Example Group Report

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Energy and Fluid Systems: Alfa Laval Spiral Heat Exchangers.

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Prue explains the group�s planning approach to report writing (27s):

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1 Summary:

The purpose of this project is to examine and evaluate the performance of the Alfa Laval Spiral Heat Exchanger (type 122) rig. This was done by changing the conditions in the flow rate or steam pressure within various parts of the rig, and recording the measurements of temperature and flow rate, which were then collated, compared and analysed. The spiral heat exchangers used in the rig have a channel width and height of 0.2m and 0.005m respectively, and a channel wall thickness of 0.002m, as well as a mean radius of 0.125m for the spiral [1].

The heat duties were calculated form the experimental data. From the result it was observed that in spiral heat exchanger 1 (SHE 1), the heating duties calculated at steady state ranged from 6000 to 110000 kJ.s-1 in the hot loop, and 7000 to 110000 kJ s- in the main loop. In (SHE 2), the values ranged from 12000 to 110000 kJ.s-1 in both the cold and main loops.

In addition the percentage of discrepancies in heat balances among the groups ranged from 60% for the hot loop, 59% and 57% for the main loop and 58% for the cold loop. However, according to the results from the calculations, it was found that the heat duty did not make sense as we had a large discrepancy with the slurry loop. It was found that the best way of dealing with this was to change the calibration curve for the flow rate of the slurry loop. A possible reason for the discrepancy could be because of the wear and tear caused by a viscous liquid containing insoluble solid lumps used in other experiments.

The heat transfer coefficients were calculated and compared with the predicted U values. For finding the predicted values the Morimoto and Hotta correlations was used.

In addition, comparing the results between the calculated and experimental heat transfer coefficients, there were about 100 to 500 (W/m^2.K) differences. Therefore, the Morimoto and Hotta correlation was useful and relevant as the results were close to each other and the above-mentioned correlation helped us to predict the heat transfer coefficient to a greater extent.

However, according to the inaccuracies in devices and incomplete recorded results from some of the groups, some errors occurred in the projects. The main measurement contribution to error was found to be due to the flow rates as the error of the temperatures were limited to their relative boundaries � the hot loop having the highest temperatures, the cold loop having the lowest temperatures, and the main loop having temperatures in between. Due to this the error in temperature is minimal; allowing us to conclude that the discrepancies between the experimental and theoretical calculations was largely due to flow rate.

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2 Introduction:

Alfa Laval Heat Exchangers

Heat Exchangers are a commonly used piece of equipment, which transport heat from one fluid stream to another. The aim of this experiment is to evaluate the heat exchangers, which are commonly used in industry.

Spiral Heat Exchangers are a form of heat exchanger where two coils are wrapped around each other, creating parallel counter current heat transfer, as the effect of the spiral shape is minimal. This design is shown in Figure 1.

The aim therefore is to evaluate the performance of two Alfa Laval Spiral Heat Exchangers in operation at steady state and when the flow rate is altered.

Figure 1: Diagram of a Spiral Heat Exchanger Made by Alfa Laval [1]

(FIGURE NOT INCLUDED)

2.2 Equations used

2.2.1 Heat Transfer Theory

The math required for this experiment includes the fundamental concepts and equations used for heat transfer. Experimentally, the flow rates and temperatures were recorded, allowing us to calculate the heat duty of the individual channels in each heat exchanger in the rig according to the equation:

Where:

2.2.4 Time Constants

The time-constant tau (τ) is a relative constant that describes the time characteristics of the rig. It compensates for the 'time lag' associated with start up and shut down times of the rig. The concepts of time constants are further explained in the discussion.

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3 Equipment and Method:

Figure 2: Process Flow Diagram of Rig used for the Experiment.

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The rig in operation, shown in Figure 2, contains two Spiral Alfa Laval Heat Exchangers (type 122) (SHE 1 and SHE 2) and one Plate Heat Exchanger (PHE). The hot water loop is a closed loop and as it circulates is heated by steam through a Plate Heat Exchanger from a boiler and then ...

3.2 Group Roles and Procedure

  1. When entering the lab each group waited for the rig to reach steady state, taking two or three measurements (depending on the group) to ensure that the fluctuations were minimal.
  2. Each group changed an element of the rig and monitored its effect on all sections of the rig.
  3. ...

4 Results

4.1 Basic Calculations

Results shown in figure 3 are conducted using equations described in 2.2. Heat Transfer Theory ...

