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unloaded, the lag compressor will shut down under 2
conditions:
1. When chilled liquid temperature drops below the Neu-
tral Zone.
2. When chilled liquid temperature drops below the "TAR-
GET" temperature and the rate of change exceeds the
internally programmed rate limits.
The micro may also shut down the lag compressor be-
fore it is totally unloaded to avoid cut-out of the entire
chiller on a Low Water Temp. Fault. This will occur under 2
conditions:
1. The leaving chilled liquid temperature falls below the
low end of the CONTROL RANGE (CR) for 37 sec-
onds.
2. The leaving chilled liquid temperature drops below the
low end of the CONTROL RANGE (CR) minus CR/4.
Individual compressor loading is computed by the micro
based on counting load/unload pulses. The micro bases
loading on a 50 pulse scale. After 50 loading pulses,
from total unload, the micro assumes full load at "50" on
the scale of 0 - 50 pulses. Unload pulses subtract from
the numerical value on the scale, whatever it may be. "0"
is fully unloaded on the 0 - 50 pulse scale. As loading
and unloading pulses are sent, the micro continues to
keep track by adding and subtracting pulses on the 0 -
50 pulse scale.
After the lag compressor is shut down, the lead com-
pressor loading will be adjusted up or down to control
temperature within the Neutral Zone. As load drops, the
micro will keep track of the load pulses sent to the lead
compressor. At minimum loading, the lead compressor
will shut down when temperature drops below the low
limit of the CONTROL RANGE (CR).
The lead compressor may also shut down while partially
loaded to avoid cut-out on a Low Water Temperature Fault,
if temperature drops 2 degrees below the low limit of the
CONTROL RANGE (CR) or if temperature drops CR/2
below the low limit of the CONTROL RANGE (CR).
Selecting The LOAD SHARE BASED ON ?
Selection of the LOAD SHARE BASED ON ? is accom-
plished under the PROGRAM key. To control from % FLA
motor current, key in a "2" as shown below and press the
ENTER key.
L O A D
S H A R E
1
=
S V % ,
This programmable display is not password protected.
YORK INTERNATIONAL
B A S E D
O N ?
2
=
% F L A
2
Selecting The LAG COMPRESSOR START POINT %
Selection of the LAG COMPRESSOR START POINT %
under the PROGRAM key primarily allows the user to
either allow the lead compressor to fully load before the
lag compressor is started, or start the lag compressor is
fully loaded.
Key in a start point % and press the ENTER key. A
sample display is shown below:
L A G
C O M P R E S S O R
S T A R T
P O I N T
This programmable display is not password protected.
Selecting a LAG COMPRESSOR START POINT % of
99% assures that the lead compressor is fully loaded
before the lag compressor is brought on. If very low loads
are expected, less compressor cycling will be noted by
selecting a high slide valve percentage. 99% would be
recommended for absolute minimum cycling. If due to
system conditions, the lead compressor does not reach
the programmed %, even though the lead compressor is
fully loaded, the lag compressor will start regardless of
the programmed % after 5 minutes of operation when-
ever the lead compressor cannot bring the leaving chilled
liquid within 2.0°F of the high end of the CONTROL
RANGE (CR). This assures that the lag compressor starts
if demand is present, regardless of the program points.
Once the lag compressor is started, equalized loading
and unloading will occur as previously described. This
assures that the chiller will fully load and maintain chilled
liquid temperature.
Selecting a LAG COMPRESSOR START POINT % less
than 99% increases the efficiency of the chiller. At lower
percentages, the lead compressor is loaded to the pro-
grammed % and the lag compressor is started and
brought up to a % motor FLA equal to the lead compres-
sor as load permits. At this point the loading of both com-
pressors is adjusted up or down to maintain capacity
and equalize loading. Running both compressors partially
loaded makes more efficient use of the evaporator bundle
than running one compressor fully loaded with the other
idle. This assures that the entire evaporator is being uti-
lized and maximum chiller efficiency is achieved. How-
ever, keep in mind that as more capacity is required, the
lag compressor will start regardless of the programmed
% and equalized loading at part load will still be achieved,
even with a programmed % of 99%.
If a LAG COMPRESSOR START POINT % of less than
99% is desired, a value of 70% is recommended. This
will typically assure that the lag compressor will start
before the lead compressor is fully loaded in most
operating conditions.
FORM 201.10-NM1
7 0 %
101
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