Module No. 32
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1) Boyle's law states that the pressure and volume of a gas are inversely proportional at constant temperature. Charles' law states that the volume of a gas is directly proportional to its temperature at constant pressure. 2) A refrigerant is a substance used in refrigeration to absorb heat from the space being refrigerated and release it elsewhere. Common refrigerants like Freon gas are used in refrigerators. 3) A refrigerator uses a vapor-compression cycle to cool its interior. Freon gas is compressed, condenses while releasing heat outside, then evaporates in the interior, absorbing heat and cooling the refrigerator. This cycle repeats continuously.
Module No. 32
- 1. 1 Module # 32 Boyle’s Law, Charles’ Law & Refrigeration Boyle (1627-1691) Famous physicist Boyle is well known for his Boyle's law of gases. Boyle’s Law This law was formulated by Robert Boyle in 1662. This law states that the absolute pressure of a given mass of a perfect gas varies inversely as its volume when the temperature remains constant. OR Boyle's law states that “the volume of a given mass of a gas varies inversely as its absolute pressure provided temperature remains constant”. If P = absolute pressure of a gas V = its volume Then according to Boyle's law temperature remaining constant V l / P OR
- 2. 2 PV = constant In other words, the product of absolute pressure and volume of a given mass of a gas is constant when there is no change in its temperature. If a gas initially at a pressure P1 and volume V1 is expanded or contracted at a constant temperature such that its new pressure and volume are P2 and V2 respectively then according to Boyle's law. P1V1=P2V2 No known gas obeys Boyle's law perfectly. There is always a deviation which can be over looked in case of air, hydrogen and nitrogen etc. except at very low temperature or at very high pressure. If we draw a graph between volume and pressure, we get a curve called Hyperbola. V Fig: (1) Graph of pressure and volume
- 3. 3 Verification of Boyle's Law Boyle's law can be verified by means of an experimental arrangement shown in the figure (2). It consists of two glass tubes A and B of uniform bore connected by a long rubber tubing. Tube A is open and the tube B is closed at the top. They are supported vertically by the side of a vertical scale such that tube A can be moved upward or downward. Mercury is poured in tube A until it partially fills both the tubes, trapping a mass of air in B. The volume V of the air enclosed in B is noted and the difference of heights of two mercury Columns is measured. The tube A is then moved to another position and the readings are noted. This process is repeated for a number of different positions of tube A. Suppose that mercury level D in the tube A is higher than that in tube B. The pressure P on the air would be equal to the sum of atmospheric pressure measured by a barometer and the difference between the levels of two mercury columns. If D were lower than the mercury surface in B, the pressure on the air in B would be equal to atmospheric pressure minus the difference between the levels of two mercury columns. The product PV, for each reading is found to be the same.
- 4. 4 Fig: (2) Verification of Boyle’s Law Charles (1747-1823) All substances expand when they are heated, but, gases expand much more than solids or liquids. Jacques Charles observed experimentally that when the pressure of a gas is kept constant, then, its volume increases with temperature at a constant rate. That is, if the absolute temperature of the gas is doubled, then, the volume also becomes doubled and vice versa. Charles’ Law This law was formulated by a Frenchman Jacques A.C Charles in about 1787. It may be stated in two different forms (1) The volume of a given mass of a perfect gas varies directly
- 5. 5 as its absolute temperature, when the absolute pressure remaining constant. If V= volume of a gas T= absolute temperature in °K Then according to Charlie's law, pressure remaining constant, V T OR V/T = constant V1 / T1 = V2 / T2 = V3 / T3 =.......Constant Where suffixes 1, 2, and 3.... refer to different sets of conditions. (2) All perfect gases change in volume by l / 273th of its original volume at 0°C for every 1°C change in temperature, when the pressure remains constant. If we plot volume against temperature, we get a straight line. This means that as T decreases, V decreases correspondingly. If this straight line is produced towards low temperatures, it meets temperature axis at -273° C or 0K indicating zero volume.
