James Joule worked out the mechanical equivalent of heat by stirring up water in a thermally insulated container. The apparatus uses two sets of vanes. One set is fixed to the inside wall of the container and the other set forms a rotor. The container is filled with water.
When the rotor is rotated, the water starts to rotate with it but is impeded by the fixed vanes causing a great deal of turbulence and drag.
The agitation of the water is converted into heat. If the rotor is operated by a pulley and a falling weight, it is possible to derive the amount of energy imparted to the water from the volume of water used, the size of the weight, and the distance the weight falls. The heat generated is the product of the temperature rise and the mass of water used. The mechanical energy expended is equal to the size of the weight times the falling distance.
A version of this device can be used as an effective heat generator. At sea, there is a large amount of mechanical energy in the waves. Some of this energy can be extracted by the combination of a balloon and a drogue. The heat pump illustrated works linearly. As a wave rises, the ballon lifts a perforated piston in a cylinder that is restrained from moving upwards by the drogue. The piston is returned by a spring or a strong piece of elastic rubber. As the piston moves downward, flap valves open, allowing the piston go downwards easily. The drogue is weighted and shaped somewhat like a half-opened umbrella. When the balloon falls, the drogue folds up and is pulled down by the weight. When the balloon rises the drogue opens and forms a sort of inverted parachute.
The water can only get through the perforated piston with difficulty on the upward stroke. The mechanical energy expended in lifting the piston is converted into heat. In a situation where the waves are one metre high from trough to peak, a balloon one metre in diameter can provide a lifting force of more than 300kg. Such waves occur at the rate of one wave in 3 to 4 seconds. The energy available is between 80 and 100 kilogram metres per second. If all this energy was converted into heat, the heating effect would be the same as an 800 watt heater or more. In practice it is likely that little more than a quarter of this heat can be used to warm a survivor at sea. A typical human generates around 80 watts of heat without moving. If the human can be given some protection from free-flowing water, the rate of heat loss when compared to the open sea is almost halved. The survival time for an unprotected person in a cold sea may be less than 15 minutes. Scuba divers can survive in a thinnish rubberised suit for more than half an hour. A small amout of water inside the suit effectively adds a signifcant amount of insulation from the cold water. This effect is made use of in the lifebag described in a separate article.
The lifebag provides some insulation by trapping a thin layer of water around the survivor. The bottom of the bag is open and allows heat from a heat pump to rise into the water around the survivor. The heat pump described could raise the temperature of the water around the survivor by 10 degrees Celsius in 10 minutes. The rate of temperature increase will fall as the difference between the warmed water and surrounding water increases. The limit is likely to be when the water around the survivor is between 20 and 25 degrees Celsius warmer than the surrounding sea. It is thought that this difference in local sea temperature will be sufficient to prevent hypothermia.
Copyright (C) W. H. James 14/6/1998
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