GB1381113A - Automatic control for electrode boilers - Google Patents

Abstract

1381113 Automatic control of electrode boilers R H EATON-WILLIAMS 10 May 1972 [19 May f971 10 March 1972] 15886/71 and 11287/72 Heading G3R An electronic controller detects the current flowing in an electrode boiler, which depends on the water level, and operates feed and drain valves in a predetermined sequence at set current values. In the controller shown in Fig. 1, the electrode current is applied through a transformer to terminals 7, 8 connected by a variable resistor for setting the required levels. A control voltage is thus produced across resistor R3 and applied to inverting inputs of differential amplifiers in the form of integrated circuits IC1, IC2, IC3. Initially, with no electrode current flowing, all the amplifiers will produce negative outputs, so that a transistor Q1 is off, transistor Q2 is on, and triac GR1 is triggered to open a feed valve so that water flows into the boiler. At the same time, transistor Q3 is off, transistor Q4 is on, and a triac GR2 would be fired to open a drain valve, except that its gating circuit is shorted by a transistor Q7 turned on by transistor Q2, so that the drain valve is held closed. As the boiler fills, the electrode current increases to a point where the voltage across resistor R3 switches amplifier IC2 to turn off transistor Q4 and remove the firing signal for the triac GR2. Further current rise switches amplifier IC1, turning off transistor Q2 and triac GR1 and thus closing the feed valve. The water now boils, but the current may rise further due to increasing conductivity and expansion of the water. A rise of, say, 10% causes amplifier IC3 to turn on a transistor Q5 which triggers triac GR2 and opens the drain valve. Resistors R37, R45 ensure that amplifier IC3 returns to its initial state only when sufficient water has been drained to reduce the current to its previous normal level. As the water boils away, the electrode current and control voltage decrease until amplifier IC2 is switched and triac GR2 opens the drain valve again. The level to which the boiler is drained before a further filling cycle begins is determined by the setting of a resistor RV1 in the circuit of amplifier IC1. The feed, boil, drain cycle will then be repeated automatically. An additional electrode is located in the boiler so that it carries current only if the water reaches a maximum permissible level. Current is then applied to a Schmitt trigger circuit Q8, Q9 to turn on a triac GR3, actuating an alarm signal and switching on a transistor Q6 which shorts triac GR1 and thus prevents opening of the feed valve. A manual drain switch is connected to terminals 3, 4, 5 to allow complete emptying of the boiler for maintenance. In a second embodiment, Fig. 2 (not shown), an additional feed period is included in the cycle, thereby allowing extra water to be drained from the boiler and thus removing more impurities. The circuit includes Schmitt triggers and a multivibrator in place of the integrated circuits of the first embodiment. Reference has been directed by the Comptroller to Specification 1139911.