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  1. #1
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    How a CRAC unit works -- DC

    I am new within the data center world and i am confused on how a CRAC unit works. Does that unit want to have 0% RH for the return air or does it want to have air return within set points (40-45% RH).

    The reason why i am confused is becaused i just listened to a presentation that talks about how a AC unit wants to have 0% RH and i find that hard to believe.

    Can someone explain this to me.

  2. #2
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    Mixing oil and fire with a big spoon.
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    you would not be able (nor should you want) to get 0% RH.

    you would be having static electricity issues at very low relative humidity levels
    "Mother" is the name for God on the lips and hearts of children....The Crow

  3. #3
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    Data center humidity should be kept between 40 to 50% RH. Jay is correct. You don't want static electricity present around sensitive electronic equipment.
    "Excellence is the gradual result of always striving to do better"
    -Pat Riley

  4. #4
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    Thanks for the reply. I know that the equipment wants 45% RH, but what the company is claiming is that a CRAC unit wants warm return air with low relative humidty and in doing so that CRAC unit will work less hard and in turn you will save on energy. Is that true?
    Last edited by Boots22; 04-03-2007 at 08:26 AM.

  5. #5
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    Let me guess you went to an ATS, Stultz or Airflow presentation. They do their design, testing, and ratings on their units at close to 0% RH return. Thats why they get dripping off the coils, and units having problems in the real world.

  6. #6
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    Mar 2007
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    Yeah, that explains a lot with the coils.

  7. #7
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    data room units are designed for 95 sensible and 5 % latent unlike homes or offices that come in at around 70/30.... have nice day..

  8. #8
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    Typical Datacenters humidity levels should be at 40-50% RH. Perimeter cooling units are designed to work within that environment. If you look at any manufactures spec sheets you will see that the capacity of those units actually goes up the warmer and dryer the return air is at the system. An Ideal system has no mixing of the supply air to the racks with the return air from those racks. Supply air at a server should be 68-74 degrees and at 40-50% RH. Coming out you'll have anywhere from a 15-35 degree rise depending on the equipment and humidity levels will be down as well.

    All CRAC units are sensible cooling. That is the reason for the fin packs being so tight and the CFM's being so high. They are designed to cool equipment within tight tolerances within a conditioned space.

    Now APC has come out with the new load neutralization systems that are installed in row. We have 2 the RC which is 100% sensible and the RP which is 95% sensible 5% latent.

    If you want so good reading on datacenters, their design and the equipment and why you should use it. Go to either APC.com or Liebert.com and read the white papers on both. Both companies will tell you about the exact same thing on datacenter design.

  9. #9
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    Mar 2007
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    That explains a lot. That is why Green Grid and Uptime are saying to keep the cold air with the cold air and keep the hot air with the hot air so your CRAC units will run at a higher effiency. It is all coming together.

    I appreciate everyeones help.

  10. #10
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    Quote Originally Posted by apc_ac_man View Post
    Typical Datacenters humidity levels should be at 40-50% RH. Perimeter cooling units are designed to work within that environment. If you look at any manufactures spec sheets you will see that the capacity of those units actually goes up the warmer and dryer the return air is at the system. An Ideal system has no mixing of the supply air to the racks with the return air from those racks. Supply air at a server should be 68-74 degrees and at 40-50% RH. Coming out you'll have anywhere from a 15-35 degree rise depending on the equipment and humidity levels will be down as well.

    All CRAC units are sensible cooling. That is the reason for the fin packs being so tight and the CFM's being so high. They are designed to cool equipment within tight tolerances within a conditioned space.

    Now APC has come out with the new load neutralization systems that are installed in row. We have 2 the RC which is 100% sensible and the RP which is 95% sensible 5% latent.

    If you want so good reading on datacenters, their design and the equipment and why you should use it. Go to either APC.com or Liebert.com and read the white papers on both. Both companies will tell you about the exact same thing on datacenter design.
    not bad... your close.. liebert runs 600 cfm per ton to make sure they get 95 / 5 ratios since in all reality most of the design load is sensible load and not latent ... why waste such valuable equipment on removing latents when most of the load is sensible anyway.

  11. #11
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    Mar 2007
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    do you have an idea on what the typical waste factor is for removing the latent air. I have heard that a CRAC unit will run at about 70% of it efficency if they are not getting the return air they like.

    Is that true?

  12. #12
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    Quote Originally Posted by Boots22 View Post
    do you have an idea on what the typical waste factor is for removing the latent air. I have heard that a CRAC unit will run at about 70% of it efficency if they are not getting the return air they like.

    Is that true?
    toatal heat is the sensible + latent. so if you had as an example 10000 btu of cooling capacity and needed 8000 btus of sensible cooling load.. you should make sure your ratio of sensible to latent reflects 80/20. if your at 70/30 youll have lower airflow per ton, and could conceivably have a sensible cooling problem. you can check this with a sling shycrometer and thermometer to check your percent of latent removal.

  13. #13
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    Mar 2007
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    you have been a great help...thanks a TON !!!

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