Fluidized particles in liquid-solid fluidized bed heat exchangers are able to remove deposits from the walls and thus to prevent fouling or scaling. This fouling prevention ability is believed to depend strongly on the frequency and force of particle-wall collisions. This paper presents piezoelectric measurements of impacts on the wall in both stationary and circulating fluidized beds of various particle sizes and bed voidages. Two types of impacts were measured, namely by collisions of particles on the sensor and by liquid pressure fronts induced by particle-particle collisions close to the sensor. The characteristics of both impact types are used to analyze the total impulse and energy exerted by impacts on the wall for various fluidized beds. (C) 2009 Published by Elsevier Ltd.