RELATIVE VALUE OF COAL AND OIL FUEL_Steam, Its Generation and Use

+------+--------+-------+-----------------------------------------------+

|Gross | Net | Net | Water Evaporated from and at |

|Effic-|Effici- |oration+-----+-----+-----+-----+-----+-----+-----+-----+

| Oil | Oil | 212 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |

| | | heit +-----+-----+-----+-----+-----+-----+-----+-----+

| | | per | |

+------+--------+-------+-----+-----+-----+-----+-----+-----+-----+-----+

| 73 | 71 | 13.54 |.3693|.4431|.5170|.5909|.6647|.7386|.8124|.8863|

| 74 | 72 | 13.73 |.3642|.4370|.5099|.5827|.6556|.7283|.8011|.8740|

| 75 | 73 | 13.92 |.3592|.4310|.5029|.5747|.6466|.7184|.7903|.8621|

| 76 | 74 | 14.11 |.3544|.4253|.4961|.5670|.6378|.7087|.7796|.8505|

| 77 | 75 | 14.30 |.3497|.4196|.4895|.5594|.6294|.6993|.7692|.8392|

| 78 | 76 | 14.49 |.3451|.4141|.4831|.5521|.6211|.6901|.7591|.8281|

| 79 | 77 | 14.68 |.3406|.4087|.4768|.5450|.6131|.6812|.7493|.8174|

| 80 | 78 | 14.87 |.3363|.4035|.4708|.5380|.6053|.6725|.7398|.8070|

| 81 | 79 | 15.06 |.3320|.3984|.4648|.5312|.5976|.6640|.7304|.7968|

| 82 | 80 | 15.25 |.3279|.3934|.4590|.5246|.5902|.6557|.7213|.7869|

| 83 | 81 | 15.44 |.3238|.3886|.4534|.5181|.5829|.6447|.7125|.7772|

+------+--------+-------+-----+-----+-----+-----+-----+-----+-----+-----+

| | | Net | |

| | | 212 | Barrels of Oil Equal to One Ton of Coal |

| | | per | |

+------+--------+-------+-----+-----+-----+-----+-----+-----+-----+-----+

| 73 | 71 | 4549 |2.198|2.638|3.077|3.516|3.955|4.395|4.835|5.275|

| 74 | 72 | 4613 |2.168|2.601|3.035|3.468|3.902|4.335|4.769|5.202|

| 75 | 73 | 4677 |2.138|2.565|2.993|3.420|3.848|4.275|4.703|5.131|

| 76 | 74 | 4741 |2.110|2.532|2.954|3.376|3.798|4.220|4.642|5.063|

| 77 | 75 | 4807 |2.082|2.498|2.914|3.330|3.746|4.162|4.578|4.994|

| 78 | 76 | 4869 |2.054|2.465|2.876|3.286|3.697|4.108|4.518|4.929|

| 79 | 77 | 4932 |2.027|2.433|2.838|3.243|3.649|4.054|4.460|4.865|

| 80 | 78 | 4996 |2.002|2.402|2.802|3.202|3.602|4.003|4.403|4.803|

| 81 | 79 | 5060 |1.976|2.371|2.767|3.162|3.557|3.952|4.348|4.743|

| 82 | 80 | 5124 |1.952|2.342|2.732|3.122|3.513|3.903|4.293|4.683|

| 83 | 81 | 5187 |1.927|2.313|2.699|3.085|3.470|3.856|4.241|4.627|

+------+--------+-------+-----+-----+-----+-----+-----+-----+-----+-----+

[Illustration: City of San Francisco, Cal., Fire Fighting Station. No.

1. 2800 Horse Power of Babcock & Wilcox Boilers, Equipped for Burning

Oil Fuel]

Many tables have been published with a view to comparing the two fuels.

Such of these as are based solely on the relative calorific values of

oil and coal are of limited value, inasmuch as the efficiencies to be

obtained with oil are higher than that obtainable with coal. Table 47

takes into consideration the variation in efficiency with the two fuels,

but is based on a constant calorific value for oil and coal. This table,

like others of a similar nature, while useful as a rough guide, cannot

be considered as an accurate basis for comparison. This is due to the

fact that there are numerous factors entering into the problem which

affect the saving possible to a much greater extent than do the relative

calorific values of two fuels. Some of the features to be considered in

arriving at the true basis for comparison are the labor saving possible,

the space available for fuel storage, the facilities for conveying the

oil by pipe lines, the hours during which a plant is in operation, the

load factor, the quantity of coal required for banking fires, etc., etc.

