KeepRite K40-CU-IM-13, Installation And Maintenance Instructions Manual, Page: 28 / 36

28
Checking Superheat
IMPORTANT SYSTEM BALANCING NOTE
To obtain maximum system capacity and insure trouble free operation it is necessary to check both the
compressor and evaporator superheat.
Compressor Superheat
Compressor suction superheat must be checked. To check the superheat at the compressor the following steps
should be followed:
(1)
Measure the suction pressure at the suction service valve of the compressor. Determine the saturated
temperature corresponding to this pressure from a “Pressure- Temperature” chart.
(2) Measure the suction temperature of the suction line about 6 inches (15 cm) back from the compressor
suction valve using an accurate thermometer.
(3) Subtract the saturated temperature (from step 1) from the actual suction line temperature (from step 2).
This difference is the
actual superheat at the compressor .
System capacity decreases as the suction superheat increases. For maximum system capacity, the suction
superheat should be kept as low as is practical. The superheat at the compressor should range within
20 to 45
o
F (11.2 to 25.2
o
C) Superheat.
NOTE: Too low of a suction superheat can result in liquid being returned to the compressor. This can cause
dilution of the oil and eventually cause failure of the bearings and rings through wash out as well as liquid
slugging.
NOTE: Too high of a suction superheat will cause excessive discharge temperatures which cause a break down
of the oil and will result in piston ring wear, piston and cylinder wall damage.
If adjustment to the suction superheat is required, it should be done either by adjusting the thermostatic
expansion valve at the evaporator, the use of liquid to suction heat exchanger or suitable use of suction line
insulation.
Evaporator Superheat
Once the refrigerated space is at its design temperature or close to design temperature , the evaporator
superheat must be checked. To check the suction superheat at the evaporator the following steps should be
followed:
(1)
Measure the suction pressure in the suction line at the bulb location by either,
(a) A gauge in the external equalizer line will indicate the pressure directly and accurately.
(b) A gauge directly in the suction line near the evaporator or directly in the suction header will suffice.
(2) Measure the temperature of the suction line at the point where the thermostatic expansion valve bulb is
clamped to the suction line.
(3) Convert the pressure obtained in step 1 above to a saturated evaporator temperature from a “Pressure-
Temperature” chart.
(4) Subtract the saturated temperature (from step 1) from the actual suction line temperature (from step 2).
This difference is the
actual superheat at the evaporator .
The superheat at the evaporator should be a minimum of 6 to 10
o
F (3.4 to 5.6
o
C) for systems with a 10
o
F (5.6
o
C) design TD (temperature difference) to a maximum of 12 to 15
o
F (6.7 to 8.4
o
C) for systems with a higher
operating TD.
Low temperature applications (freezers) should be set at superheats of 4 to 6
o
F (2.2 to 3.4
o
C).
TD = Box temperature – evaporating temperature.