[Carlo08]

Ok I have a question for which I have answered correctly. The reason I am here is to ask why the answer is as it is.

There is a heat exchanger in which one fluid - milk is flowing one way and then in the first instance milk is also flowing back the other way.

The idea is the milk flowing to the left is heated up and the milk flowing to the right is cooled down, whilst heating the other milk up.

I need to find the heat transfer rate across the exchanger.

So what i did was find the energy transfer rate Q for both streams, which as the question neglects energy loss are both equal in magnitude.

Q for heating is =72182.8 Kj/h Q for cooling is = -72182.8 Kj/h now converting the top stream, heating, to Kw the si unit (divide by 3600) i get the correct answer.

this makes sense seeing as ignoring energy loss, the energy exchange is equal.

The thing that bothers me is the second part involves water as the second stream:

So milk is heated with hot water, and the water is cooled...

so for this Q for the heating is = 93139.2 Kj/h

Q for the cooling water is = -93458.211 Kj/h

I get the correct answer if I convert the top to Kw but how come the bottom one is different?

Thank you.

[Coursework.info]

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