3 153 574 and norton et al u s pat no 4 134 750 supra and in addition although similar retention times are used in this instant invention as in the prior art work of salladay et al the pipe reactor of the present invention has a significantly more compact design configuration than the long inclined reactor of salladay et al supra this is accomplished with an initial short length large diameter vertical section with the high retention time to obtain the high nh 3 h 3 po 4 mole ratio slurry required to produce diammonium phosphate and results in a low heat flux of about 0 04 u00b7 10 6 to about 0 16 u00b7 10 6 btu h u00b7 in 2 cross sectional area while maintaining an ammonia loading of about 0 3 lb h u00b7 in 3 to about 1 0 lb h u00b7 3 this vertical section is followed by a short horizontal transfer section with a higher heat flux and short retention time upstream of the slurry distributor the heat flux for the horizontal section should be about 0 2 u00b7 10 6 to about 1 0 u00b7 10 6 btu h u00b7 in 2 and that for the distributor.