Hey this is the HVAC school podcast in this short episode is on low ambient cooling, but before we get into that, I want to thank our sponsors field piece. They make the JobLink probes with the best-in-class Bluetooth range. If you haven't taken a look at the JobLink probes, you can find them at true tech tools. Comm use the offer code gets cooled for a great discount we've been using them or making a few videos with them, showing you their use and some different applications.

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You can find out more by going to speed clean, that's speed, clean calm and always carrier carrier Tom, our first sponsor and sponsor that stuck with us all this time. Thank you to carrier all right. So today we are talking about low ambient cooling, so specifically low ambient cooling, we're not talking about running heat off in heat mode. That's a different thing: we've talked about that on other short episodes and other long episodes, but we were looking over.

We were doing a read through with our installers of some of the install manuals for some of the heat pumps that we install and straight cool units, and one thing that came up in both of them was low ambient operation. It's one of the things of which, if you're meaning to operate and piece of air conditioning equipment in this case carrier, I think 25 hce series. There says that you should not operate them below 55 degrees, outdoor ambient and, generally speaking, any time you get below 65 degrees, it can become a problem in certain types of equipment, and so you have to use certain accessories if you're trying to operate down below 50 Degrees 55 degrees in that particular piece of equipment and obviously different units vary. You will have some units that have built-in controls that will help deal with low ambient, but the first question that came up in the class one of the technicians asked well.

Why would you want low ambient cooling? What's the application and there's a couple key applications and commercial there's many applications in commercial. You generate a lot more heat inside the building with equipment that can be things like computers. Server rooms generate a lot of heat kitchen equipment. A lot of cases in commercial applications, you're gon na, have a lot more occupancy density.

Again. On average people produce about 500 BTUs per hour per person is sort of an average. I think it's actually like 470 or something like that, but right around 500 is what we'll generally think of for people who are doing just very simple basic tasks, but it can be a lot more than that if people are moving around Jim. Something like that, so you tend to have these different heat gains, so equipment, electronics, people, and so in many cases you will need to still cool a space.

Even when the outdoor ambient temperatures, the outdoor temperatures are a little bit lower, and this is where, if you think of economizers in certain markets, are used a lot where you bring in free outdoor cooling. But in a lot of cases you have equipment that can't really utilize an economizer, and so sometimes you just have to run the air-conditioning running in cool mode when the ambient temperature is low. Also in some residential situations, we had a customer who had a bunch of terrariums inside their house, so these are basically aquariums for reptiles, and so they have all these heat lamps and things and there's all this internal heat load and they were trying to operate their Ac equipment below the recommended outdoor temperatures, and so when you run into these circumstances where people are dead, set on running cooling equipment when it's cooler outside either by necessity here by desire, you've got to do some things to the equipment if it's just typical AC equipment And we also run into it, obviously in refrigeration, market, refrigeration or commercial refrigeration. Mechanics of all stripes are used to having to operate their equipment in a much wider range of conditions, because let's say that you have a remote condenser on a restaurant in a state that has snow and ice and all that stuff.

Unlike what we have here in Florida, you still have to run the freezer inside the restaurant, regardless of whether it's cold outside or not, and so you have to have controls that allow it to function in all of those different circumstances. So let's first talk about what happens when the outdoor ambient temperature goes down, because no the goal of the condenser, the condenser, we call the heat rejector in my basics class he's the heat, rejecter evaporators, the heat absorber. So you got to absorb heat into the evaporator. You got to reject it and the condenser in the way we reject and absorb heat, is through the condensing and boiling of refrigerant by the control of its pressures.

So we have to have a pressure differential between the high side and the low side. Specifically one way of saying this is we have to have a pressure drop across the metering device? One kind of old-school way of saying it is: you had to have a hundred PSI pressure drop across the metering device and, of course, that's just a rule of thumb and it varies based on the refrigerant and the conditions and a lot of other things. So that's really, you can kind of throw that out the window, but depending on the application, you of course need to have some pressure drop and, depending on the type of system you may have as low as maybe 50 psi. In some cases, and in some cases you may have higher than that, but regardless you got to have a differential between the high side and low side of the system, it doesn't work, otherwise you lose control and so in order to operate during low ambient conditions.

Because we're rejecting heat to the outside, when you have more heat rejection because of lower temperature conditions, what occurs is is you're pressure drops, that's the result, so your head pressure drops and also your condensing temperature drops mister. Now you end up with relatively or very low pressures that then don't result in the pressure drop that you need, because using refrigeration as an example, we have a pretty fixed evaporator temperature that we need in refrigeration when that system is operating. You really need that evaporator temperature to be a set temperature, and so you take the design box temperature. You subtract the design temperature difference and that evaporator temperature needs to be whatever it is.

