(a) Field of the Invention
The present invention relates to a pure water manufacturing apparatus suitable for the manufacture of ultrapure water used in the electronic and medical drug industries.
(b) Description of the Prior Art
Semiconductor devices are becoming increasingly integrated, and patterns on the order of submicrons are required. In view of this, water having an extremely high purity is required. Similarly, in the medical drug industry high purity water is required to manufacture high-quality drugs.
In view of such a need, Japanese Patent Disclosure No. 54-34545 discloses a method of manufacturing water having a high degree of purity. According to this method, pure water is manufactured by a system having a circulation system consisting of an agglomerating/clarifying apparatus, a filtration apparatus, an ultraviolet sterilization apparatus, an ion exchanger, and a heat exchanger for heating and sterilizing pure water. In this system, when water is sufficiently pure with respect to live bacteria, heat exchange at the heat exchanger is not performed, and pure water passing through the filter is supplied to a terminal device at which pure water is required.
However, when the number of live bacteria in pure water flowing in the circulation system exceeds a predetermined level, the valve is closed to stop supply of untreated water. Pure water is then circulated by a pump and heated to 80.degree. C. or higher by the heat exchanger, and sterilized.
The number of live bacteria in the pure water circulation system of a pure water manufacturing apparatus varies in accordance with external conditions, e.g., summer or winter time. Anyway, the number of live bacteria exponentially increases even after sterilization. Therefore, when the number of live bacteria in pure water exceeds a predetermined level (normally, 5 to 20/ml), sterilization must be performed immediately.
In such a conventional pure water manufacturing apparatus, the time required for sterilizing pure water varies depending upon the capacity of the sterilizing heat exchanger, the volume of the overall system, system design, and operating conditions such as sterilization temperature. However, it normally takes 3 to 7 hours due to various operations required for switching valves, heating the water and circulating the heated water. When pure water is required for use at the terminal device during the sterilization operation, only hot pure water is available. When the use of hot pure water is undesirable, the terminal device must be stopped until the pure water cools. In a manufacturing line which requires cooler pure water of about 35.degree. C. or lower, as in the case of a semiconductor manufacturing line, the entire line must be stopped during sterilization of pure water.
The time required for live bacteria to exceed in number a predetermined level is from 2 days to one month depending upon external conditions. Assuming that the manufacturing line must be stopped for sterilization on an average of once in ten days, the line capacity falls to 90% of its maximum capacity. When a manufacturing line of an annual productivity of ten million dollars is assumed, the annual productivity of the line is reduced to 9 million, resulting in a loss of one million dollars.