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Thread: delta T ?

  1. #14
    Join Date
    Apr 2002
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    Ann Arbor, MI
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    2,985
    C'mon boys, let's be more civil here.

    Delta T of the air across the evaporator is a function of entering dry and wet bulb temperature, air velocity across the coil, and evaporating temperature. umd provides a good explanation of the psychrometrics involved. Be careful about "rules of thumb". I like tinknocker service tech's response best

  2. #15
    Join Date
    Mar 2005
    Posts
    17

    Oh Yeah!

    "Ok, I'll take the bait...The dry bulb delta T across the evap coil will be a function of the amount of sensible heat removal from the airstream and the amount of air going across the coil. Now if you were only doing sensible coiling (not removing any moisture) the formula would look like this: BTU/HR(sensible)=CFM X1.08 X DELTA T. Unfortunately in the real world you are usually removing some latent heat which is not measurable by dry bulb temperature. To accurately predict the delta T, you would need to know the wet bulb temperature of the entering and leaving air. You would also need to know the airflow across the coil. Using a psychrometric chart you could predict what your delta T would be by calculating the actual enthalpy per pound of heat in your entering and leaving air and use the formula: BTU/HR (total)= CFM X 4.5 X DELTA H (actual heat content of air).
    The bottom line is this: your delta T could be anywhere from 10 to 20 degrees depending on the amount moisture in the air and the amount of air going across your coil. Low humidity equals high delta T; high humidity equals low delta T."

    umd,
    All that said, I get the feeling that you feel that delta T readings are sorta bogus, kinda like a "beer can cold and sweating" sort of measurement that is often misused/not required at all. Is there any real use for any delta T reading? Since there are so many factors involved in truly measuring this correctly (as you've written), and the results can vary so greatly (depending on so many more things than just the air temp difference), what's the use? Wouldn't proper superheat/subcooling calculations reveal air flow issues? Perhaps this was your point.
    I'm just trying to ask what I feel might be the next reasonable question to ask about this topic. I don't go to the field anymore but would like to try to keep a bit sharp about these things. So, if my questions sound like I might already know the answer to them, I may have known at one time, but have forgotten the answers from lack of use. Thanks in advance for any info you might provide!

    I love this website.
    Jim

  3. #16
    Join Date
    Jul 2001
    Posts
    40
    Jiminator writes "Is there any real use for any delta T reading?"

    Good question. I think the answer is yes. Heeding the advice of Andy regarding "rules of thumb" I think that on a system which is known to be properly installed and is operating at "steady state" conditions, delta T provides a good initial indicator of the overall performance. Here in the Northeast during the months of July and August we can generally "assume" that a home which is continously conditioned will have a relative humidity of around 50%. Bearing this in mind we might look for an initial delta T of 15 to 18 degrees. If the system has been out of service, the ambient RH will probably be higher and on start-up the delta T may be lower.

    My point would be this: It is far more important to understand the theory behind what goes into the delta T of an evaporator coil than to look for "rules of thumb" to guide us.

    Again, sorry if my initial post created any hard feelings (which I believe it did).

  4. #17
    Join Date
    Apr 2004
    Posts
    7,680
    I agree completely with UMD. His point is accurate and he makes yet another great case against rules of thumb.

    Sensible capacity can be calculated using the temperature rise method and compaired to the mfg sensible capacities found in the literature if in fact you were under the ARI conditions used. However, as he said, latent heat has priority and will cut away from the sensible capacity of the system which in turn affects the delta T.



  5. #18
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    This reminds me of a problem I had with a customer that had a "friend in the buisness" that was advising her. Unfortunatly the "friend in the buisness" was a rule of thumb kinda guy that had no education in psychometrics.

    You could check a system on a low humidity day and find a delta-t of 21, then check it again on on another day with the exact same indoor and outdoor temperatures, but with higher humidity inside and find a delta-t of 17.

    My favorite are when home inspectors come into an 85+ house, turn on the AC for 10 minutes and claim there is a problem with the AC system because the delta-t is only 12 or so. Knowing psychometrics, I read the report and think "DUH!", But most people don't understand psychometrics, so it can be interesting trying to explain why the delta-t is low on an invoice in 3 lines or less.
    If more government is the answer, then it's a really stupid question.

  6. #19
    Join Date
    Sep 2004
    Posts
    3,157
    Hey UMD:

    I am a student in this busniess. I come here to learn. I dont have all of the answers and often ask some relativily simple questions that I should know the answer to- but dont-

    It sounds like you have some knowlege to share with us but with 24 posts you mite sit back and figure out who is who here because there are some that I would bet that know enough to make you look like a student

  7. #20
    Join Date
    Jun 2004
    Location
    Florida
    Posts
    556
    Originally posted by umd
    I am a little confused...isn't this a web site for professionals in the field. The very question indicates the author knows nothing about HVAC...

    Just my opinion.
    No it's not. anyone that is interested in HVAC accept DIYers are welcome here from what I understand. I think the pro sections are a little more exclusive.

    Since your a confused I copied the part of the rules that you seemed to have missed.

    Here you go:

    HVAC Pros, please note; home owners are welcome to ask questions here, if you do not wish to be helpful that is fine but do not intimidate, obstruct or criticize those that wish to be more helpful than you. If you do not like a particular thread or topic, do not reply.

    comments like yours are what discourages the lurkers from posting.