So as an example, let's say you have a box that you want to have at will say negative 10, we'll just use that as an example and if you have a design temperature difference of 10 degrees. That means that you need to have a negative 20 degree of a protic. Well, you can't really fluctuate that too much. Otherwise, it's not going to cool the product properly in air-conditioning, we're used to having our suction temperature, or, I should say, our evaporator temperature float.

Quite a bit more because we're used to it floating with that and changes in the indoor temperature, and so we can deal with a little bit of floating in our section pressure. But still we have a very key limitation in air condition. If you think of air conditioning is high, temp refrigeration, it's the same basic thing. We have this limitation in that we don't have a defrost cycle for the evaporator coil, so you cannot run a lower than 32 degree of a protocol.

You just can't do it. If you do that, then eventually you're gon na build up ice and the coils gon na freeze over and you're gon na lose your airflow and it's not gon na work. So you can have some fluctuation in our evaporator temperature, otherwise known as our suction saturation and they're, not always exactly the same. In fact, somebody pointed out to me the other day that there's a difference between the skin temperature of the evaporator tubing and fins and the actual freezer and inside the evaporator.

And, of course, there's definitely truth to that. And it varies. But as far as we're concerned, when we say evaporator temperature we're generally talking about our suction saturation, we measure our suction pressure. We convert that to the saturated temperature depending on the refrigerant, and we call that our evaporator temperature, so for all intents and purposes, that's the way, we're going to think about that, and so we allow that to float a little bit in air conditioning.

But again you have to maintain above that 32 degrees. Otherwise, you're gon na have problems. That's the point, so we have to keep our head pressure up to a certain degree. That's what it comes down to.

So, ultimately, we have to choose how we going to keep our head pressure up and how we going to prevent our oil from freezing in air-conditioning during low temperature application. So there's some common way, so I'm just gon na go over them very quickly. First is which has nothing to do with the head pressures just to prevent the freezing, which is sort of the symptom. It's like taking a ibuprofen that is to install some form of freeze protection.

A common form of freeze protection is a clicks on type device. Call them a free stat. We mounted on a suction line right on the outlet of the evaporator coil, and then, when that suction line gets to a temperature, that's equates to freezing. Then it shuts off the condenser.

So you break! Why, with that, so you break the yellow circuit shuts off the condenser until the temperature rises enough, that it makes again and usually there's something like a 5 to 10 degrees swing in it. I'm not looking at one right now, but there's a swing there so that it before it makes again that of a protocol has to warm up significantly. So that's one way to prevent the freezing, which is sort of a symptom. But how do you deal with the actual cause? Well, the way they deal with the cause is by getting our head pressure up.

We essentially have three different ways that we get our head pressure up in a regular operational manner and it's all based on again getting our head pressure up in the liquid line. That's what we care about, because, again, you can get your head pressure up in the discharge line. It has to translate to a pressure drop at the metering device because that's where it matters, it doesn't matter what it is in the discharge line. It matters what it is in the liquid line before it gets that metering device, so you can have that required.

Pressure drop. The first two strategies rely on affecting the condenser fan in some way and the first one is a very simple one. It's called a fan. Cycling control fan, cycling control, you just set it up to turn the fan on and off and usually it's a pressure activated device.

I imagine you could do it with temperature in some way, but we do it in a pressure activated manner. So we try to maintain a fixed pressure in that condenser, so you kind of set a minimum that the condensing pressure is going to be, and so what will happen is once it drops below that it's going to shut off the condenser fan it'll. Let it get up to a point and then it will turn it back on again and it'll just keep going on and off on and off on and off until you are low enough ambient conditions that, potentially speaking, you would have a situation where the condenser fan Would just stay off and that does happen in some circumstances. So that's fan cycling.

Challenges fan cycling. Is it's sort of jarring to the system because of these constantly changing pressures? And so your expansion valves are going to be hunting and you're gon na have some challenges associated with that. It's especially frowned upon in refrigeration, because gon na throw around your evaporator temperatures, they're gon na be fluctuating. It's not gon na be good for the product in air-conditioning and residential applications, especially its popular you'll, see it in commercial applications as well in air-conditioning, where you just have fan cycling controls.

One way that works better is when you have multiple condenser fans. So once you have a rtu or something a large split that has multiple condenser fans, then you can fan cycle one or two of them and still have some others or you can stage the fan cycling where you have to go off first and then the other Two or whatever the case may be, and so that way you don't have quite the massive fluctuations. You still have some fluctuations, but it won't be so extreme as shutting off the condenser fan completely. The next way is where you're modulating your condenser fan.

