Rev.01 03-27-17
Page 14 of 71
Compu-Aire
Cooling-Direct Expansion
Direct Expansion Cooling is designed to operate when the water flow switch senses that there is
no water flow. A digital signal is sent to the system from the controller to modulate the three (3)
way valve to cut water flow to the chilled water coil and the backup system direct expansion
cooling should start. The controller signals the compressor to start cooling. The outdoor
condenser gets energized as needed.
3.5
Air Side Economizer
Summary of Equipment
The Economizer Mixing Box is factory provided; however, it might be installed in the field by
others.
Sequence of Operation
The evaporator fan and a set of dampers for the economizers are energized depending on heating
or cooling demands. The controller commands the economizer box to bring either the minimum
amount of outside air or only outside air based on outside air temperature and humidity. The
controller determines whether the outdoor air temperature and humidity is suitable for
“economizer-cooling”. If the outdoor air is suitable, mechanical cooling shall be locked out by the
outdoor enthalpy control. The set of economizer damper actuators shall be energized, operating
the outdoor air and the return air dampers. The economizer damper actuators shall be regulated
to maintain proper discharge air temperature. When outdoor air is not suitable for “economizer-
cooling”, the Economizer shall be locked out and the outdoor air damper shall maintain minimum
position while the indoor fan is operating. Upon unit shutting down or power loss, the spring
return motor actuator shall close the outdoor air damper. The economizer shall be automatically
locked out during the heat mode (if applicable).
The Air Side Economizer box shall include: prewired modulating spring return damper actuators,
economizer control logic with compressor assist option, minimum outside air damper position
control, economizer control sensors including outdoor air temperature and humidity (enthalpy)
sensor, supply air and/or mixed air sensors. The supply air temperature sensor is used to maintain
the desire supply air temperature using DX and Economizer cooling together. The exhaust of
room air during economizer cooling mode shall be done by others in the field.
3.6
Water Side Economizer Energy miser (EM) Unit Using 2-Way Valves
Summary of Equipment
Energy Miser units are provided with Dual Cooling options. DX cooling using Compressor based
system and Economizer cooling using Water Side Economizer Coil.
The primary system shall be Direct Expansion Cooling. The free cooling Energy Miser coil is
provided together with the DX cooling coil. If the Water temperature drops below the Energy
Miser setpoint, the condenser water is diverted to the free cooling coil and DX cooling will be
programmed to either turned off or made available to assist based on demand.
