the stack at the higher altitude should have the same frictional resistance as that used at sea-level. This new diameter is determined by multiplying the diameter obtained on the basis of sea-level assumption by the ratio r of barometric heights raised to the 25th power as above deduced.

An Example of Chimney Design at Altitude. Since it is now seen that the factor or ratio of sea-level pressure to the pressure at altitude enters as a first and a two-fifths power, a chart is herewith given by means of which this factor may be quickly raised to the power desired for altitudes up to 10,000 ft., without any reference to barometric pressures.

As an example, let us find the proper proportions of a chimney to amply provide for a 1000 boiler horsepower installation situated 8000 ft. above sea-level.

We have hitherto found that the proper dimensions at sealevel for such an installation are 66 in. in diameter for a height of 90 ft. Applying our rule set forth above, we find from the chart that r for 8000 ft. is 1.357. Hence the proper height is 122 ft. at this altitude, and since r raised to the 25th power is found from the chart to be 1.130 the proper diameter is 74.5 in.

CHAPTER XXX

ACTUAL DRAFT REQUIRED FOR FUEL OIL

For every kind of fuel and rate of combustion there is a certain draft with which the best general results are obtained. A comparatively light draft is best for burning bituminous coals and the amount to use increases as the percentage of volatile matter diminishes and the fixed carbon increases, being highest for the small sizes of anthracites. Numerous other factors such as the thickness of fires, the percentage of ash and the air spaces in the grates bear directly on this question of the draft best suited to a given combustion rate.

For fuel oil, the question of draft required is greatly simplified by the fact that the air does not have to be drawn in through a thick bed of fuel and there are no ashes or clinkers to further complicate the matter. The resistance offered to the entrance of air to the furnace is caused by the checkerwork furnace floor, and as the openings in the checkerwork can be altered at will, it is evident that the amount of draft required in the furnace will depend largely on the arrangement of checkerwork adopted.

For a furnace arrangement such as shown on page 158, in which the total net area of free air space amounts to 3 to 311⁄2 sq. in. per rated horsepower of the boiler, the draft required in the furnace amounts to the following, approximately:

The draft in the furnace is only a small proportion of the total draft that must be supplied by the chimney, for it is necessary to add to the furnace draft the draft loss caused by the friction of the gases in passing through the boilers, breechings and flues leading to the chimney.

DRAFT LOSSES IN STEAM POWER GENERATION

The loss of draft is greatest in boilers having the longest path of gases, the greatest velocity, and the greatest number of changes in direction of flow of gases. A boiler having a single pass with the hot gases entering at the bottom and leaving at the top has a minimum draft loss. In most designs of boilers, however, this arrangement cannot be adopted as the area of gas passage would be too large. This would result in the gases short circuiting, that is passing in a narrow stream from one corner to the other without coming in contact with all of the heating surface. To make the heating surface effective in absorbing heat from the gases it is therefore necessary to provide baffles in the boiler, which deflect the gases and cause them to travel back and forth until their temperature has been reduced as much as possible.

The arrangement of baffles is a feature of boiler design and need not be entered into here. It is well, however, to refer briefly to the general principle involved, namely, that the higher the velocity of gases traveling over the heating surface the greater will be the coefficient of heat transfer. Consequently it would seem that in order to insure maximum efficiency of the boiler there should be a large number of passages of small area, so as to insure high velocity to the gases. This is true up to certain limits, but unfortunately it is soon found that the additional loss of draft caused by increased friction and extra changes in direction of the gases makes the production of the required draft both difficult and expensive.

In the majority of water tube boilers the baffles are arranged for three passes, that is the gases are forced to travel the length or height of the boiler setting three times before reaching the stack. With this arrangement the areas of passes are such as to give the gases a velocity of 10 or 15 ft. per second when the boiler is operating at its rated capacity. By increasing the number of passes to four or five the velocity may be increased to 20 or 30 ft. per second. This results in a higher rate of heat transmission so that more heat is absorbed from the gases, reducing their temperature and resulting in less waste to the chimney.

