York Millennium YCAS 310, Installation Operation & Maintenance

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YORK INTERNATIONAL
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compressor groups will occur until each has reached
100% load, if demand allows. A setting of 40 - 60% is
recommended for the Lag Panel Start Point % for most
applications.
To program the Lag Panel Start Point %, key in the
required setting and press the Enter key to store the
value into memory and scroll to the next display.
NOTE: Care is needed not to set the Lag Panel Start
Point % too high. If this occurs the lead group
of compressors will attempt to chill the liquid
flowing through their evaporator below the Low
Limit Water Temperature (See Section 6) to
compensate for the unchilled flow through the
other evaporator. The mixed chilled liquid tem-
perature will remain above the Control Range
until the lag group of compressors start.
NOTE: Situations may occur in low ambient conditions,
when control is based on %FLA, where cur-
rent drawn at full load (100% slide valve) will
be quite low. The actual operating current for a
fully loaded compressor may be as low as 65 -
85% of the motor FLA at ambients below 95°F.
If due to system conditions, the lead compres-
sor does not reach the programmed %, even
though the lead system is fully loaded, the lag
compressor will start regardless of the pro-
grammed % after 5 minutes of operation when-
ever the lead compressor can not bring the
LWT to within 2.0°F of the high end of the Con-
trol Range (CR). Once the lag compressor is
started, equalized loading and unloading of the
two compressor will occur. This assures that
the chiller will fully load and maintain chilled
liquid temperature.
Lag Panel Differential Off (YDAS Only)
On falling demand for cooling the Lag Panel Differen-
tial Off % is the overall load level for both groups of
compressors below which equal load sharing will be
stopped. This load level is calculated as the Lag Panel
Start Point % minus the Lag Panel Differential Off %.
Below this level of loading, the lead compressor group
(SYS 1 and 2) maintains its loading while the lag com-
pressor group (SYS 3 and 4) continues to unload, then,
if necessary, shut down. The lead group load level will
then be adjusted to maintain correct liquid tempera-
ture control.
The micro will accept a range of programmable values
between 0 - 50% for this control, provided the Lag Panel
Start Point % minus the Lag Panel Differential Off % is
not less than 20%. A setting of 20 - 30% is recom-
mended for the Lag Panel Differential Off % for most
applications.
To program the Lag Panel Differential %, key in the
required setting and press the Enter key to store the
value into memory and scroll to the next display.
Lag Compressor Start Point
On rising demand for cooling, the Lag Compressor Start
Point % is the load on the lead compressor (% FLA or
% Slide Valve, whichever is programmed, see Load
Sharing Basis later in this section) at which a lag com-
pressor is allowed to start.
The micro will accept values between 40 - 99% for this
control. Selecting a Lag Compressor Start Point % of
99% requires the lead compressor to be fully loaded
before the lag compressor is allowed to start. This is
desirable in order to minimize compressor cycling if very
low or very variable loads are expected. Once the lag
compressor has started, the loading of lead and lag
compressors will be adjusted to share the cooling load
evenly in order to maximize the efficiency of the chiller.
Selecting a Lag Compressor Start Point % of less than
99% increases the efficiency of chiller over a wider range
of operation. At lower percentages, the lag compressor
is started and loading is equalized between the lead and
lag compressors at lower overall cooling loads. This
makes more efficient use of the heat exchangers than
running one compressor highly loaded with the other
idle.
If a lower % is desired for efficiency purposes, a value of
60% 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 required setting and press the Enter key to store
the value into memory and scroll to the next display.
NOTE: Situations may occur in low ambient conditions
when control is based on %FLA, where cur-
rent drawn at full load (100% slide valve) will
be quite low. The actual operating current for a
fully loaded compressor may be as low as 65 -
85% of the motor FLA at ambients below 95°F.
If due to the system conditions the lead com-
pressor group does not reach the programmed
% even though the lead group is fully loaded,
the lag group will start regardless of the pro-
grammed % after 10 minutes of operation
whenever the lead group can not bring the LWT
below the high end of the Control Range (CR).
Once the lag group started, equalizing loading
L A G P A N E L
D I F F E R E N T I A L O F F 3 0 %
L A G C O M P R E S S O R
S T A R T P O I N T 7 0 %