If in order to overcome the difficulty of having the valves slammed hard against their seats by the reversed currents, a weight or spring was applied to them to force them to their seats, the pressure due to this weight or spring must be added to the difference between the internal and external pressures, with corresponding further loss of power. It is obvious that as pressures rose and speeds increased these difficulties must be augmented.

There were also other conditions of great importance to be met. First, the clearance should be kept small so that as nearly as possible all the air displaced by the piston may be driven out through the discharge valves instead of simply being compressed into the clearance space, and then re-expanding at the beginning of the next stroke, to the exclusion of air which should be taken in from outside.

Second, the area of the valve openings must obviously bear a definite ratio to the area and speed of the piston so that a valve area which is quite sufficient to permit relatively complete filling and ready discharge of a given cylinder at a low or moderate speed may be quite insufficient for the same cylinder at a higher speed. It must be remembered particularly in this connection that these losses are frictional and therefore vary as the square of the velocity, so that a cylinder which will fill with two ounces of suction at thirty revolutions will require one-half a pound at sixty, and a pound and one-eighth at ninety.

To meet as well as possible these conditions of low clearance and large area, a great multiplicity of valves, both for inlet and discharge, were adopted, and this was standard practice for many years, there being in some designs more than a hundred such valves in each end of an air cylinder of moderate size.

With increasing rate of driving came increasing pressure, and this brought about another difficult condition. It modified the shape of the indicator diagram for the air cylinder very much for the worse, from the point of view of the engine. With a blast pressure of four pounds the piston travels only about 20 per cent. of its stroke before the air is compressed to the required pressure and begins to flow through the discharge valves, so that with reasonably good design of valves, the work done upon the air cylinder was quite uniformly distributed throughout the stroke and subject to a minimum of variation. With a blast pressure of twenty pounds the piston travels more than 50 per cent. of its stroke against a pressure which rises constantly more rapidly, with the result that about three-fourths of all the work of the stroke is done in its last half.

On the other hand, the older types of uneconomical steam cylinders used but little expansion and so the steam pressure was but little lower at the end of the stroke than it was at the beginning, and the drop which took place was quite gradual with no sharp or sudden changes.

The demand for increased economy grew, as has been pointed out in the last chapter, and this could only be obtained by one means, much more complete expansion of the steam. This means that under ordinary conditions nearly one-half of the total work of the stroke must be developed in its first quarter, exactly the opposite to the way in which it is consumed, the greatest portion, as we have just seen, being expended in the latter portion of the stroke.

Some form of equalizer has to store the surplus work done in the steam cylinder in the early portion of the stroke, and give it out in the latter portion, and this equalizer is the inertia of the fly-wheel and of the reciprocating parts, but it is obvious that in a running machine which must have a certain amount of play in its moving parts a complete reversal of the stress from one direction to the opposite, somewhere near the middle of the stroke, could not contribute to its smooth and easy working.

It will be seen from the above that the conditions under which the blowing engine worked in the early days of the long cross-head type grew steadily worse with the development of modern furnace practice. As a consequence of these conditions it is probably safe to say that, next to the furnace itself, blowing engines have given more trouble and furnished more difficult problems to furnace builders and operators than any other portion of the equipment.

Air Valves

To meet the increasing difficulties to which changing conditions subjected the automatic types of valves, many designs of positively actuated valves were brought out, of which the test of time has left but few, and it is not possible in a single chapter to treat any except those which have had some considerable degree of industrial success.