The only exact method of estimating the relative advantages and costs of

the two fuels is by considering the operating expenses of the plant with

each in turn, including the costs of every item entering into the

problem.

Burning Oil Fuel--The requirements for burning petroleum are as follows:

1st. Its atomization must be thorough.

2nd. When atomized it must be brought into contact with the requisite

quantity of air for its combustion, and this quantity must be at the

same time a minimum to obviate loss in stack gases.

3rd. The mixture must be burned in a furnace where a refractory material

radiates heat to assist in the combustion, and the furnace must stand up

under the high temperatures developed.

4th. The combustion must be completed before the gases come into contact

with the heating surfaces or otherwise the flame will be extinguished,

possibly to ignite later in the flue connection or in the stack.

5th. There must be no localization of the heat on certain portions of

the heating surfaces or trouble will result from overheating and

blistering.

The first requirement is met by the selection of a proper burner.

The second requirement is fulfilled by properly introducing the air into

the furnace, either through checkerwork under the burners or through

openings around them, and by controlling the quantity of air to meet

variations in furnace conditions.

The third requirement is provided for by installing a furnace so

designed as to give a sufficient area of heated brickwork to radiate the

heat required to maintain a proper furnace temperature.

The fourth requirement is provided for by giving ample space for the

combustion of the mixture of atomized oil and air, and a gas travel of

sufficient length to insure that this combustion be completed before the

gases strike the heating surfaces.

The fifth requirement is fulfilled by the adoption of a suitable burner

in connection with the furnace meeting the other requirements. A burner

must be used from which the flame will not impinge directly on the

heating surface and must be located where such action cannot take place.

If suitable burners properly located are not used, not only is the heat

localized with disastrous results, but the efficiency is lowered by the

cooling of the gases before combustion is completed.

Oil Burners--The functions of an oil burner is to atomize or vaporize

the fuel so that it may be burned like a gas. All burners may be

classified under three general types: 1st, spray burners, in which the

oil is atomized by steam or compressed air; 2nd, vapor burners, in which

the oil is converted into vapor and then passed into the furnace; 3rd,

mechanical burners, in which the oil is atomized by submitting it to a

high pressure and passing it through a small orifice.

Vapor burners have never been in general use and will not be discussed.

Spray burners are almost universally used for land practice and the

simplicity of the steam atomizer and the excellent economy of the better

types, together with the low oil pressure and temperature required makes

this type a favorite for stationary plants, where the loss of fresh

water is not a vital consideration. In marine work, or in any case where

it is advisable to save feed water that otherwise would have to be added

in the form of "make-up", either compressed air or mechanical means are

used for atomization. Spray burners using compressed air as the

atomizing agent are in satisfactory operation in some plants, but their

use is not general. Where there is no necessity of saving raw feed

water, the greater simplicity and economy of the steam spray atomizer is

generally the most satisfactory. The air burners require blowers,

compressors or other apparatus which occupy space that might be

otherwise utilized and require attention that is not necessary where

steam is used.

Steam spray burners of the older types had disadvantages in that they

were so designed that there was a tendency for the nozzle to clog with

sludge or coke formed from the oil by the heat, without means of being

readily cleaned. This has been overcome in the more modern types.

Steam spray burners, as now used, may be divided into two classes: 1st,

inside mixers; and 2nd, outside mixers. In the former the steam and oil

come into contact within the burner and the mixture is atomized in

passing through the orifice of the burner nozzle.

[Illustration: Fig. 28. Peabody Oil Burner]

In the outside mixing class the steam flows through a narrow slot or

horizontal row of small holes in the burner nozzle; the oil flows

through a similar slot or hole above the steam orifice, and is picked up

by the steam outside of the burner and is atomized. Fig. 28 shows a type

of the Peabody burner of this class, which has given eminent

satisfaction. The construction is evident from the cut. It will be noted

that the portions of the burner forming the orifice may be readily

replaced in case of wear, or if it is desired to alter the form of the

flame.

