How would you calculate the static pressure? 1

[Mehtesham]

Hello,

I want to calculate the static pressure of a zone. I know the cfm the fan is delivering, I know the motor HP. I was just wondering if there is any way to calculate the static pressure in the system based on these values or do you have to use a pitot tube or a manometer to determine that.

Also, Is there any way that you can know how much fresh air and how much return air your system is using.

Any links, papers or answers will be greatly appreciated.

Thanks

 

[MintJulep]

When you sat "of a zone", do you mean the conditioned space, or the the supply duct?

Regardless, with only cfm and fan HP, you can't get there.

 

[Mehtesham]

Yeah I know I need to know the fan curve and the system curve but the system I am handling with is too old built in 1950s and we don't have any OEM data for that. So I was just trying.

 

[SAK9]

Assume your fan takes in Y cfm fresh air at T1 and mixes with return air to produce Q cfm at T2.Your return air will be Q-Y cfm at T3.You can measure T1,T2&T3.You know Q(fan cfm).

Use equation QxT2=YxT1+(Q-Y)xT3 to determine Y.

 

[urgross]

If it's an existing system, then it should be more accurate, and probably faster, to use a manometer. For fresh air and return air, run traverses at the OA intake and return duct.

 

[317069]

why do you want to calculate static pressure?

 

[YuriB]

If you already know "how many cfm the fan is delivering", then you know "how much fresh air ... your system is using".

 

[willard3]

Find somebody where you work to help or hire an Engineer, you can't learn air system pressure drops on a web forum.

As you can see, your question is so general that no one can figure out what you want to know because you don't know what you want to know.

 

[Ziggypump]

Yuri, how can you tell "how much fresh air" is delivered if you know "how many cfm ... your system is using"?

Am I wrong, or is this only obvious when using a 100% makeup/outside air unit.

Mehtesham, two words "Balancing Contractor".

 

[Shophound]

>>>>Also, Is there any way that you can know how much fresh air and how much return air your system is using.<<<<

I would first start by finding the percentage of outdoor air, since you know the total CFM:

% outdoor air = (return air temp - mixed air temp)/(return air temp - outdoor air temp)* 100

Example: @ 2000 CFM total air...

(75 - 80)/(75-90)* 100 = 33.3% outdoor air. @ 2000 CFM x 0.33 = 660 CFM fresh air induction

As for the static pressure of the zone in question; since this is an existing system, get a manometer and static pressure probe and measure it. What you calculate may not match what's actually in the field in this case, since airflow in ducts is seldom laminar. That is why a traverse measurement is recommended.

 

[miningman]

Whatever happened to the formula

HP = cfm x Static Pressure ( inches WG) / 6350 x Fan Efficiency??


Ive been usuing that for so long in mine ventilation , ive forgotten how it was derived

 

[urgross]

A problem with trying to derive a leakiness factor empirically is that some occupants can actually absorb air through their skulls. You may have to introduce an airhead factor based on casual observations. Is this for a management function?