GEA Grasso - Page 63

Grasso
Refri
g
eration Division
GRASSO SYSTEM CONTROL
01.2004/1 638660E_chapter2.doc Chap. 2 - 27
2.7.2.15 High discharge temperature limitation
The discharge temperature limitation is intended to prevent a high discharge temperature alarm.
If the discharge temperature reaches its initial limitation value, the compressor capacity cannot be decreased any
further. If the discharge temperature reaches the increase limit, the compressor capacity is increased by 1
capacity step immediately.
The signal dominates compared to the capacity control and is effective until the discharge temperature has
become less than the limit value and the limit timer has expired.
The same limitation is applied for the intermediate temperature at the low pressure side of a two stage
compressor. This temperature is also called intermediate temperature LP (T
int
).
The discharge temperature limitation function is illustrated in the following diagram.
T
dis
Discharge temperature
A.. Time characteristic of the discharge
temperature
1 = calculated value for the next
capacity step (in case of capacity
decrease this line lies above the actual
value line)
2 = actual value
V.. Forced increase of the capacity due too
high discharge temperature
B.. High discharge temperature limitation
active
T.. Limit timer active; After that this timer
has expired the capacity control is
enabled again and the compressor
capacity can be decreased.
b.. Limit value for disabling capacity
decrease
f.. Limit value for forced increase of
capacity
W.. Warning limit
Al.. Alarm limit
Figure 2.31, High discharge temperature limitation
) Line 1 is valid for single stage compressors only. In case
of a two stage compressor the ϕ value is used to
determine which capacity steps have to be blocked. For
detailed information is referred to chapter 2.7.3
’Specific control of two stage piston compressors’.
Tdis hi alarm 
High limit - 
Incr limit - 
Tdis hi alarm: Alarm limit for high discharge temperature in °C
High limit: Begin of the discharge temperature limitation —
enter value as an offset to the alarm limit in K
<< Limit values 01 >> Incr limit: Begin of the forced capacity increase —
enter value as an offset to the alarm limit in K