2.4.1 Design / Operating principle
The operating principle of piston vacuum pumps is one of the oldest in the history of vacuum generation. Its principle is that of the classical positive-displacement pump. Otto von Guericke, the father of vacuum technology, used a pump incorporating this design for his experiments.
Figure 2.7:
Operating principle of a piston pump
Like diaphragm pumps, classical piston vacuum pumps are equipped with an inlet valve and an outlet valve. The arrangement of these valves produces a dead volume above the piston in the cylinder head, which limits the maximum compression ratio. Moreover, ultimate pressure is limited by the force that must be applied to open the inlet valve. These two disadvantages are avoided through the special design of the piston pump described below.
New material pairings enable operation without oil between the piston seals (4) and the cylinder wall. Since the entire cross section of the cylinder is formed as an outlet valve plate (5), the harmful space (dead volume) between the piston (2) and the cylinder head tends toward zero.
A crankshaft-driven connecting rod moves a piston up and down in a cylinder. The inlet flange (1) communicates with the swept volume via the intake holes (3) when the piston (2) is in its bottom-most position. As the piston moves upward, the inlet holes (3) close off again, and the incoming gas is compressed. After reaching the opening pressure, the valve plate (5) lifts and the gas flows to the inlet holes (3) of the second stage via the overflow channel (7) and the crankcase housing. The second seal (4) prevents the inlet channel from communicating with the crankcase during the compression stroke. The second stage operates in the same manner as the first, and discharges the gas to the atmosphere via the outlet channel (10) and the silencer (11).
Gas ballast air can be admitted to the crankcase via the gas ballast valve and the throttling port behind it in order to displace water vapor though the pump without condensation (see also 2.1.6, Gas ballast).
In the case of dry piston pumps, wear occurs on the piston seals during operation, particularly at high average piston speeds. Once the required inlet pressure is reached, seal wear can be significantly reduced by lowering the RPM.