Liquid heaters and vaporizers – Water tube – Having forced circulation system
Patent
1999-06-17
2000-11-28
Ferensic, Denise L.
Liquid heaters and vaporizers
Water tube
Having forced circulation system
1224065, 122468, 60653, F22D 700
Patent
active
061520853
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of operating a forced-circulation boiler, especially for a steam turbine, said boiler comprising at least a first heat exchanger, the inlet of which is connected to a water feed line and the outlet of which is connected, via a regulated valve, either to the inlet of a second heat exchanger, the outlet of which is connected to the steam turbine, or directly to the steam turbine. The invention also relates to a boiler for implementing this method.
The invention is aimed more particularly, without being limited thereby, at boilers supplying steam turbines used in thermal power stations for generating electricity. This is because such power stations include a boiler producing pressurized steam which actuates a steam turbine which drives an electricity generator.
The boiler may be heated by a burner which burns fossil fuel or a fuel coming from industry. The boiler may also be a waste-heat boiler used in a so-called combined-cycle thermal power station. In this type of power station, a fuel, for example natural gas or fuel oil, is burnt in a gas turbine which drives an electricity generator. The exhaust gases from this gas turbine, in large volume and rich in thermal energy, are recovered in a so-called waste-heat boiler in order to produce pressurized steam which, via a steam turbine drives an electricity generator.
The pressurized steam produced in the boiler, instead of actuating a turbine, may optionally be used for other purposes.
These boilers always include heat exchangers operating as an evaporator (in the case of water) or as a superheater (in the case of steam), these being placed horizontally or vertically in a stream of hot gases. Several types of boilers may be distinguished depending on their type of heating, their arrangement, their operating principle, etc.
In a so-called natural-circulation boiler, the water is gradually converted into steam in an evaporator where the water and the water/steam mixture circulate by the difference in density, one with respect to the other. The evaporator is followed by a superheater in which the steam produced in the evaporator is heated to the desired temperature. Given that the operating principle is based on the difference in density between water and steam at a given temperature and a given pressure, these boilers cannot operate when this difference becomes too small, i.e. when the pressure increased. This operating principle can only operate at pressures below 150 to 160 bar.
Assisted-circulation boilers also include several exchangers, but here the water and the steam flow through the evaporator due to the effect of an external force, for example that of a pump. Assisted-circulation boilers may operate at higher pressures than natural-circulation boilers but when the pressure comes too close to the critical pressure, which is a 221.2 bar, it is no longer possible to separate the water and steam effectively, in order to allow normal operation of the plant, so that the principle of assisted circulation is limited to pressures less than approximately 180 bar.
It should in fact be recalled that both natural-circulation and assisted-circulation boilers include, between the evaporator and the superheater, a separator or drum necessary for separating the steam from the water, since the superheater and, above all, the turbine operate only using steam. In this separator, the water is separated by gravity from the steam and sent to the evaporator where it therefore makes several passes.
Although both these types of boilers are limited from the pressure standpoint, it is, on the other hand, well known that the efficiency of a steam turbine is better the higher the steam pressure. This is why most conventional thermal power stations use a so-called forced-circulation boiler, more often termed a "once-through boiler" which, in fact, better describes this type of boiler given that the water is heated in it, converted into steam and finally superheated during one pass through the boiler. In this case, there is no longer any precis
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Patent Abstracts of Japan, vol. 15, No. 505 (M-1194), Dec. 20, 1991.
Dethier Alfred
Grandjean Pierre
Cockerill Mechanical Industries S.A.
Ferensic Denise L.
Wilson Gregory A.
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