CHAPTER XVI.

CONDENSING APPLIANCES.

THE primary object of condensing the exhaust steam from an engine is to increase the range of temperature between which it works. By reducing the temperature, it lowers the pressure against which the steam is exhausted, replacing the pressure of the air with that of the saturation pressure of the steam. It has been found that the effect of the condenser is produced on that portion of the steam consumption which is not dependent on the power developed, ie. that used in overcoming the friction of the engine, cylinder condensation, radiation, etc. From this it is evident that the importance of condensing increases as the load on a given engine diminishes, the saving made by the condenser varying from, say, 18 per cent. at full load to nearly 60 per cent. at a very small load. From the saving due to the condenser must, of course, be deducted the cost of providing and working it.

There are other advantages attendant on the use of a condenser, which, though theoretically of less importance, are in practice as great. The condenser obviates the noise and vibration caused by atmospheric exhaust, and prevents the spray caused by the partial condensation of the exhaust steam when discharged into the air. The nuisance so produced is very great, and often gives rise to serious trouble.

With some types of condenser there is a substantial saving made in the consumption of water, since they allow of the same water being used over and over again for boiler feeding. Not only does this economise water, which in itself is a great gain where it has to be purchased from the town mains, but the water, being distilled, is pure and free from air, while the quantity of heat retained by it represents the saving of a substantial amount of fuel.

A condenser has, then, two main duties to fulfil it must cool the steam, and at the same time exclude air from the chamber into which it is exhausted. There are various kinds of condensers, but all fall broadly into two classes, viz.: (1) those in which the condensing water mixes with the steam to be condensed; and (2) those in which the admixture is not made, the action taking place through conducting walls.

The first-named class has the advantages of simplicity, low first cost, and convenience; but, unless the condensing water is suitable for boiler feed, it necessitates the wasting of the condensed water, and, in any case, the loss of the greater part of the heat. Also the vacuum attained is lower than with the other class.

The second group of condensers are much more expensive in first cost, in maintenance, and in running; but they give a higher vacuum, enable the condensed steam and the whole of the heat it contains to be utilised, and, incidentally, they afford a ready means of testing, when desired, the steam consumption of the engines to which they are attached.

The class in which the water is mixed with the steam comprises the Jet, the Ejector, and the Barometric.

The Jet condenser is one of the oldest, and is but rarely used for central station work. It comprises a chamber to which the exhaust is connected and into which water is introduced in the form of a fine spray. The water so injected, and whatever air may have been entrained in the steam, are removed by means of an air pump.

The Ejector condenser is one that has met with much favour for central station work, on account of its great simplicity, cheapness, compactness, and reliability.

The Ejector condenser is made in several forms, but all depend on the water being introduced as a solid cylindrical jet into the vacuum chamber connected with the exhaust. The requisite form is given to the water by means of a long cylindrical tube, trumpet-mouthed at both ends. The steam is introduced on to the external surface of the water by means of a number of oblique holes in the tube, and is condensed, imparting its momentum at the same time to the water. The air contained in the steam is removed with the condensed steam by the water. The water may be supplied under a certain head, or the whole of the momentum may be supplied by the steam, according to the circumstances under which the condenser is used.

For an irregular, or frequently varying, load, it is necessary that the water should have a head of not less than 15 feet; and this type of the ejector condenser, being the one best adapted to the needs of central stations, is the only one that need be described.

The condenser is of fixed capacity, which must be that necessary for condensing the steam used by the engines at full load; and, inasmuch as the head is capable of giving the water sufficient velocity without the help of the steam, it can be started before the engine, and so provide a vacuum at once. The vacuum will be thus practically independent of the load, but will fall slightly as that increases.

A fall of not less than 1 feet from the bottom of the condenser to the level of the discharge is required, and the discharge pipe must be taken

below the surface of the water. The exhaust pipe from the engine should have a gradual fall, and a non-return valve is fixed in this pipe as near the condenser as possible, the object being to prevent the risk of water being accidentally drawn into the cylinder.

The water may be supplied directly from a pump or from a tank into which the pump discharges. The latter is greatly to be preferred, as not only does it provide a reserve in the event of the pump failing, but it allows much of the air thrown by the pump to escape before the water enters the condenser. When centrifugal pumps are used this is especially

necessary.

These condensers are most valuable for moderate powers, up to, say, 500 H.P., or rather higher; but for large powers they are not so satisfactory. They require the water to be as cold as possible, a few degrees increase of temperature causing a substantial fall in the vacuum produced.

The Barometric condenser consists of a vacuum chamber into which the exhaust steam is introduced at the bottom; while the condensing water enters at the top, being pumped in by means of a circulating pump. The vacuum chamber has attached to its bottom a pipe not less than 34 feet long, and dipping below water at its base. This tube being taller than the column of water which the air will support, the mixture of steam and water falls down it by gravity. The air entrained in the steam rises to the top of the condenser, and is removed therefrom by means of an air-pump, which need not be of large size, since, unlike that used with the jet condenser, it has only air, and not water, to remove.

