PRINCIPLES OF NAVAL ENGINEERING 



by a compressor/ In a refrigeration cycle, the 

 refrigerant must alte mate between low tempe ra- 

 tures and high temperatures. When the refrig- 

 erant is at a low temperature, heat flows from 

 the space or object to be cooled to the refrig- 

 erant. When the refrigerant is at a high tem- 

 perature, heat flows from the refrigerant to a 

 condenser. The energy supplied as work is used 

 to raise the temperature of the refrigerant to a 

 high enough value so that the refrigerant will be 

 able to reject heat to the condenser. This point 

 is discussed in more detail later in this chapter, 

 but should be noted now since it is basic to the 

 understanding of a mechanical refrigeration 

 cycle. 



Because the energy transformations in a 

 refrigeration cycle occur in an order that is 

 precisely the reverse of the sequence in a power 

 cycle, the refrigeration cycle is sometimes said 

 to be one in which heat is pumped "uphill." 

 This view of a refrigeration cycle is entirely 

 legitimate, provided the "reverse order" of 

 energy transformations does not imply actual 

 thermodynamic reversibility. True thermody- 

 namic reversibility is here, as elsewhere in 

 the observable world, considered to be an im- 

 possibility. A refrigeration cycle does not give 

 us something for nothing. Instead, we must put 

 energy into the cycle in order to extract heat at 

 a low temperature and discharge it at a higher 

 temperature. 



DEFINITION OF TERMS 



Some of the standard terms used in the 

 discussion of refrigeration are defined in this 

 section. A few of these terms have been defined 

 in chapter 8 of this text but are briefly noted 

 here because of their importance in the study of 

 refrigeration. 



UNIT OF HEAT. -The British thermal unit 

 (Btu) is the standard unit of heat measurement 

 used in refrigeration, as in most other engineer- 

 ing applications. By definition, 1 Btu is equal to 

 778.26 foot-pounds. 



A compressor provides the required energy in a 

 vapor-compression refrigeration cycle, which is the 

 cycle most commonly used in naval refrigeration 

 plants. Other kinds of refrigeration cycles use other 

 forms of energy to accomplish the same purpose- 

 namely, to raise the temperature of the refrigerant 

 after it has absorbed heat from the space or object to 

 be cooled. 



SPECIFIC HEAT. -The specific heat of a 

 substance is the quantity of heat required to 

 raise the temperature of unit mass of the sub- 

 stance 1 degree. In British systems of measure- 

 ment, specific heat is expressed in Btuperpound 

 per degree Fahrenheit. 



SENSIBLE HEAT.— Sensible heat is the term 

 used to identify heat that is reflected in a change 

 of temperature. 



LATENT HEAT OF VAPORIZATION .-The 

 heat required to change a liquid to a gas (or, on 

 the other hand, the heat which must be removed 

 from a gas in order to condense it to a liquid) 

 without any change in temperature is called the 

 latent heat of vaporization. 



LATENT HEAT OF FUSION .-The heat which 

 must be removed from a liquid in order to 

 change it into a solid (or, on the other hand, the 

 amount of heat which must be added to a solid 

 to change it to a liquid) without any change in 

 temperature is called the latent heat of fusion. 



REFRIGERATING EFFECT.-Since the heat 

 removed from an object that is being refrig- 

 erated is absorbed by the refrigerant, the re- 

 frigerating effect of a refrigeration cycle is 

 defined as the heat gain per pound of refrigerant. 



REFRIGERATION TON. -The unit which 

 measures the amount of heat removal and thereby 

 indicates the capacity of a refrigeration system 

 is known as the refrigeration ton. The refrigera- 

 tion ton is based on the cooling effect of 1 ton 

 (2000 pounds) of ice at 32= F melting in 24 hours. 

 The latent heat of fusion of ice (or water) is 

 approximately 144 Btu. Therefore, the number of 

 Btu required to melt one ton of ice is 144 x 2000, 

 or 288,000 Btu. The standard refrigeration ton is 

 defined as the transfer of 288,000 Btu in 24 

 hours. On an hourly basis, the refrigeration ton 

 is 12,000 Btu per hour (288,000 divided by 24 

 equals 12,000). 



It should be noted that the refrigeration ton 

 is not necessarily a measure of the ice-making 

 capacity of a machine, since the amount of ice 

 that can be made depends upon the initial tem- 

 perature of the water and other factors. 



COEFFICIENT OF PERFORMANCE .-The 

 coefficient of performance of a refrigeration 

 cycle is essentially comparable to the thermal 



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