Air Vents for Steam

In the article Removing Air from Steam Equipment, discussion mostly revolved around the density of air and steam, as well as how system configuration can influence the position of air vents. The following article will focus on balanced pressure air vents for steam systems.

How Hot Air is Removed from Steam

Balance pressure type air vents are usually recommended for removing air in steam systems. Bimetal type air vents also exist, however these are not recommended for removing hot air from steam systems because the valve opening temperature is more or less fixed. This is different from the valve on balanced pressure type air vents, which opens at a temperature that is a certain number of degrees lower than the saturation temperature of steam. Hot air is therefore discharged at all times regardless of fluctuations in steam pressure.

Balanced Pressure Type (X-element) Air Vents

Balanced Pressure Type (X-element) Air Vents Graph

Balance pressure air vents feature an element which usually contains an alcohol based mixture. The mixture has been carefully selected to boil at a specific temperature below the boiling point of water. When the temperature approaches the boiling point of water at any pressure, the mixture begins to boil which generates a vapor pressure within the element that causes it to expand, pushing the valve onto its seat and closing the air vent. This type of mechanism quickly responds to differences in temperature, virtually eliminating steam blowing.

X-element Mechanism

The X-element is a multi-diaphragm valve filled with a thermoliquid which opens and closes at a temperature below the boiling point of water, allowing the discharge or air or other non-condensable gases.

Removing hot air and other non-condensable gases in equipment is just as important during operation as at start-up. For example, gases can be generated during regular operation due to insufficient treatment of the boiler feed water which causes carbon dioxide or other non-condensable gases to be fed to the equipment during operation. Also, residual air inside equipment can be heated by the steam to high temperatures and interfere with heat transfer. If the air and/or non-condensable gases are not completely removed, the efficiency of the steam process can be severely impaired. Air venting should thus be ensured throughout start-up and during regular operation.

It is important to note that air vents are particularly important when equipment configuration causes air build-up or when the capacity of the air venting function of a steam trap is insufficient (i.e. usually during start-up). When selecting an air vent for steam, the balanced pressure type of air vents are recommended over the bimetal type, and should be fitted in collection areas where air will be pushed on start-up.