The Author has had no experience of this condenser, but it would appear that it is simple, and should not be costly to work, since, once started, the pump has merely to lift the water through the difference in height between the top of the condenser and the barometric height of the water, i.e., only some ten feet. It is obviously rather less convenient than the ejector condenser, and it requires the addition of the air-pump. It is claimed that it requires less power to work than any other type of condenser; that for a given vacuum the temperature is higher, and less condensing water is required; that there is no possibility of water entering the cylinder; that the vacuum attained is higher than with other jet condensers; and that the air being moved from the coldest part necessitates the handling of a smaller volume of air by the pump; and, lastly, that the capital cost is low.

Turning now to the second class, we find that it includes the Surface, the Evaporative, and the Dry Air condensers.

The Surface condenser comprises a chamber into which the steam is exhausted, and through which a number of thin brass tubes pass. A circulating pump keeps up the flow of water through these tubes, while an air-pump connected with the chamber removes the condensed steam, together with any air that may have come over with it from the boilers or have leaked

into the condenser. The water discharged by the air-pump has a temperature of from 110° F. to 130° F., and the excess of this above that of the atmosphere is a measure of the heat returned to the boilers.

In some cases, the steam passes through the tubes and the water round them, the principle, however, remaining the same.

Inasmuch as heat has to pass through the substance of the tubes, there must be a difference of temperature between their walls; and the condensed water is, therefore, about 20° F. above the temperature of the condensing water, so that more water will be required to effect the condensation.

The Evaporative condenser is a surface condenser, in which the tubes are very greatly increased in size; instead of being immersed in water, they are exposed to the air, while a comparatively very small quantity of water is allowed to trickle over the surface of the tubes, and to dissipate the heat by its evaporation.

While retaining the main advantages of the ordinary surface condenser, this condenser is open to several serious drawbacks, notably, the enormous space required, and the great distance from the engine at which it has in consequence to be placed, the long pipes entailing great risk of leakage. The steam arising from the evaporation is very objectionable in many situations, and is nearly as troublesome as if the engines were working non-condensing. The only case in which such a condenser would be considered is that of dearness or scarcity of condensing water, and even then its use does not appear to have been attended with much

success.

The Dry Air condenser is one that has been proposed, though it has been but little used, if at all. It is in effect an evaporative condenser without any water. The surfaces are very much greater, and radiation and convection currents in the air are relied upon to dissipate the heat. On the whole, it would probably be better to make no pretence of condensing than sink money in such a contrivance.

When engines are worked non-condensing, it is a common practice to utilise a portion of the waste heat to warm the feed water. Although not condensers, properly so-called, feed-water heaters may conveniently be referred to here. They are, in effect, surface condensers, having a cooling surface very much less than is required to effect condensation of the whole amount of steam, the surface being only from 1 to 2 square feet for every 30 lbs. of feed water pumped per hour. No air-pumps are provided, while the circulating pumps are replaced by the boiler feed pumps. This class of apparatus, although giving little or no vacuum, effects an immense saving of heat, and, when economisers are not used, serves the useful purpose of relieving the boilers from the stresses caused by cold feed, besides depriving the water of much of its hardness. The heater may either be

placed on the suction or the delivery side of the pump. In the latter case, it will, of course, be under pressure, in the former not. Heaters, when used with small auxiliary engines, do something to mitigate their low efficiency, and they are especially to be recommended for use with steam feed pumps.

Having briefly examined the chief kinds of condensers, we may now consider their relative advantages, together with their general arrangements.

The first point to determine is whether each engine should have its own condenser, or whether one condenser should be common to a number of engines. This necessarily depends to some extent on the size of engine, and on the type of condenser. Unless the engines be very small, undoubtedly a separate condenser to each gives the best results, since, by this means, the length of the passage from the cylinder to the condenser is reduced to a minimum, and therewith the risk of leakage. If ejector or jet condensers be employed, there is no difficulty about this, however small the engine, but with surface condensers the cost may be prohibitive. In the case of very large engines of, say, 3000 H.P. and upwards, it is wise, in the opinion of the Author, to subdivide the condenser into two halves, so that, should one set be disabled, it is still possible to obtain a very fair vacuum.

The next question is whether the circulating and air pumps (if the condensers be of a type to require the latter) shall be driven from the engine, or independently of it. This will depend in large measure on the speed of the engine. If this is greatly in excess of 100 revolutions per minute, it is not desirable to take power directly from the engines, since the pumps are best worked at a moderate speed; but for slow speed engines it is certainly cheaper in first cost, and more economical in steam consumption, to run, at all events the air-pump, directly from the main engine. There is, of course, the objection that the engine cannot have the advantage of the vacuum at starting, but this is not a sufficiently serious drawback to outweigh the advantages of this course. The circulating pumps in any case are probably best run

separately.

When separately driven, the pumps may be worked either by motors or by steam engines. There are many very compact arrangements of circulating and air pumps steam-driven, some arranged to work compound or even tripleexpansion, and themselves condensing, and, when this is the case, they are no doubt very economical; but, on the whole, the Author is of opinion that the simplest and most convenient course is to use centrifugal pumps, driven by motors, for the circulating water, and, if any way possible, to drive the airpumps from the main engine; for large engines, the circulating pumps should certainly be subdivided into two.

The cost, floor space, quantity of condensing water per lb. of steam condensed, cooling surface, power required for driving, and vacuum obtained,