Our refrigeration cycle, we have our outdoor unit, which is our compressor and condensing coil, and our indoor unit, which is our evaporator and our metering device. We have two kinds of metering device that are most commonly used. There's many metering devices, but the two most commonly used will be a fixed orifice meter device and a thermostatic expansion valve.
First let's talk about the fixed orifice metering device. Here if we unscrew this fitting right here and we see that the other side there's our distributor and distribution tubes. Once they unscrew this fitting and and pull this out, there's a hole in the very center of this piece.
This hole is fixed. Another word for a hole is an orifice. Another word for a hole that doesn't change is fixed. So we call this a fixed orifice.
It's a fixed hole. The size of that hole doesn't change. It's a fixed orifice metering device.
So it's a pretty simple thing. It's just a fixed hole that's inside of these pipes. I can't take this one out because all the refrigerant would come out of the system. So it's a metering device. It's just restricted.
restricting it from a high pressure to a low pressure. That metering device, that fixed orifice, or fixed hole metering device, must match the one that came with the outdoor unit. So you'll see that the outdoor unit may have one in it, and the indoor will also have one in it. You always want to use the one that comes with the outdoor. The compressor raising it from a low pressure to high pressure needs to match this changing it from a high pressure to a low pressure inside.
If we get them mismatched, they're not gonna pump properly. They're not gonna have the right pressures that we need. So it's important to always use the metering device, the fixed orifice that came with the outdoor unit.
So when the system's new, you can just unthread this, pull this one out, take the one that came in the package from the new unit, you can drop it in facing the correct way, slides in, hook it up, and finish installing your system. A lot of people like a fixed orifice metering device because there's no moving parts. It's very simple. There may just be a screen here or a hole to protect that hole from dirt, but it's simple.
there's no moving parts. Charging a fixed orifice media device takes a few more steps. So a fixed orifice media device, we're going to focus more on superheat.
Now we're still going to get superheat and subcooling on any system we work with. But with a fixed orifice, superheat is going to be more important. And our formula for finding a superheat is going to be our actual suction line temperature, blank degrees Fahrenheit, minus our suction saturated.
temperature blank degrees Fahrenheit equals super heated vapor. So I have my digital gauges hooked up so this is cheating a bit. It's nice and fast. My actual suction line temperature is 86 degrees.
My suction saturated is 62.8 and it tells my superheat is 30. 38.5 degrees, 38.5 degrees Fahrenheit. That is my superheat. So the next question is, is that number good or is that number bad?
So we need to compare it to something. On a thin... fixed over this meter device, even though it's a simpler meter device, there's a few steps involved.
As the outdoor temperature goes up, the pressure goes up. As the outdoor temperature goes down, the pressure goes down. So the outdoor temperature is going to affect the outdoor pressure. That outdoor pressure is going to affect how much refrigerant is coming through this meter device. If there's more pressure out there, more refrigerant is going to come through this hole.
If there's less pressure out there, less refrigerant is going to be forced through this hole. So the pressure... outside is going to be important.
So to find out what our superheat should be, we need to know the temperature of the air coming into the outdoor unit because it's going to play a part in it. So I got my thermometer here. I'm going to put it on the outside and it says the temperature of the air coming in to this unit right here is 98 degrees Fahrenheit. So this is what we call ambient, the temperature around this unit. So the outdoor, I'm going to abbreviate OD for outdoor ambient which means the ambient temperature around the unit outside outdoor ambient temperature this is also known as not just a temperature outside I don't just get the weather service and see what they say I want the temperature specifically of the air coming into this coil not coming out into the coil.
So the temperature of the air coming into this coil was 98 degrees Fahrenheit. That's one number that I am without a doubt going to need. But there's another number I'm going to need.
I want to also need the air that's coming into this. As the temperature of the air goes up, the suction pressure is going to go up. As the temperature of the air goes down, the suction pressure goes down. So we know that temperature is going to affect the pressure that's coming in on the suction.
side. But not only that, Willis-Keyer invented AC to not only cool, but also dehumidify. So the humidity, the latent heat is also going to affect how much the pressure is going to be on this evaporator coil.
If the air is more humid, there's more latent heat happening. We're having to take the moisture from the air, turn it into a liquid and drain it out this drain line. That's condensate drain line. If the air is dry, we're going to spend more temperature changing the dry temperature of the air. So there's two numbers that affect that.
So one thing we need is we're going to need a new number that we call a wet bulb temperature. A wet bulb temperature takes into account regular temperature, but not only that, it also takes into account latent heat, and we call this wet bulb. If I take a thermometer, a regular thermometer, and I put a wet wick around that thermometer, and I was to move that thermometer around in the air, the temperature of that thermometer would drop.
As the water changed state from a liquid to a vapor, it would absorb latent heat. And we used to actually do that, and you can still do this. You can take a thermometer, put a shoestring over it, and put room temperature water on that shoestring. It's gonna be soaking wet. And as you sling this thermometer in the air, or use a fan to move air across it, that water's going to evaporate, or change state from a liquid to a vapor.
And we know the definition of evaporation. Every time a substance changes state from a liquid to vapor, it absorbs heat. It will absorb heat away from this thermometer.
The temperature of this thermometer will drop. That's why a lot of the military, thank you for your service, they would put moisture or put water on a sock, put the sock over a water bottle, or their canteen would be made out of canvas. They'll put water on that canteen. They'll sling it around in the air.
As that water is evaporating, it's absorbing heat. It'll actually cool that water down. But how far that water cools down depends on humidity.
