QCS Suppose you build a two-engine device with the exhaust energy output from one heat engine supplying the input energy for a second heat engine. We say that the twO running in series. Let e and e represent the ef- engines are ficiencies of the two engines. (a) The overall efficiency of the two-engine device is defined as the total work output divided by the energy put into the first engine by heat. Show that the overall efficiency e is given by

QCS Suppose you build a two-engine device with the exhaust energy output from one heat engine supplying the input energy for a second heat engine. We say that the twO running in series. Let e and e represent the ef- engines are ficiencies of the two engines. (a) The overall efficiency of the two-engine device is defined as the total work output divided by the energy put into the first engine by heat. Show that the overall efficiency e is given by

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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The tip of a soldering iron consists of a 0.6-cm- diameter copper rod, 7.6 cm long. If the tip must be 204C, what are the required minimum temperature of the base and the heat flow, in watts, into the base? Assume that h=22.7W/m2KandTair=21C.
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.
An inventor claims to have a solar powered heat pump that receives energy as heat from the sun at the rate of 10 kW and extracts energy as heat from the environment at the rate of 7 kW. This system does not require any shaft or electrical power input. If you think this device is impossible, explain why using basic principles to support your argument. If you think it might be possible, what would be the steady-state rate of transfer of energy as heat to the house? (Assume TH = 20 Cand TL = -15 C).
It has been suggested that a heat engine could be developedthat made use of the temperature difference between waterat the surface of the ocean and water several hundredmeters deep. In the tropics, the temperatures may be 27°Cand 4°C, respectively. (a) What is the maximum efficiencysuch an engine could have? (b) Why might such an enginebe feasible in spite of the low efficiency? (c) Can you imagineany adverse environmental effects that might occur?
Suppose you have two kettles --a plug-in electric kettle and a stovetop kettle. The electric kettle uses electricity from a natural gas fired powerplant, while you boil water in your stovetop kettle on a natural gas burner. Based on the following information, which of these kettles demonstrates a higher energy efficiency? Please calculate the efficiencies of each kettle and express answers as percentages -The natural gas power plant converts chemical energy of the natural gas to electrical energy with 58% efficiency. -High-voltage power lines from the power plant to your house convey electricity with 92% efficiency. -The electric kettle converts electrical energy to thermal energy in the water with 85% efficiency. -The stove burner converts chemical energy of natural gas to thermal energy with 98% efficiency (the other 2% is light). -The stovetop kettle transfers 35% of the stove’s thermal energy to the water in the kettle.
A student buys a 5000 Btu window air conditioner for his apartment bedroom. He monitors it for one hour on a hot day and determines that it operates approximately 60 percent of the time (duty cycle = 60 percent) to keep the room at nearly constant temperature. (a) Showing all your work and using unity conversion ratios, calculate the rate of heat transfer into the bedroom through the walls, windows, etc. in units of Btu/h and in units of kW. (b) If the energy efficiency ratio (EER) of the air conditioner is 9.0 and electricity costs 7.5 cents per kilowatt-hr, calculate how much it costs (in cents) for him to run the air conditioner for one hour.
In the following problem, consider the human body as a heat engine with efficiency e = 0.2 that can take in energy as food Qh, perform work W, and generate heat Qc. This heat expelled from our muscles (Qc) is an important component of maintaining a constant body temperature, which necessitates a balance of energy intake and output. In order to calculate the energy intake Qh, you will use the table above listing activity factors for selected activities, for a person with a BMR of 1440 kcal/day. For example, the energy required to stand per hour would be (fstand)(BMR) = (1.7)(1440 kcal/day) (1day/24 hours) = 102 kcal/h. For each scenario, calculate the 24 h number of bowls of cereal that must be consumed per hour (1 bowl = 100 kcal) to perform the required activity (Qh) Determine whether you have an energy deficit or surplus by considering the heat generated by your muscles (Qc), the heat loss from conduction, and the heat loss/gain from radiation We will consider heat loss from…
The Nevada Solar One solar thermal power plant covers a land area of 140 hectares and has an estimated peak output of 64 MW. If the peak solar ﬂ ux reaching the surface of that part of Nevada is about 630 W/m2, what is the net efﬁciency of the power plant? Compare your answer with some of the higher efﬁ ciencies of semiconductor solar cells.
I need very QUICK answers, PLEASE. A solar collector meets the hot water needs of a family for 7 months, excluding 5 months of the winter season. This collector provides hot water at an average temperature of 55°C and an average cold water temperature of 16°C. The family uses an average of 14 tons of hot water per month from the solar collector, and in winter, hot water is provided by an electric heater. Since the unit price of electricity is 0.85 TL/kWh and the efficiency of the electric heater is 91%, calculate the annual electricity and money savings provided by the solar collector to the family. (Specific heat of water at room temperature=4.18kJ/kg*C)
An air conditioning unit works by taking heat out of a room. As the refrigerant gas expands in the cooling coils, it absorbs heat from the circulating air in the room. This heat is then released as the refrigerant is compressed by the part of the unit that is directed out of the room. The unit serves like a heat pump for the room. Since a refrigerator works in a similar manner, can it also be used to maintain a cool temperature inside a room? Justify your answer.
3) When a person enters his house, which is completely closed in summer, he sees that the temperature is 34°C. After turning on the air conditioner, the temperature of the house becomes 22 °C within 15 minutes. Since the efficiency coefficient of the air conditioning system is 3.2, a) the electrical power drawn by the system, b) heat dissipated to the environment, and c) Assuming that it works 8 hours a day and the electricity price is 0.6 TL per kWh, find its cost for 4 months. Assume that the entire mass inside the house is equivalent to 1000 kg of air. For air, take Cv = 0.73 kJ/kgK, Cp= 1.0051 kJ/kgK.
A heating system must maintain the interior of a building at TH = 20°C when the outside temperature is TC = 2°C. If the rate of heat transfer from the building through its walls and roof is 16.4 kW, determine the electrical power required, in kW, to heat the building using (a) electrical-resistance heating, (b) a heat pump whose coefficient of performance is 3.0, (c) a reversible heat pump operating between hot and cold reservoirs at 20°C and 2°C, respectively.