DK3037741T3

DK3037741T3 - Method for avoiding dry fire by electric heat exchanger

Info

Publication number: DK3037741T3
Application number: DK15197543.0T
Authority: DK
Inventors: Martin Lebernegg; Axel Schöps
Original assignee: Vaillant Gmbh
Priority date: 2014-12-23
Filing date: 2015-12-02
Publication date: 2018-06-06
Also published as: PL3037741T3; TR201806927T4; PT3037741T; DE102014226892A1; ES2670358T3; EP3037741B1; EP3037741A1

Description

The invention refers to a method for the avoidance of dry fire of electrical flow heaters.

In the event of dry fire, the lack of fluid causes the radiator of an electrical flow heater to be destroyed by overheating. DE 4142838 A1 shows an electrical flow heater with an impeller for measuring the flow rate through the electrical flow heater. If the flow rate falls below a certain minimum, the power supply should be switched off. In order to take the inertia of the impeller into account, a timer is provided which only uses the signal of a volume flow that is too low to switch off the energy supply after one and a half seconds. The electrical flow heater known from EP 948015 A2 is also protected by a temperature monitor, which reacts at impermissibly high temperatures due to insufficient water supply. DE 1440482 discloses an electrical flow heater in which fusible conductors are attached in series to the heating conductors and melt earlier than the heating conductors in order to protect them.

To prevent dry fire in electrical flow heaters, they often have a safety temperature limiter consisting of a fuse and a thermal switch which opens at a certain temperature and closes again when it cools down. The thermal switch is mostly a bimetal switch which opens a circuit at a predetermined temperature Ts. The fuse of the safety temperature limiter melts at a higher temperature Tsch and must then be replaced. If the melting temperature is exceeded significantly at a critical temperature Tkrit, this can lead to the molten material forming an electrical bridge again.

The object underlying the invention is to ensure reliable prevention of dry fire and, in this case, preventing reheating.

This is solved by a method according to the invention in accordance with claim 1 in that the electrical flow heater 1 is switched on only briefly for a first time period Ati, and after switching off the electrical flow heater, a second time period At2 is waited.

Thus, according to a first alternative, the electrical flow heater is switched on again after the second time period At2 has passed. If a current flow is measured immediately after the second time period At2 has passed, this indicates that the safety temperature limiter has not tripped and that sufficient fluid is therefore flowing through the electrical flow heater. Since everything is functioning properly, the current flow remains switched on.

However, if no current flow is measured immediately after the second time period At2 has passed, but this current flow occurs within a predetermined third time period At3, it is concluded that the volume flow is too low. An associated error message is then optionally issued.

In the event that no current flow is measured immediately after the second time period At2 has passed and this also does not occur within a predetermined third time period At3, it is concluded that there is dry fire. A corresponding error message is then optionally issued. Furthermore, the electrical flow heater should then be switched off completely into a fault position.

According to a second alternative, it is examined whether or not the safety temperature limiter has tripped during the second time period At2. If it has not tripped, it is concluded that the electrical flow heater is operating properly.

Advantageous designs of the invention are found in the features of the dependent claims.

The invention will now be explained in detail using the figures, wherein:

Figure 1 shows a device in which the method according to the invention can be used and

Figure 2 shows the temperature profile of the electrical flow heater with and without fluid when the method is being carried out.

Figure 1 shows the environmental heat source circuit 3 of a heat pump. In this circuit 3, an electrical flow heater 1, an environmental heat source 8, an evaporator 9 and a circulation pump 4 are connected in series. The electrical flow heater 1 has an electrical heating block 10, which is connected to a control unit 6. Furthermore, the electrical flow heater 1 has a safety temperature limiter 14 in the form of a thermal switch 2 and a fusible plug 5, which are connected in series to the control unit 6. The thermal switch 2 is embodied in the form of a bimetal switch and set in such a way that it opens above a predetermined switching temperature Ts. If the thermal switch 2 cools down again, the contact is closed again. The fusible plug 5 melts at a higher melting temperature Tsch· A temperature sensor 7 downstream of the electrical flow heater 1 and the circulation pump 4 are also connected to the control unit 6. The control unit 6 has means for determining whether the electrical heating block 10 is consuming electricity and whether the circuit is opened or closed by the thermal switch 2 and the fusible plug 5. If the control unit 6 has released current for the electrical heating block 10, said control unit 6 can simultaneously measure whether a current is flowing or whether the circuit to the electrical heating block 10 is interrupted.

When the heat pump not shown in more detail is operating, the circulation pump 4 of the environmental heat source circuit 3 is started. Environmental heat is transferred from the environmental heat source 8 to the environmental heat source circuit 3 and transferred in the evaporator 9 to the refrigerant circuit of the heat pump not shown.

If there is a risk of the environmental heat source circuit 3 freezing which is detected by the temperature sensor 7 downstream of the electrical flow heater 1, the electrical heating block 10 of the flow heater 1 is switched on to prevent this.

