Heat exchanger problem Hot water enters the tube-side of a two-shell pass, four
ID: 1843370 • Letter: H
Question
Heat exchanger problem Hot water enters the tube-side of a two-shell pass, four tube pass shell-and-tube heat exchanger at 90 degree C at a How rate of 2.4 kg/s. Cold water enters the shell side of the same heat exchanger at 17 degree C at a mass flow rate of 2.8 kg/s. The cold side surface area is known to be 12.2 m^2. The exit temperature of the hot stream was recently measured to be 34 degree C. From your job training, you were told that in its pristine condition at these flow rates and inlet conditions this heat exchanger would be expected to have a cold-side overall heal transfer coefficient of 4000 W m^-2 K^-1 and that it should be scheduled for a maintenance cleaning if internal fouling leads to more than a 10 % decrease in this metric. Based on the measured hot stream outlet temperature, is the heat exchanger in need of cleaning yet? Assume no external heat is lost or gained by the heat exchanger. Support your answer with engineering analysis. If you are within 0.25 K of a line in the property table, you do not have to interpolate for this problem. Also, if iterations to update fluid properties is required, you only need to do this once.Explanation / Answer
Cold fluid outlet temperature at 34oC hot water outlet temperature considering heat balance
T2=(90-34)x2.4/2.8+17 = 65oC
Temperature profiles(oC)
LMTD = (25-17)/ln(25/17) = 20.70 C
Heat transferred Q = mCp(T2-T1) = 2.8x4.186x(48) = 562.7 kW
Q = FTAUxLMTD
Where A= heat transfer area = 12.2 m2
U= overall heat transfer coefficient, kW/m2-K
FT =LMTD correction factor
For our 2-4 exchanger
R= (T2-T1)/(t1-t2) = (65-17)/(90-34)=0.857
S= (t1-t2)/(t1-T1) = (90-34)/(90-17)=0.77
From Fig 17 of 'Process Heat Transfer' by Kern
FT= 0.8
Hence
U = 562.7/(12.2x0.8x20.7)
= 2.785 kW/m2-K
=2785 W/m2-K
The difference from original U
=(4000-2785)/4000 = 0.303 = 30.3 % > 10 %
Hence significant fouling has occured and hence the heat exchanger needs cleaning.
Hot fluid cold fluid difference 90 Higher Temp, 65 25 34 Lower Temp 17 17 56 Difference 48 8Related Questions
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