American Standard UV05 manual Compressor Operation, Ashrae Cycle, Compressor Envelope

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 CO2 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

Evaporator temperature (indoor air coil)

DX cooling

160˚F (71.1˚C)

2

40˚F (4.4˚C)

1

28˚F (-2.2˚C) 120˚F (48.9˚C) Condenser temperature

(outdoor air coil)

Area for compressor operation envelope.

1Area where liquid slugging could occur.

2Area where starving could occur.

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.

McQuay OM 751 35