period two
Chilled-Water System Design
notes
Face-and-Bypass Damper Control
| bypass |
| damper |
airflow | |
face | |
damper | Figure 36 |
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:
nThe temperature of the water returning from the system varies as the cooling load varies.
nThe 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.
