24
C6.2.26/1013-1116/E
be taken to ensure that wiring or other materials that could be damaged by these temperatures
do not come in contact with the shell.
5.10 Pump-down cycle
A pump-down cycle for control of refrigerant migration may be used in conjunction with a
crankcase heater when the compressor is located so that cold air blowing over the compressor
makes the crankcase heater ineffective.
If a pump-down cycle is used, a separate external check valve must be added
. The scroll
discharge check valve is designed to stop extended reverse rotation and prevent high-pressure
gas from leaking rapidly into the low side after shut off. The check valve will in some cases leak
more than reciprocating compressor discharge reeds, normally used with pump-down, causing
the scroll compressor to recycle more frequently. Repeated short-cycling of this nature can result
in a low oil situation and consequent damage to the compressor. The low-pressure control
differential has to be reviewed since a relatively large volume of gas will re-expand from the high
side of the compressor into the low side after shutdown.
Pressure control
setting: Never set the low-pressure control to
shut off outside of the
operating envelope.
To prevent the compressor from running into problems during such
faults as loss of charge or partial blockage, the control should not be set lower than 12 to
15K equivalent suction pressure below the lowest design operating point.
5.11 Pump-out cycle
A pump-out cycle has been successfully used by some manufacturers of large rooftop units.
After an extended off period, a typical pump-out cycle will energize the compressor for up to one
second followed by an off time of 5 to 20 seconds. This cycle is usually repeated a second time,
the third time the compressor stays on for the cooling cycle.
5.12 Minimum run time
Emerson Climate Technologies 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 Climate Technologies) 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.
5.13 Shut-off sound
Scroll compressors incorporate a device that minimizes reverse rotation. The residual
momentary reversal of the scrolls at shut off will cause a clicking sound, but it is entirely normal
and has no effect on compressor durability.
5.14 Supply frequency and voltage
There is no general release of standard Copeland Scroll compressors for use with variable
speed AC drives. There are numerous issues that must be considered when applying Scroll
compressors with variable speed, including system design, inverter selection, and operating
envelopes at various conditions. Only frequencies from 50 Hz to 60 Hz are acceptable.
Operation outside this frequency range is possible but should not be done without specific
Application Engineering review. The voltage must vary proportionally to the frequency.
The last digit of the model motor code indicates which frequency and voltage must be applied -
see paragraph 2.2 "Nomenclature". The availability of codes per compressor model can be
checked in paragraph 2.1 "Common information about Copeland Scroll
™ compressors".
If the inverter can only deliver a maximum voltage of 400V, the amps will increase when the
speed is above 50 Hz, and this may give rise to nuisance tripping if operation is near the
maximum power limit and/or compressor discharge temperature limit.
