There is no theoretical reason why one iron-pipe stove should not be sufficient for a furnace, because it is an apparatus which works continuously by the conduction of heat through metal walls exactly as a boiler does. Barring repairs it could run on indefinitely, and small furnaces in the early days were frequently run with only one stove.

As a matter of fact, especially at large furnaces, the number of stoves is increased for two reasons: First, it is difficult to build a single unit large enough to heat the quantity of blast required for a large or even a moderate-sized furnace, and at the same time to design it so as to give good control of the heat in all parts. Second, after a certain point is passed it costs no more to build the required stove capacity in two or three units than it does in one, while if there be, say, three stoves on a furnace, then, when one of them requires repairs the other two are still able to heat the blast to a very valuable extent; whereas, if only one stove be provided, when this goes down for repair the furnace must shut down because it is commercially impossible to run a furnace on cold blast.

For these reasons two or three iron-pipe stoves are commonly provided per furnace when that type is used.

In regard to the capacity and efficiency of these, little or nothing is known. As a matter of experience it can be stated that the stove illustrated in Figs. 126, 129 is capable of heating six or eight thousand feet of blast per minute up to 850° F. without such overcrowding as will destroy it too rapidly. In regard to economy and efficiency we have not even the guide of general experience to help us, because there is no way to get at the quantity of gas consumed by one of these stoves without careful experiments on gas flow, etc., which, as far as I know, have never been conducted.

It may be safely stated that the efficiency of the Durham type of stove, that is the parallel-current type, is quite low, because the chimney gases must be discharged at a temperature of twelve hundred or more, and even with theoretically perfect combustion this involves a loss of over 50 per cent. of the heating value of the gas.

Turning now to the firebrick stove, the conditions are entirely different from what they are in the case of the iron-pipe stove, since it is impossible to get along with less than two of these, one of which is being heated up while the other is giving up its heat to the blast. Furnaces were originally operated with only two stoves of this type but this condition has long since passed away. Less than three are never contemplated, four is only ordinary good practice, and five have been installed with extremely satisfactory results at some plants.

The reason for the increased number of stoves is twofold. First, to have one in reserve, or at least be in a position to spare one for repairs without crippling the plant; second, because the heating of the blast by this process is an intermittent operation, and therefore the temperature of the blast fluctuates with the condition of the stove. When the stove is first put on it has been heated up to a high temperature, which falls as it gives up its heat and the blast temperature is therefore constantly falling.

When only two stoves are provided these variations are so great as to cause serious irregularities in the work of the furnace. By adding a third stove and burning the gas so as to give them what steel men know as a "soaking heat," their temperature falls much less rapidly while they are on blast. The fact that the relative time on gas is doubled while the time on blast remains the same contributes much to this result. This greatly reduces the irregularities due to the intermittent action of the apparatus.

But, when one stove requires to be taken out of service for repair the furnace is then thrown back onto the two-stove condition. This cuts down the heating surface to two-thirds and reduces the average blast temperature materially as well as making serious irregularities in the temperature curve. Therefore, in order always to have the benefit of three stoves, four must be provided; this gives three hours on gas for one on blast, which is a step better than the condition existing when three stoves are used.

Four stoves introduce the possibility of operating along a somewhat different plan, with the object of eliminating inequalities of blast temperature still more. This plan is to blow through two stoves simultaneously, leaving them each on blast one hour, but changing them at alternate half hours. This reduces the time of the stove on gas to one-half the total time, but halves the amount of heat taken out of it at any period. It involves somewhat more work for the stove tender, but practically halves the variations of blast temperature from maximum to minimum.

Here, again, we come to the necessity of occasional repairs necessitating a return to three-stove conditions, and this has caused many operators to consider five stoves a good investment, because then four stoves can always be maintained in operation, even when one is being repaired, and when all five are available each one can be left on gas for three hours and on blast for two hours, with still better results in eliminating irregularities in the blast temperature and maintaining a high average temperature without driving the stove too hard when it is on gas.

It will be seen, then, that the number of firebrick stoves to be supplied per furnace depends to some extent upon natural conditions, which we cannot control, and that if the best results are to be attained under present conditions, not less than four must be supplied. But, since the introduction of clean gas at some plants, it has been found that the stoves do not need to come off during an entire blast for either cleaning or repair, and this fact in addition to the increased economy and greater heating surface attainable as a result of clean gas have a tendency to reduce the number of stoves required, and it may well be doubted if five stoves will in future be installed at plants originally designed to use clean gas. It may be that the number required will fall to three.