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NOTE: All adjustable controls and valves must be field
adjusted to meet desired operation. There are no
factory preset controls or valve adjustments.
System Balancing - Compressor Superheat
IMPORTANT: In order to obtain the maximum capacity
from a system, and to ensure trouble-
free operation, it is necessary to balance
each and every system.
This is extremely important with any refrigeration system.
The critical value which must be checked is suction superheat.
Suction superheat should be checked at the compressor
as follows:
1. Measure the suction pressure at the suction service
valve of the compressor and determine the saturation
temperature corresponding to this pressure from a
“Temperature-Pressure” chart.
2. Measure the suction temperature of the suction line
about one foot back from the compressor using an
accurate thermometer.
3. Subtract the saturated temperature from the actual
suction line temperature. The difference is superheat.
Too low a suction superheat can result in liquid being returned
to the compressor. This will cause dilution of the oil and
eventual failure of the bearings and rings or in the extreme
case, valve failure.
Too high a suction superheat will result in excessive discharge
temperatures which cause a break down of the oil and results
in piston ring wear, piston and cylinder wall damage.
It should also be remembered that the system capacity
decreases as the suction superheat increases. For maximum
system capacity, suction superheat should be kept as low as
is practical. Copeland mandates a minimum superheat of
20˚F at the compressor. Heatcraft Refrigeration Products
recommends that the superheat at the compressor be
between 30˚F and 45˚F.
If adjustments to the suction superheat need to be made,
the expansion valve at the evaporator should be adjusted.
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
entire system operation should be made as follows:
(a) Check compressor discharge and suction pressures.
If not within system design limits, determine why and
take corrective action.
(b) 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.
(c) 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.
(d) Using suitable instruments, carefully check line voltage
and amperage at the compressor terminals. Voltage
must be within 10% of that indicated on the
condensing unit nameplate. If high or low voltage is
indicated, notify the power company. If amperage draw
is excessive, immediately determine the cause and
take corrective action. On three phase motor
compressors, check to see that a balanced load is
drawn by each phase.
(e) The maximum approved settings for high pressure
controls on Heatcraft air cooled condensing equipment
is 400 psig. On air cooled systems, check as follows:
Disconnect the fan motors or block the condenser
inlet air. Watch high pressure gauge for cutout point.
Recheck all safety and operating controls for proper
operation and adjust if necessary.
(f) Check winter head pressure controls for pressure
setting.
(g) Check crankcase heater operation if used.
(h) Install instruction card and control system diagram for
use of building manager or owner.