Refrigeration cycle Carnot

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... 

Referring to Fig. 1.5, the system undergoes a Carnot heat pump or Carnot refrigerator cycle in the following manner ... 

Determine the COP, horsepower required, and cooling load of a Carnot vapor refrigeration cycle using R-12 as the working fluid and in which the condenser temperature is 100°F and the evaporation temperature is 20°F. The circulation rate of fluid is O.llbm/sec. Determine the compressor power required, turbine power produced, net power required, cooling load, quality at the inlet of the evaporator, quality at the inlet of the compressor, and COP of the refrigerator. 

Why is the Carnot refrigeration cycle executed within the saturation dome not a realistic model for refrigeration cycles ... 

Figure 6.39 Ts diagram of ideal nonazeotropic refrigeration cycle and Carnot cycle.

The word refrigeration implies the maintenance of a temperature below that of the surroundings. This requires continuous absorption of heat at a low temperature level, usually accomplished by evaporation of a liquid in a steady-state flow process. The vapor formed may be returned to its original liquid state for reevaporation in either of two ways. Most commonly, it is simply compressed and then condensed. Alternatively, it may be absorbed by a liquid of low volatility, from which it is subsequently evaporated at higher pressure. Before treating these practical refrigeration cycles, we consider the Carnot refrigerator, which provides a standard of comparison. 

The effectiveness of a refrigeration cycle is measured by its coefficient of performance. For given values of Tc and TH, the highest possible value is attained by the Carnot refrigerator. The vapor-compression cycle with reversible compression and expansion approaches this upper limit. A vapor-compression cycle with expansion in a throttle valve has a somewhat lower value, and this is reduced further when compression is not isentropic. The following example provides an indication of the magnitudes of coefficients of performance. 

Soave modification of, 491-492 Reduced coordinates, 85, 477 Refrigeration, 274-290 Refrigeration cycles absorption, 288-289 Carnot, 275-276 cascade, 286-287... 

The Carnot refrigeration cycle is reversible and consists of adiabatic (isentropic due to reversible character) compression (1-2), isothermal rejection of heat (2-3), adiabatic expansion (3-4) and isothermal addition of heat (4-1). The temperature-entropy diagram is shown in Fig. 11-70. The Carnot cycle is an unattainable ideal which serves as a standard of comparison and it provides a convenient guide to the temperatures that should be maintained to achieve maximum effectiveness. 

The COP in real refrigeration cycles is always less than for the ideal (Carnot) cycle and there is constant effort to achieve this ideal value. 

In comparing the performance of a real cycle with that of a Carnot cycle, one has in principle a choice of temperatures to use for the Carnot calculation. Consider a vapor-compression refrigeration cycle in which tlie average fluid temperatures m the condenser and evaporator are Th aiid Tc, respectively. Corresponding to Th and Tc, tlie heat transfer occurs with respect to surroundings at temperature T , and Which provides the more conservative estimate of j(Camot) a calculation based on Th and Tc, or one based on Ts and... 

The COP is a strong function of the temperature range over which the refrigeration cycle operates. For an ideal refrigeration cycle (a reverse Carnot cycle), the COP is... 

The COP of real refrigeration cycles is always less than the Carnot efficiency. It is usually about 0.6 times the Carnot efficiency for a simple refrigeration cycle, but can be as high as 0.9 times the Carnot efficiency if complex cycles are used. Good overviews of refrigeration cycle design are given by Dincer (2003), Stoecker (1998), and Trott and Welch (1999). 

Estimate the annual cost of providing refrigeration to a condenser with duty 1.2 MW operating at —5°C. The refrigeration cycle rejects heat to cooling water that is available at 40 °C and has an efficiency of 80% of the Carnot cycle efficiency. The plant operates for 8,000 hours per year and electricity costs 0.06/kWh. 

An ideal refrigeration cycle could be based on a Carnot cycle, for which the ideal coefficient of performance is ... 

For a reversible cycle, the lost work would be zero, and this form of the availability balance is the classical result for the refrigeration (reverse Carnot) cycle. To prove this, the first law gives... 

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