water when having a speed equal to the piston speed, and that the valves must be large enough to permit the water to pass through them without its velocity being great-the maximum velocity of ordinary practice being 300 feet per minute. But as the height of the air pump centre above the bed plate has already been decided, and also its stroke, we see from the end view, Fig. 1976, that we could not get larger suction valves in without increasing the width of the condenser, nor must we have a less length of pump chamber than the air pump-stroke, as seen in Fig. 197a, if we are to let the pump displace a quantity of water equal to its own volume, nor, again, can we get in a sufficiently large delivery valve with a less width of hot well. It is such considerations as these which decide the sizes of this particular condenser. = = In order to find the air pump diameter knowing its stroke, wệ must know the volume of condensing water required per minute, and then obtain such a diameter, so that volume of pump × revolutions per minute volume of condensing water per minute; the volume of steam when condensed being so comparatively small that it may be neglected. The amount of condensing water required is found as follows:-Find the volume of steam used per minute at greatest speed volume of cylinder revolutions per minute. Reduce this volume to weight in lbs., by finding from tables of the Properties of Steam the volume of 1 lb. at the maximum terminal pressure. This terminal pressure must be calculated knowing the greatest cut-off, and the highest initial pressure, it being sufficiently near to assume that "pressure varies inversely with volume." Find also from the tables the total heat in 1 lb. of steam at the exhaust pressure from 0° F, which we will call T units. We then require to know the temperature of the condensing water, and the temperature of the hot well. The former of these is usually taken as 60° F. = t°, and the latter as 120° t1. Then if w lbs. of steam used per minute, W of condensing water required per minute, we know that = = lbs. Then from which we can find W, knowing w, T, t, and t. express W in cubic inches, knowing that 1 cubic foot of water weighs 62.5 lbs. Let this volume V cubic inches. We must now make an allowance for the fact that the pump does not displace an amount of water equal to its own volume per revolution, owing to what is called "slip." The usual rule for singleacting plunger pumps is to allow a pump efficiency of 0.5, so that we have 0.5 { volume of pump × revolutions per minute } that is d from which we can find the diameter d,; also to find area of suction or delivery valves we have— Area of value in sq. ins. x velocity of water in inches per minute = V, from which we can find the valve diameter knowing the area and the velocity of the water. This area is, of course, the arca through the grating (see § 139). The diameter of the inlet for the condensing water depends upon the height from which the water flows, and, therefore, upon its velocity through the pipe. But both the inlet and outlet should be designed so that the velocity of water passing through shall not be greater than 200 feet per minute. Hence, if d diameter of pipe, then If the student will work out these sizes for the engine of Ex. B, taking pump stroke 12′′, revolutions per minute 150, initial pressure of steam 98 lbs. per square inch absolute, cut-off ", condensing water 60° F., hot well water 120° F., velocity through valves 300 feet per minute, he will find that the results agree with the actual sizes. EXAMPLE. EX. B 8. Make working dimensioned drawings, half size, of the jet condenser for the horizontal engine of Ex. B, having the sizes given, and as in Fig. 197. Outside measurements 1' 10" long, 1' 3" wide, 1' 11" high, thickness of metal". Hot well 81" wide, 8" deep. Suction valve openings 43" diameter, centres 63" apart, and 4ğ" below air pump centre. Delivery valve opening 6g" diameter, centre 23" to one side of centre line of plunger. Faces for suction valves 63" from outside of condenser front. Plunger 33" diameter, thickness ", total length 1' 11," diameter outside barrel 64", length from face to face 7". Diameter of cast-iron spray pipe inside 1". Outlet from hot well 21" diameter; outlet to feed pump " diameter. Steam inlet 24" diameter; water inlet 3" diameter. Crossbar for suction valve 14" square; rod for delivery valve 14" diameter. Height of centre line of pump above base 9"; size of exhaust steam inlet 21". Holding-down bolts 1" diameter. Faces for machining to be provided at covers, stuffing box face, and for suction valve and pipe flanges. Make a detail drawing in another part of the paper of one of the suction valves. Rubber 44" diameter, " thick; guard 33" diameter (see § 139). (First draw in centre lines and the outside outline, then draw in plunger and barrel; next show thickness of metal and division plates; then draw in suction valves, delivery valves (in outline only), and inlet and outlets. Make the covers on the pump chamber and hot well as large as possible, and show the bolts. Draw in stuffing box for plunger (proportions as in Section xxiv.) and flanges for bolting down; width to allow for use of spanner on nuts. Finish details. Notice that the lugs for crossbar and upright rod must be open at one side, and that sides and ends of condenser come below bottom to give parts for machining. Work all views together.) SECTION XXXII. ENGINE FRAMES AND BED PLATES-GENERAL ARRANGEMENT DRAWINGS (212) Engine Framing. Having completed the drawings of the cylinder and its connected parts, it now remains to design the engine framing or bed plate. We start by knowing that the framing is required to support the different parts in as convenient a manner as possible, generally in as little space as is consistent with the requirements of a rigid base, and that the distribution of metal must be such that when the engine is working at its maximum speed and power, there shall be no lack of stiffness and strength. It is probable that engine framings are designed from proportions and forms suggested by the results of successful practical experience, rather than by theoretical considerations of the special stresses acting in certain parts, which stresses are in many cases almost beyond determination, so that we shall only approach it from the aspect indicated above. As a preliminary step, we must decide the length from the centre line of the cylinder to the crank-shaft centre. To do this we make a rough sketch of the parts when the piston is at the back end of the stroke, and the crank on one dead centre as in Fig. 178. We then see that the distance is equal to— Length of crank (or piston-stroke) + length of connecting-rod + distance from centre of crosshead to end of crosshead nearest cylinder + clearance between end of crosshead and gland studs + distance from end of gland studs to centre of cylinder.