Draw Thru vs Blow Thru Air Handler

[anthonyh123]

Any recommendations or pros and cons for draw thru vs blow thru configuration?

Auditorium
20,000 cfm @ approx 3.0"
Cooling Coil w/Reheat Coil
VFD Fan

Office / Classrooms
15,000 CFM @ aaprox. 2.5"
Cooling Coil
VAV w/Reheat
VFD Fan
Thanks

 

[imok2]

Draw through will give you a more even flow pattern as it has a tendency to pull air through the full coil surface. make sure you consider the required in. of water column on the condensate trap to get a good drain.

 

[Yorkman]

I think you're probably aware; that if the blower motor is in the air stream a blow through cooling coil, will see the heat load of the motor. Where with a draw through cooling coil this heat load goes directly to the supply air stream. This could increase supply air temperatures 2-3 degrees. Just something to be aware of in the selection process.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[lilliput1]

If there is requirement for final filter downstream of the fan & coils, as typically required for hospitals, don't use blow thru. You will have moisture collect on the filter as the saturated air off the coil encounters the filter pressure drop.

 

[BronYrAur]

I don't have any great advice on one over the other, but I can tell you how Carrier classifies their custom AHUs. I had an application where I was installing some HEPA filters downstream of a 24,000 CFM rooftop AHU. In order to do so, I has to increase the supply fan motor size. The return fan was obviously blow-thru, but I was convinced that the supply was draw-thru. After all, it was downstream of the hot and chilled coils. As it turns out, the supply fan was in fact blow-thru because I had a downturn plenum right after the fan. Carrier uses blow-thru when anything (not just a coil) is downstream on the fan. The fan curves were slightly different and it was important because I would have been too far left on the curve for draw-thru after increasing the motor size. I just made it on the blow-thru curve.

 

[anthonyh123]

Thanks for the input...I am going draw thru due to even coil airflow distribution plus some reheat from the motor to eliminate moisture carryover. My biggest concern now is making sure noise is not a factor and making a good connection to the supply duct to reduce fan system affect. Nothing downstream but conditioned air.

Thanks

 

[Yorkman]

Fan noise has a lot more to do with proper duct design, than the fan itself, which is probably what you mean. Refering to a good connection to the supply duct

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[AbbyNormal]

If the system has 3 or more inches of total static it could have an impact on loads that are highly sensible such as an office building.

If the load is highly sensible you may prefer blow through, as it allows the coil to remove the fan/motor heat. More heat for a coil to grab, easier to remove the fan heat.

With draw through, the air has to be overcooled to compensate for the reheat of the fan/motor. Sometimes the overcooling will end up on the saturation curve, and to get the air cool enough, you are forced to dehumidify air more than needed. Another option is to use more airflow but then this only adds to the static.

So sometimes it is possible that you may want a blow through coil on a load that is highly sensible.

 

[lzh007]

Be aware, in cold weather, blow thru will provide more air flow due to change of density. 2 senior guys were beaten by by me on this topic, then I was fired.

 

[Yorkman]

Izh007,
I'm don't agree with you on the issue of blow through versus draw through and how one is going to affect the air density any differently than the other. The density or specific volume of the air is a function of its temperature and relative humidity, not whether the fan is a draw through or a blow through. The problem that you cited sounds like the air balancer did not adjust the fan properly.
Typically the air side CFM should be balanced to the design CFM with an active wet cooling coil. If the CFM is adjusted to design in the heating mode, when the system is in cooling mode the CFM delivered will be short.


I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI