Droplet size

Many technical processes, such as gas scrubbing, depend on producing the largest possible contact surface area between a fluid and a gas flow. Achieving the greatest possible contact surface area requires the fluid to be broken up into the smallest possible droplets. Technically, this can be realized by means of the following parameter changes:

  • Increase in pressure
  • Reduction of volume flow
  • Enlarging the spray angle
  • Use of two-fluid nozzles

In practice, a nozzle produces droplets from a compact flow of fluid within a specific size range. Since it is impractical to list all produced sizes, the droplet size (in µm) normally is specified as D32 Sauter diameter. Further, it is important to be familiar with the various diameter-related terms.

The definitions of the most frequently used diameters are listed in the following:

Volume Median Diameter DV0.5

The diameter which divides the overall volume of the droplets into two equal halves. Correspondingly, one-half of the overall volume consists of droplets with a smaller diameter, and the other half of droplets with a larger diameter.

A DV0.5=200µm means that 50% of the total volume of fluid sprayed is in droplets which have diameters smaller than 200µm. This does not mean that, in terms of number, 50% of the droplets are smaller than 200µm.

At this point, it should be noted that the data of a single characteristic droplet diameter is by no means sufficient to describe the entire droplet spectrum of a spray comprehensively. Two sprays with exactly the same volume diameter DV0.5 do not necessarily have the same specific surface area.

Volume Mean Diameter D30

The diameter of a droplet, whose volume, when multiplied with the total number of droplets, equals the total volume of the sample.

Arithmetic Mean Diameter D10

The arithmetic mean of the diameter of all droplets in the spray sample.

Sauter Mean Diameter D32

The Sauter mean diameter is best suitable for characterizing a spray. The volume-to-surface ratio for the mean diameter is the same as for the entire spraying volume. For this reason, one frequently applies the calculation of substance and thermal transition processes of the Sauter mean diameter.

90% Volume Diameter DV0.9

90% of the overall volume of droplets is less than or equal to the specified droplet diameter. A DV0.9=200µm, for example, means that 90% of the overall volume consists of drops that are less than or equal to 200µm. This characteristic droplet diameter is of vital importance to the evaporation and cooling processes where complete evaporation is required.

10% Volume Diameter DV0.1

10% of the total volume of droplets is less than or equal to the specified droplet diameter. For example, a DV0.1=200µm means that 10% of the total sprayed volume of droplets is less than or equal to 200µm. This characteristic droplet diameter frequently serves as a decision-making criterion as to whether droplets of a specific size are carried along by a gas flow. The DV0.1 can serve to select a suitable droplet separator.

Swirl nozzles generally create larger droplets than spiral nozzles. Air-atomizer nozzles, such as the XA or SA series, typically produce the smallest droplets of all nozzles.