This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
137. All air vents or traps on steam-heating systems are thermostatic in principle, and are controlled by a difference in temperature between the steam and the air which is to be expelled from the heating apparatus.
Fig. 53 shows a simple form of air vent. The shank a is screwed into a radiator tube, and the nozzle d is connected to a suitable drip pipe. The valve c is a rod composed of some expansible material which is adjusted against the steam orifice by means of the screw b. When air instead of steam enters the orifice e, the rod c cools off and shortens slightly, thus opening the orifice and permitting the air to flow through. As soon as hot steam arrives, however, the rod expands, and again closes the vent.
The length of the expansible element which is exposed to the air or steam is very small, consequently the opening of the vent will be very slight, and the operation will be quite slow. There are many different forms of air vents on the market, but in principle they all resemble that shown in Fig. 53.