Understanding Boiler Efficiencies - Archives - Process Heating

Understanding Boiler Efficiencies - Archives - Process Heating

08/08/2007 08:50:01 AMhttp://www.process-heating.com/CDA/Archives/2781d1f1cc129010VgnVCM100000f932...

Having a goodunderstanding of the keyterms to evaluate whenselecting a boiler can helpyou make apples-to-applesevaluations.

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Understanding Boiler Efficienciesby Jim AlbrightJanuary 31, 2006

The efficiency of a boiler shouldbe an important part of apurchase evaluation because theannual cost of fuel can easily betwo to three times the installedcost of the equipment.Therefore, a difference inefficiency and the resultingdifference in fuel cost can easilyoffset a difference in installedcost.

While it is important to consider efficiency in an equipment

purchase, it is equally important to understand efficiency to

the point that the purchaser can be assured that values are

being compared on an apples-to-apples basis. The subject of

efficiency for a boiler is rather complex when all of the

elements that affect efficiency are considered and a complete

thermodynamic analysis is performed. Fortunately, it is not

necessary to understand the process in detail, but a basic

understanding of the terms can help ensure a good apples-to-

apples efficiency evaluation.

Efficiency Terms

Several terms are used to qualify efficiency when used in the

context of a boiler. These include simply efficiency, boiler

efficiency, thermal efficiency, combustion efficiency and fuel-

to-steam efficiency. The terms efficiency and boiler

efficiency by themselves are, essentially, meaningless since

they must be qualified in order to understand their

significance.

In general, the term thermal efficiency refers to the

efficiency of a thermal process. This is as opposed to

mechanical efficiency -- the efficiency of a mechanical

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process. When used in conjunction with boilers, thermal

efficiency sometimes refers to the efficiency of the heat

exchanger. In any event, this term is not significant for

purposes of comparing one boiler or steam generator to

another. While the thermal efficiency of the heat exchanger is

an important factor, its importance lies in its contribution to

the fuel-to-steam efficiency.

While the terms efficiency and thermal efficiency are not

meaningful for comparing one boiler to another, the terms

combustion efficiency and fuel-to-steam efficiency are. Of

these, fuel-to-steam efficiency is the most significant, but it is

difficult to measure or calculate in real-world situations.

Therefore, combustion efficiency, which can be easily

computed using a combustion gas analyzer, is, frequently,

used for performance comparison purposes.

Combustion efficiency equals the total heat released in

combustion, minus the heat lost in the stack gases, divided by

the total heat released. For example, if 1,000 BTU/hr are

released in combustion and 180 BTU/hr are lost in the stack,

then the combustion efficiency is 82 percent: (1,000 180)/

1,000 = 0.82 or 82 percent.

Fuel-to-steam efficiency is the most important because it is a

measure of the energy that is converted to steam and that is,

after all, the reason a user installs a steam boiler -- to produce

steam. Fuel-to-steam efficiency is equal to combustion

efficiency less the percent of heat losses through radiation and

convection. To continue the example above, suppose 20 BTU/

hr are lost to convection and radiation. Then the convection

and radiation losses are 2 percent: 20/1,000 = 0.02, or 2

percent. Because we know that in this example, combustion

efficiency is 82 percent, we can calculate the fuel-to-steam

efficiency by subtracting the heat losses due to convection and

radiation from the combustion efficiency. Numerically, it is 82

percent - 2 percent, which equals 80 percent fuel-to-steam

efficiency.

A word of caution: when comparing efficiencies, it is important

to know if the efficiency is based on the high heat value (HHV)

or low heat value (LHV) of the fuel. Both are essentially

correct, but comparing an efficiency based on HHV to one

based on LHV would not be correct. In the United States,

boiler efficiencies are typically based on the HHV. In Europe,

they are typically based on the LHV and result in a higher

value than when based on HHV. The general relationship is:

Efficiency based on LHV = Efficiency based on HHV multiplied

by 1.11 for natural gas and multiplied by 1.06 for diesel fuel

oil.

