Fan Static Pressure

[jedax81]

Hi everyone,

I'm currently generating a static pressure calculation of a duct system.

If total pressure at section 1 (Pt1) merges with Pt2 and Pt3 is not equal to Pt1 + Pt2, what is Pt3 then?

Note: Pt = Ps (static) + Pv (velocity)

Can Bernouli equation still applies on this case? I'm not sure about the pressure but Q=AV will apply.

If you happen to see the ASHRAE 1999 page 32.22, with a graph fig. 14, it shows that:

section 1: 35 Pa (merges with section 1)
section 2: 57 Pa (merges with section 2)
section 3: 103 Pa

In the graph it shows resulting pressure at section 3 = 160 Pa

Can someone explain this?

[Brian_UK]

It depends on the duct size I expect.

[jedax81]

Ok forget about my question above.

Here's my new question:

A fan has a rating Ps=500 Pa (Fan Static Pressure) and Q=2000 L/s

The ESP (external static pressure) you initially get is 300 Pa from the highest static pressure generated in the duct system.

Imagine that there are 3 branches having:
L1=200 Pa @ 500 L/s
L2=200 Pa @ 500 L/s
L3=500 Pa @ 1000 L/s

Situation:
If you close L1 & L2, then L3 will have more airflow equivalent to 2000 L/s and static pressure required will go up, say 1000 Pa.

Question:
Do you think the fan can deliver with 2000 L/s and 500 Pa rating over the increased static pressure of 1000 Pa?

My understanding is that, you wont feel any airflow at L3 because the static rating of the fan is below of the new requirement static pressure. In actual there might be small airflow but you wont feel anything.

My colleague insist that there will be an increased of airflow and that is gonna come out at L3.

What's your idea guys?

[RSTC]

If I'm understanding your question clearly you have a fan moving 2000l/s @ 500pa.

You have 3 branches of some sort of plenum box L1& L2 are 500 l/s @200Pa and L3 1000 l/s @ 500Pa

In this instance the Fan+plenum box + L1 = index run. (assuming no ducting on far side of fan). the index run is the total Pressure drop.

If you change the ducting eg blank off L1&L2 you will change the system. You can calculate what the new pressure drop of the ducting system is

...but...

you don't know what that will do to the fan - you need the fan curve for that! While the fan is doing 2000l/s @ 500pa, it will do something less at the new higher pressure drop. It depends on the fan type and arrangement. Every model of fan has a unique curve. Typically axial fans Q (airflow) will drop off steeply with increased pressure and centrifugal fans will have a 'longer' curve.

In your instance you will have somewhat less than 2000l/s through L3. The actual duty is where the 'fan' and 'system' curve intersect (on the same access).

Expect alot of noise though!

[Brian_UK]

Relate your examples to the Fan Laws but do not expect your fan to produce your final expectations.

Relate fan pressures to the fan curve and the system resistance curve for the results of your damper closures.

[Magoo]

I tend to agree with Brian UK. check the fan laws.
Dependent on the total static across the fan, and the type of fan is important, some are self limiting, others are not and will blow out ducting or stall motor.
As a preference I prefer the static regain / trunk duct type design.
magoo

[jedax81]

Guys,
Ok I understand your point and thanks to your ideas but I don't think fan laws will apply on this.

Remember the fan has a fix ampere, power, static pressure and air flow. (ideal conditions)

If L3 goes up with a static pressure of 2000 Pa and your rating of fan is only 500 Pa, you can't expect the fan to get more power just to get 1000 Pa new static pressure so it can deliver 2000 L/s otherwise the motor of fan will trip.

Fan Laws:
Ps (new)/ Ps (old) = [Q(new)/Q(old)] ^2

That formula only relates when you do something on the fan.


By the way RSTC,
Index Line is always the line that has the highest static pressure as per ASHRAE and that's L3.

[RSTC]

Sorry just read that back... yes fan (intake or discharge) + connection (plenum or otherwise) + L3 (not L1 which is now blanked off).

The short answer to your question is NO, You cannot get more out of the fan. In fact any fan which can produce 2000l/s @ 500 pa is going to barely readable at 2000 pa. In reality this is not going to work cause (as per first question) even if the fan physically could do it- the velocity (Q=AV or V=Q/A) is going to be nearly 20 m/s.

you'd have the world's biggest vuvuzela.

[Brian_UK]

Guys,
Ok I understand your point and thanks to your ideas but I don't think fan laws will apply on this.

Remember the fan has a fix ampere, power, static pressure and air flow. (ideal conditions)

If L3 goes up with a static pressure of 2000 Pa and your rating of fan is only 500 Pa, you can't expect the fan to get more power just to get 1000 Pa new static pressure so it can deliver 2000 L/s otherwise the motor of fan will trip.

Fan Laws:
Ps (new)/ Ps (old) = [Q(new)/Q(old)] ^2

That formula only relates when you do something on the fan.


By the way RSTC,
Index Line is always the line that has the highest static pressure as per ASHRAE and that's L3.
Sorry, but Fan Laws are Fan Laws; if you don't believe in them then try a different line of work.:eek:

[mad fridgie]

Guys,
Ok I understand your point and thanks to your ideas but I don't think fan laws will apply on this.

Remember the fan has a fix ampere, power, static pressure and air flow. (ideal conditions)

If L3 goes up with a static pressure of 2000 Pa and your rating of fan is only 500 Pa, you can't expect the fan to get more power just to get 1000 Pa new static pressure so it can deliver 2000 L/s otherwise the motor of fan will trip.

Fan Laws:
Ps (new)/ Ps (old) = [Q(new)/Q(old)] ^2

That formula only relates when you do something on the fan.


By the way RSTC,
Index Line is always the line that has the highest static pressure as per ASHRAE and that's L3.
Firstly you fan is not fixed anything, it gives what you state a one point only, make any change then all other date will change.
Re your original question, your higher static pressure is based upon the higher flow rate. The fan will not produce that pressure and flow rate (without making changes) What will happen is that equalibrium will be reached, more than likely a lesser flow than your maximum at a higher pressure than 500pa. This does all depend upon the specifics of the fan. If the max pressure developement of the fan is only 500pa, then you will only get 1000L down the open leg, Your fan is likely to be designed around optimum performance on the curve, at this not normally at max pressure .

[dougheret0]

And if you have a big fan operating way out on its curve, if you severely restrict the supply by shutting off a major branch, you can convert rectangular duct to round.