Abstract The results of the performance test of a polyphosphate-based scale control additive, carried out for a period of 8 months during 1978, showed that the additive was only moderately effective as a scale inhibitor. The Gained Output Ratio, which was close to the design value of 8.0 at the start-up, declined by 30% during the test period while the distillate output fell off by about 25% in the same period. Even though the brine heater steam temperature was progressively increased from 93.3°C to 118.3°C, a steady decline was observed in the heater inlet and outlet temperatures which decreased from 81.7 to 73.3°C and 86.7 to 79.4°C respectively. It took only 40 days of operation for the heater fouling factor to exceed the design value. The fouling factor registered a five to seven-fold increase in 8 months.