This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
In this system the suction is connected with the air valves instead of the returns and the vacuum is produced by means of a steam ejector instead of a pump. The returns are carried back to a receiving tank and pumped back to the boiler in the usual manner. The ejector in this case is called the "exhauster."
A A are the returns from the air valves and connect with the exhausters as shown. Live steam is admitted in small quantities through the valves B B and the mixture of air and steam is discharged outboard through the pipe C. D D are gages showing the pressure in the system and E E are check valves. The advantage of this system depends principally upon the quick removal of air from the various radiators and pipes which constitutes the principal obstruction to circulation; the inductive action in many cases is sufficient to cause the system to operate somewhat below atmospheric pressure.
Where exhaust steam is used for heating, the radiators should be somewhat increased in size owing to the lower temperature of the steam. It is common practice to add from 20 to 30 per cent to the sizes required for low pressure live steam.
In a system of forced circulation by means of a fan or blower the action is positive and practically constant under all usual conditions of outside temperature and wind action. This gives it a decided advantage over natural or gravity methods which are affected to a greater or less degree by changes in wind pressure, and makes it especially adapted to the ventilation and warming of large buildings such as shops, factories, schools, churches, halls, theatres, etc., where large and definite air quantities are required.