Experiment : 5/03/2012 Presentation : 12/03/2012 Group B1/B Vartak Shankul Shisheer 10D Abhishek Mathur 10D Kunal Bhoyar 10D Monika Kanwat 1

 To determine the overall heat transfer coefficient.  To find the heat transfer coefficient for inner pipe with hot fluid from overall heat transfer coefficient.  To verify the Dittus-Boelter equation for turbulent flow heat transfer. 2

A wide variety of chemical processes involve heat transfer during one or more o the basic steps of mixing, reaction and purification. Dimensionless variables like Reynolds number and Nusselt number characterize the heat transfer processes and are used for scaling an experimental condition to the pilot scale and the plant scale. Thus determination of the nature of fluid flow in various regimes becomes essential from an industrial point of view. 3

 Heat exchanger is a process device used to effect heat transfer between fluids. Here, we are using a double tube heat exchanger, in which hot fluid flows in a smaller diameter tube and cold fluid flows around these smaller tubes.  The flows is anti-parallel for efficient heat transfer.  Heat transfer on both sides of the wall (in the fluid) is generally by convection and by conduction inside the metal. 4

 The system is equivalent to the system of three resistances connected in series so the net resistance will be the summation of individual resistance. (1)  Ai, A0= Inner and outer surface area of the inner pipe,  hi, ho =Heat transfer coefficient between hot fluid and inner pipe and inner pipe and cold fluid respectively,  Ui =Overall heat transfer coefficient for the inner pipe.  Alm is the logarithmic mean surface area given by (2) 5

 The thermal conductivity of metal is very high. Hence for all practical purposes the resistance offered by the metal is neglected as compared to that offered by convection. (3)  The heat transfer rate from the hot fluid is given by Q = ρVC p (T 1 - T 2 ) (4)  Also, from the definition of the overall heat transfer coefficient (U), we have Q = U1A1ΔTlmtd (5) i.e., U1 = Q/A1ΔTlmtd (6) 6

 where log mean temperature difference, Tlmtd is given by  (7)  Where, Q = Heat transfer rate (W)  ρ = Density of hot fluid (monoethylene glycol) (kg/m3)  V = Volumetric flow rate (m3/s)  Cp = Specific heat capacity (J/kg-K)  T1 = Inlet temperature of hot fluid (K)  T2 = Outlet temperature (K)  t1 = Inlet temperature of cold fluid (K)  t2 = Outlet temperature of cold fluid (K) 7

 Heat transfer coefficient of fluid in the inner tube can be found using Dittus-Boelter equation  For different flow rates, if the mean temperature of the inner fluid does not differ much, then all physical properties remain almost constant. Hence,  Or equivalently, 8

9 Switch temperature indicator. Check for zero error for zero flow rate. Set the flow rate of cold fluid at constant value of 240 LPH. Start the heating of hot fluid & set the flow rate at 800 LPH initially. Note the inlet & outlet temperatures of hot & cold fluid for particular flow rate of cold fluid. Repeat for 6 reading of flow rate till 300 LPH. Measure length of the heat exchanger, radius of the inner and outer diameters of pipes.

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11 Calculate Prandtl number for the hot fluid. Calculate LMTD from the inlet & outlet of hot & cold fluids. Calculate the velocity of hot fluid flow. Calculate the heat transferred from the hot fluid & overall heat transfer coefficient. Calculate Re from flow rate & Nu from U & LMTD. Plot the curve between 1/U and 1/v0.8 & use the intercept to calculate hi. Plot curve between ln(Nu) & ln(Re) and compare the coefficient of ln(Re) with theoretical value.

 Table Temperatures and flow rates 12 S. No.Flow rateTemperatures (°C) ΔTlmtd (K) (LPH)Cold Fluid Hot Fluid InletOutletInletOutlet

 Table Heat Transfer Rate (Q hot ), Heat Transfer Coefficient (U) and Velocity (v) 13 S. No. Flow rate V LMTDQhotUivi1/vi^0.81/Ui (lph)(K)(W)(W/m 2 K)(m/s) (10 -4 m 2 K/W)

 Table Inner Heat Transfer Coefficient (h i ), Nusselt Number (Nu), Reynolds Number (Re) 14 S. No.hi (W/m 2 K)Nu =h i d i /kln(Nu) Re= d i u i  /  ln(Re)

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17  Overall Heat Transfer Coefficient= W/m2K  Outside fluid heat transfer coefficient ho=45491 W/m2K  Thus the value of the constant c1 in the Dittus- Boelter Equation (1.9) is  Expected (theoretical) Value is 0.8.  Relative error is ( )/0.8*100= 2%.

 On decreasing the volumetric flow rate, and hence the velocity, of hot fluid, Reynolds number (Re) decreases, while, Prandtl number (Pr) remains almost constant as it primarily depends on the fluid properties and is not sensitive to flow rates.  If the heat transferred from hot fluid, Q decreases, Nusselt number decreases as overall heat transfer coefficient, U decreases.  As the velocity, i.e. volumetric flow rate decreases, total heat transferred, Q & overall heat transfer coefficient U decrease. This is because of the relationship between Reynolds number & Nusselt number.  The Dittus-Boelter Equation for heat transfer in turbulent flow is verified with an error of 2 %. 18

 Fluctuation in flow rate of cold and hot fluid by +/-10 l/hr.  Some heat is lost from water to the surroundings while undergoing heating in the heat exchanger, so to remove this error, consider only the change in temperature of the hot fluid for calculation of amount of heat transferred and not of the cold fluid.  Proper insulation of the pipes is required to ensure no heat is lost to the surroundings.  Voltage fluctuations lead unsteady flow rates.  At high flow rates, the measurement of temperature of the hot liquid can be inconsistent. 19

 Arrange for appreciable difference between the flow rates of two readings.  Flow rate of the cold fluid should be kept constant throughout the experiment.  Readings should be taken only when the inlet and outlet temperatures stabilize.  Minimize the parallax error in noting the flow rates.  The volume flow rate of hot fluid should be in the turbulent regime. 20