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    Unhappy Calculating Reheat Capacity - For Humidity Control



    Hello,
    I am designing a HVAC system for an Equipment Room. Design set point is 22 deg C with 55% humidity. When I set the parameters and run in HAP, it says reheat is not required (@ setpoint of 55% RH).

    Now, there are instances when the internal sensible loads are not high enough to raise the supply air temperature (13 deg) to room setpoint, and I end up in a mess with very high RH in room, sometimes even condensation .

    I need to introduce a heater to control this. The issue is, the heater capacity is damn high if I consider the temperature difference of 9 (22 - 13), with the airflow of 12000l/s! Its more than the chiller capacity and its ridiculous!

    What would be the best solution to reduce this reheater capacity to an optimum level?

    N.B
    Im using Q=mCp(T2-T1) for calculating the capacity.

    Thanks
    Nevin.



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    Re: Calculating Reheat Capacity - For Humidity Control

    12000 l/s seems a lot of airflow.

    How big is the room and what is the air change rate?
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    Re: Calculating Reheat Capacity - For Humidity Control

    Yes, its huge. Approx 530 sq.m, with internal heat dissipation of 90kW, unfortunately most of the time these electrical panels are in partial load, so no much heat dissipation from those! Approx 1ACH.

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    Re: Calculating Reheat Capacity - For Humidity Control

    So, with your 530 sq.m and an airflow of 12000l/s I calculate the height of your room to be 23 metres, is that correct?
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    Re: Calculating Reheat Capacity - For Humidity Control

    No, height is only 5m. That ACH I said was for fresh air replenishment, and not for supply air. Fresh air for the space is only 693l/s..

    See attached pics for input and output...
    Input sheet.jpg

    Results.jpg
    Last edited by nevinjohn; 25-03-2018 at 07:01 AM.

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    Re: Calculating Reheat Capacity - For Humidity Control

    Have you thought of using a 3 port control valve on the chilled water circuit so that you can regulate cooling capacity to load?
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    Re: Calculating Reheat Capacity - For Humidity Control

    Quote Originally Posted by frank View Post
    Have you thought of using a 3 port control valve on the chilled water circuit so that you can regulate cooling capacity to load?
    Hmm... that is a possibility. But, what would you suggest if the client insists on a reheat?

    As per the present control philosophy, humidity control overrides temperature control. Currently, even though the room temperature set point is reached, the RH is high. This calls for the valves to fully open supply to the AHU, and 'dehumidify' the air. Although dehumidification happens at the coil, I end up with a higher RH in the room!! So that 'philosophy' is not helping me. .
    Last edited by nevinjohn; 26-03-2018 at 11:21 AM.

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    Re: Calculating Reheat Capacity - For Humidity Control

    If most of your load is from electrical panels, where is your high humidity coming from?
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    Re: Calculating Reheat Capacity - For Humidity Control

    I was wondering that as well Frank.

    Surely the only source of moisture is from the fresh air intake so you only need reheat on the 693l/s to rematch the rooms 55%RH.

    Do you have a separate cooling/dehum coil in your fresh air makeup duct?
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    Re: Calculating Reheat Capacity - For Humidity Control

    Nope, I don't have a separate system to dehumidify the fresh air. I want re-orient you guys. This humidity is due to reduced internal loads. The OFF COIL air conditions, at 13deg C is with 95 - 90% RH. The internal sensible load is minimal, so the room RH is very high. This is my issue. It has nothing to do with fresh air. I am trying to increase this sensible load with a heater. :-(

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    Re: Calculating Reheat Capacity - For Humidity Control

    So the equipment is producing moisture is that correct?

    We are trying to establish where the moisture is coming from, is the room not air tight?
    Last edited by Brian_UK; 28-03-2018 at 05:59 PM.
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    Re: Calculating Reheat Capacity - For Humidity Control

    13c @ 95%RH????

    If you have a recirculating cooling system (air) with a small amount of fresh air make up, the humidity level should reduce with each pass of the evaporator, providing that your fresh air make up is not saturated and the evaporator is sized correctly.

    The electrical load in your room will not produce water vapour so where do you get 95% RH from?
    Last edited by frank; 28-03-2018 at 08:34 PM.
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    Re: Calculating Reheat Capacity - For Humidity Control

    Physchrometric chart the condition change is almost a straight line so I'm confused.
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    Re: Calculating Reheat Capacity - For Humidity Control

    The offcoil humidity will be high due to the low off coil temperature. however it has relatively little water content so as soon as it mixes with the room air the room humidity will be low.

    to confirm we are still at the design stage here and not actual operating conditions?
    Last edited by r.bartlett; 29-03-2018 at 01:39 PM.

