Thermophoresis

:: Section 3

Application

A thermal precipitator is an instrument that collects aerosol particles using thermophoresis to deposit the particles onto a surface. It employs a heated element, such as a wire, and a collection surface. Aerosol passing between the heated element and the cooler surface will be driven to deposit on the surface. The animation below shows a simple example of a thermal precipitator, the wire-and-plate design of Green and Watson.

Since the thermophoretic velocity is not dependent on particle size among small particles, a thermal precipitator is a very efficient way to collect small particles without size segregation. However, there is some size segregation among large particles because the velocity decreases as particle size increases. The smallest particles always deposit first, and the larger ones are deposited farther downstream. For a properly designed precipitator, the collection efficiency is virtually 100% for particles smaller than 10 μm. Since thermophoretic precipitators have very low work flow rates, they are particularly used in collecting small quantities of particles for observation in microscopies.

The thermal precipitator is widely applied in many aerosol related fields such as dust sampling or even collection of nano-particles. Thermal precipitation also occurs commonly in many industrial processes. For example, fly ash from coal combustion or incineration can deposit in the heat exchanger. In some cases, such a deposition is undesirable, as the built-up may cause scaling and low energy recovery.

1. How does the thermal precipitator work to collect aerosols? 2. In the animation above, what will happen if the distance between the heated rod and the collection plate is reduced to 0.06 mm?