Thermo King 098916, Maintenance Manual

Page: 109 / 196

Operating Theory
108
Continuous Temperature Control
Operation
Chill Loads (Controller Setpoint at -9.9 C
[14.1 F] and Above):
The controller regulates the compressor, Digital
Control valve and electric heaters based on a
Control Temperature Differential (see “General
Theory of Operation” in this chapter for more
detail). This means the unit operating mode can
not be predicted based only on the setpoint and
supply air temperature.
At setpoints of -9.9 C (14.1 F) and above, the
controller operates the unit on:
• Cool mode with Modulation.
• Controller energizes the vapor injection valve
continuously when the cool capacity is 100
percent.
• Heat mode (electric heaters pulse on and off
on a 60 second duty cycle).
• Defrost mode (electric heaters on, evaporator
fans off).
A.
Cool with Modulation (control temperature
differential is above setpoint)
B.
Heat (electric heaters pulse on and off on a 60
second duty cycle if the control temperature
differential is below setpoint.)
C. In-range (based on supply air temperature)
1. Decreasing Temperature
2. Setpoint
3. Increasing Temperature
Figure 57: Chill Load Control Sequence
(Setpoints at -9.9 C [14.1 F] and Above)
AXA0156