NCEES 2001 Prob 140

[P-E]

Prior to my exam prep, I hadn't had much exposure to HAVAC, so when I came across this problem, I tried for a long time to figure out why the answer given in the book used 1.09, when MERM uses 1.08. Little things like this bother me, so I use them as a learning opportunity. If you are unfamiliar with HVAC calcs as well (as it seems from your question), I think it's important for an engineer to know how that factor is derived. The MERM points out that some references use 1.09 and others 1.1 but does not explain how that factor is calculated. So here you go:

It's a ratio of the specific heat to specific volume of air multiplied by the minute to hour conversion factor required since the heat load is given in units per hour (i.e.: (cp/specific volume)*60 min/hr).

Results depend upon precision used in the calculation, and upon the conditions of the air (temp and moisture content).

In the end, whether you use 1.09 or 1.08 is pretty inconsequential because it doesn't really affect your answer enough to matter for the exam.

Yup you have to get by the minutiae and go with the rule of thumb sometimes. Another one is using 2.31 as the conversion from psi to feet. Most of the time you'll get close enough. Sometimes time management is more important on these exams. If you have a shortcut, use it.

 

[JHW 3d]

The 1.08 is just a conversion factor for the units.

The governing equation is just sensible heat transfer on air:

q_dot = m_dot * cp * Delta T (MERM Eq 40.18, as well as many other places)

 

[Audi Driver P.E.]

The 1.08 is just a conversion factor for the units.

The governing equation is just sensible heat transfer on air:

q_dot = m_dot * cp * Delta T (MERM Eq 40.18, as well as many other places)

As I noted, it's more than just a conversion factor for the units, it is a ratio of the specific heat to the specific volume which has dependencies upon the air/moisture conditions.