Fan Intake Sizing

[mrcjh98]

I recently was involved in the installation of a radiator/fan unit. The unit, with a 42" fan (19,500cfm), was installed in place of an older unit, with a 36" fan (12,400 cfm). The new radiator/fan unit is also taller than the older unit. The fan shroud of the new unit is less than 8" from the roof of the enclosure that it is installed in (the lovred intake is on the side of the enclosure). The motor for the new fan is also drawing close to 15 Amps, when its spec'd FLA is 13 Amps.
I was wondering what the appropriate methods for calculating the minimum clearance for a unit such as this were. Also, would it be better to just install an intake on the roof, and if so, what is the best method for sizing that intake.
Im an electrical engineer, and have never delt with HVAC type engineering before. I'd appreciate any help you can give.

THANKS

 

[rogzog]

You Don't specify what type of fan this is but generally a high amp draw would indicate that either the drive pulleys are incorrectly sized or adjusted or the fan is setup to perform at a specific static pressure condition and the installation is not meeting that condition. (i.e. no resistance to the air flow allowing the fan blades to do the maximum air moving that their design can perform) The fan should have been supplied with some performance data, like a fan curve. If you have it check the actual fan speed with a tachometer and reference the fan curve. It can give you an idea of what is going on with the fan. Check to see if the fan is not running too fast. If the fan RPM is correct find a way to check the total static pressure accross the fan and again refer to the fan curve chart. Maybe this will produce some answers.

TOK

 

[rogzog]

After re-reading your post another thought came to mind.If the discharge side of the fan is less than 8" away from an obstruction that will likely be choking the fan, causing a high static pressure and increasing the amp draw on the motor. The manufacturer should have some recomendations as to the minimum clearances. Check the installation manual for the equipment.

 

[ddace]

I think your problem has to do with the CFM.
you went from 12400 to 19500 in the same enclosure.

Your intake and/or exhaust surface area are probably to small for that much flow.

Please provide more details on sizes of those areas and if there is loovers and/or dampers.

Will be easier to help evaluate/estimate static preasure.

 

[ddace]

the rule of thumb says you target approx 500ft/min (speed)at intake: 19500/500 = 39 sq ft of free surface area at intake if there is no loovers.

With loovers you should consider them like 50 % of free area
so you would need 2*39 sq ft.

at exhaust is the same but you can target 1000ft/min.

these are ball park figures. You can increase those speeds to reduce surface area needed, but then you increase static preasure and amp draw on motor.

It also depends on noise level (faster = noisy)
And if rain can be sucked in at intake (if enclosure is outdoor)

if this is not clear let me know

 

[Accystan]

I think that ddace is probably correct. If the intake and exhaust louvre area has not increased then the velocity through the existing louvres has increased by almost 60%. This will have resulted in the static pressure drop through the louvers increasing by almost 250%! If your louvre press. drop is a significant portion of your “system” delta P then, depending upon your fan characteristics, that would explain the increased current draw.

I’m not sure what you mean by “fan shroud” but if your fan inlet is now positioned such that the incoming air is unevenly loading the blades and it’s a propeller bladed fan, (rather than a centrif.) you might have problems down the road. Whatever type of fan, if the air is entering at an oblique angle you’ll not be getting the performance you require due to system effect. The minimum distance you need is one fan diameter.

Sounds like you need an additional intake (on the roof?). As ddace said, you need about 40 square feet of free area.

Cheers.