Air vent on steam exchangers: How?

[rollingstein]

Often on high points of steam heated Shell&Tube heat exchangers etc. I've seen a vent specified to allow air and other non condensables to vent.

e.g. this sketch http://www.spiraxsarco.com/images/resources/steam-engineering-tutorials/14/6/fig_14_6_1.gif

What's this device (I mean the one labelled "Air vent") in practice? It doesn't seem like a regular steam trap. Or is it? Or just a weep hole? But that'd constantly bleed steam.

In general, how is venting of steam enclosures achieved.

 

[Baluncore]

http://www.plastomatic.com/air-gas-differences.html

 

[rollingstein]

Differences between a Degassing Valves and Air Release Valves

Ah! That must be what I need.

Thanks!