EEV vs TXV Metering Device Operation, Efficiency, Additional Parts, & Evaporator Coil Function!

[00:00:00]In this HVAC training video, we're going over how this coil right here with an EEV thermistors and a pressure transducer differs from one with a thermostatic expansion valve. And make sure to join our online HVAC Academy at acservicetech.com/academy.

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[00:00:27]Now, let's get into the video. Both of these coils are equipped with a R32 leak detection sensor. So, there's one here, there's one here. We're going to go over this basic coil with a thermostatic expansion valve first so that you understand the refrigerant aspects, and then we're going to move over into the more complicated one that has all of the extra components. To discuss what's happening here, you have a thermostatic expansion valve metering device. Its job is to meter or lessen the pressure of the refrigerant traveling through it during air conditioning mode. In heating mode, this thermostatic expansion valve acts in bypass because the refrigerant will be coming through these distributor tubes through around this check valve and then coming right back through as a subcooled liquid refrigerant. During heating mode, you'll have that high-pressure vapor refrigerant about right in here, and it's going to be lowering in temperature as it flows through the coil. So, if it enters at 140°, maybe by down here it's at about 95°, and it's going to be traveling through this whole coil at about 95°, and because it's a saturated mix, which means there's liquid and vapor, it's a phase change occurring, and that's why it's going from primarily vapor, a little bit of liquid, to higher amount of liquid and a lower amount of vapor down here, and then it changes to the fully liquid state, and then it's going to be lowering in temperature. So, maybe 95° here, and then it comes out over here at about 84° as the refrigerant's traveling. Now, that's during heating mode. In air conditioning mode, the thermostatic expansion valve is going to do its job to hold what's called the superheat steady. And so, let's just simplify this a little bit because we've got additional wires and things basically just hanging here.

[00:02:09]Uh this right here, this is an A2L refrigerant sensor. And so, this is a acoustical resonance um A2L refrigerant sensor. So, it's checking for the auditory resonance change based on the refrigerant being in this area. And so, this is a Sensata one. And so, this is going to be able to detect, say, R32 or R454B. In this case, it's R32. And so, I'm just going to actually get this out of the way. That's always down low in the coil because if the uh the air handler is positioned upwards like this, it would need to be at the bottom because refrigerant density is uh heavier than air. And so, the refrigerant would gather down to lower section. If this was upside down, you'd have to change the the refrigerant sensor location to another location.

[00:02:57]That's going to be in the lowest spot.

[00:02:59]You'd have in air conditioning mode, high pressure, high temperature subcooled liquid refrigerant. And it's going to enter this metering device. And it's going to lower the pressure of the refrigerant. And so, maybe where it comes through here, it'll be at about 38° F, right? But it's already saturated because it was subcooled, meaning fully in liquid state and lowered in temperature a little bit.

[00:03:22]It's considered a high pressure, high temperature refrigerant. It comes through this metering device and comes through on the other side. It's a pressure lowering device. Anytime you lower pressure, you're going to lower temperature. So, now it's a primarily a liquid and maybe 20% a flash gas mix.

[00:03:39]So, 80% liquid, 20% flash gas. It's already saturated. It's at, say, 38°.

[00:03:44]It's going to be at 38° all the way up this coil, all the way to here. So, 38° this entire time as you have, say, 70° air crossing through this coil, and so maybe it's 50° as it exits out the top of the coil. And once again, you're removing humidity from inside the building cuz the temperature of the coil's at 38°, and it's condensing the water. It's the water's trickling down and then draining out of the pan. Now, the whole point is this is 38°. All this refrigerant section in here, maybe it's about 90% vapor and 10% liquid. And by the time it gets to right about here, it's in a fully vapor state. And then, as the refrigerant's traveling through here, it's going to increase in temperature from 38° up to a higher temperature, and maybe it's going to come out of this coil as maybe, we'll say, 50°. That means that if it was 38° in this whole rest of the coil, and it exits at 50°, it will be 50 - 38, and that's 12° of superheat. The TXV's job is to maintain superheat. So, somewhere's between 8 to 12° of superheat. You may measure somewhere's between 6 and 14 or 6 and 16° of superheat. But basically, it's the superheat is the temperature increase of the vapor refrigerant. And so, the TXV does this job mechanically based on three pressures. One is this little capillary tube. It's a stainless steel capillary tube connects over here to this bulb. And the bulb is mounted tight to this line. Now, I've removed all of the insulation from this tubing because it's going to normally be insulated, especially this bulb where the bulb touches the suction line at.

