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    Dry bulb or wet bulb


    When we measuring DT at evaporator, what should we use as temperature measurement, dry bulb or wet bulb temperature of entering and leaving air?
    I presume wet bulb, but I am not 100% sure that I am right.



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    Re: Dry bulb or wet bulb

    I would say dry bulb below freezing and wet bulb above freezing

    rule of thumb as freezer / fridge / ac is the difference
    Last edited by Quality; 15-09-2011 at 08:22 PM. Reason: typo

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    Re: Dry bulb or wet bulb

    I mean mostly for airconditioning and evaporation temperature above freezing.

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    Re: Dry bulb or wet bulb

    I misunderstood with regards to your first post. I thought you were refering to moisture (I am a fridge man) its dry bub on and off

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    Re: Dry bulb or wet bulb

    I have example from today measuring (cooling mode) of one Mitsubishi Electric (MSH-GA60VB) split system 6,5kW , which, by my calculations, works perfectly.

    Indoor unit inlet DB 32C
    Indoor unit inlet WB 21C
    Indor unit out DB/WB 12C
    Evapration temperature 6C
    Superheat 10K
    Indoor unit air flow 770 m^3/h (factory data but unit is clean and fan at max speed)
    Outdoor air temperature 32C DB
    Uotdoor unit air out 41C
    Unit nameplate current 10,4A
    Measured current 10A
    Condensate removal 2l/h

    Therefore evaporator
    TD is 21-6=15K
    DT is 21-12=9K
    Approach is 12-6=6K
    Does approach look little low regarding rule of thumb of 8-10K? (i know that rule of thumb is just that)

    If I use DB for DT than I got 32-12=20K
    DT and approach summed should give TD, and that does not match in this case.
    Last edited by nike123; 15-09-2011 at 09:03 PM.

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    Re: Dry bulb or wet bulb

    Its the outdoor dt of 9 which is low in my book

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    Re: Dry bulb or wet bulb

    Last edited by nike123; 15-09-2011 at 09:35 PM.

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    Re: Dry bulb or wet bulb

    I checked it but believe dt 9 is still low

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    Re: Dry bulb or wet bulb

    DT of 9K with airflow of 2760m^3/h gives 8 kW of rejected heat which is almost exactly 6,5 kW cooling capacity and 1,9 kW of compressor adsorbed power added together. Take in consideration measuring errors and you got perfectly good Air conditioner with fairly sized condenser. That confirm its visual apearance which is simmilar in size as some 8-10kW (cooling capacity) units of cheaper brands.

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    Re: Dry bulb or wet bulb

    Hi Nike 123.
    the total heat formula using delta h will give an actual system performance, shows up some cheaper systems that have low coil surface areas in evaps and low SHR factors.
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    Last edited by Argus; 16-09-2011 at 02:28 AM.

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    Re: Dry bulb or wet bulb

    Here is performance calculation of my example:




    OK, then to rephrase my original question.
    When we evaluating evaporator performance of air conditioner we use 3 temperature intervals:
    TD
    DT
    Approach
    Approach + DT = TD
    Am I correct?

    What temperatures we use to calculate any of these intervals. Dry bulb or wet bulb.
    From my example if we use wet bulb for everything:
    "TD is 21-6=15K
    DT is 21-12=9K
    Approach is 12-6=6K"
    6K+9K=15K
    and that fit in equation.

    If we use dry bulb for everything, than:

    TD is 32-6=26K
    DT is 32-12=20K
    Approach is 12-6=6K

    Equation Approach + DT = TD match, but numbers for perfectly good AC doesn't look good.

    If we use dry bulb only for DT then:
    TD is 21-6=15K
    DT is 32-12=20K
    Approach is 12-6=6K
    we than have equation 6K+20K=15K which doesn't match and therefore my conclusion that we should use only WB temperatures for evaluation of performance of AC evaporator (not for calculation of actual absorbed heat).

    Than that brings new question: Is this rule of thumb based on wet bulb temperatures for all measurements?

    Tai-Tao = 8 to 10 K Evaporator Δt
    Tai-Te=16 to 20 K (evaporator TD) take low TD for high relative humidity
    Tao-Te=8 to 10 K Evaporator approach

    Also, what if we don't have any condensation on evaporator coil? What temperature we should use than for DT interval?
    Last edited by nike123; 16-09-2011 at 06:54 AM.

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    Re: Dry bulb or wet bulb

    Another one!

    When we use data from tehnical manuals for air volume flow, what air volume flow is stated there?
    Air volume flow before or after heat exchange since air density is changed by heating or cooling of air.
    I presume before heat exchange as they probably measure it without cooling or heating action on heat exchanger, but we always/usualy measure air volume flow after coil.
    Am I correct here?

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    Re: Dry bulb or wet bulb


    Therefore evaporator
    TD is 21-6=15K
    DT is 21-12=9K
    Approach is 12-6=6K
    Does approach look little low regarding rule of thumb of 8-10K? (i know that rule of thumb is just that)
    In this unit evaporator air flow is 760m^3/h for 6,5kW of capacity. That is cca 240cfpm/TR.
    Rule of thumb of 8-10K for approach is probably based around 400cfpm/TR rule of thumb which is rare case in domestic ductless split systems.

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    Re: Dry bulb or wet bulb

    Quote Originally Posted by Magoo View Post
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    Is this your page? Could we (members of RE) get some discount.
    What is Webram share?

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    Re: Dry bulb or wet bulb

    Nike, the way i check air-conditioners is more simple than what you do. I always measure with digital, the air on temperature, and air off.( Dry bulb). If you want to calculate moisture removal and performance lost, (with condensate flowing down the drain), then use wet bulb. Average TD across evaporators here, with clean fan and clean evaporator, is 14*C with fan at medium speed. I know you like to work the calculations out but some A/C's are fitted with quality compressors and others are standard. 20K dry bulb TD is good..Mike.
    To the optimist, the glass is half full. To the pessimist, the glass is half empty.

