ELECTRIC INSTANTANEOUS BOILER

Abstract: An electric instantaneous boiler wherein a first opening portion as the hot water outlet opening (18a) of an output hot water pipe (18) is provided in the lower portion within the tank (16) to reduce the overshoot of the output hot water temper­ ature through the after-boiling, further a second opening portion (18b) of the output hot water pipe as the air vent opening is provided in the upper portion within the tank to prevent the abnormal excessive rise of the sheath heater (17). Also, the heat sensing portion of the temperature detector (19) is provided in the position of the first opening portion where the thermal influences of the sheath heater (17) are not directly applied so that the thermal response property is superior, and the stable output hot temperature is provided. Further­ more, the temperature sensing portion of the excessive temperature rise preventing apparatus (21) is provided on the tank top-face on the shaft center of the input water (18) pipe to quickly stop the energization of the heating heater during the abnormal operation.

Description: The invention relates to an electric instantaneous boiler comprising an electric heater placed within a tank, input and output water pipes having openings leading into the tank, the opening of the water input pipe being placed in the lower portion and directed upwards within the tank. Such boilers are preferably used in connection with showers or the like. From US-patent 4,358,665 an electric instantaneous boiler is known, which will now be described in connection with Figure 7. If a valve 1 is opened, a pressure switch 2 is operated to turn on a sheath heater 3 into a heating condition. The water flows from the valve 1 to the lower portion of the tank 5 through the water pipe 4. The water flows to the upper portion of the tank 5 while being heated by the sheath heater 3 and flows out from the outflow opening 6a of the hot water pipe 6 provided at the upper portion. However, if under such construction, the valve 1 is fully closed after the use of the flow of 3 ℓ per minute at an input water temperature of 25°C, at the output a hot water temperature of 40°C will be obtained through adjustment of the valve 1. When the valve 1 is opened again after one minute, an overshoot called after-boiling is caused as shown in Fig. 9 so that hot water of 50°C is temporarily outputted immediately after the valve 1 has been opened. The reasons are as follows. When the valve 1 is dosed the water flow within the tank 5 also stops to turn off the pressure switch 2 to cut off the energization of the sheath heater 3, but the water within the tank 5 is further heated by-the remaining heat of the sheath heater 3 so that the water is stable at such hot water distribution, as shown by the solid line of Fig. 10, with respect to the depth of the tank. Namely, the highest portion of the tank becomes about 50°C in temperature. The transition temperature grade is caused between the upper portion and the lower portion of the tank so that the temperature becomes lower with decreasing depth of the tank, to about 25°C( the input water temperature) near the input water opening 4a. As the hot water is outputted through the hot water output pipe 6 from the high-temperature water of the tank upper portion when the valve 1 is opened, the overheating of the output hot water temperature becomes large. It is natural that this tendency becomes larger with smaller tank water amount. In the "abnormal condition" where the sheath heater 3 remains conductive even if the valve 1 is closed without the operative cooperation between the valve 1 and the pressure switch 2, the water temperature within the tank 5 and the temperature of the sheath heater 3 rise. A thermostat 7 for preventing excessive temperature rise operates to stop the energization to the sheath heater 3. However, in such construction as described hereinabove, it took more time before the thermostat 7 for preventing excessive rise of temperature operated in the abnormal condition. The boiling water was jetted from the output hot water pipe 6 or the case was deformed, thus resulting in a dangerous condition. The reasons are as follows. Namely, the water within the tank 5 near the temperature sensing portion 7a of the thermostat 7 during the normal use is the highest in the water temperature within the tank during heating operation by the sheath heater 3. The temperature sensing portion 7a is normally retained highest in temperature by the transfer heat from the U-shaped heater portion 3a. On the other hand, in the abnormal condition, the heat of the U-shaped heater portion 3a is taken by the surrounding water, so that the temperature sensing portion 7a is slow in responding to the abnormal condition. Also, although the operation off temperature of the temperature excessive-rise preventing operation is set with some tolerance (10°C or more) for the error operation prevention with respect to the highest temperature during the normal use, thermal transfer dispersion is caused because of the contact condition between the brazing area between the U-shaped heater portion 3a and the inner face of the tank 5, so that a tolerance is required. As shown in Fig. 11, the temperature of the heat sensing portion 7a of the thermostat 7 during normal use becomes higher as the output hot water amount becomes smaller, so that the operation off temperature of the thermostat 7 has to be set at the high value. Thus, more time is necessary before the thermostat 7 switches off in the abnormal condition, thus resulting in a dangerous condition such as boiling water within the tank 5, jetting from the output hot water pipe 6, or a deformed case. Another embodiment of a conventional electric instantaneous boiler is shown in Japanese Patent Publication No. 59-53450, and as shown in Fig. 8. The temperature sensing portion 8a of a temperature detector 8 composed of a thermistor or the like for