Thermostatic Trap
The bellows type thermostatic traps contains a fluid-filled thermal element (bellows). The operation of the bellows follows the steam saturation curve, always discharging condensate a few degrees cooler than the steam temperature. As long as steam is present, the valve will remain closed. Only when subcooled condensate or air is present will the valve open.
Sub-Cool: The sub-cooling of condensate prior to discharge can have certain beneficial effects. In the majority of tracing applications, the sub-cooling of condensate is highly desirable because of the additional energy that is extracted from the Hot condensate. If the trap did not sub-cool condensate, this energy would be wasted. In Batch style process applications such as jacketed kettles, plating tanks and heating of outdoor storage tanks, the sub-cooling of condensate is generally not a factor to consider since the amount of condensate back-up requires less than 1% of the heat transfer surface area and is therefore considered negligible.
Air
When air, which is cooler than steam, is present, the bellows is retracted and the seat is open, allowing large quantities of air to be discharged.
Condensate
When condensate, which is cooler than steam, is present, the bellows retracts and the seat opens, allowing condensate to be discharged.
Steam
When steam reaches the trap, the bellows expands, closing off the seat and preventing the steam from escaping.