Boiler Basics

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Fire tube boilers consist of a shell, which contains the water and steam, and the heating tubes, which pass through the shell. The hot gas passes through the tubes to heat the water and generate the steam. The most common configuration of the fire tube boiler is the horizontal return tube boiler, or the HRT boiler.

A second type of heat source is the unfired boiler, where no direct burning of fuel is used to generate steam. The waste, or exhaust heat from another system is utilized. Combined cycle or cogeneration plants are gas turbine plants that are used to generate electricity or to drive other large process machinery. The exhaust from these turbines is exhausted at approximately one thousand degrees. Supplemental firing can raise gas temperatures to between twelve hundred and sixteen hundred degrees, allowing steam to be generated at the highest pressure level. A second waste heat boiler may be installed downstream of the high pressure boiler to produce low pressure steam if required.

Several characteristics that determine a boiler's classification were discussed in this section. These include use, pressure, materials of construction, size, tube content, heat source, and circulation. Also, this section covered two major types of boilers generally found in today's industry: water tube and fire tube.

3 . STEAM PRODUCTION

The primary purpose of the boiler is to produce steam. Because of its heat or energy content, steam is an efficient means of moving energy from the boiler to its intended end use.

Steam and condensate losses from a boiler system increase operating costs. If steam is lost by leakage or inefficient use, the boiler is required to produce more steam than necessary. More steam means more fuel. Similarly, if condensate is lost, it must be replaced with make-up water which requires treatment prior to being used and again more fuel is used to heat the cooler make-up.

Treatment of the water is necessary to remove impurities that naturally exist in all water, regardless of its source. The type and quantity of the impurities may vary and require different methods of removal. If left in the water, these impurities would eventually affect the steam generating process and cause damage to the boiler system.

In the boiler, heat is applied and water is converted to steam. The steam is continuously discharged from the boiler at a controlled temperature and pressure. Boiler feedwater is fed to the boiler as a continuous supply of water to makeup for the steam that is lost. As the steam leaves the boiling water, dissolved solids originally in the feedwater are left behind. These solids become increasingly concentrated and must be removed by a process known as blowdown or bleed-off.

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One of the major concerns in the production of steam for a specific process is the quality and purity of the steam. Steam quality refers to the moisture content of the steam, or how much water is mixed with the steam. Steam purity deals with contaminants in the steam. The presence of either of these can cause significant damage to components in the steam system.

In the perfect boiler system, all the water that enters the boiler would leave as steam, and all of the steam would return to the boiler as condensate to be reused. As we have seen, however, losses of steam and condensate occur for various reasons requiring make-up water to be added. We also know that water contains impurities which must be removed by blowdown. We will look at how the amount of steam produced, amount of feedwater added, plus losses and additions to the system are related to each other to provide valuable operating information.

Because the water entering the boiler contains impurities and the steam produced in the boiler is free of impurities, the impurities in the boiler begin to concentrate. To measure the concentration, ratio the amount of impurities in the boiler to the amount of impurities in the feedwater. This ratio is known as the concentration ratio, or cycles of concentration.

CR = Concentration of solids in Boiler Water / Concentration of solids of feedwater

If allowed to concentrate indefinitely, the impurities would eventually damage the boiler. Consequently, the accumulated impurities must be removed from the boiler to allow for continued operation. This is done through blowdown. A portion of the boiler water is removed to control the amount of impurities in the boiler. The amount of water blowdown is usually expressed as a percentage of the feedwater, as is the amount of make-up water used. These equations show how to arrive at the values known as percent make-up and percent blowdown.

%Make-up= (Quantity of Make-up / Quantity of Feedwater ) x 100

%Blowdown= (Quantity of Blowdown / Quantity of Feedwater ) x 100

Mass Balance

Not all flows in a boiler system are measured. Typically, steam flow will be measured, but other flows may not. Using the concept of mass balance or, in other words, understanding that gains and losses must be equal, the flows in other portions of the system can be determined using the relationships shown here.

Feedwater = Make-up + condensate return

Feedwater = Steam Rate + Blowdown

Steam Rate = Returned Condensate + Non-Returned Condensate

Fuels

A variety of fuels can be used for steam generation. The type of fuel and the boiler's intended use are the governing factors in its design. When solid or liquid fuels are used, the boilers will be larger because of the increased furnace volume necessary for combustion. Because these fuels produce ash and other by-products during combustion, boilers are designed to be cleaned periodically in order to maintain efficiency and meet environmental standards.

Various grades of fuel oil and a variety of coal are available for use as fuels depending on the temperature requirements of the boiler. A variety of process waste gases, nuclear energy, or electricity can also be used.

The uses for steam in today's industry seems almost endless. Because of this, it is becoming increasingly important to conserve and recycle water whenever possible. Through the rest of the lessons on boilers, you will become familiar with many aspects of boilers and their operation, such as problems associated with boilers, servicing, chemistry, and treatment programs. With

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