Heat transfer in a double pipe HE

In summary, the experiment involved a double pipe heat exchanger with readings taken at steady state. Cold water entered the inner tube at 9°C and exited at 81°C, with its flow rate measured. The outer tube contained steam at 133°C, with a constant temperature, and the condensate flow rate was also measured. The equations used in the attempt to solve the problem were q=U A ΔT(lmtd), q=m h_{fg}, and q=m Cp ΔT(bulk). The first two calculations yielded results of 28kW and 59kW, respectively, and the discrepancy between the two can be attributed to heat losses. Further data is needed for a more accurate energy balance.
  • #1
nod32
14
0

Homework Statement


I performed an experiment using a double pipe heat exchanger. Readings were taken at steady state.

Cold water entered the inner tube at 9°C, and its exit temp was 81°C. (flow rate was measured).

The outer tube contained steam at 133°C and this temperature remained constant. Condensate flow rate was measured.


Homework Equations



i) q=U A ΔT(lmtd)
ii) q=m h[itex]_{fg}[/itex] (where hfg is heat of vaporization)
iii) q=m Cp ΔT(bulk)

The Attempt at a Solution


Since the steam did not change temperature but condensed, there was latent heat transfer. q was calculated using ii). (result q=28kW)

I then calculated q using equation iii) applied to the cold water stream. (result q=59kW)

I don't understand why these two results are so far apart. In a perfect system, wouldn't they be equal?
 
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  • #2
nod32 said:

Homework Statement


I performed an experiment using a double pipe heat exchanger. Readings were taken at steady state.

Cold water entered the inner tube at 9°C, and its exit temp was 81°C. (flow rate was measured).

The outer tube contained steam at 133°C and this temperature remained constant. Condensate flow rate was measured.


Homework Equations



i) q=U A ΔT(lmtd)
ii) q=m h[itex]_{fg}[/itex] (where hfg is heat of vaporization)
iii) q=m Cp ΔT(bulk)

The Attempt at a Solution


Since the steam did not change temperature but condensed, there was latent heat transfer. q was calculated using ii). (result q=28kW)

I then calculated q using equation iii) applied to the cold water stream. (result q=59kW)

I don't understand why these two results are so far apart. In a perfect system, wouldn't they be equal?

Yes. And, even if there were heat losses, q ii would be higher than q iii, rather than lower. Show us the data.
 
  • #3
Yes my mistake, I mixed them up. The higher q came from equation ii). I guess that makes sense since not all the released energy from the condensation ends up warming the cold stream.
So this large difference can be attributed to heat losses?

I'm also asked to do an energy balance to verify the accuracy of the heat transfer rates. Isn't that what I just did when I compared the two q values? How would that verify anything, as they would always be different?
 
  • #4
nod32 said:
Yes my mistake, I mixed them up. The higher q came from equation ii). I guess that makes sense since not all the released energy from the condensation ends up warming the cold stream.
So this large difference can be attributed to heat losses?

I'm also asked to do an energy balance to verify the accuracy of the heat transfer rates. Isn't that what I just did when I compared the two q values? How would that verify anything, as they would always be different?

Let's see the data. Flow rates, heat of vaporization at 133, Cp used?
 
  • #5


As a scientist, it is important to remember that real-world experiments may not always produce results that align perfectly with theoretical calculations. In this case, it is possible that there were some external factors that affected the heat transfer in the double pipe heat exchanger, such as heat loss to the surroundings or imperfect insulation. Additionally, the equations used may not take into account all the complexities of the system, leading to discrepancies in the results. It is also important to consider the accuracy of the measurements and any potential errors in the experimental setup. Overall, it is normal to have some variation between theoretical calculations and experimental results, and further analysis and refinement of the experimental setup may be necessary to improve the accuracy of the results.
 

Related to Heat transfer in a double pipe HE

1. What is a double pipe heat exchanger?

A double pipe heat exchanger is a type of heat exchanger that consists of two pipes, one inside the other. The hot fluid flows through the inner pipe while the cold fluid flows through the outer pipe. This design allows for efficient heat transfer between the two fluids.

2. How does heat transfer occur in a double pipe heat exchanger?

Heat transfer in a double pipe heat exchanger occurs through convection, where the hot fluid transfers its heat to the cold fluid as they flow in opposite directions. The larger the temperature difference between the two fluids, the greater the rate of heat transfer.

3. What factors affect the efficiency of a double pipe heat exchanger?

The efficiency of a double pipe heat exchanger is affected by several factors, including the temperature difference between the two fluids, the flow rate of the fluids, the material and thickness of the pipes, and the design of the heat exchanger.

4. How can the heat transfer rate in a double pipe heat exchanger be increased?

The heat transfer rate in a double pipe heat exchanger can be increased by increasing the surface area of the pipes, increasing the flow rate of the fluids, or by using materials with higher thermal conductivity. Additionally, improving the design of the heat exchanger can also increase its efficiency and heat transfer rate.

5. What are the advantages of using a double pipe heat exchanger?

Double pipe heat exchangers have several advantages, including their compact size, ease of maintenance, and ability to handle high temperature and pressure differentials. They are also cost-effective and can be used for a variety of applications, making them a popular choice for heat transfer in industries such as chemical processing, HVAC, and refrigeration.

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