If the air is 100% humid, there will be no evaporation. At least it will be the same temperature. If the air is very dry, there's going to be a lot of evaporation, that temperature will drop a lot.
So a wet bulb temperature is just simply a thermometer with a wet wick on it. Nowadays, due to technology, we don't actually have to do that anymore. We can have what we call a digital psychrometer.
We turn this on, it will give us a wet bulb temperature, along with a lot of other numbers that we can use later on. So the regular temperature of the air coming into this, on this side, we have 92 degrees. If I change this mode, I can turn this to wet bulb temperature. The wet bulb temperature of the air coming into this unit is right at 71 degrees Fahrenheit. It takes into account sensible heat and latent heat.
It's taking into account the moisture in the air and the regular air temperature. So if I put a wet wick on this thermometer and spun it around in the room, it would give me the same temperature as this, which is 71 degree wet bulb temperature. So I'm going to call this my indoor. Return air, because it's the air returning to the unit, the air that's actually entering that unit.
Indoor, return, wet bulb. So these are the two numbers we're going to need, and that temperature was 71 degrees Fahrenheit. Indoor return air wet bulb, the outdoor ambient, these are the two numbers that's going to be affecting our pressures.
So since I have these two numbers, there's several ways I can go about this. This is going to give us a target superheat. Method number one is a formula.
I could put these numbers into a formula and when I do that what I can do is I have my return air wet bulb temperature it's a formula sorry let me write this out target superheat is idwb indoor wet bulb temperature that number multiplied by three you do that first because it's in parentheses and then you subtract from that the ODT which is the outdoor temperature and then with that number you subtract 80 and then you take all of that and you divide that by two and it tells us what we call our target superheat and that's where we want the superheat to be. So if I was to plug these numbers that we got in here, my indoor wet bulb temperature was 71, 71 and my outdoor ambient temperature was 98, so I put 98 here. So 71 times 3 minus 98 minus 80 that divided by 2 gives me target superheat. Some of you guys are gonna be great. formulas and memorizing formulas.
Others of you guys, maybe formulas aren't your thing. We have given you a chart. It's called a target superheat chart that does the same thing for you. It's pretty much all these formulas done in one chart. return air wet bulb is 71 so across the top it says return air air entering the evaporator well here's my evaporator here's the air entering the evaporator wet bulb temperature so that number was 71 so I want to find over here 71 which is right here so somewhere Up and down, that's 71 degrees.
And then also it says the outdoor ambient is 98. So what I'm going to do is go over here and find 98 degrees. And 98 degrees is going to be right here, which is going to be right along this line here. So now where these two numbers cross, that is my target superheat. What I want the superheat to be under these operating conditions.
So if I follow these two numbers, I got 98, 98 degrees, and 71 degrees is 17.5. Target superheat what I want the superheat to be So if you did the formula did all the math it should come out to 17.5 Or if we use the charts we get a return air wet bulb comes down Our outdoor dry bulb goes across where those two numbers cross is our target superheat. What our superheat should be at, give or take three degrees. That's where we want to be at.
There's a third method. There's an app called the Super Cool app. The Super Cool app, you can put in here what kind of beater device, and you put these numbers in there and it does does the calculations for you.
Those are the three ways to find out what your superheat should be at. So there's no set superheat. It's going to change with conditions if you have a fixed orifice metering device.
It's gonna change numbers. This kind of meeting device, there's no set superheat. Would it be easier?
Sure, it would be easier. It just doesn't happen. As conditions change, that's going to change. For the conditions that we have in this lab right now, our superheat should be at 17.5. plus or minus 3. So this is where we want to be.
Now let's look and see where we're at. We're at 80, sorry, let me write that backwards. Yeah, 38.5.
We're at 38.5. We want... to be at 17.5. So is our actual superheat higher than our target? Is our actual superheat lower than our target?
Or is our actual superheat equal to the target? So if we look at this, 38 is much higher than 17. So this is what we call a system with a high superheat. So our superheat is high.
We have a high superheat. A high superheat is called a starved Evaporator. Starved evaporator.
So this evaporator means it's starved. There's not enough refrigerant in this evaporator. Now it doesn't mean that we're fully low on charge. There's a good chance of that. I need to know what's happening with sub cooling.
But it means if I If I have high superheats, that means there's too much vapor. Let's say my superheated vapor is this much. That means my saturation is very low here. I don't have enough refrigerant in my evaporator for the conditions that we have today. Now tomorrow these conditions could change, but this superheat will also change.
The difference in the two will still going to be the same. So if I add refrigerants, if that's what it needs, if I get my superheat to be within three degrees of this number, I'm going to be good. So maybe tomorrow the condition should be at say 30. My superheat will be at 30. Maybe another day my superheat will be 10 and my actual target will still reflect 10. So this number changes with conditions, but this number will always change the match.
it. If it doesn't, we have a problem. If this number was lower than this number, we would have what we call a low superheat, a low superheated vapor.
Low superheat would mean that we had a flooded evaporator. There was too much refrigerant in the evaporator. So anytime we do superheat or sub cooling, we also make sure there's plenty of airflow first.
This system has plenty of airflow, so it's not going to be the issue. But I've just given you an example of how to find superheat. So even though it's a simple metering device, there's a few few more steps involved to knowing what it should be. And here we got the outdoor ambient temperature, return air wet bulb temperature.
We can put that in the formula. We can use the chart. We can use the app.
But we need to find our target. In this case, my superheat is higher than the target. We have a starved evaporator.
High superheat is a starved evaporator. We'll get a little bit more detail what to do with these numbers, but that's just how to charge with a fixed over speed meter device and how that affects superheat and subcooling.