AIR AND VACUUM PUMPS.

An air pumpis an apparatus for, 1, the exhaustion; 2, compression or transmission of air.

A vacuum pumpis an apparatus consisting of, 1, a chamber or barrel; 2, a suction pipe with a valve to prevent return flow; 3, a discharge pipe which has a valve which is closed when the chamber is emptied and, 4, a steam induction pipe provided with a valve that is opened when the chamber is filled with water and closed when the chamber is filled with steam.

It is not right to call an air pump a vacuum pump,as the latter does not moveair alone; it removes water, vapor and air from the condenser to form a vacuum. An air pump is designed to pump air alone.

A vacuum is a space entirely devoid of matter.That is, it is a space that contains nothing—no oxygen, no hydrogen, no air, no water, no pressure. It is for this reason that a perfect vacuum in practice is very difficult to obtain, especially as applied in a steam engine, as a liquid when in the presence of a vacuum generally gives off some vapor, owing to the fact that the surface is more or less in tension, besides its usual evaporative quality. Among all the liquids it has been found that mercury, on account of its very high specific gravity, can be best used to produce a vacuum and maintain it, and it is for this reason that the words “vacuum” and “inches of mercury” are synonymous.

Note.—The pressure of the atmosphere will also balance a column of water in a vacuum the same as a column of mercury but the height of the water column must necessarily be greater on account of the lesser weight of the water. A cubic inch of water weighs 13.6 times less than a cubic inch of mercury, so that the column of water which the atmosphere must balance must be 13.6 higher or 13.6 × 30 = 408 inches which is equivalent to 34 feet.A water barometercan be made in a similar manner to a mercury barometer except that instead of a tube slightly over 30 inches in length, a tube over 34 feet in height must be used. Advantage of this fact is taken in the so-called gravity condensers which require no air pump, the condensing apparatus being placed about 34 feet above the level of the hot well, the discharge pipe being sealed by always keeping its lower end below the level of the water in the hot well.

A water barometercan be made in a similar manner to a mercury barometer except that instead of a tube slightly over 30 inches in length, a tube over 34 feet in height must be used. Advantage of this fact is taken in the so-called gravity condensers which require no air pump, the condensing apparatus being placed about 34 feet above the level of the hot well, the discharge pipe being sealed by always keeping its lower end below the level of the water in the hot well.

The particular feature that makes steam valuable in producing a vacuum is the fact that when it is condensed, it decreases 1600 times in volume and except for this small quantity of water and some vapor which even cool water gives off in a vacuum, a perfect vacuum would be established and it is only necessary to draw off the condensed steam and vapor by proper apparatus to enable the vacuum to be maintained which the condensation has created. The apparatus for doing this is called the air pumpand the reservoir in which this condensation takes place is called the condenser.

The condensation of steam in the condenser is effected in two ways. The exhaust steam either meets in direct contact the water which is to condense it, or, the steam impinges upon cool metallic surfaces the temperature of which is kept down by circulating cool water through them. In the first case the condensed steam and the condensing water meet and mingle. The condenser is an iron pot or shell into which the steam is exhausted and the cooling water enters it in the form of a sheet or spray. Such condensers are calledjet condensersfor this reason, and the cooling water is calledthe injection. All water that is used for condensing steam is therefore calledthe injection water.

When the exhaust steam strikes cool surfaces and is condensed by those surfaces, such condenser is calleda surface condenser. The cooling surface is usually a series of pipes or tubes made of brass or copper to secure a rapid transfer of heat. These tubes are usually tinned inside and outside to prevent corrosion and in marine practice are made5⁄8″ in diameter. In most cases, condensation is effected by bringing the exhaust steam in contact with the outside of the tubes, the circulating water being inside.

In the surface condenser, as the circulation does not mingle with the condensed steam, the air pump has nothing to do with this water but is only required to pump out the condensed steam and air which enter the condenser; the pump which takes care of the circulation is calledthe circulating pump.When large quantities of water are used and the difference in level through which the water must be raised is slight as on board ship,centrifugal pumpsare generally used.

In the jet type of condenserwhere the water acts directly on the steam, the injection water will cause a lower temperature with less water and less apparatus than a surface condenser. The amount of injection water varies from 20 to 30 times the weight of steam to be condensed in cool seasons and from 30 to 35 times the amount in summer season. With fresh water this can be pumped into the boiler when the oil is extracted from it. It is for this reason that surface condensers are universally used for sea-going vessels to avoid salt water. They are also much used on land in places where the feed water contains mineral salts and is injurious to the boiler.

In places where the cost of hydrant water is excessive, it is of importance to use the same injection water over and over again, but this cannot be done until the water is first cooled. There are numerous methods by means of which this is done. All of these methods utilize the principle of scattering the injection water in the way best calculated to bring the greatest surface in contact with the largest quantity of air so that evaporation may take place quickly and effectively.

