Heat Exchangers

Heat exchangers are used to transfer heat from one stream to another. They are used to heat streams and to cool streams. The streams can be heated or cooled by other process streams, or they can be heated by steam and cooled by cooling water.

Design Constraints

1. Cost – wish to have an exchanger that costs the least.

2. Efficiency – wish to have an exchanger that operates most efficiently, with minimum loss of energy in the transfer, and minimum drop in pressure of the fluids.

3. Space – wish to have an exchanger that is small.

4. Materials – want an exchanger built from materials that are compatible with the process streams and don’t cost a lot.

5. Maintenance – want an exchanger that can be easily cleaned.

6. Ease of construction.

Heat Exchangers

Typical questions that chemical engineers might have about heat exchangers:

1. What size heat exchanger do I need in a new process?

2. What type of heat exchanger do I need in a new process?

3. What are the heat transfer coefficients on the tube side and on the shell side?

4. Is an existing heat exchanger operating properly?

5. How much fouling can the exchanger withstand before the performance is degraded? How do I know if this has occurred with an operating heat exchanger?

Heat Exchangers

The driving force for the operation of a heat exchanger is the temperature difference between the fluids. The higher the temperature difference, the smaller the heat exchanger. However, the temperature difference is limited by boiling points of the liquids, scaling, materials of construction, etc.

The proper design of a heat exchanger is a trade-off of price and performance.

Heat Exchangers: Chapter 15 in McCabe et al., 6th edition.

Heat Exchangers - Types

Heat exchangers come in many different types:

1. Co-current.

2. Counter-current

3. Double pipe

4. Spiral

5. Finned

6. Compact

7. Shell and tube (most common in chemical process industries)

Heat Exchangers - Types

Co- and Counter- current: The exchanger is constructed of concentric tubes. One fluid occupies the outer pipe, the other the inner.

Co-current: fluids flow in same direction

Counter-current: fluids flow in opposite direction

Co-current Counter-current

Heat Exchangers - Types

Double-Pipe: A heat exchanger built from concentric tubes.

These are simple to construct, but may require a lot of physical space to achieve the desired heat transfer area. Can be used for gases and liquids.

Heat Exchangers - Types

Double-Pipe Heat Exchanger in our UO Lab

Heat Exchangers - Types

Double-Pipe Heat Exchanger in our UO Lab

Heat Exchangers - Types

Spiral: Constructed from sheets of metal wound in a circular fashion. The fluids flow in adjacent chambers between the sheets of metal.

This design is based on the ease of fabrication.

Heat Exchangers - Types

Spiral Heat Exchanger in our UO Lab

Heat Exchangers - Types

Finned: contain fins on one heat exchange surface to increase the heat exchange surface.

Heat registers is a home hot water heating system is an example of this type.

Heat Exchangers - TypesCompact heat exchangers: constructed from round or square channels. Car radiator is an example of this type.

Heat Exchangers - Types

Shell and Tube: Most common in chemical process industries. This type provides a large heat transfer surface in a small space, can operate at high pressures, are easy to clean and can be made of a wide variety of materials.

1-1 Counter Current Exchanger (1-shell, 1 tube pass)

Heat Exchangers - Types

Shell and Tube Heat Exchanger in our UO Lab

Heat Exchangers - Types

1-2 Heat Exchanger: 1 shell pass, 2 tube passes.

Heat Exchangers - Types

2-4 Heat Exchanger: 2 shell passes, 4 tube passes.

Heat Exchangers - Temperatures

Heat Exchangers - Types

Shell and Tube Heat Exchanger cutaway in computer lab

1 shell pass

2 tube passes

Heat Exchangers - Types

End detail in heat exchanger

Heat Exchangers - Types

Heat Exchangers – Problems with Calculations

Temperature of fluid changes (possibly on shell and tube side).

Baffles and leakage around baffles.

Easy part is tube side – use existing correlations to determine heat transfer coefficient.

Difficult side is shell side as fluid flows across the tubes.