1. Field of the Invention
This invention relates generally to shaft seals that are used in hydrogen inner-cooled turbine-generators, and more particularly to methods and apparatus for cooling the shaft seals in such turbine-generators.
2. Statement of the Prior Art
Conductor cooling is a conventional process used in very large turbine-generator systems for dissipating the armature and field coil losses of the turbine-generators to cooling media within their coil insulation wall. The turbine-generators using such conductor cooling are also referred to variously as "inner-cooled," "supercharged," or "direct-cooled", and the cooling medium that is most often used in such turbine-generators is hydrogen.
Hydrogen inner-cooled turbine-generators typically operate in a pressurized hydrogen atmosphere that provides cooling for all of the turbine-generator except, in some instances, its armature coils. Such hydrogen inner-cooled turbine-generators are often operated at 60 lbf/in.sup.2 (i.e., about 4219 grams per square centimeter) or more in order to increase the mass flow of the hydrogen, and to reduce its temperature rise.
In order to minimize leakage of pressurized hydrogen cooling medium from an operational turbine-generator, shaft seals are typically used in hydrogen inner-cooled turbine-generators for maintaining an oil film under pressure in a small clearance between the rotating shaft of the turbine-generator and a stationary member surrounding the shaft at both ends of the turbine-generator. The construction of such shaft seals may be similar to a journal bearing with a cylindrical oil film or similar to a spring-loaded thrust bearing with the oil film in a plane at right angles to the shaft axis. In either case, the oil film is maintained by an oil supply pressure that is higher than the hydrogen pressure.
Oils used in such shaft seals can absorb about 10% by volume of either hydrogen or air. It is important that the flow of oil in those shaft seals toward their hydrogen side be minimized in order to reduce both the amount of air that is carried into the hydrogen inner-cooled turbine-generator and the amount of hydrogen that is carried out. Moreover, it is important to minimize temperature differences between the "hydrogen side" of the shaft seal and its "air side" so that differential thermal expansion of the seal ring which comprises the stationary member can be minimized.
Prior art shaft seals have typically employed separate supplies of oil for their hydrogen side and their air side, each such oil supply including a heat exchanger operated in one of two general fashions. One method selects a cooling media (e.g., water), and carefully sizes the heat exchanger of each oil supply to provide for open loop control of seal oil temperatures at the outlets on either side of the shaft seals. Any problems with heat exchanger fouling, decreased oil flow or decreased water flow when experienced with this method requires recognition and subsequent correction by an operator.
Another method utilizing separate heat exchangers for the hydrogen side and the air side of prior art shaft seals provides automated control valves to sense the temperature difference between the hydrogen side oil supply and the air side oil supply, and operates the particular heat exchanger in the oil supply having the higher temperature. Not only is this other method complicated in its design by virtue of the additional components that are required (e.g., two heat exchangers and many automated control valves), but it is also difficult in its implementation due to the necessity for precise, reliable components to accomplish such control over a very narrow temperature range.