AHU Problems I have seen

[Saturninus]

Hi All,

I have seen the following:

1. Return Fans that were turned off, but left in place. What does this do? My assumption is that turning off the return fan would increase the pressure inside the building. Also it might increase pressure in the AHU return. Assuming 100% return, would this affect supply fan usage?

2. AHUs where the door is left open. They leave the door open in a 150 HP AHU at the mixed air section before it gets to the motor. They do this because the motor cannot push enough air otherwise. What is the cause of this? Any ideas of how to resolve this? Does it affect energy usage?

Thanks in advance.

 

[quark]

You should think of one basic rule first and then every thing falls in place. Generally, centrifugal fans don't build up pressure but they overcome system resistance. For example, if your system offers a resistance of 4" to the fan at rated flow, if proper fan is selected, the discharge static pressure of the fan will be 4" only.

You can over come this resistance with one fan at rated capacity with 4" static or two fans with same capacity of 2" static and so on. This is purely economic decision. Ofcourse, there are some limitation with respect to the type of fans you choose. Nevertheless, main factor is economy.

Coming to your first question, when the return fan is turned off, the supply fan has to overcome the total resistance of the duct work and thus it runs to the left of the original operating point on the curve. i.e pressure increases and flow decreases. The pressure inside the building is unaffected as long as the mass flow rate of air in the building is equal to the mass flow rate of air out of the building.

Answer to your second question is, if you open the door of the AHU at mixing section, as flow follows the least resistance path, there will be more flow from the outside than the recirculation duct. This increases load on cooling coil but the power drawn by fan remains same as long as flowrate is constant.

 

[STYMIEDPIPER]

Regarding item no. 2
1. The supply filter may be undersize creating a larger pressure drop at the filter. The filter area must be sized for velocity at 500 fpm at the system volume.
2. The filter itself can be clogged/plugged with debris for some unknown reason. The filters should be on a routine maintenence schedule. A lab that we service schedules the filters to be cleaned on a monthly schedule.
3. If there is a mixing box a few other possibilities can be problemmatic. The mixing box may of been inadvertantly closed. If there is an inlet filter on the mixing box same situation as delineated in item no. 2 listed above. The make-up air requirements at the mixing box may be grossly undersized. If the make-up air is ducted from outside check that there are not any new obstructions placed in front of the inlet.
4. The return duct system can be contributing to the problem. I have seen on very rare occasion that the coefficient losses on the return ductwork was not taken into account when specifying the fan.
5. Quark is correct about air flow following the path of least resistance