The thermostatic wax element was discovered in 1936 by Sergius Vernet (1899-1968). Its main application is the automotive thermostat used in the engine cooling system. The first applications in the pipeline and heating industry were in Sweden (1970) and in Switzerland (1971).
Thermostatic elements of wax convert heat energy into mechanical energy using thermal expansion of the wax when they melt. The principle of this wax motor also finds applications other than engine cooling systems, including thermostatic radiator valve heating systems, plumbing, industrial, and agriculture.
Video Wax thermostatic element
Automotive thermostat
The internal combustion engine cooling thermostat keeps the engine temperature near the optimum operating temperature by adjusting the coolant flow to the air-cooled radiator. This rule is now performed by an internal thermostat. Comfortably, both the thermostat sensing element and the control valve can be placed in the same location, allowing the use of a simple self-sufficient non-electrical thermostat as the primary device for accurate engine temperature control. Although most vehicles now have temperature-controlled electric cooling fans, "unassisted airflow can provide sufficient cooling up to 95% of the time" and so such a fan is not the main control mechanism of internal temperature.
Research in 1920 showed that cylinder wear was exacerbated by fuel condensation when it contacted the cold cylindrical wall that lifted the oil film. The development of automatic thermostats in the 1930s solved this problem by ensuring rapid machine heating.
The first thermostat uses a closed capsule of an organic liquid with a boiling point just below the desired opening temperature. This capsule is made in the form of a cylindrical bellows. When the liquid is boiling inside the capsule, the capsule bellows extend, opening the brass plug valve in the thermostat. Because this thermostat can fail in service, the thermostat is designed for easy replacement during service, usually by being mounted under the fittings on the top of the cylinder block. Conveniently this is also the most accessible part of the cooling circuit, providing a quick response when heating.
The cooling circuit has a small bypass line even when the thermostat is closed, usually by a small hole in the thermostat. This allows enough flow of cooling water to heat the thermostat when heating. It also provides a breakout route for the air trapped when first filling the system. Larger shortcuts are often provided, through cylinder blocks and water pumps, thereby maintaining an even rising temperature distribution.
Working on cooling high-performance aircraft engines in the 1930s led to the application of a pressurized cooling system, which became common in post-war cars. When the boiling point of water increases with increasing pressure, this pressurized system can run at higher temperatures without boiling. This improves both the working temperature of the engine, so that its efficiency, as well as the heat capacity of the cooler by volume, allows a smaller cooling system that requires less pumping power. The disadvantage of a thermostat bellows is that it is also sensitive to changes in pressure, so that sometimes it can be forced to close again by pressure, leading to overheating. The type of young wax pellets has negligible changes in its external volume, so it is not sensitive to pressure changes. This is otherwise identical in operation with the previous type. Many cars from the 1950s, or earlier, originally built with thermostat bellows were then serviced with a replacement wax capsule thermostat, without the need for any change or adaptation.
This most common form of modern thermostat now uses wax pellets inside enclosed spaces. Instead of the liquid-steam transition, it uses a liquid-solid transition, which for the wax is accompanied by a large volume increase. The wax is solid at low temperatures, and when the engine heats up, the wax melts and expands. The enclosed space operates a rod that opens the valve when the operating temperature is exceeded. The operating temperature is specified, but determined by the special composition of the wax, so thermostats of this type are available to maintain different temperatures, typically in the range 70 to 90 ° C (160 to 200 ° F). Modern engines run hot, that is, more than 80 ° C (180 ° F), in order to run more efficiently and reduce pollutant emissions.
While the thermostat is closed, there is no coolant flow in the radiator loop, and the cooling water is diverted through the engine, allowing it to heat quickly while also avoiding hot spots. The thermostat remains closed until the coolant temperature reaches the nominal thermostat opening temperature. The thermostat then becomes more open when the coolant temperature rises to its optimum operating temperature, increasing the coolant flow to the radiator. Once the optimum operating temperature is reached, the thermostat progressively increases or decreases its opening in response to temperature changes, dynamically balancing the recirculation flow of refrigerant and coolant flow to the radiator to maintain the engine temperature in the optimal range as engine heat output, vehicle speed, and beyond the change ambient temperature. Under normal operating conditions, the thermostat opens up to about half of its scratch trips, which may open further or reduce its opening to react to changes in operating conditions. A properly designed thermostat will never fully open or fully closed when the engine is operating normally, or overheating or overcooling will occur.
Machines that require tighter temperature control, because they are sensitive to "Thermal shock" caused by the cooling wave, can use a system of "constant inlet temperature". In this setting the cooling inlet to the engine is controlled by a dual valve thermostat which mixes the flow of circulating sensing with the radiator coolant flow. It uses one capsule, but it has two valve discs. Thus a very compact, simple and effective control function is achieved.
The wax used in the thermostat is specifically manufactured for that purpose. Unlike standard paraffin waxes, which have a relatively long range of carbon chains, the wax used in thermostat applications has very narrow chains of carbon molecules. The extent of the chain is usually determined by the melting characteristics demanded by the specific final application. To make the product in this way requires a very precise distillation level.
Maps Wax thermostatic element
Type of element
Flat diaphragm elements
The temperature sensing material contained in the cup transfers pressure to the piston by using a diaphragm and plug, which is held tightly by the guide. In cooling, the starting position of the piston is obtained by means of a spring return. The flat diaphragm element is highly regarded because of its high degree of accuracy, and is therefore primarily used in sanitary and heating installations.
Push-hitting element
Press-Pressing Elements contain components such as synthetic rubber sleeves shaped like 'finger gloves' that surround the piston. As the temperature increases, the pressure from the expansion of the thermostatic material moves the piston with lateral pressure and vertical impulse. As with the flat diaphragm element, the piston returns to its original position by using the spring back. These elements are slightly less accurate but give longer strokes.
Properties
Stroke is the movement of the piston in relation to its starting point. The ideal stroke corresponds to the element temperature range. By element type, it can vary from 1.5 mm to 16 mm.
The temperature range lies between the minimum and maximum operating temperatures of the elements. Elements may include temperatures ranging from -15 Â ° C to 120 Â ° C. Elements may move proportionately to changes in temperature in some parts of the range, or may open suddenly around a certain temperature depending on the composition of the wax.
Hysteresis is the difference recorded between the rising and falling curves on the heating and cooling elements. Hysteresis is caused by the thermal inertia of the elements and by the friction between the parts in motion.
See also
- Thermostatic radiator valve
- Thermostatic mixing valve
References
External links
- Vernatherm - Thermal Actuator - and Other Thermostatic Fluid Control - Rostra Vernatherm
- ThermalActuators.com - Thermal Actuators - & amp; Mechanical Function Information & amp; Products - Thermal Actuators
- Vernet.fr - Elements of Themostatic Cartridge Thermostat Electrothermic Actuator
- Raymot.com - Page wax thermostatic elements
- Ysnews.com - Vernet founded the leading Yellow Springs company
- Vernay - 1946 Vernet founded Vernay Laboratories
Source of the article : Wikipedia
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