How to Approx. balanced Total pressures and CFM?

[aeroUB]

Here is and Example.

There is a system with 2-20ft dia. fans, in parallel, pulling air through a bunch of cooling coils with a cross-sectional area of about 800 ft^2.

One fan is running and the other is isolated by backdraft dampers. In this situation the flow through the unit is 842,770 CFM and a total pressure (static + velocity) of 0.853" wg.

If the backdraft dampers on the second fan failed and air was allowed to enter through the second 20' Dia. by 10' high fan stack into the unit...how can I approx. the new total pressure inside the unit and the resulting airflow. Also what % of that air is coming through the coils and what % is coming through the open 'hole' in the unit?

Thanks for any input
aero

 

[berkshire]

aeroUB (Aerospace)
You have already posted this question in Mechanical engineering other topics.
B.E.

 

[aeroUB]

Are you afraid that I was not aware of that?

1. It was posed after another question was already answered and could be easily missed.

2. I figured this question would be better directed towards someone who is more specialized in aerodynamics.

But thanks for the input.

 

[berkshire]

aeroUB (Aerospace)
Well I guess that OK then.
Are you talking a total failure of the system with the dampers held wide open. If that is the case your flow is going to short circuit around the open hole, that is flow out unless you have any venturi effect. The only static pressure you are going to get, is what you regain from the velocity pressure of the running fan together with any entrained air from venturi effect.
B.E.

 

[aeroUB]

Berk,

Yes, I mean a failure where to dampers are wide open. I guess there would be some resistance due to the fact that there is a 20' 9-blade fan in the way that the entering air would want to 'pin-wheel' in the opposite direction but I wouldn't expect this to add significantly to the static pressure that is built up inside the unit? Maybe an added 10% to be safe?

By Venturi Effect, are you refering to the fan discharge stacks that the air would now be entering though? If so, I also believe that this would be very small with this geometery.

As far as the velocity pressure I'm a little confused in this situation. Under 'normal' operating conditions I would have:

Static Pressure: 0.832" wg
Velocity Pressure: 0.217 wg
Velocity Recovery: 0.039" wg

Total Fan Pressure: 1.01" wg (.832 + .217 - .039)

Under normal conditions I would have a Velocity Pressure of .217"wg. Under the failed conditions I would get a higher velocity due to a lower static pressure in the unit. This should also then create a larger Velocity Pressure than the 0.217" that was there before right?

Thanks

 

[aeroUB]

One thing I noticed,

On the fan curve of Flow vs Pressure. The fan curve eventualy hits the curve for Velocity Pressure.

If I assumed that there is no static pressure in the unit, or if this fan was used in open space at a contant RPM, is this the point at where the flow would maximized? And this also would give the induced velocity pressure at that point?

Thanks

 

[berkshire]

aeroUB (Aerospace)
your quote,
"Under normal conditions I would have a Velocity Pressure of .217"wg. Under the failed conditions I would get a higher velocity due to a lower static pressure in the unit. This should also then create a larger Velocity Pressure than the 0.217" that was there before right?"

Yes as I see it your velocity pressure will be higher because there will be little or no static pressure in the unit. Axial flow fans, as you know do not generate very high static pressures to begin with. Also since you will have an open high pressure area adjacent to the working fan, air will flow from there to the back side of the working fan, further reducing the static pressure build up in the cooling chamber. Thus you would be relying on velocity pressure only, as though the fan was working in open air. Since I do not know the configuration of your duct, I cannot speculate on how much static pressure regain you would get as the velocity drops.
B.E.

 

[berkshire]

aeroUB (Aerospace)
One thing I forgot to add, you stated that your duct area was about 800 sq ft. One fan is going to give you about 314 sq ft of area into that, so your discharge area is two and a half times more than your inlet, not withstanding the open hole from the failed fan. Or are you talking surface area on the cooling coils not duct area.
B.E.