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)Checkliquidlinesightglassandexpansionvalveoperation. If there are indications that more refrigerant is required, leaktestallconnectionsandsystemcomponentsandrepair any leaks before adding refrigerant.
(b)Thermostatic expansion valves must be checked for proper superheat settings. Feeler bulbs must be in positive contactwiththesuctionlineandshouldbeinsulated. 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
4.Subtract the saturated temperature from the actual suction line temperature. The difference is Superheat.
Installation and Operations Manual
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.
