Publication: HVAC & R Research
Publication Date: 01-MAY-07
Delivery: Immediate Online Access
Author: Lambers, Klaus Jurgen ; Suss, Jurgen ; Kohler, Jurgen
Article Excerpt
Admission (the use of an economizer) is a well-known method to reduce the expansion losses of a refrigeration system. The alternative refrigerant carbon dioxide (C[O.sub.2]) has a relatively large isentropic exponent. Therefore, refrigeration systems using C[O.sub.2] as a refrigerant have a high potential for raising efficiency by admission. Reciprocating piston compressors are favored for C[O.sub.2] refrigeration applications due to the compression chamber's small sealing length and resulting low internal leakage rate.
This paper presents a methodology to investigate the influence of the admission (economizer) port design of a reciprocating compressor on the admission process. The port's geometry--especially its cross-sectional area--has a large impact on the system efficiency at operation conditions and therefore needs to be optimized. For this purpose, an optimization process has been established. It is based on indicator diagram analysis.
To exemplify, the measurements of three admission ports were compared at constant suction pressure (1.2 MPa relates to an evaporation temperature of -35[degrees]C) and varying admission pressures (1.4-2.6 MPa). Based on the results, the optimal port diameter for the targeted operation condition is obtained by interpolation.
INTRODUCTION
Since the early 1990s, the use of the natural refrigerant carbon dioxide (C[O.sub.2]) has gained acceptance, especially in refrigeration and air-conditioning applications. On the one hand, this increased acceptance can be explained by the fact that C[O.sub.2] is not harmful to the environment when released; on the other hand, it can be attributed to its potential for high system efficiency due to positive heat transfer behavior, which is especially good under supercritical conditions, and to its low sensibility to pressure drop. The latter leads to compensation of the C[O.sub.2]-specific increased exergetic expansion losses and increased exergetic heat rejection losses due to the relatively high isentropic exponent.
The use of C[O.sub.2] leads to encouraging COPs in cooling applications at evaporating temperatures around -5[degrees]C. This sparks investigations into the possible use of C[O.sub.2] in low-temperature applications at evaporating temperatures around -35[degrees]C. However, with decreasing evaporation pressure, the abovementioned exergetic expansion losses as well as the rejection losses increase significantly (Lambers et al. 2007). That is why alternative C[O.sub.2] freezing application system designs have been investigated recently, mainly focused on the reduction of expansion losses.
One approach to reduce these losses is admission, also known as the economizer principle, which can be applied as a compressor modification invented by Voorhees (1905). He called this modified compressor a "multiple effect" compressor.
For evaporation at -35[degrees]C, the theoretical potential of coefficient of performance (COP) improvement lies at approximately 17% at an ambient temperature of 32[degrees]C. This has been calculated by previous investigations. According to associated first measurements, an enhancement of 10% could be achieved (Lambers et al. 2006).
Optimization of the admission port is necessary to get closer to the theoretical limit. The admission port is one of the main compressor modifications to realize the "multiple effect" principle. Its design has a major impact on system efficiency.
VOORHEES-TYPE ADMISSION
The System
The efficiency of a system can be increased by admission. Here, flash gas is separated at the different pressure levels of a multistage throttling cycle. This flash gas is admitted to multistage compression at the corresponding pressure levels. In practice, this is done more economically by implementing a scroll or screw compressor. In that case, only one compression stage is necessary.
This advanced system design, although it causes additional expense, results in four beneficial effects: the COP increases; there is a rise in refrigeration capacity; the mass flow through the evaporator decreases, which implies reduced dimensioning; and the discharge temperature decreases, which is especially important for refrigerants such as C[O.sub.2] with larger isentropic exponents.
Reciprocating piston compressors best suit the thermodynamical demands of the refrigerant C[O.sub.2] (Suss 1998). The "multiple effect" compressor modification of Voorhees seems to be an option to apply the advantageous flash gas reduction by admission to a modern C[O.sub.2] refrigeration system.
The Reciprocating Compressor with Voorhees-Type Admission
The "multiple effect" modification can be applied to a conventional reciprocating piston compressor. The flash gas, which is separated in the two-stage expansion and is hereafter called auxiliary gas, is admitted into the compression chamber when the piston reaches the bottom dead center. Ideally, the auxiliary gas hereby compresses the suction gas from suction pressure to admission pressure.
Admittedly, this kind of admission comprises unavoidable throttling losses due to the discontinuity of the admission process. These losses don't occur by admission after the first stage of two-stage compression mentioned before.
Under certain conditions, the "multiple effect" modification allows the application of several admission pressure levels, as suggested in Voorhees's (1905) patent,...
NOTE: All illustrations and photos have been removed from this article.
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