[00:05:26]And you can have stainless steel touch copper. It's not going to have a dissimilar metal reaction, uh but you do notice over here, aluminum to copper braze joint is always insulated. They're trying to prevent against the corrosion that can occur at that joint. But here, that's not really an issue. The The insulation was over top of it so that the bulb has refrigerant that is basically sensing the same temperature as the refrigerant traveling through this line. So, the temperature of the refrigerant in the bulb is going to increase the pressure as this gets hotter. It's going to apply more force this way onto the TXV. It's going to allow more refrigerant to flow through because this is an opening force. Now, there's two closing forces as well. One is the spring pressure on the bottom right here, and the other closing force is this It's actually wrapped in like PVC right now. But, up here you can see where the PVC is is no longer there.

[00:06:18]It's just a shield, right? To stop it from accidentally rubbing on something, which I like. That's great. Uh but, we're measuring the pressure right here on this suction line. So, this TXV is going to have the external equalization pressure right here on the side. It's going to push up. The spring pressure is going to push up, and the bulb pressure is going to push down. It's going to allow a certain amount of refrigerant to come through. As you have a higher heat load crossing this coil, the temperature on this vapor line is going to increase, which means that the bulb pressure is going to increase. It's going to push down, and it's going to open this up a little bit more. This is going to increase as well. However, the bulb increase will be higher than the external equalization increase, which means that it's going to allow more refrigerant through to handle the heat load. Remember, the job of the refrigerant is to absorb heat from the air crossing over the coil. So, the TXV is mechanically uh adjusting the opening on the inside on pressures only. If the temperature on this line decreases, the bulb is going to decrease in pressure, and the external equalizer is going to decrease. But, this bulb is going to decrease more in pressure, and so you're going to have a lesser amount of refrigerant flowing through this coil than you had before. And so, the whole object is to maintain a a change to vapor and an increase in temperature of vapor in the top of this coil. And so that's called the superheat. It's a temperature increase of vapor refrigerant. So, now that we understand what's going on here with the TXV, let's move over to the other coil. So, once again, we have a 5-V powered acoustical resonance refrigerant leak detection sensor. So, what I'm going to do is I'm going to first get that out of the way.

[00:08:02]I want to explain what the purpose of all of these components are first before we get into the operation. So, here we have a strainer {slash} muffler and it really is just a hollow chamber with a strainer screen. So, it's not filled with desiccant or anything like that.

[00:08:20]It's to protect the uh EEV right here from any small particles getting inside and jamming it. As well, it's to reduce the any noise due to the frequency change of the refrigerant either uh entering this way or entering this way.

[00:08:37]So, you see we have a second one right here as well. So, we have a whole 'nother video on the function of a strainer {slash} muffler in the description section below. This right here is a thermistor. And so, there's a stainless steel clip holding this little uh tube-type thermistor right onto the tube. And so, what happens is this changes in electrical resistance based on the temperature of the tube. And so, when this is hooked to a circuit board, the circuit board can interpret what the temperature of the refrigerant flowing through here actually is. So, there's one tube thermistor mounted right here and there's another one right over here.

[00:09:13]So, that's where the uh low-pressure suction would be coming out during air conditioning mode. And as well, there is a pressure uh sensing device right here.

