© 2017 Emerson Climate Technologies, Inc.
4
AE21-1355 R4
Introduction
On refrigeration applications where the load may vary
over a wide range, some means of capacity control is
often desirable for optimum system performance and
control. In addition, compressor capacity modulation can
reduce power and energy consumption, reduce
compressor cycling, and decrease the starting electrical
load. In order to achieve the above objectives, Emerson
developed Copeland Discus
™
digital compressors as a
method for variable capacity. Digital technology permits
digital modulation of Discus
™
compressors for high,
medium, and low temperature applications.
Theory of Operation
Digital capacity control is achieved by using a proven
internal unloading method, blocked suction. Blocked
suction has been applied to multiple bank compressors
in the past, but digital modulation allows the technology
to apply to single bank compressors as well, such as the
3D. By varying the percentage of duty cycle that the
compressor is loaded and unloaded, capacity control
between 10 and 100% can be achieved. During blocked
suction operation, the flow of suction gas is blocked to
all cylinders on a single bank; therefore, there is no gas
to compress and the power consumption is significantly
lower. The unloader piston mechanism that controls flow
of suction gas into the cylinders is driven by a solenoid
valve. See the descriptions and figures below for the
unloaded and loaded states for Copeland Discus digital
operation.
Digital technology is also available on the 4D and 6D
Discus compressors. Applied to one bank of the
compressor it can provide 50-100% capacity modulation
on a 4D Discus and when combined with a blocked
suction bank on a 6D can provide 33-100% capacity on
a 6D (without the blocked suction bank 67-100% digital
modulation is available on the 6D platform).
Figure 1: Unloaded Operation
Enters Compressor
Passes Through Body
Unloader Mechanism Blocks Gas Before Entering
Valve Plate
Figure 1 Copeland Discus Digital Unloaded Operation (Shown on a Copeland 3D Discus compressor)
