This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
ANY system of heating hothouses which will at the present time mitigate or lessen the expenses connected therewith, ought to, and no doubt will, meet with every consideration while coal is so enormously enhanced in price. We have much pleasure in directing particular attention to a system combining the production of lime and the heating of some considerable extent of forcing-houses, which has for some time been in operation in the gardens at Drumore in Ireland, and which is the invention of Mr Cowan, the intelligent gardener there. This new system consists of a lime-kiln and an ordinary hot-water boiler combined, by which the usually wasted heat of the former is efficiently applied to the latter, of which fig. 12, kindly lent to us by Mr Cowan, will at once give our readers a clear idea. It represents a transverse and vertical section: a is the chamber of the kiln for lime-burning; b is a lining of fire-brick; c a backing of sand to conserve the heat; and / is the boiler placed over the mouth of the kiln to receive the heat generally wasted in the process of lime-making. The outside of the building may of course be stone or brick as most convenient. The two at work at Drumore are built with stone and faced with cement.
They are both inside the garden, and look quite as neat as any ordinary hot-water apparatus can possibly be. Mr Cowan of course admits that it would be more convenient to have them outside the gardens, were that possible; but there need not be anything unsightly connected with them in any position whatever. The patent does not bind Mr Cowan to the adoption of any form of boiler or any particular shape of kiln, the principle of the patent being the combination of the apparatus for heating horticultural or other buildings and making lime at one and the same time. Part of the kiln itself, or the whole of it, may be made of iron, and so turned into a large boiler when circumstances would warrant such an arrangement and any one can easily conceive what an immense heating power could thus be put in force. The working of this combination is said to be very simple, the chief point being the proper quantities of either limestone or chalk, and fuel with which to charge the kiln. The kiln is filled quite full before it is lighted. When once lighted it keeps burning until purposely let out. The cold lime is drawn out at the lower part of the kiln once or twice daily, and the space left at the top filled with alternate layers of fuel and limestone.
Mr Cowan finds that once during twenty-four hours is frequent enough for this operation, no further attention being required in the way of firing until the time comes for the second operation; and the time necessary to do the whole work connected with the kiln is just two hours of one man daily, including the breaking of the limestone, removing the lime, filling and putting everything right for the day. The fuel Mr Cowan recommends is anthracite coal or coke, from which there is next to no smoke; but of course any fuel which will burn lime in ordinary kilns will do for these. One of the apparatus at Drumore has been at work for more than seven months, and the other over three months. The first heats a vinery, a propagating house, and a plant-house. The other heats a range of vineries 200 feet long by 16 feet, and has 1000 feet of 4-inch pipe attached to the boiler; and both are ordinary saddle boilers, not by any means large or of the best stamp of that class of boilers. Mr Cowan has arranged for the manufacture of an improved boiler to take the place of such as he has made a beginning with - a model of which we have seen, and which must be powerful and conservative of heat when placed over the lime-kiln. But where the amount of heat required is not very great, the ordinary saddle does perfectly well, and is to be recommended on account of its cheapness.
When the kiln is required to be kept going and heat not required in the houses, if such should ever be a necessity, Mr Cowan makes provision for this by fixing a supply cistern very much larger than an ordinary one near the boiler, and connecting it with both the flow and return pipes, and by turning the valves the heat can be turned from the houses to this cistern without any danger to the apparatus.
With regard to the saving or compensating capabilities of the system, it is found that as much good lime is produced as pays the whole cost of fuel at Drumore, where coal costs 25s. per ton, and it just requires the two hours' labour of one man per day for the heating done. Of course the larger the apparatus the better it would pay; and in districts where coal is cheaper and lime dearer, it would tell much more in favour of the system. Mr Cowan just gets 5d per bushel for the lime.
These remarks, we hope, will give some idea of the system Mr Cowan has found to answer his purpose completely, where economy is of first-rate importance ; and surely it is well worthy a trial at the hands of those who can procure either chalk or limestone. Those who have seen the system in active operation at Drumore speak in the highest terms of its efficiency.