Boiler superheaters are designed to produce steam from a dry or liquid saturated composition supplied to the energy system. In this area, its development is required with a high thermal index, which is created in the process of enthalpy and steam-power cycles.
general information
In industrial superheaters of boilers of low power at a steam temperature of up to 500 ° C, convective modifications are usually installed. At high thermal indicators, as a rule, combined versions are operated in which the devices of the beam and convection type are combined. The working surfaces of the device are screens located directly in the upper compartment of the combustion chamber.
The metal of the superheater pipes operates in critical temperature conditions, even if the limit of the produced steam does not exceed 450 degrees. In any case, the combustion parameter acts on the shell more than on the fluid moving inside the pipes. Below is a general diagram of the operation of the device.
Features
In boiler superheaters, the intensity of heating the metal walls directly depends on the uniformity of supply of a similar indicator to the coils in the transverse part, as well as on the difference in the average temperature of the combustion products, material and metal quality.
It should be noted that the evaporation and economizer panels of the units under consideration with increased heat transfer coefficients from the wall to the working fluid in the most adverse conditions do not become higher at temperatures above 60 ° C. Directly in a superheater of a steam boiler, even at 450 ° C this indicator is as close as possible to the limit oriented to carbon steel.
In addition, the rate of heat transfer from the walls to the steam is much lower than to boiling liquid. Aggregation between these resources can lead to overheating, scale deposits and damage to parts of the specified device.
Service
To reduce the thermal sweep, in the superheaters of the boilers, a dispersed supply of steam is made along the entire length of the common collector through the use of pipes of small diameter. This allows you to evenly distribute the flow of the mixture over each coil. Also, to solve this problem, the unit is divided into several parts, intermediate collectors are mounted and the steam supply circuit is adjusted, depending on the required values and external features.
A significant impact on the reliability of the functionality of the metal has a steam speed. An increase in this parameter in the coils helps to reduce the temperature on the pipe walls, while the hydraulic resistance of the apparatus increases. In industrial models, the speed limit varies from 20 to 25 meters per second. In this mode, the hydraulic resistance does not exceed 6% of the nominal pressure.
Convective boiler superheater
In such modifications, various schemes are used for the mutual movement of steam and combustion products. According to this principle, the device is divided into three types:
- Direct-flow action.
- Mixed option.
- Counterflow models.
In the first case, the interaction of combustion products and steam goes in one direction. The scheme helps to compensate for the temperature difference due to leveling the low rate of metal heating with a higher parameter of the working composition. This is relevant only in the absence of salts in saturated steam. Otherwise, they will be deposited on the coils, which will lead to a sharp increase in the temperature of the metal. It is worth noting that the average thermal difference in the countercurrent unit requires a larger heating surface, and this leads to an increase in the cost of the device.
Mixed and countercurrent circuitry
The principle of operation of the boiler superheater in mixed mode involves the movement of the working mixture and combustion products, both directly and in countercurrent. The process taking place in various combinations is considered the most optimal for ensuring the temperature difference between the steam and the metal walls, which increases the efficiency of the device.
The counterflow circuit is characterized in that here the movement of steam and exhaust gases is performed in the opposite direction. Therefore, the coils, which are affected by the products of combustion, meet already superheated steam. This is clearly not enough to cool them. In this case, the metal works in the most difficult temperature conditions. Compared with previous systems, in this scheme the thermal head is larger and the heating surface is smaller, which helps to reduce the size and price of the device.
Boiler superheater device
The equipment in question is made of seamless pipes (diameter is from 28 to 42 millimeters). These elements are bent according to the type of coils, the ends of which are connected to the working drum by means of flaring, and to the collectors by welding. The transverse step in each row is 2-3.5 d. The collectors are usually round in shape, made of alloy or carbon steel, depending on the nominal pressure and type of system.
Convective modifications are located in a horizontal channel between the lowering compartment and the furnace. At the same time, the depth of each package does not exceed 1.5 meters. Between the sets leave at least 0.5 m free space. This makes it easier to service and repair the unit. The rate of combustion products varies from 6 to 14 meters per second. A lower indicator will contribute to the drift of the heated surface with fly ash, and a larger parameter increases the likelihood of pipe wear due to friction of solid particles with metal walls.
There is a horizontal or vertical suspension of parallel activated coils, depending on the type of placement in the duct. They are usually placed according to the corridor principle, which facilitates cleaning from ash deposits. The horizontal arrangement of the elements ensures good condensate removal after stopping the device. However, this requires reinforced and complex suspensions to avoid sagging parts. Vertical modifications are easier to suspend, which simplifies installation and reliability, but complicates the drainage of condensate.
Application
The main purpose of the boiler superheater is to control the temperature of superheated steam with the provision of this indicator in a wide range, taking into account constantly changing thermal loads. Correction is carried out using surface coolers, injecting water into the steam, bypassing part of the mining past the unit. In addition, recycling of combustion products, a change in the aerodynamic, chemical or radiative structure of the torch are used. A surface cooler is a common set of a pair of packages of U-shaped pipes that pass liquid refrigerant inside. Outside, the parts are bathed in steam, which, after contact with them, is cooled.