There is an application of the turbo blower to blowing engines of which no actual installation has as yet been made, but the possibilities of which have been figured out on the basis of actual practice, and specifications prepared for such an installation.

This is a combination with existing blowing engines of the turbo blower driven by a low-pressure turbine. The turbo blower is not designed to blow the full pressure required for the furnace, but only to do about one-half the work of compression, supplying the air under this partial pressure to the air cylinders on the existing blowing engines.

The advantage of this combination is that the turbine is a more efficient motor for pressures below atmospheric than the steam engine, particularly where high vacua are obtainable, and the turbo blower, while not more economical for low pressure than a good piston blower, is relatively more economical for low pressures of 6 or 7 lb. than it is for pressures of 15 lb. and upward, not because the turbo itself has a higher efficiency at the lower pressures, but because the friction of the large cylinders required for reciprocating blowers, unless they are most carefully designed, reduces the mechanical efficiency of that type of machine at low pressures, so that it is doubtful whether they are as economical below 6 lb. as are rotary blowers or perhaps turbo blowers, therefore the rotary unit as a whole is more economical than a reciprocating one for very low pressures. It is also much lower in first cost, and by combining the two types of machines and using each for the range of pressures for which it is best adapted, we obtain the highest efficiency with the lowest first cost. This is particularly true where existing installations are either uneconomical, unable to blow the pressure required, or unable to furnish the volume required.

By putting a turbine and condenser on the exhaust of a non-condensing engine we can recover a horse-power for about 30 to 35 lb. of steam, depending on the vacuum available. This power applied to a turbo will compress the quantity of air required to a pressure in ordinary cases of 5 to 7 lb. This increases the density of the air and therefore increases the capacity of the air cylinders of the reciprocating engines, and also obviously raises the pressure to which they can compress the air by at least the amount of this precompression.

The turbo blower will compress air to a pressure of 6 1/2 or 7 lb. in a single stage; therefore, in ordinary cases only a single-stage unit will be required in applications under this system.

Both the reciprocating engine and the turbo blower labor under certain disadvantages when used singly which are eliminated when they are used in combination, thus the turbo blower alone must have stages enough to blow the maximum power required, for instance, 30 lb., even though 15 only will be required 99 per cent. of the time. Similarly the air cylinder of the reciprocating engine must be large enough for the maximum volume of air to be handled, even though this be required only occasionally. The turbo blower, on the other hand, can supply any increase in quantity of air desired within reason, without any change in its construction, while the blowing engine can compress to any pressure within reason without any change in its construction.

By combining these two units we obtain the volume capacity of the one and the pressure capacity of the other without the need of excessive size in one case, or multiple stages in the other. This arrangement has also the great advantage of retaining the metering effect of the piston of reciprocating engines, which is certainly more positive than the systems of volume governing so far applied to the turbo blower.

In the combined unit the question of governing the turbo practically takes care of itself, since the quantity of steam exhausted by the reciprocating engine controls the speed of the turbo, and this steam must depend upon the volume of air blown and the pressure to which it is raised. If the blast pressure required increases, the tendency of the reciprocating engine is to slow down slightly and so slightly to decrease its discharge while the increased quantity of steam which it exhausts tends to increase the speed of the turbine and so to increase the pressure at which the turbo blower discharges, thus making up by an increase in density for any decrease in displacement of the reciprocating engine.

In the actual case for which specifications were prepared, there were three standard long cross-head blowing engines, two with good steam cylinders, the other with a poor steam cylinder, all non-condensing. The owners had obtained bids on a single low-pressure unit to take the exhaust from one of these engines and run condensing, letting one of the high-pressure engines run condensing with the other standing as a spare.

It was easy to show that on the basis of the guaranteed figures of the turbo blower manufacturers a single turbo blower could be installed which would supply the two good existing high-pressure engines with air compressed to 6 1/2 lb., and this increase in density, with a slight and entirely permissible increase in speed, would have made the weight of air discharged by these engines the same as that now discharged by all three. The exhaust from both of these would have gone into the low-pressure turbine, and from that into the condenser, and would have been sufficient with any ordinary vacuum to drive the turbo blower at a speed which would enable it to supply precompressed air to both reciprocating engines. The third engine would have stood as a spare. The air cylinder heads of the blowing engines would have required to be closed in so that they could receive this air under pressure. A large inward-opening check was to be provided in the pipe carrying the precompressed air so that if anything happened to the turbo blower it could instantly be shut down and the reciprocating engines would then get their air supply from the atmosphere. This would have given the furnace two-thirds wind until the third engine could be started up, which would be only a matter of a few minutes. Finally it was shown that this equipment could be installed complete for a considerably smaller sum than would be required for the low-pressure unit, and that its steam consumption would be smaller than that on the other plan by several thousand pounds per hour.

This possible installation is set forth here because of the advantages which I believe it to have for such cases. It may also be said that the installation above described, while not installed as yet, was not rejected by the owners on account of any defect in the system, but only because they decided not to install additional blowing power at that time.