Patent Publication Number: US-2020278133-A1

Title: Continuous-flow heater with a safety circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Application No. EP19160042.8 filed Feb. 28, 2019, the entire content of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a continuous-flow heater for heating a liquid, comprising a fluid-accessible channel arrangement with at least one heating channel, which is configured for heating the liquid with at least one electric heating element arranged therein; an electronic control system which is configured to control electronic circuit breakers in order to electrically operate the at least one heating element and to control the heat output of said heating element, the electronic control system being designed as a microprocessor controller; a flow measuring device, which is electrically connected to the electronic control system and configured to detect the volume flow rate of the liquid flowing through the channel arrangement and to provide a pulsed measurement signal, where the number of pulses of the measurement signal correlates to the magnitude of the volume flow rate. 
     BACKGROUND OF THE INVENTION 
     Such continuous-flow heaters are sufficiently well-known from the prior art. For example, document DE 10 2008 011 117 A1 shows a continuous-flow heater with a flow meter which is designed as a measuring turbine that has a pulse output. The measuring turbine is used to measure the volume flow rate or the flow rate of the fluid flowing through the heating device, the number of pulses per time unit being a function of the volume flow rate. The pulse signals are applied to a timing element which is formed by a microcontroller. If the flow rate of the fluid falls below a minimum volume, or if the flow rate signals of the flow meter are completely absent, this causes the timing element to trip. Furthermore, in this case the heating element of the continuous-flow heater is switched off to protect against overheating. 
     The disadvantage is that safety shutdown of the heating element is performed by the microcontroller of the control circuit itself. Any malfunctions of the microcontroller and/or the program executed with the microcontroller may mean that there is no guarantee that the heater will be shut down reliably in every case if the volume flow rate falls below a minimum. It is therefore the object of the present invention to propose a continuous-flow heater which guarantees reliable shutdown of the at least one heating element if the volume flow rate of the liquid to be heated is too low, even if the electronic control system for controlling the heat output is faulty or damaged. 
     SUMMARY OF THE INVENTION 
     The object is achieved by a continuous-flow heater with the features referred to hereinbefore in that the continuous-flow heater comprises a safety circuit which is connected on the input side to the flow measuring device and on the output side to the electronic control system, in order to provide a supply voltage for controlling the electronic circuit breakers depending on whether the volume flow rate detected by the flow measuring device is greater than a minimum volume flow rate specification. The safety circuit according to the invention has the advantage that, regardless of the electronic control system, in the event that the volume flow rate of the liquid flowing in the channel arrangement falls below the minimum volume flow rate specification, the heating of the at least one heating element is reliably shut down. This shutdown process takes place separately and completely independently of the electronic control system. Possible malfunctions of the electronic control system, particularly in the program sequence of the microprocessor controller, system crashes or “freezing” of the microprocessor, do not affect the proper shutdown of the heating of the at least one heating element by the safety circuit. Advantageously, the electronic control system is also designed and configured to electrically shut down the at least one heating element on detecting a drop below the minimum volume flow rate specification. This results in a high degree of redundancy. As long as the electronic control system is working properly, the at least one heating element can be shut down in the event of a lack of volume flow rate by the electronic control system which is designed as a microprocessor controller. In any case, shutdown of the at least one heating element in the event of a lack of volume flow rate is performed by the safety circuit according to the invention, which shuts down the at least one heating element under the conditions mentioned, irrespective of the functional capability of the electronic control system. 
     An expedient embodiment of the invention is characterised in that the safety circuit comprises at least one digital counter, the counter input of which is connected to the flow measuring device, such that the pulsed measurement signal is present at the counter input of the digital counter, at least one counter output of the digital counter being connected to the input of an electronic switching element, which enables or disables the supply voltage for controlling the circuit breakers according to the status of the counter output. Advantageously, in this way the supply voltage for controlling the circuit breakers is only enabled if a predetermined number of pulses from the flow measuring device has been processed via the counter input of the digital counter. This ensures that the heating elements are only heated if the flow rate measuring device has actually detected the presence of the volume flow rate in the channel arrangement. In particular, this safety function is also provided if the electronic control system is faulty or inoperative, for example, due to a faulty program sequence of the microprocessor controller. 
