186 - HVACR Economizer Certificate Course
186-02: Applied Economizers
Page 8
Building Pressurization continued:
On larger systems and where there are larger return air runs, a power exhaust system would be used in place of a barometric damper. The power exhaust system pulls air from the return air duct and expels it outside. It consists of a combination of dampers with some type of fan motor.
Power exhaust fan types could use either centrifugal or propeller blades and could be either belt drive or direct drive.
There are various control methods used with power exhaust systems.
1 – Power exhaust is controlled through the economizer control and is based on economizer position. Example: You might want the power exhaust to be energized when the damper is 25% open or 50% open. This control also has an adjustment so that you can set it up for your particular application. Click on this link to learn more about Power Exhaust systems.
Economizer Controlled Power Exhaust
2 – Power exhaust is controlled through the DDC controls on the rooftop unit. Click on the DDC Power Exhaust link below, to see the DDC Power Exhaust example we are talking about in this section. This is manufacturer specific and for this particular one the power exhaust is also controlled by the economizer damper position, but it is configured through the DDC controller. In this example there are two exhaust fans that are controlled through one relay so both fans will be energized at the same time. You could also see the economizer being controlled through a building automation system via DDC.
These are also available as a two-stage power exhaust. In this case you configure one fan to start at a given economizer damper position and the second fan to start at a different (higher) damper position. This is accomplished by controlling the two exhaust fans through two relays. Take a look through this link
3 – The power exhaust is energized through a mechanical end switch installed on the damper actuator. When the actuator reaches a pre-determined position the end switch is closed energizing a relay to bring on the exhaust fan.
4 – Modulating power exhaust. These are typically used on variable air volume (VAV) rooftop units and consist of one or more variable speed fans that are controlled by a pressure transducer that is monitoring building pressure.
These are a few examples of the many power exhaust options that we will find in the field. I encourage you to take a look at the web links provided for Micrometl and Canfab who are two suppliers of economizers and power exhaust systems for various HVAC manufacturers.
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