How do i perform heat balance / heat flow comparison to evaluate a heat exchanger efficiency? The hot side is steam and cold side is water.

This otta keep ya busy:

http://en.wikipedia.org/wiki/Log_mean_temperature_difference

Im guessing you are calculating the steam condenser efficiency. The easiest way to tell if it is getting fouled it to trend the vacuum and water temps in and out.

You should have old as built records and installation and construction requirements of the heat exchanger. CHeck your flows, orifice sized correctly, pressure at the inlet and outlet. Also the temps of the meduim thats being exchanged. any changes with that might require additional cooling water flow to maintain proper temps.

this should be moved to "cooling system" forum