NCEES 2001 Prob 140

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No one is asking you to post the book. Post a picture of the problem or simply type it out entirely.

 
dtop being sick heads and help the guy out.


Here's the step by step solution:

1. Given dry bulb conditions, we're dealing with Sensible heat.

2. Sensible flow heat equations, in English units: Qs = 1.08 * q * delta T

Qs = sensible heat flow (Btu/hr)

q = air flow (cfm)

delta T = temperature difference, °F

Given Qs, delta T in the problem, solve for q.

q = Qs / (1.08 * delta T)

Qs = 90, 000 Btu/hr

delta T = 82 - 60 = 22°F

q = 90,000/(1.08*22) = 3787.9 cfm

Nearest answer is B - 3750 cfm

 
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More of an HVAC give me points question than thermo I'd think.
Yes.
looks like thermo to me.
I'd rather deal with pounds of primary sludge being removed.
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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.

 

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