A steam trap is a device used to discharge condensates and non-condensable gases with a negligible consumption or loss of live steam. Most steam traps are nothing more than automatic valves. They open, close or modulate automatically. The three important functions of steam traps are:

1. Discharge condensate as soon as it is formed (unless it is desirable to use the sensible heat of the liquid condensate)
2. Have a negligible steam consumption (i.e. being energy efficient)
3. Have the capability of discharging air and other non-condensable gases.

1. Mechanical traps. They have a float that rises and falls in relation to condensate level and this usually has a mechanical linkage attached that opens and closes the valve. Mechanical traps operate in direct relationship to condensate levels present in the body of the steam trap. ARI-Armaturen's CONA-S and CONA-SC are examples of mechanical traps. 
2. Thermostatic traps. They have a valve that is driven on / off the seat by either expansion / contraction caused by temperature differ from mechanical traps in that their design requires them to hold back some condensate waiting for it to cool sufficiently to allow the valve to open. In some exceptional circumstances this is not desirable if condensate needs to be removed as soon as it is formed. ARI's CONA-B and CONA-M are examples of thermostatic traps.
3. Thermodynamic (TD) traps. Thermodynamic traps work on the difference in dynamic response to velocity change in the flow of compressible and incompressible fluids. As steam enters, static pressure above the disk forces the disk against the valve seat. The static pressure over a large area overcomes the high inlet pressure of the steam. As the steam starts to condense, the pressure against the disk lessens and the trap cycles. ARI's CONA-TD is example of thermodynamic traps.