detecting the output hot water temperature is provided in proximity to a mixture portion 10 for mixing the heated water of the upper portion of the tank 9, and the sheath heater 11. The water inputted into the lower portion of the tank 9 from the input water pipe 12 flows towards the upper portion of the tank 9 while being heated by the sheath heater 11, and is outputted from the output hot water pipe 13 after it has been mixed in the mixture portion 10. The temperature detector 8 detects the temperature of the water flowing to the mixture portion 10. A semiconductor power control apparatus 14, which receives the detection signal, compares the detected temperature value with a set temperature value to control pulses to the switching element 14a- such as triac or the like in accordance with the deviation value so as to control the supply of power to the sheath heater 11 so that the deviation value may be kept at zero. However, in such construction as described herein-above, the output hot water temperature becomes unstable with ripples being larger, as shown in B in Fig. 6, when the valve 15 is throttled to reduce the flow amount. The reasons are as follows. Namely, when the flow amount is reduced, the flow speed near the temperature sensing portion 8a, which is large in the flow-passage area on the sectional face of the tank 9, becomes very slow. As the sheath heater 11 and the temperature sensing portion 8a are quite near to each other for better thermal response property through the reduction of the waste time, which is caused by the distance L of the temperature sensing portion 8a from the sheath heater 11, the temperature sensing portion 8a is influenced by the surface temperature of the sheath heater 11 to render the output hot water temperature stable. From GB-patent 32 21/1914 an electric water boiler is known, comprising a tubular resistance heating element formed by a closed casing of silica containing a resistor, a casing for surrounding the resistor into which the water to be heated is first admitted, before passing centrally through the element and an outlet tube. The flow path of the water to be heated is restricted to a first path way of small annular cross section, and to a second passage within the outlet pipe, no circulation within the tank is possible, so that after-heating occurs leading to excessive output temperatures. NL-C 37277 shows a water boiler for a central heating system in which the hot water from the central heating boiler flows through a cylindrical tank. Within the tank, a heat-exchanging pipe of spiral form is provided through which the water to be heated is passed. Also, a circulating line is provided so that the hot water can circulate through a hot water system. Finally, DE-A-33 06 807 shows an instantaneous boiler comprising a heating pipe containing an electric heater. A flow sensor and a temperature sensor are placed before the heater, and a further temperature sensor is placed behind the heater. Furthermore, an excessive temperature sensor is mounted on the electric heater. All these sensors are connected to an electronic control unit for controlling the power delivered to the electric heater. It is an object of the present invention to provide an improved electric instantaneous boiler, having a relatively low water amount, which is capable of preventing an overshoot of the output hot water temperature when the valve is fully closed from the condition of normal use, and than opened again. The temperature detector should positively detect even if the flow amount of water is reduced, while the termal response property should be maintained, so that a stable output hot water temperature is provided. An electric instantaneous boiler of the kind described, for solving the object of the invention, is characterized in that the heater is provided in a position offset from the center line of the tank, that the input water pipe is placed in the lower portion of the tank in a free space outside the area of the heater, and that the opening of the output water pipe is placed within the lower area of the tank. In order to quickly stop the energisation of the electric heater in abnormal conditions, the electric instantaneous boiler according to the invention is preferably characterized in that the temperature sensing portion of an excessive temperature sensor is provided within the free space in the upper portion of the tank substantially at the same axis as the input water pipe. Further preferable embodiments are defined in the dependent claims. These and other objects and features of the present invention will become apparent from the following description taken in conjunction with a preferred embodiment thereof with reference to the accompanying drawings, in which: Fig. 1 is a longitudinal sectional view of the thermal exchange unit in one embodiment of the present invention; Fig. 2 is a characteristic chart of the output hot water temperature change during the opening and closing operation of the valve; Fig. 3 is a hot water temperature distribution characteristics chart within the tank; Fig. 4 is an enlarged longitudinal sectional view near the heater-soldered portion; Fig. 5 is a characteristic chart showing the relationship between the output hot water amount and the temperature sensing portion temperature of the excessive temperature rise preventing apparatus; Fig. 6 is an output hot water characteristics chart of the electric instantaneous boiler and a conventional electric instantaneous boiler; Fig. 7 and Fig. 8 show longitudinal sectional views of conventional thermal exchange units; Fig. 9 is a characteristics chart of the output hot water temperature change during the opening and closing operation of the conventional valve; Fig. 10 is a characteristics chart of the hot water temperature distribution within the conventional tank; and Fig. 11 shows the relationship between the output hot water amount of the conventional electric instantaneous boiler and the temperature sensing portion temperature of the excessive temperature rise preventing apparatus. Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. Referring now to the drawings, there is shown in Fig. 1 a cylindrical copper-made tank 16 according to one preferred embodiment of the present invention. A sheath heater 17, which has its coil axis center in a position offset from the axis center of the tank 16, is provided within the tank 16. The lead portion 17a at both ends of the sheath heater 17 is water-tightly soldered through the tank top-face 16b on the side of the space portion 16a of the tank 16. The coil-shaped sheath heater 17 coincides in the axis center with the coil axis center of an output hot water pipe 18. The output hot water pipe 18, whose outer diameter is close to the coil inner diameter, is watertightly soldered on the top face 16b of the tank 16 so that the first opening portion as the exit opening 18a from the tank 16 is in proximity to the tank bottom face 16c. A second opening portion 18b as an air vent, is smaller than the first opening portion 18a, and is provided in the topmost portion within the tank 16 of the output hot water pipe 18. Also, a temperature detector 19 composed of a thermistor or the like for detecting the output hot water temperature is mounted in the lower portion within the tank 16 on the tank bottom face 16c at the central axis of the output hot water pipe 18. The input water pipe 20, having a restricted input water opening 20a, is watertightly soldered into the bottom face 16c of the tank of the space portion 16a where no sheath heater 17 is provided. The temperature sensing portion 21a is provided on the tank top face 16b on the axis center of the input water pipe 20, with a thermostat 21 for preventing an excessive temperature rise, i.e. an excessive temperature sensor, being provided on the temperature sensing portion. The operation in the embodiment will be described hereinafter. A case will be described where the valve 22 communicating with the input water pipe 20 is opened to flow the water, and the hot water is continuously outputted from the output hot water pipe 18. In this case, the valve 22 is opened and the pressure switch 23 is turned on to energize sheath heater 17. The water flowing into the tank 16 from the input water pipe 20 is throttled and accelerated by the input water opening 20a of the input water pipe 20. The water reaches as far as the upper portion within the tank 16 to hit against the inner wall of the tank top-face 16b under the temperature sensing portion 21a of the thermostat 21, and is reversed and diffused to flow to the lower portion of the tank 16 while being heated by the sheath heater 17. The water heated by the sheath heater 17 is throttled, accelerated and mixed by the first opening portion 18a of the output hot water pipe 18 to flow into the output hot water pipe 18. It passes the temperature-sensing portion 19a and is outputted through the output hot water pipe 18. The temperature of the hot water outputted from the first opening portion 18a at this time is detected by the temperature detector 19. The semiconductor power control apparatus 24 to which the detection signal has been inputted compares the detection temperature with the set temperature to control pulses to the switching element 24a such as triac or the like in accordance with the deviation value to control the power fed to the sheath heater 17 so that the deviation value may be maintained at zero. When the valve 22 is closed and the pressure switch 23 is turned off, the energization to the sheath heater 17 is stopped. Although there is the space of the radius within the coil of the sheath heater 17 between the bent sensing portion 19a and the lower portion of the sheath heater 17, the heated water is throttled by the first opening portion and is accelerated and mixed so that the waste time becomes small, and a superior thermal response property is provided. As the heat sensing portion 19a is not close to the sheath heater 17, no direct thermal influences occur from the sheath heater 17. If the flow amount is made small, the hot-water temperature is positively detected without any detection of the temperatures of the sheath heater 17, so that the output hot water temperature is stable with small ripples as shown in A of Fig. 6. Also, the flow speed near the heat-sensing portion 19a is fast, the scales are hardly attached and fast control characteristics may be maintained even after a long period of service. As the output hot water pipe 18 is made large so that its outer diameter is close to the inner diameter of the sheath heater 17 within the tank 16, the volume of the heating chamber 25 is small, the flow speed near the sheath heater 17 is high to improve the thermal efficiency and the response property of the automatic control system of the automatic hot-water temperature control by the temperature detector 19. On the other hand, air contained in the input water, within the tank 16 is removed by the air pressure within the tank 16 through the output hot water pipe 18 by the second opening portion 18b as the air vent hole, so that the sheath heater 17 cannot be abnormally overheated through air exposure. Now, a case where the valve 22 is fully closed and is opened again a few minutes later, will be described hereinafter. The hot water temperature distribution within the tank 16 in the water-flowing condition before the valve 22 is closed shows such temperature distribution as shown by the dotted lines of Fig. 3, wherein the upper portion of the tank 16 is low in temperature and the lower portion of the tank is high in temperature. When the valve 22 is fully closed, the flow within the tank 16 stops, and the energization of the sheath heater 17 is switched off. The water within the tank 16 is heated by the extra heat of the heater 17, the distribution of the hot water temperature within the tank 16 becomes