Wet scrubbers are becoming increasingly popular in the industrial air quality control market. This is due to their ability to handle the industry’s more difficult pollutants, as well as combine particle and gas/vapor removal in a single system. When used correctly in the right applications, wet scrubbers can achieve impressively high collection efficiencies of over 99 percent.
The secret behind wet scrubbers is their unique method of dust collection. Instead of the filter bags or cartridges found in dry dust collectors, wet scrubbers use a scrubbing fluid to remove various pollutants from your airstream. The efficiency and performance of your wet scrubber are in part determined by the size of its scrubbing fluid’s water droplets, with smaller droplets equaling higher collection efficiencies.
However, not all applications are the same. To truly optimize the performance of your wet scrubber and improve your air quality, it’s best to pick the right scrubber for your application.
When Should I Use a Wet Scrubber?
While there is some overlap between dry dust collectors and wet scrubbers in terms of application, certain dust management needs and air quality control requirements are best met by using a wet scrubber. A wet scrubber is often a great choice when you have any of the following:
- Wet or sticky particulate
- Soluble particles
- Fine particles and mists
- Humid air and moisture in your gas stream
- Mixes of particulate and gases/vapors
- Combustible dust
That last point is very important, as the National Fire Protection Association (NFPA) is placing an increasingly heavy importance on dust explosion prevention. While baghouses and dry dust collectors can be fitted with extra measures such as flameless venting and external grounding to mitigate the chance of an explosion, wet scrubbers are inherently resistant to explosions due to their use of a scrubbing fluid. For a closer look into explosion-proofing requirements, NFPA 652 and NFPA 484 cover the most recent regulatory standards.
How Do I Pick the Right Wet Scrubber?
Many different types of wet scrubbers exist today. Choosing the right one will depend on the characteristics of your dust/pollutants and the specific needs of your application.
Here are the main types of wet scrubbers and which applications they fit best with:
Venturi scrubbers are great for small particulate removal down to the submicron range. They use the differential momentum between the free-flowing scrubbing fluid and high velocity gases to create contaminant-entrapping droplets which can suspend and contain a variety of toxic and harmful pollutants. With the right design, you can achieve high collection efficiencies of over 99% for particles with a diameter 1 µm and larger.
When to use a venturi scrubber: ultra-fine particulates, submicron particles, fumes and liquid aerosols
Impingement wet scrubbers are your go-to scrubber for removing most particles larger than 5 µm, as well as soluble gases. Also referred to as tray-tower scrubbers, impingement wet scrubbers use horizontal perforated trays to enhance the gas-liquid contact in the scrubbing process. Gas enters from the bottom of the tower, traveling upward through the holes in the trays while the scrubbing fluid flows over each tray, mixing with the gas. Impingement scrubbers can achieve efficient dust collection with low water recirculation and minimal pressure drop.
Sly’s Impinjet® wet scrubber improves upon the designs of standard impingement scrubbers by using impingement baffles above each hole in the tray design. These wetted baffles create tiny droplets around 100 microns in diameter to scrub the gas and entrap particles in the scrubbing liquid. The improved design of the Impinjet offers high collection efficiencies of 99% or more for particles 5 microns or larger and capacities as high as 200,000+ CFM.
When to use an impingement scrubber: particles 5 microns or larger, mixes of particulate and gases/vapors, heat recovery and applications with high hydraulic loadings
Eductor scrubbers are generally used for air flows less than 2000 CFM, removing soluble gases and particulate by inducing a flow of gas using high pressure liquid. They can be used for direct-contact condensation and offer a high liquid flow rate, great for more stubborn pollutants.
When to use an eductor scrubber: sticky/gummy particulate, as well as gases that decompose with water, such as SiCl4 and H2SiF6
Packed Tower Scrubbers
Also known as packed column or packed bed scrubbers, packed tower scrubbers are designed for the chemical scrubbing of gaseous contaminants. The scrubber’s internal packing provides a large, wetted surface area for the contaminated gas to come into contact with the scrubbing fluid, thus getting absorbed or reacting with the scrubbing fluid. Recirculation of the scrubbing liquid can help achieve desired outlet emissions.
When to use a packed tower scrubber: gas-phase emissions like sulfur dioxide and other odors/acids
In addition to selecting the right type of scrubber, you will also want to appropriately size your wet scrubber for your application. Factors that affect the size of the scrubber include gas flow rate, temperature, pressure, gas composition, humidity, contaminant loading and your desired outlet conditions.
It is important to note that whether or not your gas stream is fully saturated as it enters the scrubber, it will exit the system fully saturated, resulting in a change in volume, temperature and density. The overall scrubber size (diameter) is then a function of the velocity of saturated gas through the scrubber shell. Using the moisture content of your gas stream (typically expressed as pounds of water per pound of dry gas) and your inlet temperature, you can calculate the saturation temperature and saturated volume content can be calculated.
Better Wet Scrubbers at Sly
Here at Sly, with nearly 150 years of experience in air quality control, we have the right equipment and expertise to get you the perfect wet scrubber for your application. All of our base scrubber models can be further customized to meet your specific dust collection requirements, leading to optimal scrubber performance and collection efficiencies. For instance, Sly’s Venturi/Impinjet combination scrubber can be tailored to your system’s needs to achieve collection efficiencies of over 99.99%.
For more information on wet scrubbers and when to use them, check out Sly’s Wet Scrubber Application Guide.