Evaporator Heat Balance for recirculation - Help

[tetsu10]

This may be a silly question , but how do you do heat balance on a evaporator that is used for recirculation?

According to the heat balance I was looking at, the heat input of the evaporator is supplied by another stream vapour at 99 degrees celcius and comes out the same. The feed into the evaporator comes in at 70 degrees from a tank and comes out the same as well. Therefore delta T = 0. I know the Cp of the components of the mixture as well as the mass flowrates. On the P&ID the rating of the evaporator is 626 kW. (How do you come out with the rating?)

The vapour that exits the evaporator then goes back into the tank. The tank also had a feed of which I knew the flowrate. There will be vapour exiting the tank and finally I have to size a pressure safety valve. Please advise an approach.

 

[phex]

you need to consider heat of vaporisation and heat of condensation. if you check, it will not be vapor exiting your apparatus, but at least a mixed flow, more probably a liquid flow. and vice versa for the evaporator. liquid incoming, mixed or vapor outgoing.

hth,
chris