Liebert 50 & 60Hz, 8 Tons user manual Control Type Response Proportional Control, Cooling, Reheat

System Performance Microprocessor Controls

4.0SYSTEM PERFORMANCE MICROPROCESSOR CONTROLS

4.1Control 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.

4.2Cooling

4.2.1Multi-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.

4.2.2Chilled-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.

4.2.3GLYCOOL 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.

4.3Reheat

4.3.1Electric 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).

4.3.2SCR 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.

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