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previously. Key in the suction pressure cut-out value and
press the ENTER key. The new value will be entered into
memory and the display will advance to the next user
programmable value.
In the WATER COOLING MODE (SW1 OPEN), the cut-
out is programmable values between 44 - 70 PSIG (19 -
36 PSIG for R134a). In this mode, 44 PSIG is
recommended.
In the BRINE COOLING MODE (SW1 CLOSED), the cut-
out is adjustable from 20 - 70 PSIG (4.5 - 36 PSIG for
R134a).
LAG COMPRESSOR START POINT
L A G
C O M P R E S S O R
S T A R T
P O I N T
Selection of the LAG COMPRESSOR START POINT %
primarily allows the user to either allow the lead
compressor to fully load to 99% FLA or Slide Valve,
(whichever is programmed, see following) before the lag
compressor is started or start the lag compressor at a lead
compressor % FLA less than 99%.
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 the
recommended % to reduce cycling.
If due to system conditions the lead compressor does not
reach the programmed %, even though the lead system
is fully loaded, the lag compressor will start regardless of
the programmed % after 5 minutes of operation whenever
the lead compressor cannot bring the LWT to within 2.0°F
of the high end of the Control Range (CR). Once the lag
compressor is started, equalizing loading and unloading
of the two compressors will occur. 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 chiller operation. At
lower percentages, the lead compressor is loaded to the
programmed % and the lag compressor will come on and
be brought up to a % equal to the lead compressor. At this
point, the loading of both compressors will be adjusted up
or down to maintain capacity and equalize loading on the
two compressors. Running both compressors partially
loaded makes more efficient use of the evaporator than
running one compressor fully loaded with the other idle.
Running both compressors partially loaded assures that
the entire evaporator is being utilized and is the most
efficient condition for operating the chiller system.
92
However, keep in mind that once the lag compressor is
started, equalized loading at part load is always
maintained by the microprocessor even with the LAG
COMPRESSOR START POINT programmed at 99%.
If a % less than 99% is selected for efficiency purposes,
a value of 70% is recommended. This will typically assure
that the lag compressor will start before the lead is fully
loaded.
To program the LAG COMPRESSOR START POINT %,
key in the desired value and press the ENTER key. The
new value will be entered into memory and the display will
advance to the next user programmable value. This value
is not password protected and can be programmed
anytime the PROGRAM key is pressed.
The micro will accept a range of programmable values
7 0 %
between 40 - 99% for this operating control.
LAG COMPRESSOR DIFFERENTIAL OFF %
D I F F E R E N T I A L
This control allows the operator to select the range over
which the lead and lag compressor load share equally.
Once the lag compressor is brought on and loads up to a
% equal to the lead compressor, if demand allows,
sharing of the load will continue as the load drops until a
programmed differential % below the LAG COMPRES-
SOR START POINT % is reached.
For example: If a LAG COMPRESSOR START POINT %
of 70% is selected, and a LAG COMPRESSOR
DIFFERENTIAL OFF % of 50% is selected, the lead and
lag compressor will both load share the % (FLA or Slide
Valve, whichever is programmed, see page 62) of the lead
compressor drops to 20% (70% - 50% = 20%). At this
point, the micro will unload only the lag compressor until
it reaches a point at which the micro determines it is fully
unloaded and load does not require its continued
operation and it will cycle off.
The larger the % differential programmed, the more
efficient the use of the cooler. For efficiency purposes,
50% is recommended. Cycling of the lag compressor will
also be minimized since equalized unloading operates
over a wider range.
A small % differential, will increase cycling and lower
efficiency
COMPRESSOR DIFFERENTIAL OFF % is selected with
a LAG COMPRESSOR START POINT % of 70%, the lag
will start when the lead % FLA reaches 70%. The lag
loading will be brought up to equalize the lead of 70% if
demand requires. Both compressors will increase in
loading proportionately as demand increases. When the
L A G
C O M P R E S S O R
O F F
slightly.
For
example:
YORK INTERNATIONAL
5 0 %
If
10%
LAG
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