1 boiler horse power is about 42

8/7/2019 1 boiler horse power is about 42

1/9

BOILER INFORMATIONS

1 boiler horse power is about 42,000 BTUs of INPUT

1 pound of steam is about 1,200 BTUs of INPUT fuel, and about 1,000 BTUs at the point of use, depending on

the pressure of the steam

Low pressure steam is considered to be up to 15 psi; high is generally 100 psi and higher.

Superheat is a term that refers to higher temperature steam, as a result of a second special steam heat exchanger

in the boiler that allows steam pressure to increase, thereby taking on more BTUs (in excess of 500 psi is

typical of superheat). Superheated steam is very dry steam.

Smaller boilers are generally rated in horse power; larger are generally rated in thousands of pounds of steam(500 hp and under will typically be rated in hp)

Typical boiler efficiency will be in the 75 - 85% range; new highest efficiency boilers may be near 90%; newer

quick heat up types of boilers with copper heat exchangers can be more efficient, especially at startup and partload than older, heavy mass cast iron boilers.

Source: DOE 'Improving Steam System Performance - a Sourcebook for Industry' Oct.2004Click on image for larger view

Fire Tube Boilers

In fire tube boilers, the combustion gases pass inside boiler

tubes, and heat is transferred to water on the shell side. Scotch

marine boilers are the most common type of industrial fire tube

boiler. The Scotch marine boiler is an industry workhorse due

to low initial cost, and advantages in efficiency and durability.

Scotch marine boilers are typically cylindrical shells with

horizontal tubes configured such that the exhaust gases pass

through these tubes, transferring energy to boiler water on the shell side.

Scotch marine boilers contain relatively large amounts of water, which enables them to respond to load changeswith relatively little change in pressure. However, since the boiler typically holds a large water mass, it requires

more time to initiate steaming and more time to accommodate changes in steam pressure. Also, Scotch marine

boilers generate steam on the shell side, which has a large surface area, limiting the amount of pressure they
http://www.energysolutionscenter.org/boilerburner/Eff_Improve/Images/Boiler_Flow_Schem_DOE.jpg


2/9

can generate. In general, Scotch marine boilers are not used where pressures above 300 psig are required.

Today, the biggest firetube boilers are over 1,500 boiler horsepower (about 50,000 lbs/hr).

Firetube boilers are often characterized by their number of passes, referring to the number of t imes the

combustion (or flue) gases flow the length of the pressure vessel as they transfer heat to the water. Each pass

sends the flue gases through the tubes in the opposite direction. To make another pass, the gases turn 180

degrees and pass back through the shell. The turnaround zones can be either dryback or water-back. In dryback

designs, the turnaround area is refractory lined. In water-back designs, this turnaround zone is water-cooled,

eliminating the need for the refractory lining.

Water Tube Boilers

In watertube boilers, boiler water passes through the tubes while the exhaust

gases remain in the shell side, passing over the tube surfaces. Since tubes can

typically withstand higher internal pressure than the large chamber shell in a

firetube, watertube boilers are used where high steam pressures (as high as

3,000 psi) are required.

Watertube boilers are also capable of high efficiencies and can generate

saturated or superheated steam. The ability of watertube boilers to generate

superheated steam makes these boilers particularly attractive in applications

that require dry, high-pressure, high-energy steam, including steam turbine

power generation.

The performance characteristics of watertube boilers make them highly favorable in process industries,

including chemical manufacturing, pulp and paper manufacturing, and refining. Although firetube boilers

account for the majority of boiler sales in terms of units, water-tube boilers account for the majority of boiler

capacity.

Steam Generators

Steam generators are like boilers in that they are fired by gas andproduce steam, but they are unlike boilers in that they do not have

large pressure vessels and are made of light-weight materials. Thefact that they do not have pressure vessels means that in most

locations they do NOT require a boiler operator (always confirmwith local codes). This can be a substantial savings when there is no

other reason to have an operator other than the local code requires it

for a large pressure vessel. The fact that they are made out of light

weight materials means they perform well at part loads and respond

quickly to changes in loads. This greatly increases part load

operating efficiency.

