Following the hot vacuum forming of a paper-pulpmixture, the product, an egg carton, is transported on aconveyor for 18 s toward the entrance of a gas-firedoven where it is dried to a desired final water content.Very little water evaporates during the travel time.So, to increase the productivity of the line, it is proposed that a bank of infrared radiation heaters, whichprovide a uniform radiant flux of 5 000 W / m 2 , beinstalled over the conveyor. The carton has an exposedarea of 0.0 625 m 2 and a mass of 0.220 kg, 75% of which is water after the forming press. The chief engineer of your plant will approve the purchase of the heaters if they can reduce the water contentby 10% of the total mass. Would you recommend thepurchase? Assume the heat of vaporization of water is h f g = 2400 kJ / kg .
Following the hot vacuum forming of a paper-pulpmixture, the product, an egg carton, is transported on aconveyor for 18 s toward the entrance of a gas-firedoven where it is dried to a desired final water content.Very little water evaporates during the travel time.So, to increase the productivity of the line, it is proposed that a bank of infrared radiation heaters, whichprovide a uniform radiant flux of 5 000 W / m 2 , beinstalled over the conveyor. The carton has an exposedarea of 0.0 625 m 2 and a mass of 0.220 kg, 75% of which is water after the forming press. The chief engineer of your plant will approve the purchase of the heaters if they can reduce the water contentby 10% of the total mass. Would you recommend thepurchase? Assume the heat of vaporization of water is h f g = 2400 kJ / kg .
Solution Summary: The chief engineer rejects the proposal because the mass of 10% of carton is more than water evaporated.
Following the hot vacuum forming of a paper-pulpmixture, the product, an egg carton, is transported on aconveyor for 18 s toward the entrance of a gas-firedoven where it is dried to a desired final water content.Very little water evaporates during the travel time.So, to increase the productivity of the line, it is proposed that a bank of infrared radiation heaters, whichprovide a uniform radiant flux of
5
000
W
/
m
2
, beinstalled over the conveyor. The carton has an exposedarea of
0.0
625 m
2
and a mass of 0.220 kg, 75% of which is water after the forming press.
The chief engineer of your plant will approve the purchase of the heaters if they can reduce the water contentby 10% of the total mass. Would you recommend thepurchase? Assume the heat of vaporization of water is
h
f
g
=
2400
kJ
/
kg
.
Question : Studies show that the major energy consumption in Fijian villages is wood which is used for cooking over open fires. Typical consumption of wood is 1 kg person–1day–1.
(a) Estimate the heat energy required to boil a 2-liter pot full of water. Assuming this to be the cooking requirement of each person, compare this with the heat content of the wood, and thus estimate the thermal efficiency of the open fire.
(b) How much timber has to be felled each year to cook for a village of 200 people?Assuming systematic replanting, what area of crop must the village therefore set aside for fuel use if it is not to make a net deforestation? Hint: refer to Table 10.4.
(c) Comment on the realism of the assumptions made, and revise your estimates accordingly.
Answer: (a) mcDT ≈ 0.6 MJ (heat losses from pot imply actual requirement is higher). h ≈ 3%. (b) 70 tonnes; 7 ha.
Trasient Cooling
What are some of the factors and assumptions that would casue a different between the experimential convection coefficient, hexp, and the theorectical htheo?
An annealing furnace is at a temperature of 460 oC, where a water pool is used to cool the heated products. Each of the products is cylindrical, with a diameter of 25 mm and a length of 250 mm. Propagation of radiationassuming that the coefficient is 0.85, the amount of heat transfer that occurs when the product is submerged in the water poolcalculate.
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