Air Handler Units Configuration

[ssn61]

I am specifying air handling units with chilled/hot water coils, VFD supply and exhaust fans, single zone, full economizer mode. Project is a school in Dallas, Texas. What are your opinions on heating coil in preheat vs. reheat position, draw thru vs. blow thru fan location, and other thoughts on anything else. I am mainly concerned about moister control, and coil freeze-up in economizer mode.

 

[Drazen]

i assume vav-s are used at spaces?

if you have freeze-up danger, you need hydronic loop control that will either fully open control valve or have dedicated pump to ensure flow in coil all the time during dangerous outside temperatures.

mixed air low limit control helps much, additionally. air mixing is controlled by thermostat that monitors mixed air temperature, and when it reaches low limit, outside damper is closing, "normal" economizer cycle is overridden.

 

[KiwiMace]

The risk of OA freezing the cooling coil comes from the minimum OA setting on the economiser damper. Simple mixing calculation with the OA and RA. If there is a large OA quantity required such that the mixed air temperature is very cold, you need a preheat. The controls are configured to close down the OA damper (subject to min OA) if the mixed air temp gets too cold - it is not free cooling if you have to reheat.

Fans in draw-through have a more even flow over the coils. I'm not even sure what are the benefits of blow through.

 

[trashcanman]

The winter design temp. = 18 Deg.F. With 25% outside air and 70 Deg.F return air temp., your mixed air temp = 57 Deg.F. Even with 50% outside air, mixed temp = 44 Deg.F. Why are you worried about coil freezeup?
DISD control specifications tell you what to do.

 

[MTES93]

I typically use draw thru's for the reason that Kiwimace mentioned. Blow thru's typically require a diffuser plate of some sort so that the air is more evenly spread over coils, this cause an additional pressure drop. Blow thru's have a built in 'reheat' ability due to the fan heat gain, but that is small. I believe McQuay did a study on the two types of configurations years ago. Both configurations work and designers will argue the pro's and con's of each. It is really up to you. If possible, I would recommend a plug or plenum fan (draw thru) unless your architect gives you a lot of room to get fully developed flow in your duct work before major duct fittings, transitions and related....

If the system is a VAV type and you are actively controlling minimum outside air as per ASHRAE, your mixed air temp could conceivably get cold enough to warrant freeze protection. This would happen w/ fan at minimum and ventilation load at its max. For this reason we typically have a preheat coil in our northern climate. I would check you mixed air condition w/ maximum ventilation load at min fan speed assuming it is a VAV .

Your concern for moisture and possible reheat control in your climate is valid and could be of more concern than the low m.a. temp.

In economizer mode, cold air stratification could cause issues w/ freezing, I would suggest an air blender to help mix the air more evenly before it hits the stagnant cooling coil. Which, like the blow thru diffuser plate causes more pressure drop and significantly more length to your AHU. I have heard the blow thru's help prevent cold air stratification, but I am not convinced of that as I think the cold air goes to the outside of the wheel while the warm air goes on the bottom. Others may have more experience w/ that. Especially those that commission these systems.

I suppose it is possible that you would need both a reheat coil and preheat coil? You may need to check ASHRAE 90.1 on reheat stuff, I assume they talk about that in there. We rarely run into that condition in our climate so I am not familiar w/ what the energy code may say.

 

[ChasBean1]

My vote: OA/RA mixing chamber. Pre-filter. Blender. Preheat or heat recovery coil. Heating coil (blender alternate after heating coil). Cooling coil. Fan. Reheat coil. Final filter (65% plus). There are alternatives such as wrap around heat pipes now, providing better dehumidification and (sort of) free reheat...

Draw through is generally better for certain things such as pressure safeties. A high static cutout to protect a duct system designed for 4" might need to be set at 7" at a blow-through fan outlet to prevent false trips for example. The higher setting might blow a seam in your duct when the system operates at lower volumes.

 

[EnergyProfessional]

Do you employ an ERV for the ventilation requirement? If so, the air likely never gets too cold anyway.

You just need some safeguards (i.e. freezestat) in case a damper or ERV fan fail. I think draw-through gives you generally better coil flow as mentioned above. in addition you get some reheat after dehumidification, if that is what you want. but it depends on the AHU size etc. Maybe the AHU manufacturer has advice?

Coil freezing is more likely a problem at night when the AHU is off and cold air creeps in. Use low leakage dampers etc. Maybe you can run the economizer duct in a way to avoid thermo-siphoning?