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Put that in your pipe!

23 July 2017

Mike Griffin outlines some of the problems with conventional heat transfer design, and explains how technology is providing a solution that is both innovative and has the potential to offer a fast return on investment. 

Effective energy transfer is a prerequisite for the success of many industrial processes, but the failure of more conventional heat transfer technology in high-temperature hostile conditions is leading to unplanned downtime, increased maintenance costs and spiralling production costs for a number of businesses in the food and beverage sector. 

Dirt and corrosion can create thermal stresses and damage equipment; long lengths of tube can lending themselves to cold spots and temperature variations; and single tube failure can result in entire units needing to be replaced. Thermal and dew point corrosion, as well as fouling in some cases, is an unwanted set of obstacles for any plant engineer, while equipment which is notoriously difficult to clean also having knock-on effects for productivity and efficiency.

Accounts from engineers paint a bleak picture of regular corrosion and frequent failure. It is clear that a more reliable alternative is needed and the answer could well lie with heat pipe – a modern concept with characteristics capable of nullifying many of the concerns engineers will be familiar with.

A heat pipe is essentially a metal tube, sealed at both ends with a vacuum inside and filled with a small quantity of fluid. The part of the heat pipe that is immersed in the hot stream absorbs heat, causing the liquid inside the pipe to evaporate. The resulting steam then travels to the top of the heat pipe and when it reaches the top it heats up the cold stream, causing it to condense back into a liquid. This liquid then flows back to the bottom of the heat pipe and the cycle will continue as long as there is a temperature difference between the hot and cold streams.

With a well-constructed heat pipe, with a high integrity vacuum, heat transfer between the hot stream and the cold stream will be virtually instantaneous, making it suitable for process conditions where efficiency is an important factor. In particular a heat pipe would be well-suited for energy recovery applications from high temperature, corrosive or contaminated exhaust streams.

Many food and beverage application demands can be met from a modularised standard range of heat pipe heat exchangers, where 'cartridges' of heat pipes are installed into a standard heat exchanger casing that is suitable for its application. 

The problems associated with conventional heat transfer designs can be alleviated by using heat pipe technology. Advantages include:

• Energy and carbon savings: The single greatest advantage of heat pipe heat exchangers is their ability to contribute to energy and carbon reduction measures in line with a site’s decarbonisation strategy. Heat pipe heat exchangers generate hot water or steam from existing waste streams, thereby reducing utility supply consumption and costs by around 3-7% and carbon by around 4%.

• Multiple redundancy: Each pipe operates independently which ensures that the unit is not vulnerable to a single pipe failure. Within the heat exchanger each heat pipe is an individual heat exchange unit designed to operate in aggressive industrial environments. However, even if one or two heat pipes fail, this will have a very small effect on the overall performance of the heat exchanger. For example, with a heat exchanger composed of 100 heat pipes, failure of a single pipe will still result in 99% effectiveness of the entire system.

• Low fouling and ease of maintenance: The use of smooth pipes allows heat pipe heat exchangers to be used in high particulate or oily applications. These are commonly found in food processing applications and can have a severely detrimental effect on more conventional heat exchangers. This low fouling capability ensures that heat pipe heat exchangers can be easily maintained in situ with no requirement to uninstall. 

• Isothermal operation – no hot or cold spots: Heat pipe heat exchangers eliminate the cold corners that are common amongst traditional heat exchangers designed with a complex multi-tubular structure. Consequently, this also eradicates dew point corrosion issues and improves overall thermal performance of the heat exchanger.

• Robust materials and long life: Heat pipes do not rely upon thin metal surfaces for effective heat transfer and therefore can be constructed from robust materials that offer increased resistance to corrosion. What’s more, heat exchangers designed with heat pipe enable the pipes to freely expand and contract within the heat exchanger casing, which eliminates thermal stress on the structure.

• Low pressure drop: Compared to other heat recovery solutions currently available, heat pipe-based solutions have extremely low pressure drops. This is particularly significant in situations in which size or weight are a consideration, as this low pressure drop is achieved with systems that are also much smaller and lighter than traditional systems.

Conventional heat transfer equipment has been a source of frustration for many engineers in the food industry, with many choosing to shut it down or bypass it, rather than waste time trying to get it to work efficiently. Fortunately, there is now a solution. 

Energy recovery from industrial exhaust gases also has the potential to unlock large amounts of energy savings and reduce CO2 and carbon emissions. However, because traditional heat transfer equipment is so prone to failure when faced with high temperatures, corrosive or contaminated exhaust streams this opportunity has, largely, gone unexploited. Because heat pipes are better suited to use in hostile environments, they also offer a much better opportunity to recover and reuse waste process energy.

To explain more about how modern technology is making heat transfer equipment work more effectively, Spirax Sarco has published a new whitepaper ‘Corrosion, cracking and downtime: The failure of heat transfer in industrial applications’ which can be downloaded from bit.ly/2slrt35

Mike Griffin is emerging & innovative technologies manager Northern Europe at Spirax Sarco.


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