Heat Exchanger Problem

[stern]

Hello I need some help with a heat exchanger calculations.

A counterflow H Ex uses combustion gases to heat air for a process.

The data T1 in(air)= 25C , T3 in (gases)=180C and T4 out (gases)= 85C
air mass flow= 1 kg/s, gases m= 1.8 kg/s
The combustion gases lose heat to the environment by 5 KW

If I consider 4.18 Kj/KgK as Cp for both streams what would be the heat transfered to the air and the S generated?

I got Q air to be = 714.8 KW but for the enthropy no answer yet.

Is S gen = m air (Cp ln T2/T1) x Q surround/T surrondings alright? in KW/K Do I consider Ts of air here?

Thanks for your comments,

S

 

[EnergyMix]

Given that you've been on the forums for a couple years, I'm pretty sure this isn't a student post (which is why I think you've not received any answers.)

I think though that you'd be best served by investing in some software to calculate your answer. While someone could answer your questions, are you really sure you want to base your design off of what an anonymous person on the Internet tells you? Isn't it better to get the software yourself and have your own answers?

Want to know the do's and don'ts of Eng-Tips? Read FAQ731-376.
English not your native language? Looking for some help in getting your question across to others or understanding their answers? Go to forum1529.

 

[stern]

Thank you Energymix, Im sure youre a very kind human being..

 

[SNORGY]

I guess the first question from me would be, are you sure about the 4.18 kJ/kg.K?

The second question would be, what is the basis for the 5 kW heat loss?

I disagree with the Cp value. Should be about 4 times lower for air, ald lower still for the hot gas.

 

[IRstuff]

That value of specific heat is that of water, so not valid for the gases and air.

Nonetheless, if you have the correct specific heats, and the exit temperature of the air, you can calculate the amount of energy transferred.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[EnergyMix]

Yes I am Stern. I took the time to answer your question and saved it from going completely unanswered due to people saying "student post" due to the way it was worded. So you're welcome. In the two years you have been on these forums have you done that for anyone else?

Want to know the do's and don'ts of Eng-Tips? Read FAQ731-376.
English not your native language? Looking for some help in getting your question across to others or understanding their answers? Go to forum1529.

 

[stern]

Yes, Cp should be 1.005 KJ/Kg K and I found with an entropy balance m1s1 + m3s3 - m1s2 - m3s4 -Q/T =0 the answer.

Thank you all..