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Chapter 11: Problem 2

When is a heat exchanger classified as being compact? Do you think a double- pipe heat exchanger can be classified as a compact heat exchanger?

Short Answer

Expert verified
Why or why not? Answer: No, a double-pipe heat exchanger generally does not meet the criteria for being classified as a compact heat exchanger. This is because compact heat exchangers have a large heat transfer surface area per unit volume (typically greater than 700 m^2/m^3), while double-pipe heat exchangers have a low surface area per unit volume due to their relatively simple concentric tube geometry. Compact heat exchangers usually have more complex geometries and deliver higher heat transfer rates within a small volume, whereas double-pipe heat exchangers are more suitable for smaller applications with relatively low heat transfer surface area requirements.

Step by step solution

01

Understand the concept of a compact heat exchanger

A compact heat exchanger is one in which the heat transfer surface area per unit volume is large, typically greater than 700 m^2/m^3. A heat exchanger is considered compact to improve its effectiveness and efficiency by accommodating a large heat transfer surface within a small volume, resulting in higher heat transfer rates.
02

Learn about double-pipe heat exchangers

A double-pipe heat exchanger, also known as a hairpin or concentric tube heat exchanger, consists of two concentric tubes or pipes. One fluid flows through the inner tube while the other fluid flows through the annular space between the inner and outer tube. The fluid can flow in the same direction (parallel flow) or in opposite directions (counter flow). These heat exchangers are generally used for smaller applications where the required heat transfer surface area is relatively low.
03

Compare the surface area per unit volume of a double-pipe heat exchanger to the criteria for a compact heat exchanger

As mentioned earlier, a compact heat exchanger typically has a surface area per unit volume greater than 700 m^2/m^3. In a double-pipe heat exchanger, the heat transfer surface area is limited by the geometry of the concentric tubes, which primarily consists of the outer surface of the inner tube and the inner surface of the outer tube. This geometry does not provide as high a surface area per unit volume as other compact heat exchangers, such as plate, fin, or folded-tube heat exchangers, which have more complex geometries designed to increase the heat transfer surface area.
04

Conclusion

Based on the comparison of the heat transfer surface area per unit volume, a double-pipe heat exchanger generally does not meet the criteria for being classified as a compact heat exchanger. Compact heat exchangers typically have more complex geometries and deliver higher heat transfer rates within a small volume, while double-pipe heat exchangers are more suitable for smaller applications with relatively low heat transfer surface area requirements.

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Most popular questions from this chapter

A shell-and-tube heat exchanger with 2-shell passes and 8 -tube passes is used to heat ethyl alcohol \(\left(c_{p}=2670 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) in the tubes from \(25^{\circ} \mathrm{C}\) to \(70^{\circ} \mathrm{C}\) at a rate of \(2.1 \mathrm{~kg} / \mathrm{s}\). The heating is to be done by water \(\left(c_{p}=\right.\) \(4190 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K})\) that enters the shell at \(95^{\circ} \mathrm{C}\) and leaves at \(60^{\circ} \mathrm{C}\). If the overall heat transfer coefficient is \(800 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), determine the heat transfer surface area of the heat exchanger using \((a)\) the LMTD method and \((b)\) the \(\varepsilon-\mathrm{NTU}\) method.

Consider a water-to-water counter-flow heat exchanger with these specifications. Hot water enters at \(95^{\circ} \mathrm{C}\) while cold water enters at \(20^{\circ} \mathrm{C}\). The exit temperature of hot water is \(15^{\circ} \mathrm{C}\) greater than that of cold water, and the mass flow rate of hot water is 50 percent greater than that of cold water. The product of heat transfer surface area and the overall heat transfer coefficient is \(1400 \mathrm{~W} / \mathrm{K}\). Taking the specific heat of both cold and hot water to be \(c_{p}=4180 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\), determine (a) the outlet temperature of the cold water, \((b)\) the effectiveness of the heat exchanger, \((c)\) the mass flow rate of the cold water, and \((d)\) the heat transfer rate.

A 1-shell-pass and 8-tube-passes heat exchanger is used to heat glycerin \(\left(c_{p}=0.60 \mathrm{Btu} / \mathrm{lbm} \cdot{ }^{\circ} \mathrm{F}\right)\) from \(65^{\circ} \mathrm{F}\) to \(140^{\circ} \mathrm{F}\) by hot water \(\left(c_{p}=1.0 \mathrm{Btu} / \mathrm{lbm} \cdot{ }^{\circ} \mathrm{F}\right)\) that enters the thinwalled \(0.5\)-in-diameter tubes at \(175^{\circ} \mathrm{F}\) and leaves at \(120^{\circ} \mathrm{F}\). The total length of the tubes in the heat exchanger is \(500 \mathrm{ft}\). The convection heat transfer coefficient is \(4 \mathrm{Btu} / \mathrm{h} \cdot \mathrm{ft}^{2} \cdot{ }^{\circ} \mathrm{F}\) on the glycerin (shell) side and \(50 \mathrm{Btu} / \mathrm{h} \cdot \mathrm{ft}^{2} \cdot{ }^{\circ} \mathrm{F}\) on the water (tube) side. Determine the rate of heat transfer in the heat exchanger \((a)\) before any fouling occurs and \((b)\) after fouling with a fouling factor of \(0.002 \mathrm{~h} \cdot \mathrm{ft}^{2} \cdot{ }^{\circ} \mathrm{F} /\) Btu on the outer surfaces of the tubes.

In a textile manufacturing plant, the waste dyeing water \(\left(c_{p}=4295 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) at \(75^{\circ} \mathrm{C}\) is to be used to preheat fresh water \(\left(c_{p}=4180 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) at \(15^{\circ} \mathrm{C}\) at the same flow rate in a double-pipe counter-flow heat exchanger. The heat transfer surface area of the heat exchanger is \(1.65 \mathrm{~m}^{2}\) and the overall heat transfer coefficient is \(625 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). If the rate of heat transfer in the heat exchanger is \(35 \mathrm{~kW}\), determine the outlet temperature and the mass flow rate of each fluid stream.

Consider a double-pipe heat exchanger with a tube diameter of \(10 \mathrm{~cm}\) and negligible tube thickness. The total thermal resistance of the heat exchanger was calculated to be \(0.025 \mathrm{~K} / \mathrm{W}\) when it was first constructed. After some prolonged use, fouling occurs at both the inner and outer surfaces with the fouling factors \(0.00045 \mathrm{~m}^{2} \cdot \mathrm{K} / \mathrm{W}\) and \(0.00015 \mathrm{~m}^{2} \cdot \mathrm{K} / \mathrm{W}\), respectively. The percentage decrease in the rate of heat transfer in this heat exchanger due to fouling is (a) \(2.3 \%\) (b) \(6.8 \%\) (c) \(7.1 \%\) (d) \(7.6 \%\) (e) \(8.5 \%\)
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