But what about defrost? It's that pesky thing! We have to worry about in refrigeration, since we're typically running all of our evaporators below freezing, so the simplest form is just time off. In other words, we'll shut off the refrigeration for a set amount of time, and this works good on medium temperature, when the temperature of the space is warmer than freezing, and you can just move that air across the coil to get the ice off the other strategy. That's common would be electric where we just use electric heaters to warm up the coils for medium or low temp to get the ice off, but what if we want to use gas to defrost? So as you can see here, this is a gas defrost rack. So the solenoid valve is the defrost valve.

Now you can either defrost with hot gas or you can defrost with liquid, which is also referred to as cool gas defrost. It really doesn't matter either way, you're bringing warmer fluid into the coil to defrost it. So, what's going to happen during defrost is you're going to shut off the suction line, to the bottom right of the screen, going down to the suction header and you're, going to open this solenoid and flow gas into the suction line. So that valve will be closed and then the gas is going to come back down the liquid line.

So this gas is going to flow into the suction line and back down the liquid line. As the rack is running, so there's your direction of flow and then it's going to go up since this valve is closed to the coil. Go through your coil, warm it up, and it's going to come back down the liquid line into the liquid header of the running rack. So, as you can see, this rack is kind of hard to see, but that's the liquid header in the middle there, the the gas header is on the left, so the liquid is actually going up and under which is just how the piping is routed on this Particular rack, so we're going to go to the other end of the rack and identify whether this is hot gas or cool gas defrost.

So we're going to trace that header back and we're gon na look down here. We're gon na see this pipe coming off of. It is in fact going to the discharge line right after the oil separator, so this is hot gas defrost, so you might be thinking to yourself at this point. How are we going to create flow back to the liquid header, while everything else is running and cooling and the way we're going to do that is with a differential valve now it can either be on the liquid line for either hot gas or cool gas defrost Or it can be on the discharge line for hot gas defrost only in this case it's on the liquid line, so i was just pointing at it a moment ago.

This valve right here this valve, is going to create a pressure differential between the receiver side of the system and the liquid header side of the system. So the liquid header will be at a lower pressure than the receiver, which means that the discharge line is definitely going to be at a higher pressure than the liquid header. So that is going to establish flow back from the high side of the system. To the not quite high side that we've made the liquid header now, typically, these run like a 20 to 25 psi differential when they're engaged, but that's all dependent on your customers or manufacturers specifications.

But that's the trickery that we're going to use in order to get flow for our defrost. While everything else is still running and cooling, that's pretty much it as far as defrost goes as far as the three main strategies you're gon na see, thanks for watching. Thanks for watching our video, if you enjoyed it and got something out of it, if you wouldn't mind hitting the thumbs up button to like the video subscribe to the channel and click, the notifications bell to be notified when new videos come out, hvac school is far More than a youtube channel, you can find out more by going to hvacrschool.com, which is our website and hub for all of our content, including tech tips, videos, podcasts and so much more. You can also subscribe to the podcast on any podcast app of your choosing.

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