Hydraulic vs. Electric power for screen filtration

Understanding the essential differences between a hydraulically driven motor and one that is independently driven by electric power takes a bit of digging into some basic principles of physics. 

Before we get into that, here's something important but not always understood about screen filtration:

The hard part of fine screen filtration is not the filtering itself.  It is the scanning of dirt off the screen and restoring it to virtually 100% clean. 

The finer the filtration degree, the more this is true.  The ability of a screen to be restored to clean status is what enables automatic screen filtration to operate reliably, day after day.

The single most important factor in achieving effective screen cleaning is a high pressure differential between the two sides of the screen, which in turn generates a high velocity of water passing through the screen and into the nozzles of the suction scanners that “vacuum” the screen.  It is this velocity that dislodges the contaminants from the screen.  The greater the velocity, the greater the cleaning force.

In order to capture this high velocity, the suction scanner nozzles must come into direct contact with the screen.

Suction power weakens exponentially as the distance between scanner nozzle and screen increases.  Without actual contact, cross currents and turbulence - especially in a water environment - interfere with suction power.  With even a few mm distance from the screen, part of the dirt load, instead of being captured, will either reenter the filtration chamber or remain lodged in the screen.

(Think of vacuuming your floor. How effective would it be if you hovered over it instead of vacuuming directly on it?)

Note: As pore size decreases (fineness of filtration increases), the force required to capture collected particles during cleaning cycles increases.  Meaning, the finer the filtration degree you want -- 10 micron above all -- the more critical it is to maintain high "suction" velocity.

With a hydraulically driven motor, the sole source of energy is the available water pressure in the system.  Every unit volume of water under pressure conveys a given measure of energy.  In a closed system where there is no additional input of energy, the existing energy source, hydraulic power alone, must be allocated to all functions.  That is simply not enough to generate the velocity of water required to clean a fine screen.  Additionally, the rotation speed of the scanner is not independently controlled.  Higher incoming pressure causes it to spin more rapidly.  But cleaning a fine screen requires slow, deliberately calibrated rotation speed and torque independent of water pressure.

Thus in order to compensate for the diminished energy/volume ratio per unit of water, the hydraulic-type filter requires more water per second for its cleaning cycle.  To accommodate that higher volume, the suction nozzles must be larger.  This in turn reduces the velocity into the nozzles, further diminishing the effectiveness of cleaning.  (Think of sliding your finger across a garden hose: Reducing the aperture increases the velocity, and vice versa).

There are in summary two reasons for reduced water velocity:

1. The limitations on power in a hydraulic system as noted above.

2. The distance between the scanner nozzles and the screen.

It requires suspending the laws of mechanical physics to achieve adequate cleaning of a fine screen in turbid water conditions (such as prevail in New York City) if relying on a hydraulic-type filter.

Question:  Why don't the manufacturers of hydraulic-type filters provide scanners with nozzles that actually contact the screen?

Because there is no external power (electrical) to overcome the resistance caused by the friction of nozzles moving against the screen.  If there were any physical contact between the nozzles and the screen, the scanner system would not operate.

Note there are also hybrid designs -- hydraulic-type filters that add electric power to the scanner, but still maintain spinning suction scanners at a remove from the screen.  These are marginally more effective than purely hydraulic filters, but in the end, without actual contact the system lacks sufficient suction power to remove captured particles.

Now let's look at competing animated videos from two manufacturers.  First is a clip from a manufacturer of hydraulic screen filters that shows a simple sequence:  The suction scanner spins rapidly, controlled only by the available hydraulic power within the system.  Without requiring any substantial velocity of water, without nozzles in physical contact with the screen... it all just appears to work perfectly.  Dirt particles magically leave the screen and obediently enter the scanner nozzles.

Next, in this video clip from our manufacturer STF Filtros you will see operation of a suction scanner in physical contact with a screen.  Note the nylon bristle nozzle slowly suctioning off dirt from the screen as it methodically brushes across.  This creates resistance, which must be overcome by an energy source greater than the available water pressure inside the filter:  Electric power.

There is no alternate reality. A separate energy source apart from the hydraulic power of water moving in the filter - electricity – must independently control the cleaning mechanism.

Conclusion:  The first determination an engineer or other decision maker should make is recognizing whether the basis of design is hydraulic or independent electric power.