FIG. 79. MODIFICATION OF RECIPROCATING PUMP MADE BY THE SANITARY DEVICES MANUFACTURING COMPANY.
Were this compressor used in connection with a system operating through 1¹⁄₄-in. hose and a vacuum of 8 in. mercury maintained, the efficiency would drop to 31%.
A modification of the reciprocating pump was manufactured by the Sanitary Devices Manufacturing Company in which light-weight poppet valves placed in the heads of the cylinder were used, as indicated in [Fig. 79]. Curves of the watts per cubic foot and efficiency of this type of compressor are shown in [Fig. 80]. It will be noted that this compressor shows a better efficiency than the air compressor at all degrees of vacuum and it is the best reciprocating pump that the writer has ever tested.
This pump was made for several years without water jacket and no trouble was ever experienced with overheating. However, owing to the commercial air compressors being jacketed, the makers using same made this a talking point and this company was obliged to jacket its pumps.
FIG. 80. POWER CONSUMPTION AND EFFICIENCY OF MODIFIED RECIPROCATING PUMP.
The Vacuum Cleaner Company used a Clayton pump on its smaller plants which was fitted with a semi-rotary valve in each end serving as an induction and eduction valve, while the heavy poppet eduction valve of the air compressor was dispensed with. The increase in efficiency that should have resulted from this change was not realized. The reason for this can be more readily seen by inspection of the indicator cards, [Figs. 81] and [82].
[Fig. 81] is a card taken from one of the Clayton compressors fitted with combined induction and eduction valves, and [Fig. 82] a card from a compressor with light steel induction and eduction valves of the poppet type.
It will be noted that the compression line, a-d, [Fig. 81], extends above the atmosphere line, the pressure at the time of opening the eduction valve being 4 lbs. per square inch above the atmosphere. This is due to the failure of the mechanically-operated valve to open soon enough. This valve being also the induction valve, it is necessary for the eduction port to be closed before the induction port can be opened, in order to prevent a short circuit of air from the atmosphere into the separators. This fact is responsible for the sudden increase in the pressure at b, the eduction port having closed before the completion of the stroke and the air in the clearance space being compressed to 6¹⁄₂ lbs. above atmosphere. The induction port is not opened until after the beginning of the suction stroke resulting in the high degree of vacuum at c.