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  1. #51
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    Originally posted by Gary
    Normal load is about 20-35F above ambient, 20F being a light load, and 35F being a heavy load.


    I don't understand what you mean by "load", light or heavy ... 20-35F above ambient ? what does ambient have to do with load? are you assuming that the ambient is the "infiltration" load? No, by ambient, I am referring to condensing ambient. the case is indoors. typically I assume a constant 80'F for the air conditioner not working properly.

    At 28F over ambient, this condenser is getting rid of a fairly heavy load of heat. Where does that heat load come from? The evaporator.
    28'F over ambient? Where are you deriving that from?


    If the TXV's weren't somewhere in the ballpark, it would not be able to pick up this much load.
    ...... okay........

    The evaporator coil does two things. It cools the air (sensible load) and it de-humidifies the air (latent load). It isn't picking up much sensible load (low dT), so it must be picking up a heavy latent load (lots of humidity).
    Agreed.

    Make sense? [/B]
    see above.



  2. #52
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    ohhh! your referring to the condensing temperature above the condensing ambient! right?

    Therefore you are determining that the system is doing a great amount of work, and concluding that all is well!!!!

    I think I follow you now, please reaffirm.

  3. #53
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    ADP??????????

    SCT??????????

    ???? SCT = Saturated Condensing Temperature........

    NO?
    Last edited by herefishy; 16-04-2002 at 02:50 AM.

  4. #54
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    210psig head pressure = 105F SCT (saturated condensing temp) minus 77F outdoor temp = 28F over outdoor ambient.

  5. #55
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    see previous posts.... you and I are online simutaneously.

  6. #56
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    SCT = saturated condensing temp
    ADP = apparatus dew point = the dew point of the coil

    We are singing a duet

  7. #57
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    Wow!!!

    I've never evaluated a system by that criteria.

    I like it. (of course when it's good news)

    Are there any specifics to account for how much work is being done utilizing the SCT?

  8. #58
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    there seems to be some amount of frustration on behalf of the forumn, in regard to my reference to TD in addressing design humidity.

    but I reference a guide in the Bohn engineering data manual which specifies humidity applications, according to design temperatures, and relates the desired criteria in TD (SST).

    I assume that this is my miscommunication with Marc, being that he is intimately familiar with the logistics, however i keep referring to a rule-of-thumb.
    Last edited by herefishy; 16-04-2002 at 02:58 AM.

  9. #59
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    Marc could no doubt tell you to the fraction of a BTU

    And keep in mind that I have made assumptions. Given a full set of numbers, I would be going through several variables before reaching this point in the evaluation.

  10. #60
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    In regard to the SCT, and your understanding of the design load, typically in My market, we are most comfortable with SCT at or about 120'F (250psig). In august we'll live with up to 270psig without having a cow!! does that change your view on the operation.

    Mind you, what makes most sense, and the thought that occured to me when I was looking at the SCT when I was on the roof, was indeed the 110'F SCT compared to the 77'F ambient that I was standing in.

  11. #61
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    TD refers to a temperature comparison of two different things.

    dT refers to a change in temperature of one thing.

    What two temps are they comparing in reference to humidity?

  12. #62
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    TD... believe me... I live by the difference in definition, and I understand it.

  13. #63
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    You don't think the Prof. would send me to Dallas if his assurance wasn't a sure thing, do you?

  14. #64
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    In regard to the SCT, and your understanding of the design load, typically in My market, we are most comfortable with SCT at or about 120'F (250psig). In august we'll live with up to 270psig without having a cow!! does that change your view on the operation.
    Not at all. Being in a very hot climate, you probably have oversized condenser coils. In that case, 28F over ambient is a very heavy load. And as I said, I have made assumptions.

  15. #65
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    Say in the Bohn eng. manual, it would spec (I'm not looking at it now) convenience store cooler app. 70% R.H, 35'f box, TD = 13-15'F.

    Have you ever referred to that manual?

