Refrigerators and heat pumps

Another rule of tlie second law of thermodynamics is as follows heat cannot flow from one body to another body at a higher temperature without other changes being involved. The transfer of heat from a cooler body to a warmer one can take place only because of work A done under the action of external forces on the working body. The thermodynamic cycle initiated in the reverse direction is used in refrigerators and thermal pumps. 

The removal of an amount of heat Q2 from a cool body and its transfer to a more heated one (the thermal reservoir) is the task of the refrigerating machine (refrigerator). The thermal coefficient of performance K is defined by the ratio of the amount of heat gi removed from a cool body and given to the warm body to the external mechanical forces work A  

The basic difference between a thermal pump and a refrigerator is that in the thermal pump the amount of heat Q/ is given to the heated body, and in the refrigerator Q2 is removed from the cooled body. Heat pumps require less power expense for the work than electro-heating devices. 

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Active carbons can be used in both refrigeration and heat pumping cycles, but their potential for use in these applications does not necessarily merit the development of such systems. Before devoting research and development effort into active carbon-based thermodynamic cycles, the interest in both heat-driven cycles in general, and adsorption cycles in particular, must be justified. 

The best sub-atmospheric refrigerant is water. Unfortunately it is not strongly adsorbed by carbons, but refrigerators and heat pumps based on water - zeolite pairs have been built and tested in research laboratories. Methanol is adsorbed well by carbons and a solar refrigerator based on a carbon - methanol pair was marketed by Brissoneau et Lotz Marine in France. Methanol is environmentally friendly, but deeomposes at temperatures around 150°C and so camiot be used for very high temperature cycles. 

Figs. 9 and 10 below show the COP s of refrigerators and heat pumps respectively in the basic cycle deseribed in Seetion 2... 

Tamainot-Telto, Z., and R.E. Critoph, 2001. Monolithic carbon for sorption refrigeration and heat pump applications, Appl. Thermal Eng., 21 (1), 37-52. 

Carnot Refrigerator and Heat Pump Basic Vapor Refrigeration Cycle Actual Vapor Refrigeration Cycle Basic Vapor Heat Pump Cycle Actual Vapor Heat Pump Cycle Working Fluids for Vapor Refrigeration and Heat Pump Systems Cascade and Multistaged Vapor Refrigerators... 

Thermodynamic cycles can be divided into two general eategories heat engine cycles, which are discussed in Chapters 2-5, and refrigeration and heat pump cycles, which are discussed in Chapter 6. 

The working fluids, such as ammonia and freons, used in refrigerators and heat pumps are more desirable than steam for the very low-temperature Rankine cycles. The reason is that the specific volume of such working fluids at low temperature is much less than that of steam, so the resulting turbine sizes can be much smaller and less expensive. 

Why are working fluids such as ammonia and freons, used in refrigerators and heat pumps, more desirable than steam for the low-temperature Rankine cycles ... 

The Carnot cycle is a reversible cycle. Reversing the cycle will also reverse the directions of heat and work interactions. The reversed Carnot heat engine cycles are Carnot refrigeration and heat pump cycles. Therefore, a reversed Carnot vapor heat engine is either a Carnot vapor refrigerator or a Carnot vapor heat pump, depending on the function of the cycle. 

Review Problems 6.1 Carnot Refrigerator and Heat Pump... 

The desirable properties of working fluids for vapor refrigeration and heat pump systems include high critical temperature and low pressure, low specific volume, inexpensive, nonflammable, nonexplosive, nontoxic, noncorrosive, inert and stable, etc. 

Review Problems 6.6 Working Fluids for Vapor Refrigeration and Heat Pump Systems... 

Why are ammonia, carbon dioxide, and sulphur dioxide no longer used in domestic refrigerators and heat pumps ... 

List five desirable properties of working fluids for vapor refrigeration and heat pump systems. 

Refrigerators and heat pumps have the same energy flow diagram and have the same components. A domestic air-conditioning and heat pump system as shown in Fig. 6.22 can, therefore, be used as a heat pump in the winter as well as an air-conditioning unit in the summer. Notice that both the domestic air-conditioning and heat pump system share the same equipment. Thus, the investment in the heat pump can also be used for air-conditioning to provide year-round house comfort control. 

Large temperature differences can be achieved by cascaded refrigerators and heat pumps. 

Multistaged refrigerators and heat pumps reduce the compressor power. 

Terry, L.E. "Hydrogen-Hydride Absorption Systems and Methods for Refrigeration and Heat Pump Cycles" US Patent U,055,962 Nov. 1, 1972. 

Figure 5.26. Electricity derived from fossil energy (by conventional power plants), to be used for covering half the needs in the transportation sector and all requirements for stationary mechanical energy, dedicated electricity and electricity for cooling, refrigeration and heat pumps, after transmission losses. The figure shows average flows in W per m of land area for each country (Sorensen, 1999).

Plasma Catalysis of Hydrogen Production by Direct Decomposition (Pyrolysis) of Ethane. Interpreting the plasma-catalytic effect of ethane decomposition and hydrogen production illustrated in Fig. 10-10, explain why the application of thermal plasma results in an increase of gas temperature, while application of non-equilibrium plasma results in gas cooling and additional hydrogen production. Compare the thermodynamics of these systems with that of refrigerators and heat pumps. 

Be able to insert turbines and heat engines, and compressors, refrigerators, and heat pumps, to satisfy both the heating and power demands for a process. 

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