such as shown by the solid line of Fig. 3, wherein the upper portion of the tank 16 is high in temperature and the lower portion thereof is low in temperature because of convection. When the valve 22 opens again, the hot water is outputted though the output hot water pipe and the first opening portion 18a from the low-temperature water of the lower portion of the tank l6, so that the high-temperature water of the upper portion of the tank 16 is mixed with the input water from the input water pipe 20. On the other hand, the sheath heater 17 is energized, but the water within the tank 16 is not sufficiently heated at the early stage by the delayed rise. As a result, the changes in the output hot water temperature are as shown in Fig. 2. The overshoot by the after-heating is about 3°C, which hardly matters. The abnormal condition will be described herein-after by the use of Fig. 5. In this case, when the valve 22 is closed, the inflow amount W of the water into the tank 16 becomes zero, but the sheath heater 17 remains energized. The water within the tank 16 is quickly heated so that the water temperature of the upper portion of the tank 16 rises, especially because of convection. Furthermore, the temperature of the heat sensing portion 21a of the thermostat 21 quickly rises because of the thermal transmission from the lead portion 17a so that the thermostat 21 turns off at the operation off temperature T1 of the excessive temperature sensor, to stop the energization of the sheath heater 17. Namely, the temperature of the heat sensing portion 21a of the thermostat 21 is cooled by the input water during the normal use and is kept at the low temperature as shown in the solid line in Fig. 5 so that the operation off temperature T1 of the excessive temperature sensor 21 may be set low. Also, during the abnormal use, the cooling effect through the input flow is removed so that the temperature quickly rises to turn off in a short time the energization of the sheath heater 17, whereby dangerous conditions such as the jetting operation of boiling water from the output hot water pipe 18, the deformation of the case or the like is prevented. Also, in the present embodiment, as the input water opening 20a at the tip end of the input water pipe 20 is restricted, the input water pipe 20 can easily be inserted into the tank 16 during the assembly. As is clear from the foregoing description, according to the arrangement of the present invention, the electric instantaneous boiler of the present invention has the opening portion of the output hot water pipe provided, in the lower portion of the tank, so that an overshoot of the output hot water temperature by the after-boiling is reduced. Furthermore, as the air vent opening is provided in the output hot water pipe at the upper portion of the tank, an abnormal excessive heating of the sheath heater may be prevented. Also, as the heat sensing portion of the hot water temperature detector is at the output hot water opening and is located in a position with no thermal influences from the heater, the thermal response property is superior and a stable output hot water temperature is provided. Furthermore, as the temperature sensing portion of the excessive temperature sensor is provided on the tank top-face at the axis center of the input water pipe, the energization of the heating heater is quickly stopped during the abnormal operation to prevent accidents from being caused.

Claims: An electric instantaneous boiler comprising an electric heater (17) placed within a tank (16), input and output water pipes (20, 18) having openings (20a, 18a) leading into the tank (16), the opening (20a) of the water input pipe (20) being placed in the lower portion and directed upwards within the tank (16), characterized in that the heater (17) is provided in a position offset from the center line of the tank (16), that the input water pipe (20) is placed in the lower portion of the tank (16) in a free space (16a) outside the area of the heater (17), and that the opening (18a) of the output water pipe (18) is placed within the lower area of the tank (16). A boiler as claimed in claim 1, characterized in that the opening portion (20a) of the input water pipe (20) is formed as a nozzle for directing the incoming water towards the upper portion within the free space (16a) of the tank (16). A boiler as claimed in claim 1 or 2, characterized in that the temperature sensing portion (21a) of an excessive temperature sensor (21) is provided within the free space (16a) in the upper portion of the tank (16) substantially at the same axis as the input water pipe (20). A boiler as claimed in claim 3, characterized in that the heater (17) is a sheathed heater, one of the terminal connection portions (17a) being provided within the free space (16a) near the temperature sensing portion (21a) of the excessive temperature sensor (21). A boiler as claimed in any of the preceding claims, characterized in that the output hot water pipe (18) is provided through the upper and lower portion of the tank (16), having a primary opening (18a) within the lower portion and a secondary opening (18b) as an air vent hole within the upper portion of the tank (16). A boiler as claimed in claim 5, characterized by a temperature detector (19) for temperature control of the heater (17), placed in the output hot water pipe (18) near the primary opening (18a) outside the direct influence of the heater (17). A boiler as claimed in claim 5 or 6, characterized in that the heater (17) is a coil-shaped sheathed heater located between the inner wall of the tank (16) and the outer wall of the output water pipe (18), the outer diameter of the output hot water pipe (18) being close to the inside diameter of the heater (17).

Inventors: TAKAI, MITSUO, TANAHASHI, TAKASHI, UEDA, YASUKIYO, UEMURA, MASAHITO
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1986
Main classification: F24H1
IPC codes: F24H9, F24H1