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Chapter 11: Problem 3

What is a regenerative heat exchanger? How does a static type of regenerative heat exchanger differ from a dynamic type?

Short Answer

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Answer: A regenerative heat exchanger, also known as a regenerator, is a type of heat exchanger where heat from a hot fluid is stored and later transferred to a cold fluid. The main purpose is to improve the thermal efficiency of a system by reducing heat loss or waste heat. The key differences between static and dynamic types are the motion of the heat storage matrix and their efficiency. In static types, the matrix remains stationary, and the fluids flow alternately through it, while in dynamic types, the matrix is in continuous motion, transferring heat between the fluids as it moves. Static regenerative heat exchangers typically have lower capital cost and easier maintenance but may have lower efficiency compared to dynamic ones, which have higher capital costs and may need more maintenance but can achieve higher heat transfer efficiency.

Step by step solution

01

Regenerative Heat Exchanger

A regenerative heat exchanger, also known as a regenerator, is a type of heat exchanger where heat from a hot fluid is stored and later transferred to a cold fluid. The main purpose of a regenerative heat exchanger is to improve the thermal efficiency of a system by reducing the heat loss or waste heat. Regenerative heat exchangers are often used in industries and power plants where energy saving and efficiency are essential.
02

Static Type of Regenerative Heat Exchanger

A static type of regenerative heat exchanger is a system where the heat storage matrix remains stationary during the heat exchange process. In this type, the hot and cold fluids flow alternately through the heat storage matrix, which is typically made of a solid material like metal or ceramic. The heat from the hot fluid is absorbed by the matrix, which later releases the stored heat to the cold fluid as it flows through the matrix. An example of a static regenerator is a fixed matrix heat exchanger, in which the matrix does not move and the flow direction alternates between the hot and cold fluids.
03

Dynamic Type of Regenerative Heat Exchanger

A dynamic type of regenerative heat exchanger, on the other hand, is a system where the heat storage matrix is in continuous motion during the heat exchange process. In this type, the matrix rotates between the hot and cold fluid streams, allowing heat transfer between the two as it moves. An example of a dynamic regenerator is a rotary regenerator, in which a rotating matrix wheel transfers heat between the hot and cold fluids.
04

Comparing Static and Dynamic Regenerative Heat Exchangers

The primary difference between static and dynamic regenerative heat exchangers lies in the motion of the heat storage matrix. In a static type, the matrix remains stationary, and the fluids flow alternately through it. In contrast, a dynamic type involves the continuous motion of the matrix, transferring heat between the fluids as it moves. Another significant difference is that static regenerative heat exchangers typically have lower capital cost and easier maintenance due to their simple design. However, they may suffer from lower efficiency and effectiveness compared to dynamic regenerative heat exchangers. Dynamic regenerative heat exchangers can achieve higher heat transfer efficiency due to the continuous heat exchange resulting from the motion of the matrix. However, they generally have higher capital costs, require more robust materials to withstand the motion, and may need more maintenance due to their more complex design.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Thermal Efficiency
Thermal efficiency is a key metric in determining how well a heat exchanger conserves the energy within a system. It's essentially a measure of how much useful energy is extracted from the system compared to the total energy input. In the context of regenerative heat exchangers, improving thermal efficiency is all about minimizing heat loss and waste.
The main goal is to transfer as much heat as possible from a hot fluid to a cold one without losing energy to the surroundings. By doing that, regenerative heat exchangers ensure that systems consume less energy overall. This is especially crucial in industrial applications where large amounts of energy are used.
Radiation losses and the specific materials used can impact the thermal efficiency of a regenerative heat exchanger, making it an important factor in their design and implementation.
Energy Saving
Energy saving is a critical component in the design and application of regenerative heat exchangers. By utilizing waste heat, these exchangers can significantly reduce the amount of fuel needed to maintain the temperature of a system.
By capturing and reusing heat that would otherwise be wasted, regenerative heat exchangers help industries save on energy costs. This is not only great for reducing operational expenses but also beneficial for the environment as it leads to lower carbon emissions.
  • Regenerative exchangers recycle waste heat, boosting energy savings.
  • Savings lead to reduced fuel consumption and improved environmental performance.
  • This approach can make a significant impact in large-scale industrial setups.
Static Regenerative Heat Exchanger
A static regenerative heat exchanger involves a stationary heat storage matrix that alternates between being in contact with hot and cold fluids. This process is simple鈥攆irst, the hot fluid warms up the stationary matrix. Then, once the matrix has absorbed sufficient heat, the cold fluid is introduced to extract and use the stored thermal energy.
Static regenerative heat exchangers tend to be more economical and easier to maintain because they lack moving parts. However, they may not achieve the same level of heat recovery efficiency as their dynamic counterparts.
  • Easy maintenance due to stationary design.
  • Lower cost and fewer moving parts.
  • May offer lower thermal efficiency compared to dynamic types.
These exchangers are similar to fixed matrix systems, making them suitable for applications where simplicity and cost-effectiveness are priorities.
Dynamic Regenerative Heat Exchanger
In contrast to the static type, a dynamic regenerative heat exchanger features a rotating heat storage matrix, often in the form of a wheel that moves between the hot and cold streams. This continuous motion allows it to achieve higher thermal efficiency by ensuring constant heat exchange.
While dynamic exchangers often exhibit superior thermal performance, they come with increased complexity and potentially higher costs. The rotating parts require robust materials and regular maintenance due to mechanical wear and tear.
  • Higher thermal efficiency due to continuous heat transfer motion.
  • Complex design with moving parts needing regular upkeep.
  • Useful in high-performance applications where efficiency is critical.
Dynamic regenerators, like rotary systems, are ideal for applications demanding quick heat recovery and high energy efficiency rates.

