Handbook of air conditioning and refrigeration / Shan K

11.36 CHAPTER ELEVEN
compression ratio. The EER drops to 11.5. It is preferable to select two DX coils each with a face
area of 21 ft 2 (1.95 m 2 ) to reduce energy input.
Because the T con of an air-cooled condenser affects its total heat rejection Q rej, often T con is estimated
first when an air-cooled condenser is selected to determine Q rej for the compressor–DX coil
combination. If T con is estimated at 115°F (46.1°C), from Fig. 11.21, for a power input of 64 kW to
two compressors, the required total heat rejection of the selected air-cooled condenser is
Q rej � Q rc � 3413P in � 63 � 12,000 � 64 � 3413 � 974,400 Btu / h (285,500 W)
In Fig. 11.21, draw a line from point Y to point Z� whose Q rej � 974,000 Btu/h (285,500 W). Draw
horizontal line Z�Z intersecting the Q rej-T con curve of condenser size 120 at Z. At point Z, T con �
113.3°F (45.2°C). If a smaller condenser 095 is selected, then the intersecting point is Z� and the
corresponding T con is 117.5°F (47.5°C), which necessitates a higher P in to the compressor.
Using a larger condenser, the final selected capacity of this direct-expansion reciprocating
refrigeration system may be as follows:
Refrigeration load
Suction temperature
Condensing temperature
Power input to the compressor motor
Total heat rejection Q rej
64 tons (225 kW)
40°F (4.4°C)
114°F (45.6°C)
64 kW
986,000 Btu/h (288,900 W)
For thermostatic expansion valves, the selected capacity must be greater than the cooling capacity
of the compressor at a pressure drop across the expansion valve of 75 to 120 psi (517 to 827 kPa)
for HCFC-22 at design conditions.
11.8 OPERATING CHARACTERISTICS OF AIR-COOLED,
DIRECT-EXPANSION RECIPROCATING REFRIGERATION SYSTEM
Operating Balance at Design Condition
Consider an air-cooled direct-expansion reciprocating refrigeration system in a rooftop packaged
system with the same characteristics as described in the last section and shown in Fig. 11.21. If the
design refrigeration load from the evaporation of HCFC-22 inside the two DX coils is only 60 tons
or 720,000 Btu/h (211 kW) instead of the selected capacity of 64 tons (225 kW), then the volume
flow rate of the piston displacement V˙
p
to extract suction vapor is greater than that of the vaporized
refrigerant in the evaporator. This imbalance lowers the evaporating pressure and temperature in the
evaporator and, therefore, increases the specific volume vsuc of the suction vapor and the log-mean
temperature difference as well as the heat transfer to the evaporator until V˙
p is balanced with the
volume flow rate of refrigerant evaporized in the evaporator V˙ ev,
cfm (m/s). A new equilibrium between
the compressor and evaporator is reached
m˙ ev
m˙ com � V˙ p� v� suc � m˙ ev � V˙ ev
v suc
� 200Q rc
q rf
where � mass flow rate of refrigerant evaporized in evaporator, lb/min (kg/min)
Qrc � refrigeration load at evaporator, tons
qrf � refrigeration effect, Btu/lb
(11.10)
The refrigerant discharged from the compressor then enters the air-cooled condenser at mass
flow rate . Because the selected condenser can condense a mass flow rate of refrigerant equal to
m˙ com