100 % OA unit humidity control CAV box heater

[ml1367]

Hi All,

I'm involve in a Hospital design where I have this scenario I never have done before.

10 isolation rooms being serve for a dedicated AHU. All air supplied into the rooms must be exhausted out of building. Min of 12 ACH. Negative Pressure to the adjacent area. (ASHRAE 170).
Since all supplied air must be exhausted, the AHU would be 100% fresh air.
The Outdoor temp design condition: 97F/79.4 DB/WB with a Winter o/a temp design condition of 73F.(caribbean).
Now, 12 "constant" (to keep the negative air balance) ACH at 55 F could over cool the rooms, in some conditions, therefore a reheat coil shall be provided in each room... So far so good.
Now my questions would be.
Will be neccesary to provide another reheat coil at the AHU for humidity control or just leave it to drop de air temp as required for deshumidification, and then reheat the air at the terminal units (CAV BOX with reheat coil)as mentioned above.
3.My control's proposal, is to set the leaving air temp at 55 F and provide a humidity sensor at the exhaust system main duct (common to all isolation room.)set at 60% HR. if the humidity goes above that value then valve will open to cool the air...Is this a good way to do it?.
My concern is, if the air is cooled too much wihtout being re-heated it might bring condensation in the duct..in other hand it dosen't make sense to me to have to reheat coils in serie...(ahu and terminals)
Please any advice will be appreciated...thanks,

 

[317069]

your load calculation will determine your cooling coil capacity, and supply air conditions.

 

[walkes]

You will need to insulate your conditioned air ductwork (complete with a vapour barrier as well).

Your control proposal doesn't make sense. If your leaving air temperature is at 55°F and the space goes beyond 60% RH, you either have the valve open already and you won't get more dehumidification out of the coil and/or your space temperature is too cool.

As 317069 said, the calculations will determine your conditions. Terminal reheat will probably be your best bet given CAV

 

[SAK9]

I would suggest use of a heat pipe within the AHU to recover some heat from the exaust.This would help you reduce the amount of terminal reheat and save energy as well.

On a different note,it is not a great idea to have all your isolation rooms(assuming you have only 10 of them)served by a common AHU.You may need to fumigate the isolation rooms from time to time.So shutting down one room could upset the air balance for the remaining rooms.Isolation rooms are safety critical and attention should be paid to system reliability as well

 

[walkes]

SAK9 you could have the AHU on a speed drive with CAV boxes serving the isolation room. The AHU would scroll back to serve the remaining spaces without affecting air balance.

 

[BronYrAur]

You may want to consider a unit such at Aaon, which will be able to do hot-gas reheat to save you some money. A desiccant wheel may also be a good idea. Munters makes a nice unit.