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4.2 Statistical Analysis

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5 Discussion

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5.1 Accuracy of Results

The problem with sharing results with other groups is that some are more thorough than others. It is obvious from some groups results are not correct. Examples include,

5.2 Calibration of Slurry Loop

Comparison of the heat flow rate shows the systematic error in the main loop. At the steady state, heat loss in hot loop is very close to heat gained by cold loop; therefore it is more unlikely that main loop and cold loop are inaccurate. ...

5.3 Time Constants and Steady State

The time constant of a process is a measure of the time necessary for the process to adjust to a change in its input [5]. The system requires a certain amount of time to absorb the changes in flow rate or pressure. The value of the response in temperature T reaches 63.2 % of its steady state temperature when the time elapsed is equal to one time constant. Subsequently, 2, 3 and 4 time constants are equal to 86.5, 95 and 98 % of its steady state temperature respectively. This is because the temperature-time response might be fitted to a first order equation;

5.4 ANOVA'S

ANOVA was used for several different analysis objectives within this project. Two simple one-way ANOVA�s as well as an unbalanced two-way ANOVA were carried out on the heat exchangers.

The first analysis of variance was across the two heat exchangers, to see if there was any significant difference in the variances of the heat exchangers. ...

5.5 Fouling

...

5.6 Total resistance

The factors contributing to the overall heat-transfer resistance are the thermal resistance of the fluid on both sides of the heat exchanger wall, the thermal resistance of the wall itself, and the fouling factor, calculated as shown in figure 9. ...

5.7 Irrelevancy of heat exchanger efficiency

Different types of heat exchangers are being used in industry. They can be compared with each other related to their structural designs. For example, ...

5.8 Negligible Heat Losses

According to the heat balance principle one of the factors that affects heat transfer is temperature difference. ...

5.9 Heat Transfer and Heat Coefficients

During our original examination of the rig our group recorded values for flow rate and temperature, which caused significant difference in our heat duties (Q), particularly in the main loop. Differences in Q values were of significant magnitude; often the main loop values were double those of the hot and cold loops. After careful analysis of these values, it was determined that ...

From our results, it is observed that our theoretical U values and our practical U values were similar to each other. Both Theoretical and Practical values averaged around 1500 to 2000 W/m-2K-1. Though they are both within acceptable range for this experiment; the results themselves are of concern. This is so as our theoretical U values should be significantly higher than the practical. This is due to fouling, ...

However when taking into account of error range calculated, it is displayed that overall, most of our theoretical U values are indeed higher than our practical calculated U values, but only slightly.

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6 Conclusion

It was found that the Q values differed greatly from the cold and hot loops and hence a new calibration curve was constructed. This gave us a more precise Q value for the main loop which seemed to correspond with the values from the cold and hot loop. Due to this we obtained theoretical results of U to be slightly lower than practical results of U for some of the values. However these values may only range from 1400 to 1700 Wm-2K-1. Upon further analysis from the practical U values, we determined the final practical overall heat coefficient for H.E 1 was 1661 +/- 91 W.m-2.K-1 and for H.E.2 was 1719 +/- 71 W.m-2.K-1 (see Figure 5.1).

However, according to the inaccuracies in devices and incomplete recorded results from some of the groups, some errors occurred in the projects. The main measurement contribution to error was found to be due to the flow rates ...

The different resistances contributing to the overall heat transfer resistance were calculated and the limiting heat-transfer resistance was found to be due to the thermal resistance of the fluid on either side of the wall. This resistance was compared to the thermal resistance of the wall and the resistance due to the fouling but neither was found to measure up to the resistance of the water.

After calculating the overall heat-transfer coefficient both experimentally and theoretically, we observed an error, which could be due either to the flow rate or the temperature. After some evaluation, it was concluded that the main measurement contributing to the error was that of the flow rates. This is because the temperatures cannot err too greatly as they have boundaries relative to each other. That is, ...

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7 References

  1. Alfa Laval Organization.(2007).Spiral Heat Exchangers. http://www.alfalaval.com/ecoreJava/WebObjects/ecoreJava.woa/wa/showNode?siteNodeID=4673&contentID=-1&languageID=1
  2. Svrcek, W.Y., Mahoney, D.P., Young, B.R., 2000, A Real-Time Approach to Process Control, Wiley, Chicester.
  3. Engineering Edge Corporations.(2000-2007).Heat Exchanger. http://www.engineersedge.com/heat_exchanger/general.htm
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8 Appendix Figures

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