Heat loss from a hot water tank

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SUMMARY

The discussion focuses on calculating heat loss from a hot water tank using two methods: the U-value method and the Stefan-Boltzmann equation. For the U-value method, the user calculates heat loss as 1,050W using a U-value of 0.5, an area of 35m², and a temperature difference of 60°C. In the second method, the user applies the Stefan-Boltzmann equation but lacks the emissivity coefficient (E), leading to an incomplete calculation of radiation heat loss, which should yield 643W. It is concluded that both U and E values should have been provided in the exam for accurate calculations.

PREREQUISITES
  • Understanding of heat transfer concepts, specifically conduction and radiation.
  • Familiarity with the U-value in thermal insulation calculations.
  • Knowledge of the Stefan-Boltzmann law and its application in thermal radiation.
  • Basic proficiency in unit conversions, particularly between Celsius and Kelvin.
NEXT STEPS
  • Research how to determine U-values for different materials and their impact on heat loss.
  • Learn about emissivity coefficients and their significance in thermal radiation calculations.
  • Explore advanced applications of the Stefan-Boltzmann equation in engineering contexts.
  • Investigate methods for measuring and calculating heat loss in real-world scenarios.
USEFUL FOR

Engineers, thermal analysts, and students studying thermodynamics or heat transfer principles who need to understand heat loss calculations in practical applications.

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Homework Statement
Water in an insulated hot water tank is maintained at 80 degrees C. The external area of tank is 35m2. Calculate heat loss by i) convection ii) radiation
Relevant Equations
Q=UAdT
This is a past exam paper Q.
I think i might be missing info though.

For i) i use q=UAdT
A= 35m2
dT= (80-20)
They haven't given U but if i know answer should be 1,050W
So if i set U to 0.5

(0.5)(35)(60)
= 1050W

Am i missing something or could I calculate U with the info given?

Also ii)
Use stefan-boltzmann eq:

P = E(Stefan constant)(Area)(T^4 - T^4 in kelvin)
I don't have E the emissivity coefficient
Stefan constant is 5.67 x 10-8
80 degrees in K is 353.15
20 degrees in K is 293.15
P=E(162107.003)

The answer should be 643W
Can i calculate radiation from info given?
 
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Your work looks ok to me, except the decimal point is off in P=E(162107.003) .

The values of U and E should have been provided in the exam.
 
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