Abstract
D.Ing.
The heating of water with hot-water heat pumps is extremely energy-efficient. With the
refrigerant R-22 hot water temperatures of 60° C to 65° C are possible. However, these
temperatures are low in comparison with the temperatures obtained from other methods
of water heating, for instance electrical geysers. Should higher water temperatures be
obtained, the applications of hot-water heat pumps will increase. This is possible by using
a zeotropic refrigerant mixture as working fluid. A R-22 and R-142b zeotropic refrigerant
mixture shows exceptional potential in achieving hot water temperatures. The condensing
coefficients need to be predicted correctly to optimize the condenser design.
Unfortunately, there is a lack of detailed literature available on condensing coefficients
for the recommended mass fractions of R-22 with R-142b at condensing temperatures of
60° C or more.
Micro-fin tubes perform outstanding in enhancing heat transfer and are widely used to
save energy. Unfortunately, there is also a lack of detailed literature on condensing
coefficient at the recommended mass fractions of R-22/R-142b refrigerant mixtures
condensing in micro-fins, twisted tapes and high fins at temperatures of 60° C or more.
In this study condensing coefficients of R-22 and the zeotropic refrigerant mixture R-22
with R-142b were obtained in smooth tubes at mass fractions of 90%/10%, 80%/20%,
70%/30%, 60%/40%, 50%/50%. The experimental data were used to evaluate some of
the methods that are commonly used to predict condensing coefficients. Experiments
were also conducted at the same zeotropic mass fractions, to compare three different
methods of heat transfer enhancement to that of the smooth tubes namely: micro-fins,
twisted tapes and high fins.
All measurements were conducted at an isobaric inlet pressure of 2.43 MPa. The test
sections consisted of a series of eight tubes with lengths of 1 603 mm. The smooth tubes
had an inner diameter of 8.11 mm.
With the R-22/R-142b zeotropic refrigerant mixture condensing in smooth tubes, it was
observed in the sight glasses that a predominantly stratified wavy flow regime exists at
low mass fluxes, from 40 kg/m2s to 350 kg/m2s. The refrigerant mass fraction decreased
the condensing coefficient by up to a third on average from 100% R-22 to a 50%/50%
mixture of R-22 with R142b. A predominantly annular flow regime was observed at mass
fluxes of 350 kg/m2s and more. At this flow regime the condensing coefficients were not
strongly influenced by the refrigerant mass fraction, decreasing only by 7% as the
refrigerant mass fraction changed from 100% R-22 to a 50%/50% mixture of R-22 with
R142b. When the experimental data were compared with three methods that are
commonly used to predict condensing coefficients it was found that the flow pattern
correlation of Dobson and Chato (1998) gave the best predictions for R-22. The Silver
(1964) and Bell and Ghaly (1964) method gave the best predictions for the R-22/R-142b
mixtures. When the three heat transfer enhancement methods were compared with
smooth tubes it was found that micro-fins were more suitable as an enhancement method
than twisted tubes or high fins. It was also found that the condensing coefficients and
pressure drops decrease as the mass fractions of R-142b increases.