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 fluid heater comprising:
(a) a channel structure defining an inlet, an outlet and a plurality of channels extending in a downstream direction and communicating with the inlet and outlet so that fluid can flow in parallel though the channels from the inlet to the outlet, the channel structure including one or more electrical energy application elements associated with each channel;
(b) a temperature-sensing wire extending across the plurality of channels adjacent downstream ends thereof and in a direction transverse to the downstream direction; and
(c) a control circuit connected to the electrical energy application elements and the wire, the control circuit being arranged to monitor an electrical resistance of the wire and control application of power to the electrical energy application elements responsive to the electrical resistance of the wire so as to maintain fluid flowing through the outlet at a temperature near a set point, the control circuit being operative to control said application of power so that in at least some control conditions, fluid flowing through different ones of the channels will be heated to different temperatures.
2. A fluid heater as claimed in claim 1 wherein the control circuit includes a power supply connection and a plurality of switches including at least one switch connected between each said electrical energy application element and the power supply, the control circuit being arranged to actuate the switches so as to connect and disconnect the electrical energy application elements with the power supply.
3. A fluid heater as claimed in claim 2 wherein the electrical energy application elements include electrodes exposed for contact with a fluid flowing in the channels and the control circuit is arranged to actuate the switches so that electrical current passes through the fluid in at least some of the channels.
4. A fluid heater as claimed in claim 3 wherein the electrodes have downstream edges and the wire is disposed within about 10 mm of the downstream edges of the electrodes.
5. A fluid heater as claimed in claim 1 wherein said control circuit is arranged to control application of power to said electrical energy application elements responsive solely to the electrical resistance of the wire and the set point.
6. A fluid heater as claimed in claim 1 wherein said control circuit is arranged to control application of power to said electrical energy application elements without the use of a signal representing flow through the heater.