1873.] On the Condensation of Steam upon Cold Surfaces. 275 



I. " On the Condensation of a Mixture of Air and Steam upon 

 Cold Surfaces." By Osborne Reynolds, M.A., Fellow of 

 Queen's College, Cambridge, and Professor of Engineering in 

 the Owens College, Manchester. Communicated by Professor 

 Roscoe, F.R.S. Received March 22, 1873. 



1. The object of this investigation is to ascertain how far the presence 

 of a small quantity of air affects the power of a cold surface to condense 

 steam. A priori it seemed probable that it might retard condensation very 

 much ; for when pure steam comes up to a cold surface and is condensed, 

 it leaves an empty space which is immediately filled with fresh steam ; so 

 that the passage of the steam up to the cold surface is unobstructed, 

 and if the surface could carry on 2 the heat fast enough, then the rate of 

 condensation would be unlimited. If, however, the steam is mixed with 

 air, then, as the mixture comes into contact with the cold surface, the steam 

 will be condensed and the air will be left between the fresh steam and 

 the cold surface ; so that after condensation has commenced that surface 

 will be protected by a stratum of air, and fresh steam will have either 

 to displace this or pass through it before it in turn can be condensed. 



2. This question, besides its philosophical interest, has important 

 practical bearings on the steam-engine. 



First. If the quantity of air mixed with the steam affects the rate at 

 which it condenses, then the ratio which the pressure of air bears to the 

 pressure of steam in a condenser will materially affect its efficiency : this 

 is particularly important with reference to the surface-condenser. 



Second. If air prevents the condensation of steam, then by sending air 

 into the boiler of a high-pressure engine, the condensation at the surface 

 of the cylinder will be prevented, which, if allowed to occur, becomes 

 a source of great waste ; for when the steam comes into a cold cylinder 

 it condenses, heating the cylinder and leaving water, which will again be 

 evaporated as soon as the steam escapes ; and this, in evaporating, will 

 cool the cylinder. By preventing this, the mixing of air with the steam 

 would effect the same object as the steam-jacket, only in a more efficient 

 manner ; for the heat communicated to the steam in the cylinder from 

 the jacket is not nearly so effective as that which is communicated from 

 the boiler, in consequence of the steam in the cylinder being at a lower 

 temperature than that in the boiler. 



3. The experiments for this investigation were, by the kind permission 

 of Dr. Roscoe, carried out by Mr. Pasley, a student in the Chemical Labo- 

 ratory of the Owens College ; and I beg to tender him my best thanks. 



4. In making these experiments two objects were particularly kept 

 in view : — 



First. To ascertain if there is a great difference in the rate of conden- 

 sation of pure steam and a mixture of steam and air — to ascertain in fact 

 whether pure steam condenses at an unlimited speed. 



VOL. XXI. 2 A 