  16. #66
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    Marc could no doubt tell you to the fraction of a BTU
    So might the Prof, but he is perhaps less incitable than Marc.... Maybe not with hvac/r terminology, though
    Prof Sporlan

  17. #67
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    Originally posted by Gary
    Not at all. Being in a very hot climate, you probably have oversized condenser coils. In that case, 28F over ambient is a very heavy load
    Well, the coils are only what the mfgr. provides. I base the selection on the mfgr criteria according to max. ambient conditions in which I employ them. Technically, I could spec a very, very undersized condenser coil, however, merely hve a C.U. (compressor) that will provide the required capacity at the given maximum conditions with the condenser coil employed.

    Does that make sense?

  18. #68
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    Say in the Bohn eng. manual, it would spec (I'm not looking at it now) convenience store cooler app. 70% R.H, 35'f box, TD = 13-15'F.
    They are referring to the difference between SST and evap air in temp.

    So might the Prof, but he is perhaps less incitable than Marc.... Maybe not with hvac/r terminology, though
    And as usual only you and Marc would understand it.

  19. #69
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    Don't get me started in the terminology dept.!!!!!!

    I think the terminology is the hang-up.

    ....and someone has brought it up before..... that terminologies may differ (across the "pond") in their daily usage. However when that subject was at hand, I did not feel that was the case, because it was concerning the difference between dT and TD. I am not confused in that regard.

  20. #70
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    Originally posted by Gary
    [B]

    They are referring to the difference between SST and evap air in temp.
    I refer to TD as the difference between design room temp and design SST.

    If I have a room designed for 35'F space, and a "design" SST of 25'F. My TD is "inherantly" 10'F.

    When I say TD, I am referring to design.

  21. #71
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    Well, the coils are only what the mfgr. provides. I base the selection on the mfgr criteria according to max. ambient conditions in which I employ them. Technically, I could spec a very, very undersized condenser coil, however, merely hve a C.U. (compressor) that will provide the required capacity at the given maximum conditions with the condenser coil employed.

    Does that make sense?
    I think what you are saying here is that your condensers are oversized for the compressor due to high ambients, compared to the match ups used in cooler areas.

  22. #72
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    I refer to TD as the difference between design room temp and design SST.

    If I have a room designed for 35'F space, and a "design" SST of 25'F. My TD is "inherantly" 10'F.

    When I say TD, I am referring to design.
    That would be the design TD.

    Once the unit is up and running, you might want to check the actual TD.

    As I have said many times, I dont calculate and predict; I measure and analyze.

  23. #73
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    NO, I don't think so. Regardless of the coil area, the compressor must still overcome the lower volumetric efficiency due to the higher condensing temperatures (SCT). So regardless of the coil size, you cannot reduce the SCT. You can only increase the efficiency of the heat transfer, and perhaps increase the subcooling. But the threshold SCT is what determines what amount of power (less loss of compression ratio, thereof) is required.

    ...perhaps
    Last edited by herefishy; 16-04-2002 at 07:25 PM.

  24. #74
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    Originally posted by Gary
    , I dont calculate and predict; I measure and analyze.
    I calculate and employ.

    I do very well........ I think. My track record speaks .....

  25. #75
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    Originally posted by Gary
    That would be the design TD.

    10-ROGER-A-1-04

    That's what I mean.

  26. #76
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    NO, I don't think so. Regardless of the coil area, the compressor must still overcome the lower compression ratios due to the higher condensing temperatures (SCT). So regardless of the coil size, you cannot reduce the SCT. You can only increase the efficiency of the heat transfer, and perhaps increase the subcooling. But the threshold SCT is what determines what amount of power (less loss of compression ratio, thereof) is required.
    Try filling a condenser halfway up with liquid. You are in effect reducing the surface area. What happens to the SCT?

  27. #77
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    Okay, I follow you.

    SCT is NOT ambient.


    But I think my comment regarding the high ambients holds in that regard, because you do not want to empty it.

    I follow your point regarding the function of flooded condenser valves.

  28. #78
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    I calculate and employ.

    I do very well........ I think. My track record speaks .....
    I don't doubt it

    There is nothing wrong with calculating and predicting. This is necessary for designing and sizing systems. But I don't do either.

    I am a service tech. The system is already designed and sized. My job is to trouble shoot and repair it. In order to trouble shoot it, I must measure and analyze, not calculate and predict.

    It's kinda like reverse engineering.

  29. #79
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    boy...... the messes you must get stuck with.

