Steam trap being noticeably cooler than the temperature of the steam pipe inlet to the heat exchanger, can be a symptom of stalling. 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, 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 in a temperature controlled application when,

The heat load on a heat exchanger reduces or
High backpressure exists on steam trap because of a rise after the trap.
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.

A good indicator of stalling is, if the temperature of the steam trap is noticeably cooler than the steam pipe inlet to the heat exchanger. Condensate being nearly at the same temperature as steam, this condition indicates sub cooling of the condensate due to logging.