Venturi scrubber performance hinges on four key design factors: pressure drop, particle size, water flow, and entrainment separation. When a Venturi scrubber is not performing properly, it is best to review these design factors one-by-one.
Perhaps the most important aspect of a Venturi scrubber is its pressure drop. The pressure drop of the Venturi is directly correlated to the velocity of the gas passing through the throat. The higher the pressure drop, the faster the gas. The speed of the gas is important, as the success of a Venturi is due primarily to inertial impaction. In inertial impaction, a fast moving particle in the gas strikes a relatively slower moving water drop. The higher the velocity difference, the greater chance that particle is unable to "duck" into a slipstream around the water drop.
When the pressure drop of a Venturi decreases, the performance of the Venturi decreases. A drop in pressure can occur for a variety of reasons, but by far the most common is a drop in overall air flow. In a fixed throat system, the drop in air flow may be a result of a new or upset condition. In a reflux system, it may be to due to a pressure drop increase in downstream equipment, or a failure with a reflux damper. For a variable throat Venturi, that is designed to handle changes in air flow, a drop in pressure across the Venturi can signal a problem with the Venturi damper. In any of these cases, the problem must be fixed to regain the Venturi performance.
In a Venturi scrubber, the particle size actually refers to the aerodynamic size of the particle, which is much more influenced by the mass of the particle than by the diameter of the particle. Again, the reason is that the Venturi scrubber is an inertial impaction device, and thus the mass of the particle directly influences removal.
The best example of the effect of inertial impaction is a car windshield. Large, heavy particles like rocks and insects slam into a car's windshield at high speed. Plastic bags, though much larger, contain very little mass, and "slip" over the windshield. Particles in a Venturi are captured similarly. A denser particle with the same volume will be captured more efficiently in a Venturi scrubber than a corresponding lighter particle.
When the performance of a Venturi scrubber varies, it is important to look at upstream equipment to ensure that the particles themselves have not changed. Smaller, lighter particles will reduce performance of the Venturi scrubber, and necessitate either a greater pressure drop or downstream particle removal equipment.
In inertial impaction, the particles must collide with water (or some other liquid medium) to be collected. If there is not enough water to collect the particles, performance will degrade.
Water flow can be impacted by a host of common issues. Since the water is pumped, a problem with a pump can lead to performance issue with the Venturi scrubber. Plugging of the nozzles or of valves can also occur. Both of these issues can be solved by performing regular preventive maintenance on the pump and the nozzles.
The final step in a Venturi scrubber is to remove the particle-laden water drops from the gas stream. Whichever way is selected to remove the water drops, it is important that it does perform well, or the drops will continue into downstream equipment (or even exit the stack).
Waveform entrainment separators, the predominant water separation method, work by causing a change in flow, forcing the water drops to hit the entrainment separator while letting the gas pass through. The waveforms work as long as the gas travels through the waveforms at the right velocity. At too high a velocity, the water drops re-entrain, and pass through the entrainment separator. Particles can also stick to the entrainment separator, changing the waveform shape and reducing the area (thus increasing the velocity). Waveforms must be designed properly upfront to ensure that flow is uniform through the waveforms, and waveforms must be regularly cleaned to ensure success.
To read more about a specific Venturi scrubber application, please download the white paper below.