Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGby a compressor/ In a refrigeration cycle, therefrigerant must alte mate between low tempe raturesand high temperatures. When the refrigerantis at a low temperature, heat flows fromthe space or object to be cooled to the refrigerant.When the refrigerant is at a high temperature,heat flows from the refrigerant to acondenser. The energy supplied as work is usedto raise the temperature of the refrigerant to ahigh enough value so that the refrigerant will beable to reject heat to the condenser. This pointis discussed in more detail later in this chapter,but should be noted now since it is basic to theunderstanding of a mechanical refrigerationcycle.Because the energy transformations in arefrigeration cycle occur in an order that isprecisely the reverse of the sequence in a powercycle, the refrigeration cycle is sometimes saidto be one in which heat is pumped "uphill."This view of a refrigeration cycle is entirelylegitimate, provided the "reverse order" ofenergy transformations does not imply actualthermodynamic reversibility. True thermodynamicreversibility is here, as elsewhere inthe observable world, considered to be an impossibility.A refrigeration cycle does not giveus something for nothing. Instead, we must putenergy into the cycle in order to extract heat ata low temperature and discharge it at a highertemperature.DEFINITION OF TERMSSome of the standard terms used in thediscussion of refrigeration are defined in thissection. A few of these terms have been definedin chapter 8 of this text but are briefly notedhere because of their importance in the study ofrefrigeration.UNIT OF HEAT. -The British thermal unit(Btu) is the standard unit of heat measurementused in refrigeration, as in most other engineeringapplications. By definition, 1 Btu is equal to778.26 foot-pounds.A compressor provides the required energy in avapor-compression refrigeration cycle, which is thecycle most commonly used in naval refrigerationplants. Other kinds of refrigeration cycles use otherforms of energy to accomplish the same purposenamely,to raise the temperature of the refrigerantafter it has absorbed heat from the space or object tobe cooled.SPECIFIC HEAT.-The specific heat of asubstance is the quantity of heat required toraise the temperature of unit mass of the substance1 degree. In British systems of measurement,specific heat is expressed in Btuperpoundper degree Fahrenheit.SENSIBLE HEAT.— Sensible heat is the termused to identify heat that is reflected in a changeof temperature.LATENT HEAT OF VAPORIZATION .-Theheat required to change a liquid to a gas (or, onthe other hand, the heat which must be removedfrom a gas in order to condense it to a liquid)without any change in temperature is called thelatent heat of vaporization.LATENT HEAT OF FUSION .-The heat whichmust be removed from a liquid in order tochange it into a solid (or, on the other hand, theamount of heat which must be added to a solidto change it to a liquid) without any change intemperature is called the latent heat of fusion.REFRIGERATING EFFECT.-Since the heatremoved from an object that is being refrigeratedis absorbed by the refrigerant, the refrigeratingeffect of a refrigeration cycle isdefined as the heat gain per pound of refrigerant.REFRIGERATION TON. -The unit whichmeasures the amount of heat removal and therebyindicates the capacity of a refrigeration systemis known as the refrigeration ton. The refrigerationton 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) isapproximately 144 Btu. Therefore, the number ofBtu required to melt one ton of ice is 144 x 2000,or 288,000 Btu. The standard refrigeration ton isdefined as the transfer of 288,000 Btu in 24hours. On an hourly basis, the refrigeration tonis 12,000 Btu per hour (288,000 divided by 24equals 12,000).It should be noted that the refrigeration tonis not necessarily a measure of the ice-makingcapacity of a machine, since the amount of icecan be made depends upon the initial tem-thatperature of the water and other factors.COEFFICIENT OF PERFORMANCE .-Thecoefficient of performance of a refrigerationcycle is essentially comparable to the thermal472