Air Flow Question

[JoeysVee]

If the following is the Bernoulli equation for air flow...

SP1+VP1+TPfan=SP2+VP2+FP

then how can the following be true...

SP+VP=TP? (MERM eq. 20.6)

Seems like those two equations contradict each other.

What am I missing here? Thanks!

 

[HVACstevie]

If the following is the Bernoulli equation for air flow...
SP1+VP1+TPfan=SP2+VP2+FP

then how can the following be true...

SP+VP=TP? (MERM eq. 20.6)

Seems like those two equations contradict each other.

What am I missing here? Thanks!

How can it be contradictory when the variables are all different. Look at it this way

SP+VP=TP

so

SP1+VP1=TP1

and

SP2+VP2=TP2

then if

SP1+VP1+TPfan=SP2+VP2+FP

then

TP1+TPfan=TP2+FP

 

[JoeysVee]

So TP1 doesn't equal TPfan?

 

[HVACstevie]

So TP1 doesn't equal TPfan?

No.

Imagine a System

(1)------(FAN)------(2)

So lets think about this as water pipe pump problem. An open system and pressure is measured in feet.

(1) is at 10 ft

(2) is at 20 ft

and the friction lose of the entire system is equal to 40 feet of water. So lets solve for the pump (fan)

*since the pipe is the same in the entire system : VP1=VP2

SP1+VP1+Ppump=SP2+VP2+FP

since VP1=VP2

SP1+Ppump=SP2+FP

so, solving for Ppump

Ppump = SP2-SP1+FP=20-10+40 = 50ft.

if you're still lost, lets look at it this way

SP1+VP1+Pfan=SP2+VP2+FP

What happens if we remove the fan?

Pfan=0

SP1+VP1+0=SP2+VP2+FP

Would there be a velocity? no sp VP1 and VP2 both are 0

SP1+0+0=SP2+0+FP

If there is no velocity can there be friction? nope

SP1+0+0=SP2+0+0

simply

SP1 = SP2

 

[JoeysVee]

Thanks for the explanation. That helps a lot!