Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)
8th Edition
ISBN: 9781305387102
Author: Kreith, Frank; Manglik, Raj M.
Publisher: Cengage Learning
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Chapter 2, Problem 2.39P
The tip of a soldering iron consists of a 0.6-cm- diameter copper rod, 7.6 cm long. If the tip must be
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5 An ice chest is constructed of Styrofoam [k = 0.033 W/m · ◦C] with inside dimensions of 25 by 40 by 100 cm. The wall thickness is 5.0 cm. The outside of the chest is exposed to air at 25◦C with h = 10 W/m2 · ◦C. If the chest is completely filled with ice, calculate the time for the ice to completely melt. State your assumptions. The enthalpy of fusion for water is 330 kJ/kg.
Question 1.: A sample of 6.25 kilograms of water at 7.00°C is placed in the freezer. Assuming all energy removed from the freezer is removed from the water, how long does the 1/5 horsepower (HP) motor
(1 HP = 746 Watts) have to run to cool all of the liquid to ice at 0.00°C? The freezer has a COP of 5.00.
Your challenge: come up with a complete equation solved for the variable in question before you plug in any of the numerical values given above. In this way, you will be prepared to solve for any of the
variables!
Some questions to consider along the way ...
(a.) What is the relationship between power, P (in watts) and work, W (in joules)? What is the variable you are eventually solving for?
(b.) What is the relationship between the COP a freezer/refrigerator, and work?
(c.) How much energy as heat must be removed from the water to freeze it? What heat pump/refrigerator variable does this value correspond to?
freezer run time =
S =
minutes
An ice chest is constructed of Styrofoam [k = 0.033 W/m.°C] with inside dimensions of 25 by 40 by 100 cm. The wall thickness is 5.0 cm The outside of the chest is exposed to air at 25°C with N=10 W/m2°C. If the chest is completely filled with ice, calculate the time for the ice to completely melt. State your assumptions. The enthalpy of fusion for water is 330 kJ/kg
Chapter 2 Solutions
Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)
Ch. 2 - A plane wall, 7.5 cm thick, generates heat...Ch. 2 -
2.2 A small dam, which is idealized by a large...Ch. 2 - 2.3 The shield of a nuclear reactor is idealized...Ch. 2 - A plane wall 15 cm thick has a thermal...Ch. 2 - 2.5 Derive an expression for the temperature...Ch. 2 - A plane wall of thickness 2L has internal heat...Ch. 2 - 2.7 A very thin silicon chip is bonded to a 6-mm...Ch. 2 - 2.9 In a large chemical factory, hot gases at 2273...Ch. 2 - 2.14 Calculate the rate of heat loss per foot and...Ch. 2 - 2.15 Suppose that a pipe carrying a hot fluid with...
Ch. 2 - Prob. 2.16PCh. 2 - Estimate the rate of heat loss per unit length...Ch. 2 - The rate of heat flow per unit length q/L through...Ch. 2 - A 2.5-cm-OD, 2-cm-ID copper pipe carries liquid...Ch. 2 - A cylindrical liquid oxygen (LOX) tank has a...Ch. 2 - Show that the rate of heat conduction per unit...Ch. 2 - Derive an expression for the temperature...Ch. 2 - Heat is generated uniformly in the fuel rod of a...Ch. 2 - 2.29 In a cylindrical fuel rod of a nuclear...Ch. 2 - 2.30 An electrical heater capable of generating...Ch. 2 - A hollow sphere with inner and outer radii of R1...Ch. 2 - 2.34 Show that the temperature distribution in a...Ch. 2 -
2.38 The addition of aluminum fins has been...Ch. 2 - The tip of a soldering iron consists of a 0.6-cm-...Ch. 2 - One end of a 0.3-m-long steel rod is connected to...Ch. 2 - Both ends of a 0.6-cm copper U-shaped rod are...Ch. 2 - 2.42 A circumferential fin of rectangular cross...Ch. 2 - 2.43 A turbine blade 6.3 cm long, with...Ch. 2 - 2.44 To determine the thermal conductivity of a...Ch. 2 - 2.45 Heat is transferred from water to air through...Ch. 2 - 2.46 The wall of a liquid-to-gas heat exchanger...Ch. 2 - Prob. 2.47PCh. 2 - The handle of a ladle used for pouring molten lead...Ch. 2 - 2.50 Compare the rate of heat flow from the bottom...Ch. 2 - 2.51 Determine by means of a flux plot the...Ch. 2 - Prob. 2.52PCh. 2 - Determine the rate of heat transfer per meter...Ch. 2 - Prob. 2.54PCh. 2 - 2.55 A long, 1-cm-diameter electric copper cable...Ch. 2 - Prob. 2.56PCh. 2 - Prob. 2.57PCh. 2 - Prob. 2.58P
