Could the answer to a whole host of heat exchanger problems be found in the development of turbulator technology, asks Michelle Knott.
Shell-and-tube heat exchangers are among the most common types of equipment across almost all process industries. In spite of their everyday familiarity, heat exchangers can generate a range of performance-related challenges.
Perhaps you’re looking to install a new heat exchanger and need one that’s as compact as possible? Or maybe you want to solve a performance-sapping problem such as sediment, scale or reaction products building up on the walls of your existing heat exchanger? Or maybe your priority is to extend the run times between shutdowns? In any of these situations, tube inserts (also known as hiTRAN Thermal Systems technology) could provide an answer.
It’s the ‘big beasts’ in sectors such as oil and gas, refining, petrochemicals and power that can typically enjoy the biggest savings when deploying tube inserts, although more modest benefits can still be achieved in lighter industries.
“Mainly the hiTRAN [turbulator] technology is applied to the oil and petrochem sector but we have applied it to the food, power, automotive and pharmaceutical industries over the years,” says Tom Higley, technical sales manager with Calgavin, which offers a choice of solutions, including its hiTRAN Thermal Systems technology.
Twisted tape is a common type of turbulator, but today there are various, arguably more advanced alternatives that aim to enhance heat transfer still further. Which solution is appropriate for a given application depends in part on the flow regime operating inside the tubes.
Mainly the hiTRAN [turbulator] technology is applied to the oil and petrochem sector but we have applied it to the food, power, automotive and pharmaceutical industries over the years
Tom Higley, technical sales manager, Calgavin
All turbulators work by inducing shear stress and, in turn, turbulence in fluid passing through the heat exchanger tubes. The flow regime is described by the Reynolds number, which takes into account the characteristics of the fluid itself and the channel through which it flows.
Flow in a system with a low Reynolds number is laminar; the fluid flows smoothly and its velocity falls away from top speed in the centre of the tube to zero in the boundary layer at the tube wall.
As the Reynolds number rises, the flow becomes transitional until it eventually becomes fully turbulent, with good mixing across the diameter of the tube and a disrupted boundary layer.
So, for example, twisted tapes primarily work by inducing swirl flow in the tube-side process fluid. This results in higher near-wall velocities and better mixing, thereby giving the heat transfer coefficient a boost.
Choices, choices
According to Calgavin, a reasonable flow velocity is required in order to induce effective swirl flow, which means that twisted tapes are most effective in already turbulent flows with limited pressure drop. Under laminar flow conditions the improvements are limited.
Calgavin offers twisted tape, but explains that the hiTRAN wire-based alternatives are much more effective at enhancing heat transfer in an empty tube design operating at low Reynolds numbers (laminar to transitional flow).
Although the potential benefit is greatest in the laminar flow region (up to 16 times more heat transfer), benefits can be obtained in the transitional flow regime (up to 12 times) and turbulent flow regime (up to three times.)
The firm has installed hiTRAN Thermal Systems in heat exchangers operating at Reynolds numbers from 1 to 100,000-plus.
Of course, installing anything inside the heat exchanger tubes is going to increase the pressure drop across the unit, potentially offsetting any savings by increasing pumping costs.
As the technology is bespoke, the design not only takes into account galvanic corrosion aspects, but also the overall process conditions
However, such are the benefits on offer that Calgavin states that its hiTRAN Thermal Systems typically offer increased heat transfer at an equivalent or lower pressure drop compared to empty tubes through optimised geometries and modifications to heat exchanger tube passes.
Optimising the design of hiTRAN technology is anything but straightforward, cautions Higley, as the systems are tailored and bespoke for each application.
For instance, galvanic corrosion is a potential challenge in any multi-metal plant equipment handling polar liquids such as water.
However, Higley says that the company avoids this issue by manufacturing hiTRAN out of the same material as the tubes in most applications: “As the technology is bespoke, the design not only takes into account galvanic corrosion aspects, but also the overall process conditions such as flowrates, terminal operating temperatures, specified duties, available plant plot space and the physical geometry of existing units in the field, amongst many other aspects that contribute.”
As well as improving heat transfer by mixing the tube-side fluid and disrupting the boundary layer, tube inserts can also help prevent fouling, which otherwise erodes performance over the course of a long production run.
The very long run-times associated with heavy industries such as oil and gas, refining and petrochemicals is another reason why these industries find tube inserts especially appealing.
French company Petroval, partially owned by Total, sticks almost exclusively to oil and gas and refining, but can design inserts for applications on a case-by-case basis.
“[Our tube insert technology] can probably be used on many other types of applications but we haven’t tested it. When we look at the chemical industry it’s much more difficult to judge whether it will work or not, because some products can be corrosive and some may generate galvanic corrosion. We have done some chemical applications but they have required special care.
“Oil is way easier for us and there’s usually huge amounts of energy to recover on these big exchangers. We’re open to any type of evaluation,” says energy department manager Nicolas Aubin.
Petroval offers three types of insert: Fixotal [pictured above], Spirelf and Turbotal. Spirelf and Turbotal not only work by disrupting the tube-side flow to induce more mixing, but also use some of the energy of the fluid flowing through the tubes to create a scouring action that goes even further to mitigate fouling and extend production runs.
“These technologies have a brushing effect on the inside of the tube wall. The Turbotal is rotating and this rotation is generated by the fluid flow and reduces the fouling rates. The Spirelf is vibrating and these vibrations also have a cleaning effect,” explains Aubin.
Determining factors
Several factors determine which to choose for an application. Turbotal can only operate in liquids, so it can’t be used in reboilers or condensers that experience two-phase flow. It also operates across a very restricted range of flow velocities – typically between one and two metres per second – in order to harvest the necessary energy from the flow to induce the rotation.
These restrictions mean that Turbotal is used most commonly in crude oil pre-trains, whereas Spirelf can handle two-phase flows without a problem.
The very long run-times associated with heavy industries such as oil and gas, refining and petrochemicals is another reason why these industries find tube inserts especially appealing
These ‘active’ tube insert technologies can mitigate a range of fouling issues, according to Aubin: “They can handle coking, asphalt precipitation or polymerising on the tube walls. We cannot work on salt deposition because salts are very hard deposits and the kinetics [governing formation of salts] often very fast, so we have very few applications in water service.”
The most obvious advantage of the extra anti-fouling actions can be found in extended run times, with the Spirelf and the Turbotal promising to extend the time between maintenance sessions by anywhere between two and five times, depending on the application. Fixotal tube inserts don’t move. They’re made to be slightly wider than the tubes to remain in place.
“The flow is basically bouncing on the loops of the Fixotal so we can renew the boundary layer and this has a very significant effect on the wall temperature. We can increase the wall temperature and reduce the bulk temperature [of the tube-side fluid]. Turbulence also has some effect on the fouling, although not as much as the other two,” says Aubin.
He adds that it’s not unusual to double run time using Fixotal, but it’s unlikely to deliver more than that.
