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Test #2

will do that :)

Was all of the insulation gone?

Let me guess, the guy who previously fixed the leak, took it off and didn't think it was important.

I like your threads, Prof, glad you're here.

Brian.

Comments

  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Test #2

    The weather has been crazy here in the Northeast and I haven't given you the second test question. Here goes.

    You have an R-22 split central air conditioning system and the operating pressures and temperatures are as follows:

    Outside ambient temperature: 90 degrees

    High side pressure: 297 psig

    Condenser outlet temperature: 110 degrees

    Low side pressure: 73 psig

    Evaporator outlet temperature: 50 degrees

    Return air temperature: 72 degrees

    Supply air temperature: 59 degrees

    Suction line temperature at compressor: 75 degrees

    Compressor discharge temperature 300 degrees

    Compressor is Very hot to the touch.

    The customer says that the system is cooling, but not as well as it used to. When it's cooler outside, the system works fine, but when the temperature rises to about 90 degrees, the system is barely able to cool the house. THe customer also notes that, when it is hot outside, the condensing unit never seems to shut off.

    Also, there was a refrigerant leak in the system last year and, after hours and hours of searching for it, the technicians were able to locate and repair the leak.

    Is the system operating correctly now?

    If not, what are your thoughts? How would you remedy?

    Have fun!
  • Empire_2
    Empire_2 Member Posts: 2,343
    E/S?

    Is the evap outlet of 50 line temp? or right above the coil as in air temp? OK I'll be the first to crack it. Being that there is higher than normal head pressure and excessive sub-cooling and a red hot comp and also with poor split out of the evap, My guess is that there are 2 problems. I think that when the leak war repaired, the system was never evacuated so air is present in the system along with an under charge. If trying to add ref to satisfy the cust. head too hi . cooler days may be easier for the system to cool house due to a 12* split.
    PS go easy on me Professor......I gave it my best.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Nice try

    Yes, the condenser saturation temperature is higher than desired but no, the condenser subcooling is not high. The condenser subcooling on this system is 20 degrees. (The pressure of 297 equates to a condenser saturation temperature of 130 degrees and, 130 - 110 = 20 degrees).

    The superheat is also correct at 6 degrees, meaning that the refrigerant charge is correct.

    We do however, have a very hot compressor, and a high compressor discharge temperature.

    The 50 degree temperature I was referring to was the temperture of the suction line at the outlet of the evaporator coil.

    SO, we saved ourselves a trip to the truck for the refrigerant tank.

    Very nice try... keep going!
  • Randy_15
    Randy_15 Member Posts: 8


    Hi Professor! Is the 25 deg temp difference between the 50 deg evaporator outlet temp and the 75 deg suction line temp at the compressor too high. Is the suction line insulated.
  • don_152
    don_152 Member Posts: 1
    is

    this a heatpump? If not then I would have to say the relief
    valve in the comp is well you know.

    I mean 44 discharge temp entering the comp and 300 coming out,whew baby.Is it sludge yet?



  • JeffD
    JeffD Member Posts: 41


    Hmmm, temp split between return and supply is only 13º. We have established that the charge appears correct. We know that it was "torn into" to find a leak. Did the tech foget to put the panel back on the air handler? Did a return duct in an unconditoned space get knocked open? Has proper airflow been verified? Sounds to me like the unit is under a very heavy load, and with a 72º return temp it doesn't sound like that load is coming from the conditioned space. If airflow is correct and return ducts aren't leaking, my bet would be on a dirty condenser coil. Unit can't get rid of heat properly when the coil is plugged up. Is this a double row condenser by any chance?
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Awesome Randy!

    You are correct.

    Picking up 25 degrees in the suction line is responsible for the high temperature at the inlet of the comrpessor and the high compressor discharge temp.

    Pat yourself on the back!

    You are soooooo smart. Who is teaching you A/C? Hmmmmmmmmmmmm
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    It's 75 degrees in

    75 degree compressor inlet temp.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Good Observations

    Very good points, so let's examine them.

    The condenser saturation temperature is high, which could lead us to believe that the condenser is dirty. A blocked condenser would, however, cause the subcooling to drop. We have normal subcooling, so the dirty condenser scenario doesn't pan out.

    A losse or removed air handler panel would result in low superheat and a higher delta-t across the evaporator coil.

    Guys, can you see how this air conditioning stuff is like a puzzle?

    Look at the conditions you have and make your assumptions. Then, make a list of the symptoms that would be present if your assumption was correct. Now, compare the symtoms the system is exhibiting to the symtoms you predicted. If they match, chances are you solved the problem. If they don't match, don't try and force them!
  • don_120
    don_120 Member Posts: 15
    Agree

    Professor, but if the suction line was picking up that much
    heat one would have to assume you would have the same problem on the inlet side of the metering device as well.

    No?

  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Not really

    If the system is picking up extra heat in the suction line, it will take longer for the refrigerant to reject that heat in the condenser. The higher saturation temperature will increase the rate of the heat transfer so it all comes out in the wash when doing a subcooling calculation. Cool, huh?
  • Glenn Harrison_2
    Glenn Harrison_2 Member Posts: 845
    This may be a stretch, but...

