Static pressure and velocity

[marco3995]

A quick review, if a increase in static pressure,in any given duct, will decrease velocity then a increse in velocity, cfm, will decrease static pressure?

Thanks

[matt huber]

well, yes... the relationship you are describing is correct. Except the pressure is the dependant variable, eg. pressure varies as a function of velocity.

With a decrease in velocity you will notice an increase in static pressure. Its static pressure regain, or SPR.

The SPR method of duct sizing is based on Bernoulli's equation, which states that when a reduction of velocities takes place, a conversion of dynamic pressure into static pressure occurs.

[propmanage]

Think of it like a garden hose that has frozen from being out side. As it thaws out and you turn the water on it builds up pressure, a little water comes out so you have little water coming out and lots of static pressure pushing on the hose walls trying to get out. The static is the pressure of the ice water dragging on the hose walls trying to get out. As it becomes slushier you get more volume less static until you finally have free flow of water. The free flowing water is moving at what ever static design would be for that size hose and city water PSI. With air it is the same thing only the static is not caused from slushy air it is caused from some other blockage or slow down in the system.:p

[marco3995]

Thanks guys, that helps

[dash]

A quick review, if a increase in static pressure,in any given duct, will decrease velocity then a increse in velocity, cfm, will decrease static pressure?

Thanks


Depends on where the increase in static comes from.

1.If the fan motor's static ability is increased ,the cfms and velocity will increase ,if the duct duct size remains the same.

2.Now if we change the duct size or add other resistance to increase the static,the velocity will decrease with the increase is static on the same motor and duct size.

3.If we decrease the velocity and the duct size remains the same,then the cfms were reduced and the static will decrease.

Maybe you could be more specific as to what conditions or problem you have.

[marco3995]

Well its not so much a problem I have, but I am freshly out of college taking havc and related subjects. I work for a controls company now and they asked me to install a HRV for the office. I have my HRAI RASDT (residential air systems design tech) I am sizing the ducts and am just trying to make sense to my method, I need to know why I am getting these numbers.
Keep in mind they guys I work for only know controls, they wouldn`t know the first thing about installing a HRV.

I ask all of this becasue I have a chart from my fans that looks like this, for example:

Static 0.0 0.1 0.125 .250 .375
CFM 98 94 93 84 52

So why is the static pressure decrease with increase velocity?

[dash]

Well its not so much a problem I have, but I am freshly out of college taking havc and related subjects. I work for a controls company now and they asked me to install a HRV for the office. I have my HRAI RASDT (residential air systems design tech) I am sizing the ducts and am just trying to make sense to my method, I need to know why I am getting these numbers.
Keep in mind they guys I work for only know controls, they wouldn`t know the first thing about installing a HRV.

I ask all of this becasue I have a chart from my fans that looks like this, for example:

Static 0.0 0.1 0.125 .250 .375
CFM 98 94 93 84 52

So why is the static pressure decrease with increase velocity?

The chart above shows that as the static increases the cfm decreases ,velocity will also decrease ,as you will move less cfms thru the ducts and the HRV.

We Manual D to size the ducts for ERV/HRV's.

[marco3995]

Right! Gottcha

[propmanage]

Marco,
Keep us in the loop while you do this project it will exercise our minds and help you out. Remember underestimating the resistance will result in failure of the blower to deliver the required volume of air.

Proper duct size and shape will prevent wasted power, reduce noise, and deliver proper ventilation at given velocity

[dajw02]

Very well put Dash , I was gonna answer the same way you did - then noticed you beat me to it.

David W.

[propmanage]

:confused:
I would like to hear others opinions on these energy saving devices?
I would thing it would work for commercial buildings that need to exhaust a large amount of air due to pollutions but does residential receive a cost savings

[dash]

Very well put Dash , I was gonna answer the same way you did - then noticed you beat me to it.

David W.


Thanks,welcome to the forum!

I see by your website that you are close by,how is the sale of design services going?

[dajw02]

Thanks,welcome to the forum!

I see by your website that you are close by,how is the sale of design services going?

its starting slow but is picking up quite a bit. I expect by Summer time to have a considerable pick up.

David W.

[dash]

its starting slow but is picking up quite a bit. I expect by Summer time to have a considerable pick up.

David W.


Are you aware of the FPL program that pays a contractor $75.00 to do a Man. J ,when the replacement unit qualifies for their rebate??

Could be a big boost to your business if you have or can get customers in that area.

Joining ACCA could help ,the National Meeting will be in Orlando this year.The Florida Chapter has worked hard to get Florida Contractors there.You could get a booth there.