Speed carrier calls this a motor master control and, when you're modulating a typical motor by essentially just decreasing the voltage going to it, that's how they do it. That's not particularly good for the motor so carrier. For example, when you use a motor master control, they require you to put in a ball-bearing motor, because the ball bearing motor is gon na withstand those changes a little better than a typical sleeve bearing motor. It's gon na be much better on the system.

From a standpoint of stability, because it's gon na modulate the speed of that fan motor still, not my favorite and it's not a very electrically efficient way of doing it. But it does function in certain cases. You're generally going to see that on smaller equipment and then the more common one that we see now in grocery refrigeration and a lot of other applications is using a variable frequency drive. If you use a variable frequency drive with a three-phase motor, that's designed to be used with a variable frequency drive, then you can really vary the speed of those motors to greatly impact your pressure to get it really where you want it and, generally speaking, the goal Still in refrigeration, air conditioning is still to allow the head pressure to go as low as you can, because lower head pressure equates to lower compression ratio which equals better efficiency.

So we don't want to drive head pressure up any more than we need to. Our goal. Is to use what we've got and not have to mess around with the equipment and change how its functioning? Unless you get to a point where it's not going to function properly, and so the goal is to allow the head pressure to drop, the older strategies would be to hold it all the way up, at 105 degree condensing temperature, something or condensing temperature that equated to A day or something like 90 95 degree day nowadays, we generally allow that head pressure to drop to the point where it's still functional but allows it to drop down where you do take advantage of some of those lower compression ratios, especially in refrigeration. Like I said, all these things are more critical generally in refrigeration and also they tend to be operated in a much wider range of conditions.

You might have a negative 10 degree day and you still have to operate your refrigeration freezers in a grocery store. For example, the next way that's used - and this is also very common in refrigeration - would be what we call a headmaster, so motor master and headmaster two different things. Motor master varies the speed of a motor. I think that's a carrier term.

I don't know if others use that term, but headmaster is essentially just a valve that allows discharge gas to enter what we call the drop leg, which is the leg between the condenser to line in between the condenser and the receiver in large grocery store, refrigeration, you're Gon na have your condensers on top of your rack house or on top of a building and then you're, just gon na go down a drop leg which is a liquid line, but it's the liquid line between the condensers and the receiver. The receivers generally gon na be in the motor room down by the compressors, and so that's why they call it a drop leg because it drops down from the condensers to the receiver, and you would have this headmaster control and this headmaster control would open up and Allow discharge gas to enter the liquid line in order to drive up that liquid line pressure only when necessary. That is a way of helping to increase your head pressure, especially in a more extreme situations and again you're, really only gon na see Headmaster's in refrigeration applications, but I figured while we're talking about low emitting conditions. It's worth mentioning and then finally, you don't leave this installed, but if you're testing a system where you want to check a system in cool mode when it's really cold outside, so you have a straight cool system.

You'll want to check its operation. Then a common thing to do is to block off the condenser modulate. The condensing fan motor in some way or use field piece makes a jacket that you can actually put on top. It's like a little tent that you can put on top of the outside unit and you can modulate the air flow through the condenser in order to drive the head pressure up to something like 105 degree, condensing temperature.

You know that's a good general number, one. Five. One 10, something like that: it equates to a good hot day. You drive that up to that point, you just kind of hold it there.

While you let the system run, and then you check some cooling, obviously on a piste system, that's gon na be really challenging to do on a txt you're, just gon na chicks, up bullying, so getting a target. Super heat under those conditions would be exceptionally difficult. If you were doing it on a piston system, so I don't advise doing that to set a charge in perfectly, but if you just want to check and make sure the systems generally functioning properly, that's a way that you can do it. If I had to say set up a piece of equipment, I was starting a piece of equipment in the winter.

I would set up to come during the first warm day and come actually do a full commissioning on it once the outdoor temperatures above 65 degrees. So I try not to set in a cooling charge until that outdoor temperature is above 65 degrees again as much as possible. But you can use some of those strategies to kind of test, especially if you've done some work, and you just want to make sure that everything else is working. Okay, then that's a good practice is to block off that condenser or modulate that condenser air in some way and, like I said, take a look at that field.

Peace jacket, it's a pretty interesting. The little device Jim Bergman did a video on that several years. Back that was pretty convincing on its utility, so there we have it that is low ambient conditions. I'm sure there are other products out on the market that I did not mention, but hopefully you found this helpful, even if it was not fully comprehensive thanks for listening.

We will talk to you next time on the HVAC school podcast.