Is heat transfer technology failing British industry?
Published: 14 October, 2016
Mike Griffin, Emerging & Innovation Technology Manager Northern Europe at Spirax Sarco, outlines the problems with conventional heat transfer design, and explains how technology is providing a solution that is both innovative and has the potential to pay for itself in double-quick time.
Whether located in a waste processing incinerator, a cooking oven in a food processing plant, or a furnace in a metal processing application, heat transfer equipment is vital to overall performance. Yet British industry is facing mounting problems with the design of its conventional heat exchangers.
Dirt and corrosion are creating thermal stresses and damaging equipment, long lengths of tube are lending themselves to cold spots and temperature variations, and single tube failure is causing entire units 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 these engineers need a more reliable alternative, so where do they turn? 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.
The Solution
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.
As a result of the high quality construction of the heat pipe, in particular its high integrity vacuum, heat transfer between the hot stream and the cold stream is virtually instantaneous, making it ideal for process conditions where efficiency is an important factor. Specifically, heat pipe is exceptionally well-suited for energy recovery from high temperature, corrosive or contaminated exhaust streams.
Many 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’s suitable for its application.
The problems associated with conventional heat transfer designs can be alleviated by using heat pipe technology. The advantages of using a heat pipe heat exchanger are:
1. 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 circa 3-7% and carbon by circa 4%.
2. 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.
3. 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. Typically, these can have a severely detrimental effect on conventional heat exchangers, for example those found in waste incinerator applications or food processing. This low fouling capability ensures that heat pipe heat exchangers can be easily maintained in situ with no requirement to uninstall.
4. 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.
5. 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.
6. 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.
The truth of the matter is that up to now, British industry has been rightly frustrated with its conventional heat transfer equipment, often choosing to shut it down or bypass it, rather than waste any more time trying to get it to work efficiently. Fortunately, technology is providing a solution that is both innovative and has the potential to pay for itself in double-quick time.
For further information please visit: sxscom.uk/heatpipewp