Where burners of the spray type are used, heating the oil is of

advantage not only in causing it to be atomized more easily, but in

aiding economical combustion. The temperature is, of course, limited by

the flash point of the oil used, but within the limit of this

temperature there is no danger of decomposition or of carbon deposits on

the supply pipes. Such heating should be done close to the boiler to

minimize radiation loss. If the temperature is raised to a point where

an appreciable vaporization occurs, the oil will flow irregularly from

the burner and cause the flame to sputter.

On both steam and air atomizing types, a by-pass should be installed

between the steam or air and the oil pipes to provide for the blowing

out of the oil duct. Strainers should be provided for removing sludge

from the fuel and should be so located as to allow for rapid removal,

cleaning and replacing.

Mechanical burners have been in use for some time in European countries,

but their introduction and use has been of only recent occurrence in the

United States. Here as already stated, the means for atomization are

purely mechanical. The most successful of the mechanical atomizers up to

the present have been of the round flame type, and only these will be

considered. Experiments have been made with flat flame mechanical

burners, but their satisfactory action has been confined to instances

where it is only necessary to burn a small quantity of oil through each

individual burner.

This system of oil burning is especially adapted for marine work as the

quantity of steam for putting pressure on the oil is small and the

condensed steam may be returned to the system.

The only method by which successful mechanical atomization has been

accomplished is one by which the oil is given a whirling motion within

the burner tip. This is done either by forcing the oil through a passage

of helical form or by delivering it tangentially to a circular chamber

from which there is a central outlet. The oil is fed to these burners

under a pressure which varies with the make of the burner and the rates

at which individual burners are using oil. The oil particles fly off

from such a burner in straight lines in the form of a cone rather than

in the form of a spiral spray, as might be supposed.

With burners of the mechanical atomizing design, the method of

introducing air for combustion and the velocity of this air are of the

greatest importance in securing good combustion and in the effects on

the character and shape of the flame. Such burners are located at the

front of the furnace and various methods have been tried for introducing

the air for combustion. Where, in the spray burners, air is ordinarily

admitted through a checkerwork under the burner proper, with the

mechanical burner, it is almost universally admitted around the burner.

Early experiments with these air distributors were confined largely to

single or duplicate cones used with the idea of directing the air to the

axis of the burner. A highly successful method of such air introduction,

developed by Messrs. Peabody and Irish of The Babcock & Wilcox Co., is

by means of what they term an "impeller plate". This consists of a

circular metal disk with an opening at the center for the oil burner and

with radial metal strips from the center to the periphery turned at an

angle which in the later designs may be altered to give the air supply

demanded by the rate of combustion.

The air so admitted does not necessarily require a whirling motion, but

experiments show that where the air is brought into contact with the oil

spray with the right "twist", better combustion is secured and lower air

pressures and less refinement of adjustment of individual burners are

required.

Mechanical burners have a distinct advantage over those in which steam

is used as the atomizing agent in that they lend themselves more readily

to adjustment under wider variations of load. For a given horse power

there will ordinarily be installed a much greater number of mechanical

than steam atomizing burners. This in itself is a means to better

regulation, for with the steam atomizing burner, if one of a number is

shut off, there is a marked decrease in efficiency. This is due to the

fact that with the air admitted under the burner, it is ordinarily

passing through the checkerwork regardless of whether it is being

utilized for combustion or not. With a mechanical burner, on the other

hand, where individual burners are shut off, air that would be admitted

for such burner, were it in operation, may also be shut off and there

will be no undue loss from excess air.

Further adjustment to meet load conditions is possible by a change in

the oil pressure acting on all burners at once. A good burner will

atomize moderately heavy oil with an oil pressure as low as 30 pounds

per square inch and from that point up to 200 pounds or above. The

heating of the oil also has an effect on the capacity of individual

burners and in this way a third method of adjustment is given. Under

working conditions, the oil pressure remaining constant, the capacity of

each burner will decrease as the temperature of the oil is increased

though at low temperatures the reverse is the case. Some experiments

with a Texas crude oil having a flash point of 210 degrees showed that

the capacity of a mechanical atomizing burner of the Peabody type

increased from 80 degrees Fahrenheit to 110 degrees Fahrenheit, from

which point it fell off rapidly to 140 degrees and then more slowly to

the flash point.

