Best practice no 22

MONOTUBULAR GENERATOR

This steam generator, also called monotubular, uses a continuous coil, fed by a positive displacement pump. The space between the coil tubes is graduated to optimize the characteristics of combustion gas velocity, mixing and heat transfer.

The water flow through the coil in the opposite direction to the combustion gases, behaving as a cross-flow heat exchanger. The outlet of the heating unit is connected to an external container where the steam is separated. With this type of separator, you can obtain up to a steam quality of 99.5%.

Compared to a tube boiler, this steam generator is more compact, lighter in weight and therefore less susceptible to losses due to radiation and convection. Although the tubes in the steam generator are relatively smaller, the velocity of water and steam is higher. The high flow speed allows the suspended solids to be dragged to the vapor separator, where they are separated by gravity, with a high concentration of salts. Because of this only a small amount needs to be purged to keep suspended solids under control. Because the heat transfer process is carried out through a forced convection system, the amount of transfer surface required by Boiler Horse Power is 1.25 ft2. Steam separation is done in an independent container, so the entire surface of the coil is used to transfer heat. For all this, the volume and weight of a steam generator is reduced by up to 75% compared to a smoke tube boiler.

During startup, shutdown, or change in steam demand, response time is affected by thermal mass. Because the thermal mass of a smoke tube boiler is greater, its response is less.

The energy contained in the thermal mass is lost during a shutdown and can be recovered when the unit starts again, and any amount of steam can be called upon. This loss can be significant if the unit is not booted continuously. Due to the spiral design of the heating unit and the small amount of water stored, it is possible to quickly absorb the thermal stresses caused by the heating of the tubes, generating steam in just 5 minutes after starting completely cold.

LOSSES BY RADIATION AND CONVECTION:

A hot boiler, due to its high temperature, radiates heat to the outside because it is at a lower temperature. This heat loss increases when we have air circulating around it at high speed. This loss is a direct function of the outside temperature and the exposed surface. The best way to reduce this loss is by insulating the outside and avoiding having the boiler outdoors. The way to measure this loss is by taking the outside temperature of the boiler body and measuring the exposed surface. These losses are constant and do not depend on the steam generation capacity.

LOSSES DUE TO WASTE OF HOT GASES IN THE CHIMNEY:

Fuels require oxygen to achieve the combustion process, this oxygen is obtained from ambient air, unfortunately air only contains 21% oxygen, the remaining 79% is nitrogen. Nitrogen is a gas that does not intervene in the combustion process, so it is only heated and discharged into the atmosphere at high temperature through the chimney. Depending on the fuel, we use 13 to 20 kg. of air per kg. of fuel, which implies that 75% of the gases escaping through the chimney is hot nitrogen. The combustion process is not a natural process, so to guarantee that all the fuel is burned, it is necessary to handle excess air, this depends on the efficiency of the burner. The way to calculate this loss is by measuring the temperature of the stack gases and analyzing the amount of oxygen and carbon monoxide in the stack gases. These parameters are a direct relationship of the losses due to chimney gases.

FOR-IM-024

REVISION DEL FORMATO: C

FECHA DE EFECTIVIDAD DEL FORMATO:10/03/2022