COMPRESSED AIR POWER SCHEMES._Scientific American Supplement, No. 531, March 6, 1886

By J. STURGEON, Engineer of the Birmingham Compressed Air Power Company.

In the article on "Gas, Air, and Water Power" in the _Journal_ for Dec.

8 last, you state that you await with some curiosity my reply to certain

points in reference to the compressed air power schemes alluded to in

that article. I now, therefore, take the liberty of submitting to you

the arguments on my side of the question (which are substantially the

same as those I am submitting to Mr. Hewson, the Borough Engineer of

Leeds). The details and estimates for the Leeds scheme are not yet in a

forward enough state to enable me to give them at present; but the whole

case is sufficiently worked out for Birmingham to enable a fair

deduction to be made therefrom as regards the utility of the system in

other towns. In Birmingham, progress has been delayed owing to

difficulties in procuring a site for the works, and other matters of

detail. We have, however, recently succeeded in obtaining a suitable

place, and making arrangements for railway siding, water supply, etc.;

and we hope to be in a position to start early in the present year.

I inclose (1) a tabulated summary of the estimates for Birmingham

divided into stages of 3,000 gross indicated horse power at a time; (2)

a statement showing the cost to consumers in terms of indicated horse

power and in different modes, more or less economical, of applying the

air power in the consumers' engines; (3) a tracing showing the method of

laying the mains; (4) a tracing showing the method of collecting the

meter records at the central station, by means of electric apparatus,

and ascertaining the exact amount of leakage. A short description of the

two latter would be as well.

TABLE I.--_Showing the Progressive Development of the Compressed Air

System in stages of 3000 Indicated Horse Power (gross) at a Time, and

the Profits at each Stage_

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Gross | 3000 | 6000 | 9000 | 12,000 | 15,000 |

Indicated | Ind. | Ind. | Ind. | Ind. | Ind. |

Horse Power | H.P. | H.P. | H.P. | H.P. | H.P. |

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Thousands of | 1,080,000 | 2,160,000 |3,240,000 | 4,320,000 |5,400,000 |

Deduction for | 17,928 | 70,927 | 154,429 | 267,529 | 409,346 |

Estimated net | 1,062,072 | 2,089,073 |3,085,571 | 4,052,471 |4,990,654 |

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Purchase and pre-| £12,500 | (amounts below apply to extension of works) |

Machinery | 27,854 | £25,595 | £25,595 | £25,595 | £25,595 |

Mains | 10,328 | 10.328 | 10,328 | 10,328 | 10,328 |

Buildings | 8,505 | 4,516 | 4,632 | 4,614 | 4,594 |

general expenses,| 20,000 | .. | .. | .. | .. |

Engineering | 3,268 | 1,820 | 1,825 | 1,824 | 8,823 |

Previous Capit-| | 82,455 | 124,714 | 167,094 | 209,455 |

al Expenditure | .. | | | | |

Total Cap. Exp. | £82,455 | £124,714 | £167,094 | £209,455 | £251,795 |

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& general working| £6,405 | £7,855 | £9,305 | £10,955 | £12,480 |

Repairs, renewals| 2,780 | 5,198 | 7,622 | 10,045 | 12,467 |

Coal, water, &c. | 1,950 | 3,900 | 5,850 | 7,800 | 9,750 |

Rates | 370 | 674 | 980 | 1,285 | 1,585 |

horse power = 5 | 575 | 881 | 1,187 | 1,504 | 1,814 |

Total Ann. Exp. | £12,080 | £18,508 | £24,944 | £31,589 | £38,096 |

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per 1000 cub. ft.| 22,126 | 43,522 | 64,282 | 84,426 | 103,971 |

Profit |12.18 p.ct.|20.06 p.ct.|23.54 p.ct.|25.22 p.ct.|26.16 p.ct.|

|= 10,046 | = 25,014 | = 39,338 | = 52,837 | = 65,875 |

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TABLE II.--_Cost of Air Power in Terms of Indicated Horse Power_.

Abbreviated column headings:

Qty. Air: Quantity of Air at 45 lbs. Pressure required per Ind. H.P. per

Hour.

Cost/Hr.: Cost per Hour at 5d. per 1000 Cubic Feet.

Cost/Hr. w/rebate: Cost per Hour with Rebate when Profits reach 26 per

Cent.

Cost/Yr.: Cost per Annum (2700 Hours) at 5d. per 1000 Cubic Feet.

Cost/Yr. w/rebate: Cost per Annum with Rebate when Profits reach 26 per

Cent.

