Low-Pressure Control; Discharge Temperature Protection - Emerson Copeland Scroll ZRH49KJE Application Manuallines

4.5.2 Low-pressure control

IMPORTANT!
Loss of system charge! Bearing malfunction! A low-pressure control is
highly recommended for loss of charge protection. Do not bridge or by-pass
the low-pressure cut out.
Copeland Horizontal Scroll
protection. If allowed to go undetected, loss of system charge will result in overheating and
damage to the scrolls and floating seal. Prolonged operation with low charge will result in
decomposition of the oil that might require complete system replacement.
The low-pressure cut out, if installed in the suction line to the compressor, can provide additional
protection against a TXV failed in the closed position, a closed liquid line service valve, or a
blocked liquid line screen, filter, orifice, or TXV. All of these conditions can starve the
compressor for refrigerant and may result in compressor failure.
The low-pressure cut out should have a manual reset feature for the highest level of system
protection.
If a compressor is allowed to cycle after a fault is detected, there is a high probability that the
compressor will be damaged and the system contaminated with debris from the failed
compressor and decomposed oil.
If the compressor is fitted with a Rotalock valve, the low-pressure switch MUST be connected on
the compressor side of the valve.
The low-pressure cut out should be set as high as possible.
The low-pressure minimum is 1.8 bar(g) (R407C) and 0.3 bar(g) (R134a).
4.6

Discharge temperature protection

Internal discharge temperatures reached under some extreme operating conditions can cause
compressor damage. Extreme operating conditions are, for example, loss of charge, extremely
high compression ratio caused by failure of evaporator fan...
Copeland Horizontal Scroll
result, an external discharge line sensor is required for all refrigeration and heat pump
applications to provide loss-of-charge protection. This is due to the very low setting required on
the low-pressure cut outs of these systems due to their evaporating temperatures. Air
conditioning systems do not always need the discharge line sensor.
The discharge line sensor is normally not required for loss of charge protection when a low-
pressure cut out with a setting of 1.7 bar(g) (1.8 kg/cm²) or higher can be installed directly on the
suction line of the compressor.
The following loss-of-charge test is recommended for air conditioning systems to confirm that the
existing low-pressure control provides the necessary protection:
Operate the system in a 35°C ambient and monitor discharge line temperature while
slowly removing charge.
If the discharge line temperature, measured using an insulated thermocouple located 6
inches (15 cm) from the compressor, does not exceed 127°C before the low-pressure cut
out trips, then the system has adequate loss-of-charge protection. In this case, an external
discharge line sensor is not necessary.
If the test shows that the discharge temperature gets into the danger zone, then Copeland
recommends the use of a thermistor sensor strapped to the discharge line and insulated
to provide discharge temperature protection. The system controller must lock out the
compressor if the thermistor temperature exceeds 127°C. As with the low-pressure cut
out, the discharge temperature protection should have a manual reset feature for the
highest level of protection. A minimum thirty-minute lockout period should be provided for
any discharge temperature trip.
10
TM
compressors require a low-pressure control for loss of charge
TM
compressors have no internal scroll temperature protection. As a
C6.2.7/0302-1007/E