Handbook of air conditioning and refrigeration / Shan K

Temperature T, �F
Temperature
glide
(T sv -T sc )
As for a near-azeotropic refrigerant mixture HFC-404A, the concentration shift is small, the calculated
performance change due to circulation concentration shift is less than 1 percent and can be
neglected.
Corr et al. (1994) showed that direct-expansion evaporators typically have liquid fractions of 2
to 10 percent and result in a small shift in circulation concentration.
Heat Exchanger Flow Configuration
The temperature glide of zeotropic refrigerant blends can be utilized to increase the heat exchange
efficiency in the evaporator where the evaporating temperaure increases and in the condenser where
the condensing temperature drops. Rice (1993), using a modified Carnot cycle analysis, found that
for an air-source heat pump using zeotropic blend as refrigerant at cooling mode with a relative UA
(overall conductance-area product) level of 3, many-row crossflow has a potential COP gain of
about 7 percent, two-row countercrossflow about 10 percent, three-row countercrossflow 15 percent,
counterflow (or � three-row countercrossflow) 20 percent. Hwang and Radermacher (1997)
showed that the capacity of the near-counterflow heat exchanger increases about 10 percent compared
to that of the near-parallel flow heat exchanger.
Capacity Control and Part-Load Performance
REFRIGERATION SYSTEMS: RECIPROCATING, ROTARY, SCROLL, AND SCREW 11.47
T sv
T sc
0
Dewpoint
line
Saturated
liquid
Concentration
Saturated
vapor
Bubbling
point line
FIGURE 11.27 Temperature-concentration diagram of a zeotropic refrigerant blend.
There are two types of capacity control: variable-speed and variable-displacement compressors. In
a variable-speed scroll compressor, a variable-speed drive is used to reduce the speed at part-load
operation so that the cooling capacity is reduced to meet the reduced system load. The operating
frequency often varies from 15 to 150 Hz.
In a variable-displacement scroll compressor, there are porting holes in the fixed scroll. By
connecting or disconnecting the compression chamber to the suction side through the closing and
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