#507 2001 Thermo PM

[mcilenti]

Ok, I know I'm just missing something easy....

Where are they getting the '48' in the solution that they multiply with the Delta Omega?? The rest of the equation makes sense to me, but I can't see where that comes from.

I can see the '20' in the 20*250,000 from the Delta T to get the Q, but I'm stumped on the 48.

I thought we had to multiply the Delta Omega by hfg in order to get h from the water??

Like I said, I know I'm missing something simple, but somehow I still can't see it.

Thanks for the help,

Mike

 

[JoeysVee]

I looked at this and I have no idea where the 48 came from. I actually just used the 2 enthalpies and that gave me an answer closest to the correct answer. SO I got it right but more importantly I need to understand how they got their answer so we need to find out where the 48 came from.

 

[POPAC]

I looked at this and I have no idea where the 48 came from. I actually just used the 2 enthalpies and that gave me an answer closest to the correct answer. SO I got it right but more importantly I need to understand how they got their answer so we need to find out where the 48 came from.

48 is from stem table MERM A-44 stem table for water out at 80F hf.

 

[JoeysVee]

48 is from stem table MERM A-44 stem table for water out at 80F hf.

Thanks!

 

[mcilenti]

Right, but why are they using hf? I thought you multiply Delta Omega with hfg since you need to account for the energy it takes to evaporate that amount of water?

Joey- I also got the answer right by using the tables without accounting for the extra water vapor. I just want to ensure I understand how to complete the problem properly.

 

[POPAC]

Right, but why are they using hf? I thought you multiply Delta Omega with hfg since you need to account for the energy it takes to evaporate that amount of water?
Joey- I also got the answer right by using the tables without accounting for the extra water vapor. I just want to ensure I understand how to complete the problem properly.

The way I see it is: water enter 100F exit 80F with some vapor (evaporated water) air enter 85F exit 95F and more humid

the mass balance of water is: mw2=mw1-(W2-W1)ma

Energy balance is: mw1*hf1+ma*ha1=mw2*hf2+ma*ha2

I think they use hf because is water out at 80F sat liquid . If you sent your email I can scan my solution and email it to you. I hope this help.

 

[mcilenti]

Sure, if you wouldn't mind I would appreciate it.

My e-mail address is [email protected]

Thanks,

Mike

 

[Yingli]

Sure, if you wouldn't mind I would appreciate it.
My e-mail address is [email protected]

Thanks,

Mike

Could you please forward to me at [email protected]? thank you.

Yingli

 

[JoeysVee]

and send it to me too...you already have my addy