Compact and Modular Boilers

Modern materials, controls and the pursuit of ever

higher energy efficiency and reduced emissions is

leading to boilers that are smaller in physical size, have

cleaner emissions and produce dryer steam. Materials

are critical because old cast iron boilers relied on mass

to prevent them from thermal shocks that could split theboiler apart. New metals reduce mass which improves

thermal transfer and can handle the thermal stress ofgoing from cold water to steam in seconds.

Space is money, especially in new construction. Boilers of similar output capacity made smaller to

reduce their space requirements can result in overall lower first cost of equipment plus space.


3/9

Boilers have a certain efficiency curve that tends to result in the boiler having the highest efficiency at

full-fire. As the boiler is 'turned-down' to run at less than 100% capacity, efficiency typically drops. To

counter this situation, a modular boiler bank of 3 or more boilers with a programmed controller can

improve operational efficiency over a single boiler operating in a turn-down mode, and provides a certain

amount of redundancy for back-up.

Tubeless and Condensing Boilers

High energy prices along with improved material and combustion technology is

resulting in a new generation of high efficiency gas boilers. Traditional boilers aredesigned to PREVENT condensation because it is corrosive to boiler components and

the wide variations in temperatures cause problems with thermal shock. However,without condensation, boilers can not be higher than about 85% efficient. Boilers that

are designed for condensation and use advanced controls to squeeze every possible

BTU from the combustion process are able to achieve efficiencies in the high 90's.There is a first-cost premium, but when energy prices are high, paybacks are moreacceptable.

"Tubeless" Boilers use tubing coils instead of rigid tubes. "Direct Contact" water heaters have no tubes, tubing

or coils; they have heat transfer media such as spheres or cylinders and allow fluegases to come in direct contact with the water.

Steam Information

Steam is an invisible gas that's generated by heating water to a temperature that

brings it to the boiling point. When this happens, water changes its physical stateand vaporizes, turning from a liquid into a gas.

Conversely, when heat energy is removed from steam, it loses its ability to retain

a gaseous state and condenses back into a liquid. The resulting liquid is called

condensate. The temperature at which condensation takes place is known as the

dew point.

When water is heated at atmospheric pressure, its temperature rises until it reaches 212F (100C), the highest

temperature at which water can exist at this pressure. Additional heat does not raise the temperature, but

converts the water to steam.

One pound of water takes 1 BTU per Degree of Temperature rise up to 212F; to form steam, an additional 970BTUs is required for the "Latent Heat of Vaporization". Therefore, steam has (970 + (212 - Condensate

Temperature)) BTUs per pound.


4/9

EXAMPLE: If the condensate temperature is 160F, (970 + (212 - 160)) = 1,022 BTUs per pound. This

clearly shows why steam has more energy content than hot water.

NOTE: These are BTUs delivered to the water; efficiency must also be factored in to determine INPUT BTU

requirements.

Superheat

Superheat refers to the process of increasing the temperature of steam above about 400F and 100 psi to

produce a very "dry" steam with absolutely no water vapor. This feature is most common in very large powerplant boilers of watertube construction. An additional heat exchanger capable of the high temperatures and

pressures is required.

At least one company promotes a "direct fired" superheater, that could have some

advantages for facilities with smaller sized boilers that need higher temperatures and

pressures, but do not want to invest in a new boiler or use a Thermal Fluid system.

According to their web site, the Cannon Superheater can be used on new boilers and

retrofit installations. The Cannon Superheater can be used on watertube or firetube

boilers in the 25 HP to 1,000 HP range.

Boiler Stack Economizer

Flue gases from large boilers are typically 450 - 650F. Stack Economizers recover

some of this heat for pre-heating water. The water is most often used for boiler make-

up water or some other need that coincides with boiler operation. Stack Economizers should be considered asan efficiency measure when large amounts of make-up water are used (ie: not all condensate is returned to the

boiler or large amounts of live steam are used in the process so there is no condensate to return.)

