closed loop static pressure

[dvd]

I have been conducting a thought experiment and would appreciate some input. Imagine a closed-loop duct system that consists of a fan, ductwork connecting the inlet and outlet, and a filter that has, say, a 10" drop across it. How would one calculate whether the 10" drop across the filter was being pushed or pulled or a little of both?

This arrangement doesn't exist so experimental measurements aren't available. I would appreciate comments.

 

[berkshire]

dvd (Mechanical)
I think you will find that some wind tunnels have just this arrangement, although I am not sure about the pressure drop.
B.E.

 

[Engineer6512]

The movement of air through the duct system is based solely on the differential pressure between two adjacent points. Air will flow from higher pressure to lower pressure. So in this system, you have two primary sources consuming pressure - friction in the ductwork, and the filter. You have one source generating pressure - the fan.

The pressure will be at its highest point at the fan discharge. It will steadily get lower as it moves through the ductwork due to friction, and a step-drop across the filter. The pressure will be at its lowest point at the fan inlet.

If you define "pushing" as generating a high pressure that causes air to flow, and "pulling" as generating a low pressure that causes air to flow, then really you are doing a combination of both, although this is really not the best terminology to use.

 

[Auth]

I have tools and calculated this before, air temperature in the loop rised rapidly so I have to terminate the calculatiing, it needs a HX to balance temperature.

Fan inlet has the lowest pressure, fan outlet has the highest in the loop, to make sure the exactly pressure value, I think it needs to know air weight and temperature.

System design/optimization for Airplane Environment Control System
and Evaporating refrigerating system

http://www.flycarpet.net

 

[dvd]

I agree with everyone that the fan inlet will have the lowest pressure and that the outlet will have the highest pressure. It is also a good point that the temperature will rise due to the fan work.

I am still curious about what the static pressure in the duct will be at given locations. I can envision a system which would be dominated by negative pressure in the return leg of ductwork, and conversely, I can envision one that would be dominated by positive pressure. The point I can't really put my thoughts around is what are the physics which would cause one to occur over the other. Could a system such as described above be somewhat "unstable" in that the area of largest pressure magnitude would move from one side of the filter to the other?

This seems like an indeterminate problem, but I know that if I were to build an actual system that it would figure out how to perform. I would continue to appreciate comments on this, in particular I would like to hear everyone's comments on the physics involved.

Best regards,

DVD

 

[Auth]

Suppose we have this system, it has a cooler somewhere to keep system stable. by initial, fan not started, we can aerate it to positive pressure, or extract to get a negative pressure, even leave a little hole at fan inlet to keep constant inlet pressure like liquid loop system(ignore this because it is simple and not a sealed system).

After fan started, air flows along duct, pressurized in fan and decompressed by step in ducts and accessories.

For a specified fan, the flow-head characteristic is determined by its inlet pressure, we suppose this pressure as variable P0, so we get the system flow related to P0 as one equation by iterate with fan, ducts and accessories as filters, like we calculate a open system.

To determine P0 and static pressure at each point, we need system volume and initial pressure to get system air weight, this is the key point, the air weight is constant all the time, so we have another equation, calculate each segment air weight by its pressure and temperature(related to P0).

By iterate these two equations, we can resolve P0 and system flow, and so we can get all other data as each point static pressure.

That's my idea for this, why do you say "unstable"?

Regards,
Auth

System design/optimization for Airplane Environment Control System
and Evaporating refrigerating system

http://www.flycarpet.net

 

[dvd]

Thank you, Auth. I agree with you that there is conservation of mass within the total system. It seems to me that the mass within a given element may be changing however and thus mass will need to be solved for at a previous time step than the pressure and temperature. I say that I think this is an "unstable" process because the pressure at the step change may not be constant at any given location during sustained operation.

I would like to keep simplifying assumptions out of the analysis. Since the fluid knows what to do without making any assumptions, it would be nice if I could do that also.

 

[Auth]

Pressure is not uniform at different area, that's the problem for accurately calculating.

I didn't consider the dynamic effect, but it would be stable finally like open system, although some fluctuation may be left.

System design/optimization for Airplane Environment Control System
and Evaporating refrigerating system

http://www.flycarpet.net

 

[marcoh]

I think it is easy enough to establish a the relative pressure in the system, ie pressure rise across the fan, duct losses, and filter pressure loss etc.

Would have to think a little longer about how to establish the absolute pressure relative to atmospheric at each point? It is a closed system unlike a building where the room would be assumed to be atmospheric pressure (or close enough).

Wind tunnels do have large cooling systems to maintain temperatures. I think a typical F1 motor racing windtunnel has something like 3000kW motors which is a lot of energy to impart into a small apace!

 

[dvd]

I appreicate the responses to date. Cooling may be used on this type of system, but I know that it could reach a steady-state on its own.

My reason for initially asking the question was that I wanted to control the pressure at specific locations in the system. My current thinking is that if I want to control the pressure at certain locations within the system that I need to have two fans in series (obviously the step change would be between the two fans.)

 

[zekeman]

You are asking a good question and there is an answer.
First, analagously, consider an electric circuit, where you have a voltage source and one or more series resistances. Now here it is impossible to state what the absolute voltage is at any point in the circuit. You cannly get voltage differences but not absolute.

Your problem of a forced air system is different since it is not an entirely closed system, but two systems that interact, namely the fundamental fluid flow around the loop, and a fluid flow process that takes place between the outside air and the inside air whch includes room air used for furnace combustion, outflow from exhaust fans, etc. This combination yields a definitive pressure at all points with respect to the outside. And by no means is this indeterminate, which you know intuitively is not possible.