Figure 1 shows a modern dairy homogenizer. As previously mentioned, the homogenizer consists of a pump and homogenizing valve. The pump is usually a reciprocating positive-displacement pump, which delivers a relatively constant flow rate despite the pressure or restriction to flow made by the homogenizing valve. The positive displacement pump has a power end and a liquid end. The power, or drive, end converts rotating motion to reciprocating motion. In most machines, an electric motor is connected to a drive shaft by V-belts and sheaves. The drive shaft turns an eccentric shaft by means of gears. In other cases, the motor is connected directly to the eccentric shaft by V-belts and sheaves.

The eccentric shaft has cams that drive the plungers or pistons by means of connecting rods and crossheads. The crosshead couples the connecting rod to the plungers or pistons. A piston and a plunger are both solid cylinders that displace fluid; however, for a piston the sealing elements are moving with the piston, and for a plunger the sealing elements are stationary. The sealing elements are called plunger packing (6).

The liquid end includes the pump chamber, or block, and all its components. Figure 2 shows a typical pumping chamber. The plungers move back and forth in the chamber. The eccentric shaft cams that drive the plungers are offset to provide a steady flow. If the pump has three plung-

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