Membrane Type Pin Type Steel Water Wall Panels For CFB With Natural Circulation

Membrane Type Pin Type Steel Water Wall Panels For CFB With Natural Circulation

General Description

The full name is water wall tube, old name is water-cooled wall,also called riser. Usually laid vertically on the inner wall of the furnace, they are used to absorb the radiant heat by the flame and high temperature flue gas.They are the main form of heating surfaces for the various modern boilers, also the basic parts of water circulation circuit. For they have the functions of cooling and protecting the furnace wall, so called water wall panels.

When the water wall was initially designed, it was not intended to be heated, but to cool the furnace so that it would not be damaged by high temperature.Afterwards, because of its good heat exchange function, gradually replacing the steam drum to be the main heating parts for boilers. They are the evaporative heating surface consisting of many parallel tubes laid on the inner wall of boiler hearth. Their functions are to absorb the radiant heat by high temperature fire or gas flue in the furnace, and produce steam or hot water inside the tubes, and also reduce the furnace wall temperature so as to protect it. In large capacity boilers, the flame temperature in the furnace is very high and the intensity of heat radiation is very high. 40-50% even more of heat is absorbed by the water wall.Except small capacity boilers, the main evaporative heating surface for the present water tube boilers are the water wall.

Classification

If according to the structure type, there are three main types: bare tube type, membrane type, and prick tube type.Bare tube type are composed of the whole row of seamless steel tubes with the simplest structure. Membrane water wall is a kind of joint welding of many rolled finned tubes of water wall to form a sealed combined heating surface. It can not only improve the air tightness of the furnace, reduce air leakage, but also protect the furnace wall, reduce the weight of the furnace wall and simplify the structure.The water wall of bayonet pipe is a kind of pin which is 20-25mm long and 6-12mm in diameter welded on the water wall pipe, and then coated with chrome ore sand refractory plastic to reduce the heat absorption at the water wall part and increase the temperature of combustion zone. It is suitable for the occasion where the fuel ignition is particularly difficult or some liquid slag discharge furnaces. According to the layout, there are two types of water-cooling wall; wall-attached type and partition type(i.e. double sided heating).The former is mostly embedded in the inner wall of the furnace wall, which absorbs heat on one side;the later is mostly installed in the middle of the large-scale boiler hearth, which divides the whole furnace into two separate parts and is heated on both sides.

Membrane Water Wall

Membrane water wall is an air-tight structure consisting of pipes and fins.The advantages are less air leakage in furnace; can use pipe-laying wall, less load on steel structure and lower cost of boiler; easy to use suspension structure and improve thermal expansion system, suitable for large-scale mechanized production, high assembly rate, low heat storage capacity and shorten star-up and shutdown time. The disadvantage is that the manufacturing process is complex. Membrane water wall is often used in large capacity, high temperature and high pressure boilers.

According to the different manufacturing process, the membrane water wall can be divided into three types(see Fig. 1):1.rolled fin tubes are welded in rows, with less welding workload, but the fin tubes are expensive. 2.bare tube and fin(flat steel) are welded in rows, which has low material cost and can easily meet the requirements of various pitches, but flat steel requires high precision and finishing equipment.3.Bare tube fusion welding,the fins between tubes are formed by filling the welding wire, mostly used for the small pitch between the tubes. The pitch between tubes of membrane water wall is related to factors such as boiler pressure and furnace heat load, usually s/d is 1.2-1.5.

Material and pipe diameter for medium, high and ultra-high pressure boilers, under normal water circulation conditions, pipe wall can be well cooled, carbon steel pipe can be selected. For sub critical and super critical pressure boilers, doe to the deterioration of heat transfer and the high medium temperature(especially when the super critical parameters are reached), it is often necessary to use internal threaded tubes(multi-head spiral grooves on the inner wall of the tube, the typical structure shown in Fig. 2) or Cr-Mo alloy steel tubes. For membrane water wall, the max. Temperature and stress of fins should also be checked. The outer diameter of water wall tubes of natural circulation boiler is generally 42-76mm, the sub-critical pressure controlled circulation boilers are 42-51mm, and the once-through boilers are 22-42mm.

Pin type water wall

It is also called bayonet water wall. It is required to weld a certain length of round steel on the surface of the bare tube, and use pins to lay refractory coating firmly on the water wall to form a fire protection belt, a slag pool and a slag melting section for liquid slag discharging furnace as well as a cyclone drum of the cyclone furnace.The water wall of inner threaded pipe is a pipe with single or multiple spiral grooves on the inner wall of the pipe. Internal threaded tubes are used in the high heat load zone of once-through boilers,sub critical parameters and forced circulation boilers or in the water wall sections with high vaporization rate. When the working fluid flows in the inner threaded tube, strong disturbance occurs, which causes the water pressure in the steam water mixture to the tube wall and forces the bubbles away from the wall to be carried away by the water, thus destroying the formation of the wapor film, preventing the boiling heat transfer from deteriorating, and lowering the temperature of the tube wall of the water wall panel.

Double sided exposure water wall refers to the water wall arranged along the furnace height in the furnace space, which can absorb radiation heat on both sides. It is mostly used in large capacity boilers. Carbon steel is commonly used for water wall materials. Some of the boilers are made of alloy steel when the pressure is higher than 14 MPa. Tube outer diameter: natural circulation boilers generally use 51-83mm; multiple forced circulation boilers and once-through boilers usually use 22-60mm. Unlike natural circulation boilers, once-through boiler’s water wall are not necessarily vertical, they can also be horizontally surrounded or other forms. When the parameters are high, especially in once-through boilers, in order to prevent heat transfer deterioration in the high heat load zone of the furnace, internal threaded tubes or spoilers are often installed in the tubes.

Parameter

Company Introduction

Established in 1985,Zhangjiagang Hua Dong Boiler Co.,Ltd.(trading as HD Boiler),produces power plant; industrial; waste heat; waste incineration; biomass; organic heat carrier and electric steam boilers. Coupled with a comprehensive range of heating surfaces, HD Boiler brings to the market a unique service including boiler design , fabrication, installation, maintenance and replacement of boiler pressure parts.