To enable the number of passes in a boiler to be increased the chimney must be designed to suit the increased loss of draft that will occur. Thus in every case the actual draft loss should be determined as closely as possible, and the actual figures for the

particular case in hand used in designing the chimney. It is desirable in all cases to design the stack for a greater draft than is expected, for it is a simple matter to reduce the draft by closing in on the damper, whereas if the draft is insufficient nothing can be done to increase it. Again, it may be desired at some future time to increase the number of passes in the boiler, or otherwise modify the baffles in such a way as to require more draft. This would be impracticable unless the stack is large enough to produce a surplus of draft.

In order to give the reader some general ideas of computations involved in ascertaining draft losses assumed in design we shall now pass to a brief consideration of this problem.

Loss of Draft in Boilers.-The loss of draft through a boiler proper will depend upon its type and baffling, and will increase with the per cent. of rating at which it is run. For design purposes, it may be assumed that the loss through an oil fired boiler between the furnace and the damper will be 0.15 in. when it is run at its rating, 0.35 in. at 150 per cent. of its rating and 0.60 in. at 200 per cent. of its rating.

Loss in Flues and Turns. With circular steel flues of approximately the same size as the stack or when reduced proportionally to the volume of gases they are to handle, a convenient rule is to allow 0.1 in. draft loss per 100 ft. of flue length and 0.05 in for each right angle turn. These figures are also good for square or rectangular steel flues with areas sufficiently large to provide against excessive frictional loss. For losses in brick or concrete flues these figures should be doubled.

Thus the loss in draft flues and turns for an installation having a flue 100 ft. long and containing two right angle turns is

Total Available Draft Required. We are now enabled to compute the total available draft required for a boiler installation by summing up the separate components required for the furnace, for the boiler, for the flues and for the turns.

Thus, for an oil fired boiler to operate at 200 per cent. of its rated capacity, connected to a chimney through a flue 100 ft.

long and containing two right angle turns, we have the following:

Draft in furnace...

Draft loss in boiler.....

Draft loss in flue......

Draft required at base of chimney..

=

0.25 in.

0.60 in.

0.20 in.

1.05 in.

The size of chimney required to produce this draft may be determined by the method described in the last chapter. Thus, let us suppose we are considering an installation of 2500 h.p. At 200 per cent. of rating there will be 5000 h.p. actually developed, and the quantity of flue gas produced by the oil fires will be 5000 X 60 300,000 lb. per hour. From the diagram on page 258 we find that a chimney for this capacity should be 102 in. diameter. We may assume that at the capacity considered the average temperature of chimney gases will be 600°F. The diagram on page 258 gives the draft for a chimney 102 in. diameter and 100 ft. high with 300,000 lb. per hour of flue gas, 0.525 in. for 500°F., and 0.63 in. for 700°F. Interpolating we have a draft of 0.58 in. for a temperature of 600°F. Since the draft required is 1.05 in., the height of the chimney must be 1.05 100 X = 181 ft. This is the height of the chimney above 0.58 the point at which the flue enters. If the flue enters the chimney 14 ft. above the ground, then the total height of the chimney must be 195 ft.

Artificial Draft.-As we have seen draft in a stack is caused by difference in pressure between the gases inside and outside, resulting in a flow of air from the higher external pressure to the lower internal pressure. A similar difference in pressure, and consequent flow of air, may be produced by a fan or blower instead of by a chimney. When this is done we have what is known as Artificial Draft.

There are two forms of artificial draft known as Forced Draft and Induced Draft, the distinguishing feature between the two being the location of the fan in respect to the boiler.

In the case of Forced Draft the fan sucks air direct from the atmosphere and delivers it to the boiler, under pressure somewhat greater than that of the atmosphere. In the case of Induced Draft the fan is located between the boiler and the stack, sucks the gases of combustion out of the boiler and discharges them to the stack.