GN6002_Chemical eng_Heat transfer

In the context of heat and mass transfer, when considering the analogy between heat transfer in a solid by conduction and mass transfer by diffusion, which of the following statements is correct?

•  The temperature gradient in heat conduction is directly equivalent to the viscosity in mass transfer.

• The thermal conductivity of the solid is analogous to the diffusion coefficient in mass transfer.

• The rate of heat transfer in conduction is independent of the cross-sectional area, similar to mass transfer by diffusion.

• The heat transfer coefficient in conduction has no direct analog in mass transfer because diffusion is a bulk property.

Answer explanation

In heat transfer, thermal conductivity is a measure of a material's ability to conduct heat. In mass transfer, the diffusion coefficient represents the ease with which particles can move through a medium. Both coefficients play a similar role in their respective transfer processes, quantifying the rate at which energy or mass can be transported through a medium due to a gradient (temperature gradient for heat, concentration gradient for mass). The other options do not correctly represent the analogies between heat and mass transfer principles.

During the process of heat transfer, the efficiency of a heat exchanger is significantly influenced by the __________  coefficient, which is a measure of the thermal resistance between the heat exchanger surface and the fluid in contact with it.

Answer explanation

The heat transfer coefficient is crucial because it quantifies how well heat is transferred between the heat exchanger surface and the fluid. A higher coefficient means better heat transfer, making the heat exchanger more efficient at warming up or cooling down the fluid as it flows through.

Match the following

• Movement of heat with fluid currents.

• 1. Radiation

• Heat transfer by fluid's bulk movement

• 1. Conduction

• Heat via electromagnetic waves

• 1. Advection

• Direct heat transfer through contact

• 1. Convection

Answer explanation

Conduction is heat traveling through a solid.

Convection is heat circulating within fluids.

Radiation is heat spreading as waves.

Advection is fluid carrying heat as it moves.

Based on the passage, which statement best summarizes the differences between the three modes of heat transfer?

• Conduction requires a fluid medium, convection occurs through electromagnetic waves, and radiation requires direct contact between molecules.

• Radiation and convection require the movement of particles within a substance, while conduction occurs through electromagnetic waves.

• Convection requires a medium to move through, radiation does not require a medium, and conduction is the transfer of energy through a stationary substance.

• Conduction and radiation require a medium for the transfer of energy, while convection can occur in a vacuum.

In a ​ (a)   heat exchanger, the hot and cold fluids move in ​ (b)    directions, which can be more efficient than a ​ (c)    flow arrangement where fluids move parallel; however, maintenance can be more challenging if ​ (d)    occurs on the internal surfaces.

• counterflow
• opposite
• parallel
• fouling
• Heat transfer

Answer explanation

A counterflow heat exchanger allows fluids to move in opposite directions, creating a larger temperature difference that improves heat transfer efficiency compared to a parallel flow design, where fluids move in the same direction and have a smaller temperature gradient. However, the buildup of fouling (deposits of material) can obstruct the flow and heat transfer, making maintenance more challenging.

The following steps describe the process of a hot fluid being cooled in a shell-and-tube heat exchanger. Arrange them in the correct sequence from start to finish.

• The cold fluid absorbs heat, increasing in temperature, and flows out of the shell-side outlet.

• The hot fluid exits the heat exchanger through the tube-side outlet.

• The hot fluid enters the heat exchanger through the tube-side inlet.

• Heat is transferred from the hot fluid to the cold fluid across the tube walls.

• The cold fluid enters the heat exchanger through the shell-side inlet and flows around the tubes.

Answer explanation

Imagine a shell-and-tube heat exchanger is like a playground slide covered with ice (the tubes) inside a warm room (the shell). The ice cube (hot fluid) slides down the icy slide and starts to melt because it’s warm in the room. As it melts, it cools down and becomes a cold water droplet at the bottom. Meanwhile, the air in the room (cold fluid) gets cooler near the slide because the melting ice cube is making the slide chilly. So, the ice cube goes down the slide (enters the tubes), the air moves around the slide (flows around the shell), they share their temperatures (heat transfer), the ice cube comes out as water (hot fluid cools down), and the room's air feels a bit colder near the slide (cold fluid warms up).

Below are several items related to a double pipe heat exchanger. Categorize them into the groups "Components," "Physical Processes," "Performance Metrics," and "Maintenance Tasks."

Groups:

(a) Components

,

(b) Physical Processes

,

(c) Performance Metrics

,

(d) Maintenance Tasks

• Inner pipe

• Flow rate

• Scale buildup

• Routine flushing

• Outer pipe

• Gaskets

• Leak testing

• Counterflow configuration

• Annular space

• Temperature profile

Answer explanation

Components: The essential parts like pipes and sensors that form the double pipe heat exchanger.

Physical Processes: The actual science happening inside, like heat moving and fluids flowing.

Performance Metrics: The yardsticks like heat efficiency and energy use we check to see how well it works.

Maintenance Tasks: The chores like cleaning and inspecting to keep the heat exchanger healthy and efficient.

In a double pipe heat exchanger, the hot and cold fluids flow in __________ directions to exchange heat efficiently.

Answer explanation

The fluids flow in opposite directions, which makes the heat exchange more effective because the temperature difference between the fluids is greater along the length of the pipes, helping the heat to move from the hot fluid to the cold fluid more efficiently.