The savings potential is based on the existing stack temperature, the volume of make-up water needed, and thehours of operation. Economizers are available in a wide range of sizes, from small coil-like units to very large

waste heat recovery boilers.


5/9

Boiler Flue CondensersThere is a class of economizers that are designed to condense

the flue gases and/or have the water in direct contact with flue

gases. I have called them 'Flue Gas Condensers'. Stack

Economizers and Condensers should be considered as an

efficiency measure when large amounts of make-up water areused (ie: not all condensate is returned to the boiler or large

amounts of live steam is used in the process so there is nocondensate to return) or there is a simultaneous need for large

volumes of hot water.

The application difference between an economizer and

condenser is that economizers are primarily used to heat a smaller volume of water to a high temperature for

boiler feed water, and condenser units heat a larger volume of water to a lower temperature. Condensers can be

more efficient because they can have a lower outlet exhaust temperature and take advantage of the energy incondensed flue gasses (the Latent Heat of Vaporization).

Boiler Water Treatment

Origin of the Problem

The most common source of corrosion in boiler systems is dissolved gas: oxygen, carbon dioxide and

ammonia. Of these, oxygen is the most aggressive. The importance of eliminating oxygen as a source of pitting

and iron deposition cannot be over-emphasized. Even small concentrations of this gas can cause serious

corrosion problems.

Makeup water introduces appreciable amounts of oxygen into the system. Oxygen can also enter the feed water

system from the condensate return system. Possible return line sources are direct air-leakage on the suction sideof pumps, systems under vacuum, the breathing action of closed condensate receiving tanks, open condensate

receiving tanks and leakage of nondeaerated water used for condensate pump seal and/or quench water. With

all of these sources, good housekeeping is an essential part of the preventive program.

One of the most serious aspects of oxygen corrosion is that it occurs as pitting. This type of corrosion can

produce failures even though only a relatively small amount of metal has been lost and the overall corrosion

rate is relatively low. The degree of oxygen attack depends on the concentration of dissolved oxygen, the pH

and the temperature of the water.

The influence of temperature on the corrosivity of dissolved oxygen is particularly important in closed heaters

and economizers where the water temperature increases rapidly. Elevated temperature in itself does not cause

corrosion. Small concentrations of oxygen at elevated temperatures do cause severe problems. This temperature

rise provides the driving force that accelerates the reaction so that even small quantities of dissolved oxygen

can cause serious corrosion.

The Corrosion Process

Localized attack on metal can result in a forced shutdown. The prevention of a forced shutdown is the true aim

of corrosion control.

Because boiler systems are constructed primarily of carbon steel and the heat transfer medium is water, the

potential for corrosion is high. Iron is carried into the boiler in various forms of chemical composition and

physical state. Most of the iron found in the boiler enters as iron oxide or hydroxide. Any soluble iron in the

feed water is converted to the insoluble hydroxide when exposed to the high alkalinity and temperature in the

boiler.


6/9

These iron compounds are divided roughly into two types, red iron oxide (Fe2O3) and black magnetic oxide

(Fe3O4). The red oxide (hematite) is formed under oxidizing conditions that exist, for example, in the

condensate system or in a boiler that is out of service. The black oxides (magnetite) are formed under reducing

conditions that typically exist in an operating boiler.

External Treatment

External treatment, as the term is applied to water prepared for use as boiler feed water, usually refers to the

chemical and mechanical treatment of the water source. The goal is to improve the quality of this source prior

to its use as boiler feed water, external to the operating boiler itself. Such external treatment normally includes:

1. Clarification2. Filtration

3. Softening

4. Dealkalization

5. Demineralization

6. Deaeration

7. Heating

Any or all of these approaches can be used in feed water or boiler water preparation.

