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Thread: Superheat

  1. #101
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    Re: Superheat



    Thanks Tony

    I would have thought a system could hunt with a low or high superheat, hunting only indicates valve opening/closing to much or too irratic, rather than small adjustments

    CB



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    Re: Superheat

    I would have thought a system could hunt with a low or high superheat, hunting only indicates valve opening/closing to much or too irratic, rather than small adjustments
    I believe you are right CB

    If you look at "hunting" as a process then it could be described as the rate of change versus the rate of compensation.

    i.e. the TEV can only respond to the result of an action that happend previously.

    The valves own build characteristics provide a reasonable dampening effect at a specific band of operating parameters.

    The important parameters are
    1) quality of refrigerant at the entry to the valve
    2) Amount of change to the quality of the refrigerant at the point of sensing.

    So in effect the evap will flood before it is starved and will starve before it will flood.

    The TEV characteristics and the above parameters will change the degree of flooding and starving.

    I hope this helps...............otherwise it is an example of too much time on my hands X tendency to waffle about stuff .
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    Angry Re: Superheat

    Quote Originally Posted by bill1983 View Post
    i have been in the trade for 25 years but didn't start thinking about superheat until 9 or 10 years ago. i don't know how i managed to fix anything and make the repair last. In commercial refrigeration the emphasis wasn't(and still isnt) on the scientific side of the business, but on how many calls you can do in a day. When i was younger i used to pride myself on doing as many calls as i could. Now i tend to concentrate on properly fixing the 3 or 4 calls i now attend each day.
    What made me change?
    Well gather round and i'll tell you.
    It was a chance meeting with a retired engineer who was shopping in the store where i was working. I was having a problem setting a tev in a freezer case. Despite working through a 5 year apprenticeship i couldn't recall ever using eithe a guage or thermometer to set up a tev. I was always tought how to do it with the frost test; that is if the suction line is frosting back to the compressor, then shut the valve in until it stops frosting, then you would be in the right area. and this teaching came from a well respected engineer. The retired engineer (i never found out his name) told me how to measure superheat properly (and what is more impportant why); when i saw how simple and obvious it was i was amazed, and began to start questioning all i had been tought. i got as far as i thought i could with logical thought then started buying books. eventually i found this forum and others like it.When you see the kind of questions posted and the depth of knowledge and experience in some of the answers, it must make a lot of engineers realise how little a lot of us know. My advice to any members of this forum is not just to use it to ask questions for your own benefit, but every once in a while flick through the forums to see if anything catched your eye. If it doesn't, select a question at random you never know, something you think you know all about may just be being questioned, and it might just make you think. For those who like me thought they knew enough to do a good job try it somebody elses way for a change; you never know, it might just be better than the way you've been doing it for years.
    oh and the question is, if you've never bothered about superheat then how can you tell if the system is efficient, if you have a full refrigerant charge, if the tev is correctly set, if the compressor is receiving the correct amount of superheat to maintain it's cooling, if there is no liquid returning to the compressor, if the evaporator is defrosting correctly, if the controls are correctly set for the systems best and most efficient operation. in these times of energy efficiency drives, anybody who doesn't understand superheat really should ask the question: what kind of engineer do i want to be?
    i'm not shooting you down coolhibby because i used to be just like you, but it is time now to start thinking for yourself, use this forum and any other sources that are available to you to begin to become an engineer.
    sorry about the length everybody but i thought the point needed to be made
    That old guy was probably an engineer who worked for Troldahl.....it was the 1st thing we were tought
    I find it quite shocking how many engineers these days seem to have grasped a perverted version refrigeration engineering skills.
    Just goes to show....there's a certain lack of training and apprentices these days.

  4. #104
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    Re: Superheat

    Im new to this forum and ive been going through a lot of them and i find them very helpful i would like to say thanx! but i have a question regarding superheat. i got this W/I cooler R22 system i cant get the superheat right my head pressure is 200psi suction 44psi full sightglass TXV bulb is about the 4 O' clock position, i know on coolers superheat should be 8`F to 10`F. i got to 10 but then it just dropped to -18`F the closest ive got it was 2`F. could it be faulty TXV? or oversized orfice? or what else could it be? what do you suggest i check for?

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    Re: Superheat

    Are you saying that the superheat dropped to -18F?

