There are two basic designs for the Thermostatic Steam Trap such as a balanced pressure and a bimetallic design. Regarding both balanced pressure and bimetallic designs, it depends on the difference in temperature between live steam and condensate or air to control the release of condensate and air from the steam line.
BOWLING Balanced Pressure Thermostatic Steam Traps use heat element inside the capsule causing it to the diaphragm pushing the valve towards to the valve seat because of the difference in temperature.