The above methods, together with the regulation possible through

manipulation of the boiler dampers, indicate the wide range of load

conditions that may be handled with an installation of this class of

burners.

As has already been stated, results with mechanical atomizing burners

that may be considered very successful have been limited almost entirely

to cases where forced blast of some description has been used, the high

velocity of the air entering being of material assistance in securing

the proper mixture of air with the oil spray. Much has been done and is

being done in the way of experiment with this class of apparatus toward

developing a successful mechanical atomizing burner for use with natural

draft, and there appears to be no reason why such experiments should not

eventually produce satisfactory results.

Steam Consumption of Burners--The Bureau of Steam Engineering, U. S.

Navy, made in 1901 an exhaustive series of tests of various oil burners

that may be considered as representing, in so far as the performance of

the burners themselves is concerned, the practice of that time. These

tests showed that a burner utilizing air as an atomizing agent, required

for compressing the air from 1.06 to 7.45 per cent of the total steam

generated, the average being 3.18 per cent. Four tests of steam

atomizing burners showed a consumption of 3.98 to 5.77 per cent of the

total steam, the average being 4.8 per cent.

Improvement in burner design has largely reduced the steam consumption,

though to a greater degree in steam than in air atomizing burners.

Recent experiments show that a good steam atomizing burner will require

approximately 2 per cent of the total steam generated by the boiler

operated at or about its rated capacity. This figure will decrease as

the capacity is increased and is so low as to be practically negligible,

except in cases where the question of loss of feed water is all

important. There are no figures available as to the actual steam

consumption of mechanical atomizing burners but apparently this is small

if the requirement is understood to be entirely apart from the steam

consumption of the apparatus producing the forced blast.

Capacity of Burners--A good steam atomizing burner properly located in a

well-designed oil furnace has a capacity of somewhat over 400 horse

power. This question of capacity of individual burners is largely one of

the proper relation between the number of burners used and the furnace

volume. In some recent tests with a Babcock & Wilcox boiler of 640 rated

horse power, equipped with three burners, approximately 1350 horse power

was developed with an available draft of .55 inch at the damper or 450

horse power per burner. Four burners were also tried in the same furnace

but the total steam generated did not exceed 1350 horse power or in this

instance 338 horse power per burner.

From the nature of mechanical atomizing burners, individual burners have

not as large a capacity as the steam atomizing class. In some tests on a

Babcock & Wilcox marine boiler, equipped with mechanical atomizing

burners, the maximum horse power developed per burner was approximately

105. Here again the burner capacity is largely one of proper relation

between furnace volume and number of burners.

Furnace Design--Too much stress cannot be laid on the importance of

furnace design for the use of this class of fuel. Provided a good type

of burner is adopted the furnace arrangement and the method of

introducing air for combustion into the furnace are the all important

factors. No matter what the type of burner, satisfactory results cannot

be secured in a furnace not suited to the fuel.

The Babcock & Wilcox Co. has had much experience with the burning of oil

as fuel and an extended series of experiments by Mr. E. H. Peabody led

to the development and adoption of the Peabody furnace as being most

eminently suited for this class of work. Fig. 29 shows such a furnace

applied to a Babcock & Wilcox boiler, and with slight modification it

can be as readily applied to any boiler of The Babcock & Wilcox Co.

manufacture. In the description of this furnace, its points of advantage

cover the requirements of oil-burning furnaces in general.

The atomized oil is introduced into the furnace in the direction in

which it increases in height. This increase in furnace volume in the

direction of the flame insures free expansion and a thorough mixture of

the oil with the air, and the consequent complete combustion of the

gases before they come into contact with the tube heating surfaces. In

such a furnace flat flame burners should be used, preferably of the

Peabody type, in which the flame spreads outward toward the sides in the

form of a fan. There is no tendency of the flames to impinge directly on

the heating surfaces, and the furnace can handle any quantity of flame

without danger of tube difficulties. The burners should be so located

that the flames from individual burners do not interfere nor impinge to

any extent on the side walls of the furnace, an even distribution of

heat being secured in this manner. The burners are operated from the

boiler front and peepholes are supplied through which the operator may

watch the flame while regulating the burners. The burners can be

removed, inspected, or cleaned and replaced in a few minutes. Air is

admitted through a checkerwork of fire brick supported on the furnace

floor, the openings in the checkerwork being so arranged as to give the

best economic results in combustion.

