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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. Icething's Avatar
      Icething -
      I guess this is where i belong. I am not a professional refrigeration tech. I have worked in rinks for the last 20 years. I started off as an icemaker, worked up to foreman and now run the maintenance side of the building. I am trying to get a handle on some of the basics with regards to our ammonia plants. Our plantroom has three tanks stacked on top of each other-the condenser on top then a tank in the middle that is, i think, a flooded tank?? and on the bottom is the evaporator or chiller. I have been researching the theory of how the ammonia vapor compression system works and i get the concept. What i don't get is what that center tank is for. I am not trying to be a professional here but i want my guys to have some understanding of how things work in there.
    1. L4vK's Avatar
      L4vK -

      First and foremost I am still at school.

      But, I think what are your referring is an Absorption refrigeration system http://en.wikipedia.org/wiki/Absorption_refrigerator. (Someone would need to acknowledge me on this one)

      I worked a bit this summer and saw one of those, the technician I was working for told me absorption is usually done whit ammonia, plus I remember seeing those kind of tanks. ( I have very MINIMAL knowledge about absorption)
    1. L4vK's Avatar
      L4vK -
      I just don't understand why would they use this is an Arena... this is no heat to use and electricity is available. Are you sure it is Ammonia in ALL of those tanks and not glycol?
    1. Gary's Avatar
      Gary -
      Both of your assumptions are wrong.

      Not all absorption systems are ammonia and not all ammonia systems are absorbers.
    1. RANGER1's Avatar
      RANGER1 -
      Could it be

      Top vessel, shell & tube condensor on top with water pumped through it to condense it to liquid.

      Second vessel, liquid reciever for storage of ammonia if you want to pump it down for shutdown etc.

      Third vessel shell & tube evaporator with glycol or brine chilled down to circulate out to ice rink

      So compressor sucks from bottom vessel (cold).
      compressor discharges to top vessel (hot).
      top vessel drains to 2nd vessel (warm) probably has a level sight lass somewhere on it
      then fills bottom vessel (chiller, cold) Probably has a float control on chiller, turning solonoid valve on/off to control level in it, or something like it.
    1. mustaza's Avatar
      mustaza -
      carrier chiller fault codesguys good day i m sufring from the carrier chiller model 30 GB 150 showing erore code 09 can you plz define me or help me to solve the problem thanx
    1. olgadrake's Avatar
      olgadrake -
      wow. this will be a very good research paper for my kids term paper. thanks a lot!
    1. moideen's Avatar
      moideen -
      Quote Originally Posted by botrous View Post
      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
      Thanks botorus
      carryon,vey good idea,i am expecting your 'revised 101'
      all the best
    1. d.dufour's Avatar
      d.dufour -
      I, We did buy, a bit more that 10 years ago, a Viper VRGT equiped with Refrigerant Analysis. I have a problem with the gas identifier... placed at the front of the equipment. (see picture) the part number is # 655020 and date from 1990 decenies. I did a mistake when I clean the hose, I didn't note the way on the small plastic oneway connectors. No notes for the one way valves. I'm looking for a schema (explosed wiew) of the Refrigerant Analysis. Can you guide me to find that information. Thanks’ in advance and have a nice Christmas holiday. Denis DufourAuto body repair TeacherC.F.P. Verdun Q.C.
    1. rehanqatar's Avatar
      rehanqatar -
      hi sir i need airdale user passwordfor setpoint i use 4648
    1. rehanqatar's Avatar
      rehanqatar -
      hi dear sie i need airdale ac user password i use for setpoint 4648
    1. andy fridge man's Avatar
      andy fridge man -
      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?

      sounds to me that the oil pump pin has broken, this is located in between the oil pump shaft and the main crankshaft. check thrust bearings as this is the usual cause of the pin breaking
    1. bushell01's Avatar
      bushell01 -
      Quote Originally Posted by davidfonzy View Post
      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.
      David my names mark I'm a engineer from serv-ice refrigeration can you email me at mark@servicerefrigeration.co.uk so I can ask some info we have positions going for engineers in London
    1. ramesh.beemani's Avatar
      ramesh.beemani -
      hiiam having a problem with my refrigeration system .problem is that iam getting a cut off sound from my system . i have checked the fan for any rubbing by running the fan individually and it found ok at the same time i checked my drain line and valve and found ok can you suggest me the cause for that cut of sound (cut cut cut................) .
    1. _Carl_'s Avatar
      _Carl_ -
      Quote Originally Posted by L4vK View Post
      Thats just brilliant ! Haha
      You can't help someone who is not willing to dig at least 20 seconds to find something
      Agreed and very LOL
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