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Active Contributor

Join Date: Sep 2007
Posts: 19

PLENUM in an HVAC System: What Does it Do?

09/17/2007 1:37 AM

Hi all

Could some one tell me what is the purpose and fuction of PLENUM in a HVAC system. What is it made of?

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#1

Re: PLENUM (HVAC)

09/17/2007 6:21 AM
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#2

Re: PLENUM in an HVAC System: What Does it Do?

09/18/2007 1:27 AM

A plenum is a type of a space. For example, the space where outside air and return air mixes, just prior to being filtered and then cooled or heated is called a mixed air plenum. If you have a nonducted return and use the ceiling space to move return air back to an AHU, you would call the ceiling space a return air plenum. That is the best way I can think of answering your question.

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#3
In reply to #2

Re: PLENUM in an HVAC System: What Does it Do?

09/18/2007 2:02 AM

THANK YOU DEAR

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#4
In reply to #2

Re: PLENUM in an HVAC System: What Does it Do?

09/18/2007 8:12 AM

It also acts somewhat like a tank on an air compressor, or an accumulator on a variety of other machines. Even though 2" to 3" of pressure (inches of water) doesn't sound like much, the volume of the plenum dampens fluctuations in air pressure, as the ducts leaving or entering the plenum are generally smaller by a fair degree.

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#9
In reply to #2

Re: PLENUM in an HVAC System: What Does it Do?

07/08/2009 12:58 PM

thank you very much. the information was very useful

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#5

Re: PLENUM in an HVAC System: What Does it Do?

09/18/2007 10:04 AM

A return plenum can be the entire area between the roof and the ceiling or a plenum can be constructed above the ceiling in spcial situations such as clean rooms or precision labs. I have had much trouble where the entire under roof is used as a plenum due to heat gain in the summer that needs to be accounted for in the design. There can be problems where the roof and walls meet where the seal is not very good and excess outside air gets in. I have one building where the outside dew point is the same as the plenum dewpoint and this causes condensation on the duct. Not good. I am insulating the duct. Theory will say that supply duct does not need to be insulated in a return air plenum.

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#6

Re: PLENUM in an HVAC System: What Does it Do?

09/21/2007 2:02 AM

usually the centrifugal fans are delivering the total air quantity required for the AC system through the blower outlet.

since the blowers outlets are usually smaller in size to develop the static pressure etc.,

next to the blower out let,,,, the plenum comes

The size of the plenum will be decided by the velocity you want to use for the entire ducting system which depends on the application industrial practice/ design

for example

if you have an airconditioning application

you usually design the duct velocity aprox. between 1300 fpm- 1500fpm

So you have already selected the fan and static pressure required but you cannot delver or takeout the outlet connection for delivering air into the space because the velocity at the out let of the blower will be too high.

if another application demands 2000 fpm design velocity forn the mainduct. Keeping the velocity from the statndards and qty you know from the blower/ designs

so what else you need further

apply Q= A x V

you know the area now.

the first duct which has all the air qty of the entire system is called the plenum now you make branches sub branches , outlets etc., from the plenum

However, in case if you are not making a seperate ducting system for the return air,

then the space between the falseceiling and ceiling acts as return air duct.

In a AHU room all the quantity come and get collected and hence the return air plenum at AHU room

I hope the question is answered.

Sivabala

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#7
In reply to #6

Re: PLENUM in an HVAC System: What Does it Do?

09/21/2007 2:46 AM

Dear Mr. Sivabala

Many Many thanks fro the reply.. it was quite informative

rgds

biju

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Anonymous Poster
#8

Re: PLENUM in an HVAC System: What Does it Do?

12/31/2008 8:07 AM

Some one said

"I have one building where the outside dew point is the same as the plenum dewpoint and this causes condensation on the duct. Not good. I am insulating the duct. Theory will say that supply duct does not need to be insulated in a return air plenum."

The psych chart says the supply duct does not need insulation, not some theory, physics

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