[Illustration: Fig. 29. Babcock & Wilcox Boiler, Equipped with a Peabody

Oil Furnace]

With steam atomizing burners introduced through the front of the boiler

in stationary practice, it is usually in the direction in which the

furnace decreases in height and it is with such an arrangement that

difficulties through the loss of tubes may be expected. With such an

arrangement, the flame may impinge directly upon the tube surfaces and

tube troubles from this source may arise, particularly where the feed

water has a tendency toward rapid scale formation. Such difficulties may

be the result of a blowpipe action on the part of the burner, the over

heating of the tube due to oil or scale within, or the actual erosion of

the metal by particles of oil improperly atomized. Such action need not

be anticipated, provided the oil is burned with a short flame. The

flames from mechanical atomizing burners have a less velocity of

projection than those from steam atomizing burners and if introduced

into the higher end of the furnace, should not lead to tube difficulties

provided they are properly located and operated. This class of burner

also will give the most satisfactory results if introduced so that the

flames travel in the direction of increase in furnace volume. This is

perhaps best exemplified by the very good results secured with

mechanical atomizing burners and Babcock & Wilcox marine boilers in

which, due to the fact that the boilers are fired from the low end, the

flames from burners introduced through the front are in this direction.

Operation of Burners--When burners are not in use, or when they are

being started up, care must be taken to prevent oil from flowing and

collecting on the floor of the furnace before it is ignited. In starting

a burner, the atomized fuel may be ignited by a burning wad of oil-soaked

waste held before it on an iron rod. To insure quick ignition, the steam

supply should be cut down. But little practice is required to become an

adept at lighting an oil fire. When ignition has taken place and the

furnace brought to an even heat, the steam should be cut down to the

minimum amount required for atomization. This amount can be determined

from the appearance of the flame. If sufficient steam is not supplied,

particles of burning oil will drop to the furnace floor, giving a

scintillating appearance to the flame. The steam valves should be opened

just sufficiently to overcome this scintillating action.

Air Supply--From the nature of the fuel and the method of burning, the

quantity of air for combustion may be minimized. As with other fuels,

when the amount of air admitted is the minimum which will completely

consume the oil, the results are the best. The excess or deficiency of

air can be judged by the appearance of the stack or by observing the

gases passing through the boiler settings. A perfectly clear stack

indicates excess air, whereas smoke indicates a deficiency. With

properly designed furnaces the best results are secured by running near

the smoking point with a slight haze in the gases. A slight variation in

the air supply will affect the furnace conditions in an oil burning

boiler more than the same variation where coal is used, and for this

reason it is of the utmost importance that flue gas analysis be made

frequently on oil-burning boilers. With the air for combustion properly

regulated by adjustment of any checkerwork or any other device which may

be used, and the dampers carefully set, the flue gas analysis should

show, for good furnace conditions, a percentage of CO_{2} between 13 and

14 per cent, with either no CO or but a trace.

In boiler plant operation it is difficult to regulate the steam supply

to the burners and the damper position to meet sudden and repeated

variations in the load. A device has been patented which automatically

regulates by means of the boiler pressure the pressure of the steam to

the burners, the oil to the burners and the position of the boiler

damper. Such a device has been shown to give good results in plant

operation where hand regulation is difficult at best, and in many

instances is unfortunately not even attempted.

Efficiency with Oil--As pointed out in enumerating the advantages of oil

fuel over coal, higher efficiencies are obtainable with the former. With

boilers of approximately 500 horse power equipped with properly designed

furnaces and burners, an efficiency of 83 per cent is possible or making

an allowance of 2 per cent for steam used by burners, a net efficiency

of 81 per cent. The conditions under which such efficiencies are to be

secured are distinctly test conditions in which careful operation is a

prime requisite. With furnace conditions that are not conductive to the

best combustion, this figure may be decreased by from 5 to 10 per cent.

In large properly designed plants, however, the first named efficiency

may be approached for uniform running conditions, the nearness to which

it is reached depending on the intelligence of the operating crew. It

must be remembered that the use of oil fuel presents to the careless

operator possibilities for wastefulness much greater than in plants

where coal is fired, and it therefore pays to go carefully into this

feature.

Table 48 gives some representative tests with oil fuel.