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  • Refrigeration 101

    I was thinking it might be a good idea to set up a basic thread to send the newbies to:

    Refrigeration 101:

    Let's start from the beginning:

    Wet your finger and wave it in the air. What you are feeling is a refrigeration effect. When a liquid turns to a vapor it absorbs heat. In this case it is sucking the heat out of your finger.

    The opposite is also true. If that vapor then loses that heat, it will turn back into a liquid.

    In a refrigeration system, we force a liquid to become a vapor in the evaporator, thus absorbing heat from the refrigerated space.

    We then use a compressor to pump that vapor to the condenser.

    In the condenser we force that vapor to reject the heat and thus turn back into a liquid so that we can re-use it.

    We then meter the liquid back into the evaporator to complete the loop and do it all over again and again and again.

    How do we force a liquid to become a vapor?... or a vapor to become a liquid? By manipulating its boiling point.

    The boiling point is the temperature at which the liquid turns to vapor when heat is added. It is also the temperature at which a vapor turns to liquid when heat is removed.

    Boiling point = saturation temp = evaporating temp = condensing temp

    When we think of the boiling point of a liquid it is the boiling point at zero psi pressure. If we increase its pressure we raise its boiling point. If we decrease its pressure we lower its boiling point.

    In the evaporator we force liquid to become a vapor by lowering its pressure until its boiling point/evaporating temperature is lower than the air it is trying to cool.

    In the condenser we force the vapor to become a liquid by raising its pressure until its boiling point/condensing temperature is higher than the air it is trying to heat.

    Different substances have different boiling points at different pressures.

    We can tell what the boiling point/saturation temp/evaporating temp/condensing temp is at various pressures for common refrigerants by checking a pressure/temperature chart.

    Okay... let's go a step further: Superheat and subcooling.

    If we boil off a liquid into vapor and then add heat to that vapor its temperature will rise above the saturation temperature. This is called superheating the vapor. When its temp is 10 degrees above the saturation temperature it is superheated 10 degrees. When its temp is 20 above saturation it has 20 degrees of superheat, etc, etc.

    Similarly if we condense a vapor into liquid and then further cool the liquid this is called subcooling. When the temp gets 10 degrees below saturation it has 10 degrees of subcooling. When its temp is 20 degrees below saturation it has 20 degrees of subcooling.

    Refrigerant flows very rapidly through the evaporator coil into the suction line. Many people believe that you can't have superheat until the liquid has all turned to vapor, but this is not true. Because of the velocity of the refrigerant flow it is possible to have liquid droplets surrounded by superheated vapor at the outlet of the evaporator... and in fact this is what happens. All of the liquid droplets are gone by the time there is 5-10F/3-5.5K superheat.

    We want the superheat at the evaporator outlet to be low enough to ensure that we are fully utilizing the coil, thus maximizing its ability to absorb heat, but we do not want liquid droplets to be sent back to the compressor.

    Similarly, it is possible to have vapor bubbles surrounded by subcooled liquid at the outlet of the condenser. All of the vapor bubbles disappear at about 10-15F/5.5-8.5K subcooling.

    We want the subcooling to be high enough to ensure that we are sending sufficient liquid to the metering device, but not so high that we are backing up liquid into the condenser, thus reducing its ability to reject heat.

    On a cap tube system there is a fixed amount of liquid flowing into the evaporator. When the load is heavy there is warmer air flowing through the coil and thus the liquid is all boiled off long before it reaches the outlet of the coil, thus the superheat is high when the load is heavy. If properly designed and charged, the superheat will be just right when the design temperature (design load) is reached.

    Many people believe that a TXV will maintain a fixed superheat, regardless of load. This is just simply not true. When the load is heavy the superheat rises and more liquid is fed to the evaporator. The superheat remains high as long as the load remains high. And again, the superheat is just right when the design temperature (design load) is reached. But the design temp will be reached sooner because of the extra refrigerant feed.

    As we see, when the load decreases the superheat decreases... so what happens when the filter gets dirty, or the evap coil... or the blower wheel? Less airflow means less load therefore the superheat drops, even though the refrigerated space may be at design temp.

    When the load is high the superheat is high, and when the load is low the superheat is low... even with a TXV.

    Everywhere, throughout the system, there are opposing forces balancing against each other, and it can be very difficult to tell which of these forces is out of balance.

    And yes, there is more... much much more... but that's enough for now.
    This article was originally published in forum thread: Refrigeration 101 started by Gary View original post
    Comments 35 Comments
    1. botrous's Avatar
      botrous -
      Hi Gary,
      It seems for me a great Idea, please let me know if I can be any help.
      I'm an HVAC teacher in the industrial technical institute in beirut.......

      It would be a pleasure or me to contribute in such a thread.

      Best Regards
    1. Gary's Avatar
      Gary -
      Thanks, Botrous

      What I had in mind here was to do a re-write of the Refrigeration 101 thread in the Fundamentals forum:

      http://www.refrigeration-engineer.co...rigeration-101

      You might want to read through it, see what is needed and add it to that thread. Then eventually I will compile and condense it all.
    1. Makai's Avatar
      Makai -
      I am one of the newbies that such a write-up would help. What is a TXV? Overall, I note in this forum a lot of acronyms are used without definition. That is great is you are an insider, but a bit of a hassle if you are still trying to figure things out.

      Thanks for your efforts.
    1. botrous's Avatar
      botrous -
      Ok Gary, I will revise the refrigeration 101 . . . and see what i can add .......

      Can we consider a pre-schudeled refrigeration course, let's say that each monday a course/article will be posted on a pre-agreed curriclum basis ?

