GEA Grasso - Compressor Control Functions

Grasso
Refri
g
eration Division
GRASSO SYSTEM CONTROL
Chap. 2 - 8 638660E_chapter2.doc 01.2004/1
2.6 Compressor control
2.6.1 Capacity control
All piston compressor units from Grasso are equipped with a variable capacity control system in the geometric
range 0 — 100 %.
An external temperature or the suction pressure can be selected as the controlled variable.
The capacity is adjusted by switching capacity control solenoids, adjustment of which results in a reduction or
increase of the number of cylinders that are active. The number of active cylinders for the compression process is
thus altered.
The hydraulic adjustment of the number of active cylinders is controlled via 5 solenoids, which are situated
together in a block. The number of solenoids depends on the compressor type.
C.. Compressor
V.. Solenoid valve block
MV0.. Solenoid for unloaded start (NO)
MV1.. Solenoid for capacity control (NC)
MV2.. Solenoid for capacity control (NC)
MV3.. Solenoid for capacity control (NC)
MV4.. Solenoid for capacity control (NC)
MV5.. Solenoid for ’Fast Pull Down’ (NC)
Figure 2.9, Positioning of the compressor capacity control solenoids
2.6.2 Mode of operation of the capacity controller
The capacity controller is based on a 3-step controller. The capacity of the piston compressor is controlled in
dependence of process value, set point, neutral zone, proportional zone and ’step down correction’ value. The
capacity is increased if the process value is above the set point + ½ neutral zone and it is decreased if the process
value is below the set point — ½ neutral zone.
PV.. Process value
SP.. Set point
NZ.. Neutral zone
PZ.. proportional zone
SDC.. ’Step down correction’ value
A.. Time characteristic of PV
B.. Time characteristic of the 3-step control
output
C.. Time characteristic of the pulse generator
(step times)
D.. Time characteristic of the compressor motor
Figure 2.10, Mode of operation of the capacity controller