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energized and the intermediate cavity is pressurized
to load the floating seal and scrolls axially.
Please refer to Figures 2 and 3 for cross sectional
pictures of the two digital modulation mechanisms.
APPLICATION CONSIDERATIONS
Operating Envelope
Refer to the fixed capacity compressor model family
application bulletin for approved operating range.
System testing is highly recommended to verify
adequate oil return during fully loaded and modulated
operation.
Solenoid Valve and Coil
The external solenoid valve and coil specified by
Emerson must be used since this is a critical component
for the proper functioning of this compressor. The
solenoid valve and coil are designed for approximately
32 million cycles. Do not attempt to substitute
replacement coils or valves; use only the replacement
parts specified in Table 4.
Procedures section for information on changing the
modulation valves.
Pressure Fluctuations
During scroll modulation the suction and the discharge
pressure will fluctuate. This fluctuation should be
observed during unit testing. The installation and setting
of pressure controls should take this into account.
During the unloaded state, the discharge pressure will
decrease and the suction pressure will increase. This
normal pressure fluctuation has no observable effect on
the reliability of the compressor or system components.
System component manufacturers should be consulted
to ensure the proper application of their products.
Piping
Unlike a variable speed compressor whose mass flow
and gas velocity changes with its speed, the digital
scroll's pumping capacity is equal to its 100% capacity
while it is pumping. For this reason the gas velocity
remains high even during periods when the capacity
demand is low. Because the mass flow and gas velocity
remain high, piping may be designed as if it were
designed for a non capacity controlled compressor. For
vertical piping a trap every 20 feet should be sufficient
to ensure proper oil return. This recommendation is
based upon a minimum 1500 fpm velocity or higher.
When the digital scroll compressor is part of a tandem,
a double riser should be considered to assure that the
© 2015 Emerson Climate Technologies, Inc. All rights reserved.
Refer to the Service
velocity remains above 1500 fpm when only the digital
scroll is running.
Start Up and Shut Down
To improve the starting characteristics of the digital
scroll compressor, the the Emerson controllers delay
loading the compressor for 0.1 seconds. Likewise, to
eliminate the reverse rotation sound at shut down the
compressor is unloaded 0.5 seconds before shut down.
Compressor Cycling
Because of the digital scroll's seamless capacity
modulation from 10% to 100%, capacity short cycling
should not be a problem for single compressors.
Modulating below 10% is not recommended due to
possible motor overheat and inadequate oil return.
However, if the digital compressor is in tandem with a
non modulated scroll, short cycling of the non modulated
compressor may be a problem if the system control is
not designed and set correctly. The Emerson digital
controllers have a built in two minute anti-short cycle
timer to prevent short cycling.
Sound Characteristics
The sound spectrum of the loaded state and the
unloaded state are different. Special consideration
should be given to the transition sound between the
loaded and unloaded states. If the transition sound
is unacceptable, a heavy sound blanket should
be applied to the compressor. Fabricating Services
(www.fabsrv.com) is one source for scroll compressor
sound blankets.
The transition sound between the loaded and unloaded
states has no observable effect on compressor
components or reliability.
The Emerson controllers unload the compressor a
fraction of a second before shut down allowing the scroll
set to unload, ensuring a relatively quiet shutdown.
Internal Pressure Relief (IPR) Valve
WARNING
A high pressure control must be used in all
ZRD94-125KC and ZPD103, ZPD120, ZPD137-
182KC applications because these compressors
do not have internal pressure relief (IPR) valves.
High Pressure Control
As mentioned above, not all digital scrolls have IPR
5
AE4-1395 R5
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