Evaporative cooling
Evaporative cooling (evaporation cooling) in comparison with the competing methods heat conduction , convection and thermal radiation is the most effective way to a system or subject to heat or thermal energy to escape.
principle
A metal that conducts heat well, such as copper , has a large number of parallel, vertical holes into which a coolant, usually water , is pumped from below . This evaporates on the hot surface and requires a lot of thermal energy ( latent heat ), which in turn ensures very effective cooling of the metal. The steam is sucked off and passed through an air condenser at another point , which causes the steam to condense again, giving off heat.
Compared to smooth surfaces, bores have the advantage that the rising vapor bubbles generate a stronger turbulent flow and thus prevent film boiling with insufficient heat transfer from the metal to the liquid.
The great advantage of evaporative cooling compared to other cooling methods is the very high power density and also the protection against overheating as long as coolant is pumped up. Heat flux densities of a maximum of 300 kW / m 2 can be achieved with water without damaging the heat transfer surfaces.
In order to evaporate water at 100 ° C and 1013 mbar, the heat of evaporation of ΔU = 2257 kJ / kg must be used according to the table . In contrast, the specific heat capacity of liquid water is only 4.187 kJ / (kg · K). In order to achieve the same heat dissipation with conventional water cooling with identical coolant circulation, the water would have to be heated by 540 K.
Performance comparison
The following table shows the structure of the anode and the maximum specific load capacity using the example of transmitter tubes for different types of cooling.
| Type of cooling | Anode type | Max. specific load capacity |
|---|---|---|
| radiation | Graphite, molybdenum | 10 W / cm² |
| Compressed air | Outer anode made of copper with cooling fins | 50 W / cm² |
| Water or oil cooling | Outer anode made of copper, surrounded by coolant | 100 W / cm² |
| Evaporative cooling | Outer anode made of copper, water is evaporated | 500 W / cm² |
By far the greatest load capacity results from evaporative cooling. The high evaporation heat of the water is used to achieve a high power density.
Applications
Anodes of electron tubes
Lee de Forest , from the Western Electric Company in Chicago, had already proposed in 1915 in the USA to dissipate greater power from electron tubes by cooling the anode with water. At this point, however, it was not possible to create vacuum-tight connections between glass and copper (anode). This problem was only solved in the 1920s by glazing the cutting edge . In addition to air cooling, other types of cooling have also been developed and introduced over the years. The most recently introduced type of cooling is evaporative cooling (evaporative cooling). It is also the most economical form. The coolant consumption is about 0.03 l / min per kW tube output. If the steam is re-cooled, it is only necessary to supplement any water losses in the circuit.
Heat pipe
Heat pipes (heat pipes) . This method does not require any auxiliary energy and works silently.
Boiling water reactors
The fuel rods are cooled in this way in boiling water reactors .
Blast furnace jacket cooling
For jacket cooling, blast furnaces can also be equipped with evaporative cooling instead of normal water cooling. With this type of cooling, the boiling temperature is only slightly above 100 ° C, in accordance with the height arrangement for the upper drum, in which the separation into water vapor and water takes place. The main advantage is the constant material temperature for the outer jacket of the blast furnace including the piping and connections for the cooling boxes.
Evaporative cooling for internal combustion engines
The evaporative cooling (evaporative cooling) refers in internal combustion engines an open cooling system (also evaporator motor s) and was due to its simple structure without heat exchanger (cooler) and circulating pump (water pump) in the early days in engine design, primarily in single-cylinder stationary engines , tractors and construction machines, but few even with aircraft engines.
The cylinder is surrounded by a water jacket in the engine block; the required cooling water (cooling medium) is simply poured in from above through an opening in the engine block . The filling opening has a diameter of approx. 15 cm and is not closed. After some time, depending on the load, altitude above sea level and outside temperature, the heat generated during operation of the internal combustion engine causes the cooling water to boil and evaporate without any further temperature rise. The engine temperature is a constant 100 ° C (at sea level) and cannot exceed this value as long as there is sufficient water in the system. The resulting water vapor escapes through the filling opening upwards. To check the water level, there is a float at the filler opening with a dipstick that protrudes upwards from the engine block.
The advantage of the open cooling system is its simple structure. The disadvantages are the high water consumption (at full load up to several buckets of water per day) and the comparatively very large engine block required by the water jacket and the associated high weight.
Evaporative cooling has not been used in internal combustion engines in the western industrialized countries since around 1950, but engines with evaporative cooling are still manufactured in China, India, etc., because of the simple design (without water pump, hoses, cooler and thermostat) and any repairs are very easy.
swell
- Hans Heinrich Meinke , Friedrich-Wilhelm Gundlach : Pocket book of high frequency technology . Springer 1992, ISBN 3-540-54715-0
- Tom Lewis: Empire of the Air: The Men Who Made Radio . Harpercollins 1991, ISBN 0-06-018215-6
Web links
Individual evidence
- ↑ b: Collection of tables_Chemie / _Stoffdaten_Wasser
- ↑ Wolseley Motor ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
