Boiler Basics

Boiler Basics

Design and operation

A boiler is an enclosed vessel that provides a means for combustion heat to be transferred into water until it becomes heated water or steam. The hot water or steam under pressure is then usable for transferring the heat for the steam requirements of process industries or for power generation.

Combustion boilers are designed to use the chemical energy in fuel to raise the energy content of water so that it can be used for heating and power applications. Many fossil and non-fossil fuels are fired in boilers, but the most common types of fuel include coal, oil and natural gas. During the combustion process, oxygen reacts with carbon, hydrogen and other elements in the fuel to produce a flame and hot combustion gases. As these gases are drawn through the boiler, they cool as heat is transferred to water. Eventually the gases flow through a stack and into the atmosphere. As long as fuel and air are both available to continue the combustion process, heat will be generated.

Boilers are manufactured in many different sizes and configurations depending on the characteristics of the fuel, the specified heating output, and the required emission controls. Some boilers are only capable of producing hot water, while others are designed to produce steam.
Boilers can burn coal, oil, natural gas, biomass as well as other fuels and fuel combinations. Most boilers are classified as either watertube or firetube boilers, but other designs such as cast iron, coil type, and tubeless (steel shell) boilers are also produced.

Components of a boiler system

The main components in a boiler system are boiler feedwater heaters, deaerators, feed pump, economiser, superheater, attemperator, steam system, condenser and condensate pump. In addition, there are sets of controls to monitor water and steam flow, fuel flow, airflow and chemical treatment additions.
More broadly speaking, the boiler system comprises a feedwater system, steam system and fuels system. The feedwater system provides water to the boiler and regulates it automatically to meet the steam demand. Various valves provide access for maintenance and repair.

The stem system collects and controls the steam produced in the boiler. Steam is directed through a piping system to the point of use. Throughout the system, steam pressure is regulated using valves and checked with steam pressure gauges.
The fuel system includes all equipment used to provide fuel to generate the necessary heat. The equipment required in the fuel system depends on the type of fuel used in the system.

Feedwater system

The water supplied to the boiler, which is converted into steam, is called feedwater. The two sources of feedwater are condensate or condensed steam returned from the process and makeup water (treated raw water) which must come from outside the boiler room and plant processes.

Feedwater heater

Boiler efficiency is improved by the extraction of waste heat from spent steam to preheat the boiler feedwater. Heaters are shell and tube heat exchangers with the feedwater on the tube side (inside) and steam on the shell side (outside). The heater closest to the boiler receives the hottest steam. The condensed steam is recovered in the heater drains and pumped forward to the heater immediately upstream, where its heat value is combined with that of the steam for that heater. Ultimately the condensate is returned to the condensate storage tank or condenser hotwell.

Deaerators

Feedwater often has oxygen dissolved in it at objectionable levels, which comes from air in-leakage from the condenser, pump seals, or from the condensate itself. The oxygen is mechanically removed in a deaerator. Dearators function on the principle that oxygen is decreasingly soluble as the temperature is raised. This is done by passing a stream of steam through the feedwater. Deaerators are generally a combination of spray and tray type. One problem with the control of deaerators is ensuring sufficient temperature difference between the incoming water temperature and the stripping steam. If the temperature is too close, not enough steam will be available to strip the oxygen from the make-up water.

Economisers

Economisers are the last stage of the feedwater system. They are designed to extract heat value from exhaust gases to heat the steam still further and improve the efficiency of the boiler. They are simple finned tube heat exchangers. Not all boilers have economizers. Usually they are found only on water tube boilers using fossil fuel as an energy conservation measure.
A feedwater economiser reduces steam boiler fuel requirements by transferring heat from the flue gas to incoming feedwaer. By recovering waste heat, an economiser can often reduce fuel requirements by 5 per cent to 10 per cent and pay for itself in less than two years.

A feedwater economiser is appropriate when insufficient heat transfer surface exists within the boiler to remove combustion heat. Boilers that exceed 100 boiler hp, operating at pressures exceeding 75 psig or above, and those that are significantly loaded all year long are excellent candidates for econimiser retrofit.

Steam system

Steam and mud drums

A boiler system consists of a steam drum and a mud drum. The steam drum is the upper drum of a watertube boiler where the separation of water and steam occurs. Feedwater enters the boiler steam drum from the economizers or from the feedwater heater train if there is no economiser. The colder feedwater helps create the circulation in the boiler.

The steam outlet line normally takes off from this drum to a lower drum by a set of riser and downcomer tubes. The lower drum, called the mud drum, is a tank at the bottom of the boiler that equalizes distribution of water to the generating tubes and collects solids such as salts formed from hardness and silica or corrosion products carried into the boiler.