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TRG-TRC016-EN 31
period two
Chilled-Water System Design
notes
The final method of modulating the coil capacity to match the cooling load is
through the use of face-and-bypass dampers. A linked set of dampers varies
the amount of air flowing through the coil by diverting the excess air around
the coil. As the cooling load decreases, the face damper closes, reducing the
airflow through the coil and reducing its capacity. At the same time, the linked
bypass damper opens, allowing more air to bypass around the coil. A unique
characteristic of this method of load-terminal control is that the coil is allowed
to “run wild,” meaning that the water flow through the coil is constant.
Similar to the three-way valve, systems that use face-and-bypass dampers have
the following characteristics:
n The temperature of the water returning from the system varies as the
cooling load varies.
n The water flow through each load terminal and, therefore, pump energy are
constant at all load conditions.
An advantage of face-and-bypass control with a “wild” cooling coil is that it can
better dehumidifiy of the conditioned air when compared to varying the water
flow through the coil. As the airflow through the coil decreases at part-load
conditions, assuming that the temperature of the water entering the coil is
constant, the temperature of the air leaving the coil also decreases. That is,
the air is cooled further and more moisture is removed.
Face-and-Bypass Damper Control
bypass
bypass
damper
damper
face
face
damper
damper
airflow
airflow
Figure 36