Applied Heat and Mass Transfer - Heat Exchangers

A set of flashcards designed to help students review key concepts and definitions related to heat exchangers in applied heat and mass transfer.

What is the main function of heat exchangers?

To facilitate the exchange of heat between two fluids at different temperatures without mixing them.

Name two common applications of heat exchangers.

Space heating and refrigeration.

What is a double-pipe heat exchanger?

The simplest heat exchanger type, consisting of two concentric pipes with two fluids.

How do counter-flow heat exchangers differ from parallel-flow heat exchangers?

In counter-flow exchangers, fluids enter from opposite ends, making them more efficient.

What is the primary advantage of shell and tube heat exchangers?

They can handle high pressure applications, typically above 30 bars.

What is a plate heat exchanger?

A type of heat exchanger consisting of plates with corrugated passages used mainly for liquid–liquid heat transfer.

What defines a compact heat exchanger?

A heat exchanger with a ratio of surface area to its volume greater than 700 m²/m³.

What does the overall heat transfer coefficient (U) represent?

It quantifies the rate of heat transfer through a solid wall separating two fluids.

Explain fouling in the context of heat exchangers.

Fouling is the accumulation of deposits on heat transfer surfaces, which causes a decrease in performance.

What method can be used for heat exchanger analysis when outlet temperatures are unknown?

The effectiveness-NTU method.

Define the logarithmic mean temperature difference (LMTD).

It is an exact representation of the average temperature difference between the hot and cold fluids.

What is the effect of a high NTU on the size of a heat exchanger?

A higher NTU indicates a larger heat exchanger is required, but the increase in effectiveness may be small.

What is the role of a correction factor (F) in the NTU method?

It adjusts calculations based on the geometry and temperature arrangements of the heat exchanger.

What happens to heat transfer rates when both fluid streams have equal capacity rates in a heat exchanger?

Both streams experience the same temperature change.

What type of fouling results from algae growth?

Biological fouling.

What kind of heat exchanger is typically selected for low and medium-pressure applications?

Plate heat exchanger.

How does an engineer select a heat exchanger?

By achieving the desired temperature change using methods like LMTD or effectiveness-NTU.

What are the effects of fouling on a heat exchanger's performance?

It reduces heat transfer rates and increases pressure drop.

What equation is used to calculate the rate of heat transfer in a heat exchanger?

Q = U * A * F * ΔTlm.

What are the primary types of classifications for heat exchangers based on construction?

Double-pipe, shell and tube, plate, and compact.

What is the definition of capacity ratio (c) in heat exchangers?

Cmin/Cmax, where Cmin is the minimum heat capacity rate of the two fluid streams.

What happens to the effectiveness of heat exchangers as NTU increases?

Effectiveness increases rapidly for low NTU values and gradually for higher values.