    Hey, your Pm mailbox is full. I tried to PM you.

  30. #80
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    boy...... the messes you must get stuck with.

    Hey, your Pm mailbox is full. I tried to PM you.
    I think my boss searches high and low for nightmare scenarios to dump in my lap. LOL

    My PM box says empty.

  31. #81
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    Originally posted by Gary
    [B]ADP = apparatus dew point = the dew point of the coil/B]
    Is the ADP the SST? Or is that a 3-phase polynomial derived from the LMTD?

    I always assumed that the SST was the DP. I suppose that if I wasn't lazy, I could refer to a psychrometric chart and derive the answer, however I pose the question at the risk of humiliation!

    Of course, now that I've thought about it for a few seconds... the SST is subject to the amount of work being done... perhaps.... because the SH could vary (amount of refrigerant in the evap), and not truly be representaive of the LMTD?????? (or the work being done)

    Maybe I'm catching on?

    ..... I'm still trying to grasp the LMTD thingy.

    Gary, help me....LMTD=? (terminology)
    Last edited by herefishy; 16-04-2002 at 04:41 AM.

  32. #82
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    We are now leaving my turf and crossing over into the twilight zone of precision mathematics. I'll take an oversimplified shot at it, but Marc or the Prof will have to give you the real explanation.

    Imagine that you want to know the temperature of something, but find that the temperature varies at different points. You could take several readings and figure out the average temp. OR you could get really precise and use a logarythmic formula to find it's log mean temp. Think of it as the true average temp.

    In this case, we are trying to determine the true average temperature on the surface of the coil.

    I sometimes call it the lean mean TD.
    Last edited by Gary; 16-04-2002 at 12:57 PM.

  33. #83
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    Late word from John Murray concerning the Sporlan supermarket seminar being held in Arlington, TX 4/17/02. There are about 5 spaces open. herefishy is welcome to crash the party!
    Prof Sporlan

  34. #84
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    Originally posted by Prof Sporlan
    Late word from John Murray concerning the Sporlan supermarket seminar being held in Arlington, TX 4/17/02. There are about 5 spaces open. herefishy is welcome to crash the party!
    Prof., I'm leaving at 3:00a.

  35. #85
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    Originally posted by Fridgetech
    LMTD

    Section 11-9 | RJ Dossat | Pages 206 onwards in the third edition.
    Would Yunus A. Cengel, 10-6 p. 539 / 13-4 p. 717 suffice?

  36. #86
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    Hi Gary,

    We're up and rolling.... I promised you the numbers...

    OD ambient 73'F
    SST 23'F

    we backed the valves out 2.5 to 3 turns to acheive a superheat of 8'F before the outlet subcooler.

    liq. line temp entering TEV (before ALLS) approx 73'F
    liq line temp entering case (before subcooler) 86'F

    HP = 186psig (96'F SCT)

    air entering evap = 49
    air exiting evap = 42'F

    Now the readings were not simultaneous, ya' know. while we're adj. SH, the case(s) are being trimmed out and such, however when we arrived the temp was at about 33'F to 35'F. when the final readings were taken, the case had gone up in temp a bit due to all the work and door openings goin' on.

    Obviously much of the latent load had been satisfied, having gone from a 4'F dT to a 7'F dT ( I would think)

    It appeared that the inlet vs. outlet liq. temp of the ALLS was almost nil, being that I was there when the readings were taken. However, there seemed to be about a 10'F to 15'F dT in the liq. temp going through the suction line (outlet) subcooler.

    At the end, my tech was getting punchy with the refrigerant again, because he said the SG was 2/3's full. I know we have about about 28# of gas in the system, and the subcooling looks real good, so I told him to "Leave it alone". SH is good, and considering liq temp entering TEV, I assume no flash.

    Any observations are welcome.
    Last edited by herefishy; 17-04-2002 at 01:22 AM.

  37. #87
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    Is the ADP the SST?
    No, it's an air side metric, but their values are usually close to each other. It's one of those points you can easily locate on a psych chart. Locate your return air and supply air states, then draw a straight line connecting the two. Where the line intersects the 100 percent RH line is your ADP.

