Best practice no 22

Combustion occurs in the Morrison tube and the combustion gases travel through the tubes until they reach the chimney. These boilers are built with 2,3 and 4 steps. The gases pass through the tubes, giving up heat and decreasing their temperature and specific volume, so the passages decrease in total area in order to maintain adequate speeds for the best heat exchange with water and steam. Technologies such as Gas Recirculation (FGR) are currently available for smoke tube boilers. This design reduces NOx emission by recirculating a portion of relatively cold combustion gases within the combustion chamber, which reduces the combustion gas temperature. An efficient burner design must be provided in order to ensure complete combustion of fuels. The walls of the burner and the tubes can suffer from embrittlement due to high temperature if there is not a rapid transfer of heat between the steel and the water, so possible incrustation due to the salts present in the water should be avoided as much as possible, with a adequate treatment of the feed water, either by softening or demineralization as the case warrants. Tube boilers are non-condensing type, this means that there is a low limit on the temperature of the combustion gases and therefore a decrease in efficiency. If the temperature of the gases is allowed to drop too low, condensation occurs, this condensation would cause serious corrosion problems and failure of the tubes due to rupture. As a rule of thumb, the temperature of the chimney gases should be maintained around 100°F (55°C) with respect to the saturation temperature of the steam. Usually in standard designs 5 ft² of heating surface is required for each boiler horsepower (BHP). This type of boiler is designed up to a maximum of 1000 BHP. Efficiency increases with the number of steps, however, there is a limit; The addition of an extra step improves the average efficiency by 2 to 3%, however, this causes greater pressure losses, which must be compensated with an increase in fan power. Disadvantages of tube boilers: 1) High temperature differential between the chimney gases and the water/steam mixture in the upper part of the boiler. 2) Due to the large dimensions of the body, a large investment in insulation is required to avoid losses due to radiation and convection; in some places, freezing problems may occur when the boiler is stopped. 3) The large volume of water contained implies very long start-up times before steam can be generated. 4) Due to the large volume of water contained, a large amount of blowdown is required to eliminate sludge at the bottom. The transfer coefficient due to the combustion gases is a function of the speed with which they pass inside the tube, so maximum efficiency is obtained at high load ratings, dropping considerably at low loads. You must balance the relationship between the number of steps and the increase in power, avoiding lowering the temperature of the chimney gases too much.

WATER TUBE BOILER

Unlike smoke tube boilers, water tube boilers are designed with a minimum of two steel domes, one upper and one lower, joined by curved tubes, within which water is stored, in the dome. upper part the steam is separated. The combustion process takes place in the home where a part of the tubes form a wall to absorb the radiation coming from the combustion process. The combustion gases are taken to the chimney, causing them to be deflected touching the other tubes until maximum heat transfer is achieved.

Due to their design, they withstand high steam pressures and can even superheat the steam, which is why their use is widespread in power applications for the generation of electrical energy.

FOR-IM-024

REVISION DEL FORMATO: C

FECHA DE EFECTIVIDAD DEL FORMATO:10/03/2022