N. Hawkins

Fig. 577.

CONDENSING APPARATUS.

A condenser is an apparatus, separate from the cylinder, in which exhaust steam is condensed by the action of cold water; condensation is the act or process of reducing, by depression of temperature or increase of pressure, etc., to another and denser form, as gas to the condition of a liquid or steam to water. There is an electrical device called “a condenser” which must not be confounded with the hydraulic apparatus of the same name; there is also an optical instrument designated by the same term, which belongs to still another division of practical science.

A vacuum is defined very properly as an empty space; a space in which there is neither steam, water or air—the absolute absence of everything. The condenser is the apparatus by which, through the cooling of the steam by means of cold water, a vacuum is obtained.

The steam after expelling the air from the condenser fills it with its own volume which is at atmospheric pressure nearly 1700 times that of the same weight of water.

Now when a vessel is filled with steam at atmospheric pressure, and this steam is cooled by external application of cold water, it will immediately give up its heat, which will pass off in the cooling water, and the steam will again appear in a liquid state, occupying only ¹/₁₇₀₀ part of its original volume.

But if the vessel be perfectly tight and none of the outside air can enter, the space in the vessel not occupied by the water contains nothing, as before stated. The air exerting a pressure of nearly 15 pounds to the square inch of the surface of the vessel tries to collapse it; now if we take a cylinder fitted with a piston and connect its closed end to this vessel by means of a pipe, the atmospheric pressure will push this piston down. The old low pressure engines were operated almost entirely upon this principle, the steam only served to push the piston up and exhaust the air from the cylinder.

In Fig. 578 is exhibited the effect of jets of water from a spray nozzle meeting a jet of steam; the latter instead of filling the space with steam is returned to its original condition of water and the space as shown becomes a vacuum.

Briefly stated condensation and the production of a vacuum may be used to advantage in the following ways:

1. By increasing the power without increasing the fuel consumption.

2. By saving fuel without reducing the output of power.

3. By saving the boiler feed water required in proportion to the saving of fuel.

4. By furnishing boiler feed water free from lime and other scaling impurities.

5. By preventing the noise of the escaping exhaust steam.

6. By permitting the boiler pressure to be lowered ten to twenty pounds without reducing the power or the economy of the engine.

The discovery of the advantages arising from the condensation of steam by direct contact with water was accidental.

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.

Condensers are classified into surface condensers and jet condensers, both again being divided into direct connected and indirect connected condensers.

The surface condenser (see Fig. 579) is mainly used in marine practice because it gives a better vacuum, and keeps the condensed steam separate from the cooling water; it consists of a vessel, of varied shapes, having a number of brass tubes passing from head to head. The ends of this vessel are closed by double heads, the tubes are expanded into the inner one on one end, while their other ends pass through stuffing-boxes in the other inner head.

The “admiralty” or rectangular surface condenser is represented in Fig. 579. This form occupies less floor space than the round shell, and is preferred upon steam yachts and small vessels.

Steam is condensed on its introduction at the top of the apparatus where it comes in contact with the cool surfaces of the tubes. Through these water is circulated by a centrifugal pump driven usually by a separate engine.

The water of condensation leaves the condenser at the bottom and is drawn off by the vacuum pump. The water from the circulating pump enters at the bottom right-hand end; following the direction indicated by the arrows, it flows through the lower half of the tubes towards the left whence it returns through the upper half of the tubes towards the right and escapes overboard through the water outlet pipe.

It will be observed that the coolest water encounters the lowest temperature of steam at the bottom, hence the best results are reached. There is also a baffle plate just above the upper row of tubes to compel a uniform distribution of exhaust steam among the tubes, as shown in the engraving.

These tubes are usually small—¹/₂ outside diameter—of brass and coated with tin inside and outside to prevent galvanic action which is liable to attack the brass tubes and cause them to corrode.

Fig. 581 shows an end view of the right-hand head of the surface condenser here described.

A single tube is shown in detail in Fig. 580. One end of the tube is drawn sufficiently thick to chase upon it deep screw threads, while a slot facilitates its removal by a screw-driving tool. The other end is packed and held in place by a screw gland, which is also provided with a slot. In this way the tube is firmly held in one head, and, though tightly fitted in the other, is free to move longitudinally under the influence of expansion or contraction, due to the varying heat.

