Steam traps are installed in locations where condensate is formed and collects, such as all low points, below heat exchangers and coils, at risers and expansion loops, at intervals along horizontal pipe runs, ahead of valves, at ends of mains, before pumps, etc. The purpose of a steam trap is to separate the steam (vapor) side of the heating system from the condensate (water) side.

A steam trap collects condensate and allows the trapped condensate to be drained from the system, while still limiting the escape of steam. The condensate may be returned to the boiler by a gravity return system, a mechanical return system using a vacuum pump (closed system), or condensate pump (open system).

Condensate must be trapped and then drained immediately from the system. If it isn’t, the operating efficiency of the system is reduced because the heat transfer rate is slowed. In addition, the build up of condensate can cause physical damage to the system from “water hammer.”

Water hammer can occur in a steam distribution system when the condensate is allowed to accumulate on the bottom of horizontal pipes and is pushed along by the velocity of the steam passing over it. As the velocity increases, the condensate can form into a non-compressible slug of water.

 If this slug of water is suddenly stopped by a pipe fitting, bend, or valve the result is a shock wave which can, and often does, cause damage to the system (such as blowing strainers and valves apart).

Steam traps also allow air to escape. This prevents the build up of air in the system which reduces the heat transfer efficiency of the system and may cause air binding in the heat exchanger.

In a steam heating system, water enters a heat conversion unit (a heat exchanger, the boiler, etc.) and is changed into steam. When the water is boiled, some air in the water is also released into the steam and is moved along with the steam to the heat exchanger.

As the heat is released at the heat exchangers (and through pipe radiation losses), the steam is changed into condensate water. Some of the air in the piping system is absorbed back into the water. However, much of the air collects in the heat exchanger and must be vented.

Steam traps are classified as thermostatic, mechanical or thermodynamic. Thermostatic traps sense the temperature difference between the steam and the condensate using an expanding bellows or bimetal strip to operate a valve mechanism.

Mechanical traps use a float to determine the condensate level in the trap and then operate a discharge valve to release the accumulated condensate. Some thermodynamic traps use a disc which closes to the high velocity steam and opens to the low velocity condensate. Other types will use an orifice which flashes the hot condensate into steam as the condensate passes through the orifice.

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1 comment:

Unknown said...

Thermostatic traps sense the temperature difference between the steam and the condensate using an expanding bellows or bimetal strip to operate a valve mechanism.
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