- 6. 6 However, volume of gas can never be zero. We cannot draw this line anymore, because it will indicate negative volume which is a meaningless idea. Thus -273°C is the lowest possible temperature that is possible to attain. This temperature is called the absolute zero. For this reason, -273°C is taken as the zero of the absolute scale of temperature measured in Kelvin. V Fig: (1) Temperature Volume Graph Refrigerant The working substance used in a refrigerator is called refrigerant. Generally speaking, a refrigerant is anybody or substance which acts as a cooling agent by absorbing heat from another body or
- 7. 7 substance. The refrigerant must be circulated in a cyclic process during a refrigeration process and must be used again and again. Hence, vapor compression cycle and vapor absorption cycle are the two most important cycles of refrigeration. A heat carrier must be used to move heat from the interior of a cabinet or room to the outside. Thus in refrigerating systems fluids which absorb heat inside the cabinet and release it out side are called refrigerants. The ideal refrigerant would be one that could liberate all the heat that it is capable of absorbing. The refrigerant changes from liquid to vapor during the process of absorbing heat and condenses again to liquid while liberating heat, in most of the refrigerating systems. The refrigerant may be defined as a carrier of heat from a low temperature region to a high temperature region. Refrigeration In general, refrigeration is defined as any process of heat removal. More specifically, refrigeration is defined as that branch of science which deals with the process of reducing and maintaining the temperature of a space or material below the temperature of the surroundings.
- 8. 8 Since the heat removed from the refrigerated body is transferred to another body, it is evident that refrigerating and heating are actually opposite ends of the same process. Often only the described result distinguishes one from the other. Refrigeration may be defined as the process by which the temperature of a given space or a substance is lowered below that of the atmosphere or surroundings. The system maintained at the lower temperature is known as refrigerated system and the equipment which is used to maintain this lower temperature is known as the refrigerating system. In order to maintain a temperature below the surrounding atmosphere, it is essential that whatever amount of heat gets into refrigerated space, must be extracted out to maintain the desired temperature difference. Ton of Refrigeration Due to the fact that in early stages refrigeration was produced by ice, the rate of removal of heat in a cooling operation was expressed in terms of tons of ice required per unit time, usually per day (24 hours). In F.P.S. system, the latent heat of fusion of ice is 144 B.T.U. /lb., which means that one Ib. of ice absorbs 144 BTU of heat when it melts. One short ton (2000 Ibs.) of ice, therefore, absorbs 2000 x 144 or 288,000 BTU of heat. In other words, when one ton of ice
- 9. 9 melts in 24 hour it will produce cooling effect at the rate of 288,000/ 24 = 12,000 BTU per hour or 12,000/60 = 200 BTU per minute. This rate of cooling has been designated as a ton of refrigeration. So, a ton of refrigeration is not a unit of mass but it is a measurement of the rate of heat transfer. In M.K.S. system of units 72, 000 kcal per 24 hours or 3000 kcal per hour or 50 kcal per minute rate of cooling means one-ton refrigeration. Hence, a machine having its capacity to produce cooling effect of 200 BTU/min or 50kcal/min is classed as one-ton machine. Refrigerator The machine which is used to keep the things cool is commonly known as refrigerator. OR It is a device which makes the heat flow from a body at lower temperature to a body at higher temperature. Principle of Refrigerator Gas is liquefied under pressure and is then let through a valve into the evaporator. But under normal pressure this liquid evaporates and absorbs a lot of heat from the area surrounding
- 10. 10 the evaporator and again converts into gas. This process continues and things in it always remain cool. Construction Fig: The Refrigerator A refrigerator consists of three parts. 1. Evaporator 2. Compressor 3. Condenser Working of Refrigerator In the three parts of the refrigerator, the following processes occur.
- 11. 11 A refrigerator uses such gases which can easily liquefy under pressure at normal temperature. Usually Freon gas is used for this purpose. Compressor In this part, the gas is first compressed and then it is fed into the condenser. Condenser In it, gas is condensed into liquid under pressure. Here, heat of condensation is evolved. Now, the liquid gas moves into the evaporator through a valve. Evaporator In the evaporator, the liquid Freon evaporates under normal pressure absorbing a lot of heat from the area surrounding the evaporator, and, thus cooling it down. The gas is then again brought to the condenser with the help of compressor. In the condenser, it loses a large amount of heat on being liquefied. This cycle is then continuously repeated. The condenser is placed outside the body of the refrigerator so that it can lose its heat in the atmosphere. The evaporator is placed inside the body.