Figure 22 Two Single-Step Compressors With Scr Reheat Set To Tight Mode - Emerson Liebert Challenger 3000 User Manual

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 22 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 22 Two single-step compressors with SCR reheat set to Tight mode

Temp Setpoint: 70°F
Proportional Band: 8°F
Deadband: 0°F
62
-200%
Heating
Cool 1
Off
½ Proportional Band
Standard Mode
In Standard mode, the SCR reheat operates only when the Temperature Proportional Band calls for
heating. SCR reheat output is adjusted proportionally as the Temperature Proportional Band varies
the requirement for heating from 0% to -100%. Compressors operate only when there is a call for cool-
ing as described in 3.2.2 - Compressor Control.
Figure 23 illustrates how SCR reheat operates when SCR Control Type is set to Standard mode.
63 64 65 66
-100 %
Heating
½ Proportional Band
SCR On
Cool 1 On
67 68 69 70
0%
Cool 2
Off
½ Proportional Band
SCR Reheat On
Cooling 1 On
26
Cool 2
On
71 72 73 74
SCR
Off
Cooling 2 On
Operation
100%
Cooling