Coward Environmental Systems, Inc.

Requirement:

Maintain the environment in the room around a long process tunnel at 65 degree dry bulb temperature at relative humidity not to exceed 50 degrees on a year round basis.

Solution:

Employ a "split system" with a single air handling unit that contains the necessary high efficiency filtration, multi circuit direct expansion cooling coil, steam type reheat coil connected to a multi compressor air cooled condensing unit.

The unique feature of this system involves the unloading capability regardless of the cooling load.

The dew point temperature leaving a multi circuited cooling coil is electronically controlled through "Compensating Evaporator Pressure Regulating Valves", the signal from which cycles the coil circuit liquid line solenoid valves in numerical order.

The "High Side" versus "Low Side" capacity is maintained on an equal basis by interlocking the liquid line solenoid valves with compressor unloading.

Loss of control and short cycling during very light loads is prevented with the use of modulating hot gas bypass during the last step of control.

The steam reheat capacity is modulated by automatically mixing air and steam before its delivery to the steam type heating coil. Automatic supply and removal of the blending air prevents damaging "water hammer".

By: Kenneth Wicks

ASHRAE Fellow

5/3/99