CHAPTER 6. REPAIR INSTRUCTIONS - TM-5-4120-369-140123

TM 5-4120-369-14

CHAPTER 6.

REPAIR INSTRUCTIONS

REFRIGERATION SYSTEM

Section I.

6-1.

GENERAL.

The refrigerant system, illustrated by the refrigerant flow diagram (Figure 6-1), is a mech-

anical, vapor cycle type circuit consisting of the evaporator, thermal expansion valve, motor-

compressor, condenser, and the necessary valves and cutout devices for automatic control during

operation. the thermal expansion valve releases high-pressure liquid refrigerant into the evapor-

ator at reduced pressure, The liquid refrigerant begins to vaporize by absorbing heat from the

air passing over the external surface of the evaporator coil. The heated vapor is sucked out of

the evaporator section by the motor-compressor and forced into the condenser section under high-

-pressure where it is cooled and condensed back into a liquid. The heat released during conden-

sation is carried off by the condensing air stream. The liquid refrigerant flows from the conden-

ser to a receiver, to a subcooler, and then to the thermal expansion valve. If the temperature

control switch (evaporator return air thermostat) becomes satisfied, or the evaporator return air

temperature is lower than the control switch set point, the refrigerant system will switch to a

bypass condition. The temperature control switch will activate the normally-open liquid bypass

solenoid valve, closing the valve, and therefore shutting off the evaporator section of the unit.

The motor-compressor will continue to pump as usual and the suction pressure will begin to

drop. When it reaches approximately 58 psig ( 400 kPa), the pressure regulating valve will start

to open in an effort to maintain the suction pressure above 55 psig (380 kPa) (approximately) .

As the suction temperature increases, due to the pressure regulating valve opening, the quench

expansion valve will start to meter liquid refrigerant into the suction line in an effort to maintain

the suction temperature below 75F (24C) (approximately), or 30F (-1C) superheat (approxi-

mately). This action (the pressure regulator and quench valve actions) is totally automatic and

also may occur at extreme conditions in an attempt to maintain the suction pressures (even during

the cooling mode) at a condition above 55 psig (380 kPa) and the suction temperatures (measured

at the quench bulb well) below 75F (24C). The condenser louvers are operated by a refriger-

ant powered piston located in the high pressure part of the system. This piston should be full

extended (louvers open at 80F (27C) approximately) at 250 psig (1140 kPa) head pressure and

fully closed at 165 psig (1140 kPa). Failure to perform this function could result in icing of the

evaporator coil and/or cutout on the low pressure cutout.

Normal Operating Pressures.

Table 6-1.

Outdoor Ambient Temperature

75F (24C)

100F (38C)

120F (49C)

50F (10C)

At 90F (32C) DB return air to unit

60-75 psi

75-90 psi

58-70 psi

Suction

58-65 psi

(400-450 kPa)

(400-485 kPa)

(255-295 kPa)

Pressure

(515-620 kPa)

255-295 psi

175-210 psi

375-410 psi

125-160 psi

Discharge

(1750-2025kPa)

Pressure .

(860-1100kPa)

(1200-1450kPa)

(2550-2825kPa)

At 80F (27C) DB return air to unit

60-75 psi

58-70 psi

65-75 psi

Suction

58-65 psi

(450-515 kPa)

(400-450 kPa)

(400-485 kPa)

(415-515 kPa)

Pressure

250-290 psi

120-155 psi

170-205 psi

270-410 psi

Discharge

(825-1070kPa)

(1725-2000kPa)

(2550-2825kPa)

(1170-1415kPa)

Pressure

6-1