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  1. #1

    Confused Load Calc for Clean room

    Room type: Pharmaceutical Clean room

    Volume: 5008 Ft^3

    ACPH: 80

    Selected CFM: 6680

    Total Sensible Heat: 47565 BTU/Hr

    Total Heat: 49380 BTU/Hr

    Outside Condition: 102.2 F (DB); 96.1 F (WB); 80% RH

    Inside Condition: 71.6 F (DB); 50% RH

    Rm Pressure: 42.5

    Fresh Air CFM: 70

    No Pre cooling

    These are the data I have got and I need help on calculating the tonnage of the cooling coil. Please post some links on how to calculate the coil load for a clean room system where ACPH is much higher.I'm having problem on this Back cooling Tonnage calculation.

  2. #2
    Join Date
    Apr 2002
    80 air changes is pretty high, is this like like a class 10 clean room or something where they wear the bunny suits?

    It is going to be a hurricane in that little room.

    If that is infact your load and air change requirement then you would be better off getting a unit that can give you 47565 Btu/hr sensible cooling and blow the cooled air into the ass end of another fan that sucks in and HEPA filters an additional 4680 CFM of return air.

    The filter unit moves the 6700 CFM, the AC cools down say 2000 to 2100 CFM, the rest of the air is just return going along for the ride through the filters.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

  3. #3
    During this tonnage calculation, Do I need to consider only the CFM required to offset the sensible heat or the for selected CFM of the room (this results a higher back cooling tonnage )?

  4. #4
    Join Date
    Apr 2002
    you have very high air flow 5008 x 80 /60 = 6677 CFM to get the air through the filters 80 times an hour.

    typically air conditioning works on about a 20 degree F temperature drop, it can be more or less. But it is sort of rediculous to put 6677 CFM through a coiling coil just to extract 47565 Btu/hr out of it.

    So you would be better off trying to cool about 47565/1.1/20 = 2162 CFM off by 20 degrees and then supply it into a filtration unit that handles the 6677 CFM
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

  5. #5
    Join Date
    Feb 2010
    So the loads are: equipment, lights and plug loads, occupants, process, fan heat, td gain across envelope, and osa? Did i omit anything? Sort of, you'll have to de-rate your cooling cap due to low indoor and hi outdoor conditions.

    The load calc specified includes all of the above? Are you on the hook for the load calc/system capacity or relying on someone else's work.

    OSA for the posted spec is about 12,300 according to EZ air (buy it).
    cfm x 4.5 x H diff(64-25) or 70 x 4.5 x 29

    I'd imagine you'll have to do some humidification and reheat with controls to reach a tight room spec.

    You're probably already aware but I'll say it anyways, Your cooling volume and circulated volume to the hepas are two different circulated paths/fans that draw from the same low return plenum(s). The cooling volume then dumps back into the hepa path before the fan. The simple cooling fan could never approach the needed volume or SP needs of the hepa design.

    sounds like you have (9) 2 x 4 fan powered hepas.

    Pharmaceutical customer will sign off w/o stamped engineers design?

    go here for some simple Psychro concepts:
    Last edited by btuhack; 07-18-2010 at 03:26 AM.

  6. #6
    I'm trying to figure out the discrepancy between two calculations that I have been provided for the same clean room. One of them is showing 6.5 TR of back cooling and another one is showing 16.5 TR (back cooling) + 1TR of pre-cooling. As far as I have understood, first one only negotiated with CFM required to offset the sensible heat and the 2nd one counted the selected CFM for calculating the tonnage. Space loads are same but the coil load differs.

    Actually this selected CFM is for two rooms with same required conditions.
    1st room: 3946CFM [ SH: 20534 BTU/Hr ]
    2nd room: 2732CFM [ SH: 27031 BTU/Hr ]
    TOTAL SELECTED: 6680CFM [TSH: 47565 BTU/Hr]

    ACPH: 80 for both rooms

    @ carnak

    How did you calculated the return CFM? Please explain. Also please explain the procedure of checking the tonnage calculation. I'm going around circles!!

    There will be FFUs in the rooms.

  7. #7
    Join Date
    Apr 2002
    I think something is being lost in the translation here.

    I also note that your outdoor condition would tie a world record for dew point set in the Persian Gulf area, so this is a little suspect
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

  8. #8
    ok. Please explain whether to select the total CFM or only the CFM required for sensible heat offsetting for tonnage calculation??

  9. #9
    Join Date
    Feb 2010
    Is the attached what you are doing, sort of? If so, the coil cfm is what IT needs to handle the heat load, and the balance is diverted to the recirc fans. The coil/flow is matched to the load calc.

    The circulated room volume is determined by the process; laminar, ach, whatever you want to call it and has nothing to do with the heat load. As carnak states "it's along for the ride".