    I don't think so. Negative superheat is not possible.

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    Re: Superheat

    44psi = 21F
    21 + 8 = 29F
    21 + 10 = 31F

    At 44psi (21F SST) the suction line temp at the TXV bulb should be 29-31F.

  7. #107
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    Re: Superheat

    SUPERHEAT: find your saturated evaporating temperature by converting your low side pressure on your gauges to a temperature using a pressure-temperature chart for that refrigerant.Now take the temperature of the suction line about 100mm after it leaves the evaporator.The difference between these temperatures is you superheat.
    EG: evaporating temp= -2 degrees c
    suction line at evap= 4 degrees c
    Then superheat= 6K
    SUBCOOLING: is very similar,you get your saturated condensing temp by converting your high side prssure to a temperature then measure the line exiting the condenser the difference betwwen these is your subcooling
    EG condensing temp= 46 degrees c
    line exiting cond.= 42 degrees c
    subcooling= 4k
    good superheat setting is 6 k
    good subcooling is 4 k

  8. #108
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    Smile Re: Superheat

    Quote Originally Posted by moonman View Post
    Im new to this forum and ive been going through a lot of them and i find them very helpful i would like to say thanx! but i have a question regarding superheat. i got this W/I cooler R22 system i cant get the superheat right my head pressure is 200psi suction 44psi full sightglass TXV bulb is about the 4 O' clock position, i know on coolers superheat should be 8`F to 10`F. i got to 10 but then it just dropped to -18`F the closest ive got it was 2`F. could it be faulty TXV? or oversized orfice? or what else could it be? what do you suggest i check for?
    One of my guy call in with almost identical problem as you describe. Negative super heat?? I couldn't beleived my ear.

    I went to site. it turned out, Suction valve was broken - split in the middle, sometime it will pump down to negative, sometime not. (it was a semi-hermatic compressor, we take the head apart and replace the valve. problem solved).

    Do a pump down test, see if you have the same problem.
    Be Happy

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    Re: Superheat

    As Gary says, you can't have negative superheat, more like subcooling which suggests that you have liquid floodback to your compressor.
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Smile Re: Superheat

    Quote Originally Posted by Gary View Post
    The subcooling tells you if there is not enough refrigerant (low subcooling) or too much refrigerant (high subcooling) in the high side of the system.


    None of the above is accurate unless you have sufficient airflow through both the evaporator and the condenser, so always check the airflow first.
    Hi Gary,
    How does a tech know if he has proper air flow?. For discussion purpose: I will use this example; a Walk in freezer, approximately twenty years old, unknown brand - label is long gone.

    Technician notice: On evaporator; one fan motor is 1/15 HP 1550RPM, the other fan motor is 1/20HP 1500RPM. both fan blade is 14", 4 blade, approx. 23 pitch. Question; how do you determine if you have proper air flow?
    Further: On condensing unit: one fan motor is 1/10 HP, 1600RPM, one motor is 1/8 Hp 1600 RPM. How do you know if you have proper air flow?
    Be Happy

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    Re: Superheat

    Delta-T (dT) is the difference between coil entering and leaving air temperatures. As airflow decreases, dT increases. On a freezer evaporator coil, the dT should be no more than 20F/11K. On a condenser coil, the dT should be no more than 30F/16K. Excessive dT means insufficient airflow.

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    Re: Superheat

    interesting wise and nice people thanks for letting me be in the forum

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    Re: Superheat

    Hello all
    how do you masuer to superheat foor sabro compresso
    thak you

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    Re: Superheat

    Quote Originally Posted by yacoub View Post
    Hello all
    how do you masuer to superheat foor sabro compresso
    thak you
    Same as any other compressor.
    Now, when I am officialy citizen off EU, I am looking for decent job! For any job offer please check my profile!

  15. #115
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    Re: Superheat

    here's a controversial superheat question. As an evaporator coil frosts up does superheat increase or decrease? There's less heat being absorbed but the compressor may pull to a lower pressure.

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    Re: Superheat

    Quote Originally Posted by jumper View Post
    here's a controversial superheat question. As an evaporator coil frosts up does superheat increase or decrease? There's less heat being absorbed but the compressor may pull to a lower pressure.
    That depend on expansion device.
    Now, when I am officialy citizen off EU, I am looking for decent job! For any job offer please check my profile!