If there is no fluid in the electrical flow heater 1, there is a risk of dry fire. In electrical flow heaters 1 of the prior art, the thermal switch 2 and the fusible plug 5 heat up in the event of dry fire in such a way that the thermal switch 2 opens and the fusible plug 5 melts. The fusible plug 5 must then be replaced so that the electrical flow heater 1 can be operated again.

Figure 2 shows the temperature profile in different constellations. Line III shows the temperature profile of the electrical flow heater 1 in the event of dry fire in accordance with the prior art. At time t0, the heating coil 10 of the electrical flow heater 1 is switched on. After a short delay, the temperature rises. The switching temperature Ts is exceeded, so that the thermal switch 2 opens. Since the heat of the heating block cannot be dissipated by a fluid, the temperature initially rises further within the electrical flow heater 1 until the melting temperature Tsch at the fusible plug 5 is also reached, so that the latter melts. Due to the typically high heat capacity of the electrical flow heater 1, the high energy input also causes the critical temperature Tkrit to be exceeded in the further process, at which critical temperature Tkrit the fusible plug 5 may be reconnected if solder forms an electrical bridge. When cooling down, the thermal switch 2 closes again. If the solder of the fusible plug 5 has formed a contact, the control unit 6 detects this and switches the heating coil 10 of the electrical flow heater 1 on again when heat is required and the process starts again. This causes the temperature of the heating block 10 to rise until finally the heating block 10 fails completely. Possible consequences are a total failure of the device and other consequential damage.

In the method according to the invention for detecting dry fire, bridging after the fusible plug 5 melts should be avoided and the lack of fluid should be effectively detected.

For this purpose, the method according to the invention provides that the heating block 10 of the electrical flow heater 1 is only energised briefly, so that melting of the fusible plug 5 is precluded if there is fluid in the flow heater. To this end, the circulation pump 4 is first switched on. At time to, the heating block 10 of the electrical flow heater 1 is switched on and then switched off again after a first time period Ati.

If there is fluid in the environmental heat source circuit 3, the temperature initially rises, but remains below both the switching temperature Ts and the melting temperature Tsch in the further process; this is shown by line IV. After a second period At2, during which the heating block 10 of the electrical flow heater 1 is switched off, the heating block 10 is switched on again. As the thermal switch 2 is closed and the fusible plug 5 is intact, a current flows through the properly functioning flow heater 1. Accordingly, the control unit 6 knows, due to the closed circuit and through the thermal switch 2 and the fusible plug 5, that fluid is present, and leaves the heating block 10 of the electrical flow heater 1 switched on. The heating behaviour is then monitored by the temperature sensor 7 downstream of the electrical flow heater 1 and, if necessary, controlled or switched off when a predetermined temperature is reached.

If there is fluid in the flow heater 1 but there is no volume flow, for example because the circulation pump 4 is not running, the switching temperature Ts of the thermal switch 2 is exceeded, but not the higher melting temperature Tsch of the fusible plug 5 in the further process, which does not melt as a result. Further operation of the heating block 10 of the electrical flow heater 1 is possible after the fault in the lack of volume flow has been remedied, as the thermal switch 2 resets itself automatically. This is shown by line I. If no current flow is measured after the second time period Δί2 has passed, a third time period ΔΪ3 is waited. If a current flow then occurs, it is concluded that the volume flow is too low and a corresponding error message is issued.

If there is no fluid at all in the environmental heat source circuit 3, the temperature continues to rise because of the low heat capacity due to the lack of water. Both the switching temperature Ts and the melting temperature Tsch ate exceeded. Due to the short current supply during the first time period Ati, the critical temperature Tkrit is not exceeded. This is shown by Line II. No current flow is measured after the second time period ΔΪ2· Current flow does not occur within the predetermined third time period Δΐ3, so that it is concluded that there is dry fire and an associated error message is issued. The heating block 10 of the electrical flow heater 1 then remains switched off.

The first, second and third time periods Δΐι, ΔΪ2 and Δί3 are to be determined device-specifically. The heat output of the heating block 10 and the first time period Δΐι determine the amount of heat introduced. The heat capacity and mass of the heating block 10 determine its heating behaviour. Accordingly, the first time period Δίι is to be determined such that in the event of dry fire, the switching temperature Ts is exceeded in the next few seconds, but not the melting temperature Tsch. The second time period ΔΪ2 must be selected in such a way that no further increase in temperature is to be expected thereafter. The third time period Δί3 must finally be selected in such a way that in the event of a nonflowing but present fluid, the thermal switch 2 is closed again. The time periods Δΐι, ΔΪ2 and ΔΪ3 can be calculated or experimentally determined.