      Best regards
    1. botrous's Avatar
      botrous -
      Quote Originally Posted by Makai View Post
      I am one of the newbies that such a write-up would help. What is a TXV? Overall, I note in this forum a lot of acronyms are used without definition. That is great is you are an insider, but a bit of a hassle if you are still trying to figure things out.

      Thanks for your efforts.
      TXV stands for thermostatic expansion valve and this notation is widely used in books, manuals , and from manufactureres.

      Regards
    1. nike123's Avatar
      nike123 -
      I would say typical newbie!
      http://tinyurl.com/4bcqayc
    1. davidfonzy's Avatar
      davidfonzy -
      Hi all i have just passed my refrigeration and air con course and have my fgas,but have been looking at jobs and all want between 2-10 years experiance,the course cost me 3000 and wanted to get a good trade behind myself,i must say i do not feel confident enough to go out on my own,has any one got any suggestions,please email any advice to davidfonzy@hotmail.com. thanks to all those who try to help.
    1. L4vK's Avatar
      L4vK -
      Quote Originally Posted by nike123 View Post
      I would say typical newbie!
      http://tinyurl.com/4bcqayc
      Thats just brilliant ! Haha
      You can't help someone who is not willing to dig at least 20 seconds to find something
    1. mustafaalam's Avatar
      mustafaalam -
      I need help with setting up a cold storage for potatoes. Can someone please guide me? The engineers in my part of the world aren't as well versed as you guys are. They are suggesting freon as opposed to ammonia.
      I want to set up something for 25000 bags, Each bag is 120 kg. Can someone please help ?
    1. djd1010's Avatar
      djd1010 -
      I have a Carlyle 5H80 Compressor. The suction pressure and oil pressure are the same while running & cuts off on oil failure after a short time delay. Does anyone know if this is a sign of oil pump failure?
    1. nike123's Avatar
      nike123 -
      Quote Originally Posted by mustafaalam View Post
      I need help with setting up a cold storage for potatoes. Can someone please guide me? The engineers in my part of the world aren't as well versed as you guys are. They are suggesting freon as opposed to ammonia.
      I want to set up something for 25000 bags, Each bag is 120 kg. Can someone please help ?
      I really don't understand what that has to do with subject, or, you are just posting your problem without care what section, theme or else, it is compatible.
    1. monkey spanners's Avatar
      monkey spanners -
      Quote Originally Posted by djd1010 View Post
      I have a Carlyle 5H80 Compressor. The suction pressure and oil pressure are the same while running & cuts off on oil failure after a short time delay. Does anyone know if this is a sign of oil pump failure?
      Its a sign its tripping on low oil pressure, could be faulty crankcase heater, pump, wrong/no oil, blocked filter, faulty pressure switch, faulty expansion valve/low suction superheat, worn bearings etc
    1. asiakevin's Avatar
      asiakevin -
      Thanks to all that's willing to help out.
    1. toni82's Avatar
      toni82 -
      hi
      Can you help me with password for service level for Power face Hiromatic E

      THNX
    1. billy7's Avatar
      billy7 -
      cant understand why you say this, iam sure talented engineers can deal with all sorts of crazy questions. people need to learn chances should be given , so putting people into pods is not the answer or is it you seem to think your a lot higher than the average engineer. hered it all before . talk the talk but can u actually get the pack running when it fails. a little knollage is a dangerous thing, i go to work every day and listen to the bull i can do this and that, but no its engineers like me that managers ask for on the otherside of the phone. this is why no up and coming young lads are doing well. pods mate
    1. Kompulsa's Avatar
      Kompulsa -
      I decided to start doing this (just for the basics) on my website recently and spell it all out. I like how you wrote the beginning especially. My tutorial is going to be free, as all of the others are. I want a full and intuitive manual that is free for all to read. Not just a little piece of information like what people usually find online.
    1. chillerman2006's Avatar
      chillerman2006 -
      Quote Originally Posted by billy7 View Post
      cant understand why you say this, iam sure talented engineers can deal with all sorts of crazy questions. people need to learn chances should be given , so putting people into pods is not the answer or is it you seem to think your a lot higher than the average engineer. hered it all before . talk the talk but can u actually get the pack running when it fails. a little knollage is a dangerous thing, i go to work every day and listen to the bull i can do this and that, but no its engineers like me that managers ask for on the otherside of the phone. this is why no up and coming young lads are doing well. pods mate
      Hi Billy

      its not clear to me who you are referring to, so I will illiminate the doubt for you where I can 'Gary' 'Botrous' 'Nike' 'Monkey' can all walk the walk

      I have met a lot of engineers over the years, all of different abillity & have always found the ones that feel the need to 'big' themselves up are still trying to gain recognition

      maybe you could give the forum much of your free time & help others as they have & then maybe I could give you the same respect that they have already earned

      R's chillerman
    1. chillerman2006's Avatar
      chillerman2006 -
      Quote Originally Posted by Kompulsa View Post
      I decided to start doing this (just for the basics) on my website recently and spell it all out. I like how you wrote the beginning especially. My tutorial is going to be free, as all of the others are. I want a full and intuitive manual that is free for all to read. Not just a little piece of information like what people usually find online.
      Hi Kompulsa

      I can email you 100's of meg of info, whats your email download limit ?

      Will send you a good book to get you started

      R's chillerman
    1. TRASH101's Avatar
      TRASH101 -
      Si mundus non lactaverunt .... Nos omnes excidere !
      Mundus est ex magna parte lactaverunt....Ut enim multae et magnae
    1. chillerman2006's Avatar
      chillerman2006 -
      Sorry Gary......

      Hi Trash

      translation to english is not coming out right, as usual

      here's another gooden

      Absentem laedit cum ebrio qui litigat

      Chiller homo
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