CHD01 - as an HVAC engineer, my way of thinking is in standard cubic feet per minute (SCFM). I think this industry tends to be guilty of using volume flow and mass flow interchangeably.
We simplify for example by saying if 10,000 cfm is being discharged from a fan, then the sum of all the air outlets and leakage should be 10,000 cfm, neglecting actual volumetric corrections based on the fact that the fan discharge is pressurized and each outlet is essentially atmospheric. We also maintain the equivalent cfm assumption when individual zones are heating the air.
'The density at the inlet and outlet is the same' is faulty - you're right and sometimes I post before I think it through. Your previous post is accurate regarding mass and volume flow. The (true) volume flow could vary but I don't think it's the principal factor in determinining the pressure build-up in a room.
To throw a wrench in the discussion, what if reheated air at 140°F were entering the room while 70°F air was being squeezed out through cracks? Would these thermal changes in volume flow proportionally (or at least relatedly) affect changes in room pressure?
My way of thinking is very simply that air flows from a high to low pressure area. If you consider the room as a large opening in the duct with the air entry point being point 1 and the exit point being point 2, assuming the exit opening is sized equally with the inlet opening, the main reason for pressure build-up in the room is that the friction loss at the point where air is being pushed into point 2 is causing accumulation of air molecules within the room, therefore higher pressure with respect to areas downstream of point 2.
If the opening at point 2 is reduced in size, the mass (or SCFM) flow through the room would still be equal at points 1 and 2 but pressure would build further due to the added restriction (higher friction loss) at the exit opening.
When we close off point 2 completely, representing a perfectly-sealed room, pressure in the room is not a function of the volume flow, mass flow, or friction loss, but is now a static condition equal to the inlet duct pressure. If the room can take it, we can pressurize it to 5,000 psig by using any volume flow of air greater than zero assuming your fan or compressor is equal to the task.
The concept is simple from a common sense standpoint but is complex mathematically for compressible flow with friction. I've posted the Q = 2610 A dP^.5 from ASHRAE 1999 Applications (Ch. 51.5) because I like how it takes a grueling concept with unknowns, scary integrals, internal energy, relative roughness of open area surfaces, etc. and boils it down (with many assumptions, of course) to something usable for this application.
Quark, room pressures generally come to equilibrium pretty quickly (within a second or two in most applications) so if you want to re-think the steady flow energy equation with compressibility, friction, internal energy, etc. during brief transient periods, be my guest!
Now I've gotten too wordy and the more I write the more that can be used against me at a later date... CHD01 - your end cap "the more you learn, the less you are certain of" is definitely true. Best regards, -CB