Catalogue

UV-PRC003-EN 17
Application Considerations
effectively applying an active humidity control system is to use reheat only when it is needed. This
requires the sensing of both the humidity and temperature in the occupied space.
When the space humidity falls below the upper limit, the system returns to the standard cooling
mode and again operates as a traditional constant-volume system.
Basic components of Trane’s reheat system, include a (1) classroom unit ventilator with a main
coil and an auxiliary coil downstream, (2) the Tracer ZN520 digital controller, and (3) two
sensors, one for the temperature and one for relative humidity. Sensors may be located in the
zone, or in the return air stream.
If after hours operation is required, the addition of a building automation system (BAS) such as
Tracer Summit™ is recommended to coordinate the chillers, pumps, boiler and unit ventilators. It
also assures proper operation of the exhaust fans.
Reheat may come from new energy (electric resistance heat or boilers fueled by gas or oil) or
recovered energy.
Recovered energy reheat refers to the process of salvaging or transferring energy from another
process within the facility. In this case, the recovered energy is the by-product of a cooling process
which would normally be rejected or wasted. A common example may include a plate and frame
heat exchanger in the condenser water loop of a water-cooled chiller system.
Automatic Fan-Speed Adjustment
Reducing the fan speed (supply airflow) at part-load conditions is a way to improve the coincidental
dehumidification performance of a unit ventilator. The Tracer ZN520 digital controller can be
configured to automatically reduce fan speed when the sensible load is decreased, Figure 12, p. 18.
At full load, the fan operates at high speed and the control valve’s flow is wide open. As the cooling
load decreases, the controller modulates the valve to throttle the rate of chilled water flow through
the coil. At some point, based on valve position, the unit controller switches the fan to low speed.
Less airflow means that colder supply air is needed to maintain the target space temperature. The
control valve opens allowing the coil to remove more moisture from the passing air stream.
The controller will also adjust the outside air damper to help properly ventilate the classroom at
the lower fan speed condition.
Face and Bypass Dampers
Face and bypass control is a common and accepted method of capacity control.
The face and bypass damper, consists of a single blade installed immediately upstream of the
cooling coil. The bypass is sized to have the same pressure drop as the cooling coil so that a
constant air quantity can be maintained at all times during system operation.
Bypass control maintains the dry bulb temperature in the space by modulating the amount of air
flowing through the cooling coil, thus varying the supply air temperature to the space. As the face
and bypass damper begins to close some of the outside/return air mix is diverted around the coil
and mixed with air coming off the coil to obtain a supply air temperature that is proportional to the
reduction in space load. Because the chilled water valve remains wide open, the portion of the air
passing through the coil is dehumidified further, improving part-load dehumidification.
However, face and pass control does not actively control space humidity. It still allows the space
humidity level to rise at part-load, often higher than desired.
For more information on various methods for improving dehumidification performance of unit
ventilator systems, refer to Trane “Dehumidification in HVAC Systems” application manual
SYS-APM004-EN.
Dehumidification
UV-PRC003-EN.book Page 17 Tuesday, June 4, 2013 8:59 PM