226

THE COLLIERY GUARDIAN.

January 29, 1915.

reference to the figure of steam consumed in auxiliaries,
which he had mentioned in his paper, that was the
steam used for all purposes, and not only for turbine
auxiliaries. As to the use of refuse fuel at Is. 6d. a
ton, very many attempts had been made to burn such
fuel under boilers, but the results had not been
encouraging. He (Mr. Woodhouse) had come to. the
conclusion that if such fuel was to be used, it would be
in a gas producer, as suggested by Mr. Blake Walker.
Mr. Lucas’s suggestion for a joint colliery power scheme
was a recognition of the principle on which power com-
panies were founded, but, as the power companies
supplied not only collieries, but mills, steel works, and
all kinds of users, the benefits of diversity and high
load factor were much greater than could be the case if
only collieries were supplied.

The Chairman moved a vote of thanks to Mr.
Woodhouse for his paper, and this was heartily
accorded.

Col. T. W. H. Mitchell then vacated the chair, and
Mr. W. D. Lloyd presided in his stead.

A paper by Mr. J. Ivon Graham, B.A. (Cantab.), B.Sc.
(London), Tyndall Research Student of the Royal
Society, and Chemist at the Doncaster Coal Owners’
Research Laboratory, on “ The Absorption of Oxygen
by Coal,” was taken as read, and it was decided to
adjourn the discussion upon it. The paper is given
on p. 228.

The final business of the meeting was the reading by
Mr. E. A. Hailwood of what he termed “ A Reply to
Criticism on my Paper, read at the Midland Institute
of Mining Engineers, September 29, 1914, entitled
‘ Miners’ Electric Lamps Compared with the Combus-
tion Tube Lamp.’ ”

Mr. Hailwood’s contribution will be dealt with in
next week’s issue of the Colliery Guardian.

On the suggestion of the Chairman, it was agreed that
any contribution Mr. Maurice cared to make upon the
subject should, owing to the lateness of the hour, be
published in the institute’s Transactions.

The meeting then terminated.

Russia’s Fuel Problem : Pit Props for Coal.—At present,
when the importation of foreign coal via the Baltic Sea and
over the Russo-Prussian frontier has been stopped, says a
Petrograd paper, the question of supply becomes important.
It as expensive to rail coal from the Donetz basin. The
quantity of coal habitually imported was at least 300,000,000
poods a year. Less, of course, is required with reduced
intensity of industry. But the quantity lacking has to be
made good. The natural substitute in Russia is wood,
which, in any case, is the predominant fuel in the country.
The Council of the Congress of South Russian Metallurgists
has made the following statement comparing wood and
liquid fuel used with Donetz coal in poods :—

Kinds of fuel.

Wood.	Coal. Liquid fuel.

Household use... 3,312,000,000 ... 165,600,000 ...	—

Industries ......... 456,800,000	...	767,600.000	...	220,700,000

Railways ........... 109,000,000	...	380,500,000	...	126,400,000

River steamers ..	19,000,000	...	28,000,000	...	93,000,000

Now Russia exports annually enormous quantities of props,
etc. The quantity of props, etc., exported in 1913 was
45,561,000 poods, value 10,432,000 roubles. The exportation
of these goods has been interfered with in 1914. This,
added to the timber of other sorts that has had to be held
back, forms a large reserve of fuel that will more than make
up for the shortage in mineral fuel, and, says a semi-official
journal, steps should be taken to bring back from the ports
such export timber as is lying there, and use it for house-
hold fires and boiler furnaces, etc.

The Panama Canal and Coal Prices.—A correspondent of
the Birmingham Daily Post, referring to the opening of the
Panama Canal, states that a vast avenue of possibilities is
opened out for British shipping and for the coal, iron, steel,
and shipbuilding trades. One point which has received
little attention in this country relates to the comparative
prices at which coal can be supplied to vessels on the new
Panama and the old Suez routes. Where the distance
between certain world ports are about the same via Panama
as via Suez, the cost of coal will often be the consideration
that will decide which way vessels will take, and, of course,
whether they burn British or American coal. The price of
coal attains a new international significance. The difference,
for example, between the price of British coal at Port Said
and that of American coal at Panama will frequently decide
whether vessels trading to Japan, China, Australia, and New
Zealand go via 'the old canal and use our coal, or whether
they go via the new one and steam with American coal. This
.. problem of coal prices has provoked more speculation in the
United States than in our country, though we have the
most at stake. The American Government has long .since
had the whole problem of prices, stations, and supplies
thoroughly investigated. A few of the conclusions of the
investigators, together with other data on the subject, may
be summarised. Taking the 1912 contract prices—fixed, be
it observed, before we indulged in the luxury of a costly coal
strike and minimum wage legislation—it was shown that
British coal at Suez cost from 21s. a ton for Durham and
26s. for Welsh, and that beyond Suez the prices advanced
up to 35s. until Japanese competition was met, whereas New
River (U.S.A.) coal was obtainable for about 19s. at
Panama, and it was declared that this was only 5 per cent,
inferior to best Welsh. The probability was mentioned that
the United States Government might make its naval coaling
stations at both ends of the Panama Canal large enough to
supply merchant vessels, and that it might sell at or about
bare cost as an inducement to ships to use the new water-
way. This has since been confirmed. The best of coaling
accommodation is being provided, and it is anticipated that
bunkers can be filled at the Atlantic end of the canal for
about 18s. a ton, and at the Pacific end for 20s.

