Copeland 21-1216
For more free Copeland literature please visit www.HVACRinfo.com
Application Engineering Bulletin
Revised April, 2001
Blocked suction internal unloading has been successfully used for many years to unload 4 and 6 cylinder reed compressors for air-conditioning applications.
More recently, blocked suction unloading has been
applied on selected 4, 6, and 8 cylinder Discus compressors for high, medium, and low temperature applications.
duce compressor cycling and decrease the starting
electrical load. Blocked suction capacity control also
makes efficient compressor operation possible when
the compressor is unloaded.
Internal Capacity Control Valve Construction
A schematic illustration of the internal valve operation is shown in Figure 1.
Compressor capacity modulation can reduce power
consumption, provide continuous dehumidification, re-
Figure 1
© 1984 Copeland Corporation
Copeland 21-1216
In the normal (full capacity) operating position
with the solenoid valve de-energized, the needle valve
is seated on the lower port, and the unloading plunger
chamber is exposed to suction pressure through the
suction port. Since the face of the plunger is open to
the suction chamber, the gas pressures across the
plunger are equalized, and the plunger is held in the
open position by the spring.
When the solenoid valve is energized, the needle
valve is seated on the upper port, and the unloading
plunger chamber is exposed to discharge pressure
through the discharge pressure port. The differential
between discharge and suction pressure forces the
plunger down, sealing the suction port in the valve
plate, thus preventing the entrance of suction vapor
into the unloaded cylinders.
Figure 3
Figures 4 and 5 show the pressure port connections to the unloading solenoid valve. The action of the
valve determines which pressure is applied to the
unloading valve plunger. The gasket is a universal
gasket fitting all 4, 6, and 8 cylinder reed and Discus
The seal on the unloading plunger minimizes any
leakage in pressure so that a pumpdown cycle may be
used with the valve either energized or de-energized
without excessive compressor cycling.
4R-6R Copelametic Compressors
A typical capacity control valve plate for conventional Copelametic compressors is shown in Figure 2.
Figure 4
Figure 2
An interior view of the internal unloading head is
shown in Figure 3. Note that the unloader plunger
chamber has been cast in the head, but that the
gasketed surfaces of the head are the same as a
standard head. A standard head gasket is used with
the internal capacity control valve - no special unloading gasket is required.
PART # 020-0783-00
Figure 5
Copeland 21-1216
Copelametic Discus Compressors
The same basic internal unloading mechanism is
used on both Discus and conventional compressors.
Figures 6, 7, 8 and 9 show typical Discus construction.
When an unloading valve in a operating system is
switched from the loaded to the unloaded position, a
new system balance takes place. In air-cooled systems the suction pressure will rise and the condensing
pressure will drop. In water-cooled systems the suction pressure will rise, but the condensing pressure will
remain substantially the same.
The pressure or temperature control should have
as wide a differential as necessary to prevent rapid
cycling of the unloading valve. Rapid cycling can lead
to wear and premature failure of the solenoid valve
and/or internal unloader components. The unloader
cycle rates should be limited to no more than 10 to 12
per hour to ensure satisfactory unloader life. A wider
differential will be required with the pressure control
method than the temperature control method because
of the thermal mass (or flywheel effect) of the cooled
Figure 6
Application Requirements
Because of the decreased volume of suction vapor returning to the compressor from the system and
available for motor cooling, the operating range of
unloaded compressors must be restricted. Reed style
compressors should be limited to a minimum evaporating temperature of 25°F when operating unloaded.
Discus compressors can be operated unloaded below
25°F with the addition of head cooling fans and oil
coolers. See Tables 1 and 2 for specific requirements
for Discus.
The capacity control valve is in the normal discharge position (loaded) when de-energized, and in
the unloaded position when energized. The solenoid
valve may be energized either by means of a reverse
acting low pressure control or a temperature control
thermostat. On six cylinder compressors with two
unloaders, two step thermostats may be employed,
but if sequence control of unloaders from compressor
suction pressure is desired, two separate controls are
Figure 7
Figure 8
Figure 9
Copeland 21-1216
Although the leak back rate when the coil is energized during a compressor off cycle is minimal, to
prolong the solenoid coil life it is recommended that
the capacity control solenoid valve be de-energized
when the compressor is not operating.
supply. In such cases the unloading solenoid valve
and control may be connected as shown in Figure 10,
or they may be connected in parallel with the compressor contactor coil as in Figure 11.
Booster Compressors
In control circuits operating at line voltage, the
solenoid valve and control can be connected to the
load side of the contactor as in Figure 9. The unloader
solenoid valve will then be de-energized and closed
when the compressor is not operating.
See Application Bulletin 4-1288 for requirements.
Demand Cooling Applications
Demand Cooling is not approved for compressors
with modulation. See AE 4-1287 for additional information.
