Steam:
Its Generation and Use

PRIMING OR WET STEAM. A fault, frequently met with in steam boilers is the carrying over of water mechanically mixed with the steam, which water not only carries away heat without any useful effect, but, when present in any marked quantity itself becomes a source of danger and of serious loss in the engine. This is a point frequently forgotten in designing boilers, par- ticularly sectional boilers. If steam rises from a sur- face of water faster than about 2 ft. 6 ins. to 3 ft. per second, it carries water with it in the form of spray, and when a fine spray is once formed in steam it does not readily settle against a rising current of very low velocity, as a cur- rent of i ft. per second will carry with it a globule of water of an inch in dia. Steam at 95 lbs. pressure Superheated 9 degrees. The common method of determining the per- centage of moisture in steam is described in the report of the test of Babcock & Wilcox boilers at the Raritan Woolen Mill, on a subsequent page. If experiments of this kind are not made with great care by experienced hands, and with instru- ments of the utmost ac- curacy, they are liable to such errors as will render them worthless. Fuller directions for this pur- pose, together with a statement of the difficul- ties in securing accuracy in such tests, will be found in the report of the Com. on Boiler Tests, in Vol. VI, of the Transactions of the Amer. Society of Mechanical Engineers. Another method, in which the heat required to evaporate the en- trained water, has been invented, and used with excellent results, by Geo. H. Barras, M. E. Steam at 55 lbs. pressure, with 1.94 per cent, moisture. Steam at 55 lbs. pressure. Boiler Foaming Violently. Steam at 55 lbs. pressure with 1.4 per cent, moisture. Prof. J. E. Denton has demonstrated that jets of steam escaping from an orifice in a boiler or steam reservoir show unmistakable change of appearance to the eye when the steam varies less than one per cent, from the condition of sat- uration either in the direc- tion of wetness or super- heating. Consequently if a jet of steam flow from a boiler into the atmosphere under circumstances such that very little loss of heat occurs through radiation, etc., and the jet be trans- parent close to the orifice, or be even a grayish white color, the steam may be assumed to be so nearly dry that no portable con- densing calorimeter will be capable of measuring the amount of water therein. If the jet be strongly white, the amount of water may be roughly judged up to about 2 per cent, but beyond this a calorimeter only can determine the exact amount of moisture. The cuts on this page were made direct by pho- Dry Steam at 95 lbs. pressure. tography from jets un- der conditions stated, and show very clearly the effect of dryness and slight moisture on such jets. With a little experience any one may determine by this method the conditions of steam within the above limits. A common brass pet cock may be used as an orifice, but it should, if possible, be set into the steam drum of the boiler and never be placed fur- ther away from the latter than four feet, and then only when the intermedi- ate reservoir or pipe is well covered, for a very short travel of dry steam through a naked pipe, will cause it to become perceptibly moist. Steam containing not more than 3 per cent, moisture may be termed commercially “dry.” * 74