POWER COSTS.*

By WILLIAM B. WOODHOUSE.

Many papers have been presented to the institution
on the cost of steam, gas, and electric power for colliery
purposes ; and figures of performance for each particular
form of power have been claimed by their various
advocates, although these conflict seriously one with the
other. These differences appear to be due principally
to the absence of a definite basis of comparison. If a
comparison be made between different forms of power
under similar conditions, a just conclusion may be
arrived at for those conditions, but for those conditions
only; therefore,,unless the enquirer goes further, and *
considers the effect of variations of the conditions of
working, the choice made may be quite unsuited for the
actual conditions under which the power plant is to
work. It is consequently essential that the working
conditions should be known. When this is the case, it
may be said that the form of power to be preferred is
that which gives, under working conditions, a maximum
of economy, reliability, and convenience for a minimum
investment of capital.

The principal working conditions which affect the
cost of power are as follows:—(1) load factor;
(2) maximum power requirements; (3) reliability and
convenience; (4) consideration of future needs; and
(5) capital expenditure. All these conditions react one
on the other as well as on the cost, and the final
decision as to the best form of power must depend on
the accuracy with which the working conditions can be
estimated, as well as on the relative importance attached
to each of them.

Load Factor.

The load factor is a convenient measure of working
conditions, and is defined as the proportion that the
average rate of using power bears to the maximum rate;
or, if the power to be produced is electricity, and it is
borne in mind that 1 kw. used continuously throughout
the year represents a consumption of 8,760 units of
electricity, then the load factor may be expressed as—

The number of units used in the year
Maximum load in kilowatts x 8,760.

It is usually expressed as a percentage, and may be
referred to any period of time desired.

In practical working the amount of power required
varies from minute to minute and from day to day, and
the variation depends on the use to which the power is
put. The tendency of modern labour conditions is to
shorten the working hours of the greater part of the
colliery machinery so that the bulk of the power is used
in some 48 hours a week, or, allowing 50 working
weeks a year, in some 2,400 hours a year out of a total
of 8,760. It will be realised, therefore, that the load
factor of a colliery power plant as a whole is usually of
the order of from 20 to 30 per cent. There are many
cases of lower load factors on account of such influences
as holidays, labour disturbances, and variations of trade,
which are beyond the control of the colliery manager.
Some typical examples may be of interest.

Fig. 1 is a portion of a load curve of a colliery driven
electrically throughout, the use including winding,
ventilating, pumping, haulage, and coal cutting. The
load factor over a period of a week is 30 per cent.
Fig. 2 is a diagram showing the monthly variation of
power used in a colliery, and illustrating the effect of
trade, &c., mentioned above. Fig. 3 is a load curve of a
haulage plant for a period of two hours, in which it will
be noted that, although the maximum load frequently
exceeds 100 kw., the average is less than 25.

The general effect of the increased load factor is to
minimise the effect of capital and other standing
charges, and to increase the thermal efficiency of the
power plant—that is to say, to reduce the cost per unit
of every item of power cost.

Maximum Power Requirements.

The total power to be provided has an important
effect on capital cost as well as on fuel consumption,
wages, and other items of running cost; in addition,
there is a relationship between the load factor and the
total power which is greatly to the advantage of large
power plants—that is, the effect of diversity in the time
at which power is required for different uses.

Where the various machinery units about a colliery
are driven by separate engines, each engine must be
large enough for the maximum power that it has to
exert. The same rule applies if each machine is driven
by a separate electric motor; but if a large number of
motors are driven from one central power plant, it is
found that the maximum load on the power plant is
never as great as the individual motor powers added
together. There is a diversity in the use, which becomes
greater as the number of motors increases. As examples,
several cases are compared in Table I. :—

Table I.—Relationship Between Motors Installed and
the Maximum Load on the Generating Plant.

