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Chapter 11: Problem 79

For a specified fluid pair, inlet temperatures, and mass flow rates, what kind of heat exchanger will have the highest effectiveness: double-pipe parallel- flow, double-pipe counterflow, cross-flow, or multipass shell-and-tube heat exchanger?

Short Answer

Expert verified
Answer: The multipass shell-and-tube heat exchanger has the potential to have the highest effectiveness, given the specific fluid pair, inlet temperatures, and mass flow rates. However, the effectiveness of any heat exchanger depends on its specific design, materials, and operating conditions.

Step by step solution

01

Understand heat exchanger effectiveness

Heat exchanger effectiveness is the ratio of the actual heat transfer to the maximum possible heat transfer between the fluids. The higher the effectiveness, the better the heat exchanger is at transferring heat. Different heat exchanger configurations have different effectiveness values, depending on their design.
02

Investigate double-pipe parallel-flow heat exchanger

In a double-pipe parallel-flow heat exchanger, fluids flow in the same direction. The temperature difference between the fluids decreases along the length of the exchanger, reducing the heat transfer rate. Thus, the effectiveness of this type of heat exchanger is lower than other types.
03

Investigate double-pipe counterflow heat exchanger

In a double-pipe counterflow heat exchanger, fluids flow in opposite directions. This arrangement maintains a more constant temperature difference between the fluids, which leads to a higher heat transfer rate. The effectiveness of this type of exchanger is higher than the parallel-flow heat exchanger.
04

Investigate cross-flow heat exchanger

In a cross-flow heat exchanger, fluids flow perpendicularly to each other. This arrangement provides a high rate of heat transfer, resulting in higher effectiveness than both parallel-flow and counterflow heat exchangers. However, the effectiveness of a cross-flow heat exchanger depends on the specific flow arrangement (i.e., whether the flows are mixed or unmixed).
05

Investigate multipass shell-and-tube heat exchanger

In a multipass shell-and-tube heat exchanger, fluids flow through multiple passes within the tubes and shell. This configuration increases the number of heat transfer surfaces, resulting in a higher heat transfer rate and effectiveness. The effectiveness of a multipass shell-and-tube exchanger depends on the number of passes and the specific design.
06

Determine the heat exchanger with the highest effectiveness

From the information provided in steps 2-5, and knowing the specific fluid pair, inlet temperatures, and mass flow rates, one can conclude that the multipass shell-and-tube heat exchanger has the potential to have the highest effectiveness. However, it is important to note that the effectiveness of any heat exchanger depends on its specific design, materials, and operating conditions.

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Most popular questions from this chapter

Hot water coming from the engine is to be cooled by ambient air in a car radiator. The aluminum tubes in which the water flows have a diameter of \(4 \mathrm{~cm}\) and negligible thickness. Fins are attached on the outer surface of the tubes in order to increase the heat transfer surface area on the air side. The heat transfer coefficients on the inner and outer surfaces are 2000 and \(150 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), respectively. If the effective surface area on the finned side is 10 times the inner surface area, the overall heat transfer coefficient of this heat exchanger based on the inner surface area is (a) \(150 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (b) \(857 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (c) \(1075 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (d) \(2000 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (e) \(2150 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\)

Consider a condenser unit (shell and tube heat exchanger) of an HVAC facility where saturated refrigerant \(\mathrm{R} 134 \mathrm{a}\) at a saturation pressure of \(1318.6 \mathrm{kPa}\) and a rate of \(2.5 \mathrm{~kg} / \mathrm{s}\) flows through thin-walled copper tubes. The refrigerant enters the condenser as saturated vapor and it is desired to have a saturated liquid refrigerant at the exit. The cooling of refrigerant is carried out by cold water that enters the heat exchanger at \(10^{\circ} \mathrm{C}\) and exits at \(40^{\circ} \mathrm{C}\). Assuming initial overall heat transfer coefficient of the heat exchanger to be \(3500 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), determine the surface area of the heat exchanger and the mass flow rate of cooling water for complete condensation of the refrigerant. In practice, over a long period of time, fouling occurs inside the heat exchanger that reduces its overall heat transfer coefficient and causes the mass flow rate of cooling water to increase. Increase in the mass flow rate of cooling water will require additional pumping power making the heat exchange process uneconomical. To prevent the condenser unit from under performance, assume that fouling has occurred inside the heat exchanger and has reduced its overall heat transfer coefficient by \(20 \%\). For the same inlet temperature and flow rate of refrigerant, determine the new flow rate of cooling water to ensure complete condensation of the refrigerant at the heat exchanger exit.

In a parallel-flow, liquid-to-liquid heat exchanger, the inlet and outlet temperatures of the hot fluid are \(150^{\circ} \mathrm{C}\) and \(90^{\circ} \mathrm{C}\) while that of the cold fluid are \(30^{\circ} \mathrm{C}\) and \(70^{\circ} \mathrm{C}\), respectively. For the same overall heat transfer coefficient, the percentage decrease in the surface area of the heat exchanger if counter-flow arrangement is used is (a) \(3.9 \%\) (b) \(9.7 \%\) (c) \(14.5 \%\) (d) \(19.7 \%\) (e) \(24.6 \%\)

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.

A cross-flow heat exchanger consists of 80 thinwalled tubes of \(3-\mathrm{cm}\) diameter located in a duct of \(1 \mathrm{~m} \times 1 \mathrm{~m}\) cross section. There are no fins attached to the tubes. Cold water \(\left(c_{p}=4180 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) enters the tubes at \(18^{\circ} \mathrm{C}\) with an average velocity of \(3 \mathrm{~m} / \mathrm{s}\), while hot air \(\left(c_{p}=1010 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) enters the channel at \(130^{\circ} \mathrm{C}\) and \(105 \mathrm{kPa}\) at an average velocity of \(12 \mathrm{~m} / \mathrm{s}\). If the overall heat transfer coefficient is \(130 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), determine the outlet temperatures of both fluids and the rate of heat transfer.
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