GEA Grasso - Page 69

Grasso
Refri
g
eration Division
GRASSO SYSTEM CONTROL
01.2004/1 638660E_chapter2.doc Chap. 2 - 33
2.7.3.2 Monitoring of the intermediate pressure, intermediate temperature and discharge temperature
The GSC monitors the intermediate pressure, intermediate temperature at the low pressure side (T
int
) and the
discharge temperature (T
dis
) of the two stage compressor. The monitoring function uses the ϕ value. This ϕ value
is calculated as follows: number of active high pressure cylinders divided by the number of active low pressure
cylinders.
For each capacity step of the two stage piston compressor this ϕ value is fixed.
The maximum discharge temperature limit is used to calculate the lowest intermediate pressure possible. This
pressure is calculated real time and depends on the running conditions of the compressor. The lower the
maximum discharge temperature limit is set, the higher the calculated minimum intermediate pressure allowed
will be. The minimum value allowed for the intermediate pressure is used to calculate the minimum ϕ value.
The calculation of the maximum ϕ value is based on the maximum intermediate pressure limit.
As soon as the ϕ value belonging to a capacity step falls outside the minimum and maximum ϕ value, this step is
blocked. The step will be available again after that the running conditions have changed and the ϕ value of this
capacity step is valid. If the current capacity step becomes invalid, the first step available with less capacity will be
activated automatically. If this is not possible the next available step with higher capacity will be activated. In
case more capacity is needed, this sequence is reversed.
The procedure described above is illustrated in the diagram below.
Symbol Explanation
Diagram for monitoring of the ϕ value
RC7212E Compressor type
step Capacity step (0...12)
ϕ
Ratio between number of
active high and low
pressure cylinders
motor
Compressor motor; If
capacity step = 0 then the
motor is switched off.
start only
This step is available
during start up only.
min. step
Minimum capacity step;
Only during start up lower
steps are allowed. After
passing this step stopping
the compressor is the only
way to return to lower
steps.
max. step Maximum capacity step
act. step Actual capacity step
ϕ forced
step
Forced capacity step due
too changed ϕ values
cap. decr.
Capacity decrease by
capacity controller
cap. incr.
Capacity increase by
capacity controller
PV Process value
SP Set point
NZ Neutral zone
Figure 2.39, Explanation of the ϕ value monitoring