Closed Loop Air Flow Pressures

[erex engr]

Hi,

Imagine a closed loop with the following; fan, duct work, cyclone. Assume a system loss of say 6 inwc. That fan has the potential to overcome the pressure loss. If the fan is turned on and the system allowed to operate (no heat gain or loss, no air added or removed) at what pressure (gauge) would the inlet of the fan be? What would the outlet be?

One could assume the inlet would be -3 and the outlet +3 inwc. Why couldn't it be -6 at the inlet and 0 at the outlet or some other combination there of?

If you can site some technical studies or sources that support your response, please do so.

Thanks,
erex

 

[GT-EGR]

Your question may seem clearer if you think of it like piping and a pump. The pump/fan just adds the differential needed to get through the system resistance, and then the actual internal pressures depend on how you fill and seal your system.

So in a piping system, you may fill and seal the system so it’s minimum pressure holds at some high value like 10 PSI, and then the 50 PSI pump boosts it to 60 PSI and then the after goes back around the loop until it is 10 PSI again at the inlet. But if you filled your system to start at 20 PSI minimum, then the pump would boost it to 70 PSI, but essentially do the same work.

Ducts are not truly sealed so their point of reference pressure is just 0/atmospheric - which is where the air enters the atmospheric space aka zero gauge pressure. Then it comes back to the inlet of the fan where it is negative until the fan then boosts it.

The -6 inlet and 0 discharge pressure scenario you are describing would require your closed duct system to have been sealed to a vacuum sub atmospheric pressure, but since ducts really aren’t built that way, in the end, a ducted fan system just jumps positive at the fan discharge and negative at the fan inlet since it has to be above 0 pressure to push air into a space, and below 0 pressure to draw air from a space

 

[berkshire]

What you are describing sounds like an exhaust ventilation duct for removing entrained solids, rather than an HVAC problem. You may get better answers for this one on forum 1043 Local exhaust ventilation. By the way with this type of system it is unusual to return the air back to the room without extensive secondary filtration. I suspect the same people will answer.
B.E.

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