Catalogue

16 UV-PRC003-EN
Application Considerations
Constant Volume System
A typical unit ventilator is a constant-volume, variable-temperature device. It uses a constant fan
speed and modulates water flow through a chilled water coil to maintain the dry bulb temperature
in the space based off of a setpoint. Outdoor air is introduced at the back of the unit ventilator, and
distributed with the supply air. At design cooling load conditions, a system controlled in this
manner typically has a leaving air temperature that is cold enough (and, therefore dry enough) to
sufficiently dehumidify the space, but its ability to dehumidify can decrease significantly at part-
load conditions.
When the sensible load in the space decreases (part-load), the constant-volume system responds
by raising the dry-bulb temperature of the supply air. In a chilled water unit ventilator, this is
accomplished by modulating a valve to reduce the rate at which water flows through the coil.
Figure 11 shows how this affects the supply air leaving the coil—the warmer coil surface that results
from less water flow provides less sensible cooling (raising the supply air temperature) and
removes less moisture from the passing air stream.
The sensible cooling capacity of a constant volume system decreases to match the smaller sensible
cooling load. Any latent cooling (dehumidification) capacity is purely coincidental, whether the
cooling-coil medium is chilled water or refrigerant. As the load diminishes, the system delivers
even warmer supply air. Some dehumidification can occur in this situation, but only if the sensible
load is high enough.
Some designers attack this problem by oversizing the unit ventilators. This does not solve the
problem; in fact, it can make the situation worse. Increasing the capacity of the unit ventilator may
also require increasing the supply airflow. A higher-than-necessary supply airflow results in
warmer supply air and, in non arid climates, less dehumidification. It’s important to understand,
this is not just a unit, coil, or fan-sizing challenge. Rather, it’s an issue of properly controlling the
system in a manner provides sufficient dehumidification at all operating conditions. Proper
dehumidification with terminal units is a matter of proper control.
Active Humidity Control
A common method used to address this part-load humidity control challenge in a constant-volume
system is active humidity control through supply air tempering (reheat). Active humidity control
involves monitoring and controlling both the dry bulb temperature and humidity in the occupied
space. Whenever the space humidity is below the preset upper limit (typically 60 percent relative
humidity), the system operates just like a normal constant-volume system. However, if the space
humidity reaches or exceeds the upper limit, the cooling coil control valve is driven open regardless
of the need for sensible cooling in the space. The coil over-cools the air, increasing the
dehumidification capacity of the system.
With this control sequence, a reheat coil is placed downstream of the cooling coil to temper (reheat)
the cold, dry air leaving the cooling coil in order to avoid over-cooling the space. The key to cost-
Figure 11. Part-load dehumidification with modulated chilled water
Dehumidification
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