An important parameter in wet scrubbing systems is the rate of liquid flow. It is common in wet scrubber terminology to express the liquid flow as a function of the gas flow rate that is being treated. This is commonly called the liquid-to-gas ratio (L/G ratio) and uses the units of gallons per 1,000 actual cubic feet or liters/cubic meter (l/m³).
Expressing the amount of liquid used as a ratio enables systems of different sizes to be readily compared. For particulate removal, the liquid-to-gas ratio is a function of the mechanical design of the system; while for gas absorption this ratio gives an indication of the difficulty of removing a pollutant. Most wet scrubbers used for particulate control operate with liquid-to-gas ratios in the range of 4 to 20 gallons per 1,000 actual ft³ (0.5 to 3 liters per actual m³).
Depending on scrubber design, a minimum volume of liquid is required to "wet" the scrubber internals and create sufficient collection targets. After a certain optimum point, adding excess liquid to a particulate wet scrubber does not increase efficiency and in fact, could be counter-productive by causing excessive pressure loss. Liquid-to-gas ratios for gas absorption are often higher, in the range of 20 to 40 gallons per 1,000 actual ft³ (3 to 6 litres per actual m³).
L/G ratio illustrates a number of points about the choice of wet scrubbers used for gas absorption. For example, because flue-gas desulfurization systems must deal with heavy particulate loadings, open, simple designs (such as venturi, spray chamber and moving bed) are used. Also, the liquid-to-gas ratio for the absorption process is higher than for particle removal and gas velocities are kept low to enhance the absorption process.
Solubility is a very important factor affecting the amount of a pollutant that can be absorbed. Solubility governs the amount of liquid required (liquid-to-gas ratio) and the necessary contact time. More soluble gases require less liquid. Also, more soluble gases will be absorbed faster.