Frequent cycling of the control valve between its open and close positions can be a symptom of stall in your heat exchanger. Stall increases steam consumption and process time of the heat exchanger.

Fluids flow under the influence of difference in pressure. Stall is the reduction or cessation of condensate flow from the heat exchanger. This typically 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 case the control valve reduces the steam pressure (less than 1 bar g) to meet the falling heat load (temperatures of less than 80degC). Now the pressure at the inlet of heat exchanger may become equal to or less than the pressure at the outlet of the steam trap. Thus in such cases, differential pressure across the steam trap reduces, which impedes the flow of condensate, causing condensate to waterlog the steam space.

This logging of condensate reduces the surface area available for steam to condense. The impeded heat transfer coupled with low heat loads leads to frequent cycling of the control valve.

Stall reduces the heat flow and it now requires more steam and longer time to heat the secondary fluid. One solution is to ensure condensate evacuation by gravity. Else specialized steam traps are available that prevent occurrence of stall in heat exchangers. These traps have provisions for forced evacuation of the condensate by pumping it out of the heat exchanger.