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Most popular questions from this chapter

Saturated water vapor at \(100^{\circ} \mathrm{C}\) condenses in a 1 -shell and 2-tube heat exchanger with a surface area of \(0.5 \mathrm{~m}^{2}\) and an overall heat transfer coefficient of \(2000 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). Cold water \(\left(c_{p c}=4179 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) flowing at \(0.5 \mathrm{~kg} / \mathrm{s}\) enters the tube side at \(15^{\circ} \mathrm{C}\), determine \((a)\) the heat transfer effectiveness, \((b)\) the outlet temperature of the cold water, and \((c)\) the heat transfer rate for the heat exchanger.

Glycerin \(\left(c_{p}=2400 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) at \(20^{\circ} \mathrm{C}\) and \(0.3 \mathrm{~kg} / \mathrm{s}\) is to be heated by ethylene glycol \(\left(c_{p}=2500 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) at \(60^{\circ} \mathrm{C}\) and the same mass flow rate in a thin-walled double-pipe parallel-flow heat exchanger. If the overall heat transfer coefficient is \(380 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) and the heat transfer surface area is \(5.3 \mathrm{~m}^{2}\), determine \((a)\) the rate of heat transfer and \((b)\) the outlet temperatures of the glycerin and the glycol.

Saturated liquid benzene flowing at a rate of \(5 \mathrm{~kg} / \mathrm{s}\) is to be cooled from \(75^{\circ} \mathrm{C}\) to \(45^{\circ} \mathrm{C}\) by using a source of cold water \(\left(c_{p}=4187 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) flowing at \(3.5 \mathrm{~kg} / \mathrm{s}\) and \(15^{\circ} \mathrm{C}\) through a \(20-\mathrm{mm}-\) diameter tube of negligible wall thickness. The overall heat transfer coefficient of the heat exchanger is estimated to be \(750 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). If the specific heat of the liquid benzene is \(1839 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\) and assuming that the capacity ratio and effectiveness remain the same, determine the heat exchanger surface area for the following four heat exchangers: \((a)\) parallel flow, \((b)\) counter flow, \((c)\) shelland-tube heat exchanger with 2 -shell passes and 40-tube passes, and \((d)\) cross-flow heat exchanger with one fluid mixed (liquid benzene) and other fluid unmixed (water).

11-100 E(S) Reconsider Prob. 11-99. Using EES (or other) software, investigate the effects of the inlet temperature of hot water and the heat transfer coefficient on the rate of heat transfer and the surface area. Let the inlet temperature vary from \(60^{\circ} \mathrm{C}\) to \(120^{\circ} \mathrm{C}\) and the overall heat transfer coefficient from \(750 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) to \(1250 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). Plot the rate of heat transfer and surface area as functions of the inlet temperature and the heat transfer coefficient, and discuss the results. 11-101E A thin-walled double-pipe, counter-flow heat exchanger is to be used to cool oil \(\left(c_{p}=0.525 \mathrm{Btu} / \mathrm{lbm} \cdot{ }^{\circ} \mathrm{F}\right)\) from \(300^{\circ} \mathrm{F}\) to \(105^{\circ} \mathrm{F}\) at a rate of \(5 \mathrm{lbm} / \mathrm{s}\) by water \(\left(c_{p}=\right.\) \(1.0 \mathrm{Btu} / \mathrm{lbm} \cdot{ }^{\circ} \mathrm{F}\) ) that enters at \(70^{\circ} \mathrm{F}\) at a rate of \(3 \mathrm{lbm} / \mathrm{s}\). The diameter of the tube is 5 in and its length is \(200 \mathrm{ft}\). Determine the overall heat transfer coefficient of this heat exchanger using (a) the LMTD method and \((b)\) the \(\varepsilon-\mathrm{NTU}\) method.

A shell-and-tube heat exchanger with 2-shell passes and 12 -tube passes is used to heat water \(\left(c_{p}=4180 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\) with ethylene glycol \(\left(c_{p}=2680 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\). Water enters the tubes at \(22^{\circ} \mathrm{C}\) at a rate of \(0.8 \mathrm{~kg} / \mathrm{s}\) and leaves at \(70^{\circ} \mathrm{C}\). Ethylene \(\mathrm{glycol}\) enters the shell at \(110^{\circ} \mathrm{C}\) and leaves at \(60^{\circ} \mathrm{C}\). If the overall heat transfer coefficient based on the tube side is \(280 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), determine the rate of heat transfer and the heat transfer surface area on the tube side.
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