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Pressurised container or metering pump?

The suitability of pressure container and metering pump for double lines (line combinations) will be described in the following:

The disadvantage of pressure container delivery is that the outflow volume is dependent on the changes of the total discharge cross-section of all connected application units.

When the total discharge cross-section is changed, the discharged total volume also changes - however, not always as desired. 

When opening a second application unit – for example when generating the line combinations  – it requires double the material volume. This however doesn’t happen automatically when using a pressure container.

With a higher flow rate, the resistance in the line system rises up to the intersection V, which could only be compensated by increasing the air pressure.

The required double volume would only be reached when the container pressure is 1 bar higher, that is at 4 bar.

When opening the spray header for the second line, the container air pressure would have to be increased by 1 bar within milliseconds and when closing it again, it would have to be lowered by 1 bar just as quickly. That this is not possible needs no further explanation.

Systems with HOFMANN metering pumps behave differently on principle. A pump is not simply a substitute for a pressure container because: 

The discharge flow is proportional to the speed and this independent of the viscosity and the viscosity changes of the material, as well as independent of pressure and changes of pressure.

When the speed is doubled abruptly, the discharge flow also doubles abruptly. 

The cause-effect correlation is fundamentally different: While in the case of the pressure container method the discharged volume of material is the result of air pressure, viscosity and resistances in the system, pressure, viscosity and resistance play no role whatsoever in the case of metering pumps. The material flow rate is servo-hydraulically controlled. 

The material flow rate can thus be adjusted to the required quantity within a few milliseconds (double, triple - halve, third).

Since 1976 HOFMANN has had a pump principle available which was specially developed for highly-abrasive discharge media, the absolutely wear-free bellow pump principle, which is even suitable for temperatures up to 250° C. Since 2007 these pumps have also been available for medium-sized marking machines and especially for highly-viscous plastics. The principle of servo-hydraulic control has been available since 2017.

The pump has no sliding parts which are exposed to the discharge medium and which could wear out and continuously impair the efficiency of the pump.

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