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Pumping liquids with side channel pumps

The earth is surrounded by an envelope of air which is subject to weather-related pressure fluctuations and has an average pressure of 1 bar (10 N/cm2) at sea level. All bodies on the earth's surface and thus all liquids are exposed to this atmospheric pressure.

If this atmospheric pressure is reduced by means of a technical device, the resulting pressure difference is capable of causing a column of liquid to rise to a certain height. This height is given in m Fl.S (metres of liquid column). In relation to water as a pumped medium with a density of 1000 kg/m3 , this means that at a 100 % vacuum (which is of course not technically feasible) the height of the water column would theoretically be 10 metres.

Good self-priming side channel pumps achieve a suction height of about 7 - 8 m, due to mechanical losses and leaks.

Operating principles of pumps

Pumps are divided into two superordinate types:

• non-self-priming positive displacement pumps (example centrifugal pump)
• self-priming hydraulic pumps (example piston pump)

Positive displacementpumps

A pump of this type consists of a casing containing a rotating impeller driven by an electric motor. However, before the pump is to be operated, the casing with the impeller must be filled with the medium to be pumped. This can be done by installing the pump geometrically lower than the liquid level of the medium to be pumped. Then the liquid would automatically fill the pump casing, even before the pump is put into operation.

If, due to other criteria, the pump has to be placed higher than the liquid level, a one-time filling of the pump suction side with the medium to be pumped is necessary before start-up.

When the pump is running, the liquid medium is pressed against the wall of the pump casing by the centrifugal forces created and escapes through the pressure outlet of the pump.

Self-priming hydraulic pumps

A self-priming pump is capable of venting the suction line to a certain degree during operation without any additional measures, thus initiating the delivery of the liquid medium. Self-priming hydraulic pumps are, for example, piston pumps. Here too, however, the suction capacity is limited by the inevitable air gaps between the piston and the cylinder wall. Especially when a piston pump is not in operation for a longer period of time, it is therefore advisable to first fill the suction side of the pump with the medium to be pumped. The liquid then acts as a kind of seal between the piston and the cylinder wall. This considerably increases the suction capacity.

Special features of side channel pumps

An impeller equipped with blades directs the medium to be pumped through gaps and openings into a side channel located in the casing. As the impeller rotates, the liquid passes through the impeller blades and the side channel several times per revolution. If the liquid is in the impeller, it must inevitably also assume its circumferential speed. If the liquid is directed into the side channel, its speed slows down considerably. This constant change in velocity in the impeller and in the side channel also changes the pressure in both the impeller and the side channel.

Physical background

In a flowing medium, its total energy is composed of velocity energy (kinetic energy), pressure energy and position or height energy. The sum of these three partial energies is constant at every point of the flowing medium. Mathematically, this law of nature is summarized in the energy equation:

W12/2 + g * h1 + P1/ρ = W22/2 + g * h2 + P2/ρ = constant.

In this energy equation (also called Bernoulli's equation), the

• specific velocity energy = W2/2 (in Nm/kg)
• specific height energy = g * h (in Nm/kg)
• specific pressure energy = P/ρ (in Nm/kg)

Conclusions from this energy consideration and findings for the side channel pump

If the velocity of the flowing medium increases, its pressure decreases at the same time because of the constancy of the total energy.
When the impeller is rotating, the flowing medium has a high velocity in the impeller because it must assume the circumferential velocity of the impeller. In the side channel, on the other hand, the velocity drops sharply. This means that the pressure of the flowing medium in the impeller is lower than in the side channel because of the higher velocity.

If the velocity of the flowing medium decreases, its pressure increases at the same time, also because of the constancy of the total energy.

Advantages of the side channel pump

Large delivery heads can be achieved. Pumping is possible without any problems even under unfavourable conditions. The side channel pump is particularly suitable for substances with a low boiling point, such as liquefied gas, which changes its state of aggregation from liquid to gaseous even at low pressure.
Side channel pumps are insensitive to the unpleasant phenomenon of cavitation.

Conclusion

The effect of velocity change of the flowing medium at the transition from the impeller to the side channel can be further increased by series connection. The arrangement of several impellers with downstream side channels improves the positive characteristics of side channel pumps.

Side channel pumps are suitable for high heads due to the higher pressure in the side channels.

Side channel pumps are preferably used for pumping media with a low boiling point, such as liquid gases. Their aggregate state changes from liquid to gaseous and vice versa do not cause any problems for side channel pumps.