     A preferred development of the invention is characterised in that the digital counter has at least one counter disabling input and in that this counter disabling input is electrically connected to the at least one counter output. In other words, the digital counter is switched so that it goes into self-holding as soon as a counter value is reached, causing the relevant counter output to change from a low level to a high level. In this way, it is achieved—without there being any need for further components or elements—that the safety circuit is configured to be monostable, i.e. it automatically maintains the status of providing the supply voltage for controlling the electronic circuit breakers, as long as the volume flow rate detected by the flow measuring device is greater than the minimum volume flow rate specification. This state is only left again if the detected volume flow rate falls below this minimum volume flow rate specification or even drops completely to zero. 
     According to a further preferred embodiment of the invention, the digital counter is designed as a ring counter. This embodiment as a ring counter has the advantage that the counter outputs of the digital counter are not configured to be binary-coded, but rather every counter reading is linked to a change in the output level of one of the counter outputs. After counting a pulse, for example, the output zero is activated, while after counting n pulses every nth counter output is activated. Thus, by connecting one of the counter outputs to the counter disabling input, it is particularly easy to determine at what counter reading the digital counter circuit should go into self-holding. By selecting a different counter output, it is particularly easy to adjust which counter reading has to be reached before the safety circuit goes into the self-holding state for providing the supply voltage for controlling the electronic circuit breakers. 
     A further expedient embodiment of the invention is characterised in that the digital counter has a reset input, which is electrically connected to a digital output of a triggerably configured monostable multivibrator, the trigger input of the monostable multivibrator being configured to control whether or not the digital level at a digital output of the monostable multivibrator changes once, and the trigger input of the monostable multivibrator being electrically connected to the flow measuring device, such that the pulse measurement signal is present thereat. This has the effect that, if the pulses of the measurement signal are absent or in the event that the time lag between the pulses exceeds a predetermined time interval, the digital counter is reset and thus the supply voltage for controlling the electronic circuit breakers is switched off. Advantageously, the heating of the at least one heating element is thus switched off within a very short time if the flow measuring device does not detect any volume flow rate or detects one that is lower than the minimum volume flow rate specification. The monostable multivibrator is also referred to as a mon-stable trigger circuit. 
     According to a further preferred embodiment, the monostable multivibrator is configured in such a manner that the digital level of the digital output changes once as soon as the trigger input is free from pulses of the measurement signal of the flow measuring device. In other words, the digital level at the digital output of the monostable multivibrator only changes if the flow measuring device does not detect any flowing volume flow. For example, the monostable multivibrator is configured in such a manner that the digital output has a low level while pulses of the measurement signal are present at the trigger input, as a result of which the digital counter is in “counting mode”. If the pulses of the measurement signal are absent, the digital output of the monostable multivibrator assumes a high level and the digital counter is reset. 
     A further expedient embodiment of the invention is characterised in that the time constant of the monostable multivibrator is selected in such a manner that said time constant is a multiple of the maximum pulse length of the measurement signal in each case. This ensures that the digital level at the digital output of the monostable multivibrator does not change as long as pulses of the measurement signal of the flow measuring device are present at the trigger input. A level change at the digital output of the monostable multivibrator only takes place if no pulses of the measurement signal are received at the trigger input at least for the time determined by the time constant. 
     According to a further preferred embodiment of the invention, the safety circuit has a digital output which is configured to signal to the microprocessor controller whether or not the supply voltage for controlling the electronic circuit breakers is provided by the safety circuit. In this way, it is also additionally possible to use the microprocessor controller to process and evaluate the respective supply voltage status for controlling the electronic circuit breakers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of an exemplary embodiment of the safety circuit according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Continuous-flow heaters for heating a liquid are already sufficiently known from the prior art. The drawing therefore does not include a separate illustration of such a continuous-flow heater. The continuous-flow heater comprises a fluid-accessible channel arrangement with at least one heating channel configured for heating the liquid. At least one electric heating element, preferably a bare wire heating element, is arranged in the heating channel. 
