Air Handler Units Configuration
Air Handler Units Configuration
(OP)
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.





RE: Air Handler Units Configuration
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.
RE: Air Handler Units Configuration
Fans in draw-through have a more even flow over the coils. I'm not even sure what are the benefits of blow through.
RE: Air Handler Units Configuration
DISD control specifications tell you what to do.
RE: Air Handler Units Configuration
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.
RE: Air Handler Units Configuration
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.
RE: Air Handler Units Configuration
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?