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- Calculate the heat capacity of a gas sample from the following information: The sam- ple comes to equilibrium in a flask at 25°C and 121.3 kPa. A stopcock is opened briefly, allowing the pressure to drop to 101.3 kPa. With the stopcock closed, the flask warms, returning to 25°C, and the pressure is measured as 104.0 kPa. Determine Cp in J mol- K- assuming the gas to be ideal and the expansion of the gas remaining in the flask to be reversible and adiabatic.arrow_forwardAn ice chest is constructed of Styrofoam [k = 0.033 W/m.°C] with inside dimen sions of 25 by 40 by 100 cm. The wall thickness is 5.0 cm The outside of the chest is exposed to air at 25°C with N=10 W/m2°C. If the chest is completely filled with ice, calculate the time for the ice to completely melt. State your assumptions. The enthalpy of fusion for water is 330 kJ/kgarrow_forwardConsider two coffee cups A and B. Small coffee cup heaters are placed in cups A and B and heat is transferred to keep cup A at 45°C and cup B at 35°C. Room temperature is 25°C. The cups contain the same amount of water (100 grams). Which answer best describes the rates at which heat must be transferred to maintain the temperatures described above?: * A. Cup A will require heat at about five times the rate of B. B. Cup A will require heat at about twice the rate of B. C. Cup A will require heat at a slightly faster rate than B. D. Both cups will require heat at the same rate.arrow_forward
- Carbon steel cubes (side = 8 mm, ρ = 7833 kg/m3, k = 54 W/m·°C, Cp = 0.465 kJ/kg·°C, and α = 1.474x10-6 m2/s) are annealed by heating them first to 850°C in a furnace and then allowing them to cool slowly to 95°C in ambient air at 28°C. What is the total rate of heat transfer from the cubes to the ambient air, when 2700 cubes are to be annealed per hour (in Watts)?arrow_forward2 lbm of refrigerant-134a occupy a 3 ft^3 system at 50 psia. In what state is the refrigerant present?arrow_forwardIn determining the average rate of heating of a tank of 20% sugar syrup, thetemperature at the beginning was 20°C and it took 30 min to heat to 80°C. Thevolume of the sugar syrup was 50 ft3 and its density 66.9 lb/ft3. The specific heat ofthe sugar syrup is 0.9 Btu lb-1°F-1.(a) Convert the specific heat to kJ kg-1°C-1.(b) Determine the rate of heating, that is the heat energy transferred in unit time, in SI units (kJ s-1)arrow_forward
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- A guy shot a bird with a steel rod. When the bird and rod fell on the ground, the bird has been well cooked by the hot rod. The bird is about 0.51kg and has a specific heat of 3.53 kJ/(kg-K), and the rod's weight is 0.2kg. The bird needs to be heated to 150 °C to be well cooked. The melting point of steel is 1510°C. Justify the statement using the knowledge of thermodynamics. Assume that the live bird's body temperature is 35°C.arrow_forward3500J / s of energy is transferred from a heat source to the surface of the part. The heated region is circular and the heat density decreases as the radius increases. Find out whether the heat density is sufficient to melt the metal since 70% of the heat is concentrated in a circle with a diameter of 3.75 mm.arrow_forwardWhat is the total required heating energy in raising the temperature of a given amount of water when the energy supplied is 1000 kwh with heat losses of 25%?arrow_forward
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