Air Plenum - what's the effect without it? 2

[Journeyman1972]

I'm a mechanical designer in the truck/bus industry, and a colleague decided to redesign the front defroster by getting red of the air plenum above the 2 blowers. He said that it's better for performance but I'm wondering why I can't seem to buy it. Any HVAC expert input please.
Thanks.

 

[pennpoint]

Journeyman1972
Philosophically:
Having space available to get the very best performance from a auto-defrost system is a rare thing in your industry. Historically, it seem to me, most designers take their best shot and hope the thing works.
Having said that. You should try to optimize the system based upon past successful experience, stay strong in supporting your design as the opposition will try to beat you down. Don't give in to cheap. This could very well relate to safety issues. Stand your ground.
I cannot make a judgement of a design with or without the plenum because air volume and static pressures prevail and do effect the overall performance. Look for some historical data of which you might have access. Seek out the advice of experienced engineers in the company or information in the company library.

I wish You Good Luck

pennpoint

 

[Journeyman1972]

thanks pennpoint!

I was telling my colleauge that the plenum was there so that enough pressure will build up to overcome friction losses along the ducts. He argued nothing written will support my claim. Anyways, I guess I have to prove it scientifically to older, experienced colleague.

 

[pennpoint]

Journeyman1972
Read plenum as "distribution" on the outlet side and as a "collector" on the inlet side.
Support for your position is in ARI, seek and you will find. If you are not in the USA. Well oops?
Your colleauge most likely has no data that supports his/her claim.
Shame on your colleauge for not documenting data that supports his/her position. You are both on even ground.
Have him/her prove your design is false. Stand your ground.

Best Regards
pennpoint



PS. You think "Journeyman1972" is going to be employed next week?

 

[lilliput1]

The velocity profile comimg out the fan is not uniform. The plenum gives it a chance to equilize as it encounters the pressure drop.

 

[Journeyman1972]

Thanks guys for your input. I've tried connecting to ARI's website, but to no avail. Maybe because I'm in Canada? Anyways, what I would like to do is to approach the issue scientifically as an engineer should do, and that is through
calculations and testing. However, I would like to do some reading first and I've been browsing some books but couldn't get a good topic on plenums and it's relationship to air distribution. Could it be that the old fella' is right? Naahh.....I couldn't convince myself. I know that the plenum is there for reasons you've already pointed out.

 

[SAK9]

It is quite unlikely that you will find much information on plenum application as it is not the best way to design an air distribution system.In order to get the best fan performance, sufficient straight lengths should be provided at the fan suction and discharge side.Off the top of my head it is something like 6D at suction and 10D at discharge D being the diameter of duct or bigger dimension in case of a rectangular duct.

Plenums are used when you can not achieve even half of this.The problem with plenum is that there are sudden entry and exit losses when the duct enters or leaves the plenum.Further for the plenum to be effective it should have a very low velocity ideally zero because you need to convert all your velocity pressure from the fan into static pressure(most of the time this is not practical because of space constraints).A very low velocity in the plenum will also ensure that air flows evenly into all the take offs.

The reason for the use of plenums in automobiles is probably because you can not achieve the required straight runs of ducts due to lack of space.

When a fan is installed with sharp bends immediatly at suction and discharge side, its performance can drop as much as 50%.So if your friend is going to replace the plenum with sharp bends,there could be problems.

If you look into air distribution fundamentals in the ASHRAE, SMACNA or Carrier handbook,it will be a lot clearer to you.

 

[chicopee]

effects of sharp bends can be minimize w/ short and long radii elbows!!!

 

[ChasBean1]

I have to cast my vote with lilliput1 on this one. If you think from a cost-benefit perspective, subtracting a fraction of a fraction of fan horsepower from what your 500 HP bus motor is already guzzling so you can have two clear streaks on the windshield versus a whole clear windshield (resulting in numerous dead pedestrians from poor bus driver vision) doesn't make sense. OR, you can diffuse the air directly from the blower, which adds a bit of static pressure, which gets us right back where we started...