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Greenheck. Building Value in Air.
Welcome to Greenheck’s Virtual Lab. In controlling of laboratory exhaust fan systems the main control set point is typically duct static pressure. This is typical of most lab fan systems that utilize laboratory exhaust valves above each hood.
The two main ways to control this are bypass damper, and fan speed. As we can see on the screen the fan is actually staying at constant speed. As we open the lab hoods the bypass damper is modulating in which to maintain duct static pressure.
If we take a look at this on a flow chart, we can see that we can adjust the fan speed and static pressure, but we only adjust one at a time. So typically you'll adjust fan speed first and then, if the fan is at its minimum, maintaining outlet velocity, then we can use the bypass damper.
Looking at this on a fan curve, we can visually see where our fan speed changes and where our bypass damper can adjust.
Redundancy is also very common in these fan systems. There are two main types of redundancy that can be selected. N+1, and N-1 redundancy. Think of N as your normal operation and you're either going to plus one add a fan, or minus one, where the system is designed such that you can lose a fan and it can maintain performance, similar to fan arrays in a built-up air handler.
If we look at normal and redundant operation the chart here shows a depiction of what happens in normal and emergency mode with each type of control system.
We can take this to an example. We're going to look at a fan system that's running 12,000 cfm, one-and-a-half inches of static pressure.
We can see that with a three-fan system running N-1, we're going to run slightly smaller fans that are going to be running quieter in normal operation, but louder in emergency.
In N+1, we have the same operation, no matter what if we're in normal or emergency mode because we still have that one single fan operating.
