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Conditioners' efficiency

Discussion in 'Electrical Engineering' started by abu, Apr 28, 2008.

  1. abu

    abu Guest

    Hi all,
    we need to cool down a server room.
    It is known that we need 3500 BTU every KW of computers (plus some more
    for the room form factor, ok)

    I was reading that conditioners nowdays have an EER rating of 10, that
    is they draw 10000 BTUs for every KW of consumed electricity. Hence, I
    computed that we would need 350W to cool down 1KW of computers.

    However, our electrician tells us that this is not true, because the
    power drawn by conditioners is in facts much higher than what is stated
    on the box. He says that a fully running 12000 BTU conditioner actually
    draws 3 KW, not 1.2 KW like it is written on the box.

    If this is true, then we would need 875W of conditioners in order to
    cool down 1KW of computers, instead of the 1.2 KW declared on the
    conditioner specs.

    This would be a problem for us, because we would then need to place new
    electric cabling in order to bring more current to the server room.

    Do you confirm that conditioners are so much more inefficient than what
    is stated?

    Thank you.



    Oh, another question: where are these conditioner watts dissipated? I
    hope that is on the external fan unit, not on the in-room AC split,
    otherwise we would need to also cool down those watts, and the needed
    conditioning power would be immensely superior...
     
  2. I think your electrician is getting mixed up. He will have to size
    cables, breakers, supply, etc for higher current as aircon takes a
    surge when starting, but this reduces considerably once they've
    started up.

    It's not a good idea to run your aircon from the same supply as the
    servers though. This startup surge can generate quite a dip in the
    voltage at the end of a supply cable. Depends on your supply impedance
    at point of use.
    The external heat exchanger gets rid of both the heat pumped out of
    the room, and the energy consumed by the aircon.
     
  3. James Sweet

    James Sweet Guest


    The quoted efficiency is accurate under one specific set of conditions. Amp
    draw of the compressor is pretty much directly related to condenser
    temperature. On a hot day outside, or if the coils are blocked by debris,
    the head pressure and amp draw will be much higher than on a cool day. The
    only one I've actually measured is the 36,000 BTU unit at my house, I've
    found it draws between 10A and 14A at 240V, yours is probably somewhat lower
    efficiency, but even the 1.2KW sounds a bit high and is probably measured on
    a hot day.

    Your electrician likely has a poor understanding here, refrigeration isn't
    something a lot of them understand well. He's probably taking other factors
    into account to choose the wire size and breaker capacity rather than the
    actual power draw.


    It's almost entirely the compressor in the outside unit. The indoor fan
    draws perhaps a few hundred watts.
     
  4. abu

    abu Guest

    I don't understand what you say here. If the A/C might draw more than
    specified on the nameplate when it starts up, and we size the breaker
    for the nameplate specs, at the AC startup the breaker will trigger and
    break the circuit!
    ??
     
  5. abu

    abu Guest

    Is this surge higher than the nameplate specs?
    This is interesting. All of you have said this.
    We certainly would have this problem, as the cable reaching the server
    room is like 50 meters long, and both the A/C and servers are attached
    at the end of it.

    I am a CS engineer, not electric, hence I have some understanding of
    electrical engineering but not a deep one, and I cannot really
    understant how this "dip" happens.

    If we size the cables for the worst case, that is, maximum A/C
    (air-conditioner) current draw + maximum servers current draw, even
    during the A/C engine start the voltage available to the servers should
    not fall below the specs of the power supplies. I understand that there
    would be "interferences" in the sinusoidal shape of the alternated
    current but, who cares: the power supplies of the computers AFAIK work
    like a rectifier + charge pump and not like induction transformers, so,
    sinusoidal interferences are irrelevant in this type of design, they
    should practically have no effect at the output of the power supplies...?

    Thanks a lot
     
  6. James Sweet

    James Sweet Guest

    Breakers do not trip instantly.

    The spec you're looking for is LRA, Locked Rotor Amps. A typical 3 ton
    residential AC unit may draw 12A operating, but the LRA can be over 100A.
    That's why the lights in the rest of the house will dim momentarily whenever
    the unit starts up. This current surge is not long enough to cause the
    breaker to trip.
     
