Consider a two-stage cascade refrigeration cycle with a flash chamber as shown in the
figure with refrigerant-134a as the working fluid. The evaporator temperature is -10°C
and the condenser pressure is 1600 kPa. The r
...
Consider a two-stage cascade refrigeration cycle with a flash chamber as shown in the
figure with refrigerant-134a as the working fluid. The evaporator temperature is -10°C
and the condenser pressure is 1600 kPa. The refrigerant leaves the condenser as a
saturated liquid and is throttled to a flash chamber operating at 0.50 MPa. Part of the
refrigerant evaporates during this flashing process, and this saturated vapour is mixed
with the refrigerant leaving the low-pressure compressor. The mixture is then
compressed to the condenser pressure by the high-pressure compressor. The liquid in
the flash chamber is throttled to the evaporator pressure and cools the refrigerated
space as it vaporizes in the evaporator. The mass flow rate of the refrigerant through
the low-pressure compressor is 0.11 kg/s. Assuming the refrigerant leaves the
evaporator as a saturated vapour and the isentropic efficiency is 86% for both
compressors, first sketch the cycle
on a T-s diagram, then determine
a) the mass flow rate of the
refrigerant through the highpressure compressor, in kg/s,
b) the rate of refrigeration (?ሶ)
supplied by the system, in kW,
and
c) the COP of this refrigerator.
Bonus question: determine the
rate of refrigeration and the
COP if this refrigerator operated
on a single-stage vapourcompression cycle between the
same evaporating temperature
and condenser pressure with
the same compressor efficiency
and the same flow rate as
calculated in part (a).
Refrigerators and Heat Pumps: Go over the example problems in the text, including the basic vapour‐
compression refrigeration cycle (C&B ex. 11‐1 and 11‐2; the latter example problem also deals with
non‐ideal effects, i.e., isentropic efficiency which you should review), 2‐stage cycle (C&B ex. 11‐4),
and 2‐stage system with a flash chamber (C&B ex. 11‐5)
3. (C&B 11‐53). A two‐stage compression refrigeration system with an adiabatic liquid‐vapour
separation unit as shown below uses refrigerant‐134a as the working fluid. The system operates the
evaporator at ‐40ºC, the condenser at 800 kPa, and the separator at ‐10.1ºC. This system is to serve a
30‐kW cooling load. Determine the mass flow rate through each of the two compressors, the power
used by the compressors, and the system’s COP. The refrigerant is saturated liquid at the inlet of each
expansion valve and saturated vapour at the inlet of each compressor, and the compressors are
isentropic.
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