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  1. #1
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    Jul 2006
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    Ok,

    I'm trying to understand the derating to be applied to equipment capacity when using an IAT different than 80F. I've seen it a couple different ways on this site.

    Let's say there's a 3-ton unit rated at 36K Btuh with 27K sensible (at the ARI 80F IAT & 1200 cfm). If you design for an IAT of 75F, what is the total capacity and sensible capacity of the equipment at those conditions?

    Using the derating formula, I would assume

    Derate=835*(1200/1000)*(80-75)=5000(approx.) Btuh

    So, do you derate the total capacity and figure sensible based on SHR (I assume SHR=.75 for this example) like:

    Tot Capacity=36k-5k=31k & Sensible=31k*.75=23.25k

    OR

    do you derate the sensible only like:

    Sensible=27k-5k=22k and I'm not sure what the Total Capacity is in this case(I could argue for 31k btuh or 29.3 btuh).

    I'm thinking the latter but not sure. I assume this is covered in Manual S - I have J and D but maybe need to spring for S to do all my own calcs before talking to some contractors.

  2. #2
    Join Date
    Feb 2003
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    Huntsville,AL
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    derating is unit dependent -- probably different for each model of each manufacturer, at least each family of models

    and, the curves are not linear over the entire range
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  3. #3
    Join Date
    May 2000
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    Indianapolis, IN, USA
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    34,168
    I would think much would depend upon metering device. A fixed orifice system has reduced flow as the outdoor temps go down so it would lose capacity, especially latent, as it cools off outside. A TXV reacts to actual conditions and maintains a cold coil even when it gets cooler out keeping capacity up.

  4. #4
    Join Date
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    The entering drybulb temperature has no effect on total capacity.It does however decrease the sensible as entering drybulb is lowered,latent increases the same amount ,so total stays the same,and you have excess latent,in most all cases.



    Now with excess latent capacity,the indoor humidity will be a little lower then what was designed for in the Manual J.This causes a slight decrease in total capacity , a large decrease in latent,and large increase in sensible capacity.

    With a lower (than Manual J) wet-bulb,approximately half the excess latent will be converted to sensible,that can be added to the mfrs data ,that is at a higher wet bulb.




    Best bet is to adjust for the sensible loss,understanding that you will have excess latent.Though you could adjust for the excess latent ,I don't think I would,too little to be gained.

    If you do it, understand that from the mfrs. data ,you must look at four things that effect capacity;
    Airflow
    Outdoor temperature
    Entering coil dry bulb temp.
    Entering coil wet bulb temp.

    [Edited by dash on 08-22-2006 at 05:01 PM]

  5. #5
    Join Date
    Nov 2000
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    Memphis TN USA
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    That is simalar to a formula I used to have. (I think I lost it when my computer crashed once.)
    The formula I had used wet bulb temps @ 50%RH. I bet your formula has that built into the 835 number.
    The number you estimated seems pretty good exept one thing. Lower IAT will lower evap temp and increase SR. This will add a little of that 5000K in lost capacity back. Since you are getting a ballpark number I would say 31K total capacity

    Loonie, He is refering to maximum capacity. It will have part capacity at part load.

    If you do a load size and spec a 3-ton unit, you need to be able supply 36K at peak load. It is best to carry 1 or 2 brands, so you can easily find good data to spec equipment. And be used to reading that type of chart.
    If the superheat ain't right it ain't charged right.

  6. #6
    Join Date
    Feb 2006
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    468
    I'm suprised to hear that a decrease in indoor DB does not decrease total capacity.

  7. #7
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    It would lower evap temp. That would lower compresion ratio and capacity. Even with a TXV system would lose capacity, but not as much
    If the superheat ain't right it ain't charged right.

  8. #8
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Originally posted by watercop
    I'm suprised to hear that a decrease in indoor DB does not decrease total capacity.
    Take a look at mfr. data like Carrier's or better yet maybe Markbieser,will post the rating from Trane that let you change conditions and see capacity,computerized version.

    Carrier data same system , same cfm same 95 outdoor ;

    80 indoor db,62 wb,=20,200 total,19,800 sensible

    75 indoor db,63 wb,=20,400 total,15,700 sensible

    To bad they don't have them at the same wet bulb


  9. #9
    Join Date
    Jul 2006
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    SoCal-OC
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    Thanks for the info, guys. So it looks like I derate the sensible by roughly the amount from the formula. I saw the same formula basically in some specs on a Bryant compressor data sheet. That derating really seems to have an impact on sizing - for a 75 IAT it seems like it would generally mean you'd have to go up about a 1/2 ton to get your sensible number back. Another question, do you guys also adjust for the OAT? Some data sheets show multiple outdoor temps and you could theoretically adjust for your design temp. For instance, my summer design temp is 92 - would you interpolate the published capacities between 85 an 95 to be more accurate or is that overkill?

  10. #10
    Join Date
    Feb 2006
    Posts
    468
    [/B][/QUOTE]

    Take a look at mfr. data like Carrier's or better yet maybe Markbieser,will post the rating from Trane that let you change conditions and see capacity,computerized version.

    Carrier data same system , same cfm same 95 outdoor ;

    80 indoor db,62 wb,=20,200 total,19,800 sensible

    75 indoor db,63 wb,=20,400 total,15,700 sensible

    To bad they don't have them at the same wet bulb

    [/B][/QUOTE]

    I understand now...it's ALL about WB...I was mentally lowering WB along with DB...bad assumption.

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