English
中文Humidification Section
1) Describe three general classes of instrument used for the industrial measurement of the Humidity of an air sample giving the major principle each one depends upon.
a) Sling Psychrometer. Used in measuring relative humidity, the sling psychrometer is the simplest type of hygrometer, which consists of a dry bulb and a wet bulb. To make a reading, the wick at the end of the wet bulb thermometer is dipped in water and whirled about a handle or sling, hence the name. The difference in the temperatures of both thermometers is then read and referred to a psychrometer chart to obtain the relative humidity. The principle behind the instrument is that the more water is released from the wick to saturate the surrounding air, the less humid is the air. The energy used to evaporate the water cools the thermometer, so that the dryer the day, the more water evaporates, hence, the lower the temperature of the wet bulb (Dunn, 2005, p.148). This results to a larger difference between the readings of the thermometers, which when referred to the psychrometer chart returns a lower relative humidity value.
b) Hygristor. This is a humidity sensor using a moisture sensitive film, lithium chloride, which is used to find humidity values at different altitudes. The hygristor is usually attached to a small, balloon-borne instrument platform called radiosonde, which is capable of transmitting meteorological data. The principle governing this instrument is that the hygristor’s resistance changes with humidity in a non-linear way (Fraden, 2004, p.66).
c) Hair Hygrometers. This is the oldest and simplest type of hygrometer, which was first built by the Swiss physicist and geologist Horace Benedict de Saussure. This is based on the principle that organic materials such as hair expand with humidity. According to Dunn (2005, p.147) “the human hair lengthens by 3 percent when humidity changes from 0 to 100 percent.” Changes in hair length move the control pointer in the humidity scale where readings can be obtained.
2) Describe the general construction, operating principles and advantages/ disadvantages of each type of cooling tower. a) Natural draught, b) Forced draught c) Induced draught
a) Natural Draught. This is an enclosed device, which has a “waisted” design. Due to its large size, it is the most prominent structure in many nuclear and thermal power stations. According to Andhra Pradesh State Electricity Board, natural draught cooling towers rank among the largest reinforced concrete thin shell structures. With a thickness as little as 170 mm for a height of over 150 m, the shell is an astonishing structure, hence, considered to be one of the greatest structural innovations of all times (APSE Board, [n.d.], p.2). Water is cooled through direct contact with air. Air flows through the tower due to the density differential between the less dense air outside and the ambient air outside the tower. Although dependable in its thermal performance, this type of cooling towers is more expensive, and would not be appropriate in arid and high altitude regions.
b) Forced Draught. A forced draught is square type mechanical tower where a fan is mounted at its base and the air is blown through and discharged at the top. Special applications like cooling corrosive liquids usually use this type of cooling tower. One of the advantages of this type is the minimal vibration since mechanical assembly is mounted on a solid foundation near the ground. It is also more efficient than the induced draught type, can be installed indoors, and unlikely for the mechanical assembly to corrode since its location is in a dry air stream. On the other hand, this type requires more horsepower inasmuch as air is forced up which is subjected to the resistance of gravity. Noise level is high because the placement of the motor is within the level of the observer. They are also extremely susceptible to recirculation and are therefore considered to have less performance stability than the induced draught (Hensley, 2006, p. 9).
c) Induced Draught. As opposed to the forced draught type, induced draught can be square or round and usually has its fan located at the top. Air is drawn through the tower by the fan located at the exiting air stream. A fill of wood or plastic material increases water surface, which is distributed over fills by spray. Induced draughts are relatively popular inasmuch space requirement is less than what natural draught towers consume. Unlike the forced draught, noise level is low since the fan motor is placed at the top of the tower. The potential for recirculation on an induced draught tower is not self-initiating and, therefore, can be more easily quantified purely on the basis of ambient wind conditions (Hensley, 2006, p. 9). On the demerits side, vibration is high and proper fan assembly balancing is required since it is located at the top of the tower. It is also essential to observe regular fan maintenance since it is always in contact with hot and humid air.
