156 | Thermodynamics
until the temperature is 15°C. Determine (a) the final pres-
sure, (b) the change in the volume of the cylinder, and (c) the
change in the enthalpy of the refrigerant-134a.
3–38E The temperature in a pressure cooker during cooking
at sea level is measured to be 250°F. Determine the absolute
pressure inside the cooker in psia and in atm. Would you
modify your answer if the place were at a higher elevation?60 percent of the heat generated by the burner is transferred
to the water during boiling, determine the rate of evaporation
of water.
3–42 Repeat Prob. 3–41 for a location at an elevation of
1500 m where the atmospheric pressure is 84.5 kPa and thus
the boiling temperature of water is 95°C.
3–43 Water is boiled at 1 atm pressure in a 25-cm-internal-
diameter stainless steel pan on an electric range. If it is
observed that the water level in the pan drops by 10 cm in 45
min, determine the rate of heat transfer to the pan.
3–44 Repeat Prob. 3–43 for a location at 2000-m elevation
where the standard atmospheric pressure is 79.5 kPa.
3–45 Saturated steam coming off the turbine of a steam
power plant at 30°C condenses on the outside of a 3-cm-
outer-diameter, 35-m-long tube at a rate of 45 kg/h. Deter-
mine the rate of heat transfer from the steam to the cooling
water flowing through the pipe.
3–46 The average atmospheric pressure in Denver (eleva-
tion 1610 m) is 83.4 kPa. Determine the temperature at
which 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 filled
with water and covered with a 4-kg lid. If the local atmo-
spheric pressure is 101 kPa, determine the temperature at
which the water starts boiling when it is heated.
Answer:100.2°CPressure
cooker
250 °FFIGURE P3–38E3–39E The atmospheric pressure at a location is usually spec-
ified at standard conditions, but it changes with the weather
conditions. As the weather forecasters frequently state, the
atmospheric pressure drops during stormy weather and it rises
during clear and sunny days. If the pressure difference between
the two extreme conditions is given to be 0.3 in of mercury,
determine how much the boiling temperatures of water will
vary as the weather changes from one extreme to the other.
3–40 A person cooks a meal in a 30-cm-diameter pot that is
covered with a well-fitting lid and lets the food cool to the
room temperature of 20°C. The total mass of the food and the
pot is 8 kg. Now the person tries to open the pan by lifting
the lid up. Assuming no air has leaked into the pan during
cooling, determine if the lid will open or the pan will move
up together with the lid.
3–41 Water is to be boiled at sea level in a 30-cm-diameter
stainless steel pan placed on top of a 3–kW electric burner. If
Vapor3 kW60% 40%FIGURE P3–41Patm = 101 kPa
mlid = 4 kgH 2 OFIGURE P3–483–49 Reconsider Prob. 3–48. Using EES (or other)
software, investigate the effect of the mass of the
lid on the boiling temperature of water in the pan. Let the
mass vary from 1 kg to 10 kg. Plot the boiling temperature
against the mass of the lid, and discuss the results.
3–50 Water is being heated in a vertical piston–cylinder
device. The piston has a mass of 20 kg and a cross-sectional
area of 100 cm^2. If the local atmospheric pressure is 100 kPa,
determine the temperature at which the water starts boiling.