Liquid heater with temperature control
A liquid heater such as a direct electrical resistance liquid heater having multiple flow channels is provided with a temperature-sensing element in the form of a wire extending across numerous channels, preferably all of the channels, near the downstream ends of the channels. The resistance of the wire represents the average temperature of the liquid passing through all of the channels, and hence the temperature of the mixed liquid exiting from the heater. A bubble suppressing structure is provided in the vicinity of the wire.
1. A liquid heater comprising a chamber having an inlet and an outlet, and at least three electrodes disposed within said chamber so that the at least three electrodes are in contact with liquid flowing from said inlet to said outlet and wherein the liquid is heated by electrical current flow through the liquid between two or more of the at least three electrodes; an electrical power supply connection; and at least one switch for each of the at least three electrodes, the at least one switch being arranged to removably connect each of the at least three electrodes to the electrical power supply connection independent of the remaining ones of the at least three electrodes, the at least three electrodes being constructed and arranged to provide different current paths through the liquid between different pairs of the at least three electrodes, at least one of the current paths including two of the at least three electrodes connected to the electrical power supply connection and at least one of the at least three electrodes disconnected from the electrical power supply connection and disposed between the two electrodes of the at least three electrodes included in the current path and connected to the electrical power supply connection, different ones of the current paths having different conductances so that connection of different sets of the at least three electrodes to the electrical power supply connection provides different levels of current passing through the liquid, the levels of current including levels defining a stepwise progression between zero current when none of the at least three electrodes are connected and a maximum current when all of the at least three electrodes are connected, the progression having substantially uniform ratios between the currents of adjacent steps with non-zero current levels.
2. A liquid heater as in claim 1 wherein the at least three electrodes define a plurality of adjacent channels for liquid flow from said inlet to said outlet.
3. A liquid heater as in claim 1 wherein the at least three electrodes are non-uniformly spaced apart from one another with different distances between different pairs of mutually-adjacent ones of the at least three electrodes.
4. A liquid heater as in claim 1 , wherein a greatest difference between the levels of current in any two adjacent steps of the progression is no greater than 10% of the maximum current.
5. A liquid heater as in claim 1 , wherein a greatest ratio between the currents in any two adjacent steps of the progression having non-zero currents is no more than 1.22:1.
6. A liquid heater as in claim 5 , wherein the greatest ratio is no more than 1.1:1.
7. A liquid heater as in claim 1 , wherein a greatest difference between the levels of current in any two adjacent steps of the progression is no greater than 5% of the maximum current.
8. A liquid heater as in claim 1 wherein said stepwise progression includes at least 60 different current levels.
9. A liquid heater as in claim 1 wherein a ratio between the maximum current and a minimum non-zero current in said progression is at least 250:1.
10. A liquid heater as in claim 1 wherein said electrical power supply connection is a three-phase connection having three terminals and the at least one switch for each of the at least three electrodes is operable to connect each of the at least three electrodes to any one of the terminals of the three-phase connection.
11. A liquid heater as in claim 1 wherein the electrical power supply connection has two opposite terminals and said at least one switch for each of the at least three electrodes is operable to connect each of the at least three electrodes to one or the other terminal of the electrical power supply connection.
12. A liquid heater as in claim 1 further comprising a controller controlling the operation of the at least one switch for each of the at least three electrodes and thereby controlling connection of the at least three electrodes to the electrical power supply connection.
13. A liquid heater as in claim 12 further comprising a temperature sensor sensing the temperature of the liquid after it contacts the at least three electrodes, wherein the controller controls the operation of said at least one switch based upon information received from the temperature sensor.
14. A liquid heater as in claim 13 further comprising an electric current sensor sensing the amount of electric current being utilized by the liquid heater, wherein said controller controls the operation of said at least one switch for each of the at least three electrodes, based upon information received from said electric current sensor.
15. A liquid heater as in claim 13 , wherein said controller controls the application of electrical power to the at least three electrodes based upon the temperature sensed by said temperature sensor and the rate of change of the temperature sensed by said temperature sensor.
16. A liquid heater as claimed in claim 13 wherein said controller periodically determines if the combination of the at least three electrodes connected to the electrical power supply connection needs to be changed to raise or lower the current level applied to the liquid, based upon information received from said temperature sensor and a desired liquid temperature at said chamber outlet.
17. A liquid heater as in claim 16 wherein said controller limits the rate of change of electrical current applied to said at least three electrodes, by adjusting the current applied to the at least three electrodes only to a next higher or a next lower current level in the progression, each time said controller periodically determines if there needs to be a change in the current level applied to the liquid.
18. A liquid heater as in claim 17 wherein said controller limits the rate of change of electrical current applied to said at least three electrodes by changing the combination of said at least three electrodes connected to the electrical power supply connection no more than once during every cycle of an alternating current supplied to the electrical power supply connection.