Step change for heat exchangers

With the oil price still bumping along at a subterranean low, some argue that there is less pressure to invest in energy-saving equipment today than there has been for some time.

However, cheaper energy does not mean free energy, and the continuing drive for lower bills and improved environmental performance, together with global growth in process industries such as nuclear and renewables mean that the market for heat exchangers remains buoyant, according to Global Industry Analysts (GIA).

The firm’s 2015 report on heat exchangers suggests that the market will reach US$24.3 billion (£16 billion) by 2020.

“PHEs are much more efficient and can reach - depending on type and application - up to five or six times higher overall heat transfer coefficients, reducing for that reason surface area and costs."

And, while Asia Pacific will experience the fastest increase at 8.9% compound annual growth, Europe remains the biggest market overall.

One of the key trends that GIA identifies within this picture is a continuing shift from shell-and-tube exchangers, which are the traditional workhorse of the process industries, towards plate heat exchangers (PHEs).

“Currently, welded PHEs are cannibalising market opportunities in the shell-and-tube heat exchanger market, given their potential to enhance heat recovery by over 55% as compared to the latter,” GIA told Process Engineering.

“Factors driving the adoption of PHEs as an alternative to shell-and-tube include large and effective heat transfer surface, compact size and small form factor and superior thermal efficiency in comparison with shell-and-tube.”

Falk Mohasseb, director of research and development with Kelvion (formerly GEA Heat Exchangers), agrees, saying that the transition began in the relatively mild process conditions of food and beverage and is now progressing to more demanding applications thanks to brazed and welded versions of PHEs.

These eliminate the need for gaskets between the plates, which otherwise limit the temperature and pressure ranges of regular PHEs.

“The replacement of shell-and-tube technology started mid of the last century, first within the dairy and beverage industry where PHEs were used for the heat treatment of dairy products like milk, cream and yoghurt or for the beer processing like wort cooling and heating,” says Mohasseb.

“Due to [this] success, other industries started to replace shell-and-tube heat exchangers by PHEs. With the addition of brazed PHEs and laser-welded cassette PHEs, refrigeration applications followed this trend in the nineties.

“Currently this trend is ongoing within the chemical industry, as well as within the oil and gas industry, mainly by fully-welded PHEs.”

Mohasseb believes that this change is primarily performance-driven: “PHEs are much more efficient and can reach - depending on type and application - up to five or six times higher overall heat transfer coefficients, reducing for that reason surface area and costs. The footprint is smaller and filling volume is less as well. Further, due to the higher efficiency, PHEs can reach closer temperature approaches, which will improve the overall process efficiency and hence PHEs help in reducing primary energy consumption.”

On the other hand, shell-and-tube technology offers greater flexibility in more demanding temperatures and pressures, as well as resulting in a lower pressure drop – and thus pumping costs.

The narrow gap between the plates of a PHE also means that they’re not suitable when handling fluids with a high solids content, which excludes them from some of today’s key growth areas, such as certain renewable energy applications.

In fact, Stephen Wooler, senior technical design engineer with HRS, says that the limitations of the more-common gasketed PHEs mean that plates are gaining ground only in certain applications.

“Gaskets have limited pressure and temperature, typically 10-16 bar and around 160oC (they can go higher, but the cost increases greatly),” he says.

Gasketed PHEs also have cost implications going forward, since the gaskets have a limited life of between six months and two years, depending on the duty.

“A spare set of gaskets can cost up to half the price of a new heat exchanger, so the long term costs are high,” adds Wooler.

While he concedes that PHEs offer advantages in terms of space savings, energy efficiency and initial cost, he believes that the skills shortage and the resulting trend towards outsourcing engineering functions to contractors is another big driver in the growing popularity of gasketed PHEs.

“For example, if you need to generate hot water from steam on site, an installer will use a PHE. It’s cheaper and has a smaller footprint [than a shell-and-tube unit], so it’s easier to win contracts with lower Capex costs.

“However, the end user is left with the spares costs for the lifetime of the unit. If the same duty is being bought by an end user who is aware of the difference, they will normally use a tubular as they believe that Capex is less important; Opex is king.”

Mohasseb agrees that ongoing maintenance and ease of use are significant considerations.

“Comparing the plain heat exchange abilities, a shell-and-tube heat exchanger shows some drawbacks compared to a PHE, but other aspects like service ability, cleaning, adaptation to various flow conditions and fluids for each side individually compensate the disadvantages.”

There are also techniques that suppliers can use to try and plant equipment improve the heat transfer properties of shell-and-tube units.

For instance, HRS, which manufactures and supplies both shell-andtube and PHE units, always uses corrugated tubes in its shell-andtube exchangers.

“Corrugated tubes add extra turbulence to the fluid and break the boundary layer at the tube wall. This results in better mixing and an improved heat transfer,” says Wooler.

“Typically we can reduce the surface area required by between 20-40% compared to smooth tube applications. However the corrugated tubulars do not have as good heat transfer as plates.”

Similarly, Chemineer combines its Kenics static mixer with its shell-and-tube exchangers to achieve a thoroughly mixed, plug flow regime.

Kenics’ global business development manager Neil Cathie explains that the resulting turbulence offers additional advantages because it ensures a consistent residence time.

Fluids do not run the risk of being held up in ‘dead zones’, which can be an issue in PHEs operating in transitional, laminar and creeping flow regimes.

“[Kenics units] offer plug flow characteristics, which means equal residence times and equal heating/cooling for all parts of the fluids. This is particularly important when handling reactive or degradable fluids such as polymers, pharma or food products, when PHEs could burn or freeze the fluids if they get stuck in the dead zones for long periods,” he says.

“A properly selected, designed, installed and maintained heat exchanger can be – along its life-cycle - the most trouble-free component in a process plant or system.”