HVAC - Why Not Constant Volume

[PaperFolder]

I'm new to HVAC and thermodynamics in general, and am struggling with something pretty basic. All the examples I've seen with the the psychometric chart imply the cooling and heating process of a environment is constant pressure. Even for a house, why isn't constant volume more accurate?

I'm assuming it is because of air infiltration/leakage? If this is the case, then how does one go about determining how much infiltration will occur? Do you normally use a rule of thumb?

 

[MechEngNCPE]

Infiltration occurs at the cracks and joints of structures, and has to do with pressurization of the building or room. If you know the room pressure, the crack widths and lengths, the velocity pressure across the cracks, then you can determine the cfm going in or out. Constant volume is not as accurate due to the compressibility of air with changing temperature.

 

[Xera]

Check ASHRAE's handbooks. They cover all of it.

 

[PaperFolder]

Xera can you recommend a ASHRAE book? I got my hands on the 2009 fundamentals one but the first chapter is already using the psychometric chart assuming everything is constant pressure.

 

[Xera]

I will check for constant volume. I was referring to infiltration/leakage. But the fundamentals is a good starting point for system design.

 

[KiwiMace]

The pressure has much less effect on the psychrometric properties than the temperature. This chart is useful for most normal elevations - 1000ft will give less than 4% error on air density calcs based at std P&T. If you are working at high altitude or cold temps then you can find or generate an altitude specific chart from software.

The saturation pressure of moist air is a function of temperature only. Refer the two formulae on pg 1.2 of your 2009 Fundamentals.

In practice the localised ambient pressures on a building vary considerably due to wind and stack effect, but this is often transient and otherwise averages out. Deliberate mechanical pressurisation is insignificant.

 

[Xera]

Check out section 21.8.1 of the 2009 Fundamentals.

 

[chicopee]

If you get a classical thermodynamics textbooks, it will explain the fundamentals of a psychometric chart.

 

[Drazen]

If you have certain electrical background - Navier-Stokes equations exist there as well, and knowing some ropes about them, you will have no doubt why every simplification offers much.

In hydrodynamics, it is hard to reach assumption how acceptable error from simplifications is, but once you have quantifiable results, mostly as combination of theory and long-time observations - than new technical field is open.

Most of air handling works in range up to 700-800 Pa, and up to 1000 Pa air can still be assumed to be incompressible (like water), which enables much of simplifications.