    I'm thinking there is a restriction in the suction line, causing the increase of suction line temp at the compressor, and the unit was overcharged in an attempt to increase the suction pressure and lower the superheat.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    could be...

    But then the subcooling would be higher...

    Nice thinking though.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    That's it!

    Exactly!

    And BRian, I'm glad that you're glad I'm here!
  • Empire_2
    Empire_2 Member Posts: 2,343
    Professor if you can?

    In the near future, can you please run off a P/E chart and also go thru the plotting of the chart. Been 22 years since I've used one. In school, I thought it was great, it's almost like a psyical exam without all the patient complaints.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    I Would Love to...

    Mike, I would love to but, the main problem is that the chart becomes imposible to see, let alone understand when reproduced for instructional purposes.

    In the past, I have attempted to go through the whole process online, and the results were not as expected.

    In the near future, I will have a small book (about 100 pages) devoted to pressure enthalpy plotting, system evaluation and other goodies, so keep your eyes peeled.

    Also Mike, I will be taking my P/E class on the road so, if you let me know where you are located, I will make every attempt to accomodate you and yours. In the class, I tone down the "engineering-ese" of pressure enthalpy to make it a useful troubleshooting tool for the seasoned and not-so-seasoned technician.

    As a younger lad (thanks Dan) I would plot out systems on pressure-enthalpy charts and keep a running history of my long-term customer's systems. It gave me the opportunity to predict system failures long before they actually arose. Vey cool stuff.

    In the meanwhile, if you have any specific P/E questions, feel free to ask and I will do my best to answre them in an internet-environmentally friendly manner.

    At any rate, thanks for expressing your interest in P/E.
  • enthalpy
    enthalpy Member Posts: 13


    From info given you can calculate:
    Delta t = 13/ 72 - 59
    Delta t of the coil = 37 (72 - 35)
    90 + 30 = approximate condenser sat temp of 110 to 115
    depending on condition of the coil, which is 230 to 240.
    If evap temp is 50 at the outlet you can figure yor superheat. 50 - 44 = 6 (44 corresponding to the 73)
    If the suction line temp is 75 at compressor and subtract the 44 the superheat is about 34.
    297 converts to about 130, subtracting the 110 looks like about 20 subcoolling.

    The Delta T across the coil is to me low even if there is alot of humidity. The low superheat at the evaporator is not the greatest but not terrible.
    The first thing I would do is see if my outdoor coil is clean, this would aid in a reduced refrigeration effect and contribute to the high head pressure. Then I would re-examine the pressures.
    I would look to see how well my suction line is insulated to see if I am picking up to much heat on the return side causing higher head,possibly causing extra heat in the compressor.
    This system is not working to the proper capacity, would like alittle more info such as metering device, but will also throw in the posiblity of an overcharge.
    Please check to see if above data calculate correctly and any help would be greatly appreciated.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Correct Calculations

    Your calculations are correct and the suction line insulation is indeed damaged/removed/eaten by rats/disintegrated/ripped/or otherwise rendered ineffective.

    The main cause for concern is the 25 degrees of sensible heat that is eing picked up in the suctione line. This is a tell-tale sign that the suction line insulation is not, well, insulating.

    Good call, Dawn.

    By the way guys, enthalpygal (also known as Dawn) is one of my HVACR students in the AAS degree program at Suffolk County Community College. She has 3 years in the field and is one of the sharpest technicians I have met. She e-mailed me with a whopper of a service call situation yesterday and we will be posting it soon for all of you to ponder.

    Everyone, say hi to Dawn and welcome her to our little community.
  • Brian (Tankless)
    Brian (Tankless) Member Posts: 340
    Hi Dawn

    > From info given you can calculate: Delta t = 13/

    > 72 - 59 Delta t of the coil = 37 (72 - 35) 90 +

    > 30 = approximate condenser sat temp of 110 to

    > 115 depending on condition of the coil, which is

    > 230 to 240. If evap temp is 50 at the outlet

    > you can figure yor superheat. 50 - 44 = 6 (44

    > corresponding to the 73) If the suction line

    > temp is 75 at compressor and subtract the 44 the

    > superheat is about 34. 297 converts to about

    > 130, subtracting the 110 looks like about 20

    > subcoolling. The Delta T across the coil

    > is to me low even if there is alot of humidity.

    > The low superheat at the evaporator is not the

    > greatest but not terrible. The first thing I

    > would do is see if my outdoor coil is clean, this

    > would aid in a reduced refrigeration effect and

    > contribute to the high head pressure. Then I

    > would re-examine the pressures. I would look to

    > see how well my suction line is insulated to see

    > if I am picking up to much heat on the return

    > side causing higher head,possibly causing extra

    > heat in the compressor. This system is not

    > working to the proper capacity, would like

    > alittle more info such as metering device, but

    > will also throw in the posiblity of an

    > overcharge. Please check to see if above

    > data calculate correctly and any help would be

    > greatly appreciated.



  • Brian (Tankless)
    Brian (Tankless) Member Posts: 340
    Hi Dawn

    Welcome to the Wall.

    Funny how all those calc's can result in "eaten insulation on the suction line, eh?)

    Brian in Swampland.
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