Electric heating – Heating devices – With heater-unit housing – casing – or support means
Patent
1997-02-21
1999-01-12
Walberg, Teresa J.
Electric heating
Heating devices
With heater-unit housing, casing, or support means
219544, 219546, 392501, H05B 306
Patent
active
058594116
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a compact heater particularly but not exclusively for aquariums. The heater according to the invention can also be used in other technical fields, such as in photographic studios and laboratories, where it is necessary to heat a certain volume of liquid and keep it at a substantially constant temperature.
Conventional heaters are generally constituted by a tubular glass container that is shaped like a test-tube and internally accommodates three essential components, namely: an electric heating component, which is series-connected to a thermostat; an electric circuit for connecting the heating component to an external source of electric power; and a thermostat regulator that can be accessed from outside by a user.
Examples of heater of the type mentioned above are described in U.S. Pat. Nos. 3,895,217, 4,072,847, 4,124,793, 4,149,067, 4,225,648, and 4,812,626, in Swiss patent no. 269723, in European patents Nos. 0 022 149 and 0 247 676, and in Italian utility model applications Nos. 6194B/85 and 30694 B/86.
In all this prior art, the heating component is generally constituted by an electric resistor that is accommodated in the lower part of the container, whereas the thermostat is formed by a sensitive part that includes a bimetallic strip mounted on an appropriate support which is located in the upper part of the test-tube.
The electric resistor is kept at a certain distance from the glass wall of the test-tube, generally more than a millimeter, in order to protect it against thermal shocks and to comply with safety rules regarding electrical devices in contact with liquids.
This conventional arrangement produces a series of drawbacks.
First of all, heat exchange between the water and the resistor occurs only by convection and irradiation, consequently limiting the thermal efficiency of the device. Furthermore, in order to dissipate significant power levels it is necessary to create very steep thermal gradients, on the order of a few hundred degrees Celsius, consequently raising the maximum temperature of the device.
For closing the circuit, the bimetallic strip is calibrated when the resistor is not powered and is therefore actually sensitive to the temperature of the water.
Due to the longitudinal displacement of the heating component with respect to the thermostat, part of the heat produced by the heating component unavoidably flows upward into the thermostat, and therefore the circuit opens not so much in response to the temperature of the heated medium but due to the heat that originates directly from the resistor. The circuit opens at a temperature that is a few degrees higher than the closure temperature, typically by about ten degrees. This interval can be controlled by a magnet-fitted contact that attracts an iron plate located at the end of the bimetallic strip. The magnet retains the contact until the strip has curved beyond a certain limit, which can be adjusted at will with various systems, correspondingly varying the opening temperature.
If opening were due only to the heat from the water, the temperature of the heated medium would oscillate over a wide range between the closing and opening temperatures of the bimetallic strip. Since the bimetallic strip is instead affected directly by the temperature of the heating system, the temperature of the water oscillates between the closure temperature of the bimetallic strip and a slightly higher one. This temperature gets closer to the closure temperature as the flow of heat, from the resistor to the bimetallic strip, increases. In extreme cases, the bimetallic strip opens and closes the circuit continuously, since the heat supplied by the resistor tends to deform the strip immediately. In this manner one would obtain excellent temperature control, but an extremely long time would be needed to heat the water and the number of opening and closing actions would become excessive and would shorter the life of the contacts. In order to prevent this phenomenon, in practice a thermally insulating part, or heat shield, is i
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patent: 3319049 (1967-05-01), Ulanet
patent: 3803386 (1974-04-01), Rodriquez
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patent: 4994780 (1991-02-01), McQueen
patent: 5220154 (1993-06-01), Gunther
patent: 5389184 (1995-02-01), Jacaruso
patent: 5598502 (1997-01-01), Takahashi
patent: 5633978 (1997-05-01), Hofsass
Bresolin Valerio
Ragazzon Daniele
Coleman Henry D.
Hydor S.R.L.
Nguyen Quan
Sudol R. Neil
Walberg Teresa J.
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