Boiler types watertube boilers

High pressure 2-50 MW Yes All types (mainly classes G and H) 6-20 20 3 1 to 12 1 Mainly wide-angle sprays in WT boilers, low excess operation Large shell boiler and watertube boiler process apphcations... 

The overall efficiency of the condensing thermal cycle, as discussed, is dictated primarily by the steam conditions used. There are some small industrial stations with outputs up to 2 MW using shell-type boilers for the generation of steam. Here the steam conditions are limited to approximately 17 bara and 250°C. For larger installations these conditions will rise sharply when watertube boilers become attractive and more common steam conditions are of above 60 bara and 540°C. 

There are two basic types of watertube boilers assisted and natural circulation. Assisted circulation might apply where heat is from convection rather than a radiation source such as a waste heat application. Natural circulation is more suited where radiant heat and high gas temperatures are present. 

Starting with the boiler(s), these will be set out giving due consideration to space between boilers and other items of plant in order to give adequate access to all equipment, valves and controls for operation and maintenance. For small- to medium-sized boilers 1 meter may be considered a reasonable space between items of plant where access is required. With large boilers (including watertube), this may be increased up to 3 m. The width of firing aisle will be dependent upon the size and type of boiler. 

Table 23.5 shows the recommended water characteristics for shell boilers and Table 23.6 the water quality guidelines for industrial watertube boilers. Due to the wide parameters encountered in the quality of feedwater, it is not possible to be specific and to define which treatment suits a particular type and size of boiler. The quality of make-up and percentage of condense returns in a system will both have to be taken into consideration. 

Fluidized beds 1-50 All types of coal and waste sohd fuels 3.0 0.3-1.0m 2 1 and then by bed sectoring Removal and clean-up from bed Special vertical shell boilers and watertubes... 

Roller Water The steam purity limits define boiler-water limits because the steam cannot be purified once it leaves the boiler. For a once-through boiler, the boiler water must have the same specifications as the steam. A recirculating boiler is a still, and there can be considerable purification of the steam as it boils and is separated from the water in the steam dmm. The process of separation is not perfect, however, and some water is entrained in the steam. This water, called mechanical carryover, contains impurities in the same proportions as the boiler water, and its contribution to steam impurity is in those proportions. Typical mechanical carryover is less than 0.25% and often less than 0.1%, but operating conditions in the boiler can affect the mechanical carryover. In addition to mechanical carryover, chemicals can be carried into the steam because of solubility. This is called vaporous carryover. Total carryover is the sum of mechanical and vaporous carryover. The boiler-water specification must be such that the total carryover conforms to the steam purity requirements. For salts, such as sodium phosphate and sodium chloride, vaporous carryover is not a significant problem below approximately 15 MPa (2175 psia). As boiler pressures approach the critical point, vaporous carryover increases rapidly. Above 15 MPa (150 bar), boiler solids concentrations must be carefully controlled to minimize vaporous carryover. Most boilers operating over 18 MPa (180 bar) use all volatile treatment to prevent deposition of salts in turbines. Boiler-water limits for utility boiler are Us ted in Table 2. Recommendations from American Boiler Manufacturers Association (ABMA) for boiler-water limits for drum-type boilers and associated steam purity for watertube boilers are listed in Table 3. 

Characteristically boiler types are generally classified as either firetube or watertube. 

In the above example, a relatively complex steam generator of the watertube type has been adopted. Where lower-quality steam for process or fuel heating is required, a simpler shell (or firetube) design may be appropriate. In some cases, supplementary firing may be provided for the boiler, so further increasing plant complexity and with it the need for enhanced control and maintenance requirements. 

One manufacturer uses single-drum, watertube type waste heat boilers on incineration systems. Watertube bodets are also used by other manufacturers in installations where high steam pressures and flow rates are required. Another manufacturer offers heat recovery systems with water wall or radiant sections in the primary chamber. These water wall sections, which are usually installed in series with a convective type waste heat boiler, can increase overall heat recovery efficiencies by as much as 10 to 15%. 

This is usually referred to as the boiler or convection section of the unit. Closely spaced tubes are arranged to allow passing of the products of combustion around the tubes or through the tubes, depending on the type of unit. Most of the steam is generated in the boiler portion of the unit. In watertube units, if additional steam temperature is required by the process, the steam is then routed to a superheater. 

---