Concentric Tube Heat Exchanger Experiment Report

Abstract

The Concentric Tube Heat Exchanger experiment aims to demonstrate the working principles of industrial heat exchangers under co-current and counter-current flow conditions. The study also investigates the effect of flow rate variations on heat exchanger performance.

The experiment was conducted using a specifically designed apparatus to facilitate measurements and calculations.

Introduction

Heat exchangers are vital devices in various industries, allowing efficient heat transfer between fluids of different temperatures while preventing their mixing. Different types of heat exchangers exist, including shell and tube, plate and shell, adiabatic wheel, plate fin, and pillow plate, each with specific configurations and applications.

Heat exchangers can operate in two main flow arrangements: parallel (co-current) and counter-flow.

The concentric tube heat exchanger used in this experiment was designed to illustrate the working principles of industrial heat exchangers. The experiment is conducted with a minimal setup requiring only a cold-water supply, a single-phase electrical outlet, and a benchtop. It provides students with the opportunity to measure temperature profiles, understand energy balances, calculate log mean temperature differences, and determine heat transfer coefficients.

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Objectives

The objectives of this experiment are as follows:

1. Demonstrate the working principles of a concentric tube heat exchanger under co-current flow conditions.
2. Demonstrate the working principles of a concentric tube heat exchanger under counter-current flow conditions.
3. Demonstrate the effect of flow rate variation on the performance characteristics of a concentric tube heat exchanger under counter-current flow conditions.
4. Study the effect of co-current and counter-current flow on heat exchangers.
5. Investigate the effect of fluid temperature on counter-flow heat exchanger performance.
6. Examine the impact of fluid flow rates on heat exchanger performance.
7. Calculate the efficiency for co-current flow, counter-current flow, and fluid flow rate variation.
8. Determine the overall heat transfer coefficients for co-current flow, counter-current flow, and flow rate variation.
9. Calculate flow rates for different co-current and counter-current flow conditions in the heat exchanger.

Theory

Before calculating the overall heat transfer coefficient (U), the power emitted and power absorbed must be determined. The efficiency of the heat exchanger is also calculated.

The log mean temperature difference (LMTD) is used in heat transfer calculations and is calculated using the following equation:

LMTD = (^ΔT1₁ - ^ΔT2₂) / ln(^ΔT1₁ / ^ΔT2₂)

The overall heat transfer coefficient (U) for a double pipe heat exchanger is expressed as:

U = 1 / (¹_hi + ^Δx_k / ^do_ho + ^di_ho)

Where:

• hi = Inside film coefficient for heat transfer across the tubes
• ho = Outside film coefficient for heat transfer across the tubes
• Δx = Thickness of the wall
• k = Thermal conductivity of the wall
• do = Outer diameter of the annulus
• di = Inner diameter of the annulus

The film coefficients (hi and ho) are calculated using the following equations:

hi = G' / (ρ * Cp)

ho = G' / (ρ * Cp)

Where:

• G' = Mass flowrate per unit area
• ρ = Density
• Cp = Specific heat

Experimental Methods

The experimental procedure involved the following steps:

1. Connect the pipes to ensure co-current flow.
2. Record temperatures at the inlets and outlets of both the hot and cold streams once the flowrate stabilizes.
3. Keep the cold flowrate constant and vary the hot flowrate to obtain three sets of measurements, recording temperatures at each flowrate.
4. Keep the hot water flow constant and change the cold stream flowrate, recording temperature values again.
5. Repeat the experiment with counter-current flow of the fluids in the pipes.

Results

Co-Current Flow

The following tables and graphs present the results for co-current flow:

Table 1.1: Co-Current Flow Temperatures

Table 1.2: Co-Current Flow Power and Efficiency

Counter-Current Flow

The following tables present the results for counter-current flow:

Table 2.1: Counter-Current Flow Temperatures

Table 2.2: Counter-Current Flow Power and Efficiency

Discussions

In this experiment, we investigated the performance of a Concentric Tube Heat Exchanger under both Co-Current and Counter-Current flow conditions. The primary objective was to analyze the impact of flow rate variations on the heat exchange efficiency and overall heat transfer coefficient.

Co-Current Flow Findings:

Table 1.2 presents the data for Co-Current flow, showing the power emitted and the efficiency at different flow rates. We observe that as the flow rate increases, the power emitted also increases, indicating higher heat transfer rates. The efficiency remains consistently high, exceeding 100% for some flow rates. This suggests that the Co-Current flow arrangement is highly efficient in transferring heat between the hot and cold fluids.

Counter-Current Flow Findings:

Moving on to the Counter-Current flow experiments, Table 2.2 displays the data for power emitted and efficiency under varying flow rates. In this case, we notice that the power emitted increases as the flow rate rises. However, unlike the Co-Current flow, the efficiency remains relatively stable and consistently high without exceeding 100%. Counter-Current flow appears to offer stable and efficient heat exchange even at different flow rates.

Table 2.1 and Table 2.2 visualize the data, emphasizing the relationship between flow rate and both power emitted and efficiency in Counter-Current flow. These graphs illustrate that Counter-Current flow maintains a stable heat transfer efficiency despite changes in flow rate.

Comparison of Co-Current and Counter-Current Flow:

Comparing the results of Co-Current and Counter-Current flow experiments, several key observations can be made:

1. Efficiency Stability: Co-Current flow exhibits higher efficiency, with some values exceeding 100%, indicating an effective heat transfer process. In contrast, Counter-Current flow maintains stable efficiency levels without surpassing 100%. This suggests that Co-Current flow may be more suitable for applications requiring maximum heat transfer efficiency, while Counter-Current flow offers consistent performance across a range of flow rates.
2. Effect of Flow Rate: Both Co-Current and Counter-Current flow show a positive correlation between flow rate and power emitted, signifying that higher flow rates result in increased heat transfer rates. However, Co-Current flow experiences a more significant increase in power emitted as flow rate increases compared to Counter-Current flow.
3. Overall Heat Transfer Coefficient (U): The U values, representing the overall heat transfer coefficient, are crucial indicators of a heat exchanger's effectiveness. The U values for Co-Current and Counter-Current flow conditions can be compared to assess the overall performance of the heat exchanger.

Interpretation of the Data:

The experimental data demonstrates that the choice between Co-Current and Counter-Current flow configurations depends on specific application requirements. Co-Current flow excels in achieving higher heat transfer rates and efficiency, making it suitable for applications where maximizing heat exchange is crucial.

On the other hand, Counter-Current flow offers stable and reliable heat exchange performance across varying flow rates. This makes it a suitable choice when consistent heat transfer efficiency is needed, even with changing operating conditions.

The results also emphasize the importance of considering the overall heat transfer coefficient (U) when designing or selecting a heat exchanger. U values provide a comprehensive measure of a heat exchanger's effectiveness, considering factors such as fluid properties, flow rates, and geometry.

In practice, engineers and designers must carefully evaluate the specific requirements of their heat exchange application to choose the most appropriate flow arrangement, whether Co-Current or Counter-Current, to achieve the desired level of heat transfer efficiency.

Conclusions

In conclusion, this experiment has provided valuable insights into the performance characteristics of Concentric Tube Heat Exchangers under different flow conditions and flow rates. These findings can serve as a basis for optimizing heat exchanger designs and selecting the most suitable configuration for various industrial applications.

References

1. Kern, "Process Heat Transfer"
2. ThermExcel, https://www.thermexcel.com/english/tables/eau_atm.htm
3. JM Coulson, JF Richardson, RK Sinnott, "Chemical Engineering Volume 6," Elsevier Science & Technology