McQuay OM 751
35
Description of Operation
ASHRAE Cycle II
The UVC supports ASHRAE Cycle II operation. The basis of ASHRAE Cycle II is to
maintain the required minimum amount of ventilation whenever possible, which can be
increased during normal operation for economizer cooling or CO
2
DCV control or reduced to
prevent excessively cold discharge air temperatures.
A discharge air temperature sensor is installed in all unit ventilators. If necessary, the
ASHRAE II control algorithm overrides room control and modifies the heating, ventilating,
and cooling functions (as available) to prevent the discharge air temperature from falling
below the VCLL set point.
Compressor Operation
The UVC is configured to operate the compressor as secondary (mechanical) cooling when the
economizer is available. When the economizer is not available and the compressor is
available, the UVC uses the compressor when cooling is required.
Compressor Envelope
The compressor envelope protects the compressor from adverse operating conditions that can
cause damage and or shortened compressor life by ending compressor operation if coil
temperatures exceed the defined operating envelope.
For self-contained units, the UVC is configured to monitor both the inside air/refrigerant and
outside air/refrigerant coil temperatures to prevent compressor operation under adverse
conditions.
For split-system units, the UVC is configured at the factory to only monitor the inside air/
refrigerant coil as part of the compressor envelope function.
Figure 23: Compressor envelope in self-contained units
Compressor Cooling Lockout
The UVC is configured to lockout compressor cooling when the OA temperature falls below
the Compressor Cooling Lockout set point (63.5°F/17.5°C). Below this point, only
economizer cooling is available.
2
1
Condenser temperature
(outdoor air coil)
E
vap
or
ator
tem
per
at
ur
e
(indoor
a
ir
c
oi
l)
DX cooling
1
Area where liquid slugging could occur.
2
Area where starving could occur.
Area for compressor operation envelope.
160˚F (71.1˚C)
40˚F (4.4˚C)
28˚F (-2.2˚C)
120˚F (48.9˚C)
