System Performance Microprocessor Controls; Control Type Response Proportional Control; Cooling; Multi-Step Cooling, Compressorized Direct Expansion (Dx) Systems - Liebert 8 Tons User Manual

4.0 S P
YSTEM ERFORMANCE
4.1

Control Type Response Proportional Control

The percent requirement is determined by the difference between the return air temperature and the
temperature setpoint. As the return air temperature rises above the temperature setpoint, the per-
cent cooling required increases proportionally (from 0 to 100%) over a temperature band equal to the
temperature sensitivity plus 1 degree Fahrenheit. The heating requirement is determined in a simi-
lar manner as the temperature decreases below the setpoint. With this control type the temperature
at which the room is controlled increases as the room load increases. At full load the room would be
controlled at a temperature equal to the setpoint plus the sensitivity.
4.2

Cooling

4.2.1

Multi-Step Cooling, Compressorized Direct Expansion (DX) Systems

The system will use the 3-ton (compressor #1) and 5-ton (compressor #2) compressors in an 8-ton sys-
tem. The control will determine the average cooling requirement updated every hour and select the
lead compressor or, the user can select the lead compressor through the "Setup Operation" menu. At
startup, the 3-ton compressor will be the lead compressor. The compressors will be staged on with hot
gas bypass energized at 50 and 100% cooling requirements. The compressors will turn off at 75% and
25% requirements.
4.2.2

Chilled-Water Cooling (8 Ton)

The chilled-water control valve is adjusted proportionally as the temperature control varies the
requirement for cooling from 0% to 100%. This is based on the full valve travel time programmed in
the "Setup Operation" menu.
4.2.3

GLYCOOL Cooling (8 Ton)

When GLYCOOL is available, the temperature control will calculate a total cooling requirement of
200%. Assuming that full GLYCOOL capacity is available, the GLYCOOL valve opens proportionally
as the requirement for cooling rises from 0 to 100%. If the call for cooling continues to increase, the
control will energize the compressors as needed to match the average cooling requirement. As long as
GLYCOOL is available, the control will leave the valve 100% open. If GLYCOOL cooling is not avail-
able, the temperature control will operate the compressors in the same manner as the Multi-Step
without GLYCOOL.
4.3

Reheat

4.3.1

Electric Reheat - Staged

For 2-stage electric reheat, they are activated when the temperature control calculates a requirement
of 50% and 100%. They are deactivated when the requirement decreases to 75% (reheat 2) and 25%
(reheat 1).
4.3.2

SCR Electric Reheat

The SCR (Silicon Controlled Rectifier) controller shall proportionally control the stainless steel
reheats to maintain the selected room temperature. The rapid cycling made possible by the SCR con-
troller provides precise temperature control, and the more constant element temperature extends
heater life. During operation of the SCR control, the compressor(s) operate(s) continuously. The heat-
ers are modulated to provide temperature control. If overcooling occurs, the compressor(s) will be
locked off when the temperature drops to the low temperature alarm. SCR reheats are 15 kW.
M
ICROPROCESSOR
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System Performance Microprocessor Controls

C
ONTROLS