The method according to the invention can alternatively also be carried out without the third time period ΔΪ3 if the control unit 6 can detect during the second time period At2 whether the thermal switch 2 and the fusible plug 5 are closed. This requires a device according to Figure 3 comprising a control relay 11 and a safety relay 12. The control relay 11 is located in the control unit 6 in the circuit of the heating block 10 and serves to switch off the heating block 10 after the first time period Ati. The safety relay 12 is also located in the control unit 6. The safety relay 12 is energised by the circuit using the safety temperature limiter 14. If the safety relay 12 is de-energised, it opens the circuit using the heating block 10. A voltage meter 13 is present parallel to the safety temperature limiter 14.

After the first time period Δΐι, the voltage meter 13 measures continuously during the second time period Δί2 whether the thermal switch 2 is closed and the fusible plug 5 is intact. If the safety temperature limiter 14 is closed, the voltage is zero. However, if the safety temperature limiter 14 is open, the mains voltage is present. If the thermal switch 2 and the fusible plug 5 are closed during the entire second period Δί2, this is synonymous with proper operation (flowing fluid is present).

If there is fluid in the flow heater 1 but there is no volume flow, the switching temperature Ts of the thermal switch 2 is exceeded, but, in the further process, not the higher melting temperature Tsch of the fusible plug 5, which does not melt as a result (line I). Opening the thermal switch 2 causes the safety relay 12 to open. The control unit 6 permanently switches off the heating block 10 by means of the control relay 11. If the thermal switch 2 cools down again, the safety relay 12 is energised again, but the heating block 10 remains switched off.

If there is no fluid at all in the environmental heat source circuit 3 (line II), at least the thermal switch 2 is opened and probably also the fusible plug 5, whereby the safety relay 12 opens and the control unit 6 also switches off the heating block 10 permanently.

In the case of experimental determination, the times can also be determined experimentally using a defined procedure as an alternative to simulations. At least the factors initial/ambient temperature at the start of the commissioning of the flow heater and the supply voltage have an influence on the temperature profile in the flow heater in the event of dry fire. First, the flow heater is operated at the maximum permissible supply voltage with at least three different, predetermined time periods Ati in the event of dry fire and the temperature occurring is continuously recorded at the position of the fusible plug 5. This measurement is also carried out at different initial/ambient temperatures. For each measurement, the maximum temperature that has occurred is determined at the position of the fusible plug 5. This makes it possible to determine a suitable time periods Ati, At2 and At3.

List of reference signs 1 Flow heater 2 Thermal switch 4 Environmental heat source circuit 5 Circulation pump 6 Fusible plug 7 Control unit 7 Temperature sensor 8 Environmental heat source 9 Evaporator 10 Electrical heating block (heating element) 11 Control relay 12 Safety relay 13 Voltage meter 14 Safety temperature limiter

Claims

A method for avoiding dry fire by an electric through-flow heat exchanger (1) with a heating block (10) in a hydraulic circuit (3) in which a circulating pump (4) is located, and a melt solenoid (5) and a thermal contact (2) electrically coupled in series and thermally conductive to the heating block (10), whereby the thermocouple (2) opens above a predetermined contact temperature Ts, and the fusible fuse (5) melts at a higher, predetermined temperature Tsch, whereby the current through the fuse fuse (5) and the thermocouple (2) are measured, and in the event of a power failure, the heating block (10) is switched off, whereby the circulation pump (4) is switched on, the heating block (10) is then switched on for a first period of time Ati, and after switching off the heating block ( 10) a second time period At2 is expected, whereby, either after the expiry of the second time period At2, the heating block (10) is switched on, and in the case of electrically measured risk current through the fuse block (5) and the thermocouple (2) immediately after the second time period At2 has expired, the electrical current through the heating block (10) remains switched on, o while in case no electric current is measured through the fuse block ( 5) and the thermocouple (2) immediately after the second time period ΔΪ2 has expired, but electric current occurs in a third period ΔΪ3, it is concluded that there is too little volume flow, or it is checked during the time period ΔΪ2 if electric current occurs through the fuse fuse (5) and the thermocouple (2), and in the event that no electric current occurs through the fuse fuse (5) and the thermocouple (2) during the second time period Δί2 concluded dry fire, o while in the event that electrical current occurs through the fuse (5) and the thermocouple (2) throughout the second time period At2, the electrical current is reconnected Remove the heat block (10).

Dry fire avoidance method according to claim 1, characterized in that, if the heating block (10) is reconnected after the second time period At2 has expired, dry fire is concluded in the event of no electric current being measured immediately after second time period At2 has expired, and no electric current occurs within a predetermined third time period ΔΪ3.

Dry fire avoidance method according to claim 1 or 2, characterized in that, if too little volume flow or dry fire is concluded, a corresponding error message is sent.

Method of avoiding dry fire according to any one of the preceding claims, characterized in that if dry fire is concluded, the heating block (10) is switched off in the electric through-heater (1).