    Prof., I'm leaving at 3:00a.
    LOL!!!!!

    It appeared that the inlet vs. outlet liq. temp of the ALLS was almost nil
    It's likely the distributor pressure drop during pulldown is maintaining saturated temperature between the TEV and distributor near 70°F, making heat exchange at the ALLS ineffective. You may need the case closer to design temperature before you see a temperature drop here.

    However, there seemed to be about a 10'F to 15'F dT in the liq. temp going through the suction line (outlet) subcooler.
    That's reasonable.

    At the end, my tech was getting punchy with the refrigerant again, because he said the SG was 2/3's full. I know we have about about 28# of gas in the system, and the subcooling looks real good, so I told him to "Leave it alone".
    The Prof would also wait to see what happens. Hopefully, a restriction upstream of the sightglass isn't causing this problem.
    Prof Sporlan

  38. #88
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    evap airflow not insufficient
    cond airflow not enough data
    subcooling not excessive
    load good
    TEV sizing appears ok...not enough data
    no restrictions
    TEV superheat not sure...not enough data
    TEV may be overfeeding...not enough data
    compressor efficiency probably ok...not enough data

    The numbers are far from simultaneous and therefore inconclusive. Several numbers are missing.

  39. #89
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    The Prof would also wait to see what happens. Hopefully, a restriction upstream of the sightglass isn't causing this problem.
    More likely a result of TEV overfeed.

  40. #90
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    It's likely the distributor pressure drop during pulldown is
    maintaining saturated temperature between the TEV and
    distributor near 70°F, making heat exchange at the ALLS
    ineffective. You may need the case closer to design temperature
    before you see a temperature drop here.
    A keen observation, prof. I wonder if that is the case.

    I looked up the Tyler reach-ins over the weekend and noticed that the evaporators and fan motors and blades are the same for D5FG's and D5NG's (hoping I have the nomenclature right) as I suspected.

    More likely a result of TEV overfeed.

    Pardon the pun, Gary, but could you expand on that?

  41. #91
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    I can't be sure because the data is all over the place, but it is highly unlikely that the TEV's would need to be opened 2 1/2 to 3 turns, particularly given the previous data. And inability to clear the sightglass would tend to support this.

    TEV's should be made non-adjustable. I cringe every time I hear someone say they adjusted a TEV. It is almost always a mistake.
    Last edited by Gary; 17-04-2002 at 03:24 AM.

  42. #92
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    A keen observation, prof. I wonder if that is the case.
    The Prof would give it a 9.7 out of 10...

    but it is highly unlikely that the TEV's would need to be opened 2 1/2 to 3 turns
    When the case reaches temperature, the TEV will likely need to be adjusted back in.

    TEV's should be made non-adjustable
    Interestingly, the vast majority manufactured in the US are.... Residential a/c and heat pump systems almost exclusively use non-adjustable TEVs, and they outnumber refrigeration systems significantly....
    Prof Sporlan

  43. #93
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    I am listening.

    It was reported to me, that upon arrival, the SH was 16'F to 18'F
    Last edited by herefishy; 17-04-2002 at 08:31 AM.

  44. #94
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    Originally posted by Gary
    The numbers are far from simultaneous and therefore inconclusive. Several numbers are missing.
    After I get back from the seminar, I will report back. I will personally take all readings with the system undisturbed. Indicate any additional information you would like to see, Gary.

  45. #95
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    Originally posted by Gary
    ...it is highly unlikely that the TEV's would need to be opened 2 1/2 to 3 turns
    I find that discerning, also.

  46. #96
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    With evap air in temp near design temp:

    evap air in temp
    evap air out temp
    SST (saturated suction temp)
    coil outlet suction line temp
    compressor inlet suction line temp

    cond air in temp
    cond air out temp
    SCT (saturated condensing temp)
    receiver outlet liquid line temp
    TXV inlet liquid line temp
    discharge line temp near compressor

    Preferably, all readings should be taken at the same evap air in temp.

    I suspect the superheat readings were taken with the doors open and/or the evap air in temp above design temp. Superheat is highly sensitive to evap air in temp. Open the door for a few seconds and up goes the superheat.
    Last edited by Gary; 17-04-2002 at 09:40 PM.

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