Service manual

SERVICING
37
Units having capillary tubes or flow control restrictors can
be checked against the Desired Superheat vs. Outdoor
Temperature Chart in this section. Coils with thermostatic
expansion valves (TXV's) must be checked by subcooling.
See "Checking Subcooling and Superheat" sections in this
manual.
If a restriction is located, replace the restricted part, replace
drier, evacuate and recharge.
S-104 CHECKING COMPRESSOR EFFICIENCY
The reason for compressor inefficiency is broken or dam-
aged suction and/or discharge valves, or scroll flanks on
Scroll compressors, reducing the ability of the compressor
to pump refrigerant vapor.
The condition of the valves or 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-
⇒ Below normal high side pressure.
⇒ Above normal low side pressure.
⇒ Low temperature difference across coil.
⇒ Low amp draw at compressor.
-and the charge is correct. The compressor is faulty - re-
place the compressor. NOTE: THIS TEST CANNOT BE
DONE IN THE HEATING MODE
S-105 THERMOSTATIC EXPANSION VALVE
The expansion valve is designed to control the rate of liq-
uid refrigerant flow into an evaporator coil in exact propor-
tion 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 refrigerant in the coil.
This regulation of the flow prevents the return of liquid re-
frigerant 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 pe-
riod. The valve will shut off completely at approximately
100 PSIG Pressure.
Good thermal contact between the feeler bulb and the suc-
tion line is essential to satisfactory valve control and perfor-
mance.
The bulb must be securely fastened to a clean straight sec-
tion of the suction line. Application of the bulb to a horizon-
tal run of line is preferred. If a vertical installation cannot
be avoided the bulb should be mounted so that the capil-
lary tubing comes out at the top.
THE VALVES PROVIDED BY AMANA ARE DESIGNED
TO MEET THE SPECIFICATION REQUIREMENTS FOR
OPTIMUM PRODUCT OPERATION. DO NOT USE SUB-
STITUTES.
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 cool-
ing performance charts in the servicing section.
2. Check the operation of the power element in the valve
as explained in S-26 Checking Expansion Valve Op-
eration.
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-26 Checking Expansion Valve Op-
eration.
S-108 SUPERHEAT
The expansion valves are factory adjusted to maintain 12
to 18 degrees superheat of the suction gas. Before check-
ing the superheat or replacing the valve, perform all the
procedures outlined under Air Flow, Refrigerant Charge,
Expansion Valve - Overfeeding, Underfeeding. These are
the most common causes for evaporator malfunction.
CHECKING SUPERHEAT
Refrigerant gas is considered superheated whenever its
temperature is higher than the saturation temperature cor-
responding to its pressure. The degree of superheat equals
the degrees of temperature increase above the saturation
temperature at existing pressure. See Temperature - Pres-
sure Chart.
1. Attach an accurate thermometer or preferably a ther-
mocouple type temperature tester to the suction line at
a point at least 6" from the compressor.
2. Install a low side pressure gauge on the suction line
service valve at the outdoor unit.
3. Record the gauge pressure and the temperature of the
line.