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the regulating field rheostat should be automatically short circuited on starting and automatically cut in circuit to a predetermined setting when the armature current reduces to a safe

amount.

Conclusion.

It would appear from the description in the foregoing paragraphs of the number of types and principles available for consideration in the starting and control of D. C. motors, that the selection of the most suitable combination for satisfactory results is somewhat involved. Although there may possibly be more than one general type or principle which will give equal results for a specific application, nevertheless, it is apparent that considerable study of the characteristics involved is necessary in order that the purchaser may be conversant with the characteristics of the various types on the market. For Naval service, it is believed as a guide to engineers who deal with the specification, selection and application of starters and controllers, the following general features are essential in Naval application:

Manual operated starters for frequent duty, not to exceed 4 horsepower.

Automatic starters to be of the current limit principle.

Upon shut down of motor from overload or low voltage, the starter to require manual re-set.

Excessive peak currents should be avoided at any stage of starting.

The effect of high or low line voltage to a reasonable degree should be negligible.

Ship vibration and shock of gun fire should not result in the starting or stopping of the motor.

Protection from the effect of moisture and corrosion should be carried to a high degree.

FUEL OIL VISCOSITY.*

By LIEUT. COMDR. G. B. VROOM, U. S. NAVY, MEMBER.

A present want, and one that will be felt increasingly as heavier and more viscous oils are marketed, is a simple and accurate means of determining, recording and interpreting the viscosity curves of fuel oils. As an operating problem, the handling of viscous fuel oil, with respect to its fluidity, must be considered not only as regards the supply system to the atomizers, but also its handling from the tanks, or bottoms, to the service tank from which the supply to the atomizers is taken. It has been found that preheating of the more viscous oils, in the storage tanks, is necessary in order that the pumps will take a suction, particularly when the temperature of the oil is below 50 degrees F.

The two definite parts of the problem are:

(a) Temperature to which oil in tanks or bottoms must be heated to permit ready handling.

(b) Temperature to which oil should be heated in the supply system to the atomizers.

There are many factors entering into the ultimate solution of these problems, about which little is known; these factors may be mechanical, in so far as they concern the design of the pumps, the fuel lines, the "hot oil recirculating" system, and the heaters; they may have to do with the characteristics of

The purpose of this discussion is to demonstrate to the operating engineer, a practical application of certain known facts in petroleum technology, as a ready means of solving an ever present operating problem: i.c.. having a certain grade of fuel oil in the bottoms, what temperatures are necessary to reduce it to the prescribed Engler viscosities? It is well recognized by the author that the solution demonstrated will not produce laboratory exactness; on the other hand such a degree of refinement could hardly be utilized under operating conditions.

The use of the logarithmic scale in no way complicates the practicability of the methods demonstrated; logarithmic cross-section paper is readily procurable and, for that matter, can be easily constructed to any desired unit scale, by methods described in "Kent" and similar hand-books.

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the oils procurable, and finally they may be collateral, such as allowable temperature limits in tanks contiguous to magazines; and increased fire hazards due to heating oils above their flash points with consequent release of highly inflammable vapors in tanks and piping.*.

It would seem that the temperature of preheating of fuel oils should, for the reasons above enumerated and also for reasons of economy, be controlled within definite limits depending upon the characteristics of the oil in use. Assuming that the viscosity required for greatest efficiency in handling is known within limits-present practice sets those limits between 75 and 150 seconds Saybolt Universal or 2 and 4 degrees Engler-the operating engineer must have a viscosity curve for the oil he is using. Preferably a series of curves should be available covering the several tanks and storage bottoms. This, it is believed, is necessary, since at each refueling, or transfer of oil from one tank to another, there is inevitably a mixing of oils of differing characteristics. This would be particularly the case when vessels fuel at different points. Even though suppliers furnished sufficient dataviscosities at two or more temperatures-with which to plot curves, the curves would be inaccurate immediately when two or more oils of different characteristics were mixed in the tanks. Since it appears highly desirable that users have accurate viscosity curves, and further that the only way such curves may be obtained readily and accurately is by the determination of viscosity data of the oil as it lies in the tanks, an investigation has been made of the feasibility of obtaining such data with sufficient accuracy for practical purposes without recourse to the use of expensive apparatus requiring considerable skill to operate. Referring to Figure 1, a viscosity curve is shown, plotted for viscosity (kinematic) versus temperature F. The 2-4 degrees Engler limits corresponding to the kinematic viscosity scale are shown in dotted lines. It is evident from this Figure that, for the given oil represented by the

* The Fire Hazards of Fuel Oil-"Marine Engineering and Shipping Age," May, 1925.

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