The sprays are assumed to be spherical with radius , even though the assumption is valid for solid particles(liquid droplets) when their shape has no consequence on the combustion. For liquid droplets to be nearly spherical, the spray has to be dilute(total volume occupied by the sprays is much less than the volume of the gas) and the Weber number , where  is the gas density,  is the spray droplet velocity,  is the gas velocity and  is the surface tension of the liquid spray, should be .
The equation is described by a number density function , which represents the probable number of spray particles (droplets) of chemical species  (of  total species), that one can find with radii between  and , located in the spatial range between  and , traveling with a velocity in between  and , having the temperature in between  and  at time . Then the spray equation for the evolution of this density function is given by
where
: is the force per unit mass acting on the  species spray (acceleration applied to the sprays),
: is the rate of change of the size of the  species spray,
: is the rate of change of the temperature of the  species spray due to heat transfer,
:  is the rate of change of number density function of  species spray due to nucleation, liquid breakup etc.,
: is the rate of change of number density function of  species spray due to collision with other spray particles.