  7. Guest

    | Paul Hovnanian P.E. wrote:
    |>> However, our electrician tells us that this is not true, because the
    |>> power drawn by conditioners is in facts much higher than what is stated
    |>> on the box. He says that a fully running 12000 BTU conditioner actually
    |>> draws 3 KW, not 1.2 KW like it is written on the box.
    |>
    |> Your electrician is incorrect in the narrow context of his statement.
    |> All equipment draws no more than what it says 'on the box' (on the
    |> nameplate, actually) per code requirements. It may draw more on startup,
    |> but panel, breaker and circuit sizing are based on nameplate ratings as
    |> dictated by code (I'm assuming the NEC for your jurisdiction).
    |
    | I don't understand what you say here. If the A/C might draw more than
    | specified on the nameplate when it starts up, and we size the breaker
    | for the nameplate specs, at the AC startup the breaker will trigger and
    | break the circuit!

    The typical circuit breaker has two trip elements. One is thermal, which
    requires reaching a certain temperature to trip the breaker. The thermal
    element very closely mimics the heat buildup in the wiring it protects.
    A very brief 100A starting current will heat things up more than a 50A one
    of the same duration, but this won't be all that much for just a second or
    two. The other trip element is a magnetic one. It is supposed to catch
    short circuit faults more quickly. It is adjusted to a point expected to
    be well above any motor starting current in the targeted type of usage.
    Many industrial circuit breakers, where very large motors are involved,
    have an adjustment for this right on the front of the breaker.

    If you have a circuit with a 20A breaker, and then start drawing 30A on that
    circuit, it will not trip the breaker immediately. You may have a few minutes
    before it trips. For higher current levels, that time is shorter. But it
    should be long enough to get most motors started just fine.

    If you have an air conditioner that is rated to draw 1.2KW, but draws 3.0KW
    all the time, then either it is defective, or the manufacturer lied about the
    rating. But I would not be surprised if it draws 6.0KW for a second to get
    started. For an air conditioner that large, I'd get one for 240V, not 120V,
    and wire the circuit accordingly. That would reduce the starting current in
    half, resulting in half the voltage drop, which would be split between the
    two 120V "sides".
     
  8. abu

    abu Guest

    Thanks to all of you, also to Michael replying on the other sub-thread:
    I think I understand the whole problem now!

    Now, this problem is going to be really nasty for us because due to some
    policies of the building that hosts us, it seems that we cannot bring
    two separate power lines to our server room.

    In addition, I think we cannot afford an UPS so big to protect all the
    equipment. We accept to have the equipment go down if there is a
    blackout (this is a computation cluster, we don't need 24/7 guaranteed
    availability), however we would like to protect the equipment from the
    damage that can be caused by the frequent brown-outs from the A/C. Is
    there anything we can do that comes to your mind?

    Two ideas come to my mind but I am not sure of the feasibility: would
    you please comment?

    1 - We could use a current limiter before the A/C. Does it exist? What's
    the technical name? We need one that can be put before a 50,000 BTU air
    conditioner (probably three-phase).

    2 - We could use a buck-boost voltage stabilizer before the computers.
    For high powers I think this costs 1/10 of what an UPS costs (we need
    around 15000 KVA).

    Thank you
     
  9. James Sweet

    James Sweet Guest

    You can't do this, if you limit the current, the compressor motor will stall
    at startup, this will cause the current draw to sit at the max limit until
    the breaker trips or the thermal cutout in the motor opens.


    That should certainly help. You could also pick up a smaller UPS to run the
    most important equipment and leave some of it on just a stabilizer.
     
  10. abu

    abu Guest

    15000 KVA is the power of the computers, not of the A/C
     
  11. James Sweet

    James Sweet Guest


    You have 15 megawatts of computers?!
     
  12. abu

    abu Guest

    WHOPS ROTFL!!!
    We have 15 kVA of computers not 15000
    sorry.... :)
     
  13. abu

    abu Guest

    Hello again,
    on this matter, I have just learned about the "inverter" technology:
    this seems what we are looking for, doesn't it?
    http://en.wikipedia.org/wiki/Inverter_(air_conditioning)
    Do you confirm that buying this type of air conditioning unit should
    avoid us the problem of the dip at startup (potentially damaging for the
    servers) that everybody has mentioned?

    This would be immensely cheaper for us than any other type of solution
    like a current stabilizer or UPS...

    Thank you
     
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