Bryan explains why you would measure discharge line temperature. He goes over the information that you could learn from discharge line readings and how to reduce excessive discharge line temperatures and compression ratios.
The discharge line is the smaller line that exits the compressor and eventually feeds into the condenser. On a heat pump, it normally goes into the reversing valve from the top.
The discharge line should not be confused with the liquid line, which goes out of the condenser. You can expect the discharge line pressure to be a bit higher than the liquid line pressure, but its temperature will be way higher than the liquid line temperature. That’s because the condenser desuperheats that very hot vapor, condenses it to a liquid, and cools it below the saturation temperature (subcooling).
Like the suction line, the discharge line contains superheated vapor. However, the temperature of the discharge line is much, much higher than that of the suction line.
You can use Bluetooth probes to measure the discharge line. Several things impact the discharge line temperature, including the location. Normally, we want to measure the discharge line temperature about six inches away from the compressor; the temperature can vary quite a bit depending on where we take our measurements, so the industry standard is typically lower than 225 degrees six inches from the compressor outlet. However, individual units may have higher or lower specifications. (The compressor may be around 70 degrees hotter, and the area near the condenser will be significantly cooler.)
Normally, when the compressor exceeds 300 degrees, oil breakdown may occur. Higher temperatures result in oil with reduced viscosity, meaning the oil becomes thinner and cannot lubricate the bearings as well as it should. Mechanical wear in the compression chamber is another problem that can be discovered by finding a high discharge line temperature. So, we want to run compressors as cool as possible.
High compression ratios may cause compressors to overheat. So, we want to do whatever is in our power to reduce the compression ratio. We can find the compression ratio by dividing the absolute head pressure by the absolute suction pressure, and we typically want that quotient to be ~3 for modern air conditioning equipment (refrigeration equipment tends to have higher compression ratios).
We can reduce compression ratios by keeping the compressor cooler, such as by using interstage cooling or liquid or vapor injection to reduce discharge temperatures. Dirty condensers that restrict airflow also raise the head pressure, increasing the compression ratio and resulting in higher discharge pressures. Significantly overcharged systems may also raise head pressure.
Low suction pressure also leads to higher compression ratios; low suction pressure may be caused by low loads or metering devices that feed the evaporator coil poorly. However, cooler suction lines don’t necessarily cause the discharge line to be cooler, especially if the mass flow rate is also low.
Copeland Mobile can show you how operating a system outside its operating envelope can contribute to poor system performance (assuming you’re working with a Copeland compressor). You can adjust the conditions within the app to see what the discharge temperature, compressor capacity, and mass flow rate will be under various operating conditions. You may find it exceptionally helpful when used with the AE Bulletins app, which contains useful information about Copeland compressors.
Read all the tech tips, take the quizzes, and find our handy calculators at https://www.hvacrschool.com/.
The discharge line is the smaller line that exits the compressor and eventually feeds into the condenser. On a heat pump, it normally goes into the reversing valve from the top.
The discharge line should not be confused with the liquid line, which goes out of the condenser. You can expect the discharge line pressure to be a bit higher than the liquid line pressure, but its temperature will be way higher than the liquid line temperature. That’s because the condenser desuperheats that very hot vapor, condenses it to a liquid, and cools it below the saturation temperature (subcooling).
Like the suction line, the discharge line contains superheated vapor. However, the temperature of the discharge line is much, much higher than that of the suction line.
You can use Bluetooth probes to measure the discharge line. Several things impact the discharge line temperature, including the location. Normally, we want to measure the discharge line temperature about six inches away from the compressor; the temperature can vary quite a bit depending on where we take our measurements, so the industry standard is typically lower than 225 degrees six inches from the compressor outlet. However, individual units may have higher or lower specifications. (The compressor may be around 70 degrees hotter, and the area near the condenser will be significantly cooler.)
Normally, when the compressor exceeds 300 degrees, oil breakdown may occur. Higher temperatures result in oil with reduced viscosity, meaning the oil becomes thinner and cannot lubricate the bearings as well as it should. Mechanical wear in the compression chamber is another problem that can be discovered by finding a high discharge line temperature. So, we want to run compressors as cool as possible.
