Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGtemperatures are usually considered. These arethe dry-bulb temperature and the wet-bulb temperature.The dry-bulb temperature is the temperatureof the air as measured by an ordinary dry-bulbthermometer. The dry-bulb temperature reflectsthe sensible heat of the air.The wet-bulb temperature is the temperatureof the air as measured by a wet-bulb thermometer.A wet-bulb thermometer is an ordinarythermometer with a loosely woven cloth sleeveor wick placed around the bulb and then wetwith water. The water in the sleeve or wick ismade to evaporate by a current of air at highvelocity. The evaporation lowers the temperatureof the wet-bulb thermometer. The differencebetween the dry-bulb temperature and thewet-bulb temperature is called the wet-bulbdepression . When the air is saturated, so thatevaporation cannot take place, the dry-bulbtemperature is the same as the wet-bulb temperature;the condition of saturation is unusual,however, and a wet-bulb depression is normallyto be expected.The wet-bulb thermometer and the dry-bulbthermometer are usually mounted side by sideon a frame. A handle or a short chain is attachedto the frame so that the thermometers may bewhirled in the air, thus providing an air currentof high velocity to facilitate evaporation. Such adevice is known as a sling psychrometer. (Seefig. 19-13.) Motorized psychrometeTi areprovided with a small motor-driven fan and drycell batteries. Motorized psychrometers aregradually replacing the sling psychrometers.An exposed view of a hand electric psychrometeris shown in figure 19-14. With either type ofpsychrometer, the wet-bulb temperature mustbe observed at intervals as the water is beingevaporated. The point at which there is no furtherdrop in temperature on the wet-bulb thermometeris the wet-bulb temperature of thespace.As may be inferred from this discussion,the wet-bulb depression is an indication thatlatent heat of vaporization has been used tovaporize the water in the sleeve or wick aroundthe wet-bulb thermometer.When the air contains some moisture but isnot saturated, the dew-point temperature islower than the dry-bulb temperature and thewet-bulb temperature is between the dew-pointand the dry-bulb temperatures. As the amountof moisture in the air increases, the differencebetween the dry-bulb temperature and the wetbulbtemperature becomes less and less. Whenthe air is saturated, the dew-point temperature,the dry-bulb temperature, and the wet-bulbtemperature are identical.AIR MOTION.— In perfectly still air,a layerof air adjacent to the body absorbs the sensibleheat given off by the body and increases in temperature.This layer of air also takes up theGRIPFigure 19-13.— A standard sling psychrometer,5.65486