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    Re: Calculating Reheat Capacity - For Humidity Control

    Quote Originally Posted by r.bartlett View Post
    The offcoil humidity will be high due to the low off coil temperature. however it has relatively little water content so as soon as it mixes with the room air the room humidity will be low.

    to confirm we are still at the design stage here and not actual operating conditions?
    Yes, this is the point. The supplied air is almost at 95%RH, as it is with all air conditioning applications. But, the set point is reached, when this air absorbers the heat inside the room as represented as a straight line (between off coil and room conditions). The issue is, I don't have enough loads, for this conditioned air to absorb heat and this causes high Relative Humidity. The specific humidity is obviously low. . . Unfortunaly, its not in design stage, the system is operational. It gives me trouble during off peak period. Summers are good. I am worried about winters and rain, when the building envelop load is also less. . .

    See the attached psy chart. Don't bother the calculations, see the SHR, at part loads. scan_00032.jpg

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    Re: Calculating Reheat Capacity - For Humidity Control

    Quote Originally Posted by Brian_UK View Post
    Physchrometric chart the condition change is almost a straight line so I'm confused.
    Yes, this straight line is under the assumptions that air picks up internal room loads, and achieves this set point. But what happens if there are no internal loads?

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    Re: Calculating Reheat Capacity - For Humidity Control

    I am not an HVAC guy, so consider this a question more than a comment.
    If the conditioned space is at the required temp why cool the air going into it?

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    Re: Calculating Reheat Capacity - For Humidity Control

    Quote Originally Posted by NH3LVR View Post
    I am not an HVAC guy, so consider this a question more than a comment.
    If the conditioned space is at the required temp why cool the air going into it?
    My next thought, control the space temperature better.

    As Frank suggested earlier use a three port valve on the cooling coil.
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    Re: Calculating Reheat Capacity - For Humidity Control

    Can you reduce the air volumes to match the load better.

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    Re: Calculating Reheat Capacity - For Humidity Control

    Quote Originally Posted by NH3LVR View Post
    I am not an HVAC guy, so consider this a question more than a comment.
    If the conditioned space is at the required temp why cool the air going into it?
    Good thought. The reason is again high RH. The space needs to be pressurized, hence I need some percentage of fresh air supply through out the year. This fresh air will causes varying RH (sometimes very high during winter and rain). To control these, and to meet the design RH set point of 50%, I need the chillers to be functional through out. . .

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    Re: Calculating Reheat Capacity - For Humidity Control

    Quote Originally Posted by r.bartlett View Post
    Can you reduce the air volumes to match the load better.
    No, unfortunately the AHUs and ducts are already in place. . I cannot change air flows... Heater is my only option.

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    Re: Calculating Reheat Capacity - For Humidity Control

    Quote Originally Posted by Brian_UK View Post
    My next thought, control the space temperature better.

    As Frank suggested earlier use a three port valve on the cooling coil.
    Thank you Brian, I am trying the same. Apparently, there are some issues with the controls. The client however insisted on a heater to be on a safer side... Its a panel room, and any downtime due to condensation is money lost.

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    Re: Calculating Reheat Capacity - For Humidity Control

    So, from the data posted so far, I get that the room size is 530m2 x 5m high. Volume therefore is 2650m3.

    With a total air flow volume of 12000 l/s or 12 m3/s (16.30 ACH)

    Fresh air make up is 1 Air Change /Hour which equals to 2650m3/h or 0.736m3/s

    So,

    recirc air = (12 - 0.736) = 11.26m3/s
    fresh air = 0.736m3/s

    With the electrical VSD's producing very little, or any water vapour, we can assume that the excessive RH is coming from the fresh air make-up. If so, then it is only this element that needs treating and not the 12m3/s, either by shutting off the fresh air via humidity sensors when temp set point is reached or by dehum and re-heat.

    Either way, you will have to modify the existing duct system.
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    Re: Calculating Reheat Capacity - For Humidity Control

    Quote Originally Posted by frank View Post
    So, from the data posted so far, I get that the room size is 530m2 x 5m high. Volume therefore is 2650m3.

    With a total air flow volume of 12000 l/s or 12 m3/s (16.30 ACH)

    Fresh air make up is 1 Air Change /Hour which equals to 2650m3/h or 0.736m3/s

    So,

    recirc air = (12 - 0.736) = 11.26m3/s
    fresh air = 0.736m3/s

    With the electrical VSD's producing very little, or any water vapour, we can assume that the excessive RH is coming from the fresh air make-up. If so, then it is only this element that needs treating and not the 12m3/s, either by shutting off the fresh air via humidity sensors when temp set point is reached or by dehum and re-heat.

    Either way, you will have to modify the existing duct system.
    Yes, I did the reheat option here.

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    Re: Calculating Reheat Capacity - For Humidity Control

    Quote Originally Posted by nevinjohn View Post
    No, unfortunately the AHUs and ducts are already in place. . I cannot change air flows... Heater is my only option.
    a couple of VSD's will reduce energy consumption too?

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    Re: Calculating Reheat Capacity - For Humidity Control

    I would consider addin a smaller coil before the ac coil with evaporating temp of -8 c to freeze the water in the air with natural defrost every hour.

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