Hey thanks for watching in this video we're going to talk about one of the neglected components off neglected components as far as understanding goes in the refrigerant circuit, and that is the metering device. Now one of the often blamed components in the refrigeration circuit is the TX V and the TX V stands for a thermostatic expansion valve it's one type of metering device and it is very, very misunderstood. So first thing I want to say here is that if you are new to the trade under no circumstances, are you allowed to condemn or replace a TX v unless you fully understand it, because it is very misunderstood now, this video is not specifically about everything related To TX V's, you can go to HVAC our school comm and find more information on TX vs. I have all sorts of articles and you will know everything you need to soup-to-nuts if you study there, but a TXE is a type of metering device and the purpose of the metering device is to act as a pressure dropper.

It's there to create a pressure drop, and so it separates the high side and the low side of the system. If you look at the the basic refrigerant circuit, you'll notice that the compressor and the metering device kind of split that circuit into where the compressor is, the pressure increase her and then the metering device is the pressure drop er before it goes into the evaporator coil. So think of it in this way, the compressor has to pressurize the refrigerant before enters the condenser in order to get the heat out and the metering device has to drop the pressure before it goes into the evaporator coil to get the heat in, because when you Drop pressure, you drop temperature when you increase pressure, you increase temperature and you have to manipulate temperature in order to get heat in and out of things, which is what we're doing with the refrigeration circuit is we're getting heat in and out of things. So there are several different types of metering devices: you have capillary tubes, you have Pistons, you have TX V's and you have electronic expansion valves, there's a few other, more exotic types of metering devices, I'm that you'll read about in books and we're not going to talk About those here, automatic expansion valves, high side, low side float systems like that, but we're going to focus on these four and just talk a little bit about what the general purpose of the metering device is.

So we know it's a pressure dropper. How does it do it? Well, it drops pressure by basically creating a small passageway that the refrigerant has to go through so is so. Pressure is built up on the inlet side and then on the outlet side. There is a drop because it goes through a very small area and then it opens up into a larger volume.

So when we think about the relationship between pressure volume and mass, we know that if you increase volume, the new decrease pressure and what we're doing is we're increasing volume, because it goes into an evaporator coil out of a liquid line. The liquid line is a smaller line, goes into this larger volume of space into the evaporator coil, so there's an increase in volume and but we're also controlling the amount of flow moving in so we're controlling how much pressure is able to make it into that of A protocol all that said, we end up seeing a pretty significant pressure drop across that metering device. That's what its job is now another thing in order for a metering device to do its job, the pressure coming into it has to be appropriate. It varies on what required pressure drop.

You have to have across the metering device for it to do its job commonly stated. One is in a 100 psi, some people will say: 80 psi depends on the segment of the business you're in and the type of valve that you've got, but there has to be a difference in pressure across that valve for it to do its job. The other thing that valve needs is, it needs a full line of liquid, so you need to have a hundred percent liquid entering that metering device. I keep saying valve and when I say valve, I'm using that as sort of a generic term for a metering device, but a meter device isn't necessarily a valve there's: thermostatic expansion valves, electronic expansion valves those are both types of valves, but some of the most simple Metering devices like capillary tubes and Pistons are literally just a small orifice that the refrigerant has to go through in the case of a piston.

It's a little hunk of brass that can be easily replaced with different sizes, and it's allowed to slide back and forth. In order to seen an unseat depending on the mode of operation and that's what we call it, a piston is because just like a piston in the car engine, it slides back and forth. That's sort of the definition of that a capillary tube is literally just a piece of tubing that creates a pressure drop across it, but then we have the more complicated versions like the electronic expansion valve and the thermostatic expansion valve in both of those devices. What they're doing is is they are measuring the superheat at the outlet of the evaporator coil they're, using pressure and temperature in the case of electronic expansion valve they're, using an electronic temperature sensor, an electronic transducer which is transducers just a fancy word or a a pressure.

Sensing electronic device at the outlet of that evaporator coil, so that we can see what is the superheat at the end of that of a protocol translation. If you don't know what super he means, superheat is just a measurement that we take to show. Is it fully vapor? At the end, we talked a little bit about compressors. We've talked a little bit about evaporator coil, so we know that it does have to be fully vapor when it leaves that evaporator coil.

So that way we don't end up with liquid inside the compressor. That's a big no-no. So that's why, in the case of the electronic expansion valve using those electronics or in the case of the thermostatic expansion valve, it uses a refrigerant filled, bulb and then just a little probe called an external equalizer that tap into that suction line. What what they're both doing is they're measuring superheat, it's making sure okay, how far through this evaporator coil are we feeding refrigerant and we want to make sure that we're not accidentally feeding liquid refrigerant into the compressor.

So we want liquid in the evaporator coil. We need boiling liquid in the evaporator coil for it to evaporate and absorb much more heat. Because that's that's the point, that's why we call it an evaporator coil, but we don't want too much that we end up with a liquid refrigerant running down that suction line and entering that compressor, so that that's sort of the balance that we're trying to achieve and Those more advanced valves, the electronic expansion valve and the thermostatic expansion valve they have that modulation so, rather than just having a fixed sized orifice, that's a single size like you have in an epistle or in a capillary tube. It has a fluctuating size and its job is to maintain superheat or in other words, to maintain of a protocol feeding again, if you're not familiar with the word superheat sub cooling saturation, I've done a lot of content on those you can find those at HVAC our School comm learn more about those words, but at this point I want you to just mostly understand that metering device as the pressure drop device it's reliant on liquid refrigerant coming in so we've got to give it liquid coming down the liquid line.

We have to have appropriate pressure coming into it if our pressure gets too low coming into it, then we're gon na lose control, and also you have some that are just a fixed, orphis eyes, and then you have some that modulate to open and close in order To try to maintain that superheat or that feeding through the evaporator coil, if you will in order to maintain efficiency and also in order help protect the compressor. Hopefully you found that helpful. We'll get you in the next video.