GEA Grasso - Page 55

Grasso
Refri
g
eration Division
GRASSO SYSTEM CONTROL
01.2004/1 638660E_chapter2.doc Chap. 2 - 19
2.7.2.7 Low external temperature limitation (Chiller outlet temperature)
This limitation is only active if an external temperature sensor is installed.
If the external temperature reaches its initial limitation value, the capacity of the compressor cannot be
increased any further. If the external temperature reaches the decrease limit, the compressor capacity is reduced
by 1 capacity step immediately.
The signal dominates compared to the capacity control and is effective until the external temperature has
become less than the limit value and the limit timer has expired.
In case of Chiller application this limitation is intended to prevent the evaporator from freezing (frost
protection).
The external temperature limitation function is illustrated in the following diagram.
T
ext
External temperature
A.. Time characteristic of the external
temperature
V.. Forced reduction of the capacity due
too low external temperature
B.. Low external 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 increased.
b.. Limit value for disabling capacity
increase
f.. Limit value for forced reduction of
capacity
W.. Warning limit
Al.. Alarm limit
Figure 2.23, Low external temperature or Chiller outlet temperature limitation
Text low alarm 
Low limit + 
Decr limit + 
Text low alarm: Alarm limit for minimum external temperature in °C
Low limit: Begin of the temperature limitation -
enter value as an offset to the alarm limit in K
<< Limit values 01 >> Decr limit: Begin of the forced capacity reduction — enter value as an
offset to the alarm limit in K