JOHNSON CONTROLS
216
FORM 201.23-NM2
ISSUE DATE: 3/9/2015
SECTION 7 - OPERATION
When compressors start, the load and unload timers
will be set to 30 seconds. During the first 30 seconds of
operation after a compressor reaches the start frequen-
cy, loading/unloading is inhibited. After 30 seconds,
the control logic looks at the LWT temp, compares it
to the Setpoint plus CR, and makes a decision to load
or unload.
For precise capacity control, the Chiller Control Board
microprocessor loads and unloads compressors quick-
ly, as fast as every 2 seconds, in increments of 0.1 to
1 Hz each time a load or unload change is required.
Fixed load and unload timers of 2 sec. are set, after a
speed change of 0.1 to 1 Hz, to minimize undershoot
and overshoot.
As additional cooling is required (LCHLT more than
Setpoint plus CR), the Chiller Control Board micro-
processor will increase the speed of the compressor at
the rate of 1Hz every 2 seconds until the load is satis-
fied. Loading will continue to occur as long as leaving
chilled liquid temperature is above the Setpoint plus
CR.
The chiller control board will be make decisions re-
garding speed changes under conditions where the “er-
ror” and “rate” conflict. Under these conditions, load-
ing/unloading follows the guidelines described in the
Fuzzy Logic Control on page 214
If chilled liquid temperature is not satisfied and above
Setpoint plus CR, the microprocessor looks to see if
any of the lag compressors are not running. If any lag
compressor(s) is off, the Chiller Control Board micro-
processor looks at the VSD output frequency. If the
VSD output frequency is greater than the ADD COM-
PRESSOR FREQUENCY plus 15 Hz or equal to the
maximum chiller speed (frequency), the microproces-
sor starts an additional compressor. The ADD COM-
PRESSOR FREQUENCY is calculated as:
ADD = Minimum Start Freq x (Number of Compressors R1)
COMPRESSOR Number of Compressors Running
FREQUENCY
Example - A single compressor had been running with-
out satisfying cooling demands. Assume the minimum
VSD start frequency based on ambient is 50 Hz for this
example. The number of compressors running in the
formula will equal to 1. Placing the values into the for-
mula: 50 Hz x (1+1)/1 = 100 Hz. The add compressor
frequency will equal 100 Hz. Since the controls are de-
signed to add a compressor at a frequency 15 Hz above
this point, a compressor will be added if the speed
reaches 115 Hz.
When a compressor is to be added, the Chiller Con-
trol Board microprocessor decelerates the compressor
VSD frequency to 5 Hertz. This enables the differential
between discharge and suction pressure to be reduced
to a point where it will not affect motor current when
the compressor is restarted. It also reduces the chance
for backspin on the running compressor. The next lag
compressor is activated and all compressors are accel-
erated to the START FREQUENCY. The START FRE-
QUENCY is calculated as:
START = Current VSD Freq x (Number of Compressors Running –1)
FREQUENCY
Number of Compressors Running
With 2 compressors now running and a current VSD
frequency of 115 HZ, the start frequency will be com-
puted as:
115 Hz x (2-1) = 115 = 58 Hz
2 2
When the compressors restart, loading and unload-
ing is inhibited for 30 seconds after the compressor(s)
reaches the start frequency, as is the case on any com-
pressor start. The anti-recycle timer will be set to 120
sec.
In a situation where a single compressor on a 2 com-
pressor chiller is running and is in load limiting for
any reason, and LCHLT more than Setpoint plus CR
for less than 5 minutes, but more than 30 seconds, the
microprocessor will reset the load/unload timers to 2
seconds every “potential” load cycle. When LCHLT
more than Setpoint plus CR for more than 5 minutes,
the microprocessor will enable the lag compressor just
as it were not satisfied and determine a second com-
pressor was required to handle the load, since the lead
compressor is load limited.
If the cooling capacity exceeds the demand (LCHLT
less than Setpoint – CR/2) and multiple compressors
are operating, the Chiller Control Board microproces-
sor will decrease the speed of the compressors at the
rate of 0.1 to 1 Hz every 2 seconds until the LCHLT
rises to within the Control Range. If temp remains be-
low Setpoint – CR/2, rate is falling, and speed falls to
the minimum VSD frequency as determined by the
ambient, the VSD will decelerate all compressors to 5
Hertz. The last lag compressor will be shut down. The
remaining compressors will be restarted minus the lag
compressor. The lead compressor will restart and ac-
celerate to the STOP COMP FREQ designated as:
Summary of Contents for YCIV0157
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