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4.3 Diaphragm vacuum pumps

4.3.1 Design / Operating principle

Diaphragm vacuum pumps are dry positive-displacement pumps. A crankshaft-driven connecting rod (4) moves the diaphragm (1) that is tensioned between the head cover (2) and the housing (3). The space between the head cover and the diaphragm forms the suction chamber (5). Diaphragm pumps require inlet valves and outlet valves (6) to achieve targeted gas displacement. Pressure-controlled shutter valves made of elastomer materials are used as valves. Since the suction chamber is hermetically sealed off from the drive by the diaphragm, the pump medium can neither be contaminated by oil nor can aggressive media corrode the mechanics. The dead volume between the outlet valve and the suction chamber results in a restricted compression ratio which means that with just one pumping stage it is only possible to achieve an ultimate pressure of approximately 70 hPa. Connecting multiple pumping stages in series makes it possible to attain an ultimate pressure of 0.5 hPa. Lower pressures cannot be achieved, as in this case there is no longer sufficient force to open the inlet valve. The principle of the diaphragm pump is particularly well suited for low pumping speeds of up to approximately 10 m3· h-1.
  • Diaphragm vacuum pump diagram

    Figure 4.5: Operating principle of a diaphragm vacuum pump

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4.3.2 Application

Their hydrocarbon-free suction chambers make diaphragm pumps particularly well suited as dry backing pumps for turbomolecular pumps with a Holweck stage. Even two-stage diaphragm pumps that can reach an ultimate pressure of approximately 5 hPa. This is sufficient for backing of pumps for Holweck turbopumps. The clean vacuum is particularly useful for analytical and R&D applications. Diaphragm pumps, too, do not displace water vapor without gas ballast. Even the low volumes of water vapor that desorb from the walls of high vacuum equipment can allow the ultimate pressure of a diaphragm pump to increase dramatically. However, some diaphragm pumps are equipped with a gas ballast valve that operates in accordance with a patented process. For this purpose, gas is admitted into the connection channel between the first and second stages of two-stage diaphragm pumps, and this is connected with the suction chamber of the first stage via a small hole.
    Diaphragm Pumps
    Model
    Pumping speed
    Ultimate pressure
    Applications
    MVP 003-2
    0.25 m³ · h-1
    ≤ 7.0 hPa
    Small turbopump pumping stations (ideal with HiPace 10 and HiPace 80), mobile analysis devices, MiniTest helium leak detector regeneration
    MVP 006-4
    0.25 m³ · h-1
    ≤ 2.0 hPa
    Small turbopump pumping stations (ideal with HiPace 10 and HiPace 80), mobile analysis devices
    MVP 015-2
    0.5 m³ · h-1
    ≤ 3.5 hPa
    Turbopump pumping stations, leak detectors, research laboratories, analytical and chemical applications
    MVP 015-4
    0.5 m³ · h-1
    ≤ 0.5 hPa
    MVP 030-3
    1.8 m³ · h-1
    ≤ 2.5 hPa
    MVP 040-2
    2.3 m³ · h-1
    ≤ 4.0 hPa
    MVP 070-3
    3.8 m³ · h-1
    ≤ 1.0 hPa
    MVP 070-3 C
    3.4 m³ · h-1
    ≤ 1.5 hPa
    Corrosive gas applications requiring a hydrocarbon-free vacuum

    Table 4.12: Diaphragm pump performance data

If greater volumes of moisture accumulate and diaphragm pumps without gas ballast are used, suitable separators or cooling traps must be connected upstream to prevent significant condensate formation in the pump. However, the ultimate pressure will nevertheless increase.

4.3.3 Portfolio overview

Diaphragm pumps from Pfeiffer Vacuum differ in terms of their ultimate pressure, pumping speed and their suitability for pumping corrosive gases. The pumping speeds of the pumps are between 3 and 160 l · min-1 (0.25 to 9.6 m3· h-1). Ultimate pressures of less than 4 hPa for two-stage pumps and less than 0.5 hPa for four-stage pumps can be attained. Their pumping speed and the attainable final pressure depend on the mains frequency.
Corrosive gas pump models with coated diaphragms and corrosion-resistant housings are available for pumping corrosive gases.
The designations for the pumps are selected in such a manner as to indicate the pumping speed in l · min-1 and the number of pumping stages. Corrosive gas pumps have the letter C as a suffix to the model designation.

4.3.4 Accessories

The following accessories are available for diaphragm pumps:
  • Screw-in flanges and flange adapters
  • Purge gas connection
  • Fore-vacuum relay boxes
  • Power supply packs for wall or standard rail mounting
  • Cable for connection to turbopump drive