one direction and returned through another pass by another gang of men at the opposite end.

A variation of this method consists in the use of bristle brushes instead of the leather discs, using water under high pressure instead of compressed air. Still another company makes use of specially made steel plugs with a wire spring attached, which are claimed to give good results. The plugs are used in the same manner as the brushes described before and are driven by water under high pressure. A sketch of one of the plugs is attached.

FIG 48-CONDENSER CLEANING PLUG

The baking of condensers has been successfully done, principally where the circulating water is fresh and the small amount of leakage from the circulating water into the condenser is not prohibitive. This is effected by allowing steam to enter the condenser either through the prime mover or from the special lines connected to the bottom of the condenser, heating the entire condenser to a temperature of approximately 180 deg. F. and maintaining it at that temperature for a period of about two hours.

Previous to the heating, the hand-hole plates are removed to allow the circulation of air through the tubes and any heavy deposit is removed by washing with a high-pressure water line. With this method of treatment the accumulation on the inside of the tubes is dried so that it curls up and peels off, and is washed out when the circulating water is put through the tubes.

In cleaning a condenser in this manner it is essential that the tubes should be pitched or should be high at the center so that they will drain perfectly, as the time required for baking will thus be very materially reduced and the drying effect on the tube packing will be reduced.

Where it is necessary to clean condensers only once or twice each year, and it is feasible to remove the water box covers, sand blasting has been used with considerable success where the deposit is in the nature of a scale and cannot be removed by the use of brushes, plugs and the like.

FIG 49-INCREASE IN ABSOLUTE CONDENSER PRESSURE AND COAL COST DUE TO DIRTY CONDENSER 20 000-KW UNIT

This method, which has been described in the previous report, consists in driving a high pressure water jet through each tube and admitting sand to the jet so that the tubes will be scoured. Each tube is treated with a measured quantity of sand. Considerable care must be used, however, that the inner surface of the tubes is not eroded by the sand blast and considerably roughened or even cut through.

Another method is to push or pull a wire brush through

each individual tube, using a long rod. This, of course, takes considerable time, but there is no danger of damaging the tube, and when properly done it results in a perfectly clean condenser.

In one of the larger stations where a certain part of the equipment is much more efficient than the rest, and is consequently operated as continuously as possible and with a load factor which is nearly 90 percent, it was desired to determine just how often it was advisable to take the more efficient equipment out of service for condenser cleaning, carrying the load meanwhile on the less efficient equipment. It was estimated that six hours were required for cleaning the condenser in question, and that to carry the load of this machine on the less efficient equipment would cost approximately $100 in excess for this length of time. Readings were taken which showed the increase in back pressure on this condenser for each day under the particular season conditions, and another curve showed the cost per day, due to this increase in absolute pressure. The cumulative curve showing the cost to date due to loss in vacuum shows immediately at what point a saving could be made by taking this unit out of service.

From the accompanying curve it will be seen that under the conditions described this unit should not be run for a period longer than seven days with an increase of 0.3 inch in back pressure.

STEAM VELOCITIES IN PIPING AND FITTINGS

CONSEQUENT PRESSURE REDUCTIONS

The tendency of engineers and manufacturers engaged in designing high-pressure boilers, piping and generating equipment, has been steadily toward increased steam velocities, as the working pressure and temperature of the steam have been increased. This has probably been due to the fact that recent developments have shown steam generating equipment to be capable of capacities far in excess of what had been originally contemplated, and these increased capacities have resulted in certain conditions, one of which is increased steam velocity, which have thus in a sense changed the previous rules and ideas pertaining to this part of the industry, and have become standard practice. Where formerly steam velocities of 2000 to 3000-ft. per minute were commonly

met with, we now find velocities as high as 7500-ft. per minute, and certain authorities are advocating velocities in excess of 10 000-ft. per minute.

There is no question of the saving in the original cost of piping, fittings and coverings by increasing velocity, but it must not be forgotten that increased velocities through openings and in the piping mean increased pressure drop in every case, and this in turn means a marked decrease in the capacity of the prime mover. It may not necessarily mean loss of energy as the pressure drop may be compensated for by increased temperatures where the piping is properly insulated. Smaller steam piping means greater flexibility, resulting in decreased maintenance troubles. Higher velocity also results in more agitation in the steam line with better mixing, and prevents the deposit of water along the bottoms of headers, with resultant difficulty in maintaining tight joints.

The fact remains, however, that not enough consideration has been given to the subject of pressure drop in installing steam generating equipment, piping and valves. While there are arguments in favor of small piping and high velocities from the standpoint of investment, the fact should not be overlooked that with a fixed pressure at the prime mover the excessive pressure drop in the fittings and piping may result in such high pressure at the boiler that the cost at this point may be materially increased. Further than this, higher boiler pressures and consequently higher steam temperatures generally mean decreased boiler efficiency, assuming that the flue gases leave the boiler with the same temperature head difference between the gases and the steam temperature.

Beginning at the boiler, it is often the case that the openings in the dry pipes are so restricted that excessive pressure drops result as the steam leaves the drum. The claim is, of course, made that the dry pipes are so designed to dry the steam, but under the extreme conditions occasionally found this claim is not justifiable. Even in well designed equipment pressure drops exceeding 5 pounds are frequently found in the dry pipes when a boiler is operating at maximum capacity.

Superheaters also generally occasion considerable drop in pressure which is generally excused on the ground that the super heater is surrounded by hot gases, hence there can be no heat

loss in the superheater regardless of the pressure drop. Under ordinary operating conditions therefore, there is frequently a considerable pressure drop before the steam finally leaves the boiler, and this is directly chargeable to the manufacturers of boilers and superheaters.

When steam was generated for use in reciprocating engines it was necessary to select a non-return valve or automatic check valve small enough to cause such a drop in pressure and consequently sufficient lift that the pulsations of the steam would not. pound out the valve seats. With turbines requiring a uniform steam flow this trouble is not present and valves may safely be supplied with a corresponding smaller pressure drop.

The use of separators at the turbine end of the steam lines was another frequent source of pressure reduction, as this apparatus from its very purpose almost necessarily causes some decrease in pressure. The use of separators is probably

decreasing.

The strainer in the steam line as usually furnished by the manufacturer also deserves careful consideration. The mesh of the screen is frequently made smaller than actual conditions warrant, and where in addition to this, incrusting material is carried over with the steam and deposited on the screen, excessive pressure drops may result.

All of the losses occurring between the steam drum and the header increase, of course, with the quantity of steam being delivered, and at high boiler ratings the boiler drum pressure must be increased so as to maintain uniform pressure in the header. If this practice is followed the boiler feed line pressure must always be carried high enough to put water into the boilers at maximum rating. The greater the loss in pressure between the boiler drum and the steam header the greater the difficulty in regulating the supply of boiler feed water, since when the maximum quantity of feed water is required a minimum pressure difference exists, and vice versa.

Just how far it is advisable to go in the matter of steam velocities is at present a debatable point and is, of course, open to argument. The following curves are intended merely to give an idea of what may be expected in the way of decrease in pressure, using modern equipment, laid out according to conservative practice. The measuring apparatus used in these tests consisted.