Thermodynamics

cen84959_ch03.qxd 4/1/05 12:31 PM Page 156156 | Thermodynamicsuntil the temperature is 15°C. Determine (a) the final pressure,(b) the change in the volume of the cylinder, and (c) thechange in the enthalpy of the refrigerant-134a.3–38E The temperature in a pressure cooker during cookingat sea level is measured to be 250°F. Determine the absolutepressure inside the cooker in psia and in atm. Would youmodify your answer if the place were at a higher elevation?Pressurecooker250°FFIGURE P3–38E3–39E The atmospheric pressure at a location is usually specifiedat standard conditions, but it changes with the weatherconditions. As the weather forecasters frequently state, theatmospheric pressure drops during stormy weather and it risesduring clear and sunny days. If the pressure difference betweenthe two extreme conditions is given to be 0.3 in of mercury,determine how much the boiling temperatures of water willvary as the weather changes from one extreme to the other.3–40 A person cooks a meal in a 30-cm-diameter pot that iscovered with a well-fitting lid and lets the food cool to theroom temperature of 20°C. The total mass of the food and thepot is 8 kg. Now the person tries to open the pan by liftingthe lid up. Assuming no air has leaked into the pan duringcooling, determine if the lid will open or the pan will moveup together with the lid.3–41 Water is to be boiled at sea level in a 30-cm-diameterstainless steel pan placed on top of a 3–kW electric burner. If60 percent of the heat generated by the burner is transferredto the water during boiling, determine the rate of evaporationof water.3–42 Repeat Prob. 3–41 for a location at an elevation of1500 m where the atmospheric pressure is 84.5 kPa and thusthe boiling temperature of water is 95°C.3–43 Water is boiled at 1 atm pressure in a 25-cm-internaldiameterstainless steel pan on an electric range. If it isobserved that the water level in the pan drops by 10 cm in 45min, determine the rate of heat transfer to the pan.3–44 Repeat Prob. 3–43 for a location at 2000-m elevationwhere the standard atmospheric pressure is 79.5 kPa.3–45 Saturated steam coming off the turbine of a steampower plant at 30°C condenses on the outside of a 3-cmouter-diameter,35-m-long tube at a rate of 45 kg/h. Determinethe rate of heat transfer from the steam to the coolingwater flowing through the pipe.3–46 The average atmospheric pressure in Denver (elevation 1610 m) is 83.4 kPa. Determine the temperature atwhich water in an uncovered pan boils in Denver.Answer: 94.6°C.3–47 Water in a 5-cm-deep pan is observed to boil at 98°C.At what temperature will the water in a 40-cm-deep pan boil?Assume both pans are full of water.3–48 A cooking pan whose inner diameter is 20 cm is filledwith water and covered with a 4-kg lid. If the local atmosphericpressure is 101 kPa, determine the temperature atwhich the water starts boiling when it is heated.Answer: 100.2°CP atm = 101 kPam lid = 4 kgH 2 OVaporFIGURE P3–4860%3 kWFIGURE P3–4140%3–49 Reconsider Prob. 3–48. Using EES (or other)software, investigate the effect of the mass of thelid on the boiling temperature of water in the pan. Let themass vary from 1 kg to 10 kg. Plot the boiling temperatureagainst the mass of the lid, and discuss the results.3–50 Water is being heated in a vertical piston–cylinderdevice. The piston has a mass of 20 kg and a cross-sectionalarea of 100 cm 2 . If the local atmospheric pressure is 100 kPa,determine the temperature at which the water starts boiling.