Calculation tasks #5
Calculation #12: Heat transfer through a roof
Given:
A one-floor level house has a flat roof that is made from concrete (k = 0.8 W/m·K) is
10 m long (l), 8 m wide (w) and 25 cm thick (t) and is the main source for heat loss.
Inside the house a temperature of 18 °C present during the whole day and in the
winter the outside temperature is 4 °C. The heat inside the house is created by electricity (ideal heating process) and the price of the electricity is 4 DKK/kWh.
Searched:
a) How much heat is transferred through the roof to the outside in a winter day?
b) How much money does it cost to keep the temperature inside the building constant?
Calculation #13: Convective heat transfer around a wire
Given:
An electric wire (l=2 m, d=3 mm; U=90 V, I=1 A) is producing heat due to the resistance. The surface temperature of the wire is 152 °C and the surrounding temperature is T∞= 15 °C. An assumption is that the temperature distribution of the wire is
constant (steady state) and radiation is neglected.
REG-no. DK 30 06 09 46
Searched:
a) How much heat is transferred to the surrounding?
b) How high is the convective heat transfer coefficient?
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Calculation #14: Heat transfer of a human to the outside by
convection and radiation
Given:
A person with a skin temperature of 29 °C and a surface of 1.6 m2 is present in a
room with a temperature of 18 °C. The convective heat transfer coefficient is
hconv=6 W/m2·K and emissivity of this person is ε=0.95. Only convective and radiative heat transfer have to be considered and all other effects will be neglected.
Searched:
a) How much heat is transferred by this person due to the single processes of convection and radiation?
b) How much heat is then in total transferred from this person to the room?
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