line indicating the location of the transit cross-hairs on the target with the ship afloat.

The errors noted were corrected by reboring struts and fitting new strut-bearing bushings.

In conclusion, it is stated that this method of optical alignment has been successfully used at this Yard for establishing the shaft lines on vessels under construction. If the coördinates of any two points along the shaft line are given-for instance, a point on a bulkhead and the location of the center of the stern-tube bearings-the location of all the other bearings along the shaft line can be determined, it being only necessary to set up the transit in line with the two given points, and, without changing the setting of the transit, focus it on any other point in line and set the centers of the spiders coincident with the cross-hairs of the transit. The center of the spiders then form a locating center from which to lay off points preliminary to boring.

Instead of using the target which is described above, when establishing a shaft center line, a special target (see Plate 5) or light-cross is used. As will be seen from Plates 5 and 6, this target is a bracket-like device on the back of which is mounted a parabolic reflector carrying a low candlepower electric light and on the front a cap. Through this cap are cut two 1/32" slots forming a cross through which the light from the reflector is projected onto the lenses of the transit. This target is mounted on a bulkhead with the center of the lightcross as one established point in the center line of the shaft. The cross-hairs of the transit may be very accurately located in the light-cross and the reflected light from the target (with the cap removed) allows of the work being carried on at night when the hull is of a uniform temperature and interference by other workmen on the ship is eliminated.

The shaft lines of Destroyer No. 70, Craven, were thus located, the time required for both shafts being about three hours. The alignment was later found to be very good.

The details and general arrangement were perfected by Mr. L. C. Campbell, Junior Inspector of Naval Construction, to whose interest and initiative the successful development of this method of shaft alignment is largely due.

AN IMPROVED METHOD OF OPERATING
EVAPORATORS.

BY M. C. STUART,* ASSOCIATE.

Marine Engineers will agree that the evaporating plant has the unenviable reputation of requiring more attention and causing more trouble than any other of the ship's auxiliaries. The determination of the proper coil pressure, water level and other conditions for the production of a desired capacity forms a source of continual speculation. Priming, reduction in capacity and the rapid accumulation of heavy masses of scale are the accompaniments of the most careful operation. Recent improvements in design of shell and coil leave untouched the many problems and difficulties of operation.

A new method of operating evaporators, developed at the U. S. Naval Engineering Experiment Station, Annapolis. Md., shows such remarkable and positive results in solving the questions and lessening the difficulties of operation as to warrant its careful consideration, and, it is believed, its adoption, by Engineer officers who are interested in maintaining the ship's fresh-water supply with the least possible effort.

The essential feature of the method is the production of fresh water at a constant rate and at any desired capacity within the limit of capacity of the evaporator. This result is obtained by the application of a steam orifice to the coils, which, by reason of the law of constant flow of steam, as expressed by Napier's formula, effects a constant production of vapor without the need of additional regulating appliances. In addition to the constant capacity feature, the use of the method reduces to a minimum the liability of priming, con

*Mechanical Engineer, U. S. Naval Engineering Experiment Station, Annapolis, Md.

siderably lessens the difficulty with scale, and stabilizes and simplifies the entire operation.

Before describing the method it will be desirable to review the process of evaporation, and discuss at some length the usual method of operation, in order that the principle of the new method and the manner in which it remedies some of the difficulties and defects of the usual method may be clearly understood.

In a

The production of fresh water from sea water is accomplished by the vaporization of sea water by means of the condensation of steam in coils which are surrounded by the sea water or brine contained in a shell. The condensed steam is drained from the coils and the vapor is condensed in a separate distiller condenser, usually called the distiller. multiple-effect plant the vapor produced is carried along to serve as steam in the coils of a second evaporator or effect. As evaporation continues, the water level of the brine in the shell must be maintained by the continuous introduction of sea water as feed. This increases the amount of salt in the brine and the accumulated salt must be removed, either by the continuous discharge of a small amount of brine (continuous · blow-down), or by the periodic discharge of the entire volume of brine when the salinity reaches a certain limiting value (intermittent blow-down).*

HEAT TRANSFER COEFFICIENT.

An unavoidable result of the process of evaporation is the gradual accumulation of scale on the brine side of the coils. This scale greatly affects the ability of the coil to transmit heat, and is one of the most troublesome features of evaporator operation. The ability of the coil to transmit heat is measured by the heat transfer coefficient, and inasmuch as the entire process of evaporation is so closely bound up in the

*For a complete discussion of blow-down, see "Salt Water Evaporators," by W. L. De Baufre, A. S. N. E. JOURNAL, Nov., 1915, page 946.

transmission of heat through the coil the heat transfer coefficient may be considered at some length with profit.

By definition, the heat transfer coefficient is equal to the B.t.u.'s transferred per hour per square foot of surface per degree temperature difference. This may be expressed by the formula,

W

=

Steam condensed in coils, pounds per hour.

H. Heat in steam entering coils, B.t.u. per pound. qa Heat in condensate from coils, B.t.u. per pound. S= Evaporating surface, square feet, measured on the outside of the coils.

T. D. = T ̧-T,, in which, T, is the temperature corresponding to the pressure of the steam in the coils, and T, is the temperature of the vapor corresponding to the vapor pressure in the shell.

The numerical value of the coefficient for any given set of operating conditions may be computed from the equation. Physically, the value of the coefficient depends upon a number of factors, which, in the probable order of their importance, are condition and amount of scale accumulated, salinity of brine, water level, design of coils, vapor pressure and coil pressure. In a In a given evaporator, values of K ranging from 1.400 to 100 are possible. One thousand two hundred is an average value when the coils are clean and with sea water in the shell, and 200 is a probable average value when the scale has accumulated on the coils to a considerable extent.

In the practical operation of an evaporator by the usual method a most important problem is the determination of the proper coil pressure for the production of a desired capacity. The temperature difference required for the production of any