Abbreviated row headings:

CASE 1.--Where air at 45 lbs. pressure is re-heated to 320° Fahr., and

expanded to atmospheric pressure.

CASE 2.--Where air at 45 lbs. pressure is heated by boiling water to

212° Fahr., and expanded to atmospheric pressure.

CASE 3.--Where air is used expansively without re-heating, whereby

intensely cold air is exhausted, and may be used for ice making, &c.

CASE 4.--Where air is heated to 212° Fahr., and the terminal pressure is

11.3 lbs. above that of the atmosphere

CASE 5.--Where the air is used without heating, and cut off at one-third

of the stroke, as in ordinary slide-valve engines

CASE 6.--Where the air is used without re-heating and without expansion.

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| Cub. Ft. | d. | d. | £ s. d. | £ s. d.|

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CASE 1 | 125.4 | 0.627 | 0.596 | 7 1 1 | 6 14 0½|

CASE 2 | 140.4 | 0.702 | 0.667 | 7 17 11 | 7 10 0 |

CASE 3 | 178.2 | 0.891 | 0.847 | 10 0 5½ | 9 10 5½|

CASE 4 | 170.2 | 0.851 | 0.809 | 9 11 5½ | 9 1 10½|

CASE 5 | 258.0 | 1.290 | 1.226 | 14 10 3 | 13 15 9 |

CASE 6 | 331.8 | 1.659 | 1.576 | 18 13 3 | 17 14 7 |

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The great thing to guard against is leakage. If the pipes were simply

buried in the ground, it would be almost impossible to trace leakage, or

even to know of its existence. The income of the company might be

wasting away, and the loss never suspected until the quarterly returns

from the meters were obtained from the inspectors. Only then would it be

discovered that there must be a great leak (or it might be several

leaks) somewhere. But how would it be possible to trace them among 20 or

30 miles of buried pipes? We cannot break up the public streets. The

very existence of the concern depends upon (1) the _daily_ checking of

the meter returns, and comparison with the output from the air

compressors, so as to ascertain the amount of leakage; (2) facility for

tracing the locality of a leak; and (3) easy access to the mains with

the minimum of disturbance to the streets. It will be readily

understood, from the drawings, how this is effected. First, the pipes

are laid in concrete troughs, near the surface of the road, with

removable concrete covers strong enough to stand any overhead traffic.

At intervals there are junctions for service connections, with street

boxes and covers serving as inspection chambers. These chambers are also

provided over the ball-valves, which serve as stop-valves in case of

necessity, and are so arranged that in case of a serious breach in the

portion of main between any two of them, the rush of air to the breach

will blow them up to the corresponding seats and block off the broken

portion of main. The air space around the pipe in the concrete trough

will convey for a long distance the whistling noise of a leak; and the

inspectors, by listening at the inspection openings, will thus be

enabled to rapidly trace their way almost to the exact spot where there

is an escape. They have then only to remove the top surface of road

metal and the concrete cover in order to expose the pipe and get at the

breach. Leaks would mostly be found at joints; and, by measuring from

the nearest street opening, the inspectors would know where to break

open the road to arrive at the probable locality of the leak. A very

slight leak can be heard a long way off by its peculiar whistling sound.

[Illustration: COMPRESSED AIR POWER]

The next point is to obtain a daily report of the condition of the mains

and the amount of leakage. It would be impracticable to employ an army

of meter inspectors to take the records daily from all the meters in the

district. We therefore adopt the method of electric signaling shown in

the second drawing. In the engineer's office, at the central station, is

fixed the dial shown in Fig. 1. Each consumer's meter is fitted with the

contact-making apparatus shown in Pig. 4, and in an enlarged form in

Figs. 5 and 6, by which a current is sent round the electro-magnet, D

(Fig. 1), attracting the armature, and drawing the disk forward

sufficiently for the roller at I to pass over the center of one of the

pins, and so drop in between that and the next pin, thus completing the

motion, and holding the disk steadily opposite the figure. This action

takes place on any meter completing a unit of measurement of (say) 1,000

cubic feet, at which point the contact makers touch. But suppose one

meter should be moving very slowly, and so retaining contact for some

time, while other meters were working rapidly; the armature at D would

then be held up to the magnet by the prolonged contact maintained by the

slow moving meter, and so prevent the quick working meters from

actuating it; and they would therefore pass the contact points without

recording. A meter might also stop dead at the point of contact on

shutting off the air, and so hold up the armature; thus preventing

others from acting. To obviate this, we apply the disengaging apparatus

shown at L (Fig. 4). The contact maker works on the center, m, having an

armature on its opposite end. On contact being made, at the same time

that the magnet, D, is operated, the one at L is also operated,

attracting the armature, and throwing over the end of the contact maker,

l¹, on to the non-conducting side of the pin on the disk. Thus the whole

movement is rendered practically instantaneous, and the magnet at D is

set at liberty for the next operation. A resistance can be interposed at

L, if necessary, to regulate the period of the operation. The whole of

the meters work the common dial shown in Fig. 1, on which the gross

results only are recorded; and this is all we want to know in this way.

