Duty of heat Exchangers

[padovano]

I would like to ask 3 questions.
1 -Is the duty found on the data sheet of a heat exchanger that of the heat released by the hot fluid or that absorbed by the cold fluid or the average of both?
2 - In an attempt to monitor the heat exchangers of the preheating train of a distillation column I found that the heat absorbed by the cold fluid in certain Heat exchagers is greater than that release by the hot fluid. Can this be possible? is it a fouling indicator? My intention is to calculate the heat transfer coefiecient(U) hence fouling resistant(Rs) of the heat exchangers to identify those that are foul.
3- I would like to ask if the duty obtained by PROII is the mean of the heat released and heat absorbed.
Thanks
formula used = Q = U.A.F.Dtlm

 

[siretb]

1) both should be the same
2) no , not possible ; a fouling indicator will be the drift of the overall U coefficient, assuming the velcities do not change much.
Are you sure when you do your calculations you do not have an unaccounted for phase change.
Are you sure of the instruments?
3) I do not know. Often software check that the heat load (cold side) equal the heat load (hot side) at better than 5% (on input) but their output ids the same for both sides.

 

[SNORGY]

Is there a variation in Cp between states 1 and 2 for each fluid such that averaging them (the Cp values) produces different results on each side?

In any event, the duty should be equal on both sides.

Regards,

SNORGY.

 

[25362]

In a heat "exchanger", the enthalpy changes of both fluids, released and absorbed, should be equal, assuming no heat loss to, or heat gain from, the surroundings. In fact, this is generally attained applying good thermal insulation.

 

[padovano]

Thank you all for your contributions. It means if I have to consider phase change, I will need to use a simulator like PROII? or is the any other means I can estimate the presents of two phases?