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    Clearity about AIR CHANGE in AIR CONDITIONING



    Hi everyone,

    i am new to this forum.I need a help from you guys. I always get confused by the term Air Change in air conditioning. I mean what is this? the recirculation rate or an ordinary AC or supply and exhaust of fresh air? if it is recirculation rate , then how cad it add latent and sensible heat to the room? if it is the second one, how can we practically supply fresh air in a closed bedroom where a small single split unit is being used?

    please help. thanks in advance



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    Air changes only refer to how often the air in a room is changed (or replaced).

    In other words, it is a measure of the supply and extract of fresh air from outside.

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    Re: Clearity about AIR CHANGE in AIR CONDITIONING

    but if we use a split unit in a bed room which we keep closed, how can we supply air from outside there as we do not use any fresh air fan?

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    Let's take a step backwards.

    Why these questions?
    Have you been given a spec that include air changes?

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    Re: Clearity about AIR CHANGE in AIR CONDITIONING

    ok lets take it this way: I need to design the cooling load for an office , so i definitely have to consider the ACH...but how can i consider that when CLEARLY i'm not making arrangements of supplying fresh air in the office from outside by any fan or something? or should i not consider the ACH?

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    OK,
    Somehow the people in that office need the air replaced.
    If no mechanical means of providing fresh air exist people will start to open windows.

    I don't know where you are based but in UK there is a minimum fresh air requirement unless there are windows that can be opened.

    Normally the best option is to install fresh air units with heatexchangers in them, google VAM or lossnay to see what I mean.

    However, in real life, for most smaller split installations the fresh air requirements are ignored.

    How you calculate for it?
    What software are you using?
    Have you been given a spec for the job?

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    Re: Clearity about AIR CHANGE in AIR CONDITIONING

    As Viking says, ACH is relative to how many times per hour the air volume in the space is changed.
    Air conditioners (split type) work on a recirculation basis, so no ACH. If you are designing for an office, you should be incorporating some form of ACH into your design as the occupants need fresh air to breathe.

    Your total air volume should consist of the recirculation volume required to meet the cooling load and the fresh air load.

    For instance, if your cooling load for the space requires 1.5m3/s for the load and your occupancy ventilation rate requires 0.3m3/s, then your total ventilation rate is 1.53m3/s, and this will also increase your cooling load as you are bringing outside air into the space.

    You must take into consideration the air required for occupants when sizing the office cooling load, as well as fabric gains, electrical heat gains, lighting gains and infiltration
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    Re: Clearity about AIR CHANGE in AIR CONDITIONING

    Thanks to both Viking and Frank

    I guess i'm grabbing the fact a little bit now.check if i've got you correct or not.you wanted to say:

    1)In home use we need not to arrange external air supply as it will come from infiltration and through window , door etc.

    2) for an office or other commercial spaces we must consider proper ACH and make arrangements for the supply of fresh air?

    3) if i do not arrange any supply fan then that must be compensated by open windows or doors.

    have i got it right ?

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    Re: Clearity about AIR CHANGE in AIR CONDITIONING

    Yes, that's about the basis of it.

    Don't forget to add the cooling load of the incoming air to your room calculations.

    In the UK, CIBSE give recommended amounts of fresh air input in conditioned spaces as between 10 and 20litres/second per person.
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    Re: Clearity about AIR CHANGE in AIR CONDITIONING

    Quote Originally Posted by frank View Post
    For instance, if your cooling load for the space requires 1.5m3/s for the load and your occupancy ventilation rate requires 0.3m3/s, then your total ventilation rate is 1.53m3/s, and this will also increase your cooling load as you are bringing outside air into the space.
    Frank, just a little correction in your math:
    1,5m^3/s +0,3m^3/s= 1,8m^3/s

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    Re: Clearity about AIR CHANGE in AIR CONDITIONING

    ok, now one more thing.

    In a office, i'm not setting up any supply fan or anything like that. The supply air will come through the door and window.....how can i assume what is the value of ACH by this?

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    Re: Clearity about AIR CHANGE in AIR CONDITIONING

    Quote Originally Posted by nike123 View Post
    Frank, just a little correction in your math:
    1,5m^3/s +0,3m^3/s= 1,8m^3/s
    Obviously I'm no good at Math after Harvest Pale

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    Re: Clearity about AIR CHANGE in AIR CONDITIONING

    Quote Originally Posted by tanveer.aziz View Post
    ok, now one more thing.

    In a office, i'm not setting up any supply fan or anything like that. The supply air will come through the door and window.....how can i assume what is the value of ACH by this?
    We work on anything between 1/2 to 1+1/2 ACH for infiltration depending on how well constructed the building is (new or old)

    If you are hoping to use open windows in an air conditioned space, you had better up your cooling load to compensate as the cool, dense air will tend to dump out of the window
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    Re: Clearity about AIR CHANGE in AIR CONDITIONING

    Thanks a lot Frank. I think i've got this. You guys helped a lot. I'm gonna disturb you again at any step i stumble.

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