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Abstract
R22 is the most widely used refrigerant around the world since 1936, but it is scheduled for phase -out by the year 2015 due to its harm effect on both ozone layer and global warming. As result, many environmental issues related to
chloro-fluoro-carbons (CFCs) and hydro-chloro-fluoro-carbons (HCFCs) alternatives are reported in literature.
The present investigation aimed at evaluating a possibility of using R1270/R290, R1270/R600a and R1270/R600 blends with different concentration, R290 and R407C as DROP-in working fluids to replace R22 in a window air conditioner. Hydrocarbons (R290, R600, R600a and R1270) are environmentally friendly refrigerants because they have zero ozone depletion potential and negligible global warming potential. In order to achieve the above objective, simulation of vapor compression refrigerant system (VCRS) and sizing of capillary tube for a window air conditioner are carried out through the present work.Performance characteristics of the window air conditioner were predicted over a wide range of condensing temperature (40 to 60°C) and evaporating temperature (5 to 10°C) for various working fluids such as R22, R407C, R290 (propane), R1270 (propylene), R1270/R290 blend, R1270/R600a combination
and R1270/R600 mixture with various propylene mass fractions. Performance
characteristics of the considered window air conditioner were identified by the
coefficient of performance (COP), volumetric cooling capacity, condenser heat
load, power consumption, discharge temperature, vapor pressure, pressure ratio
and refrigerant mass flow rate.
Results showed that pure butane (R600) and pure isobutene (R600a) yield
many desirable characteristics, but require compressor change. Coefficient of
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performance of the window air conditioner using R1270, R290 and R22 are
approximately the same. Comparison among the considered working fluids
confirmed that average refrigerant mass flow rate of R290 and R1270 is lower
than that of R22 by about 41.1 % and 42.3 %, respectively. Also, results
indicated that R22 and R1270 have approximately the same values of volumetric
cooling capacity. The saturation pressure, input power, condenser heat load and
cooling effect of R1270 are higher than those of R22 by nearly about 5.7 %, 4.6
%, 1.0 % and 73 %, respectively, while its pressure ratio and discharge
temperature are lower than those of R22 by about 7.5 % and 13.3 %,
respectively. Finally, the reported results confirmed that R1270 is the best DROPin
replacement for R22 in the window air conditioner under different operating
conditions.
Consequentally, a further work is conducted to size a capillary tube of the
window air conditioner working with R1270. The model is based on the mass,
energy and momentum conservation equations for a separated refrigerant flow
under different inlet conditions, such as sub-cooled, saturated and two phase
flow. The effects of the inlet pressure (14-20 bar), inlet vapor quality (0.001%-
15%), inlet sub-cooling degree (0-15°C), refrigerant mass flow rate (10-40 kg/h),
capillary tube inner diameter (0.72-2.8 mm) and the tube surface roughness
(0.002-0.03) on the capillary tube length are predicted.
Results showed that present model predicts data that are very close to the
available experimental data in the literature for R22, R12 and R134a with an
average error of 1%. The pressure of R1270 decreases, while its vapor quality,
specific volume and specific entropy increase along the capillary tube. Also, the
results indicated that the capillary tube length is largely dependent on the
capillary tube diameter. Other parameters such as mass flow rate, inlet pressure,
sub-cooling degree (or quality) and relative roughness influence the capillary
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tube length in that order. Also, capillary tube selection charts either to predict the
mass flow rates of R1270 through adiabatic capillary tubes or to select the
capillary tube size according to the required applications are developed. The
comparison between R22, R407C and R1270 indicated that for a given mass
flow rate, the pressure DROP per unit length is about 7.41, 7.99 and 17.1 bar/m for
R22, R407C and R1270, respectively. Ratios of the average actual mass flow
rate of R1270 to those of R22 and R407C are about 0.57 and 0.63, respectively.
The average capillary tube length for R1270 is longer than those of R407C and
R22 by about 6.3 % and 28 %, respectively.