Heatcraft Refrigeration Products Unit Coolers H-IM-UC - Manual (Page 6)


Installation and Operations Manual
Operational Check Out
After the system has been charged and has operated for at least
two hours at normal operating conditions without any indication of
malfunction, it should be allowed to operate overnight on automatic
controls. Then a thorough recheck of the evaporator operation should
be made as follows:
(a) Check liquid line sight glass and expansion valve operation.
If there are indications that more refrigerant is required,
leak test all connections and system components and repair
any leaks before adding refrigerant.
(b) Thermostatic expansion valves must be checked for
proper superheat settings. Feeler bulbs must be in positive
contact with the suction line and should be insulated. Valves
set at high superheat will lower refrigeration capacity.
Low superheat promotes liquid slugging and compressor
bearing washout.
(c) Check defrost controls for initiation and termination
settings, and length of defrost period. Set fail safe at
length of defrost + 25%.
Example: 20 minute defrost + 5 minutes
= 25 minute fail safe
(d) Check drain pan for proper drainage.
(e) Install instruction card and control system diagram for
use of building manager or owner.
Evaporator Superheat
Check Your Superheat. After the box temperature has reached or is
close to reaching the desired temperature, the evaporator superheat
should be checked and adjustments made if necessary. Generally,
systems with a design TD of 10 ˚ F should have a superheat value of 6 ˚
to 10 ˚ F for maximum efficiency. For systems operating at higher TD’s,
the superheat can be adjusted to 12 ˚ to 15 ˚ ˚F as required.
NOTE: Minimum compressor suction superheat
of 20˚F may override these recommendations
on some systems with short line runs.
To properly determine the superheat of the evaporator, the following
procedure is the method Heatcraft recommends:
WARNING: If the condensing unit has no flooded
condenser head pressure control, the
condensing unit must have the discharge
pressure above the equivalent 105˚F
condensing pressure.
1. Measure the temperature of the suction line at the point
the bulb is clamped.
2. Obtain the suction pressure that exists in the suction line
at the bulb location by either of the following methods:
a. A gauge in the external equalized 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 of the
evaporator will yield the same reading as 2a above.
3. Convert the pressure obtained in 2a or 2b above to
saturated evaporator temperature by using a
temperature-pressure chart.
4. Subtract the saturated temperature from the actual
suction line temperature. The difference is Superheat.
Alternative Superheat Method
The most accurate method of measuring superheat is found by
following the previous procedure, Temperature/Pressure method.
However, that method may not always be practical. An alternative
method which will yield fairly accurate results is the temperature /
temperature method:
1. Measure the temperature of the suction line at the
point the bulb is clamped (outlet).
2. Measure the temperature of one of the distributor
tubes close to the evaporator coil (inlet).
3. Subtract the inlet temperature from the outlet
temperature. The difference is Superheat.
This method will yield fairly accurate results as long as the pressure
drop through the evaporator coil is low.
Figure . Bulb and Contact Location
Figure 7. Multiple Evaporators