    Am I on the right track or are you asking something else?
    Attached Images Attached Images

  10. #10
    Join Date
    Apr 2002
    Quote Originally Posted by ruzfactor View Post
    ok. Please explain whether to select the total CFM or only the CFM required for sensible heat offsetting for tonnage calculation??
    your total CFM is the air flow needed to give you the 80 air changes per hour. It means your clean room requires that you run all the air in the clean room through the filters 80 times each hour. I have never had to have this high of an air change in anything I have ever done.

    The sensible heat calculation, determined how much air flow had to be cooled off by 20 degrees F, to give you the required sensible cooling.

    So I suggested you get a system that gives you the required sensible cooling and moves an amount of air similar to what I calculated out. This gives you your cooling and saves energy compared to pulling the additional 4000 plus CFM through a cooling coil. The schematic btuhack posted shows a cooling system supplying air to individual laminar flow hepa units. The cooled air is fed to these units and at the same time, return air is pulled through these units and filtered as well

    I am not quite sure what you mean by "TR" and I have no idea what "back cooling" is
    Last edited by Carnak; 07-18-2010 at 04:27 PM.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

  11. #11



    TR= Ton of Refrigeration

    Let me just explain what I got from the summary sheet of the two rooms. I need to find out what's the basis of this design:

    Room Grade: B

    ACPH: 80

    Pressure: 40-45 Pa

    Design Temp.: 22(+- 3) C and RH= 55 (+-5)

    Outdoor: 39 C RH= 80%

    Room Data from BTU Estimation Sheet:

    Room 1:
    Area- 301 FT^2
    Volm - 2959 FT^3
    RSH=20534 BTU/Hr
    RTH=20597 BTU/Hr
    Grand Total Heat (including outdoor air heat): 21376 BTU/Hr
    Air Flow (Dehumidified): 1003 CFM
    Air Flow (Volumetric): 3946 CFM

    Room 2:
    Area- 208 FT^2
    Volm - 2049 FT^3
    RSH=27031 BTU/Hr
    RTH=27455 BTU/Hr
    Grand Total Heat (including outdoor air heat): 28004 BTU/Hr

    Air Flow (Dehumidified): 1325 CFM
    Air Flow (Volumetric): 2732 CFM
    Heat Load Estimation Summary:

    Selected CFM: 6680
    Infiltration CFM: 100
    Return CFM: 6780
    Bypass CFM: 4350
    Fresh Air CFM: 70
    Purge CFM: 170
    Tonnage: 6.5 TR
    Static mm of wc: 125
    Blower: 7.5 KW
    Reheat: 2.0 KW

    Can some one please explain this design procedure?? Because for the same design condition I have received a tonnage of 16.5 TR !!!!

  12. #12
    Join Date
    Apr 2002
    You should start by listing out, how you are arriving and the room sensible heat loads.

    The 49000 sensible heat in such small areas sounds intense, like it is a hyrodponic grow-op -- indoor agriculture with a lot of lights.

    So you work out the heat load generated inside of the rooms and add it to any heat coming in from the walls, windows, through the roof and or up to the floor.

    That sensible load will define how much air you need for the cooling ( or what the load sheets call Dehumidified CFM)

    So you work out the dehumidified air flow from the room sesnible load. This gives you your CFM and your required supply dry bulb and wet bulb.

    Now with clean rooms, you pressurize them so that clean air leaks out of the clean space- this keeps the dirty air out of the clean space. I noted "Infiltration" listed in the calcualtions. If you have infiltration in the clean room, you are in big trouble.

    So you work out how much outdoor air you need to build up the pressure, in order to make dry clean air leak out of your clean room. You then work out your entering air condition for the cooling part of your process.

    So you draw in room air plus your volume of outside air. You get the mixed air condition and your grand total heat is the cooling required to take this mixed air down to the dry bulb and wet bulb you need to supply to the space.

    Anyways, I think the 49,000 Btu/hr is high to begin with and I do not see how that becomes 6.5 tons.

    16.5 tons seems even more far fetched to me, maybe you are off by a decimal place on that world dew point record fresh air volume, you are carrying it at ten times higher than it is
    Last edited by Carnak; 07-24-2010 at 11:47 AM.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

  13. #13
    I have checked the cooling load calculation and found it correct. How do I calculate dehumidified CFM?
    From this>>
    QS = 1.08 x CFM x T (Interior Design Temp Cooling Coil Temp)

    What should be the cooling coil temp? Is it from the psychrometric chart using the SHF?

    For example, If the rooms have pressures of 45 Pa and the say another room adjacent to that is having 50 Pa pressures, then there should be some infiltration of air from 50pa room to 45 pa room. Isn't it?

    Also, ACPH*Volume= CFM : gives me the required CFM for the room. Is this formula applicable here?

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