NCEES 2001 Prob 140

[slickjohannes]

can someone please explain the steps involved in dolving this problem?

 

[Ramnares P.E.]

Would be helpful if you posted the problem...

 

[P-E]

...so we could help dolve it.

 

[slickjohannes]

I knew someone would say that.

I'd rather not post copyrighted material. If someone has got the book and can dolve the problem, great- but if not then no worries.

 

[Ramnares P.E.]

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

 

[P-E]

You could copy it wrong.

 

[Ramnares P.E.]

You could copy it wrong.

But then I'd dolve it wrong :dunno:

 

[matt267 PE]

dtop being sick heads and help the guy out.

 

[P-E]

dtop being sick heads and help the guy out.

Shouldn't you be studying?

 

[matt267 PE]

Nope. The April 2016 exam is months away.

 

[Ramnares P.E.]

That's the spirit Matt

 

[Ramnares P.E.]

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

 

[matt267 PE]

Yuck, I hate thermo.

 

[Ramnares P.E.]

More of an HVAC give me points question than thermo I'd think.

 

[P-E]

More of an HVAC give me points question than thermo I'd think.

Yes.

 

[matt267 PE]

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.

 

[P-E]

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.

 

[matt267 PE]

Ah yes, Uncle Eddy.

 

[slickjohannes]

Dhanks guys.

 

[Audi Driver 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.