In some cases engineers prefer the ordinary arrangement of screw glands at both ends of the tubes, with the usual wick packing.

The mechanism illustrated in Figs. 582 and 583 shows a combined condenser and feed-water heater. A compact and efficient method of heating the feed-water from the hot well is of great importance; this is the case in cold weather when the circulating water is at a low temperature.

The Volz apparatus is a combined condenser and feed-water heater; the shell or exhaust steam chamber contains a set of tubes, through which the feed-water passes, while the lower part contains the condensing tubes, both parts being in proper communication with their respective water chambers. The heater tubes being located immediately adjacent to the exhaust inlet, are exposed to the hottest steam, and the feed-water becomes nearly as high temperature as that of the vacuum. Pages 304 and 305 show the sectional and outside views. The enclosing shell containing the combined heater and condenser is a well ribbed cylindrical iron casting; free and independent access is provided to either set of tubes by removing corresponding heads.

The illustration, Fig. 584, is a longitudinal section of one side of the condenser pump, and also a section of the condenser cone, spray pipe, exhaust elbow and injection elbow. “A” is the exhaust to which is connected the pipe that conducts to the apparatus the steam or vapor that is to be condensed. The injection water is conveyed by a pipe attached to the injection opening at “B.” “C” is the spray pipe, and has, at its lower extremity, a number of vertical slits through which the injection water passes and spreads out into thin sheets.

The spray cone “D” scatters the water passing over it, and thus ensures a rapid intermixture with the steam. This spray cone is adjustable by means of a stem passing through a stuffing-box at the top of the condenser, and is operated by the handle “E.” The cone should be left far enough down to pass the quantity of water needed for condensation.

All regulation of the injection water must be done by an injection valve placed in the injection pipe at a convenient point.

The operation of this condensing apparatus is as follows: steam being admitted to the cylinders “K,” so as to set the pump in motion, a vacuum is formed in the condenser, the engine cylinder, the connecting exhaust pipe, and the injection pipe. This causes the injection water to enter through the injection pipe attached at “B” and spray pipe “C” into the condenser cone “F.” The main engine being started, the exhaust steam enters through the exhaust pipe at “A,” and, coming in contact with the cold water, is rapidly condensed. The velocity of the steam is communicated to the water, and the whole passes through the cone “F” into the pump “G” at a high velocity, carrying with it, in a comingled condition the air or uncondensable vapor which enters the condenser with the steam. The mingled air and water is discharged by the pump through the valves and pipe at “J” before sufficient time or space has been allowed for separation to occur.

The exhaust steam induction condenser is based upon the same principle heretofore explained under the section relating to injectors. See Fig. 585.

The exhaust steam enters through the nozzle, A. The injection water surrounds this nozzle and issues downward through the annular space between the nozzle and the main casting. The steam meeting the water is condensed, and by virtue of its weight and of the momentum which it has acquired in flowing into the vacuum the resulting water continues downward, its velocity being further increased, and the column solidified by the contraction of the nozzle shown. The air is in this way carried along with the water and it is impossible for it to get back against the rapidly flowing steam in the contracted neck. The condenser will lift its own water twenty feet or so. When water can be had under sufficient head to thus feed itself into the system, and the hot-well can at the same time be so situated as to drain itself, it makes a remarkably simple and efficient arrangement. In case the elevation is so great that a pump has to be used to force the injection, the pump has to do less work than the ordinary air pump, and its exhaust can be used to heat the feed water.

The Bulkley “Injector” condenser is shown in Fig. 586, arranged so that the condensing water is supplied by a pump. The condenser is connected to a vertical exhaust pipe from the engine, at a height of about 34 feet above the level of the “hot-well.” An air-tight discharge pipe extends from the condenser nearly to the bottom of the “hot-well,” as shown in the engraving.

The condenser is supplied by a pump as shown, or from a tank, or from a natural “head” of water; the action is continuous, the water being delivered into the “hot-well” below. The area of the contracted “neck” of the condenser is greater than that of the annular water inlet described above, and the height of the water column overcomes the pressure of the atmosphere without.