} All these separate sizes can be obtained from drawings previously made, except the clearance between crosshead and gland, and this we see must be determined by knowing how near the end of the slipper or guide block, if it extends beyond the crosshead may come to the cylinder, and also that until this distance is settled we cannot rightly fix the length of the pistonrod. In the vertical engine of Ex. A (Fig. 178) we see that the slipper cannot conveniently approach nearer the cylinder than the distance shown, owing to the position of the guides below the top flange of the standard, and also that in Ex. B (Fig. 179) the least distance is decided by the length required for the lugs on the cylinder end to which the guide bars are bolted, although in this case it will be found that a clearance between the crosshead and gland studs gives ample clearance for the guide block. It will be necessary then for the vertical engine of Ex. A to design and draw out the framing before the limiting position of the crosshead can be fixed. From the drawing of Fig. 178 it will be seen that the bottom framing for the crankshaft bearings, the back standard, and the top flange for the cylinder form one casting, the shape of which will be clearly seen from the views given. The bottom box section framing is raised at the sides to receive the shaft bearings CB, and to provide bosses for the front columns, it is also recessed under the crank as shown by the dotted line e, f, g, h, Fig. 178, B, to provide clearance for the crank and its attached parts. The back standard is known as the A pattern, rising from the bottom frame as two legs, which join at the line mn, and then continue as one piece up to the top flange, the shape of which is seen in the plan and which is also provided with bosses for the front columns. A section plan across the line ab is shown in Fig. 178, D, which will make the construction of that part clear, and which shows how the flat face p is left to form a surface for the slipper guide. In determining the height to the top of the flange from the shaft centre, the connecting-rod and crosshead with the slipper should be drawn in outline, showing merely limits of lengths, and then the clearance of about 43′′ allowed between the slipper and the top of the flange. Care should be taken that this gives sufficient clearance between the extreme top parts on the crosshead and the cylinder gland. EXAMPLES. EX. A 8.-Make working dimensioned drawings of the casting for the framing and standard of the vertical engine of Ex. A, as in Fig. 178, showing at least three views. Scale, 3" = 1 Height from base to centre of crank-shaft bearings, 83′′; thickness of metal,!"; of bottom and top flange, 11"; around bearing 11"; around front columns at bottom, 1"; bosses at top, 3" dia meter, 31" long. Columns, 13" diameter. Holding-down bolts, " diameter. Other sizes as in Fig. 178. = (Obtain the distance between the bearings centres from crank-shaft drawing, Ex. A 7, knowing crank pin, webs, and length of journals. Draw in circle for journal in Fig. 178, A, allow thickness for brasses (see § 115), and thus obtain height to top of crank bearing frame. The width of the frame should be less than total width across the brasses, by an amount equal to (twice the flange thickness on the brasses + allowance for facing for flanges), as these faces should be machined without touching the surrounding metal. The distance from the centre line through the piston-rod to the face for the slipper guide is found from the crosshead drawing, and also the length of face required, thus giving the position for the joining of the two legs at the line mn (least length of face length of slipper + stroke, less allowance for slipper to overlap guide plate at ends of stroke). See Fig. 187 for sizes of slipper-guide plates. The flange diameter at the top need only be about 1" larger than the flange on the cylinder which is bolted to it. The front columns, as seen in Fig. 178, B, are fixed so that the distance of their centres apart is equal to the distance between the centres of the bearings. The top ends are turned smaller in diameter and screwed into the bosses on the flange, and the bottom ends are fixed by round pins, about " diameter, slightly taper, driven through rod and boss. Flange width at base to suit holding-down bolts. The least distance from the engine centre to the legs of the A frame, where they leave the bottom framing, is settled by the length of the bearing cap, which should, therefore, be drawn in position as a guide.) = 5" EX. B 9.-Make working dimensioned drawings, three views, of the bed plate for the horizontal engine of Ex. B, as in Fig. 179. Scale, 3" 1'. Total depth 4", 1" less at condenser end to allow for extra depth of condenser below engine centre. Flange, 11′′ thick; thickness of metal, 5′′; seven cross ribs, " thick. Holding-down bolts, 12-11" diameter, passing through each corner of plate, and through ribs which are enlarged by bosses where they join sides of plate, to allow of bolt holes. Width over all at condenser part, 2'; at cylinder part and for rest of length, 2′ 73′′ Distance from shaft centre to end 16". Bed plate to be recessed under crank for clearance, the recess to continue for a less width, and decreasing in depth, forming a groove 4" wide from the crank recess up to the cylinder face, and under the glands of the cylinder and steam chest, to form a dish for the oil. Facings for planing " thick to be provided under condenser, cylinder, guide standard, valve-rod guide, and pedestals for shaft. Distance between centre of cylinder and centre of condenser, 3′ 10′′. Other sizes as in Fig. 179. (The design of this bed plate should present no difficulty, but it will be necessary to refer to previous drawings. The best way of proceeding will |