     The continuous-flow heater further comprises an electronic control system which is configured to control electronic circuit breakers, for example semiconductor switching elements, such as thyristors or triacs, in order to electrically operate the at least one heating element and to control the heat output of said heating element. The electronic control system is designed in particular as a microprocessor controller, thus having at least one microprocessor with the usual further electronic components which are regularly required for the operation of such a microprocessor. 
     The continuous-flow heater further comprises a flow measuring device, which is electrically connected to the electronic control system and configured to detect the volume flow rate of the liquid flowing through the channel arrangement and to provide a pulsed measurement signal, the number of pulses of the measurement signal correlating to the magnitude of the volume flow rate. 
     The flow measuring device is preferably designed as a flow turbine which is arranged in the flow path of the liquid either within the channel arrangements or in a liquid inflow or outflow route. The flow measuring device is designed in particular to provide a pulsed measurement signal. The number of pulses per time unit is a measure for the magnitude of the volume flow rate of the liquid flowing through the continuous-flow heater. If the volume flow rate is equal to zero, the measurement signal generated by the flow measuring device is pulse-free, i.e. is at low or high potential. The number of pulses of the measurement signal, when the volume flow rate present is not equal to zero, is preferably proportional to the magnitude of the volume flow rate. 
     The continuous-flow heater according to the invention comprises a safety circuit  10 , a possible switching variation of which is shown by way of example in  FIG. 1 . On the input side, the safety circuit  10  is connected to the flow measuring device. Thus, the pulsed measurement signal is present on the measurement signal line  11 . On the output side, the safety circuit  10  is connected to the electronic control system, not shown in the drawing, via a supply voltage line  12 . 
     The safety circuit  10  according to the invention provides a supply voltage for controlling the electronic circuit breakers via the supply voltage line  12 . Whether or not the supply voltage for controlling the electronic circuit breakers is provided via the supply voltage line  12  depends on whether or not the volume flow rate detected by the flow measuring device is greater than a minimum volume flow rate specification. The safety circuit  10  is thus configured in such a manner that it is only possible to control the electronic circuit breakers if it is determined that a minimum volume flow rate in accordance with the minimum volume flow rate specification is flowing through the fluid-accessible channel arrangement. 
     If the volume flow rate detected by the flow measuring device remains below this minimum volume flow rate specification, no supply voltage for controlling the electronic circuit breakers is provided via the supply voltage line  12 , thus at any rate preventing heating of the at least one heating element when the volume flow rate is too low. The heating element or heating elements are also reliably switched off in any case by the safety circuit  10  as soon as the volume flow rate detected falls below the minimum volume flow rate specification. This switching off takes place independently of the microprocessor controller. The safety circuit according to the invention ensures that the heating elements are reliably switched off even if the microprocessor controller is faulty. 
     The safety circuit  10  according to the invention preferably has at least one digital counter  13 . The counter input  14  of the digital counter  13  is electrically connected to the flow measuring device via the measurement signal line  11 , such that the pulsed measurement signal is present at the counter input  14  of the digital counter  13 . The digital counter  13  preferably has a plurality of counter outputs  15 . As shown in  FIG. 1 , the digital counter  13  has, for example, nine of the counter outputs  15  which are designated as outputs Q 0  to Q 9  in the circuit diagram according to  FIG. 1 . 
     At least one of the counter outputs  15  is electrically connected to the input of an electronic switching element  16 . The electronic switching element  16  is configured to enable or disable the supply voltage, which is present by way of example at pin  17  in  FIG. 1 , for controlling the electronic circuit breakers. By way of example,  FIG. 1  shows how one of the counter outputs  15 , namely Q 3 , is connected via the series circuit of the resistor  18  and the series resistor  19  to the base  20  of the switching element designed as NPN transistor  21 . The presence of the supply voltage for controlling the electronic circuit breakers on the supply voltage line  12  may optionally be displayed via the light-emitting diode  22  with its series resistor  23 . 
