ASME Standard Alloy Steel Superheater Coil For Coal Power Plants

ASME Standard Carbon / Alloy Steel Boiler Superheater Coil For Ultra-Supercritical Coal Power Plants

Superheater Coil Introduction

The main advantages of using a superheater are reduced fuel and water consumption but there is a price to pay in increased maintenance costs. In most cases the benefits outweighed the costs and superheaters were widely used.

A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, steam engines, and in processes such as steam reforming. There are three types of superheaters: radiant, convection, and separately fired. A superheater can vary in size from a few tens of feet to several hundred feet (a few metres to some hundred metres).

Types

1. A radiant superheater is placed directly in radiant zone of the combustion chamber near the water wall so as to absorb heat by radiation.
2. A convection superheater is located in the convective zone of the furnace usually ahead of economizer (in the path of the hot flue gases). These are also called primary superheaters.
3. A separately fired superheater is a superheater that is placed outside the main boiler, which has its own separate combustion system. This superheater design incorporates additional burners in the area of superheater pipes. This type of superheater is rarely if ever used, because of poor efficiency and steam quality that is not better than other superheater types.

Superheaters and reheaters are specially designed to increase the temperature of the saturated steam and to help control the steam outlet temperature. They are simple single-phase heat exchangers with steam flowing inside and the flue gas passing outside, generally in the cross flow. Superheaters and reheaters are often divided into multiple sections to help control steam temperature and optimize heat recovery, and their heating surfaces can be arranged either horizontally or vertically. The physical design and location of the surfaces is dependent on the desired outlet temperature, heat absorption, fuel ash characteristics and cleaning equipment. The main components are usually manufactured from steel alloys, as both the superheater and reheaters are operated at high temperature.

One of the main differences between the superheater and reheater is the steam pressure. The outlet pressure of the superheater in a subcritical drum boiler, for example, is for example at 18.6 MPa, while the outlet pressure of the reheater is only 4.0 MPa. When a supercritical boiler is operated under normal load, the heating surfaces of the reheater and superheater have to increase in order to meet the heat requirement for superheating, which is more than 50% of the total steam heat. The heat absorbed by the superheater is principally in the form of radiation when the temperature of the superheated steam is controlled by the ratio of furnace water flow to fuel input. When the water-cooled wall works under supercritical pressure, however, the temperature of the working medium in the pipes does not increase with heat absorption and there is no clear thermodynamic state boundary. If the temperature of the outlet flue gas is controlled by the absorption of heat by the water-cooled wall, design requirements clearly cannot be met.
To solve the problem of too small a heating surface, the superheater is usually placed above or close to the furnace, creating a superheater system with radiation and convection heat transfers in the supercritical boiler. The heat transfer characteristics of the radiant superheater allow an improvement in the rate of utilization of the radiant heat.
At low pressure, the steam has poor cooling ability: to address this, the reheater is usually positioned after the superheater where the temperature of the flue gas is relatively low and principally relies on convection heat transfer. The temperature is gradually reduced from the outlet of the furnace to the tail of the boiler, while the volume of gas is reduced with no air leakage. In order to improve the convective transfer in the convective passes, the superheater and the reheater are placed close to the economizer in the flue gas path of relatively low temperatures.

Superheater Coil