Lesson 8 - Heat Exchangers

When designing a heat exchanger, there are two main steps. The first is to select the type and configuration of the heat exchanger, and the second is to size the exchanger. In the next few sections, we will consider each step in more detail.

Choosing a type of heat exchanger

In this lesson so far, you have learned about the unique characteristics and designs of the two main types of heat exchangers in a brewery, namely plate heat exchangers and shell and tube exchangers. However, when tasked with selecting an exchanger for your brewery, which type would you choose? Let us look at the relative advantages and disadvantages of these exchanger types.

• Heat transfer rates

  • Plate heat exchangers have better heat transfer rates than shell and tube exchangers. The design induces high turbulence in both the product and utility fluid. As you have learned, this translates to better heat transfer rates and thermal efficiencies.

  • This means that plate heat exchangers are far more compact than a shell and tube exchanger for the same duty.

  • It also means they can achieve much closer approach temperatures; in other words, the product fluid temperature can get very close to the utility fluid temperature.

• Maximum operating temperature and pressure

  • Plate heat exchangers are not suitable for pressures of greater than  400 kPa or temperatures above 200°C. In comparison, shell and tube exchangers can be used for applications with pressures of up to  1000 kPa and temperatures of up to 500°C.

  • In plate heat exchangers to avoid bending the plates, the absolute pressure and the flowrates of the product and utility fluid must be similar. These requirements can limit the applications of plate heat exchangers.

  • For the same capacity heat exchanger, plate heat exchangers have a higher pressure drop than shell and tube exchangers. So, for applications where there is limited system pressure shell and tube exchangers are a more suitable choice.

• Flexibility

  • Once the shell and tube exchangers are designed their capacity, and flow configuration cannot be altered. By comparison, capacity or flow configuration of plate heat exchangers can be changed simply by adding, removing, or replacing plates.

• Holding volume

  • The volume of fluid held in a plate heat exchanger is far less than a shell and tube. This means that plate heat exchangers are more responsive to process variations and can achieve far tighter temperature control. If the utility fluid is hazardous, for example, ammonia, the large volume of fluid in the shell of a shell and tube heat exchanger presents a safety risk that has to be correctly managed.

• Fouling and cleaning

  • Because of the narrow space between plates, plate heat exchangers are more prone to fouling, but both sides of the exchanger are easily cleanable.

  • Tube side cleaning of a shell and tube heat exchanger is easy, but it's difficult to near impossible to clean the outside of the tube bundle.

• Maintenance

  • Shell and tube heat exchangers have low maintenance requirements and costs. Also, the risk of cross-contamination between the fluids is far smaller. However, if there is a leak, it can be more difficult to locate and costly to fix.

  • As you have learned the gaskets of plate heat exchangers degrade over time and require regular replacement. Correctly reassembling and tightening a plate heat exchanger requires specialised skills and can be time consuming. 

Configuration of heat exchanger

Once you have selected the type of exchanger you want, before you can size it, you need to consider its configuration.

1. Firstly, you must decide which fluid flows on which side. For plate heat exchangers, this is not so important as both passages are the same. However, for shell and tube, this is a key decision.

2. Secondly, you must decide between co-current, countercurrent or hybrid flow configuration. This will be governed by any specific temperature limitations there may be on the product temperature, and if the exchanger has multiple passes.

3. Thirdly, the temperature profile of the product stream may also dictate that a multi-pass exchanger is required.

We now know what to consider when selecting a heat exchanger. In the next few sections of this lesson we will learn how heat is transferred in a heat exchanger and about the concepts involved in quantifying this heat transfer.