198 DETAILS OF (PADDLE-WHEEL)
suspend these latter valves laterally in the ex-haust passage, rather than vertically as indi-cated. By such an arrangement of the detailsin question, less space can be occupied toproduce the required effect than when situatedas shown; and the door for access, instead ofbeing above the valves as represented, wouldbe opposite the same. Another cause forlaterally suspending the valves is, that thecloser these details are to the air pump themore effectual is the discharge from the same.This is a matter of recognised importance withthose who understand the subject under notice.
For a pair of air pumps and one condenser,for oscillating engines of 250 horse powernominal collectively, the practice has latelybeen as follows relative to the proportions :—
Ft. In.
Diameter of Connecting Kod . . .03
Diameter of Air Pump . . . 2 10
Diameter of Trunk.0 101
Length of Stroke.19
Area of Foot Valves (7 for 1 pump) . 276 sq. in.
Area of Piston Valves (7 for 1 pump) 241 ■ 5 sq. in.Area of Discharge Valve (8 for 1 pump) 276 sq. in.Diameter of Injection Pipe (2) . . .05
In some instances, Messrs. Penn and Messrs.Maudslay have put the discharge valve directlyover the air pump piston, allowing the trunk topass through the top valve seating, and studsor bolts passing through the top cover retainthe seating in question in its position. This,although objectionable as an arrangementtheoretically, is not found practically incon-venient, as the entire portion above the footvalves with small engines, below fifty horsepower nominal collectively, can be removedwithout a separate disconnection.
Notwithstanding these facts relating to smalltypes, and that doors are arranged to admit
access to the foot valves in larger examples,it is obvious that in either case an independentmeans of access to the separate portions is notavailable. Another question also presentsitself for due attention—the position of thecondenser relative to the air pump. It is seenthat the general practice, from the two ex-amples previously noticed, is to arrange thecondenser and pump on the same level inprinciple, and thus the water will not rise inthe pump—by gravitation—above that in thecondenser. The drainage of the condenser isonly admitted by the rising of the airpump piston, and should the latter not be inperfect order, a great loss of vacuum is theresult. The main cause for the universal adop-tion of the arrangements in question, combinedwith the knowledge of their several defects,is that the motion imparted to the pistons isfrom the crank shaft, and thus an almostvertical action imperative. The condenseralso, with large engines of great weight,must, or should be, as low in the hull ofthe vessel as practicable. The pumps are,as a rule, with little exception, single acting,and thus the foot valves are at the bottom ofthe condenser, at the base of the pump ; therecessed portion, directly below the barrel,admitting an accumulation of the condensedsteam and water under the valves in question.
On the piston rising, the foot valves open; thewater leaves the condenser and ascends in thepump, although the natural course of the fluidis to descend. The piston, when descending,closes the foot valves by causing a load onthem, due to the water in the pump; the weightis next relieved from the valves by the risingof the piston, and the final discharge effected.
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