Power plant auxiliaries and their relation to heat balance

In the larger central station steam plaids, efforts to increase the over-all economy and ease of operation were responsible for the use of motor-driven auxiliaries receiving their power from auxiliary turbogenerators, called house turbines, the exhaust from which is used for heating feed water. This scheme is reliable and economical. It is handicapped by the fact that over a considerable range of load some of the auxiliary power must be taken from the main bus or some of the energy generated must be fed to the main bus. This resulted in transfer motors generators, etc., which were additional complications. Still greater economy is possible by bleeding steam from the low-pressure stages of the main unit to heat the feed water. Considerable heat that would otherwise be rejected with the condensing water is reclaimed, thereby allowing of a smaller condenser than would otherwise be required, the performance of the main turbine also being improved due to somewhat relieving the congestion of steam in the low-pressure stages. To utilize fully the advantages of stage bleeding, the auxiliary power must be obtained from the main turbine. In order to insure an uninterrupted supply of auxiliary power to the essential drives it is suggested that an auxiliary generator be connected to and driven by the main turbine, thereby supplying the necessary reliability as long as the main turbine is available for load. Due to the advantages of variable speed drive for circulating pumps, boiler feed pumps, etc., and the necessity of having direct current for excitation, a direct-current generator may be used to advantage. The use of the direct-connected auxiliary generator with stage heating, gives a maximum of flexibility, is especially reliable in that it entirely eliminates all small turbine and gear troubles, and permits of the use of the unit scheme of grouping and supplying auxiliaries, with what appears to be a maximum of economy at no apparent increase in cost. Furthermore, the use of closed heaters with the fresh water storage located within the coiidenser hoi well seems to offer an economical and highly satisfactory solution for the de-aeration of the feed water and to eliminate the possibility of the water picking up further air after leaving the condenser.