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Cn3108 H1 Double Pipe Heat Exchanger

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by Fredrick Timotius on 16 November 2012

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Transcript of Cn3108 H1 Double Pipe Heat Exchanger

A very special presentation for a very special someone brought by ... Group Th14 And the very special someone is ... Assistant Professor Praveen Linga Who taught us the wonders of mass transfer and radiation in CN2125. Eng Ping the Leader
Fredrick the Experimenter
Arjun the Analyst
Cheryl the Analyst
Jia Mei the Reviewer Objectives To investigate the characteristics of double-pipe heat exchanger for counter and co-current flow, by varying hot water flow rate and keeping cold water flow rate constant
To investigate the relationship between Nusselt and Reynolds numbers Summary Temperatures of the inner tube hot water and outer tube cold water at various positions were measured Heat transfer for counter-current system was found to be more efficient Inner wall heat transfer coefficient was larger than outer wall heat transfer coefficient Nusselt number determined from inner film heat transfer coefficient and Reynolds number from hot water flow rate. A relationship is formed between these 2 and compared to Dittus-Boelter Correlation Theoretical Background Heat Exchangers Main purpose is to regulate temperature in industrial processes Types include shell and tube, double-pipe, plate heat exchangers Double-pipe heat exchanger consists of 2 concentric tubes Energy exchange occurs by conduction and convection Plate Heat Exchanger Shell and Tube Heat Exchanger Theoretical Background Dimensionless Constants ratio of inertial forces to viscous forces ratio of momentum diffusivity to thermal diffusivity ratio of convective to conductive heat transfer ratio of heat transferred to thermal capacity Theoretical Background Correlations Dittus-Boelter correlation: Where:
n = 0.4 if fluid is heated, n = 0.3 if fluid is cooled
All fluid properties measured at arithmetic mean bulk temperature
Reynolds > 10000
0.7 < Prandtl < 100
L/D > 60 Theoretical Background Correlations Sieder-Tate correlation: Where:
All fluid properties except viscosity at wall temperature are evaluated at bulk temperature
Reynolds > 10000
0.7 < Prandtl < 17000
L/D > 60 Apparatus and Equipment There are 2 main equipment used in the experiment:
Double-pipe heat exchanger (H952)
Water tap Apparatus and Equipment Hot water level indicator Hot water temperature controller Cold water flow rate controller Hot water flow rate controller Equipment power switch Double-pipe heat exchanger where heat exchange takes place Valves to control cold water flow direction Temperature measurement at designated location Apparatus and Equipment Cross section of double-pipe heat exchanger The 10 temperatures to be monitored throughout the runs Experimental Procedures Results For counter-current flow: For co-current flow: Discussion Check the energy balance for the heat exchanger. Are the assumptions required for the validity of Q = UA(log mean T) satisfied? Discussion List of assumptions:

Steady-state
Adiabatic system
No appreciable heat generation or accumulation in heat exchanger wall or in fluid
Constant overall heat transfer coefficient
Constant fluid properties for the bulk flow
No phase changes in the fluid
No shaft work
Incompressible fluid Discussion Steady-state

Time was allowed for the flow rate and fluid temperature to stabilize before readings were taken. In terms of flow, flow of fluid in the pipes is assumed to be fully developed.

Very minor fluctuations in the flow rates further allows us to approximate a steady state condition
Adiabatic system

Under adiabatic system, there is no heat loss.
Qloss = 0
Qhot = Qcold
Heat loss by hot water = Heat gain by cold water Counter-current: Co-current: Discussion Discussion No heat generation and accumulation

No heat generation is expected in the heat exchanger as the 2 streams only contained water and they were streamed at different temperatures. Thus no heat generation assumption holds.

Friction could be generated as water flows through the pipe, however, the amount of heat contributed through this is small relative to the amount of heat transferred in the main exchanger process and can be neglected. (Still relatively valid)


Discussion Constant overall heat transfer coefficient

Uo is a function of temperature and since temperature varies along the length of a heat exchanger, the value of Uo would fluctuate non-linearly throughout the exchanger and thus some degree of deviation is expected when comparing the experimental value and the theoretical value, which is under the steady state assumption.

