It is not right to call an air pump a vacuum pump, as the latter does not move air 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 barometer can 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 pump and 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 called jet condensers for this reason, and the cooling water is called the injection. All water that is used for condensing steam is therefore called the injection water.
When the exhaust steam strikes cool surfaces and is condensed by those surfaces, such condenser is called a 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 made 5⁄8″ 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 called the 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 pumps are generally used.
In the jet type of condenser where 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.