Air Cooler Bug Screen 3

[Brandon181]

We have requested a bug screen be placed at the inlet fan shrouds of our air coolers (mounted on pipe rack). The vendor has come back saying this cant be done, due to performance and maintenance issues. However, they have proposed an alternate scenario where the bug screen is placed in the plenum, between the fan and bundle.

First, is it uncommon to have bug screens placed at the inlets of air coolers? Second, am I correct in thinking that a pressure drop caused by the inlet bug screen would also be present if that bug screen was on the outlet? If so, why would the second option be more viable than the first.

Please do not consider fouling or clogging of the bug screen in your answers. And we have discarded the idea of full bug screen around the air cooler substructure.

 

[ko99]

Brandon,

For turbulent air, pressure drop is proportional to velocity[sup]2[/sup]

I can only speculate on your setup but if the screen is butted up against the fan, you'll have a ring of high velocity air racing past a small cross-section of the screen. If the screen is placed further away, such as downstream in a plenum, the air is distributed over a larger cross-section, which reduces the velocity (assuming CFM is constant) and greatly reduces the pressure.


ko (

http://www.msthermal.com)

 

[Brandon181]

@ko99: Thank you so much! I was being dense and not taking into consideration the varying geometries. What you say makes perfect sense when I actually think

Any experience as to whether it is common to place bug screens on inlets of air cooler fans? Although I see the problem with the higher pressure drop, it seems that maintenance on inlet screens would be much much easier than on plenum screens.

 

[GHartmann]

I believe it depends on the type of arrangement that you have.

If you have the forced fan type (see attached JPEG picture), then you can see the fan motor interferes with the your "bugscreen". However, downstream the fan it is fairly easy to place the screen, but as you said the maintenance access is more difficult.

I believe your best source of information will be your prospective vendor.

 

[SNORGY]

I think it is very common to put a bug screen on the air cooler inlet, but I have absolutely never done it between the fan and the bundle. The face velocity across the screen at that location will be so high that the only things you are going to accomplish, in my mind, are dramatically increase the static pressure required, dramatically increase the fan power required, and ultimately wreck the screen anyway. Is it a cloth screen or a metallic screen?

On first inspection, I don't like this idea at all.

 

[willard3]

Screens at fan inlets will also cause pre-rotation and other undesirable geometric effects and reduce the performance of the fan.

 

[GHartmann]

At the risk of hijacking the OP's thread, I am not sure I understand whether the fan cares if the screens are at its suction or discharge. A screen on the inlet would be no different than having the fan after the bundle.

I am also not understanding the comment about putting screens inlet the air cooler, but worried about between the fan and cooler. Where else could they be in such an installation?

Just looking on the web, I am not seeing many screen installations. I did see some complete "housings" around the inlet plenum and even some air inlets where the air entered radially (from circumference) into the fan (not very common).

 

[speco]

Brandon181,

I think you should reconsider the idea of putting the bug screen around the perimeter of the struture below the plenum. As was pointed out previously, the pressure drop through a screen (or about any obstruction) is proportional to the velocity squared. Since a bug screen has a relatively small percentage of open area, you need a lot more surface area to keep the static pressure drop low. The cost of a bug screen is relatively small compared to the cooler. Also, the capital cost of the screen is extremely small compared to the cost of the fan horsepower over the life of the cooler, especially if you have a small flow area for the screen.

Speco (

http://www.stoneprocess.com)