System Performance with Advanced Microprocessor Controls
4.1.4Heating Operation
Electric Reheat
The two heat stages are activated when the temperature control calculates a requirement of 50% and 100%, respectively. Each stage is deactivated when the heat requirement is 25% less than the activa- tion point.
Hot Water Reheat
The solenoid valve opens when the requirement for heating is 100% and closes when the requirement drops below 50%.
SCR Electric Reheat (Requires Special Control Software)
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 improves heater life. During operation of the SCR control, the compressor operates continuously. The heaters are modulated to provide temperature control.
On units equipped with SCR Reheats, unit operation can be controlled even further with DIP switch #7 on the control board. With DIP switch #7 in the “OFF” position, or “Tight Control,” the compressor is locked on and the SCR Reheats are modulated for very tight temperature control. In the event of a malfunction of the SCR Reheats, the compressor will be turned off if the temperature drops to the Low Temperature Alarm Setpoint. With DIP switch #7 in the “ON” position or “Standard Control,” the compressor will cycle on and off as required to satisfy the requirements for cooling and dehumidi- fication in a similar manner as a standard unit without SCR Reheats, i.e., Temperature Setpoint PLUS Temperature Sensitivity PLUS 1°F. The SCR Reheats are modulated from 0% to 100% as required to satisfy the requirements for heating over a control band from the Temperature Setpoint MINUS the Temperature Sensitivity MINUS 1°F.
4.2Humidity Control
4.2.1Dehumidification/Humidification Required, in Percent
The humidity control program for the Advanced Microprocessor is based on a calculated % require- ment for dehumidification/ humidification.
4.2.2Response to Control Types Proportional Control
The % requirement is determined only by the difference between the return air humidity and the humidity setpoint. As the return air humidity rises above the humidity setpoint, the % dehumidifica- tion required increases proportionally from 0 to 100% over a humidity band equal to the humidity sensitivity setting. The converse is true for % humidification requirement.
PID Control (Chilled Water or SCR Reheats only)
If PID control is selected, humidity is controlled in the proportional mode with the sensitivity band being determined by the humidity sensitivity setpoint.
Intelligent Control (Chilled Water only)
If intelligent control is selected, the return air humidity is controlled at or near the humidity setpoint. The % dehumidification/humidification required is calculated based on a set of logical “rules” that simulate the actions of an expert human operator (see 4.3.3 - Intelligent Control (Chilled Water only)).
4.2.3Dehumidification Operation
Dehumidification with the standard configuration is accomplished by operating the compressor with- out hot gas bypass active. If system has part coil, dehumidification is accomplished by using only part coil. If the installation has a very light sensible load, dehumidification will be inhibited at the point at which 125% heating would be required to prevent overcooling. Dehumidification would be enabled when the temperature increases to the point where only 50% heating is required.
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