Refrigeration & Air Conditioning Technology, 8e - 72

72

Section 1 Theory of Heat

Figure 3.50 Adding subcooling increases the net refrigeration effect of the system.

Source: E. I. DuPont

R - 22

68 Btu/lb
NET REFRIGERATION EFFECT

R - 22

53 BTU/LB

57 Btu/lb
NET REFRIGERATION EFFECT

is raised due to a dirty condenser, Figure 3.51. Using the first
example, Figure 3.48, and raising the condensing temperature
to 140°F (337.2  psig or 351.9  psia), we see the percentage
of liquid leaving the expansion valve to be about 64% (36%
flash gas) with a heat content of 53 Btu/lb. Using the same
heat content leaving the evaporator, 110  Btu/lb, we have a
net refrigeration effect of 57 Btu/lb. This is a net reduction in
refrigeration effect of about 7% from the original example,
which contained 49 Btu/lb at the same point. This shows the
importance of keeping condensers clean.

110 BTU/LB

Source: E. I. DuPont

ABSOLUTE PRESSURE (PSIA)

Figure 3.51 A rise in system head pressure will result in a reduction of system capacity.

Figure  3.52 shows how increased superheat affects the
first system in Figure 3.48. The suction line has not been insulated and absorbs heat. The suction gas may leave the evaporator at 50°F and rise to 75°F before entering the compressor.
Notice the high discharge temperature (about 200°F). This
is approaching the temperature that will cause oil to break
down and form acids in the system. Most compressors must
not exceed 225°F. The compressor must pump more refrigerant to accomplish the same refrigeration effect, and the condenser must reject more heat.



Refrigeration & Air Conditioning Technology, 8e

Table of Contents for the Digital Edition of Refrigeration & Air Conditioning Technology, 8e

Contents
Refrigeration & Air Conditioning Technology, 8e - Cover1
Refrigeration & Air Conditioning Technology, 8e - Cover2
Refrigeration & Air Conditioning Technology, 8e - i
Refrigeration & Air Conditioning Technology, 8e - ii
Refrigeration & Air Conditioning Technology, 8e - iii
Refrigeration & Air Conditioning Technology, 8e - Contents
Refrigeration & Air Conditioning Technology, 8e - v
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