Marine Högberg

Calculation methods for comparing the performance of pure and mixed working fluids in heat pump applications

Three methods for comparing cycle performance of working fluids, pure as well as non-azeotropic mixtures, are investigated for two applications and for two mixture pairs, HCFC22-CFC114 and HCFC22-HCFC142b, and their pure components. The methods differ in the way of calculating the heat exchange processes. They assume, respectively, equal minimum approach temperatures, equal mean temperature differences and equal heat transfer areas. Changes of coefficient of performance (COP) with composition are explained for all methods. It is shown that transport properties must be taken into account when making rigorous comparisons between working fluids. To predict the relations between fluids with high accuracy, one must use the method with equal heat transfer areas. By the method with equal mean temperature differences, the COP can be estimated with the same accuracy for mixtures as for pure fluids, and can be used for rough estimations of the COP level with different fluids. The method of equal minimum approach temperatures should be avoided for non-azeotropic mixtures.

Non-azeotropic mixtures as working fluids in two-stage economizer-type heat pumps

Abstract The thermodynamic differences in behaviour between non-azeotropic mixtures and pure fluids in two-stage cycles are theoretically investigated for two applications and over the whole composition range of three mixture pairs, HCFC22 HFFC152a, HCFC22 HCFC142b and HCFC22 CFC114. Twe two most common types of two-stage plant are covered:those working with a flash vessel at an intermediate pressure, and those in which an economizer heat exchanger is used. The coefficient of performance (COP) and the capacity of the two types with different working aluids are compared with each otheras well as with the performances of a one-stage cycle. It is found that the COP and the capacity of a non-azeotropiic mixture increase more than those of a pure fluid upon changing from a one-stage cycle to a two-stage cycle with economizer heat exchanger. In the search for the ‘best’ working fluid, pure or a mixture, for a given application, the optimal choice depends on which type of cycle one intends to use.

CFC Alternatives for High-Temperature Heat Pump Applications

Two typical high temperature heat pump applications have been examined, production of low-pressure steam, and production of high temperature hot water. Heat pump cycle calculations have been carried out for several possible CFC alternatives. Two different methods of comparing the performance of the fluids have been used, and the importance of including the transport properties in the comparison is pointed out. It is concluded that in the near future the CFC alternatives for high temperature applications will either be flammable, or chlorine containing. Flammable fluids may give higher COPs than non-flammables and are therefore worth considering. One way to use the benefit of a flammable fluid can be to mix it with a non-flammable, in such proportions that the mixture is non-flammable. The use of mixtures can also extend the working range of fluids that otherwise will have too high condenser pressures.

Nonazeotropic Mixtures as Substitutes for Today's CFC Fluids

ABSTRACT Due to the CFC problem the interest for nonazeotropic mixtures as working fluids in heat pump/refrigeration/air-conditioning plants has increased considerably during recent years. Traditionally, mixtures have been considered as being of interest mainly in applications with large gliding temperatures, but as CFC substitutes they are now considered also in several types of applications with small gliding temperatures. In this paper thermo-dynamical, practical, and performance aspects of mixtures are discussed and results from cycle calculations are given. The main results are: ○ Mixtures can compete with both CFC12 and HFC134a in performance. With a proper choice of mixture this is true also when the external temperature gradients are small; ○ Near-azeotropes can be used also in applications traditionally not considered for mixtures; ○ The influence of the performance of a working fluid leakage should, in most cases, be marginal; ○ Even when taking into account the fact that mixtures can have considerably lower heat transfer coefficients, they can still compete with pure fluids. Hence, this problem has probably been exaggerated.

Impact of uncertainties on estimations of heat pump cycle performance

Sensitivity analysis is used to show how uncertainties in basic factors, such as heat transfer, pressure drop, compressor efficiency, and thermophysical properties, influence the result of heat pump cycle performance estimations. By comparing the results of this analysis with uncertainty levels of estimations and experimental determinations, it is established to what extent improvements in accuracy are needed. The investigation was carried out for several different applications and, as working fluids, both pure substances and non-azeotropic mixtures were examined. It was found that evaporation heat transfer and compressor efficiency are two areas where improvements are needed. It is also shown that estimates using very accurate, substance-specific, equations of state differ from those using more general types by up to 1 % for COP and 2% for capacity.

Experiences of a nonazeotropic mixture as replacement for CFC12

Abstract In the paper theoretical and experimental comparison between the performance of a two-stage heat pump working with the nonazeotropic mixture HCFC22-HCFC142b and that of one working with CFC12 is presented. An experimental full-scale plant consisting of two heat pumps of 1 MW each, one working with the mixture HCFC22-HCFC142b and the other with pure CFC12, has been in operation, since May 1991, in a small district heating system. The main aims of the project are: to verify theoretical calculations; to get practical experience of the handling of nonazeotropic mixtures; to experimentally study the influence of different parameters. The HCFC22-HCFC142b mixture was chosen since it is one of the few non-flammable mixtures, suitable for CFC12 replacements, that was available at the beginning of the project. Three problems have occurred: darkening of the oil; instability of the suction gas superheat; and problems with the initial sealing material. These problems are now solved. A rigorous computer program has been developed and used to calculate the COP and capacity for mixtures and pure fluids for the same heat output, under the same operating conditions and assuming equal heat transfer areas for pure and mixed fluids. The measurements of the COP have in principle confirmed the results from the theoretical calculations.