Evaporation pressure

The optimum intermediate pressure for the two-stage refrigeration cycles is determined as the geometric mean between evaporation pressure (pi) and condensing pressure (p/, Fig. 11-79) ... 

Gurgel and Grenier s results showed the bed conductivity to increase from 0.14 to 0.17 W/mK as the pressure was raised from 4 mbar (evaporating pressure) to 110 mbar (condensing pressure). The principle reason stated for this small variation is the reduction in the gas conductivity with decreasing pressure (Knudsen effect) in the macropores. The solid grain conductivity varied linearly from 0.61 to 0.65 W/mK as the methanol concentration varied from 0 to 31%. 

At the low absolute pressure of the flash chamber, the entering water partially evaporates and in so doing absorbs heat from the bulk of the water in the compartment. The latent heat of steam (greater than 1,000 Btu/lb) at the evaporator pressure is removed and the water in the compartment is cooled an equivalent amount. Figure 11-8 indicates the conditions for one system. 

Enthalpy,Btu/lb., h Temperature at 3 i Condensing Pi is Evaporating Pressure P2 is Condensing Pressure... 

System is more flexible as to changes in required evaporator pressure. 

A low-pressure cut-out switch is usually fitted to stop the compressor under these circumstances. Settings may be 0.6-1.0 bar below the design evaporator pressures, but depend very much on the type of system. The cut-out setting should be above atmospheric pressure if possible to avoid the ingress of air through any leaks. 

Back pressure regulation valves (Figure 9.5) can be used in the suction line, and their function is to prevent the evaporator pressure falling below a predetermined or controlled value, although the compressor suction pressure may be lower. 

Modulate the evaporator pressure according to a varying load, controlled by the load temperature. 

Figure 10.2 Use of back pressure regulating valve to maintain evaporator pressure (and temperature)...

Vacuum operation. The second consideration, illustrated in Figure 24.32, is that at the evaporator temperature, evaporator pressure below atmospheric pressure should be avoided. An evaporator pressure above atmospheric avoids potential problems with the ingress of air into the cycle, which can cause performance and safety problems. However, special designs can use evaporator pressures below atmospheric. The boiling points of some common refrigerants are given in Table 24.3 at atmospheric pressure. 

II2 = specific enthalpy at the evaporator outlet (saturated vapor enthalpy at the evaporator pressure (J kg-1, kJkg ) H4 = specific enthalpy at the condenser... 

Calculation of the power requirements for the three refrigerants requires the flowrate to be calculated for a duty of 3 MW. This can be calculated from the enthalpy difference across the evaporator (H2 — H1). The enthalpy difference across the evaporator is assumed to be the difference between the saturated vapor enthalpy at the evaporator pressure and the saturated liquid enthalpy at the condenser pressure. This assumes no subcooling of the refrigerant. 

Determine the COP, horsepower required, and cooling load of a basic vapor refrigeration cycle using R-12 as the working fluid and in which the condenser pressure is 130 psia and the evaporation pressure is 35 psia. The circulation rate of fluid is 0.1 Ibm/sec. The temperature of the refrigerant at the exit of the compressor is 117°F. Determine the compressor power required, cooling load, quality at the inlet of the evaporator, and COP of the refrigerator. 

A steady-flow ideal 0.4 ton refrigerator uses refrigerant R-134a as the working fluid. The evaporator pressure is 120 kPa. The condenser pressure is 600 kPa. Determine (a) the mass rate flow, (b) the compressor power required, (c) the rate of heat absorbed... 

Heat Requirement of the Process. Heat is required for vaporization in the extractive distillation column, and for the reconcentration of magnesium nitrate solution. Overall thermal effects caused by the magnesium nitrate cancel out, and the heat demand for the complete process depends on the amount of water being removed, the reflux ratio employed, and the terminal (condenser) conditions in distillation and evaporation. The composition and temperature of the mixed feed to the still influence the relative heat demands of the evaporation and distillation sections. For the concentration of 60 wt% HNO3 to 99.5 wt% HNO3 using a still reflux ratio of 3 1, a still pressure of 760 mm Hg, and an evaporator pressure of 100 mm Hg, the theoretical overall heat requirement is 1,034 kcal/kg HNO3. 

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