Internal Treatment

Even after the best and most appropriate external treatment of the water source, boiler feed water (including

return condensate) still contains impurities that could adversely affect boiler operation. Internal boiler watertreatment is then applied to minimize the potential problems and to avoid any catastrophic failure, regardless of

external treatment malfunction.

Feed Water Preparation

The basic assumption with regard to the quality of feed water is that calcium and magnesium hardness,migratory iron, migratory copper, colloidal silica and other contaminants have been reduced to a minimum,

consistent with boiler design and operation parameters.

Once feed water quality has been optimized with regard to soluble and particulate contaminants, the next

problem is corrosive gases. Dissolved oxygen and dissolved carbon dioxide are among the principal causes of

corrosion in the boiler and pre-boiler systems. The deposition of these metallic oxides in the boiler is frequently

more troublesome than the actual damage caused by the corrosion. Deposition is not only harmful in itself, but

it offers an opening for further corrosion mechanisms as well.

Contaminant products in the feed water cycle up and concentrate in the boiler. As a result, deposition takes

place on internal surfaces, particularly in high heat transfer areas, where it can be least tolerated. Metallicdeposits act as insulators, which can cause local overheating and failure. Deposits can also restrict boiler water

circulation. Reduced circulation can contribute to overheating, film boiling and accelerated deposition.

The best way to start to control pre-boiler corrosion and ultimate deposition in the boiler is to eliminate the

contaminants from the feed water. Consequently, this section deals principally with the removal of oxygen, the

impact of trace amounts of contaminants remaining in the feed water, and heat exchange impact.

Feed water is defined as follows:

Feed water (FW) = Makeup water (MW) + Return condensate (RC)

The above equation is a mass balance (pounds or kilograms).

Deaeration (Mechanical and Chemical)


7/9

Mechanical and chemical deaeration is an integral part of modern boiler water protection and control.

Deaeration, coupled with other aspects of external treatment, provides the best and highest quality feed water

for boiler use.

Simply speaking, the purposes of deaeration are:

1. To remove oxygen, carbon dioxide and other noncondensable gases from feed water

2. To heat the incoming makeup water and return condensate to an optimum temp

3. Minimizing solubility of the undesirable gases

4. Providing the highest temperature water for injection to the boiler

Deaerators

Mechanical deaeration is the first step in eliminating oxygen and other corrosive gases from the feed water.

Free carbon dioxide is also removed by deaeration, while combined carbon dioxide is released with the steam

in the boiler and subsequently dissolves in the condensate. This can cause additional corrosion problems.

Because dissolved oxygen is a constant threat to boiler tube integrity, our discussion on the deaerator will be

aimed at reducing the oxygen content of the feed water. The two major types of deaerators are the tray type

and the spray type. In both cases, the major portion of gas removal is accomplished by spraying cold makeup

water into a steam environment.

Tray Type Deaerating Heaters

Tray-type deaerating heaters release dissolved gases in the incoming water by reducing it to a fine spray as it

cascades over several rows of trays. The steam that makes intimate contact with the water droplets then scrubs

the dissolved gases by its counter-current flow. The steam heats the water to within 3-5 F of the steam

saturation temperature and it should remove all but the very last traces of oxygen. The deaerated water then

falls to the storage space below, where a steam blanket protects it from recontamination.

Nozzles and trays should be inspected regularly to insure that they are free of deposits and are in their proper

position.

Spray-Type Deaerating Heaters

Spray-type deaerating heaters work on the same general philosophy as the tray-type, but differ in theiroperation. Spring-loaded nozzles located in the top of the unit spray the water into a steam atmosphere that

heats it. Simply stated, the steam heats the water, and at the elevated temperature the solubility of oxygen is

extremely low and most of the dissolved gases are removed from the system by venting. The spray will reduce

the dissolved oxygen content to 20-50 ppb, while the scrubber or trays further reduce the oxygen content to

approximately 7 ppb or less.

