Data were obtained from 21 days of the operation of a plant for the oxidation of ammonia to nitric acid. There were three independent variables.
x1 was air flow. This represents the rate of operation of the plant - the nitric oxides produced are absorbed in a counter current absorption tower.
x2 was the cooling water inlet temperature. This is the temperature of the cooling water that circulates through the coils in the absorption tower.
x3 was the acid concentration. This is 10(concentration of the acid circulating -5)
y was 10 times the percentage of the ingoing ammonia to the plant that escapes from the absorption column unabsorbed. This is an inverse measure of the overall efficiency of the plant.