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Commercial hydrogen has the degree of purity required for cooling purposes. It is inert, non-explosive in concentrations of <4 percent and >70 percent, and does not support combustion in a purity range of 95 percent to 99 percent - normally maintained in hydrogen generators.
Because hydrogen coolant is more effective at high density, generators generally are designed to operate at 30 psig. However, generators with hydrogen gas pressures up to 75 psig are commonly in use today. The increased ratings have resulted from larger rotor sizes, improvements in forging materials, and higher conductor current capacity - resulting from improved ventilation schemes.
Although some heat is dissipated from the generator frame by radiation, it represents only a relatively small portion of the heat moved by generator coolers mounted in the frame. Liquid coolers installed outside the generator frame can also provide some relief from this heat source.
Additional heat can be removed from frames by operating generators at high hydrogen gas purity, as well as by installing generators designed to operate at higher ambient gas temperatures.
Bearings & Lubrication
Another major generator heat loss results from the system's roller bearings, for moving parts, and the proper lubrication of the bearings. Although roller bearings are usually grease lubricated, they require very little lubricant. However, specifications for power plant applications usually require regreasable bearings, since too much grease on roller bearings produces fluid friction and may cause the bearings to fail from overheating.
An alternative measure is to use anti friction bearings. Although they are sensitive to dirt, corrosion, vibration, and poor fit, when anti friction bearings are properly maintained they require almost no additional maintenance and have a very long life. They are best used in slow-speed applications, where sliding type bearings are not suitable. But anti friction bearings are not suitable for very high-speed equipment because of the difficulty to lubricate them.
In addition, at high speeds, fluid friction may cause excessive heating problems. Horizontal sleeve bearings are lubricated with oil supplied by small oil rings, having larger diameters than the shaft, that ride on the shaft of the rotor with the lower ring part immersed in the lubricating oil. The turning rotor causes the oil rings to ride up over the shaft and provide a continuous oil supply. The oil carries heat away from the shaft and returns it to the reservoir. Heat exchangers may also be required in the oil reservoir. Oil can also be provided by lubrication pumps that force oil to the sliding surfaces. In large rotating devices, such as generators (and turbines), a machine may have several forced lubrication pumps.
When determining the means of removing heat from turbine-generators, plant operators should consider not only the losses that must be dissipated when the machine is operated under steady-state conditions at rated load, but also the thermal response of machine components under transient load conditions that may exist during operation at other than rated voltage, frequency, and machine pressure.
Resources: Black & Veatch, Overland Park, KS; ITT Industries, Santa Ana, Calif.
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