How Do VFDs Save Energy (Video)?

By Craig Hartman on 27th Jun 2014

Video Transcript

Today we are going to talk about how VFDs save energy.

I’d like to refer to the diagram on the screen. This is a diagram that shows a typical fan application. Let’s say in a commercial building, the bottom line is called the system curve. This represents how much pressure it takes to circulate a certain amount of air through the building. If the building is dark, there is no one there, maybe you don’t need any flow and therefore you don’t need any pressure. 0% pressure uses 0% flow. On the other hand, if you need a 100% flow, then you’re going to need 100% pressure. But the fascinating thing is what that curve looks like in between. You can see that if you only need 50% flow, you’ll only need 25% pressure. If you need 70% flow, you only need 50% pressure. And this is wonderful for giving us energy savings.

Let’s assume that we need 75% flow, just as an example. There are two ways to do this: (1) we can use dampers. Dampers go in front of the fan and those dampers basically restrict the air flow. When you put dampers in front of the fan, the pressure goes up a little bit because you are restricting the air flow out of that fan. So at 75% flow, we ended up requiring 107% pressure. As you can see, a good percentage of that pressure is dropped across those dampers and then ends up being wasted pressure. So you can see the amount that is wasted, and you can see the amount that is used. On the other hand, if we use a variable frequency drive, we can simply slow that fan down. We only need 56% pressure to get 75% flow rate. So we slow down the fan down to 75% speed, and that will give us just the 56% pressure that we require – No waste.

Next of all, let’s take a look at a pump application. The pump application works pretty much the same except for the fact that we have what’s called static head. Assume that you have a reservoir in a big tank that is used for water supply to a city. It may take a certain amount of pressure before you get any water flow up into the tank. So in this case, I just picked a value of 50% as a nice convenient point. We don’t get any water flow until we have 50% pressure. And then everything else looks just like the fan curve in the preceding graph. Now in this case, we’re using a throttling valve. So we’d use either a throttling valve or we’d use a variable frequency drive.

Let me tell you about a recent compression application that we did in the eastern part of the United States. It was a bio gas situation, in which we were taking gas from a land field, and that gas was then put into a compressor and then used to drive turbines to create electricity. In this case, the compressor was operated at full speed and they were wasting a great deal of power. We installed a 2500 Horse Power, 4000 Volts variable frequency drive. That drive then was used to slow down the compressor to produce only the amount of pressure that was actually required by the turbines at any given time. In this way, they were able to achieve an excess of $100,000 dollars per year in energy savings. So they actually paid off the drive in energy savings. This is actually very common to see an application where you pay off the cost of the variable frequency drive in less than a year with the energy savings.

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