York YMPA 0130 - Page 114

FORM NO.:6U6K-B01E-NB-EN
JOHNSON CONTROLS
114
LIQUID TEMP. CONTROL
The setpoint, when programmed for Return Liquid Temp.
Control or Leaving Liquid Temp. Control, is the temperature
the unit will control to within +/- the (control) cooling or
heating range. A Shift Value “D” is applied to dene the range.
Figure 36 should be utilized to aid in understanding the
following description of Liquid Temp. Control.
The acquisition interval of liquid temperatures is 20 seconds
and then the microprocessor will further calculate the demand
for loading or unloading. The difference
△
Ts and change rate
As shown in the gure, there’re roughly four regions to dene
the energy control. The X-axis,
△
Ts, is divided by Shift Value
“D” (programmable from 1 to 3 °C). And the Y-axis,
△
Tw, is
divided by Temp. Change Rate “N” (selectable from 0 to 6).
If the liquid temperature is far above the setpoint in cooling
mode or below the setpoint in heating mode, the control
parameters will locate in “Load” region, thus the lead
compressor on the lead system will be energized. Upon
energizing any compressor, the 40 second Anti-Coincidence
△
Tw are applied herein.
Cooling:
△
Ts=Twi –Tset,
△
Tw=Twi –Twi-1
Heating:
△
Ts=Tset –Twi,
△
Tw=Twi-1 –Twi
Where:
△
Tw is dened by °C/min;
Twi – Current liquid temperature detected;
Twi-1 – Liquid temperature in previous cycle;
Tset – Target liquid temperature;
△
Ts – Temperature deviation from Tset;
△
Tw – Rate of temperature change.
timer will be initiated to prevent multiple compressors from
turning on.
If after 40 seconds of run-time the liquid temperature is still not
reaching the Hold region, the compressor will increase its speed
if the lead compressor is an inverter one, or the next compressor
in sequence will be energized.
The Temp. Change Rate,
△
Tw, is introduced to prevent
the liquid temperature from changing too fast. If the liquid
△Ts
△
Tw
High
Low
Low
High
Hold
Load
Unload
Hold
FIGURE 43 – CAPACITY CONTROL MATRIX
SECTION 9 – UNIT OPERATION
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