Operating Efficiency


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Each of the terms discussed above refers to the efficiency of a

boiler when operating at a fixed condition such as at 100

percent load, with specified air and feedwater temperatures,

etc. These efficiencies are, unquestionably, important, but

there are operational factors that affect the annual fuel bill and

can have an affect that may be greater than the difference of a

point or two in the efficiency of the equipment when, for

instance, operating at 100 percent. Operational factors include

boiler design, time required to startup, steam quality and level

of blowdown required.

Steaming Rate

The steaming rate, or the rate at which a boiler produces

steam, normally is expressed in terms of pounds per hour or

kilograms per hour. It is frequently misunderstood, and such a

misunderstanding can lead to the purchase of the wrong size

boiler. It is, therefore, essential that the steaming rate be

qualified when selecting a boiler size. The three common

steaming rate terms are:

From and at 212F (100C) Steaming Rate.

Gross Steaming Rate.

Net Steaming Rate.

From and at 212F is the steaming rate for a boiler producing

steam, at the outlet flange, at 212F, and 0 psig, with

feedwater at the inlet flange at 212F and 0 psig. This is the

most common steaming rate term and is used most often

when steaming rate information is provided. And, by definition,

one boiler horsepower (BHP) is equivalent to 34.5 lb of steam

per hour, from and at 212F.

Gross Steaming Rate is the rate at which a boiler produces

steam, at the outlet flange, based on application specific

feedwater conditions at the inlet flange and application specific

steam conditions. The gross steaming rate typically differs

from the From and at 212F steaming rate because both the

feedwater inlet and the steam conditions are different than

212F and 0 psig. A typical application may, for instance, have

feedwater at 190F (88C) and produce saturated steam at

100 psig (338F). Because the inlet temperature is less than

212F and the outlet temperature is greater than 212F, the

amount of heat needed to produce a pound of steam, at these

conditions, is greater than the amount needed to produce a

pound of steam with inlet and outlet temperatures of 212F.

The gross steaming rate is, therefore, frequently less than the

From and at 212F steaming rate. It may, however, actually be

greater if the feedwater receiver is a pressurized deaerator

that heats the feedwater to a temperature above 212F.

Net Steaming Rate is the steaming rate at which a boiler

produces steam to your plant or process and, thus, is the most



important steaming rate to consider. Net steaming rate differs

from gross steaming rate in that it takes into account the

amount of steam needed to heat the feedwater in the

feedwater receiver (deaerator or hotwell): Specifically, the net

steaming rate equals the gross steaming rate minus the

steaming rate to the feedwater receiver. Except for some very

unusual applications, the net steaming rate is less than the

gross steaming rate or the From and at 212F steaming rate.

Take, for example, a 100 BHP boiler operating with 100

percent makeup water at 60F (16C) and producing steam at

125 psig. In this case, the From and at 212F (100C)

Steaming rate is 3,450 lb/hr, but the net steaming rate is only

2,874 lb/hr -- 17 percent less than the From and at 212F

steaming rate.

The effect of feedwater heating is applicable in all applications

and thus should always be considered. There is another factor

that has an effect and can be significant in some applications:

blowdown, which is required for the boiler to operate

effectively. In this case, blowdown refers to the amount of

water that must be removed from the boiler system, on a

regular basis, in order to control the level of total dissolved

solids (TDS) in the boiler. Water that is removed to control

TDS has been heated, and the amount of energy needed to

heat this water reduces the amount of energy that is available

to produce steam.

In summary, users should be certain to qualify steaming rates

when using them to define the size of a boiler. Boiler

horsepower is a specific term and no further information is

needed to select the size of a boiler. However, if a steaming

rate is used to specify boiler size, then which steaming rate is

being used must be qualified.PH

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Jim [email protected] Albright is national sales manager for Clayton Industries,City of Industry, Calif., a manufacturer of industrial processboilers and steam generators. For more information, call (800)423-4585; e-mail [email protected] or visitwww.claytonindustries.com.

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