‘Stalling’ in a heat exchanger reduces the heat transfer rate, wasting steam and increasing process time. Identify stall condition and take corrective actions.
Fluids flow under the influence of difference in pressure. Stall is the reduction or cessation of condensate flow from the heat exchanger, and occurs when the pressure at the inlet of the heat exchanger is equal to, or less than, the pressure at the outlet of the steam trap.

This condition may occur when,

The heat load on a heat exchanger reduces or
High backpressure exists on steam trap because of a rise after the trap.
In the first condition the control valve reduces the steam pressure to meet the falling heat load. Now the pressure at the inlet of the heat exchanger becomes equal to or less than the pressure at the outlet of the steam trap. Thus, in both conditions mentioned above, the differential pressure across the steam trap reduces, impeding the flow of condensate and causing condensate to waterlog the steam space.

Logging of condensate in the heat exchanger reduces the surface area available for steam to condense. Hence the heat flow reduces and it now requires more steam and longer time to heat the secondary fluid.

Thus, stall should be identified and remedial measures must be adopted to reduce steam consumption and process times. Specialised steam operated pump traps are available that prevent occurrence of stall in heat exchangers.