paraphrase the report
Talldos
hex_prelab.docxObjective
In this experiment, the behavior of a shell and tube heat exchanger will be investigated and modeled. To do this, the flow rates of both the heated and cooled stream will be set, and the inlet and outlet temperatures of the streams will be taken. By comparing the difference in temperature of the streams, it is possible to calculate the transfer of energy from the machines. This, in turn will allow us to calculate the total heat duty and relate it to the overall heat transfer coefficient. The tube-side and shell-side coefficients can be back calculated out of the overall heat transfer coefficient. We will also be checking the validity of the Donohue Equation, by comparing the experimental heat transfer to the heat transfer predicted by the model
Safety
The main chemical used in this experiment is H2O (water). Water is a fairly safe fluid to come in contact with but there are some safety precautions to take. Wearing proper lab attire is crucial which includes a hardhat, safety glasses, and close toed shoes. A major rule to consider while handling this is this water is not for ingestion. If for any reason it is needed be aware of the location of the safety showers and eye wash stations which are located by the door on the first floor of the lab and directly above that on the second floor.
Being that this experiment may potentially involve high heat, one should be extremely cautious around any heated or potentially heated streams. Pipes with an unknown temperature should be assumed to be hot until their temperature can be verified. The pipes should not be touched to check their temperature; equipment temperature gauges should be used if at all possible. Lastly, any flammable materials should be kept away from any heating equipment. Volatile solvents and other chemicals should be stored in fire cabinets if needed, and the location of the fire exits and extinguishers should be memorized. If a fire does occur and it is too large to safely manage, the lab should be evacuated and the fire department alerted rather than fighting the fire directly. If the fire department is needed, they should alerted of any chemicals that may cause potential harm or other issues to firefighting personnel.
Overuse of water can also impact the environment by the amount of energy used to purify, supply, and pump water to where the experiment will take place. On the other hand water is plentiful in America and also very inexpensive to supply. A very situational idea to consider is if the area where the experiment was being performed was in a drought. The over usage of water will decrease the amount available to the community as well as diminish habitats for animals in the area.
Theory
Basic heat transfer coefficients are useful for calculating data in theoretical systems or estimating values for real systems, heat exchanger systems are complex and the actual results will likely vary from these theoretical results. If more specific data becomes necessary to analyze and regulate a process, the heat transfer coefficients can be calculated based on experimental data found from that exact process.
If the shell side flow is in the same direction as tube side flow, then the flow is considered cocurrent. If the shell side flow is in the opposite direction of tube side flow, then the flow is considered countercurrent. Countercurrent flow is generally considered to be more efficient at heat transfer.
The total rate of heat transfer from the tube side fluid is:
Equation 1
= Tube heat transfer ()
= Flow rate ( )
= Specific heat of the fluid ()
= Entering temperature of the fluid ()
= Exiting temperature of the fluid ()
Equation 1 is the most valid for all scenarios. Once you solve for you can also obtain the overall heat using the heat transfer coefficient.
Equation 2
= overall heat duty (BTU/hr)
= Overall heat transfer coefficient based on the tube side area ()
= Inside heat transfer area of a heat exchanger ()
Y = Correction factor for average temperature difference in multipass exchangers
= Log mean temperature ()
In this experiment there are no cross-flow or multi-pass flows, so the factor Y will be taken as unity, Y=1. To solve for log mean temperature, this equation must be used:
= Log mean temperature ()
= Entering temperature of cold fluid ()
= Exit temperature of cold fluid ()
= Entering temperature of hot fluid ()
= Exiting temperature of hot fluid ()
= Overall heat transfer coefficient based on the tube side area )
= Tube side heat transfer coefficient ()
= Shell side heat transfer coefficient ()
= Inside diameter of a tube or pipe ()
= Thermal conductivity of pipe/tube wall ()
= Outside diameter of a tube or pipe ()
Nusselt number is vital while analyzing heat transfer processes. The Nusselt number is the ratio of conductive to convective heat transfer in a system. Assuming the flow of fluid is laminar, the Nusselt number can be solved by the following equation:
Equation 5
When solving this equation, the usage of Reynold’s number and Prandtl’s number is required. The Reynolds Number is a ratio of momentum forces to viscous forces which in turn helps determine the flow pattern of a fluid by classifying it as laminar or turbulent. Prandtl’s number is the ratio between momentum diffusivity and thermal diffusivity.
Equation 6
Equation 7
D= inside pipe diameter (m)
v= average velocity of the fluid (m/s)
ρ= density of the fluid (g/L)
µ= viscosity of the fluid (g/ms)
= Specific heat of the fluid ()
Thermal conductivity []
Calibration Procedure:
1. Plug the flow meter box into a power source.
2. Complete calibration of shell and tube flow rate using timed weighings
a. Open all shell and tube heat exchanger valves.
b. Choose either co-current or countercurrent to begin.
c. Ensure that all the lines of the shell and tube heat exchanger lined up and the water is drained to the drain continually.
d. Start collecting data at different flow rates in the shell side by using the timed weighings with a tared bucket.
e. Weigh water in bucket.
f. Using time and weight of water in the bucket, calculate actual flow rate of the fluid.
3. Repeat steps d through f but this time in the tube side.
a. Close all shell and tubes heat exchanger valves.
b. Open all plate and frames heat exchanger valves.
c. Ensure that all the lines of the shell and tube heat exchanger lined up and the water drained to the drain continually.
4. Repeat steps d through f.
a. Unplug flow meter box from the wall.
b. Create calibration curve using this data.
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