This is sometimes done by pumping the water through a number of spray nozzles up into the air, allowing it to fall into a lake or cold well below, or, as is more usually the case, the injection water is allowed to descend in a tower in a fine state of division over tiles or wire gauze or corrugated surfaces. A current of air, either forced by a fan or drawn up through it, causes a vaporization of the film of warm water pouring over the different surfaces, and the air cooling and the evaporation combined withdraw the heat from the water so that when it reaches the bottom it is in condition to be used again.

Fig. 344.

Fig. 345.

Cooling towers are used with either jet or surface condensers and can be used either with or without a fan, depending upon the design. In general these towers usually lower the temperature of the water from 120 degrees to 80 degrees, whichis sufficient to maintain a vacuum of about 26 inches. As they depend chiefly upon the results of evaporation to do the cooling, they work better on a dry day than when the air is humid.

The figures on the opposite page are designed to illustrate the use ofan air pump in connection with a jet condenser; this combination is properly called a vacuum pump because it not only pumps air, but water and vapor as well. Thesteam endof this apparatus is described in Part One, page 324, of this work.

The air and vacuum endhas a cylinder lined with composition-brass bored smooth; the piston has square rubber and canvas packing. The discharge—as shown in cut—is located sufficiently high, so that the cylinder retains a large portion of water. This forms a seal and causes the pump to work more advantageously than it would with air alone. A small pipe leads from the discharge chamber to the piston-rod stuffing-box. This contains a double packing and the water which flows through this small pipe forms a continuous seal around the piston-rod and thus prevents air from entering.

The injection water enters the elbow at the top and is drawn through an annular opening into the condenser. This opening may be regulated by the small hand wheel shown at the top end of the stem.

The exhaust from the steam end flows into the condenser through the pipe as may readily be observed—or escapes into the atmosphere by throwing the switch valve.

Note.—Utilizing hot discharge water.In manufacturing establishments where large quantities of water are required, advantage can be taken of the fact that in condensing apparatus of this and similar pumps, the water, after performing useful work in the condensing chamber, can be elevated to a tank in any portion of the building,and used over again for another purpose, such as washing, cooling metal plates, rolling-mill rolls, etc. The fact that the temperature of this discharge water will range from 100° to 120° will, in many cases, be advantageous, and effect a saving in the cost of heating other water for purposes in which this discharge water will answer equally well.When the water is not required in the tank, the stop-valve may be opened, and the water allowed to escape into a drain, or any other convenient place.

A ball-float attached to an air valve is located at the right hand of the condenser so that in case the pump should fail to operate from any cause, the injection water will lift the ball-float, which in turn will open the air valve and by discharging the vacuum will prevent the flooding of the engine cylinder with water.

It is a well-known fact that the atmosphere exerts what is usually termed “back pressure” of 14.7 pounds per square inch upon the piston area of a steam engine, also that water converted into steam, may be converted into its original state by condensation. Now, if this back pressure, which is, in reality, the weight of the surrounding atmosphere, be removed from the piston of a steam engine, the steam on the opposite side of the piston would have that much (14.7 lbs.) less work to do.

Applying this to steam engines means conveying the exhaust, or expanded steam, which would otherwise be allowed to escape into the open air, into a closed chamber, where it is met by a spray of cold water, which so rapidly absorbs the heat contained in the steam that it ceases to retain its gaseous form, and is again reduced to its original bulk as water. A great change has now taken place, and the steam is reduced to its liquid form. As this water of condensation only occupies about1⁄1600of the space filled by the steam from which it was formed, the remainder of the space is vacant, and no pressure exists.

The difference in volume accounts for the atmospheric pressure on the outside of the chamber, and as the vacuum extends throughout the whole distance which the exhaust steam originally occupied, it, of course, is made available in the cylinder of the engine in the shape of a decreased pressure on the exhaust side of the piston; the atmospheric pressure remains constant, therefore we have the atmospheric pressure acting on one side of the piston, and absent on the other; the gain being 14.7 pounds per square inch, if a perfect vacuum could be secured. It amounts in average engineering practice to from 12 to 13 pounds, or 24 to 26 inches of mercury, as the graduations usually read on vacuum gauges.

Jet and Surface Condensers are further described and illustrated in a special allotted section of this work. The vacuum pump is usually of the reciprocating order, although other methods have been employed for emptying condensers, but not with equally satisfactory results.

The gain to be secured by using a condensing apparatusmay be measured in two ways: first, by the decrease in fuel consumption over that necessary when running non-condensing, which will represent a constant decrease in running expenses; or, second, by the increase of power working quite up to its economical limit, in a non-condensing engine.

By the use of a condenser a further increase of power is realized in raising the mean effective pressure of steam within the engine cylinder without increasing the demand upon the boiler.

The application of a condenser to a steam engineincreases its economy from 20% to 25% depending upon circumstances, while by compounding and condensing an economy of 35% to 40% is effected.

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