     The digital counter  13  preferably has a counter disabling input  24 . The counter disabling input  24  of the digital counter  13  is configured to interrupt or continue the counting function of the digital counter  13  as a function of the signal level present at this counter disabling input. The counter disabling input  24  is electrically connected, as shown in  FIG. 1 , to one of the counter outputs  15  via the resistor  18  which may also be designed as a 0-ohm bridge. 
     If, due to the incoming pulses at counter input  14 , the digital counter  13  reaches a counter value which results in Q 3  of the counter outputs  15  changing from a low level to a high level, the counter disabling input  24  is also set to a high level and the counter value of the digital counter  13  is “frozen” as it were. In other words, the digital counter  13  is switched such that it goes into self-holding on reaching a certain counter value. The counter value at which this happens depends on which of the counter outputs  15  is fed back at the counter disabling input  24 . The drawing shows the resistors or 0-ohm bridges  25 ,  26  for this which are optionally fitted instead of the resistor  18 . 
     The safety circuit  10  further comprises a monostable multivibrator  27 . The monostable multivibrator  27  is configured to be triggerable, this means that it is possible via a trigger input  28  to control whether or not the digital level at a digital output  29  of the monostable multivibrator  27  changes. The trigger input  28  of the monostable multivibrator  27  is electrically connected via the measurement signal line  11  of the flow measuring device such that the pulse measurement signal of the flow measuring device is present at a trigger input  28 . 
     The digital output  29  of the monostable multivibrator  27  is connected to a reset input  30  of the digital counter  13 . If no liquid flows through the fluid-accessible channel arrangement, the measurement signal provided by the flow measuring device on the measurement signal line  11  is pulse-free, and thus has a constant electrical level. 
     The time constant of the monostable multivibrator  27 , that is the delay time which elapses until the digital level at the digital output  29  changes after a trigger process via the trigger input  28 , is determined by the capacitance  31  and the size of the resistor  32 . If, as previously described, a signal with constant level is present at the trigger input  28 , the digital output  29  changes its digital level after the time constant set by the capacitance  31  and the resistor  32  at the latest. 
     These digital pulses reach the reset input  30  of the digital counter  13  via the line  33 , such that if pulses of the measurement signal are absent via the measurement signal line  11 , the counter value of the digital counter  13  is reset, all outputs Q 0  to Q 9  of the digital counter assume a low level and the supply voltage for controlling the electronic circuit breakers via the switching element  16  is switched off, so that the supply voltage line  12  is de-energised. 
     Further preferably, the monostable multivibrator  27  comprises the resistor  34  as well as the capacitor  35  which form a timing element via which the reset input  36  of the integrated switching circuit  37  is moved into a defined state during the switching on process of the safety circuit  10 . The integrated circuit, for example, is a re-triggerable monostable multivibrator  27  with reset function, for example an integrated circuit of the type  74  HC123. Further preferably, an integrated circuit of the type  74  HC 4017 is used as the digital counter  13 . 
     The previously described monostable multivibrator  27  is consequently configured in such a manner that the digital level at the digital output  29  changes if the trigger input  28  is free from pulses of the measurement signal of the flow measuring device. 
     Preferably, the time constant of the monostable multivibrator  27 , which determines the cycle duration of the digital pulses, is selected in such a manner that said time constant is a multiple of the maximum pulse length of the measurement signal in each case. It is achieved by selecting the said time constant of the monostable multivibrator  27  so that a change of the digital level only takes place at the digital output  29  if no more pulses come from the flow measuring device via the measurement signal line  11  or, if the time gap between pulses has grown so large because of a low volume flow rate that said time gap is greater than the time constant defined by the capacitance  31  and the resistor  32 . This ensures that the digital counter  13  is only reset if no volume flow rate or only an extremely low volume flow rate is determined by the flow measuring device. In both the latter two cases, the supply voltage for controlling the electronic circuit breakers is switched off in each case via the electronic switching element  16 . 
     Optionally, a counter output line  38 , which is electrically connected to one of the respective counter outputs  15 , is routed out to a connection  41  via a resistor  39  with a downstream diode  40 . The status of each counter output  15  can be queried as a status signal at the connection  41  by the electronic control system. The diode  40  prevents feedback via the electronic control system.