20
C6.2.27/0716-0219/E
6.8
Starting sound
During the very brief start-up, a clicking sound, resulting from initial contacting of the spirals, is
audible; it is normal. No start-assist devices are required for single-phase compressors, even if a
system uses non-bleed expansion valves. Due to the design of the Copeland scroll, the internal
compression components always start unloaded even if system pressures are not balanced. In
addition, since internal compressor pressures are always balanced at start-up, low-voltage starting
characteristics are excellent for Copeland Scroll compressors. Moreover, if low voltage conditions
exist at start up, protector trips could result.
6.9
Deep
vacuum
operation
CAUTION
Vacuum operation! Compressor damage!
Copeland Scroll compressors
should never be used to evacuate refrigeration or air-conditioning systems.
The scroll compressor can be used to pump down refrigerant in a unit as long as the pressures
remain within the operating envelope. Low suction pressures will result in overheating of the scrolls
and permanent damage to the compressor drive bearing or will cause the ASTP to activate.
Copeland Scroll compressors incorporate internal low vacuum protection; the floating seal unloads
when the pressure ratio exceeds approximately 10:1 for ZB* and 20:1 for ZF* compressors.
6.10 Shell
temperature
The top shell and discharge line can briefly but repeatedly reach temperatures above 177°C if the
compressor cycles on its internal protection devices. This only happens under rare circumstances
and can be caused by the failure of system components such as the condenser or evaporator fan
or loss of charge and depends upon the type of expansion control. Care must be taken to ensure
that wiring or other materials that could be damaged by these temperatures do not come in contact
with the shell.
6.11 Pump-down cycle
To control refrigerant migration a pump-down system should be used. The discharge check valve
with a refrigeration scroll compressor is designed for low leak back and will allow the use of a
pump-down without the addition of an external check valve.
If the compressor is stationary for prolonged periods, refrigerant could migrate into the compressor
and therefore a crankcase heater must be installed.
If cold air is constantly drawn over the compressor this could make the crankcase heater
ineffective and therefore a pump-down system is recommended.
For ZB* models care should be taken because the scroll sets will unload at a pressure ratio of
approximately 10:1. If the unit fails to pump down, the pump-down pressure should be reset to a
higher value. The low-pressure control differential for all models needs to be reviewed since a
relatively lower volume of gas will re-expand from the discharge plenum of the compressor into
the low side on shutdown.
6.12 Minimum run time
Emerson recommends a maximum of 10 starts per hour. There is no minimum off time because
Scroll compressors start unloaded even if the system has unbalanced pressures. The most critical
consideration is the minimum run time required to return oil to the compressor after start-up. To
establish the minimum run time obtain a sample compressor
equipped with a sight tube (available
from Emerson) and install it in a system with the longest connecting lines that are approved for
the system. The minimum on time becomes the time required for oil lost during compressor start-
up to return to the compressor sump and restore a minimal oil level that will ensure oil pick-up
through the crankshaft. Cycling the compressor for a shorter period than this, for instance to
maintain very tight temperature control, will result in progressive loss of oil and damage to the
compressor.
