be used the advantage that the concentrator offers is that it removes fluid from the slurry after the slurry has passed through the fluid pumps f under normal and usual conditions the choking device h will be set so as to allow about 50 percent of the fluid to be removed from the slurry in such a case a 10 pound per gallon proppant fluid slurry passed through the fluid pumpers f would be increased by the invention to a concentration of 20 pounds per gallon of fluid when a 20 pound per gallon slurry is foamed to a mitchell quality of 75 percent by the addition of 3 parts gas to 1 part fluid by volume the proppant concentration becomes 5 pounds of proppant per gallon of foam such a concentration is twice that attainable using conventional procedures without a concentrator it is normal procedure during fracturing operations to vary the proppant concentrations over the course of the job the conventional method of varying proppant concentrations is to vary the proppant input at the blender a while maintaining a constant rate of fluid flow the use of the choking device h to vary the percentage of fluid removed from the slurry is an additional.