[00:09:24]This up here is called a pressure transducer and there's three wires attached to that. So, it's a 5-V pressure transducer. And so, what happens is it'll output a signal that's between, say, 0.5 and 4.5 V based on the pressure it reads right here and the circuit board interprets that voltage as a certain pressure. Now, up here, this is just a valve core, so it's a port with a valve core in it in case pressure needed to be measured. This is also likely where they added the nitrogen in for the dry nitrogen pressure test from the factory. Here we have an electric expansion valve metering device and so this has a detachable head on it and it has iron teeth on the inside and so it's got maybe this is about 10 teeth here and then 10 teeth face downwards and then another 10 teeth face upwards and then another 10 teeth face downwards right here, so you have 40 teeth right here and there's probably about say 250 12-V steps that this EEV can apply to the internal permanent magnet on the inside to either reduce the size of the opening down here or enlarge the size of the opening and so the circuit board is going to send those 12-V pulses in a sequence and so this is a five-wire EEV, so there's one common and there's four of these coils and each one is going to be powered with 12-V, 12-V, 12-V, 12-V, that's one pulse, that's all it needs and so basically this is turning into an electrical magnet magnetizing 10 teeth and it's aligning to the 10 norths on the inside of this and it's spinning it slowly until it fully fully closes or fully opens it. So, in effect, this is measuring the superheat across this coil and it's going to be maintaining the superheat by adjusting the EEV, that's an electric expansion valve, it's not an electronic expansion valve, there's no electronics in this head, it's just turning into four electromagnets that are powered one at a time in sequence, but basically the circuit board is going to adjust the the of refrigerant flowing into here based on the air, which is the heat load crossing the coil.

[00:11:35]And it can do that by monitoring the saturated temperature as the refrigerant's exiting the metering device, because remember it's already mainly liquid, low pressure liquid, and it's a little bit of vapor.

[00:11:48]Now, it doesn't even necessarily need the sensor. It's actually using this more for heating mode, because over here it has a thermistor for measuring the temperature, and it has a pressure transducer right here for measuring the saturated temperature. So, this pressure transducer, basically it you could convert pressure to saturated temperature.

[00:12:11]And so, then you have the temperature increase in vapor form. So, this will be how the circuit board is measuring the superheat across the coil, which is the amount of vapor that's flowing in this coil, compared to the amount of saturated refrigerant. The more saturated refrigerant you have in the coil, the greater the the heat absorption is during air conditioning mode. And so, something like this could operate at a at a low superheat. It could control it very very precisely.

[00:12:39]Now, we have a video on thermistors and how they work, and also pressure transducers. So, those videos are linked down in the description section below.

[00:12:47]We also have a video on how EEVs work in the description section below, as well.

[00:12:51]Now, the circuit board is taking pre-programmed algorithms. I'm just explaining this so, like in human terms, because realistically it's not going to be measuring the pressure and converting that into electrical signal, and then converting the electrical signal into a a pressure signal, and then converting the pressure to saturated temperature to know what the saturated temperature is. It's all done through algorithms that are pre-programmed in that circuit board up there.

[00:13:16]But, it is able to measure the superheat. So, if this temperature was 10° higher than whatever the saturated temperature was, then say it would be 10° of superheat across the coil because the pressure measured here is the same pressure that's measured in the middle of the coil. That pressure is not going to change. The only thing that's going to change is the temperature measurement. And so, let me just show you that. So, we have our thermistor, which is right here. So, that is sandwiched right up against the the tube right here to measure that tube temperature, which is therefore going to give us the refrigerant temperature there. So, the inside of the EEV is a very small hole where it's able to control the amount of refrigerant that's flowing through. And so, it's making very very minute changes to the amount of refrigerant flowing into this coil.

[00:14:08]Now, technically, to measure superheat, you could do it with just this tube thermistor and this tube thermistor.

[00:14:14]You're going to have a little bit of a pressure drop, however, when you go through over to the coil. And so, it's going to be using the pressure transducer. Now, the pressure transducer and this is also here for during um heating [snorts] mode, as well. And what's going to happen is you're going to have the discharge gas from the compressor. It's going to be high pressure, high temperature vapor refrigerant traveling into here. And you're going to be able to measure the pressure and also the tube temperature right here. So, it knows the discharge temperature. It's going to be flowing through the coil. It's going to be rejecting the heat from the refrigerant into the air crossing the coil. It's going to subcool. It's going to do the complete phase change from fully vapor state up here to fully liquid state down here. And as it is coming out of here at the fully liquid state, it's lowering in temperature in this bottom part of the coil. So, say if it's 95° here, maybe by the time it comes out, it's at 85° as a liquid. And so, it can measure right here. It can measure the temperature decrease of the refrigerant. Now, this EEV may be the sole metering device for the entire system for both the indoor coil and the outdoor coil or there may be two.

