Thermodynamics, positive pressure argument

[Heimisson]

I'm having a argument with a mechanical engineer concerning positive pressure in a room. I was hoping someone could shed light on this.

We both agree (of course) that pumping air into a room that will keep the room under positive pressure (relative to the outside) given that the room isn't completely open (i.e. has small holes or cracks compared to the total surface area of the room).

However I think heating the room to a temperature higher than the outside air (and maintaining that temperature via heating) will increase the positive pressure even more because the air pumped inside it will expand when it gets warmer and increase the the total pressure. So there is is constant expansion of air going on.

However he claims that higher temperature will not increase the pressure. I'm not really sure why. Something to do with this being a open system (although I don't agree that this is a completely open system)

So what do you say?

 

[edgepflow]

The pressure would increase in the room only if it is air tight. Since it is vented, air escapes to the environment.

 

[Mapes]

I agree with Heimisson. If one heats any of the air that is or has been pumped in, the volume of that air will increase. This increase will result in a higher volumetric flow at the interface, which is the same result that would arise from having a higher pressure inside the room. The heating thus augments the pumping effect, as you intuited.

If the air cools down quickly in the process of leaving, however, then the increase in effective pressure may be negligible. And one must continue to pump in air, or continue to heat the internal air to higher and higher temperatures, to maintain this positive pressure.

 

[russ_watters]

Assuming you guys are on the same page about the assumptions, you are correct. The key is, as Mapes said, that the volume of air added is measured before it is heated.

 

[256bits]

However I think heating the room to a temperature higher than the outside air (and maintaining that temperature via heating) will increase the positive pressure even more because the air pumped inside it will expand when it gets warmer and increase the the total pressure. So there is is constant expansion of air going on.

However he claims that higher temperature will not increase the pressure. I'm not really sure why. Something to do with this being a open system (although I don't agree that this is a completely open system)

If you are using a fan to pressurize the room, that fan can push only a certain volume of air into the room at a particular pressure. Heat the room to expand the air as you say and the fan will push less and less air, as the pressure does increment somewhat. The fan is feeling a feedback from the heated air. Thus, as Mapes says, the heating will augument the pumping effect.

But economically, is it more effective to spend extra dollars on heating with what could be achieved by a slightly more powerful fan?

If you would be using a positive displacement pump, then that could be considered a measured quantity of air being entered into the room. Then again, a fan is much cheaper to buy and operate.