  8. #21
    Join Date
    Apr 2004
    Posts
    7,680
    Originally posted by ct2

    It sounds like you have some knowlege to share with us but with 24 posts you mite sit back and figure out who is who here because there are some that I would bet that know enough to make you look like a student
    Not to start or continue anything here ct2 but outside of his initial response, he answerd the question as completely and accurately as it could be answered in such a forum. Maybe I missed something?

  9. #22
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,510
    bananaboy
    here is the deal with out getting technical or over your head. t/d across the coil is only a measurment for other things pertaining to the ac system and only a fool who use it as the only means of checking the refrigerant level in a system. you can geta 20 degree t/d after 10 minutes of running time and say it fine. then 3 hours later it is iced up and it wasnt fine. with out gauges, t/d. subcool, and with lenox approach method witch seems to me to be an easy way of subcool. now outside temp, inside temp, humidy inside and out side, air flo across the coil to much or to little, filter and other factors need to be taken into account. i have seen guys charge a system on t/d and way over charge it. norminaly under ideal donditions if you get between 15 and 20 degrees you should be ok. like i said should be dosent mean it is ok
    25 degree t/d night be to good. not enought air across the coil and might cause it to ice up under heavy conditions and not. is filter cleam, a coil clean, blower to low or warn and not moving enough air. or is it set up for that and is working fine. only true way to know what is going on and if it is working properly is in the beginingof this post.

  10. #23
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,817
    Originally posted by umd
    Jiminator writes "Is there any real use for any delta T reading?"



    Again, sorry if my initial post created any hard feelings (which I believe it did).

    I thought it was alittle long.LOL...

    Nice post, hope you post more answers.


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  11. #24
    Join Date
    Nov 2004
    Location
    CA
    Posts
    238
    tinknocker service tech,



    "...bananaboy
    here is the deal with out getting technical or over your head. t/d across the coil is only a measurment for other things pertaining to the ac system and only a fool who use it as the only means of checking the refrigerant level in a system. you can geta 20 degree t/d after 10 minutes of running time and say it fine. then 3 hours later it is iced up and it wasnt fine. with out gauges, t/d. subcool, and with lenox approach method witch seems to me to be an easy way of subcool. now outside temp, inside temp, humidy inside and out side, air flo across the coil to much or to little, filter and other factors need to be taken into account. i have seen guys charge a system on t/d and way over charge it. norminaly under ideal donditions if you get between 15 and 20 degrees you should be ok. like i said should be dosent mean it is ok
    25 degree t/d night be to good. not enought air across the coil and might cause it to ice up under heavy conditions and not. is filter cleam, a coil clean, blower to low or warn and not moving enough air. or is it set up for that and is working fine. only true way to know what is going on and if it is working properly is in the beginingof this post...."


    Thank you for that long post.

    The reason I posted the question was that I switched company and the new company wants me to do maintenance (i.e. cleaning or "tune-up") for the upcoming A/C season all WITHOUT pressure gauges. One of the thinks on the check list is to check "delta T".

    By no means, I would ever charge up A/C ONLY on TD, in fact in the past 9 years (yes, that long UMD), I have always relied on Subcooling, most importantly SUPERHEAT (my favourite one) in corelation with the Outside T.

    I, also, do not forget the amount of Air passing trough the Evap. Coil. Probably the most important of all. Yes.

    I always use the formula BTU(Output)=CFM x 1.08 x delta T.
    From this formula I derive the CFM. Trust me it is acurate way of determining your CFM.

    In all fairness, I have to admit - I have not used DB or WB method in my way of servicing and charging the A/C.

    People, here said - I know nothing about this business, well I know how much I know.

    So, I will find a way to explain to this new company that I need gauges (which I am sure I will get later on) to properly assess an A/C.

    What frustrates me a lot is the fact that I have all necessary tools, I mean all (and have used them all the time, I am a tool-lover and believe that tools make your life easier) but this new company I recently move to - want s to keep me as a maintenance tech. for this season.
    The good news is that I got company truck (brand new baby) company benefits and "the whole 9 yards".

    When I was independant, man, I did almost everything.
    But times have changed. With the ever increasing gas prices, I decided rather to work for somebody else.

    Anyway, I enjoyed the discussion.

    See you around.


  12. #25
    Join Date
    Jan 2004
    Location
    Midlothian, Virginia
    Posts
    195


    hey bananaboy make sure you pop the door on that there airhanler. see, then you lookie towards the coil. and iffen you see a strange little bulb clamped on the big line and it runs backie to a funny little gizsmoe in the liquid line.

    well then you have found whats call an gall durn expansion valve. aka TXV

    this will broaded your horizons to subcooling. get yourself a dual thermometer and two fluke clamps. clamp one on little line at unit along with one on big line. and study sub cooling and super heat. hell man you can see the world of the refrigeration circuit in a whole new light.

    never just look at superheat. you are blind if you do.
    Goodbyee stranger it's been nice. Hope you find your paradise! Hey it aint rocket science, "It's a Trade !"

  13. #26
    Join Date
    Nov 2004
    Location
    CA
    Posts
    238
    hey, resolutetech

    Thanks for the post, eventhough I did not learn anything from it. In fact you got me confused. Are you saying to measure Subcoling at the Evap. coil ?

    Tell me, on Salad cooler (at 40 F)running on R12, what would be the Superheat (yes, it is using TXV) ?

    Also, when switching from R12 to R409 - how do you adjust that "gizmo" as you call the TXV ?

    Is the Superheat meaningfull on a fixed metering device, if so , then what is the proper Superheat on a domestic fridge ?

    Thanks boddy (I meant buddy)

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