The decision of which type of heat exchanger to install is one of the most critical that chemical processing engineers can make when designing a processing plant. As with other critical process equipment decisions, it should be made early in the plant's design. Not only does the heat exchanger selection impact process performance and efficiency, it also affects the size and the configuration of piping and supports. What usually happens, however, is that engineers wait until a later design phase to choose the specific type of heat exchanger, limiting their options and possibly negatively affecting process.

Addressing heat exchanger options at the beginning of a plant's design opens up a wide range of possibilities, enabling designers to use the most efficient exchanger for the job. Not yet hemmed in by design restrictions that crop up later, engineers can look beyond the traditional shell-and-tube type and consider alternatives such as plate-and-frame, spiral, plate-in-shell, printed circuit, plate-fin and welded-plate type heat exchangers.

To begin with, it's important for the engineers to single out which are viable options for their plant. Each type should be considered in relation to the plant's process. Factors to look at include pressure and temperature limitations of the exchanger and the suitability of the exchanger to handle the liquids used during processing. This will minimize fouling. Also important is the compatibility of the enlarger with other equipment in the plant. Lastly, the employee skill level required to handle the machinery needs to be evaluated and either staffing or equipment changes made to accommodate the choice.

Engineers should next focus on keeping the plant's tasks and design as simple as possible. Large numbers of heat exchanger units are complicated and take up space. The number of units needed depends on heat exchanger type. Some types need only one unit. Shell-and-tube exchangers often require more. One reason for this is that shell-and-tube types usually don't have countercurrent flow and must make multiple passes to complete the process. The lower heat transfer rates that result require the purchase of additional units. Some of the alternate types do have countercurrent flow and can achieve higher heat transfer rates with fewer units. In addition, many alternate heat exchangers have multi-streaming. This means that a single unit of these types has more than one hot stream and one cold stream. They're often thought of as containing an entire heat exchanger network. Depending on the process, there can be situations where one alternate heat exchanger type could do the work of several shell-and-tube types, thus cutting down on both the piping and the cost.

Heat recovery is an important factor in minimizing plant complexity. Some of the alternate heat exchanger types available require less heat recovery. Thus the amount of heat recovery equipment can be reduced. If shell-and-tube heat exchangers are used, the larger number of units that are needed will slow heat recovery. This means a hot utility input will have to be purchased, which adds to the plant's complexity and drives up installation costs. An extra hot utility input is usually no longer needed when an alternate heat exchanger type with multi-streaming is installed. Also, the smaller size of most alternate heat exchanger types usually means less plant floor space and less piping.

Engineers should also look to control the heat exchanger's lifetime cost. It's important to consider more than just the initial investment. In many cases, the cost of installation can be more than the price of the unit itself. The installation cost for a single unit of an expensive type can be less than the installation cost of multiple units of an inexpensive type. Oftentimes, the heat exchanger types that perform the best also prove to be the less expensive option. Engineers should also consider the possibility of plant downtime. Downtime often racks up costs greater than the price of any heat exchanger type. Yet it can usually be avoided if the right heat exchanger is selected in the design phase. Downtime often occurs when the heat exchanger becomes fouled, a condition that some types are less prone to suffer from than others. Plate-and-frame type heat exchangers have an especially good reputation in this respect.

Finally, engineers should think about the plant's operability. This means considering the conditions under which a heat exchanger must perform. Engineers also need to be aware of the changes that impact throughput and the changes that occur in the plant's climate. Reducing throughput can alter the heat exchanger's flow regime and cutting it to half can force the flow regime into transition and reduce heat exchanger performance. Engineers should look beyond designs that can only handle a narrow spectrum of throughput conditions. Instead they should envision designs that ensure steady heat exchanger performance across many possible conditions. This could mean incorporating tube inserts, twisted tubes or plate-and-frame exchangers. Climate conditions are also important in the plant setting. Colder climates generally result in greater liquid viscosity than warmer climates. If a plant's climate creates a wide span of viscosity levels, the engineers should select a heat exchanger type suited to handle all of them. Shell-and-tube type heat exchangers are generally considered a bad match for viscosity levels greater than 5 cps. Plate-and-frame or spiral exchangers have better track records.

Whatever type of heat exchanger the engineers decide on, the time for them to think about it is early in the plant's design. Putting the decision off until later narrows the choices they have and could leave them stuck with less than the best option.

Source: Cast Heat Exchangers in a Leading Role
Graham T. Polley
Chemical Engineering, March 2002
http://www.che.com/mag/cecover.htm