A large fridge freezer is to extract 80 kJ of energy every second from inside of the fridge, maintained at temperature of -3 0C, and to dump it to the air outside which is at 30 0 To do so, the fridge consumes 7 kW of electrical energy which is also converted and dissipated as heat to the surrounding air. If the electrical power used is to remain at 7kJ/s, but we still require to extract 80 kW of heat from inside, what is the lowest temperature in the fridge that we could theoretically hope to achieve?

A large fridge freezer is to extract 80 kJ of energy every second from inside of the fridge, maintained at temperature of -3 0C, and to dump it to the air outside which is at 30 0 To do so, the fridge consumes 7 kW of electrical energy which is also converted and dissipated as heat to the surrounding air. If the electrical power used is to remain at 7kJ/s, but we still require to extract 80 kW of heat from inside, what is the lowest temperature in the fridge that we could theoretically hope to achieve?

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter1: Basic Modes Of Heat Transfer

Section: Chapter Questions

Problem 1.67P: 1.67 In beauty salons and in homes, a ubiquitous device is the hairdryer. The front end of a typical...

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1.67 In beauty salons and in homes, a ubiquitous device is the hairdryer. The front end of a typical hairdryer is idealized as a thin-walled cylindrical duct with a 6-cm diameter with a fan at the inlet that blows air over an electric heating coil as schematically shown in the figure. The design of this appliance requires two power settings, with which the air blown over the electric heating coil is heated from the ambient temperature of to an outlet temperature of and with exit air velocities of 1.0 m/s and 1.5 m/s. Estimate the electric power required for the heating coil to meet these conditions, assuming that heat loss from the outside of the dryer duct is neglected.
A plane wall, 7.5 cm thick, generates heat internally at the rate of 105 W/m3. One side of the wall is insulated, and the other side is exposed to an environment at 90C. The convection heat transfer coefficient between the wall and the environment is 500 W/m2 K. If the thermal conductivity of the wall is 12 W/m K, calculate the maximum temperature in the wall.
8.12 An electric room heater has been designed in the shape of a vertical cylinder 2 m tall and 30 cm in diameter. For safety, the heater surface cannot exceed . If the room air is at , find the power rating of the heater in watts.
2.46 The wall of a liquid-to-gas heat exchanger has a surface area on the liquid side of with a heat transfer coefficient of . On the other side of the heat exchanger wall flows a gas, and the wall has 96 thin rectangular steel fins 0.5 cm thick and 1.25 cm high as shown in the accompanying sketch. The fins are 3 m long and the heat transfer coefficient on the gas side is . Assuming that the thermal resistance of the wall is negligible, determine the rate of heat transfer if the overall temperature difference is .
The interior wall of a large, commercial walk-in type meat freezer is covered under normal operating conditions with a 2-cm thick layer of ice. One day, a power outage cuts electricity to the refrigeration system of the freezer. Estimate the time required to melt this layer of ice if it has a mass density of 700kg/m3 and a latent heat of fusion of 334 kJ/kg. Consider the air temperature inside the freezer to be 20C with a heat transfer coefficient of 2W/m2K for convection from the freezer surface to air, and clearly state the assumptions made in your calculations.
The temperature of a gym is wanted to be kept at 297K. The environment is 312 K and the gym unit has a heat input of 80kW from this environment. 8 kW of heat is produced per unit time inside the gym. The total electrical energy given to the electronic devices in the gym is 292kW. With a heat machine operating between 312K environment and 1200K hot heat source, it is desired to obtain both the electricity requirement of the center and the work required to operate the cooling machine.600Kw of heat is drawn.A) If the thermal efficiency of the heat engine is 55%, find W,W1,Q2,Q3,Q4 and the cooling coefficient of the influence cooling machine and the efficiency of the system.B)f the gym and the cooling machine are considered as the control area, is it possible to release Q3=560KW heat from the cooling machine to the environment? Show by calculating.
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A refrigerator is connected to a generator powered by a fuel with a calorific value of 35000 kJ/kg and a density of 0.85 g/cm3 . The inside temperature of the refrigerator is 0°C, and the outside temperature is 20°C. a. How much work is required to freeze 500 g of water? ΔfreezingH° = 6.02 kJ mol– 1 ) b. How much internal energy change occurs during the process? c. Since the generator consumes 5 L of fuel per hour, calculate the thermal efficiency of this engine.