Chapter 6. REPAIR INSTRUCTIONS - TM-5-4120-352-140103

TM 5-4120-352-14

CHAPTER

REPAIR INSTRUCTIONS

Section I.

REFRIGERATION SYSTEM

tally automatic and also may occur at extreme con-

6-1.

General

ditions in an attempt to maintain the suction pres-

The refrigerant system illustrated by the refrig-

sures (even during the cooling mode) at a condition

erant flow diagram (fig. 5-1), is a mechanical, vapor

above 55 psig (380 kPa) and the suction tem-

cycle type circuit consisting of the evaporator, ther-

peratures (measured at the quench bulb well) below

mal expansion valve, motor-compressor, condenser,

75F (24C). The condenser louvers are operated by

and the necessary valves and cutout devices for au-

a refrigerant powered piston located in high pres-

tomatic control during operation. The thermal ex-

sure part of the system. This piston should be fully

pansion valve releases high-pressure liquid refrig-

extended (louvers open 80F (27C) (approx.) at 250

erant into the evaporator at reduced pressure. The

psig (1140 kPa) head pressure and fully closed at

liquid refrigerant begins to vaporize by absorbing

165 psig (1140) kPa). Failure to perform this func-

heat from the air passing over the external surface

tion could result in icing of the evaporator coil and/

of the evaporator coil. The heated vapor is sucked

or cutout on the low pressure cutout.

out of the evaporator section by the motor-com-

pressor and forced into the condenser section under

6-2.

Pressure Testing the Refrig-

high pressure where it is cooled and condensed back

erant System

into a liquid. The heat released during condensation

is carried off by the condensing air stream. The liq-

a. General. A pressure test will indicate

uid refrigerant flows from the condenser to a re-

whether the air conditioner is operating at normal

ceiver, to a subcooler, and then to the thermal ex-

or at abnormal pressures. When the air conditioner

pansion valve. If the temperature control switch

is not operating at normal pressures the cause

(evaporator return air thermostat) becomes satis-

should be ascertained and corrected. Refer to table

fied, or the evaporator return air temperature is

5-1 for troubleshooting chart.

lower than the control switch set point, the re-

b. System Pressure Test. Remove caps from

frigerant system will switch to a by-pass condition.

high and low pressure service valves (fig. 5-7), con-

The temperature control switch will activate the

nect suction and discharge pressure gages to their

normally-open liquid bypass solenoid valve, closing

respective service valves. Compare the gage reading

the valve, and therefore shutting off the evaporator

with the normal range of system pressure shown in

section of the unit. The motor-compressor will con-

table 6-1.

tinue to pump as usual and the suction pressure will

begin to drop. When it reaches approximately 65

6-3.

S e r v i c i n g Refrigerant System

psig (450 kPa), the pressure regulating valve will

a. General. When the air conditioner must un-

start to open in an effort to maintain the suction

dergo maintenance that requires opening the sys-

pressure above 55 psig (380 kPa) (approximately).

tem, the system must be discharged prior to mainte-

As the suction temperature increases, due to the

nance, then purged, a new dehydrator installed, and

pressure regulating valve opening, the quench ex-

the system charged after maintenance. Basic pro-

pansion valve will start to meter liquid refrigerant

cedures involved in servicing the refrigerant sys-

into the suction line in an effort to maintain the

tem are as follows:

suction temperature below 75F (24C) (approx.), or

b. Discharging and Purging System. Refer to

30F (-1C) super-heat (approx.). This action (the

figure 6-1 for instructions on discharging or purg-

pressure regulator and quench valve actions) is to-

ing the refrigerant system.

6-1