FORM 201.10-NM4
107
YORK INTERNATIONAL
perature, called the Panel Target Temperature. This is
performed by programing the acceptable deviation or
range of leaving temperatures above and below both
Target Temperatures. The tops and bottoms of the
ranges (Control Range, CR) are called the High and
Low Temperature Limits. The Target Temperatures are
midway between these programmed limits.
The Mixed Temperature Limits are programmed first
based on the desired Target Temperature for the chiller
application. For example, if the desired Mixed Target
Temperature is 46.0°F and the allowable deviation from
the temperature is +/ 2.0°F, then the programmed ac-
ceptable Mixed Temperature Low and High Limits are
44.0°F and 48.0°F respectively. This can be viewed pic-
torially as follows:
It is normal, when cooling demand is low, for only one
compressor group to be running and chilling liquid flow
through its evaporator while the other compressor group
is off. In this case, the micro will cause the liquid flow-
ing through the operating evaporator to be overcooled
so that when it mixes with flow through the non-operat-
ing evaporator, the resultant Mixed leaving chilled liq-
uid temperature is within the desired limits.
To allow
this to occur, individual control panel Control
Ranges must be programmed at slightly lower val-
ues than the Mixed leaving chilled liquid Control
Range. 2.0 - 4.0°F is recommended. This can be
viewed pictorially as follows:
Mixed Temperature Control
A mixed leaving liquid temperature control algorithm
sends signals to each compressor group to load, main-
tain constant load (balance), or unload compressors
based on the programmed Mixed High and Low Tem-
perature as follows:
Load if temperature is above High Limit
Balance if temperature is within the Control Range
Unload if temperature is below the Low Limit
The signal to load, balance, or unload is sent to the indi-
vidual compressor groups based on the calculated total
load of the two compressors in each group as follows:
If no compressors are running, all safeties and run
permissives are satisfied, and the mixing leaving liquid
temperature rises above the upper limit of the Mixed
Control Range, the Master compressor group lead com-
pressor will start and load. If demand continues to rise,
the lag compressor will start and load. Load signals will
be sent to the lead compressor group until cooling de-
mand is matched.
If demand continues to rise and the total load of the
lead compressor group reaches a programmed percent-
age of full load (the Lag Panel Start Point), balance sig-
nals will be sent to the lead group and load signals will
be sent to the lag group to start and match the cooling
demand.
If the lag compressor group reaches the same total
percentage of full load as the lead group, and demand
continues to rise, equalized loading of the two groups
will take place.
NOTE: Under some conditions, the lag compressor
group may be sent load signals before the lead
compressor reaches the required percentage
of full load, if chilled liquid temperature is not
brought within the control range after 10 min-
utes of run time. For more details, see Section
8.2 Lag Panel Start Point.
If the chiller is near full load and cooling demand falls,
the compressor groups are unloaded equally until they
reach the programmed percentage of full load see
Section 8.2 Lag Panel Differential Off.
If demand continues to fall, the lead group will be main-
tained at constant load and the lag group will continue
to be sent unload signals. If the lag compressor group
shuts down and demand continues to fall, the lead group
will then be sent unload signals until cooling demand is
matched or the lead group shuts down.
Mixed High Limit Water Temperature
Mixed Low Limit Water Temperature
MIXED
CONTROL
RANGE
➤
➤
48.0°F
46.0° F
44.0°F
MIXED
TARGET
temp.
(User
acceptable
leaving chilled
liquid
operating
range)
48.0°F
47.0°F
46.0°F
45.0°F
44.0°F
43.0°F
42.0°F
Mixed High Limit
Panel High Limit
Panel Low Limit
PANEL
TARGET
Temp.
N/Z
PANEL
CONTROL
RANGE
MIXED
TARGET
temp.
Mixed
Low
Limit
MIXED
CONTROL
RANGE
Содержание Millennium YDAS 410
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