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. | at Central | | | | | | Works: | | | | | | -----------------------------------------------------------------------------
Thousands of | 1,080,000 | 2,160,000 |3,240,000 | 4,320,000 |5,400,000 | Cubic Feet at 45 | | | | | | lbs. pressure | | | | | | at engines | | | | | | Deduction for | 17,928 | 70,927 | 154,429 | 267,529 | 409,346 | friction and | | | | | | leakage | | | | | | Estimated net | 1,062,072 | 2,089,073 |3,085,571 | 4,052,471 |4,990,654 | delivery | | | | | | -----------------------------------------------------------------------------
CAPITAL | | | | | | EXPENDITURE-- | | | | | | Purchase and pre-| £12,500 | (amounts below apply to extension of works) | paration of land | | | | | | 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 | Parlimentary and | | | | | | general expenses,| 20,000 | .. | .. | .. | .. | royalty, etc. | | | | | | 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 | -----------------------------------------------------------------------------
ANNUAL CHARGES-- | | | | | | Salaries, wages, | | | | | | & general working| £6,405 | £7,855 | £9,305 | £10,955 | £12,480 | expenses | | | | | | Repairs, renewals| 2,780 | 5,198 | 7,622 | 10,045 | 12,467 | etc.(reserve fund)| | | | | | Coal, water, etc. | 1,950 | 3,900 | 5,850 | 7,800 | 9,750 | Rates | 370 | 674 | 980 | 1,285 | 1,585 | Contingencies of | | | | | | horse power = 5 | 575 | 881 | 1,187 | 1,504 | 1,814 | per cent on above| | | | | | Total Ann. Exp. | £12,080 | £18,508 | £24,944 | £31,589 | £38,096 | -----------------------------------------------------------------------------
Revenue at 5d. | | | | | | per 1000 cub. ft.| 22,126 | 43,522 | 64,282 | 84,426 | 103,971 | (average) | | | | | | 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 | -----------------------------------------------------------------------------

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, etc.

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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| Qty. Air | Cost/Hr. | Cost/Hr. | Cost/Yr. | Cost/Yr. |

| | | w/rebate | | w/rebate |

| 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|