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Can Final Temperature of Hot fluid be lower than cold fluid in a heat exchanger? |
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#1 |
New Member
Aaditya
Join Date: Apr 2021
Posts: 19
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Hello everyone,
(This is an urgent problem so please have a look at it and suggest me if you have a solution as soon as possible) I am having a trouble calculating the final temperatures of fluid in a heat exchanger. The problem is as follows: I have a heat exchanger with effectiveness of 0.67 The fluid inside the tube is water with following properties: inlet temperature: 303 K Specific heat capacity (water) = 4184 J/kg K flow rate (water)= 0.00002 m^3/sec density = 1000 kg/m³ The fluid outside is air with following properties inlet temperature: 298 K Specific heat capacity (air) = 1006.3 J/kg K flow rate (water)= 0.10372 m^3/sec density = 1.1845 kg/m^3 My approach: Cp_water = 4184 * 1000 * 0.00002 = 83.68 Cp_air = 1006.3 * 1.1845 * 0.10372 = 123.63 Q_max = Cp_min * (T_water_inlet - T_air_inlet) =83.68 * 5 Q_max = 418.4 W Actual heat flow = Q_act = Q_max * Effectiveness = 418.4* 0.67 = 280.328 W For both the fluids the difference in outlet and inlet temperatures is referred below as T_diff with _a and _w as suffix for air and water respectively T_diff = T_inlet - T_outlet Q_act = Cp_water * T_diff Thus, T_diff_w = Q_act / Cp_water = 280.328 / 83.68 = 3.35 K T_diff_a = Q_act / Cp_air = 280.328 / 123.63 = 2.267 Here as you can see the final temperature of water is 299.65 K and for air is 300.267 K. Have I made any mistake in calculations? or is this practically possible? Please help me! Thank you! |
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#2 | |
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Sam
Join Date: Nov 2022
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#3 |
Senior Member
Erik
Join Date: Feb 2011
Location: Earth (Land portion)
Posts: 1,173
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This is possible on a counter-flow heat exchanger only.
If the two fluids are traveling the same way, then no it is not possible. With a counter flow heat exchanger, the outlet of one fluid is exchanging heat with the inlet of the other. |
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Tags |
heat and mass transfer, heatequation, heatexchanger, temperature calculation |
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