HEATING ELEMENTS: Go with the Flow
July 1, 2006
The increasing popularity of hot water dispensing fixtures in homes and hotels, along with rapidly growing sales of single-serving coffeemakers, has fuelled interest in technologies that can heat fluids quickly.
One such technology can be found in a flow-through heater unit, developed by Ferro Techniek B.V., Gaanderen, The Netherlands, and marketed in North America by Davidon Industries, Warwick, R.I. The unit's heating element combines an extremely low mass with high power-density to deliver true electric instantaneous water heating. In addition to coffeemakers and hot water dispensers, the compact units are also ideally suited for commercial appliances such hot beverage vending machines, where the flow-through heaters can eliminate the need to maintain a storage tank of heated water.
The flow-through heater is a sealed, circular device with a diameter of 90 mm (3.54 in.) and a height of 35 mm (1.38 in.). It contains a flat, thick-film heating element at the bottom, on top of which rests an intricate, narrow water channel (see photo) that provides a tortuous path for the water from the inlet at the outer edge to the outlet at the center.
Flowing through the channel at high speed, water is quickly heated by contact with the thick-film heater below it. Ferro says that the benefit of this approach is that the heater can raise water temperature from 15 DegC (59 DegF) to as high as 95 DegC (203 DegF) within 5 sec., without suffering from the effects of steam creation or rapid scaling. Desired outlet temperature is settable from 30 DegC (86 DegF) to 95 DegC (203 DegF).
NTC temperature sensors are fitted at the water inlet and outlet. These sensors are used to precisely control the water flow rate, with the help of an electronic controller provided with the package.
With the existing model, the power can be configured up to 2 kW and the unit can handle pressures up to 2.5 bar (36 psi), which permits a very good flow rate. In practice, Ferro says that a 2 kW module can provide a mug (236ml/8oz) of hot coffee in 35 sec.
The low mass of the thick-film heating element is what enables the fast response time, as well as good energy efficiency.
The flow-through heater is just the latest application for Ferro's thick-film heaters, which have been used in numerous appliance applications, including electric water kettles, milk frothers, humidifiers, steamers, toasters, irons, hot plates, grill plates, fryers, and warming plates.
Compared to conventional tubular heating elements, thick-film heaters offer a number of advantages. The most obvious difference is the mechanical configuration that gives thick-film heaters a lower physical and thermal mass that is spread out over a wider surface area. This results in:
- Faster response for heating up.
- Faster response for cooling down.
- More uniform heat distribution.
- Higher energy efficiency.
A typical thick-film element consists of several layers, beginning with a sheet metal carrier that provides structural support for the component. The carrier is coated with a glass ceramic dielectric material called KERDI, which was developed by Ferro. The KERDI is applied to a thickness of 160 microns, providing a dielectric strength of 1,650 V. Then circuitous tracks of the thick-film resistor material are printed on top of the dielectric. A protective, insulating cover coat is then placed over the circuit traces.
The coefficient of thermal expansion of the ceramic dielectric is tuned to the CTE of the metal carrier to give the component a high resistance to thermal shock. The materials used also provide the component with a high mechanical strength.
Depending on the configuration, the package can deliver a surface temperature up to 300 DegC (572 DegF) and achieve power densities up to 70 W/cm2. The layered construction of the element directs most of the heat into the metal carrier, providing a directional heat, ensuring that most of the heat is directed toward the load and less heat gets wasted going off in other directions.
Another option available with the Ferro thick-film elements, including the one in the flow-through heater, is an overheat protection feature called E-Fast, which stands for Electronic Full-Area Sensing Technology. E-Fast is a desirable option on a high-powered, fast-response element because any lack of heat dissipation due to abnormal use, dirt, or scaling can lead to severe overheating and irreversible damage to the element. The E-Fast system is designed to detect such conditions immediately and shut down the element before damage occurs. The protective system can be designed for either grounded or non-grounded appliances. In both cases, the concept involves detecting leakage current from the thick-film circuit. Because leakage current rises exponentially with temperature, a locally overheated area on the heater will quickly generate a large leakage current.
In the non-grounded configuration of E-Fast, a low-temperature version of the dielectric (KERDI LT) is used to separate the thick film from the metal carrier, which serves as the sensor. If leakage current passes through the KERDI LT, it is detected by the sensor, which then reports to the controller, which then shuts down the element. In the grounded configuration, a metallic sensor grid is sandwiched between a layer of KERDI LT and a high temperature version of the material, KERDI HT, which coats the metal carrier. The thick-film traces are applied to the top of the KERDI LT. Any leakage current from the thick-film circuit is detected by the metallic sensor grid. The E-Fast method provides two big advantages over other overheating protective devices such as NTCs, bimetal switches or thermal fuses. Firstly, E-Fast gives a faster response to overheating conditions. Secondly, it is designed so that the sensing mechanism never loses thermal contact with the element, because it is an integral part of the construction.
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