The auxiliaries of the steam plant at a blast-furnace do not differ from those of any other steam plant of similar size and character, and for that reason it is not necessary to discuss most of them here, but the condenser is important enough to deserve some attention.

These for steam engines are of two types, the direct contact or jet condenser, and the surface condenser. Surface condensers are high in first cost and having an enormous number of packed joints are difficult of maintenance for continuous operation. Moreover, the water available at blast-furnaces is not always of such character as to enable them to be kept as clean as they should be for best results, and when foul they soon lose their efficiency and the vacuum drops. For this reason when reciprocating blowing engines are used the elevated syphon condenser, working on the counter-current principle, and with positive means for removing the air, is very generally preferred.

Fig. 113. Weiss condenser.

The first of these brought into use was the Weiss, introduced by the Southwark Machine Company, shown by Fig. 113. This is of the elevated type with a vertical tail pipe about 35 feet long, high enough, in other words, to carry out the condensing water and water of condensation without pumping, against any vacuum which the condenser may produce. The air is taken off from the condenser by a dry-air pump, G in the illustration, the condenser being designed to discharge the air after it has received the maximum cooling possible, so as to make its density at this low pressure as high as possible and enable the air pump to handle it to the best advantage. The air-pump engine drives by belt or gear drive a rotary pump, known as the Bibus pump, which supplies the water for the condenser. The Bibus pump is used on account of its high efficiency, but owing to its gearing, etc., this pump is not as well adapted to run submerged as is a plain centrifugal pump, and as condenser pumps in the nature of things are extremely likely to be submerged from time to time, I personally prefer the simpler if somewhat less efficient centrifugal machine which can run submerged indefinitely.

Several condensers of the same general style as the Weiss are now on the market, of which one of the best and most favorably known is the Helander (Fig. 114), designed by the late A. H. Helander, formerly chief engineer of the Mesta Machine Company, afterward vice-president of the William Tod Company. This condenser dispenses with the supporting tower used by the Weiss, the tail pipe of the condenser itself being made strong and stiff enough to support it. The air is taken off at the high point of the condenser body, as in the Weiss, different types of apparatus being used to pump it out.

In some installations a steam jet or modified exhauster has been used whose steam economy is not very much lower than that of a mechanical pump, which is low in first cost, and completely eliminates moving parts. In other cases a crank and flywheel vacuum pump is used as in the Weiss, Water is generally supplied by a centrifugal pump driven either by a motor or by a small direct-connected steam turbine.

In blowing engine plants one condenser is generally installed for several engines, and this system has much to commend it, at the same time if steam for many units is to be condensed, the distance through which the exhaust has to be carried to the condenser becomes great and the piping large and expensive, while the vacuum realized on the further engine is probably never as good by an appreciable amount as in the nearer engines, so that this system must not be carried to extremes, especially as the high cost of the exhaust piping in a large installation would go far to pay for additional condensers. Neither must it be forgotten that several units of this kind have the advantage of greater flexibility and more freedom from general interruption than a single large unit.

Fig. 114. Mesta Helander barometric condenser.

Fig. 115. Vertical cross-section of Helander condenser vessel.

For turbines the barometric condenser is sometimes used, but to get the best results these machines require vacua which measured as absolute pressures are only from a half to a third of those used with steam engines, therefore it is highly desirable to have the exhaust connection extremely short and very large so as to avoid both friction losses and leaks. For these reasons the condenser for the turbine is, in the best practice, placed immediately beneath it and in some cases forms the base upon which the turbine rests. For this service either a surface condenser is used or some sort of jet condenser with water-discharge, as well as dry-air pumps. The former is particularly desirable with turbines for the reason that their exhaust contains no oil and may be used for boiler feed without treatment, and for the high vacua desired in this service it has the advantage of not introducing any air dissolved in the condensing water, which is an important factor under these conditions.

As a result of these considerations new types of jet condensers have been invented and new designs of surface condensers brought out so that condensers as a whole have received a vast development in recent years. In relation to blowing power for furnaces, however, these conditions are not sufficiently important to justify more extensive treatment here.