Venturi scrubbers are commonly used in pollution control systems as particulate control devices. Particles are collected primarily according to their aerodynamic size through inertial mechanisms. Good particle collection is achieved by maintaining a high differential velocity between particles in the gas stream and water droplets in the Venturi throat. A high differential velocity is created by reducing the cross sectional area in the Venturi throat and thereby creating a pressure drop. The reduction in area accelerates the particles relative to water that is injected into the throat perpendicular to the gas flow. As particles collide with the water droplets they become entrained. The particle laden droplets are then collected in the Venturi sump and are purged in a blowdown stream.
A key to Venturi performance is therefore maintaining a constant pressure drop across the throat. This is relatively straightforward if you have a process with a constant flow rate. However, many processes have variable flow rates. An incinerator or kiln comes to mind where there are changing flow rates throughout the process cycle. In many cases the variation may be as high as 4:1 or 6:1 from the maximum to minimum flow rate. This ratio is often called the turn-down ratio. Three methods of maintaining a constant pressure drop for variable flow conditions are discussed below:
- Reflux Damper
- Variable Throat
- Manual Inserts
Reflux Damper - A reflux damper is often used on Venturi scrubber systems for solid waste combustors. A solid waste combustor can be an incinerator, kiln, gasifier, or plasma reactor. The Venturi is designed for the maximum flow condition. When the gas flow decreases, ambient air is recycled to the Venturi inlet through a pneumatically actuated damper to make up the difference. The ambient air is recycled from the downstream side (clean side) of an induced draft fan which is used to pull the gas through the system. The damper modulates to maintain the combustor draft pressure based on a 4-20 mA control signal from a draft sensor mounted in the combustor chamber.
The flow rate is equal to the design gas velocity times the cross sectional area. As the flow rate decreases the cross sectional area must be reduced to maintain the design gas velocity. For this reason a reflux damper is particularly recommended for smaller gas flows because it is easier to modulate than for a variable throat. This is because the gas velocity of a reflux damper is about 1/6th the gas velocity of a Venturi throat. A reflux damper is therefore less sensitive to flow rate variation. This makes it easier to tune and maintain the control loop. Another advantage of a reflux damper is the recycled gas is clean because it has already passed through the Venturi. Therefore there is no potential for fouling the damper blade from particulate in the gas.
The adjacent photo shows a 400 lb/hr medical waste incinerator scrubber with a Venturi inlet flow rate of 1,200 scfm. The reflux damper can be seen as the white horizontal duct from the ID fan outlet to the Venturi inlet on the right hand side of the rectangular condenser/absorber box.
Variable Throat - A variable throat Venturi is another common method of maintaining a constant pressure drop across a Venturi scrubber system. A valve is integrated into the Venturi throat. At maximum flow, the valve is fully open. As the flow decreases, the valve closes to reduce the cross sectional area accordingly. The variable throat can be a damper blade, butterfly valve, plumb bob, or pinch valve. As discussed above, variable throats are generally more suitable for larger gas flow processes. Consideration should be given to the potential for fouling from particulate build up on the valve. Particulate can accumulate and get stuck behind a butterfly valve, damper blade or on the shaft of a plumb bob. This can impede the ability to adjust or modulate the throat. The potential for this type of fouling may depend on the nature of the particulate. Envitech often uses variable throat Venturi's on industrial dryer applications. Variable throat Venturi's were discussed in a previous blog post, Venturi Scrubber: Adjustable Throats.
Manual Inserts - A third approach for maintaining a constant pressure drop is the use of manual inserts. This approach might be taken for a process that has distinct flow rates for long periods of time. It might also be used in situation where the design conditions are uncertain, say for a pilot or demonstration plant. The use of manual inserts provides a way of designing flexibility into the equipment.
Please click on the icon below to view a video of a variable throat Venturi.
