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Heat exchanger selection advice

04 March 2019

Matt Hale offers advice on choosing the best heat exchanger to meet the needs of your application. 

The first question to ask is whether the heat exchanger technology is actually the right solution for you. They are a good solution in many situations, but can prove to be relatively complex and costly. Heat exchangers work best where a heat (or cooling) source is already available, or where the heat source could be used for more than one purpose. However, in simple situations a straightforward heating element or simpler refrigeration system may offer a better solution.

The simplest forms of heat exchangers are plate heat exchangers, which consist of a number of plates separated by gaskets through which the product and the heating or cooling medium move. They are relatively simple and cost effective and can do a good job with simple fluids like milk and thin oils. However, for more viscous substances, non-Newtonian fluids, products containing particles and processes requiring high levels of heat transfer, tube-in-tube heat exchangers may be a better option.

Tubular heat exchangers come in different forms including corrugated tubes to increase product turbulence, which prevents fouling and improves operating efficiency. For high fouling and viscous fluids, scraped surface heat exchangers are also available in both reciprocating and rotary versions. These allow different products to be handled carefully, maintaining quality characteristics or increasing product mixing, while also being highly efficient. 

It is important that the heat exchanger is capable of providing the right amount of heat transfer. Different materials will have different thermal properties and this needs to be considered. Factors such as viscosity, solids content and texture will need to be assessed alongside product flow rates to ensure that the product receives the correct treatment. For example, if the heat exchanger does not deliver sufficient heat it may result in an incomplete process, which could have severe implications for product safety. 

If the system does not handle certain products correctly it can change or damage their quality; for example, rough handling of viscous sauces can have a negative effect on their texture.

Finally, the heat exchanger set-up should be capable of handling the maximum amount of product required at any time. While there will be physical constraints on the size of individual heat exchanger elements, in most cases it is possible to combine multiple units into suitable arrays in order to increase treatment capacity.

Key benefits
One of the key benefits of many heat exchangers is their ability to recover heat from the end of the process and re-use it. In many cases this enables the system to be more efficient, reducing the amount of heat which needs to be supplied in the first place. However, in some situations – for example, where the heat source is plentiful – then the recaptured heat can be used for another process or for something else altogether, such as heating offices or buildings.

Maintenance and servicing requirements will vary according to the type of heat exchanger chosen. More important than the actual maintenance requirements is how easy maintenance is. Consider how complicated it will be to access key components and look are the cost of routine spare parts such as seals and gaskets. 

For double and triple tube heat exchangers, it is useful to be able to remove individual tubes without dismantling the entire unit and to service parts of the unit without shutting down the whole process.

Cost considerations
Cost is usually one of the most key factors in making an investment decision. However, it is important to compare both the capital cost of different units and their anticipated operating costs and service life. For example, a 25% higher purchase price can easily be recouped by greater product efficiency and reduced servicing costs over the same, or even longer, operating life. Only by considering all the associated costs will you be able to make an accurate investment decision.
 
Matt Hale is international sales & marketing director at HRS Heat Exchangers.


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