Emerson iCOM user manual SCR Reheat, Tight Mode

Operation

3.5.2SCR Reheat

SCR reheat is a type of electric reheat that provides tighter temperature control than staged electric reheat. SCR reheat capacity modulation is achieved by pulsing the reheat On and Off. Full capacity is achieved by constantly energizing the reheat. Units equipped with SCR reheat can operate in Tight or Standard mode. By default, cooling units with SCR reheat are factory-set to operate in Tight mode.

The mode of operation can be set by adjusting the SCR Control Type parameter (Service Menu, Set- points).

Tight Mode

In Tight mode, the compressors and reheats are operated at the same time to provide maximum tem- perature control. The temperature deadband is set to zero at the factory. In a cooling unit with SCR reheat and two single-step compressors, the first single-step compressor is started and full reheat capacity is provided at 0% calculated output from the Temperature Proportional Band. As the call for cooling increases from 0% to 100%, the reheat capacity is slowly reduced by pulsing the reheat. At 100% call for cooling, the reheat is deactivated and the second single-step compressor is started. As the call for cooling is reduced, the reheat capacity is slowly increased. When the call for cooling returns to 0%, the second single-step compressor is deactivated.

If the Temperature Proportional Band calculates a call for heating from 0% to -200%, the first sin- gle-step compressor remains activated and full reheat capacity is provided. Based on the factory default settings, the first single-step compressor is deactivated when the control reaches -200% call for heating. The compressor remains deactivated until the control calls for 0% heating. The compres- sor activation and deactivation points can be adjusted in the Service menu under Setpoints.

Figure 20 illustrates how a cooling unit with two single-step compressors and SCR reheat operates when the SCR Control Type is set to Tight mode.

NOTE

Some cooling units are not suited for a strict NO LOAD application. These cooling units require a minimal load in the space. Consult factory for verification.

Figure 20 Two single-step compressors with SCR reheat set to Tight mode

Temp Setpoint: 70°F												Cool 2
Proportional Band: 8°F
												On
Deadband: 0°F
								SCR On
							Cool 1 On
62	63	64	65	66	67	68	69	70	71	72	73	74
-200%			-100%				0%					100%
Heating Cool 1			Heating									Cooling
								Cool 2				SCR
Off								Off				Off

½ Proportional Band	½ Proportional Band	½ Proportional Band
	SCR Reheat On
	Cooling 1 On

Cooling 2 On

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