I have had success finding tube leaks using shaving cream.
Pressurize the shell side with air or nitrogen.
Remove both channel covers.
Coat the entire tube sheet with foamy shaving cream (both tube sheets).
The leaking tube will blow out foam.
If the unit is in the field, wind and rain may reduce the methods effectiveness.
Also the method can be confusing if you have multiple leaks.

Thank you Yitbos for the response,

The issue is when the shell side design pressure is way lower than Tube side design pressure, say 5 bar vs. 30 bar.

I don't think pressurizing the Shell side would work!

Thanks,

If you know or pretty sure you have a leak you can use one of the following instruments to locate the offending tube/tubes while in place.  
I have used Ultrasonics and Helium Leak testing to locate leaks under similar conditions.  

Pressure Testing

http://estgroup.cwfc.com/productsServices/spokes/01b_heatExchg_10-TubeTestingGuns.htm

Vacuum Testing

http://estgroup.cwfc.com/productsServices/spokes/01b_heatExchg_10d-650-TubeTestingGuns.htm

Prior to any hydrostatic testing we would do quick air bubble test to get the big ones.

We would then install the shell clamps and test the shell while watching the tubes. Add the channel sections and use pressure decay to hydrotest the tubes while watching the shell side.  

I use an ultrasonic leak detector for pressure or vacuum leaks on heat exchangers, has worked well for me. Also helium testing.

http://www.uesystems.com/prod_up_2000.asp

• http://www.uesystems.com/prod_up_2000.asp