Heat exchanger calculation

  • Thread starter katchum
  • Start date
  • #1
119
0

Homework Statement



What happens when in counter flow heat exchangers, you diminish the mass flow of the low temperature stream?

Homework Equations



UA = cte
m2 = cte
Tin = cte
tin = cte
Q=cm1(tout-tin)
Q=cm2(Tout-Tin)
Q=UA(Tout-tout)+(Tin-tin)
----------------------
ln(Tout-tout/Tin-tin)

T = warm stream
t = cold stream

The Attempt at a Solution



I've inputted these equations in mathcad, but I am unable to find the mass flow m1 in function of Q. I get weird numbers like W(...) and root(...)

I want to know if m1 decreases, Q will increase or decrease.
So basically:

When I want to heat up 1 kg of cold water as high as possible.
Should I use a low flow or a high flow of cold stream?

It looks like I always get warm water in bath when I use low inlet flows.

Hmm, I also think, that the hot stream will decrease a little bit in temperature when the cold stream has a low flow, so the temperature difference will be very high, so basically the cold stream should be higher in temperature at the outlet.

So is it true that if m1 decreases, the Q will increase?

Edit:

This is weird:

Apparently when you decrease m1, then Q will decrease too. So less heat will be transferred.
But, the temperature of the outlet streams will be higher.

Is there any explanation for this?

My conclusion is, to have an efficient heat transfer you should open the inlet valve fully open to your bath. But when you want warm water you should close the inlet valve a little bit.

Funny to know is, when you want to save energy you should use a low inlet flow to your bath, you'll get warm water and there is less heat transfer so you'll use up less energy. Or am I wrong?
 
Last edited:

Answers and Replies

  • #2
18
0
Apparently when you decrease m1, then Q will decrease too. So less heat will be transferred.
But, the temperature of the outlet streams will be higher.

Is there any explanation for this?

Well, if you look at it from a first law perspective for one of the streams.
Q=m(h(out)-h(in)) If you decrease m, regardless of the factors of h, Q will decrease.
 
  • #3
119
0
How do you know that h(out)-h(in) doesn't increase more than m decreases?

h(in) = cte and h(out) will increase.
 
Last edited:

Related Threads on Heat exchanger calculation

  • Last Post
Replies
0
Views
2K
  • Last Post
Replies
9
Views
1K
  • Last Post
Replies
14
Views
2K
  • Last Post
Replies
0
Views
4K
  • Last Post
Replies
13
Views
1K
  • Last Post
Replies
0
Views
898
  • Last Post
Replies
6
Views
920
  • Last Post
Replies
9
Views
5K
  • Last Post
Replies
12
Views
2K
  • Last Post
Replies
10
Views
456
Top