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    Thumbs up Re: Superheat

    On Starting new Equipment, Only check superheat when you are close to Thermostat Setpoint. Under High load and of course your superheat will higher than target range. Do not change Superheat Setting just to change it unless it is really needed. Of course as Gary as stated, Consider Air Flow and Dirty Coils first but than again Dirty Coils on new Equipment shouldn't be happening. But most Air Conditioning Manufacturers are more concerned with Subcooling on charging their systems correctly. If you notice most installation Manuals will have Subcooling Chart but not a Superheat Chart. But simply making a note of Superheat on new Equipment is probably good practice. Are you going to need to change setting on new Equipment with proper sized TXV. Probably not.

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    Re: Superheat

    hi,iam new to this site,i want to know the what will be the correct super heat for evaporator and compressor in f or c

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    Re: Superheat

    Quote Originally Posted by venkatesan View Post
    hi,iam new to this site,i want to know the what will be the correct super heat for evaporator and compressor in f or c

    It depends on the system and compressor, what system are you working on?

    Jon

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    Re: Superheat

    its a cold room temp is -22'c

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    Re: Superheat

    what are basic tips of an installation engineer before instaling the cold storage panels and pipe routing

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    Re: Superheat

    I learn from you guys, thank you

  23. #123
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    Re: Superheat

    hey sir can u tell me v can measure the super heat with out the gauge as u said earlier so whtz the thing which v can come to find the superheat with out gauge.

  24. #124
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    Re: Superheat

    You cannot measure superheat without a gauge only temperatures.
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    Re: Superheat

    You could do it with 2 temperature meters (TM): The 1st TM measures the temperature of the first bend where there's no SH realized yet and the other TM measures the temperature on the outlet of the evaporator.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Superheat

    Why are temps expressed in C and superheat in K, I assumed it was Celcius and Kelvin but thats not right is it, and quite apart from How we measure and where what ,in simple terms are the desired figures as far as I can make out it's 6C superrheat and 5 to 8 c subcooling is that right, it,s very near to what I learned
    Devlin

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    Re: Superheat

    Hi devlin,
    'K used in superheat refers to the delta/ temperature difference. Check the kueba.de web site for super TXV super heat checks and settings.

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    Re: Superheat

    Hi, devlin maguire

    Quote Originally Posted by devlin maguire View Post
    Why are temps expressed in C and superheat in K, I assumed it was Celcius and Kelvin but thats not right is it, and quite apart from How we measure and where what ,in simple terms are the desired figures as far as I can make out it's 6C superrheat and 5 to 8 c subcooling is that right, it,s very near to what I learned
    Devlin
    see this ....

    http://www.refrigeration-engineer.co...efinition-reqd

    http://www.refrigeration-engineer.co...tive-superheat

    you can search RE forums too .... for Kelvin, SC&SH ... a lot of posts ..

    Best regards, Josip

    It's impossible to make anything foolproof because fools are so ingenious...

    Don't ever underestimate the power of stupid people when they are in large groups.

    Please, don't teach me how to be stupid....
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  30. #130
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    Re: Superheat

    Hi Frank,
    I have installed Cold Room its set point in -22 C but the compressor does not off except defrosting time means remains on and some times temperature reaches to -18 C I measured its super heat which details are below.
    Suction line pressure 20 PSI convert to PT chart R-22 temperature is -5 F
    Suction line temperature is 71 F so that according to calculation super heat is (71 - (-5) ) = 76 F ? (71-5) = 66 F ?
    In both condition the super heat is too high so my question is that in cold room what super heat is ideal at evaporator and compressor side as in A/C
    Compressor side Super heat 20~25 is ideal kindly reply I am really confused.

  31. #131
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    Re: Superheat

    Hi Ismail,

    I try to get the superheat between 4-8K when working in Celcius. This will let you know that the evaporator is fully flooded with refrigerant but you have enough superheat to make sure the refrigerant is in a vapor state when it enters the compressor.

    For a freezer room you should be evaporating at least -25'C. This could just be short of gas but you would need to take more readings and find out what your sub cooling value is. That should also be between 4-8K.

    Low subcooling + High Superheat = low gas charge

    Also check for clean condenser with correct fan speed. Did you remove all the air with a vacuum pump before charging with refrigerant?

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