Liquid or Solid Expansion Steam Traps (Wax Capsule Steam Trap) are thermostatic steam traps actuated by temperature sensitive devices, responding to changes in condensate temperature.

Thermostatic steam traps respond to changes in temperature and therefore discriminate very well between steam and cooler non-condensable gases. They can rapidly purge air from a system, especially on a cold start-up, and can be installed in various positions. Most frequently, actuation is by means of a bimetallic element or a bellows-like capsule filled with a vaporizing liquid.

Bimetallic actuated devices are characterized by their high resistance to damage from freeze-ups, water hammer and superheat. They are relatively small in size and lend themselves to high pressure designs. The condensate discharge temperature, however, does not follow the saturation curve very well, and the bimetallic elements are subject to corrosion with some reduction in closing force over time.

Bellows actuated steam traps, on the other hand, discharge condensate at a temperature which follows the saturation curve. The weak point is the bellows itself which can be damaged by superheat, water hammer or freeze-ups.

Thermostatic traps respond slowly to changing conditions even though the cause is usually misunderstood. It is not the heat sensitive element that is slow to respond. Rather it is the heat energy in the condensate inside the trap, which is slow to dissipate, that causes the time delay. Insulating thermostatic traps reduces their responsiveness even more. Mounting the trap at the end of a cooling leg in an area where air can circulate improves responsiveness and is the basis for installation instructions recommending a cooling leg at least three feet in length.

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Liquid or solid expansion steam traps are not widely in application in the industry. The opening and closing of these traps is a function of temperature and balanced return spring forces. Elevated temperatures cause an expansion of the thermostatic element which closes the valve, while low temperatures cause a contraction of the element, aided by the spring, which results in opening the valve.

Traditionally, the thermostatic actuator has been in the form of a metal rod, having a high thermal coefficient of expansion, or an elastic metallic capsule (bellows) filled with a liquid which expands when heated. In the past, design innovation has introduced a small diaphragm actuator filled with a wax-like substance which expands rapidly at a pre-selected temperature. This has significantly reduced trap size and increased the speed of response relative to the more traditional design. Above figure shows the working internals typical of a newer wax capsule expansion trap.

Regardless of design variations, these steam traps have one characteristic in common. The temperature of the condensate they discharge remains constant at a predetermined point and is not a function of steam supply pressure. All other steam trap types have a condensate discharge temperature that increases with steam supply pressure.

In general, these constant discharge temperature traps respond slowly to changes in temperature and should only be specified where subcooled discharge with resultant condensate back-up is desired.

Prior to selecting a steam trap for your application, review steam traps selection with its advantages versus disadvantages and additional steam trap types: Disc Steam Traps, Piston Steam Traps, Lever Steam Traps, Closed Float Steam Traps, Inverted Bucket Steam Traps, Open Bucket Steam Traps, Bimetallic Steam Traps, Bellows Steam Traps and Orifice Steam Traps.