The heat exchange rate of the fluid inside tube is different from that of the fluid out of tube but within shell. The heat exchange coefficient means the heat volume exchanged between the liquid and shell by decreasing one Celsius degree of differential temperature. It is an important index to evaluate the heat exchange capability of a heat exchanger.
The heat exchange efficiency of water condensing upon the shell ranges from 10000 to 20000 w/(m2.℃).
The heat exchange efficiency of water boiling upon the shell ranges from 5000 to 10000 w/(m2.℃).
The heat exchange efficiency of water passing across the shell without status transferring ranges from 2000 to 10000 w/(m2.℃)
The heat exchange efficiency of the air or smog passing across he shell ranges from 20 to 80 w/(m2.℃).
The heat exchange efficiency of air convection ranges from 5 to 10 w/(m2.℃).
Based on the figures above, we can see the nature of fluid determines its heat exchange efficiency.
Let’s suppose that there exists a situation like this. The pipe has flowing water inside with the heat exchange coefficient is 5000 w/(m2.℃), but has the flowing smog outside with the heat exchange coefficient is 50 w/(m2.℃). The differential reached a hundred times. Where is the biggest thermal resistance when the energy transfer happened either inwards or outwards? Definitely, the side of smog is the answer, for the smog has much lower heat exchange coefficient.
For example, a circuit with series resistances inside will get its current size determined by the biggest one among the resistances. The only way to increase the current is to decrease the biggest resistance.
How to change the heat transfer efficiency? The best way is to enlarge the contact area between smog and pipe. The finned tube is good solution to achieve the goal without sacrificing the metallic material, area occupation and high cost performance rate.