In the earliest construction of steam-engines the desired vacuum was produced by the circulation of water through a jacket around the cylinder. This was a slow and tedious process, the engine making only seven or eight strokes per minute. “An accidental unusual circumstance pointed out the remedy, and greatly increased the effect. As the engine was at work, the attendants were one day surprised to see it make several strokes much quicker than usual; and upon searching for the cause, they found, says Desaguliers, ‘a hole through the piston which let the cold water (kept upon the piston to prevent the entrance of air at the packing) into the space underneath.’ The water falling through the steam condensed it almost instantaneously, and produced a vacuum with far less water than when applied to the exterior of the cylinder. This led Newcomen to remove the outer cylinder, and to insert the lower end of the water pipe into the bottom of the cylinder, so that on opening a cock a jet of cold water was projected through the vapor. This beautiful device is the origin of the injection pipe with a spray nozzle still used in low-pressure engines.”
The apparatus described above is called the jet-condenser and is in use up to the present day in various forms. In the Fig. [577], page 298, the jet is shown at C. It will be understood that steam enters through the cock D and comes in contact with a spray of cold water at the bottom, where it is condensed and passes into the air pump through which it is discharged.
By this diagram, Fig. [577], may be understood in a simple yet accurate manner the course of steam from the time it leaves the boiler until it is discharged from the condenser.
Referring to the upper section of the plate, a sectional view of a steam cylinder, jet condenser, air pump and exhaust piping is shown. The high pressure steam “aa” is represented by dark shading, and the low pressure or expanded steam “bb” by lighter shading.
The steam enters the side “aa,” is cut off, and expansion takes place moving the piston in the direction of the arrow to the end of the stroke. The exhaust valve now opens and the piston starts to return. The low pressure steam instead of passing direct to the atmosphere, as is the case of a high pressure engine, flows into a chamber “C,” and is brought in contact with a spray of cold water. The heat being absorbed by the water, the steam is condensed and reduced in volume, thus forming a vacuum. It is, however, necessary to remove the water formed by the condensed steam together with the water admitted to condense the steam, also a small amount of air and vapor. For this purpose, a pump is required, which is called the air pump.
Fig. 579.
