| The eductor |
Optimised according to the percentage of aspirated product, its viscosity and the duration of the operation, there are several techniques for proportionally mixing various liquids.
The simplest, least expensive and most used by firefighters uses the Venturi effect.
This effect is the consequence of a reduction in the flow section of the moving water which lowers the pressure until another liquid can be sucked in, then the premix passes through a regularly increased section which allows the pressure to rise again.
The difficulty lies in the fact that a change in the supply pressure of the unit changes the proportion of suction. To remedy this, some manufacturers (including POK®) insert a valve between the water inlet of the appliance and the inlet of the liquid to be sucked in, which regulates the suction percentage. In particular, this valve makes it possible to adjust the pressure loss (pressure drop) between the inlet and outlet of the appliance.
The POK proportioners are set for a pressure drop of 35%.
Other proportioners on the market have a pressure drop of around 38%.
All other units operate without a control valve and are called injectors. They all have a pressure drop greater than or equal to 40%, allowing them to operate over a wide range of supply pressures. These valveless injectors are very economical and at the same time relatively imprecise, which is not necessarily a disadvantage with cheap foamers. These injectors are mainly used by the British and American fire services.
Valve proportioners are used in Germany and in Germanic regions with a better sense of economy. The percentage of emulsifier in the premix is usually 3% with a knob that ranges from 1% to 6%, and even up to 7% on the Polish standard, but I can sense that the Poles have chosen this rule to avoid introducing into their country the German, Austrian, Czech, and Russian productions that throughout history have invaded them; but some people claim that this is because of the poor quality of their emulsifier production.
Below a percentage of 1% a system called “around the pump” is used, which consists of taking the liquid under pressure from the pump outlet, passing it through an injector and returning the premix to the pump feed. The injector’s dosing adjustment gives proportions that can vary from 0.1% to 1% and is used to introduce retardants into the liquid to extinguish forest fires.
There are also other processes where energy from pressure and flow rate is taken to operate a pump and allows very precise dosing with generally high costs and little used for foam production but rather in animal feed.
Naturally, if we were to find a market for POK for such a device, we would carry out the design and production.
To do so, we would have to find a fire officer capable of acquiring it in series and willing to leave his name in the history.
B.G.