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    Re: Dry bulb or wet bulb

    Quote Originally Posted by nike123 View Post
    Is this your page? Could we (members of RE) get some discount.
    What is Webram share?
    MAGOO! REALLY! And Nike123, you shouldn't be looking at that, some say it will send you blind, Ha! Good observation none the less . Now ( tapping the cane around ), where did i place my beer ??
    To the optimist, the glass is half full. To the pessimist, the glass is half empty.

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    Re: Dry bulb or wet bulb

    Quote Originally Posted by mikeref View Post
    Average TD across evaporators here, with clean fan and clean evaporator, is 14*C with fan at medium speed.
    Mike, is this 14K average TD or DT and is this for small domestic ductless split systems?
    If that is TD, it looks low to me? What I expirence here is TD more in range of 18-20K if we use DB.

    What I frequently read in manuals of their manufacturers is DT in cooling from 8-15K and in heating from 18-25K (for max speed of fan) and notice that smaller numbers are with high humidity, and that mean that they adressing DB temperatures.
    Last edited by nike123; 16-09-2011 at 08:17 AM.

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    Re: Dry bulb or wet bulb

    Yes Nike, 14*C air temperature drop through the coil, dry bulb, and on ductless systems. If humidity is high, than i expect less sensible heat reduction, somewhere around 12 K, but on low humidity days, it could be as high as 16 K. Humidity here can be anywhere from 30% to 95%. Depends on the time of year. If we had one brand a/c working in one location, and another brand in the next house, with same rated value, say 12,000 BTU, or 3.45KW cooling, and both having the same area to cool, then i would not be surprised to see different temperature reduction across the evaporators. Must say that sometimes condensing units i've seen, are fitted in areas that allow short cycling of % of ambient air that obviously causes problems . Corrosion of Evaporator and condenser fins, especially on the coast, weakens a/c ability, as you would know..Mike.
    To the optimist, the glass is half full. To the pessimist, the glass is half empty.

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    Re: Dry bulb or wet bulb

    OK than that is DT (Δt) and not TD as you previously wrote, wich is OK figure.
    DT of 12 with, say, 8K Approach gives 20K TD GOOD (high humidity)
    DT of 16 with, say, 5K Approach gives 21K TD GOOD (low humidity)

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    Re: Dry bulb or wet bulb

    Sorry Nike, what i failed to mention was the efficiency of the a/c compressor at certain ambient temperatures. Of course, the volumetric efficiency at certain ambients has an effect on it's overall performance, so at higher ambient temperatures, up to say, 35*C, and assuming (bad word), the condensing unit is rated to this, than i would expect maximum performance from this compressor. However, in lower ambients, efficiency will drop as less gas pressure will be available to compress. Please note, i have jumped many variables in my response, as in materials used, air flow, pressure reduction if non-variable speed condenser fans are used, or quality of insulation, to say the least..Mike.
    To the optimist, the glass is half full. To the pessimist, the glass is half empty.

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    Re: Dry bulb or wet bulb

    Quote Originally Posted by mikeref View Post
    Nike, the way i check air-conditioners is more simple than what you do. I always measure with digital, the air on temperature, and air off.( Dry bulb). If you want to calculate moisture removal and performance lost, (with condensate flowing down the drain), then use wet bulb. Average TD across evaporators here, with clean fan and clean evaporator, is 14*C with fan at medium speed. I know you like to work the calculations out but some A/C's are fitted with quality compressors and others are standard. 20K dry bulb TD is good..Mike.
    I suppose that's the problem doing it so simply. Using dry bulb air on/off in Perth you'd expect 20K differential with fan on high speed for ductless wall split.
    Less moisture to remove, so it's going to be a regional thing based on local ambients and Rh if you do it that way just based on dry bulb.

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    Re: Dry bulb or wet bulb

    From Daewo service manual:


    Aux:
    ②Indoor unit air inlet and outlet temperature difference: Normal temperature difference shall be within 12-14 Celsius degree, but they will also be impacted by temperature and wind velocity.

    I think one manual is DT in WB (Daewo) and other is in DB(Aux).
    Last edited by nike123; 16-09-2011 at 12:02 PM.

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    Re: Dry bulb or wet bulb

    Quote Originally Posted by paul_h View Post
    I suppose that's the problem doing it so simply. Using dry bulb air on/off in Perth you'd expect 20K differential with fan on high speed for ductless wall split.
    Less moisture to remove, so it's going to be a regional thing based on local ambients and Rh if you do it that way just based on dry bulb.
    Hi, paul h, 20k on high fan speed? Wow, that's good. I'm smack in the middle of rainforest here, leaches and marchflies, mossies and Bandicoots,( Bandicoots, food stealing rodents ). http://www.youtube.com/watch?v=JQjCSlEhZhc
    To the optimist, the glass is half full. To the pessimist, the glass is half empty.

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    Re: Dry bulb or wet bulb

    Most evaporators of ductless split systems are designed around 300cfpm/TR.
    So what would be rule of thumb for DB and WB temperature differences for this design?

    According to my calculations:
    DB evaporator temperature differential should be 7K to 20K depending on humidity 95%-30% (respectively)
    and
    WB evaporator temperature differential should be 7K to 8K depending on humidity 95%-30% (respectively)

    and therefore, evaporator performance should be judged by WB temperature as long as it produces condensate.



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    Re: Dry bulb or wet bulb

    Ah.. Sorry Nike.. Have to view, read your post with a clear head..Regards, Mike.
    To the optimist, the glass is half full. To the pessimist, the glass is half empty.

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