A float and thermostatic trap (see Figure 10-23) has both a thermostatic element to release air and a float element to release the condensation. As such, it combines features of both the float trap and the thermostatic trap.
These traps are recommended for installations in which the volume of condensation is too large for an ordinary thermostatic trap to handle. Float and thermostatic traps are also used in low-pressure steam heating systems to drain the bottom and end of steam risers (see Figures 10-24 and 10-25). Other applications include the draining of condensation from unit heaters, preheat and reheat oils in air conditioning systems, steam-to-water heat exchangers, blast coils, and similar types of process equipment.
Check the trap-seat pressure before installation to make sure its rating is equal to or greater than the steam supply of the boiler.
If the trap is operating properly, it will immediately and continuously discharge condensation, air, and noncondensable gases from the system that enter the inlet orifice of the trap.
The condensation is handled by the ball float, which is connected by a level assembly to the main valve head. Condensation entering through the trap inlet causes the ball float to rise, moving the level assembly and opening the valve for discharge.
Air and noncondensable gases are discharged through the thermostatic air vent. The thermostatic element is also designed to prevent the flow of steam around the float valve.
Float and thermostatic traps operate under pressures ranging from vacuum to a maximum pressure of 200 psig; however, the great majority of them are designed for 40 psig or less.