NCEES 2008 #536 (HVAC Afternoon)

[Bman]

The problem states:

In a refrigerated warehouse, 10,000 ob of whole, lean, cured ham is brought in at 40 degrees F, cooled to 28 degrees F, frozen, and cooled to o degrees F. The refrigeration required (BTU x 10^3) to do this is most nearly:

There is a similar question in the HVAC SMS (#24). The solutions to these two problems differ when determining the latent heat removal. The SMS solutions multiplies the mass by the percentage water content before multiplying by the latent heat of fusion, but the NCEES solution doesn't take into account the percentage of water content. The "hint" for the SMS even states that the "latent heat removal will depend upon the water content of the pork."

Does anyone know why you wouldn't multiply the mass by the percentage water content? It seems to be that the NCEES solution is incorrect...

Since we've now seen this type of problem twice, I'm figuring there is going to be a similar question on the exam and I am not sure whether to account for the water content or not....

 

[MechGuy]

The problem states:
In a refrigerated warehouse, 10,000 ob of whole, lean, cured ham is brought in at 40 degrees F, cooled to 28 degrees F, frozen, and cooled to o degrees F. The refrigeration required (BTU x 10^3) to do this is most nearly:

There is a similar question in the HVAC SMS (#24). The solutions to these two problems differ when determining the latent heat removal. The SMS solutions multiplies the mass by the percentage water content before multiplying by the latent heat of fusion, but the NCEES solution doesn't take into account the percentage of water content. The "hint" for the SMS even states that the "latent heat removal will depend upon the water content of the pork."

Does anyone know why you wouldn't multiply the mass by the percentage water content? It seems to be that the NCEES solution is incorrect...

Since we've now seen this type of problem twice, I'm figuring there is going to be a similar question on the exam and I am not sure whether to account for the water content or not....

I remember this question quite well! It stuck out as being an odd question to me. But from what I remember, I would account for the water content. I trust the SMS books more than the NCEES exam personally.

Do you knw which ASHRAE book to look in to get the info you need for this problem?

 

[Bman]

The info is in the 2006 Refrigeration book, table 3, page 9.5. The % moisture content is provided along with the specific heats and latent heat of fusion, so I'm not sure why you wouldn't account for it.

 

[MechGuy]

The info is in the 2006 Refrigeration book, table 3, page 9.5. The % moisture content is provided along with the specific heats and latent heat of fusion, so I'm not sure why you wouldn't account for it.

Yep, I think you have a good handle on it. Follow your instincts. Is there any errata from NCEES on that question?

 

[Bman]

I looked up the errata, but it doesn't list anything for this problem. I will send them an email once they fix their "contact us" link, but right now it just brings you to a server error page. Thanks for the help!

 

[SMTx]

You are correct, but the NCEES question does not address the moisture content so you just multiply the mass by the latent heat of fusion.

 

[Bman]

If the information is readily available, why wouldn't you apply it? If the question had stated to ignore the moisture content, that would be understandable, but it doesn't say either way.... After all, the latent heat of fusion only applies to the water content and not the entire mass, so techically the unit would be over sized.

If I come across this during the exam, I'll probably just work it both ways and see which answer best matches up with the answer choices. In the case of the NCEES question, the answer where the moisture content was neglected exactly matched one of the choices where my answer (where I accounted for the MC) was only sort of close to one of the choices. So I guess the I would pick the exact match over another answer...

 

[SMTx]

I was mistaken. I didn't notice that the ASHRAE book gave you the moisture content. I thought the SMS problem must have stated the moisture content. My mistake. Don't have the SMS book.

 

[Bman]

Ok, I think I figured it out. The ASHRAE book gives you the %MC and also the latent heat of fusion. This latent heat of fusion appears to have already accounted for the MC; it is 104 for this particular problem vs. 143.4 (latent heat of fusion for water). So, it looks like you can solve it by simply multiplying the mass by the LHF in the ASHRAE book, or by multiplying the mass x %MC x the latent heat of fusion for water. Both answers come out to approx 104,000. So both approaches are correct.