Heat transfer fundamentals (3 of 5)

What are corrugated tubes?

When the overall heat transfer rate for a given heat exchanger is limited by the tubeside partial heat transfer coefficient (α inside) the overall surface area of the heat exchanger can usually be reduced if this coefficient is improved in some way. Many methods of artificially enhancing this coefficient have been tried, some successful other less so.

• Wire or strip inserts are sometimes used pushed into each tube to stir the boundary layer liquid away from the tube wall into the bulk of the fluid. This type has the disadvantage of a substantial increase in pressure loss per unit length and any particles or product pieces entrained in the fluid render them useless.
• Internal ribs or fins along the length of the tube which are designed to increase the internal surface area per unit length of tube. Even moderate viscosity fluids can bridge the gap between the fins to give a layer of cold static fluid which negates any benefits that may be present.
• Deformed tubes where the tube is flattened to reduce the effective hydraulic diameter and increase the coefficient accordingly. Once again any particles or product pieces entrained in the fluid render them inadvisable as blockage can easily occur.
• The HRS Heat Exchanger' type of shallow deformation which causes the boundary layer to be disrupted without too large a reduction in flow area. Providing the working diameters are chosen carefully this does not prevent the free passage of particles and pieces and does not allow even the most viscous fluids to bridge adjacent corrugations.

Why corrugated tubes were developed?

An increasing requirement for food products to be pasteurised for long term storage and general hygiene led to the realisation that the heat transfer characteristics of a large proportion of food products were poor. In addition to this they commonly contain pieces of fruit, vegetable or meat which must maintain their integrity during processing to keep the quality of the product at an acceptable level. Without enhancement the heat exchangers required for processing even modest quantities of some food products can be unrealistically large and expensive.

Because of these inherent operational requirements, methods of reducing size and cost for this type of heat exchanger led to the development of tubes which can be used with fluids containing large size particles but still increase the rate of heat transfer by disrupting the boundary layer to give values of heat transfer coefficient higher than would normally result from the flow conditions being used.

Experimentation with different styles and types of tube deformation led to a general form consisting of a shallow spiral deformation down the length of the tube which has been optimised by further experimentation by HRS Heat Exchangers into our "Hard" and "Soft" standard types. Whilst being deep enough to disrupt the boundary layer they are not deep enough to constitute a barrier for any solids content which could cause blockage.

Our two standard corrugations differ only in the angle which the spiral indentation takes down the tube, the "Hard" being a steeper angle to the longitudinal centre line of the tube.

Experimentation with high viscosity food products has led to further development as an intermittent indentation which has proved more effective at values of Reynolds number between 40 and 200 and is now used when high viscosity causes these low Re values. The so called "dimple" corrugation is shown in the image.