Returning condensate by trap pressure often results in reduced condensate evacuation and thus higher steam consumption.
It is a common practice to connect the steam trap outlet directly to the boiler feedwater tank. Given the pressure in steam it is expected that the condensate will travel the distance easily and does not necessitate an additional pump. While the condensate may reach the feedtank this is not the complete story.
The condensate discharge capacity of a trap and thus adequate condensate evacuation depends on two factors,
- The trap type and size
- The differential pressure across the trap.
When steam trap itself is used to return condensate to the feedtank, back pressure develops because of the distance that the condensate has to travel as well as bends and rises that come along its way. This reduces the differential pressure and thus the condensate discharge capacity of a trap. The result is inadequate condensate evacuation and reduced heat exchange. Also with this practice, traps operating at different pressures are connected to the same line leading to increased back pressure on the traps.
It is recommended not to return condensate by trap pressure to achieve high heat exchange and thus conserve energy. Condensate should be returned by employing a combination of a flash vessel and steam operated pump to derive maximum benefit of condensate recovery.
The illustration below shows the variance in the condensate discharge capacity of Float traps (15, 20 & 25 NB) with differential pressure.
As seen with small changes in the differential pressure, the condensate handling capacity of the trap varies substantially.