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Bob Guest

Posted: Wed Dec 03, 2014 2:24 am Post subject: questions on heat transfer of pizza in an oven 


This problem considers the amount of heat necessary to bake a pizza. All the numbers are from a real life scenario. The idea itself is similar to what happens in some batch processes in the Chemical industry.
You have bought a pizza from a wellknown supermarket chain. It comes in a packet of two, each weighs 340 g. Below is some relevant information.
Nutritional value per 100 g
Carbohydrate 25.7 g
Protein 11.3 g
Fat 10.1 g
Assume the rest is water
Cooking instructions:
Preheat oven to 220 Celcius
Take the pizza directly out of the freezer, place it in the middle shelf of the oven and bake for 15 mins
Useful data:
Specific heat capacities in kJ/kg K for some components
Carbohydrate 1.420
Protein 1.550
Fat 1.680
Ice 2.000
Latent heat of melting of ice 335 kJ/kg
Assumptions
The temperature of the freezer is 10 Celcius
The specific heat capacities provided above will not change with temperature. Note that they don’t include water.
No damage will occur to the carbohydrates, proteins and fat, i.e. they won’t breakdown to other components. Hence their quantities will remain the same from start to finish.
Observation:
On weighing the pizza right after taking it out of the freezer, then again on taking it out of the oven, it was noticed that there was a 26 g loss in its weight.
Using all the information above (nutritional values, instructions, data, assumptions,
observations, etc), in addition to any other assumptions you make (state them clearly),
determine the following:
a) Amount of heat necessary to bake the pizza
b) If the oven power is 3000 W, how long would it take to bake the pizza, i.e. supply the amount of heat necessary?
c) Is this number different from the one specified in the cooking instructions?
d) If so, what is your explanation of this difference? 

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