Sometimes, the most powerful forces are the ones we can’t see. Take air, for example. Colorless and completely invisible, it exerts constant pressure on everything it touches. It can be as gentle as a spring breeze or as mighty as a hurricane.
In addition, air is widely used as an insulator because of its low conductivity. For example, double-pane glass used in today’s windows is simply two layers of glass with an insulating layer of air sandwiched between them.
However, air isn’t too helpful in steam systems. Any place air occupies, steam cannot. Air is at a much lower temperature than steam — thus, when air is mixed with steam and flows along with it, pockets of air will remain at the heat exchange surfaces where the steam condenses. Gradually, a thin layer builds up to form an insulating blanket, hindering heat transfer. A film of air on the steam side of a heat transfer surface is resistive to the flow of heat, reducing the rate of heat transfer.
The removal of air from a steam system is essential to its success, but can present a challenge. Air is present within steam pipes and steam equipment at startup. Even if the system is filled with pure steam when it’s in use, the condensing steam causes a vacuum and draws air into the pipes at shutdown.
The most efficient means of air venting is with an automatic device. Air mixed with steam lowers the mix temperature. This enables a thermostatic device (based on either the balanced pressure or bimetallic traps) to vent the steam system. An air vent fitted on the steam space of a vessel or at the end of a steam main will open when air is present and close in the presence of steam.
For maximum removal of air, the discharge should be as free as possible. A pipe is often fitted to carry the discharge to a safe location — preferably not a condensate return line, which could restrict the release of air and may also encourage corrosion.
During startup, air will be pushed toward the outlet of the coil where a float and thermostatic trap should be fitted, incorporating a balanced pressure air venting trap. Air typically accumulates near the top of a steam coil under normal operations, so an additional air vent should be fitted to the top of the coil.
Most modern day steam coils, such as Colmac, can be purchased with a connection for both an air vent and vacuum breaker.
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