I just need some help with this past exam question.

 

A counterflow concentric tube heat exchanger used for engine cooling in a very hot enviroment has been in servive for an extended period of time. The heat transfer surface area of the exchanger is 5m2 and the design value of the overall convection coefficient is 33W/m2k. During a test run, engine oil flowing at 0.1kg/s is cooloed from 110°C to 66°C by water supplied at a temperature of 25°C and a flow rate of 0.2kg/s.

 

a. Calculate the operating convection coefficient

b. If the ambient temperature is 50°C give a possible reason for the

discrepancy

Funny thing, I have a master in Chemical process engineering, but I have never heard of a "operating convection coefficient". Google also gives zero hits for the term. Instead, it links to this book.

 

I am sure that the "heat transfer coefficient" is meant here... but that is just because (1) heat transfer coefficients are largely determined by convection and (2) the book I just linked to also uses symbol h for the parameter, and h is the usual letter for the heat transfer coefficient and (3) the units (W/m2K) match.

 

Sadly I have no time for this little exercise today.

I found the answer, convection coefficient is just the heat transfer coefficient for convection strangly enough. All I had to do was use the equation q=m*cp*deltaT for the oil then using the same equation solve for the unknown final temp of the water. Then finding the log mean temperature and substituting it into q=U*A*deltaT and rearranging for U.

Thanks anyway.