On small heat exchangers draining to atmosphere vacuum breakers may be used to avoid “stalling”.
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 also occurs when there is a sudden drop in heat load of the plant. In this condition the control valve reduces the steam pressure to meet the falling heat load. Now the pressure at the inlet of heat exchanger becomes equal to or less than the pressure at the outlet of the steam trap. Thus 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.
On smaller heat exchangers draining to atmosphere, the stall condition can be avoided by installing a vacuum breaker on the steam inlet to the heat exchanger. When the vacuum is reached in the steam space, the vacuum breaker opens to allow condensate to drain down to the steam trap.
However in general, it is not desirable to introduce air into the steam space, since it acts as a barrier to heat transfer and reduces the effective steam temperature. This becomes a problem on larger heat exchangers, where it is not advisable to use a vacuum breaker to overcome stall. Furthermore, if the condensate is lifted after the steam trap, for example, into a raised condensate return main, the vacuum breaker cannot assist drainage. In both these cases, it is necessary to use an active method of condensate removal such as a steam operated pump trap.