The supply pump delivers cool water only, and is therefore but one-third of the size of the air-pump. The pressure of the atmosphere elevates the water about 26 feet to the condenser.

The accompanying diagrams, Figs. 587 and 588, are worthy of study. They represent a condenser plant designed by the Schutte & Koerting Co., Philadelphia, and placed on steam-vessels plying on fresh water. In these drawings the parts are designed by descriptive lettering instead the ordinary way of reference figures; this adds to the convenience of the student in considering this novel application of the condenser-injector, the action of which is described in the following paragraphs.

For steamers plying on fresh water lakes, bays and rivers it is unnecessary to go to the expense of installing surface condensers such as are used in salt water; keel condensers, however, are used in both cases.

The keel condenser consists of two copper or brass pipes running parallel and close to the keel, one on each side united by a return bend at the stern post. The forward ends are connected, one to the exhaust pipe of the engine while the other end is attached to the suction of the air pump.

In other cases both forward ends are attached to the exhaust pipe of the steam engine while the water of condensation is drawn through a smaller pipe connected with the return bend at the stern post which is the lowest part of the keel condenser.

Fig. 587 is much used for vessels running in fresh water. The illustration is a two-thirds midship section of a vessel with pipe connections to the bilge—bottom injection—side injection into the centrifugal pump, thence upward through suction pipe into the ejector condenser where it meets and condenses the exhaust steam from the engine and so on through the discharge pipe overboard. The plan of piping with valves, drain pipes and heater are shown in Fig. 588.

In case of the failure of any of the details of this mechanism to perform their respective functions a free exhaust valve and pipe is provided which may be brought instantly into use. The discharge pipe has a “kink” in it to form a water seal, as represented with a plug underneath to drain in case of frost, or in laying up the vessel in winter. A pipe leads from globe valve (under discharge elbow) to feed pump for hot water.

Condensing Surface Required. In the early days of the surface condenser it was thought necessary to provide a cooling surface in the condenser equal to the heating surface in the boilers, the idea being that it would take as much surface to transfer the heat from a pound of steam to the cooling water and condense the steam as it would to transfer the heat from the hot gases to the water in the boiler and convert it into steam. The difference in temperature, too, between the hot gases and the water in the boiler is considerably greater than that between the steam in the condenser and the cooling water.

Steam, however, gives up its heat to a relatively cool surface much more readily than do the hot furnace gases, and the positively circulated cooling water takes up that heat and keeps the temperature of the surface down, while in a boiler the absorption depends in a great measure upon the ability of the water by natural circulation to get into contact with the surface and take up the heat by evaporization. It has been found, therefore, that a much smaller surface will suffice in a condenser than in the boilers which it serves.

The Wheeler Condenser and Engineering Company, who make a specialty of surface condensers, say that one square foot of cooling surface is usually allowed to each 10 pounds of steam to be condensed per hour, with the condensing water at a normal temperature not exceeding 75°. This figure seems to be generally used for average conditions. Special cases require special treatment.

For service in the tropics the cooling surface should be at least ten per cent. greater than this estimate. Where there is an abundance of circulating water the surface may be much less, as with a keel condenser, where 50 pounds of steam is sometimes condensed per hour per square foot of surface; or a water works engine, where all the water pumped is discharged through the condenser and not appreciably raised in temperature, probably condensing 20 to 40 pounds of steam per hour per square foot of surface.

Under the division of this volume devoted to “air and vacuum pumps,” much information has been given relating to the principles of the condensation of steam and also some illustrations of working machines. Still it may be well to say this, in addition, that—

All questions in regard to a vacuum become plain when we consider that the atmosphere itself exerts a pressure of nearly 15 pounds, and measure everything from an absolute zero, 15 pounds below the atmospheric pressure. We live at the bottom of an ocean of air. The winds are its currents; we can heat it, cool it, breathe and handle it, weigh it, and pump it as we would water. The depth of this atmospheric ocean cannot be determined as positively as could one of liquid, for the air is elastic and expands as the pressure decreases in the upper layers. Its depth is variously estimated at from 20 to 212 miles. We can, however, determine very simply how much pressure it exerts per square inch.