During normal operation, the vent valve must be open to maintain a continuous plume of vented vapors andsteam at least 18 inches long. If this valve is throttled too much, air and nonconclensable gases will accumulate

in the deaerator. This is known as air blanketing and can be remedied by increasing the vent rate.

For optimum oxygen removal, the water in the storage section must be heated to within 5 F of the temperatureof the steam at saturation conditions. From inlet to outlet, the water is deaerated in less than 10 seconds.


8/9

Deaerators and Economizers

Where economizers are installed, good deaerating heater operation is essential. Because oxygen pitting is the

most common cause of economizer tube failure, this vital part of the boiler must be protected with an oxygen

scavenger, usually catalyzed sodium sulfite. In order to insure complete corrosion protection of the economizer,

it is common practice to maintain a sulfite residual of 5-10 ppm in the feed water and, if necessary, feed

sufficient caustic soda or neutralizing amine to increase the feed water pH to between 8.0 and 9.0.

Below 900 psi excess sulfite (up to 200 ppm) in the boiler will not be harmful. To maintain blowdown rates,

the conductivity can then be raised to compensate for the extra solids due to the presence of the higher level ofsulfite in the boiler water. This added consideration (in protecting the economizer) is aimed at preventing a

pitting failure. Make the application of an oxygen scavenger, such as catalyzed sulfite, a standard

recommendation in all of your boiler treatment programs.
http://www.energysolutionscenter.org/boilerburner/Eff_Improve/Images/Deaerator_Spray2.jpghttp://www.energysolutionscenter.org/boilerburner/Eff_Improve/Images/Deaerator_Tray2.jpg


9/9

Treatment

The foregoing discussion shows the importance of proper deaeration of boiler feed water in order to prevent

oxygen corrosion. Complete oxygen removal cannot be attained by mechanical deaeration alone. Equipment

manufacturers state that a properly operated deaerating heater can mechanically reduce the dissolved oxygen

concentrations in the feed water to 0.005 cc per liter (7 ppb) and 0 free carbon dioxide. Typically, plant oxygen

levels vary from 3 to 50 ppb. Traces of dissolved oxygen remaining in the feed water can then be chemically

removed with the oxygen scavenger.

Blowdown Control

The main purpose of blowdown is to maintain the solids content of the boiler water within prescribed limits.

This would be under normal steaming conditions. However, in the event contamination is introduced in the

boiler, high continuous and manual blowdown rates are used to reduce the contamination as quickly as

possible.

Because each boiler and plant operation is different, maximum levels should be determined on an individual

basis.

Bottom Blowdown

By definition, bottom blowdown is intermittent and designed to remove sludge from the areas of the boilerwhere it settles. The frequency of bottom blowdown is a function of experience and plant operation. Bottom

blowdown can be accomplished manually or electronically using automatic blowdown controllers.

Continuous Blowdown

Frequently used in conjunction with manual blowdown, continuous blowdown constantly removes

concentrated water from the boiler. By design, it is in the area of highest boiler water concentration. This point

is determined by the design of the boiler and is generally the area of greatest steam release.

Continuous blowdown allows for excellent control over boiler water solids. In addition, it can remove

significant levels of suspended solids. Another advantage is that the continuous blowdown can be passed

through heat recovery equipment.

Blowdown Control Summary

Proper boiler blowdown control in conjunction with proper internal boiler water treatment will provide thedesired results for a boiler water program. Many modern devices can automate boiler blowdown, thereby

increasing the overall efficiency of the unit.

ION EXCHANGE SYSTEMS

Ion exchange systems range from light commercial water softeners and filters to specially designed industrial

equipment. Also known as deionizations (DI) systems. These systems are considered high-end where the

highest quality of water treatment is needed, such as with steam turbines.

RO SYSTEMS

Reverse Osmosis (RO) systems are available for tap water, brackish water or seawater. These systems are

considered high-end where the highest quality of water treatment is needed, such as with steam turbines.

Documents

1 boiler horse power is about 42