The superheater is a device to increase the efficiency of a steam locomotive. In locomotives without a superheater (so-called saturated locomotives), the temperature of the steam entering the cylinders directly from the boiler is just at the boiling point for the given pressure (380°F or 195°C at 200psi or 14atm., for example). As a result, cooling in the cylinders causes condensation of part of the steam before it has had a chance to perform useful work.
The superheater functions by returning evaporated steam back in to the boiler for additional heating (hundreds of degrees) before it is led to the cylinders. Superheated locomotives have boiler firetubes of two diameters. These are visible in the bottom right hand portion of the illustration below, depicting a partially installed superheater as viewed looking back at the tube bank from inside the smokebox, just below the stack. The large tubes or flues are used to house the superheater elements, shown in the left central part of the picture. Saturated steam passes down into the elements from one side, and returns up the other side as superheated steam. The total surface area of the elements within the flues is the superheater surface.
© 2005 J.D.H. Smith