Best AHU configuration to reduce reheat?

[tikitime]

I have an industrial building in which the ahu's are being replaced. The units are 10,000 - 15,oo cfm units. The new units will be set to provide 100% OA during the occupied hours due to chemicals in the building, and minimum OA during the unoccupied hours. The new units will have VFD's. The building owner is concerned about reducing the amount of reheat. Also, due to funding, the only energy savings device on the new units will be the vfd's - as compared to using a heat wheel or the like. The ahu's will use chilled water and hot water.

The ahu vendor and building owner disagree on the the ahu configuration that will work the best. The vendor is suggesting a typical draw-thru unit with a single cooling coil and heating coil. He wants to use the control valves to control the load and use reheat when required.

The building user has suggested using a mixed air bypass unit or a unit with two cooling coils stacked on top of each other - a smaller one on top for the RA and a larger one on the bottom for the OA - the airstreams mix prior to the coil.

Any input on which is the better way to go?

 

[quark]

If latent loads are not high, you can manage with less reheating. To decide one or two cooling coils you should give us data viz, ambient conditions, required room conditions and load.

If you are maintaining two chilled water inlet temperatures, two coil arrangement is good for better energy savings. (you need not pull down chilled water temperature for high humidity condition)

Otherwise I would go with the vendor.

 

[cjw81]

The builder would have the more energy efficient design, but could be improved a little more. If you get credit for the VFD'S only, why not put one on the chilled water pump so that when you reduce your OA at night (probably to a recommended 20%) you can reduce your pumping capacity to match the reduced load on the system for additional savings. Also don't forget your exhaust system, the needs of which will be reduced at night also, the min. air cold deck coil will reduce the OA and thereby the need to exhaust 100% of your max air capability. Just make sure you have enough flow or bypass to maintain your chiller delta P.

 

[ChasBean1]

It seems the user is either thinking outside the box or has no clue. I can't tell from your post which one is the case. We have conflicting givens... 1) "The building owner is concerned about reducing the amount of reheat." Why would reheat be reduced? Does he/she mean return air or heat recovery?? And 2) "The building user has suggested using a mixed air bypass unit or a unit with two cooling coils..." What is this 100% outside air unit mixing air with? In the off-hours, when you note that mixing will occur, why complicate the controls so badly as to try to separately regulate return air and outside air? Let the airstreams mix (this, after all, is the purpose of returning air) prior to trying to condition.

Unless the owner could better explain his motives, I would rule for the defendant (vendor). -CB

 

[jherbert]

There is too little information
for anyone here to make firm recommendations.
but it seems you have recipe for disaster brewing.

You need a engineer to ascertain client
technical requirements and translate
those into specification for vendor.

If 100% OA is a requirement, why bother
with expensive VFD's - better to
redistribute load if possible. Countless
VFD running at 100% capacity for
8hr x 365days x 15 yrs seems a
total waste of resources.

Have you considered pre-purge volumes??

Too many questions and few answers I am afraid.