The action is so rapid, owing to the use of the magnetic disengaging

gear, that the chances of two or more meters making contact at the same

moment are rendered extremely small. Should such a thing happen, it

would not matter, as it is only approximate results that we require in

this case; and the error, if any, would add to the apparent amount of

leakage, and so be on the right side. Of course, the record of each

consumer's meter would be taken by the inspector at the end of every

quarter, in order to make out the bill; and the totals thus obtained

would be checked by the gross results indicated by the main dial. In

this way, by a comparison of these results, a coefficient would soon be

arrived at, by which the daily recorded results could be corrected to an

extremely accurate measurement. At the end of the working day, the

engineer has merely to take down from the dial in his office the total

record of air measured to the consumers, also the output of air from the

compressors, which he ascertains by means of a continuous counter on the

engines, and the difference between the two will represent the loss. If

the loss is trifling, he will pass it over; if serious, he will send out

his inspectors to trace it. Thus there could be no long continued

leakage, misuse, or robbery of the air, without the company becoming

aware of the fact, and so being enabled to take measures to stop or

prevent it. The foregoing are absolutely essential adjuncts to any

scheme of public motive power supply by compressed air, without which we

should be working in the dark, and could never be sure whether the

company were losing or making money. With them, we know where we are and

what we are doing.

Referring to the estimates given in Table I., I may explain that the

item of repairs and renewals covers 10 per cent. on boilers and gas

producers, 5 per cent. on engines, 5 per cent. on buildings, and 5 per

cent. on mains. Considering that the estimates include ample fitting

shops, with the best and most suitable tools, and that the wages list

includes a staff of men whose chief work would be to attend to repairs,

etc., I think the above allowances ample. Each item also includes 5 per

cent. for contingencies.

I have commenced by giving all the preceding detail, in order to show

the groundwork on which I base the estimate of the cost of compressed

air power to consumers, in terms of indicated horse power per annum, as

given in Table II. I may say that, in estimating the engine power and

coal consumption, I have not, as in the original report, made purely

theoretical calculations, but have taken diagrams from engines in actual

use (although of somewhat smaller size than those intended to be

employed), and have worked out the results therefrom. It will, I hope,

be seen that, with all the safeguards we have provided, we may fairly

reckon upon having for sale the stated quantity of air produced by means

of the plant, as estimated, and at the specified annual cost; and that

therefore the statement of cost per indicated horse power per annum may

be fairly relied upon. Thus the cost of compressed air to the consumer,

based upon an _average_ charge of 5d. per 1,000 cubic feet, will vary

from £6 14s. per indicated horse power per annum to £18 13s. 3d.,

according to circumstances and mode of application.

A compressed air motor is an exceedingly simple machine--much simpler

than an ordinary steam engine. But the air may also be used in an

ordinary steam engine; and in this case it can be much simplified in

many details. Very little packing is needed, as there is no nuisance

from gland leakage; the friction is therefore very slight. Pistons and

glands are packed with soapstone, or other self-lubricating packing; and

no oil is required except for bearings, etc. The company will undertake

the periodical inspection and overhauling of engines supplied with their

power, all which is included in the estimates. The total cost to

consumers, with air at an average of 5d. per 1,000 cubic feet, may

therefore be fairly taken as follows:

Min. Max.

Cost of air used £6 14 0½ £18 13 3

Oil. waste, packing, etc. 1 0 0 1 0 0

Interest, depreciation,

etc., 12½ per cent. on

£10, the cost of engine

per indicated

horse power 1 5 0 1 5 0

-------- ---------

£8 19 0½ £20 18 3

The maximum case would apply only to direct acting engines, such as

Tangye pumps, air power hammers, etc., where the air is full on till the

end of the stroke, and where there is no expansion. The minimum given is

at the average rate of 5d. per 1,000 cubic feet; but as there will be

rates below this, according to a sliding scale, we may fairly take it

that the lowest charge will fall considerably below £6 per indicated

horse power per annum.--_Journal of Gas Lighting_.

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