Service Manual

SERVICING
45
S-104 CHECKING COMPRESSOR EFFICIENCY
The reason for compressor inefficiency is broken or damaged
scroll flanks on Scroll compressors, reducing the ability of
the compressor to pump refrigerant vapor.
The condition of the scroll flanks is checked in the following
manner.
1. Attach gauges to the high and low side of the system.
2. Start the system and run a “Cooling Performance Test.
If the test shows:
a. Below normal high side pressure.
b.
Above normal low side pressure.
c.
Low temperature difference across coil.
d. Low amp draw at compressor.
And the charge is correct. The compressor is faulty - replace
the compressor.
S-105B THERMOSTATIC EXPANSION VALVE
The expansion valve is designed to control the rate of liquid
refrigerant flow into an evaporator coil in exact proportion to
the rate of evaporation of the refrigerant in the coil. The
amount of refrigerant entering the coil is regulated since the
valve responds to temperature of the refrigerant gas leaving
the coil (feeler bulb contact) and the pressure of the refriger-
ant in the coil.
This regulation of the flow prevents the return of liquid
refrigerant to the compressor.
The three forces which govern the operation of the valve are:
(1) the pressure created in the power assembly by the feeler
bulb, (2) evaporator pressure, and (3) the equivalent pressure
of the superheat spring in the valve.
0% bleed type expansion valves are used on the indoor coils.
The 0% valve will not allow the system pressures (High and
Low side) to equalize during the shut down period. The valve
will shut off completely at approximately 100 PSIG Pressure.
Good thermal contact between the feeler bulb and the
suction line is essential to satisfactory valve control and
performance.
The bulb must be securely fastened to a clean straight
section of the suction line. Application of the bulb to a
horizontal run of line is preferred. If a vertical installation
cannot be avoided the bulb should be mounted so that the
capillary tubing comes out at the top.
THE VALVES PROVIDED ARE DESIGNED TO MEET THE
SPECIFICATION REQUIREMENTS FOR OPTIMUM PROD-
UCT OPERATION. DO NOT USE SUBSTITUTES.
S-106 OVERFEEDING
Overfeeding by the expansion valve results in high suction
pressure, cold suction line, and possible liquid slugging of
the compressor.
If these symptoms are observed:
1. Check for an overcharged unit by referring to the cooling
performance charts in the servicing section.
2. Check the operation of the power element in the valve as
explained in S-110 Checking Expansion Valve Opera-
tion.
3. Check for restricted or plugged equalizer tube.
S-107 UNDERFEEDING
Underfeeding by the expansion valve results in low system
capacity and low suction pressures.
If these symptoms are observed:
1. Check for a restricted liquid line or drier. A restriction will
be indicated by a temperature drop across the drier.
2. Check the operation of the power element of the valve as
described in S-110 Checking Expansion Valve Opera-
tion.
S-108 SUPERHEAT
The expansion valves are factory adjusted to maintain 12 to
15 degrees superheat of the suction gas. Before checking
the superheat or replacing the valve, perform all the proce-
dures outlined under Air Flow, Refrigerant Charge, Expan-
sion Valve - Overfeeding, Underfeeding. These are the most
common causes for evaporator malfunction.
CHECKING SUPERHEAT
Refrigerant gas is considered superheated when its tem-
perature is higher than the saturation temperature corre-
sponding to its pressure. The degree of superheat equals
the degrees of temperature increase above the saturation
temperature at existing pressure. See Temperature -
Pressure Chart on following page.