Refrigeration saturation single stage cycle (SSS)
Process 1-2: Isentropic compression of saturated vapor in compressor
Process 2-3: Isobaric heat rejection in condenser
Process 3-4: Isenthalpic expansion of saturated liquid in expansion device
Process 4-1: Isobaric heat extraction in the evaporator
Refrigeration cycle with liquid-suction heat exchanger (LSHX)
Required degree of sub cooling and superheating may not be possible, if one were
to rely only on heat transfer between the refrigerant and external heat source and sink.
Also, if the temperature of refrigerant at the exit of the evaporator is not sufficiently
Superheated, then it may get superheated by exchanging heat with the surroundings as it
Flows through the connecting pipelines (useless superheating), which is detrimental to
System performance. One way of achieving the required amount of sub cooling and
Superheating is by the use of a liquid-suction heat exchanger (LSHX). A LSHX is a
Counter flow heat exchanger in which the warm refrigerant liquid from the condenser
Exchanges heat with the cool refrigerant vapor from the evaporator.
the schematic shown below of a single stage VCRS with a liquid-suction heat exchanger shows the modified cycle on T-s and P-h diagrams. Since the temperature of the refrigerant liquid at the exit of condenser is considerably higher.
Than the temperature of refrigerant vapor at the exit of the evaporator, it is possible to
sub cool the refrigerant liquid and superheat the refrigerant vapor by exchanging heat
between them.
Comment on the use of LSHX by comparing the performance of the system with a saturation single stage( SSS).
Cycle operating between the same evaporator and condensing temperatures.
Effectiveness of LSHX, εX = (Q /Q) = [(mCp) ΔT]/ [(mCp) act max min act,min minΔTmax]
= (T2-T1)/ (T4-T1); _ Cp,vapour < t2 =" T1+0.65(T4-T1)" h5 =" h4" te =" 7.2oC" tc =" 54.4oC" x =" 0.65">
a) Refrigeration effect = (h1-h6) = 161.2 kJ/kg
b) Volumic refrigeration effect = (h1-h6)/v2 = 3676.2 kJ/m3
c) Work of compression = (h3-h2) = 36.1 kJ/kg
d) COP = (h1-h6)/ (h3-h2) = 4.465
e) Temperature at compressor exit (from Pc and s3=s2) = 104.9oC
Without LSHX:
a) Refrigeration effect = (h1-h6’) = 138.1 kJ/kg
b) Volumic refrigeration effect = (h1-h6’)/v1 = 3660.2 kJ/m3
c) Work of compression = (h3’-h1) = 31.0 kJ/kg
d) COP = (h1-h6’)/ (h3’-h1) = 4.455
e) Temperature at compressor exit (from Pc and s1=s3’) = 74.23oC
Comments:
a) There is no appreciable change in COP with the addition of LSHX
b) Quality of refrigerant at evaporator inlet is significantly lower with LSHX
c) Discharge temperature is significantly high with LSHX
d) For refrigerant R-22, use of LSHX does not improve the performance of the
system significantly, however, the evaporator with LSHX performs better due to
the lower vapor fraction at its inlet
