Fridgewatch 2100 Controller Mk4
Section 3
Publication 3-58
Issue 1.1 : 01/08
Page 113 of 152
Example: use of S63, S84, S85, S86, S87, S88 and S89
25 bar g 4 to 20 mA pressure transmitters sensing discharge pressure
are wired into the P4 analogue inputs. Settings: S63 = 2, S84 = 0 bar g,
S85 = 25 bar g, S86 = -2 bar g, S87 = 15 bar g, S88 = 17 bar g,
S89 = 30 bar g.
The objective of limiting is to prevent a high pressure trip. A trip based on
the P4 reading is NOT required. Each compressor has it's own
condenser system. If any compressor cannot meet the required
refrigeration load because of high discharge pressure, it is desirable to
start another compressor.
If measured pressure rises above 15 bar g (= S87 the Hold Limit), all
loading is inhibited, irrespective of any load requirements from the PID
control algorithm, the external load request or manual load button. The
following are also initiated:
•
Limiting appears on the front panel display.
•
The Next Start Delay countdown timer before starting the
next compressor is commenced.
If the measured pressure continues to rise and passes 17 bar g (= S88
the Unload Limit), 2 = ABS (S86) unload pulses are generated. If, after
one sample time (= S60) expires, the pressure remains above the Unload
Limit, another 2 unload pulses are generated.
All loading of this compressor remains inhibited until the pressure falls
below the hold limit, the Next Start countdown continues, if the latter
expires before limiting clears, the next compressor is started.
During commissioning It is important to set the value of S86 appropriately
for the system. If too many unload pulses are generated, the pressure
will fall below the S87 value and the countdown to start the next
compressor will be cancelled. The PID loading algorithm that has been
suppressed whilst above the Hold Limit will cause the compressor to load
again and the cycle will repeat. The system will yo-yo in and out of
limiting without another compressor being allowed to start.
However high the measured pressure rises an E14 P4 trip will NOT occur
as the trip threshold limit entered into S89 of 30 bar can never be
reached with a 25 bar transmitter. If S86 was positive, indicating a low
level trip, an unreachable low value; for example, minus 2 bar would be
needed to disable tripping on P4.
Configuration Settings S90 to S93: Limiting and Trips on P1 (3MIO1
Card Analogue Input 1, Pt 100
Ω
Temperature Channel)
NOTE: S90, S91, S92 and S93 may be used to provide
Low Limits only, i.e. when the measured value falls
below a chosen value, for example, limiting on local
heat exchanger outlet temperature.
S90 P1 (temperature) limiting/trip:- unused/defined
S90 setting defines whether analogue input 1 on the 3MIO1 card is to be
used for low temperature limiting and/or trip:
S90 = 0: no limiting or trip is applied to this channel
S90 = +
x
: reserved,
DO NOT
use.
S90 = -
x
: low limit limiting and/or trips are applied to this channel
on a system wide basis not a per compressor basis.
