Heat Exchanger (Shell & Tube)

all ratios, work energy out/energy in

If you like the formula for regen, I can post that.

ok , post it pls

Do you mean efficiency in terms of design of a new exchanger or in terms of in-service performance vs design performance or clean performance? The first has more than one definition and the second can be tricky to measure, especially if both streams are not single phase and flow rates, compositions, and all temperatures are not known.

it is in service, the temp of hydraulic oil is very high , and the water temp difference is very small , so i need to know how can i calculate it?

You don't need to calculate its efficiency, Auntie, you need to improve the cooling water flowrate. Is there a valve nearly shut somewhere? A blocked strainer? Or is the heating surface so covered in crap that nothing much in the way of cooling water can get through?

all valves before it is open , & the temp of inlet water is 28c

And? Is the cooling water loop full of crud? Differential pressure compared with design? Oil flowrates compared with design? Water flowrates compared with design? Come on. Throw out a carrot!