High compression ratios may cause compressors to overheat. So, we want to do whatever is in our power to reduce the compression ratio. We can find the compression ratio by dividing the absolute head pressure by the absolute suction pressure, and we typically want that quotient to be ~3 for modern air conditioning equipment (refrigeration equipment tends to have higher compression ratios).
We can reduce compression ratios by keeping the compressor cooler, such as by using interstage cooling or liquid or vapor injection to reduce discharge temperatures. Dirty condensers that restrict airflow also raise the head pressure, increasing the compression ratio and resulting in higher discharge pressures. Significantly overcharged systems may also raise head pressure.
Low suction pressure also leads to higher compression ratios; low suction pressure may be caused by low loads or metering devices that feed the evaporator coil poorly. However, cooler suction lines don’t necessarily cause the discharge line to be cooler, especially if the mass flow rate is also low.
Copeland Mobile can show you how operating a system outside its operating envelope can contribute to poor system performance (assuming you’re working with a Copeland compressor). You can adjust the conditions within the app to see what the discharge temperature, compressor capacity, and mass flow rate will be under various operating conditions. You may find it exceptionally helpful when used with the AE Bulletins app, which contains useful information about Copeland compressors.
Read all the tech tips, take the quizzes, and find our handy calculators at https://www.hvacrschool.com/.
Brain your tutorials are excellent. I’m not glibly throwing throwing complements here, your work is truly superb. I have quite a library of the HVAC School tutorials and greatly admire your abilities as an educator.
I do however have one tiny little problem and it’s not anything that you or I can do much about. I can’t use as much of your stuff in my class as I’d like to because of the F word. Fahrenheit… there I’ve said it. The one word that I will not allow students in my classes to use.
How long would this video be if we had to recalculate every reference to temperature and how much would it break the flow of information. If only we all used the same method. I guess it won’t be last time we hear this lament and none of us will be holding our breath waiting for change.
Again, thank you for your great work.
You are a great teacher. I have spent all day watching your videos. They're not boring. They teach best practices as far as doing the job once. No call backs. Call backs are the biggest cause of me not using a contractor twice
Wow. Great explanations along with no repetition. Very difficult to speak this way. You have a teaching gift. Thanks
now this is more for Southern or western issues w more year round hot weather I'm assuming … because in New York area and the north east we have less of thay hottet weather… so would you still recommend this for us guys up this way ? Service area Barrhaven??
Good info, but I think this video flow could have been a little better organized – all your other videos I've seen were organized better, imo.
Bryan, Hi!
I recently had a pretty bad experience due to a UV light that evidently did it’s work on my eyes before I disconnected it (was unaware it was in the system prior to opening it – lovely!) that won me a trip to the hospital, along with some terror of blindness.
It’s cleared up now, but I’m looking into buying some safety shades and unsure what code or specification to look for – if my research serves correctly, ANSI Z87 with orange tint should be what I’m looking for, but would like some guidance from the community to keep safer on the job!
Thanks guys! It’s hot here in SW Florida!
As a commercial tech….you are speaking my language…..we dont hardly ever see a liquid line port. I will say though….ran into a weird situation 2 weeks ago. Mammoth WSHP it has a discharge and liquid port…..and no suction port…..i was like what the heck is this. Great video
Hey Brian! Any chance y’all could update your playlist on your Chanel?
Great video, thanks for your excellent explanation Are you in Orleans ?
Great videos 😜
I used to install a small stick-on dot on the discharge line that would change color so I could tell if the compressor had overheated.
So, the general industry standard is not to exceed approximately 250 degrees, 6 inches down the discharge line. What is the minimum temperature standard to keep from damaging the compressor? I know that most compressor mfg. say not to go below 20 degrees superheat on the suction inlet, but what is the minimum temperature on the discharge line?
I'm a technician for Fluid Chillers Inc. Damn this is a fasciating video. I have run ultra low temperature Chillers with as low as -20/ or 20Hg suction line pressure. These are two stage Bitzers with interstage subcoolers. So the discharge of the interstage H/E helps cool down the compressor?
Thanks for info
Thanks Bryan