Pressure container or metering pump?
The suitability of pressure container and metering pump for double lines (line combinations)
Pressure container delivery
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 like e.g. Spotflex® nozzle bar or paint guns. When the total discharge crosssection is changed, the discharged total volume also changes – however, not always as desired.
When opening a second application unit – e.g. when generating the line combinations shown in Fig. 1 – it requires double the material volume. This however doesn’t happen when using a pressure container as has been shown by the measuring results of a simple test (see fig.1 and 2).
The outflow volumes from only one opened Spotflex® nozzle bar, dependent on container pressure, are shown in column 1 and the values for two opened nozzle bars are shown in column 2.
With test n° 2 (fig. 2) there is a material volume of 10.7 kg/min available for the single line with a container pressure of 3 bar. The double material volume of 21.4 kg/min is necessary in the area of the double line (column 4). The actual result, however, was a volume of only 15.9 kg/min (column 2), which represents 25 % less than necessary.
A combination of lines would resemble the appearance shown in Fig. 3. In the area of the double line the agglomerates are visibly leaner, in the area of the single line they are more voluminous.
Explanation
With a higher flow rate, the resistance in the line system rises up to the intersection V (fig.1), which could only be compensated by increasing the air pressure.
As one can see from the few measuring values, 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.
Metering pump
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.
Interaction
The cause-effect correlation between pressure container and metering pump is fundamentally different:
- In the case of the pressure container method the dis- charged 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).
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