[00:15:26]Now, uh there is specialized EEVs that will actually open up to be able to bypass or there'll be a piping arrangement for it to bypass. In this example, this is the sole metering device for the system.

[00:15:37]Uh and so it'll have another temperature thermistor at the outdoor unit and another pressure transducer and likely another thermistor at the outdoor unit to be measuring right at the the inlet of that unit. Now, right here, see how I have this kind of wiggling on here?

[00:15:56]Um you want to make sure that this is down all the way. And so, I like when these are insulated. Honestly, I don't like This just has a plastic cap on it, but I like to see these fully insulated because one of the biggest problems with the EEVs is that the the teeth on the inside end up getting rusty. Uh and so, it doesn't have as much magnetic strength on those teeth and so that becomes a problem.

[00:16:19]In this instance, I also want to mention that there is a basically a piston, but the hole size is so large, it's almost it's almost not going to matter, but it definitely is there for a reason. Uh so, in heating mode, it's in bypass, so it'll go through and around it. And in air conditioning mode, it's basically going to be to the front and it's only going to let the refrigerant flow through.

[00:16:41]It's just enabling the refrigerant to flow easier than if the chamber was completely open because this is really here to be able to provide a uh the inexpensive normal method from going from the liquid line into the distributor tubes. And you can always disconnect this here and here to remove the EEV completely and replace it with a new one that will also have the new strainer screens and in case they're clogged. But it's just something to to be aware of. This is not like a normal piston that's here. This is a a very very very large hole. It's just uh helping direct the refrigerant flow for during air conditioning mode. It's not actually metering it. Now, the whole point of this is to be able to adjust the refrigerant flow here isn't going to really help much if you have a single stage compressor. It will help, but it's not necessarily worth all of the added expense and everything for all of the electronic components. This coil and system is made to be operating with a inverter outdoor unit. So, that's a compressor that will ramp up and down in its speed and operational capacity. So, it's going to deliver higher amount of refrigerant volume to the indoor coil or a lower volume of refrigerant to the indoor coil based on the heating load crossing the coil. And it's also monitoring the outdoor temperatures. The blower motor also is going to have a variable speed blower motor that can push the air across here efficiently. Remember, air flow is just as important as the refrigerant flowing through the coil.

[00:18:14]So, this is made for a variable speed system, whereas the thermostatic expansion valve coil is made for more of a single stage system. There are instances where you're going to use a thermostatic expansion valve for a two-stage system as well, but that has to have the appropriately sized TXV for that. So, this is meant to operate with a variable speed system. So, a variable speed outdoor unit. That means a variable speed outdoor fan, variable speed compressor, and also a variable speed indoor blower motor as well. And so, you also have this varying or modulating pin position here. It's just meant to for efficient operation. Now, this kind of setup could potentially be used on a two-stage system as well, but it may not be worth it with all of the accessories. It really depends on the system. So, the object of all of a variable speed system is it can run in a part load condition. So, it may only be operating at say 30% of the full speed that it could be operating in. And so, that will be a reduced energy cost and as well, it's going to be quiet for when it turns on and also for when it turns off. It will run longer and keep the uh coil at a low temperature for a longer period of time. Of course, you have less airflow going across there, so you're going to have less humidity.

[00:19:29]However, it is at a lower speed and you'll still have that uh condensation occurring. And so, the whole object is it'll be running in an energy-efficient manner for the heat transfer of the refrigerant to the air. So, I hope this video helped and if you want to learn more about HVAC, make sure to join our online HVAC Academy at acservicetech.com/academy.

[00:19:50]There's over 95 hours of content, over 38 courses, over 50 certificates of training, and there is a start here and categories option. So, you can progress right through there and you can do it on any device. Also, make sure to check out our second edition refrigerant charging and service procedures for air conditioning book. So, this book is updated for all the new refrigerants as well as the older refrigerants as well. It goes through getting a system ready for a refrigerant, airflow, measuring the refrigerant charge, and troubleshooting.

[00:20:18]We also have our mini split book as well. So, you can get these books over at Amazon and at acservicetech.com and our online HVAC Training Academy's at acservicetech.com/academy.

[00:20:28]Hope you enjoyed yourself. We'll see you next time at acservicetech channel.