GEA Omni, Instruction Manual

Page: 202 / 238

3.12.2 Types of Vi Control Systems
GEA Grasso Compressors use one of two different systems for combined Vi partial load control. All GEA Vi sys-
tems utilize an optimum Vi slide position which is calculated based on the type of refrigerant in use, the operating
mode of the compressor and the current Capacity Slide Position using a proprietary formula.
1.
Parallel Slide (PS) System
This type of control is used by compressors in the SH family (types C, D, E and G) and the MC family (types
H, L, M and N). It utilizes a pair of valves, the capacity slide and the Vi slide. These valves are placed in
parallel with each other and may be adjusted independently. SH family compressors are equipped with a sin-
gle position sensor which is used to monitor the capacity slide, while MC family compressors utilize a pair of
position sensors to monitor both the capacity slide and the Vi slide.
2.
Tandem Slide (TS) System
This type of control is used for compressors in the LT family (types P, R, S, T, V, W, Y, Z, XA, XB, XC, XD,
XE and XF) and the new M family (types HM, LM, MM and NM). It utilizes a pair of slide valves, a primary
slide and a secondary slide, which are placed one in front of the other in the same cavity. To change the
swept volume for the compressor, the primary slide will first load up to a maximum of the calculated optimum
position, after which the secondary slide will be varied to control the swept volume. Once the maximum
swept volume is reached, both valves will move together to control the Vi. The compressor is equipped with
a single position sensor on the primary slide. A second position sensor on the secondary slide is optional.
3. Vi Slide only with speed control
This type of control is used for compressors in the MC family (types H, L, M and N) that have no parallel
slide installed. It utilizes a pair of valves and the Vi slide which are placed on the compressor housing
directly. These compressors are equipped with a single position sensor which is used to monitor the Vi slide.
3.12.3 Combined Vi Partial Load Control
GEA Grasso compressors may be configured for continuous variable Vi control. The type variable Vi control
used in a given project is dependent on the manufacturer and model of the compressor. These options are set in
the
Package Details section of the Compressor Configuration .
1.
SH Family
The Vi control of the SH Family of compressors is accomplished using a series of two parallel slides that are
operated independently to maintain the desired Vi and capacity of the compressor.
The capacity slide is used to adjust the capacity of the compressor to maintain the desired control value set
point and is completely independent of the Vi slide.
The Vi slide is used to maintain the optimum Vi for the current operating conditions. The Vi slide is loaded or
unloaded based upon a calculated value that is a function of the current optimum Vi setting and high and low
Vi limits of the compressor. This calculated optimum Vi is displayed on the compressor tile in the form
X.X
Vi . If the optimum Vi for the compressor is below the midpoint of the Vi range, the Vi slide is loaded. Con-
versely, if the optimum Vi is above the midpoint of the Vi range, the Vi slide is unloaded.The Vi slide is
loaded or unload in this manner when the compressor is running. There is no positional feedback from the Vi
slide, subsequently, there is no value displayed value for the Vi slide valve on SH Family compressors.
Control
Variable Vi Instruction Manual | GEA Omni™
Control panel
202 GEA Refrigeration North America, Inc. | E_806550_2 | Generated 12.01.2016