Colliery.	7 Use made of  power.	Slumber of motors.	Total horse- power.	Maximum load horse- power
A ...	Winding, pumping, and haulage 		10	...	1,986	...	951
B ...	Screening, pumping, haulage & winding	45	...	1,933	...	950
C ...	Pumping, coal-con- veying, & crushing	18	...	602	...	470
D ...	Screening & haulage	6	...	286	...	205
E ...	Pumping, haulage, and coal-cuttin g...	6	...	179	95

The effect of driving machinery from a central source
of power is twofold: the proportion that the maximum
load on the central station bears to the horse-power

* From a paper read before the Midland Institute of
Mining, Civil and Mechanical Engineers.

required to drive the machinery becomes less, and the
irregularities of load on the individual machines merge
together and produce a steadier load and better load
factor on the generating plant. Furthermore, an
increase in the size of steam plant has an important
effect on the efficiency of the plant. As a striking
example of this, fig. 4 shows how the steam consumption
per kilowatt hour of a turbine decreases as the size of
the machine is increased.

Reliability and Convenience.

Reliability will depend on the type of plant installed
on the number of units into which the total plant is
divided, and on the amount of stand-by provided. All
these will affect the capital expenditure, and the size of
engine or turbine adopted will have an important effect
on working costs. The proportion of spare or stand-
by plant boilers, engines, producers, &c., which is
considered necessary will depend on the special circum-
stances of the case. The greater the number of
machines, the smaller the proportion of spare plant
becomes, and therefore the smaller the idle capital
expenditure. On the other hand, the larger the
individual boilers or engines are the more efficient is the
plant, and therefore a large power station is indirectly
more economical than a small one. This and the
advantage of diversity of demand form the two main
advantages of public electric-power system.

Consideration of Future Needs.

The next consideration in settling the size of the
power plant is the necessity to provide for future needs.
These are always difficult to forecast, and necessitate
either the provision of large machines which must run
for a considerable time at very light loads, or a number
of small and relatively inefficient machines to which
additions are made from time to time. In either case
some sacrifice of efficiency must be made, and in
considering working costs this point must not be over-
looked, for there is one characteristic common to all
forms of power plant—the highest efficiency is only
obtained at or about the full load.

Capital Expenditure.

In considering the capital cost of plant, the investment
in the spare plant necessary for reliability and for
meeting future needs must be included; and if, as in
the case of gas engines, the machines possess no
appreciable overload capacity, the proportion of spare
plant and the capital cost must be increased accordingly.
The capital cost of power plant depends on the size of
machine installed, being less in proportion to the power
as the size is increased. For example, the cost of steam
turbines is some 68s. per kw. for a 500 kw. machine,
36s. for a 5,000 kw. machine, and as low as 28s. for a
30,000 kw. machine. In other words, the cost of a
30,000 kw. machine is practically the same as that of
24 machines of 500 kw., or, for the same investment,
the larger single machine gives two and a-half times as
much power.

The reduction of cost which accompanies an increase
in size holds with all types of machinery, with perhaps
the exception of the gas engine, which, on account of
the enormous stresses which have to be dealt with,
weighs more per horse-power' in the large sizes, and
therefore tends to become dearer.

In addition to the main items of buildings, boilers, or
producers, engines or turbines, condensing and pumping
plant, the cost of the auxiliary plant and connections
must not be overlooked. The cost of these is an
important part of the total, more particularly if the
number of engines, &c., he large. Taking the whole
power plant together, it is obvious that for a given total
power the capital cost is reduced to a minimum if one
unit of plant is used throughout.

Working Costs.

Bearing in mind the relative importance to be
attached to reliability, future needs, and capital
expenditure, and knowing approximately the maximum
load and the load factor to be expected, a comparison of
the working costs of different forms of power for any
particular case may then be made. The total cost of
power may be divided into (1) capital charges, (2) fuel
costs, (3) maintenance and repairs, (4) wages, and (5)
management, insurance, and general charges.

Capital Charges.

In considering the capital charges, it must be borne
in mind that the annual allowance for depreciation
should not only cover the gradual deterioration of the
plant with age, but, what is of greater importance,
should provide against obsolescence.

With regard to the rate of interest that is to be
allowed on capital, it should not be overlooked that
capital spent on directly productive work in any trade
will have a greater earning power than if spent on
indirectly productive purposes. The rate of interest
charged against the generating plant should therefore
be high, and this being so, it is obvious that generating
plant which is costly in the first place must show
substantial savings in working expenses to justify its
use.

Fuel Costs.

Fuel charges are the next most important item of
power costs, and in considering these it is essential to
have regard to the effect of reduced and variable loads
on the various types of plant.

Steam engines governed by throttling and steam
turbines have the same characteristic relationship
between steam consumption and load. The total steam
used at any load may be divided into two parts—(a) a
certain fixed amount per hour which is independent of