On large installations, the control circuit may have
a power source independent of the compressor power
Figure 10
Figure 11
Copeland 21-1216
IMPORTANT - Unloader Valve Location
One very important factor to be considered in the
design of any system equipped with an unloading type
compressor is the gas velocity. It must be maintained
at a sufficiently high level to insure oil return to the
compressor when it is unloaded. When an unloading
type compressor is employed on a single evaporator
with a single suction line, it may be necessary to
employ double risers constructed as shown to insure
oil return. (See Copeland Refrigeration Manual AE104).
right bank
right bank
6R, 33%:
right bank
6R, 67%:
right & left bank
6D*1, 33%:
right bank
6D*1, 67%:
right & left bank
6D*3, 33%:
right bank
6D*3, 67%:
right & top bank
lower right & upper left banks
The right bank is the terminal box side. While unloaders
could be physically mounted on the other banks, to do
so would interfere with the internal crankcase pressure
suppression and prevent proper oil return.
If the unloading type compressor is to be installed
on a system employing multiple evaporators with solenoid valves to control the flow of refrigerant to the
evaporators, a separate suction line should be run from
each evaporator to the machine room and there connected into a common line, which must slope downward toward the compressor. See Figure 12.
Capacity Control Variations (6D*1-vs-6D*3)
There are two styles of unloader heads presently used
by Copeland. To identify the two styles see Figures 8
& 9. The hole configuration is different depending on
what model compressor you have.
Expansion valves for systems with unloading compressors must be able to control the refrigerant flow
while the compressor is unloaded as well as when
loaded. They generally cannot maintain stable control
below 25-35% of their rated capacity and, therefore
must not be oversized for the fully loaded condition.
Prior to 1991 all 4D & 6D compressors were the *1
version (see Figure 8). This style unloader used
gasket 020-0690-00. The new *3 model (see Figure
9) has different hole configurations and requires a
different style unloader gasket. The new gasket (see
Figure 5) was made to fit both the *1 & *3 models.
Field Service and Conversion 4-6 Compressors
Standard 4 and 6 compressors may be converted
to internal capacity control models by means of the
appropriate conversion kit. See Table 1 for Part Numbers.
When changing from the *1 to the *3 type head
you must do the following:
To convert a standard reed compressor to unloading, it is only necessary to remove the existing head,
install a new head gasket, install the new head and
unloader assembly, and install the solenoid valve. A
new valve plate is also required on Discus compressors. Torque head bolts to 375 inch pounds on reed
compressors and 550 inch pounds on Discus compressors.
Use gasket (Figure 5) on all 4D/6D unloader type
heads. Make sure the tab of the gasket is aligned
with the tab of the head.
The unloader valve pin fits in the small hole on the
*1 style head. For the *3 style head the unloader
valve must be turned 180 degrees and the valve
pin fits into a much larger hole. If the old style
gasket is used on a *3 head, the port to the plunger
will be covered up and the unloader will not work.
Figure 12
Copeland 21-1216
Non unloading Operation
The 4, 6, and 8 cylinder capacity control models are
shipped from the factory with a shipping plate and
gasket on the unloading pad, and the unloading valve
and coils as a “ship remote” item. These compressors
can be operated continuously full loaded without the
valve and coil installed if desired for any reason. The
gasket needs to be replaced when mounting the valve
and coil for unloading. See Figure 14.
Figure 14
The unloading valve and coil assembly (240 volt) is
included in the capacity control model standard bill of
material as a “ship remote” item. The capacity control
model can be operated without the unloading valve and
coil installed, if desired for any reason. If mounting the
valve and coil for unloading, use the correct gasket per
the following instructions.
Unloading Valve Gasket. Use this gasket when
installing the unloading valve and coil on all 4, 6, and 8
cylinder models. It is supplied in the gasket kit.
Figure 13
Shipping Gasket / Full Load Gasket. This gasket is
under the shipping plate covering the unloader mounting pad on the cylinder head. There is no need to
remove the plate or change gaskets if the compressor
is to be operated without a valve and coil. If a compressor was previously operated with valve and coil installed, but is to be converted back to non-unloading
(without valve), this gasket should be used under a
shipping plate.
Copeland 21-1216
Table 1
Medium Temperature Discus Models - Unloaded Operation Criteria
Evaporator Range +25°F. To -5°F.
Head cooling fan required.
Maximum condensing temperature............................................................140°F.
Minimum evaporating temperature........................................................... -5°F.
Table 2
Low Temperature Discus Models - Unloaded Operation Criteria
Evaporator Range -5°F. to -40°F.
Head cooling fan required.
Oil cooler required.
Maximum condensing temperature..........................................................130°F.
Minimum evaporating temperature for 4D and 6D (33% unloaded)............-40°F.
Minimum evaporating temperature for 6D (66% unloaded)........................-25°F.
NOTE: Minimum evaporating temperature for 4R, 6R, 8R models is +25°F when unloading.
Because of lower suction gas velocity when unloading low and medium temperature compressors,
adequate oil return must be assured through piping design or defrost control.
Copeland Corporation
Sidney, OH 45365-0669
Printed in U.S.A.
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF