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© 2015 Emerson Climate Technologies, Inc. All rights reserved.
AE4-1395 R5
Introduction
The 3 to 15 ton Copeland Scroll Digital
™
compressors
described in this bulletin include the follow compressor
model numbers:
R-410A
R-22 & R-407C
ZPD34 to ZPD54K5
ZRD36 to ZRD81KC
ZPD61-ZPD83, ZPD91,
ZPD104, ZPD122KC
ZRD94 to ZRD125KC
ZPD103, ZPD120,
ZPD137-182KC
ZPD and ZRD digital scroll compressors are variable
capacity compressors that can modulate down to 10%
of full load. Digital scrolls are suitable for a variety of
applications where a variable capacity compressor is
useful, such as VAV applications, dedicated outside
air units, units that typically used hot gas bypass for
capacity control, and applications that require accurate
control of temperature and humidity. Other applications
include multiple compressor systems where modulation
is required over the entire operating range of the system
and in applications where compressor starting and
stopping is unacceptable. Typical digital scroll model
numbers are ZRD94KCE-TF5 and ZPD182KCE-TWD.
This bulletin describes the operating and application
differences with respect to the equivalent fixed
capacity Copeland Scroll
™
compressors. The following
Application Engineering bulletins should be consulted
for non-modulating scroll application guidelines:
AE4-1331
ZPD34-ZPD54K5
AE4-1365
ZPD61-ZPD83KC, ZPD91KC,
ZPD104KCE, ZPD122KCE
AE4-1303
ZPD103KC, ZPD120KC,
ZPD137-182KC, ZRD94-125KC
AE4-1312
ZRD36-81KC
Nomenclature
The model number of the Copeland Scroll Digital
compressors includes the approximate nominal
60 Hz capacity at the AHRI high temperature full
load air conditioning rating point. An example is the
ZPD120KCE-TFD, which has approximately 120,000
Btu/hr cooling capacity at the air conditioning rating
point when operated on 60 Hz. Note that the same
compressor will have approximately 5/6 of this
capacity or 100,000 Btu/hr when operated on 50 Hz
power. Please refer to the Online Product Information
at
EmersonClimate.com
for more information on
performance at part load.
Digital Compressor Operation
The digital scroll is capable of seamlessly modulating
its capacity from 10% to 100%. A normally closed
(de-energized) solenoid valve is a key component for
achieving modulation. When the solenoid valve is in its
normally closed position, the compressor operates at
full capacity, or loaded state. When the solenoid valve
is energized, the two scroll elements move apart axially,
or into the unloaded state. During the unloaded state,
the compressor motor continues running, but since
the scrolls are separated, there is no compression.
During the loaded state, the compressor delivers 100%
capacity and during the unloaded state, the compressor
delivers 0% capacity. A cycle consists of one loaded
state and one unloaded state. By varying the time of
the loaded state and the unloaded state, an average
capacity is obtained. The lowest achievable capacity is
10% which equates to 1.5 seconds of pumping during
one 15 second cycle.
An example for the 15 second controller cycle: In any
15 second cycle, if the loaded time is 10 seconds and
the unloaded time is 5 seconds, the average capacity
is 66% or if the loaded time is 5 seconds and the
unloaded time is 10 seconds the capacity during that
15 second period is 33%.
See Figure 1
for a graphical
representation of the digital cycle, and
Figure 4
for
a graph showing solenoid on-time vs. compressor
capacity.
How it Works
The digital scroll compressor unloads by taking
advantage of the Copeland Scroll
compressor's axial
compliance. All Copeland Scroll compressors are
designed so that the compression elements can
separate axially a few thousands of an inch. The
ZPD34-ZPD54K5, ZPD61-ZPD83KC, ZPD91KC,
ZPD104KCE, ZPD122KCE, and ZRD36-81KC
compressors described in this bulletin use a lift piston
mechanism to separate the scrolls during the unloaded
state. When the solenoid is energized the volume on top
of the piston is vented to the low side allowing the piston
and fixed scroll assembly to move axial away from the
orbiting scroll. When the solenoid is de-energized the
piston is forced down and the scrolls are loaded axially.
The ZPD103KC, ZPD120KC, ZPD137-182KC and
ZRD94-125KC digital scroll compressors employ
a solenoid valve that is mounted on the side of the
compressor that vents the intermediate cavity to the
low side of the compressor during the unloaded state.
During the loaded state the solenoid valve is de-