However, Uo calculated on the basis of log- mean temperature difference and hence the Uo can be approximately related to a constant function for a section of the heat exchanger
Discussion Constant fluid properties

For simplicity in calculations of hi, ho and subsequently, Uo , the fluid properties such specific heat capacity (Cp), viscosity (μ) and density (ρ) are assumed to be constant. In reality, they are functions of temperature. Since temperature varies along the length of the pipe, the fluid properties will change accordingly too. However, since the temperature dependence of water properties is weak and the inlet and outlet temperatures for the respective streams are close, this assumption is justified Discussion No phase changes to the fluid

The experiments lie in the region for which the fluid remains a liquid.i.e 0-100 degrees Centigrade No shaft work


Incompressible fluid Discussion Energy balance

The fundamental energy balance equation can be expressed as follows:

Accumulation = Energy INPUT – Energy OUTPUT + Energy generation

Assuming steady-state operation with no energy generation, the energy balance for the heat exchanger can be simplified to give:

Energy INPUT = Energy OUTPUT

Which translates to the heat lost by hot fluid stream is equal to heat gained by cold fluid stream, i.e., Qc = Qh in the absence of heat lost or gained from surrounding. Discussion Temperature profiles for both hot and cold water streams for counter and co-current arrangement? Discussion The temperature points to be measured: Discussion For counter-current arrangement: Discussion For co-current arrangement: Discussion Plot the film heat transfer coefficient hi and ho and the overall heat transfer coefficient versus hot water flow for both configurations. Discussion Discussion Investigate the relationship between Nusselt Number and Reynolds Number for the hot water and compare your experimental results with those in the references. Discuss briefly the differences between shell-and-tube heat exchanger and plate heat exchanger. Discussion Error Analysis Time given for system to reach steady state

Fluctuating hot and cold water flow rates

Non-ideal insulation

Short tubes for the heat exchanger

Possible malfunction of the heat exchanger/Age of the equipment

Bends in the heat exchanger Error Analysis Human error

Assumption of zero resistance of the tube wall

Interpolation of fluid properties

Fluctuations in temperature readings

Calculation of overall heat transfer coefficient

Precision of the measuring devices in the setup Experimental Precautions Sufficient water in the hot water tank

Flow rate meters were read at eye level to eliminate parallax error

System was given time to reach steady state after each change in flow rate and temperature set point before data was recorded

Time was given to allow the temperature readings to stabilize after turning the temperature selector switch before data was recorded

Mean hot water temperature (t3+t6)/2 was maintained within ±2°C for each hot water flow rate Safety Precautions Compliance of laboratory safety dress code

Water temperature kept below 65°C at all times

Hot water control valve was kept fully open when the
circulating pump was running

In case of any spillage/leakage, all further experimental procedures should be stopped and the lab technician should be immediately informed Safety Precautions In case of spillage near electrical points, the electrical point(s) should be switched off first before informing the lab technician

After the experiment, the heat switch should be switched off, the cold water flow rate set to the highest value and the hot water control valve should be fully opened. The system should be cooled to about 40°C before the main switch and the cold water supply are turned off Conclusion Thank you! Discussion Selecting the Dittus-Boelter correlation, All 5 runs satisfy the requirements to use Dittus-Boelter correlation as Reynolds > 10000, 0.7 < Prandtl < 100 and L/D > 60 Since Pr is almost constant and for convenience of plotting, convert Dittus-Boelters into the form: Discussion Plotting the results: Counter-current arrangement: Co-current arrangement: Discussion Compare to Dittus-Boelter correlation by converting it to the form: Include Prandtl into the constant a.
Calculating Prandtl to be an average of 2.9968 in both counter and co-current arrangements, Discussion Discussion Results The hot water bulk temperature, (t3 + t6)/2, must be within 2 degree Centigrade of each other for the different runs

For the 5th run, to ensure that the hot water bulk temperature is within 2 degree centigrade of the other 4 runs, the temperature was set to be much higher than the other runs

The temperature readings would then be used in the subsequent calculations and their results discussed. Discussion Dittus-Boelter correlation:





Where:
n = 0.4 if fluid is heated, n = 0.3 if fluid is cooled
All fluid properties measured at arithmetic mean bulk temperature
Reynolds > 10000
0.7 < Prandtl < 100
L/D > 60 Discussion Significance of results:

Accuracy of Dittus-Boelter correlation expected to be within 15%, and this was found to be true for b, but not a

Reasons could be attributed to


were expected, but heat loss to surroundings ranges from 2% to 50% due to poor insulation and results in high inaccuracies in calculations

Compounded by other experimental errors In this experiment, we:

Obtained temperature profiles for both counter and co-current flow
Obtained heat transfer coefficient
Investigate relationship between Nusselt and Reynolds and compared to Dittus-Boelter correlation
Discussion of error analysis and safety procedures
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