hour per square foot of surface, and the right ordinates are vapor produced, gallons per day per square foot of surface. This ratio of steam to vapor corresponds to 0.86 pound of vapor per pound of steam, a fair value for single-effect operation. The solid-line group of curves, diverging from the lefthand origin, are lines of constant heat-transfer coefficient and show the relation between coil pressure and capacity for various values of K. These curves were computed from the heat-transfer coefficient formula. The dotted lines, which extend horizontally till a coil pressure of 81 pounds is reached, and then converge at the right-hand origin, represent steam flow through three orifices and also the capacity cespeding to these steam flows. The horizontal section of the Linflow curves are computed from Napier's equation, and the curved portion, from the critical pressure of 1 pounds to 150 pounds, is computed from a thermodynamic formula. Orifice B is of such size as to discharge sufficient steam to produce a capacity of 60 gallons per day per square foot of surface. Orifice A is of such size as to produce a capacity of 120 gallons per day per square foot of surface. Orifice A and B arranged in parallel will produce 180 gallons per day per square foot of surface. For convenience orifice A is assumed to produce 100 per cent rating, though this is double the present Navy rating. Orifice B alone produces 50 per cent rating, and orifices A and B combined produce 150 per cent rating.

It must be clearly recognized that every condition of coil as regards scale produces a definite heat-transfer coefficient at which the evaporator must operate. With the constant-coil pressure method of operating, if the attempt is made to start with clean coils and a coil pressure of 50 pounds gage, the operation will be at the point A, on the curve K = 1,200, where the capacity would be 264 gallons per day per square foot of surface. With most evaporators violent priming would occur at this capacity, and in order to produce pure water the water level in the shell would usually be lowered till the ef

fective heat-transfer coefficient was low enough (due to the reduction in surface) to produce a safe capacity. Operation at this low level soon produces an enormous quantity of scale over the exposed portion of the coils. The operation then continues down a vertical line at constant pressure and decreasing capacity as the heat-transfer coefficient decreases to some point such as B, which is for the value of K 200. Some operators start the operation at a lower pressure, say 20 pounds, in which case the coils may be submerged and the start would be made at C, within the safe capacity of the evaporator. The operation then proceeds down a vertical line to some point D, when the pressure may be increased to say 66 pounds. The operation would continue at E, at the same value of K as existed for the lower pressure, and from E the capacity would again decrease as the operation proceeded at constant pressure and decreasing heat-transfer coefficient.

With an orifice installed, operation always starts at the intersection of the steam-flow curve for the orifice, and the heat-transfer coefficient curve of the coils. For example, with orifice A installed, and with clean coils, operation would start at the point R, and the coil pressure would be 15 pounds. Operation will then proceed in a horizontal line at a constant capacity, instead of in a vertical line at constant pressure. As the value of K decreases the capacity remains constant and the coil pressure increases, instead of the capacity falling off, as in the present method. With orifice A installed, the evaporator will operate at its rated capacity till a coil pressure of 81 pounds is reached at the point L, where the value of K is 400. At this point there is a choice of several things which may be done. The coils may be scaled and the operation resumed with clean coils. (This is the point at which the coils are scaled in ordinary operation.) Or, the operation may continue down along the steam-flow curve, at increasing coil pressures and only slightly decreasing capacities, to some point such as S, where the coil pressure is 110 pounds, K is 300,

and the capacity is reduced only 10 per cent below the rated capacity. If there is no objection for structural reasons to carrying full-line pressure in the coils and steam header, the by-pass valve may be opened wide, allowing the evaporator to operate with full-line pressure of 150 pounds on the coils. This will cause the operation to move up the line of K = 300, bringing the operation to the point T, at which the capacity is only 10 per cent above the rated capacity and where the operation will therefore be safe and without danger of priming. From T, the capacity will fall at constant coil pressure as the scale further accumulates. If the coil pressure may not be increased above 81 pounds, and it is desired to continue running when this pressure is reached, orifice A may be closed and orifice B opened. This will mean dropping back along the line of K 400 to the point M, where the coil pressure is only 27 pounds, and 50 per cent capacity is obtained. Operation may now continue at 50 per cent capacity along the line M N till the value of K is reduced to 200 at a coil pressure of 81 pounds.

=

When it is desired to force the evaporator orifices A and B may be operated in parallel at the start. This will result in starting at the point G with 150 per cent of rating and continuing to the point H where K=600. From this point the operation may be either continued along the steam-flow curve at increasing coil pressures and slightly decreasing capacities, or the operation may be immediately resumed at the rated capacity by closing off orifice B, and dropping back to the point J and continuing at rated capacity.

ARRANGEMENT OF ORIFICE INSTALLATION.

The proposed arrangement of the steam piping to the firsteffect coils for orifice operation is shown by the sketch, figure 14. The three-branch construction of standard fittings is substituted for the reducing valve at the coil entrance. The upper branch contains the orifice plate "A" for usual operation at

100 per cent capacity. The center branch contains the orifice plate "B" for operation at 50 per cent capacity, or at 150 per cent when used in parallel with A. The lower branch contains the by-pass valve C, with no orifice plate at the flanges. The gages shown before and after the orifices are necessary. Gage D shows if the full-line pressure is available, and the reading of this gage is an indication of the exact amount of steam flowing to the coils. Gage E shows the pressure in the coils, and the reading of this gage is an indication of the conditions in the shell as regards salinity, scale, etc. The relief valve is necessary, as usual, to guard against excessive pressures from any cause. The orifice plates are prepared by drilling a hole of the required diameter in a brass or Monel-metal plate. The inlet edge of the orifice may be slightly rounded.

SUMMARIZED DISCUSSION OF SALIENT POINTS OF ORIFICE

METHOD.

The entire subject may be summarized and any doubtful points concerning the improved-orifice method cleared up by a discussion of the following questions which naturally arise in connection with the installation and operation of the orifice method:

(a) What is the principle of the orifice method and how does it differ from the present method?

(b) How is the proper size of orifice for a given installation determined?

(c) What coil pressure should be expected upon starting operation with clean coils?

(d) What causes the coil pressure to increase, and how are the coil pressures decreased?

(e) At what rate do the temperature differences and coil pressures increase?

(f) What maximum coil pressure shall be carried, and when shall the operation be stopped for the removal of scale? (g) What method shall be used in removing scale so that the operation may be continued without opening up the shell?