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skywalker92
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Mass Flow Control Valve

Sun Apr 10, 2016 9:20 pm

Can any one explain me how this mass flow control valve works,as per the image if the cabin pressure reduces valve will move in to a close position rather than opening it to allow more air in the cabin.
one of my friend told me that this is for mass control and not for controlling the pressurization in the cabin... Is it correct?
Somebody please explain me this and I would like to know what is the modern use of mass controller used in newer air crafts.


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Skywalker92.
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Horstroad
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RE: Mass Flow Control Valve

Mon Apr 11, 2016 3:09 am

From looking at the image:

Dynamic pressure from the compressed air pushes on the inlet mass flow effective area to close the control valve. But when the control valve is closed you have no airflow, so no dynamic pressure. Static pressure inside and outside of the control valve are equal, so the spring pushes the control valve back to open until dynamic pressure and the spring force are equal. That's how that part of the valve regulates itself.
When the cabin is pressurized you have a lower pressure difference to the compressed air than you would have with the cabin unpressurized. Less pressure difference means less flow, less mass. That's why the back pressure sensing vent helps to open the control valve depending on the pressure in the cabin to adjust the flow accordingly.

I hope I got it right so far, if not please correct me.

I'm not sure what the static vent is good for. Looks like the greater the pressure difference between the cabin and the outside pressure, the less mass flow you get as cabin pressure that would help open the control valve is vented away from the back & static pressure effective area. Not sure about that.


Cabin pressure is controlled with the outflow valve(s). You always want a constant stream of fresh air to the cabin. Imagine a perfectly airtight cabin. Once it is pressurized you wouldn't need more air, so a fictional cabin pressure control valve would close. The air would be pretty stale after a 10 hour flight 
So you just put in a constant amount of fresh air and dump what you don't need through the outflow valves.
 
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Jetlagged
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RE: Mass Flow Control Valve

Mon Apr 11, 2016 3:34 pm

As Horstroad has said, it's to regulate the mass air flow into the cabin. This needs to be fairly constant to maintain the ventilation rate. In my experience the packs have a flow control valve, typically with several flow settings, so I was interested in what aircraft this flow controller was from.

The diagram indicates that side of the piston is for back and static pressure, i.e. the highest of the two. So perhaps the static vent is there for when the cabin is at lower than ambient pressure. It would also enable the flow control valve to open if the back pressure vent was blocked.
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Horstroad
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RE: Mass Flow Control Valve

Mon Apr 11, 2016 5:06 pm

Quoting Jetlagged (Reply 2):
The diagram indicates that side of the piston is for back and static pressure, i.e. the highest of the two. So perhaps the static vent is there for when the cabin is at lower than ambient pressure.

Under normal conditions this should not happen. In a rapid descent maybe. But the negative pressure relief vents open at -0.2 psi pressure difference between cabin and outside (B777). Considering this in the flow control valve does not make too much sense imho.

Quoting Jetlagged (Reply 2):
It would also enable the flow control valve to open if the back pressure vent was blocked.

This would only be beneficial when the static pressure is higher than the air trapped inside the blocked back pressure vent. Which in the worst case is only below 8000ft.
 
113312
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RE: Mass Flow Control Valve

Mon Apr 11, 2016 5:52 pm

The point is that as RPM of the engine changes, the output of compressed air will change also. To prevent surges of airflow, the mass flow valve compensates to keep the volume of air going to the cabin constant.
 
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skywalker92
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RE: Mass Flow Control Valve

Fri Apr 15, 2016 11:48 am

thanks all...

Quoting Jetlagged (Reply 2):
so I was interested in what aircraft this flow controller was from.

This diagram was included in our aircraft systems module and even I am not aware of any practical usage of this shown valve.
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