[gera19]

1)if a surface emits 300W at a temp of T(K), how much will it emit at a temp 2T?

2)I think that in a heat exchanger, it is impossible for the exit temperature of the cold fluid to be greater than the exit temp of the hot fluid when both fluids are single phase fluids. But i'm not sure..am i right?

3)How is natural convection different from forced boiling?

Thanks pp....

[Donaldson Tan]

1)if a surface emits 300W at a temp of T(K), how much will it emit at a temp 2T?

Assuming radiation is the only mode of heat transfer, use Wien's Law

2)I think that in a heat exchanger, it is impossible for the exit temperature of the cold fluid to be greater than the exit temp of the hot fluid when both fluids are single phase fluids. But i'm not sure..am i right?

YES because you need a temperature gradient for heat transfer to occur. This holds true even if multiphase fluid is being used.

3)How is natural convection different from forced boiling?

think of density

[HCaulfield]

Regarding the second question. If the fluids go counter current, the exit temperature of the cold fluid can be higher than that of the hot fluid.

[Donaldson Tan]

Regarding the second question. If the fluids go counter current, the exit temperature of the cold fluid can be higher than that of the hot fluid.

for a counter-current heat exchanger, the maximum outlet temperature of the cold fluid is the inlet temperature of the hot fluid.

for a co-current heat exchanger, the maximum outlet temperature of the cold fluid is the outlet temperature of the hot fluid.

[michal]

IT MEANS TEMPERATURE CROSS IS GOOD PRACTICE

[Borek]

IT MEANS TEMPERATURE CROSS IS GOOD PRACTICE

http://www.chemicalforums.com/index.php?page=forumrules

Read point 5 please.