The total heat rejection to the cooling tower is the sum of the cooling capacity of the chiller + the compressor input (assuming hermetic compressor so compressor input all goes to refrigerant, not to ambient). So say the 750 ton unit compressor input is .7 kw/ton, since 1 kw is 2545 btu/hr and 1 ton is 12000 btu/hr, heat rejection of the 750 ton chiller is 2545 x.6 + 12000 = 13527 btu/hr per ton of cooling. Now typical condenser water temperature is 85°F entering and 95°F leaving or 10°F temperature rise. Solving for cooling tower gpm per ton:
Gpm = (btu/hr)/(500 x t rise) = 13527 / (500x10) = 2.71 gpm per ton. The 750 ton chiller therfore will require 2.71 x 750 = 2033 gpm at 10 degrees TD. The gpm for the 1150 ton chiller can similarly be calculated. Actually it is usually possible to select electric chiller & cooling tower combinations that would work in the 2 to 3 gpm (note absorption chillers would require about 4.6 gpm/ton)range. But the minimum entering water temperature is always 85°F based on typically 78°F wb, (occassionally 80°F wb but selection will not be CTI certified). Typical is about 3 gpm per ton but we do design for the minimum, 2 gpm per ton if we want to save on cooling tower space (existing building limits available space) and pumping energy.
