Hey thanks for watching this quick video, we're talking a little bit about liquid lines and discharge lines. Some of the difference and some of the things to know about when you are measuring pressures and temperatures on both so to start with your discharge line, is the line that leaves your compressor and goes into your condenser. It is a high pressure, high temperature vapor. Generally speaking, in most of your normal air conditioning applications, it's going to be somewhere between 160 and 225 degrees and, generally speaking, going to be closer to that 160 degree mark.

But it's going to be significantly higher temperature than your liquid line and it's going to be fully vapor. So though they are both high pressure lines, one is going into the condenser and one is going out of the condenser and if you remember our description of the condenser, the condenser is the heat rejector. So its job is to reject the heat out of the refrigerant, which then in turn changes its state, starting with fully vapor all the way down to fully liquid. So if you imagine, you have your discharge, gas vapor goes into the top of the condenser and it works its way down now again in this image.

It shows a single pass. Condensers are generally multiple passes with headers, but it creates a liquid seal down at the bottom once it fully condenses and that's where we see sub cooling because sub cooling is the temperature decrease in refrigerant once it goes below that condensing temperature or what we call our Liquid saturation temperature, so when your refrigerant is at saturation, that means it's still changing state from vapor to liquid, so it starts off fully vapor, it desuper heats, and then it goes into this phase where it's changing once it drops below that now we know that it Is fully liquid and our goal is to deliver a complete column of liquid to our metering device, which is why you're generally going to see around 10 degrees of sub cool. Now you follow your manufacturer's specifications, but around 10 degrees of sub cooling and that ensures that we're going to deliver a full line of liquid to our metering device on our liquid line liquid line temperature. We did a separate video on that how to how to locate your liquid line temperature, but in general your liquid line temperature is going to be about 5 to 10 degrees warmer than your outdoor temperature on normal modern equipment.

That's going to be typical, but again you're going to want to know something about your equipment before you come up with that and that's what we call approach. So if you take your outdoor temperature and you compare that to your liquid line temperature measured near the condensing unit, you should see that that liquid line temperature is 5 to 10 degrees warmer than your outdoor temperature and there's several things that go into that which again You can watch that video on liquid line temperature to understand that better. But a big thing to know is that you need to make sure you measure your liquid line temperature in the right place so that you don't get confused, especially when there are liquid line dryers in the circuit. So the discharge line from the compressor feeds into the top of the condenser and then it's multiple passes, but it comes together and it works its way down until you have a liquid seal at the bottom.

And so, when we're looking at our liquid line pressure, we're converting that to a condensing temperature, we're comparing that to the physical temperature of the liquid lines, what we call the actual, measured liquid line temperature. But if we were to measure on the other side of a liquid line dryer, we could potentially read in an accurate measurement, because if this liquid line dryer has a pressure drop that could result in a temperature change because of flash gas meaning changing state from liquid. To vapor within the dryer in this case, that would be pretty obvious not to do because you can see the liquid line dryer, but in some cases the liquid line dryer may be inside the condenser itself. Certain brands will put it inside the condensing unit, and so you may be measuring here and reading what you would think is a very high sub cooling, which is actually flash gas.

It's actually quite the opposite, because there's a restriction in between. So that's one of the common mistakes that we see made is measuring past a liquid line, filter dryer, rather than measuring before it on that liquid line. This refrigerant is going this direction in cooling mode towards that indoor metering device. So, as you can see, measuring your liquid line, temperature in the right location is important.

Now there is an argument to be made to measure your liquid line temperature inside and outside and compare the differential, because you really shouldn't see any significant difference. If there is a difference where the temperature is lower on the inside than it is on the outside, meaning it's lower near the metering device than it is near the condenser and that's an indication that there's flash gas in the liquid line, which means that some of It is changing back to vapor and that's not good, because that's lost efficiency, so we want to make sure that our liquid line, as it leaves the condenser and as it enters your metering device near the evaporator, is pretty much the same temperature again. You know the rules of pressures and temperatures tell us that, even on a liquid, when there is a pressure drop, there will be a temperature change, but it is an infinitesimal temperature change as long as it's still liquid, where you're going to really see that measurable temperature Change is if there's enough pressure drop, that all of the sub cooling is taken up and now it's actually beginning to boil again and that's that concept of flash gas which we've talked about a lot on the podcast and in other videos. But a common mistake.

That's made is somebody will measure the pressure and temperature potentially on a discharge line and attempt to do sub cooling and that doesn't work. Sub cooling has to be measured pressure, wise meaning your condensing temperature that comes from your pressures has to be measured on the liquid line, and then the temperature also needs to be measured on the liquid line. In some cases, certain package units rooftop units - you don't have a port on the liquid line, which makes it challenging and in those cases the only way to really measure sub cooling is to sort of guess at the pressure drop across the condenser. So if you imagine the refrigerant leaving, the compressor is had a higher pressure than it is by the time it comes out because there's a pressure drop across all that tubing in the condenser.

Now i've been told - and i haven't measured this on a lot of pieces of equipment, but i've been told that that pressure drop is generally 10 to 15 psi. Typically speaking, so if you wanted to guess at it, you could potentially do that, but there is no way to accurately measure sub cooling unless you can take a pressure on that liquid line and the ideal place to do that is right after the condenser and before Any liquid line, filter dryers, like we talked about in that little clip so key thing here is, is don't confuse your discharge line with your liquid line, they're both on the high pressure side, but they are completely different states of refrigerant and temperatures. Discharge line is going to be below 225 degrees. Ideally, if you do ever measure a discharge line, temperature, that's 225 or over that's an indication of a problem could be due to low mass flow rate through the compressor, meaning, not enough refrigerant cycling.

Through the compressor, it could be due to uh high return temperatures, meaning high superheats, some other things that can cause as well, but measuring discharge line. Temperature is a valuable thing to measure for sure and when you measure it measure it six inches away from that compressor and make sure that it's below 225 degrees. Just don't confuse that with liquid line temperature, which is what we use for sub cooling. So liquid line temperature and pressure is used for sub cooling.

Your discharge line temperature helps us see if potentially, we have a problem with compressor overheating, which is serious and causes oil breakdown and some other things. But it's important to know the difference, and hopefully you found this helpful, we'll catch you on the next video you.