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Heat exchanger considerations

29 July 2017

Determining when laminar flow becomes turbulent flow is critical to equipment performance.
Determining when laminar flow becomes turbulent flow is critical to equipment performance.

Arnold Kleijn offers advice on selecting the right heat exchanger for food processing applications. 

Thermal processing is an essential part of food production. Whether cooking; pasteurising or sterilising; or heating or cooling products, most food production lines will use a heat exchanger to carry out thermal processing. Yet, with such a variety of applications possible, it is important to select the right heat exchanger. 

There are a number of types on the market – plate, tubular, corrugated tube, scraped surface, etc. Each is suited to a particular application, so think carefully about the process, including the nature of the materials to be heated or cooled, the objective of the process (such as cooking or pasteurisation) and any restrictions of the environment where the heat exchanger is to be used. 

The driving force for heat transfer is the difference in temperature between the two substances (often, but not always fluids). In the case of a smooth tubular heat exchanger the temperature of two simple fluids, such as water and milk changes as they pass through the heat exchanger. One of the reasons for making corrugated tube and scraped surface heat exchangers is that they are suitable for fluids and materials with complex properties, such as viscous and non-Newtonian fluids like custards and sauces, or for materials containing particles or sediment such as fruit purees. 

The next step is to ensure that the model supplied is correctly sized for the job. The heat exchanger must have a large enough heat transfer area for the specified fluids and their specified inlet and outlet temperatures. Most calculations should also factor in variables such as whether the heat exchanger operates using counter-flow or parallel flow.

Breaking down barriers
Another important factor controlling heat transfer is the resistance to heat flow through the various ‘layers’ that form a barrier between the two fluids. There are effectively five layers: 

1. The inside ‘boundary layer’ formed by the fluid flowing in close contact with the inside surface of the tube.
2. The fouling layer formed by deposition of solids or semi-solids on the inside surface of the tube (which may or may not be present).
3. The thickness of the tube wall and the material used, which will govern the resistance to heat flow though the tube itself.
4. The fouling layer formed by deposition of solids or semi-solids on the outside surface of the tube (which may or may not be present).
5. The outside ‘boundary layer’ formed by the fluid flowing in close contact with the outside surface of the tube.

The values for numbers 2 and 4 can usually be supplied by the client based on experience, while the designer of the heat exchanger will select the tube size, thickness and materials to suit the application. The resistance to heat flow resulting from numbers 1 and 5 (known as the partial heat transfer coefficients) depends both on the nature of the fluids and the geometry of the heat transfer surfaces themselves.

One way to prevent the build-up of these layers is to increase the speed at which the fluid passes through the heat exchanger so that turbulence is formed and the boundary layer breaks away from the surface of the tube. This is the point at which laminar flow (with the fluid passing through in smooth layers, where the innermost layer flows at a higher rate than the outermost) becomes turbulent flow (where fluid does not flow in smooth layers but is mixed or agitated as it flows).

The speed at which this occurs is influenced by many different factors, but to quantify it for specifying heat exchangers, the Reynolds number is used. This is determined by the diameter of the tube, the mass velocity of the fluid and its viscosity. Reynolds numbers of less than 2,100 describe laminar flows, while numbers above 10,000 describe full turbulent flows. Between the two values is an area of uncertainty called the transitional zone, where there is a general transition from full laminar to full turbulent flow. In practice, engineers try to provide solutions outside of this zone. Tube deformation such as corrugation helps to increase the heat transfer performance once entered the turbulent flow area (Reynold > 2100). 

It is important that increasing turbulence in the flow of the material does not create issues with the product itself. For example, with pizza sauce, excessive turbulence can make the sauce sheer, leaving it runny and unsuitable for use. In such applications, corrugated tube heat exchangers are unsuitable and scraped-surface models can be applied, optimising the scraper speeds to optimise on shearing behaviour. In turn, this requires a different set of calculations to be performed although the thermal and physical properties of the product remains the same.

Much of the literature used to build calculations and model heat exchanger performance does not reflect the most recent science and there is very little published data on scraped surface heat exchangers. HRS has developed a software programme to calculate the size of its heat exchangers and this is producing some interesting results and is providing insights into how best to design tubular and scraped surface heat exchangers.

Arnold Kleijn is product development manager at HRS Heat Exchangers.

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