new compressor.
The steady-state characteristics of air conditioners that use R-22 and alternatives such as R-410A are well established, but the transient characteristics are crucial to the operation of the system at off-design conditions. The on-off types of controls are generally used in small-capacity units to maintain the desired indoor temperatures and regulate capacity. For real systems using such controls, it can be said that a pure steady state does not exist. Judge et al. (1996) reported that on-off cycling of the system degrades the coefficient of performance (COP) of the system to values below 75% of the steady-state values. Thus, the transient characteristics are equally as important as the steady-state performance from the point of actual energy efficiency of the systems. The transient losses during cycling are mainly attributed to two different phenomena: thermodynamic losses (heat exchangers mass) and start/stop losses (refrigerant migration) for the same compressor unit (Janssen et al. 1992). When the compressor is in the off position, the condenser gradually cools down, while the evaporator heats up, depending on the surrounding temperature. This presents itself as a load during startup as the thermal mass of the system has to be reconditioned to steady-state conditions. Also, during the off cycle, the refrigerant migrates from the high-pressure condenser to the low-pressure evaporator, especially if a capillary is used as the expansion device. The capillary is generally preferred in small-capacity units, as it equalizes the pressure on the two sides of the compressor, and thus reduces its starting torque. However, the migrating refrigerant carries energy and lubricant, disturbing the steady-state operating parameters of the system. So, both the temperature and the pressure increase in the evaporator, which affects the conditions in the room. When the compressor turns on again, it has to redistribute the refrigerant and the lubricant to bring the system and the conditioned space to its new operating conditions. This results in loss of capacity, which varies with time. The losses are the maximum at the time of start-up and gradually reduce as the system approaches steady-state conditions. Although the actual cooling capacity will also depend on the thermal mass of the conditioned space, the refrigerant capacity of the system at any instant can be obtained by carrying out a transient analysis of the complete system. Thus, it is possible to quantify the transient losses in the system due to on-off cycling.
Of the four major components of the VCR system, the dynamics of the heat exchangers (condenser and evaporator) are usually the slowest and have the largest impact on transient performance. The currently used heat exchangers (both condenser and evaporator) with microfin tubes have been selected and their transient performance compared with those using smooth tubes. Subsequently, the paper presents in detail the dynamic behavior of the complete air-conditioning system during start-up using R-410A and R-22.
LITERATURE REVIEW
The transient characteristics of small residential air-conditioning and heat pump (HP) systems have been reported by many researchers, based on numerical as well as experimental studies.
Numerical Studies
Bendapudi and Braun (2002) and Ding (2007) presented an exhaustive review of literature on dynamic modeling and the simulation of vapor-compression equipment. Selected examples from this review are discussed in this section.
Dhar and Soedel (1979) modeled the complete VCR system for a window air-conditioner unit. The heat exchangers of the system were modeled using a moving-boundary, lumped-parameter approach, taking into account the different zones in the condenser (e.g., super-heated, two-phase, and subcooled) and the evaporator (e.g., two-phase and superheated). They used the Euler method for the solution of ordinary differential equations. Chi and Didion (1982) also developed a model using a moving-boundary, lumped-parameter formulation for a residential air-to-air heat pump system. The momentum equation was also included in the heat exchanger models. The dynamic characteristics during start-up were analyzed and verified with the cooling test results. In three papers, MacArthur (1984a, 1984b, and 1984c) presented a distributed approach to model heat exchangers, rather than the lumped-parameter approach. This was a distinct approach for the heat exchangers that permitted space-time variation in the properties. The thermal response was obtained by space and time variation of temperature and enthalpy, whereas the pressure response of the heat exchangers was based on the assumption of uniform flow velocities along the heat exchanger length. This approach gave inaccurate mass distributions. MacArthur and Grald (1989) overcame this shortcoming by determining the flow fields from the conservation equations and including a void fraction model.
Murphy and Goldschmidt (1984, 1985, and 1986) presented a series of papers to address start-up and shutdown transients for a 3 ton air-to-air split air-conditioner system. In the start-up model, the compressor model was based on a steady-state correlation. The evaporator model was also based on the measured data on evaporator performance. Based on their sensitivity analysis with the condenser model during start-up, they concluded that a heavy condenser does not improve the start-up performance and the longer the liquid line, the greater the cyclic loss. In the shutdown study, the lumped-parameter approach was used to model the heat exchangers containing two-phase refrigerant. Sami et al. (1987) predicted the dynamic behavior of a heat pump using lumped-parameter models for the various system components. In 1987, the Montreal Protocol was signed under the auspices of the United Nations Environmental Program (UNEP), which led to the search for eco-friendly refrigerants. These opened new dimensions in the dynamic characteristics studies, as most of the alternatives suggested to R-22 were mixtures. Sami and Dahmani (1996), Judge et al. (1996), and Haberschill et al. (2003) published dynamic simulations of AC/HP systems with mixtures of refrigerants. Sami and Dahmani's (1996) work dealt with HFC alternatives to R-22, specifically R-407A, R-507, and a blend of R-22, R-23, and R-152a. Judge et al.'s (1996) and Haberschill et al.'s (2003) work dealt with R-407C as a refrigerant.
Experimental Studies
The majority of experimental studies focus on determining the phenomenon responsible for cyclic losses and ways to minimize them. Murphy and Goldschmidt (1979) evaluated the refrigeration capacity or efficiency of a residential unit under cyclic behavior. They concluded that the transient losses due to cycling differ for a heat pump in cooling and heating modes. The losses also depend on the thermal mass of the conditioned space and its thermostat setting. Tanaka et al. (1982) studied the dynamic change of refrigerant quantity, along with the pressure and temperature variations, in three different sections of a VCR system: (1) the indoor heat exchanger, (2) the outdoor heat exchanger, and (3) the compressor. Based on their results, they suggested light design of heat pumps, with minimum refrigerant charge and preventing refrigerant flow during off cycle for improving the start-up performance. Mulroy and Didion (1983) experimentally studied refrigerant migration with time by installing pneumatic valves in the system. They showed that during start-up more charge was in the evaporator but during steady-state operation it migrated to the condenser. Regression analysis was carried out on the experimental data to obtain the following equation for instantaneous refrigeration capacity as a function of time, accounting for the transient loss due to both the thermal mass of heat exchangers and the refrigerant migration:
[Q/[Q.sub.ss]] = [1 - exp([-t]/[[tau].sub.1])][1 + aexp([-t]/[[tau].sub.2])] (1)
Wang and Wu (1990) showed that the start-up transients are greatly influenced by the shutdown transients. By preventing the refrigerant migration during shutdown by closing a...
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