Liquid heaters and vaporizers – Steam treatment – Automatic heat regulation of superheater
Reexamination Certificate
2000-01-18
2001-11-06
Wilson, Gregory (Department: 3749)
Liquid heaters and vaporizers
Steam treatment
Automatic heat regulation of superheater
C122S487000, C122S438000, C122S451100, C122S466000
Reexamination Certificate
active
06311647
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to steam boiler systems, and more specifically, to methods and devices for controlling the temperature at the outlet of a steam superheater.
BACKGROUND OF THE INVENTION
In most steam boilers, the steam temperature is controlled at the boiler outlet by injecting water in the mostly subcooled state at one or more points in the superheater train. This water is usually removed before the economizer and evaporates after the injection point, so that the enthalpy and therefore the temperature of the superheated steam is lowered at the injection or mixing point of the respective heating surfaces. This also reduces the temperature at the outlet of the respective heater surface.
In known methods, the temperature reduction is limited by the water injection, and only such an amount of water can be injected that the steam remains sufficiently superheated after the mixing of the injection water and steam. The purpose is the prevention of temperature shocks that may be created by impermissibly large temperature drops, or caused by the impact of cold injection water droplets on the hot, steam-carrying pipes.
Known injection control loops are constructed in a cascading manner, whereby the subcontrol loop controls the inlet temperature before the respective superheater. The information of the inlet temperature is lost, however, as soon as a two-phase mixture at saturation temperature is present after mixing. The controlling then would have to be performed manually.
As a result, the possible drop in temperature through the injection of water in the designs known so far is critically limited, since it is not possible to inject an amount of water that would lower the steam temperature to the saturated steam temperature.
The known injection coolers furthermore have a very high constructive expenditure; each injection cooler between the heating surfaces shares the heating surface bundles and requires corresponding headers and connecting pipes.
In respect to energy, the past practice of injecting water between or after the superheater heating surfaces has the disadvantage that the steam temperature control on the one hand is coupled with a drop of the already achieved high steam temperature with, on the other hand, a water evaporation at a low temperature level. The result is an adverse effect on the efficiency of the superheater or boiler. Another disadvantage is, in respect to materials, the heating surface design for higher temperatures.
Of special significance is the problem of steam temperature control, e.g., in refuse incineration plants, i.e., in plants with increasing fouling of the heating surfaces in the combustion chamber. This results in a steady increase of the flue gas temperature before the superheater. In order to be able to maintain the steam temperature after the superheater, the injection water quantity must be increased. This often requires installation of several intermediate injection coolers.
SUMMARY OF THE INVENTION
It is an object of this invention to overcome the disadvantages described above. The invention is based on the objective of creating a method of the initially described type in which a respective superheater train without injection cooler can be constructed between the heating surface segments and/or at the end of the heating surfaces. Superheater and reheater should be integrated with the possibility of an at least partial operation as evaporators in the start-up process of, for example, a steam power plant.
A frontal part of the medium flowing through the superheater according to the invention works as an evaporator at a saturation temperature with a high internal heat transfer so that the metal temperature and also the degree of corrosion can be kept low. The flue gas temperature is also reduced in the area of the superheater outlet, thus increasing the efficiency of the boiler. The economizer is relieved. No intermediate headers and additional injection coolers are necessary in the superheater train.
According to the invention, this is achieved in that the injection water
m
EW
is injected into the steam at the inlet of the superheater. If the superheater is constructed of several heating surface bundles, the inlet into the superheater is in this case the inlet into the overall superheater train. A suitable temperature control is used to control the superheater outlet temperature. For this purpose, the steam parameters pressure PIST,A and temperature t
IST,A
are measured at the superheater outlet. From these parameters, the enthalpy of the steam h
IST,A
is determined and is compared with a desired enthalpy h
S,A
resulting from the desired value of the steam temperature t
S,A
the present pressure
PIST,A
. In addition to this, the steam mass flux
m
D
is determined at a suitable point. This means that an energy and mass balance at the mixing point can be used to calculate the inlet enthalpy h
E
into the superheater and can be sent to the enthalpy control in order to improve the control quality.
In a preferred embodiment, water is injected at the inlet into the superheater in such quantities that saturated steam conditions exist at the outlet of the superheater. In this case the entire superheater is operated as an evaporator. In another case, such a quantity of water is injected that wet steam conditions exist at the outlet of the superheater. The residual moisture is then separated by a separator.
In a device according to the invention for performing the method that comprises a steam generator including a superheater with superheater tubes and an inlet header, a mixing point for mixing in the injection water is arranged in the area of the inlet header of the superheater.
A steam superheater for performing the method is characterized by an injection water pipe comprising a control device, with said injection water pipe extending to the inlet area of the superheater.
A steam boiler for performing the method, where said steam boiler comprises an evaporator, a superheater, and a conduit extending from the evaporator to the superheater, is characterized by an injection water pipe comprising a control device that extends to the inlet area of the superheater.
A control for performing the method, which is used to control the position of the control device in the injection water pipe is characterized in that the enthalpy at the superheater outlet (h
IST,A
) is calculated from the parameters pressure (p
IST,A
) and temperature (t
IST,A
) at the superheater outlet and is compared with a desired enthalpy h
S,A
resulting from the desired temperature t
S,A
by the present pressure p
IST,A
.
A further embodiment of the control for improving the control quality calculates the inlet enthalpy (h
E
) into the superheater from the energy and mass balance at the mixing point and send it to the enthalpy control.
According to an especially preferred embodiment, a separator is arranged at the outlet of the superheater. If the steam at the outlet of the superheater contains residual moisture, this residual moisture can be removed by the separator, and saturated steam can be provided. The separator can be used to advantageously protect the pipes and steam turbine from erosion.
The concept of the invention will be described using the example of the superheater of a random boiler, but shall apply to all heating surfaces functioning in the superheated area, in particularly also to the reheater.
REFERENCES:
patent: 3942483 (1976-03-01), Laubli
patent: 4241701 (1980-12-01), Morse
patent: 4969084 (1990-11-01), Smith
patent: 5307766 (1994-05-01), Pearce
patent: 5605118 (1997-02-01), Sinn et al.
patent: 5924389 (1999-07-01), Palkes et al.
patent: 215 439 (1960-11-01), None
patent: 702 231 (1941-01-01), None
patent: 852 092 (1952-08-01), None
patent: 877 311 (1953-04-01), None
patent: 975 895 (1962-10-01), None
patent: 41 17 796 (1993-01-01), None
patent: 38 41 741 (1993-02-01), None
patent: 0 263 056 (1988-04-01), None
Herzog Rudolf
Liebig Erhard
Olia Hamid
Wilke Wolf-Stephan
Ziegler Georg
Alstom (Switzerland Ltd
Burns Doane Swecker & Mathis L.L.P.
Wilson Gregory
LandOfFree
Method and device for controlling the temperature at the... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and device for controlling the temperature at the..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and device for controlling the temperature at the... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2577200