Abstract
This paper gives an exposition of the various items which go to make up the leaving velocity and exhaust loss of a steam turbine. The importance of this loss and the rapidity with which it increases at high loads cause it to be a determining influence in fixing the economic rating of a machine. The several elements necessary for an analysis are each evaluated in a fairly direct, although sometimes approximate, manner. More detailed and precise estimates might be made but are beyond the intent of this paper.
The loss in question occurs in the exhaust hood between the last wheel exit and the exhaust flange to the condenser. It is made up both of kinetic energy loss and of pressure loss through the hood and each effect varies with load and with location around the wheel annulus. Moisture is allowed for; supersaturation neglected.
The total loss may be expressed in Btu per pound flow to condenser or as a per cent of adiabatic heat drop or preferably as a per cent of total energy theoretically available for conversion to switchboard power.
With a particular exhaust operating at fixed steam conditions, the leaving velocity and exhaust loss increases roughly as the square of the load (parabolic rule).
With a particular exhaust passing a fixed flow, increasing the total available energy in the higher stages of the turbine by improved steam conditions correspondingly reduces the percentage loss in the exhaust (hyperbolic rule).
With a fixed percentage loss in a particular exhaust the power may be increased by improved steam conditions as the 3/2-power of the total available energy by increasing the flow to the condenser.
Base-load operation justifies more liberal exhaust areas than peak-load service but in the ultimate the heat-rate-load curve is the characteristic which is of most importance to the operator and second only to the reliability of performance.
