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Oct-2015

Enhanced heat transfer for improved throughput and energy consumption

Customised revamps of heat exchangers optimise energy use and reduce 
costs in revamps.

TOM HIGLEY
Calgavin
Viewed : 1987
Article Summary
The recent crash in oil prices clearly demonstrated the element of volatility in what is otherwise considered to be a relatively stable market. The entire spectrum of companies across the sector has felt the squeeze that lower oil prices have put on production.

More specifically, operators and end users of oil refineries and petrochemical plants have reduced investment and in many cases even halted projects. These decisions have moved the focus and driving force of future projects, with an added emphasis on profitable 
and quick return on capital expenditure.

Furthermore, upgrades or developments of processes on existing facilities have had capital funding of such projects significantly reduced. Taking this into account, revamps of plant equipment need to be novel and within the return on investment (ROI) limit for a plant to implement any action.

Another factor to take into consideration is the underlying pressure from the industry to improve upon these existing plants to expand their processing capacity, whilst keeping energy consumption to a minimum. The ideal revamp to invest in is where the process is changed to improve efficiency, that is the energy that is consumed enables a higher increase in throughput per unit of energy. Although there are challenges with this type of revamp, it is achievable at a reduced capital cost compared to traditional means.

At Calgavin, with our knowledge of the heat transfer community and advances in research in process heat transfer engineering, we understand that there are novel technologies that can achieve efficient revamps, at a significantly lower investment to the alternative. Mainly, the traditional alternative is upgrading existing units or heat exchangers with larger pieces of plant, to handle greater throughputs and associated duties. This also includes changing ancillaries, implementing new feedback loops and cutting and changing pipework included in the commissioning process. In addition to these initial costs, the operating costs and energy consumed for these larger exchangers will increase, as the utilities needed for the larger duties follow the same trend.

Improving tube side performance
The nature of turbulent flow makes it the most effective flow regime for heat transfer. However, even in turbulent flow, transferring heat to or from a fluid flowing through a tube has limitations. The problem lies at the tube wall, where friction and viscous forces within the fluid combine to produce a thin layer of fluid that is scarcely moving. The mechanism that carries heat through this ‘boundary layer’ is conduction: a process that takes place slowly for fluids with low thermal conductivity, such as oil.

An effective way to improve tube side performance is to use hiTRAN thermal systems, which solve the problem of poor heat transfer by increasing the velocity close to the wall, thus eliminating the troublesome boundary layer. As Figure 1 shows, under laminar flow conditions streams of blue and red dye cling to the wall of an empty tube (A), but quickly mix into the bulk fluid as soon as they encounter the thermal system (B), which results in improved tube side heat transfer.

On both vertical and horizontal exchangers, including U tubes, these turbulators can be installed in situ without removing the tube bundle, or otherwise in the workshop during fabrication or maintenance.

Revamps with a difference
Calgavin has had experience with a variety of projects, working on oil refineries where revamping of existing heat exchangers has occurred without any drastic changes made to the unit. Taking this into consideration, almost immediately the forecasted capital costs are starting at a significantly lower level compared to the usual complex physical changes made.

These projects span from different areas within the plant, ranging from the CDU to further downstream units on specific streams. An example of technology that Calgavin has used as a heat transfer enhancement solution is the hiTRAN thermal systems previously mentioned. The following examples of revamps reflect the variety of scenarios that plants face and try to overcome.

Vacuum distillation unit: revamping heat exchangers
One particular revamp we encountered was within a vacuum distillation unit. The exchanger, a four-pass shell and tube heat exchanger, was used to preheat crude on the shell side by utilising heat of the VDU short residue from the tube side. After the unit was designed, the type of crude being processed was changed, which resulted in a reduction in the amount of short residue produced.

The fluid was moderately viscous and, with lower flow rates, resulted in the tube side operating at a Reynolds number in the range 700-1700 as compared to the design value of around 10000. This changed the flow regime from turbulent to laminar, resulting in a drastic reduction in the tube side heat transfer coefficient to approximately 7% of the design value. As the crude flow was not reduced, it resulted in the exchanger operating at a reduced heat duty or increased mean temperature difference. This limitation on performance naturally had an effect on overall process performance and efficiency.

The exchanger suffered significant fouling, which resulted in shutdown for cleaning every 5-6 months. It was suspected that the low velocities and, as a result, lower shear stresses at the tube wall may have caused deposition of fouling materials. However, with such low tube side heat transfer coefficients it was expected that the overall heat transfer coefficient would be very low such that the effect of fouling on the overall coefficient is relatively small.

It was clear that the low heat transfer rates were limiting the exchanger performance and this was further affected by increased fouling.

Table 1 demonstrates the enhancement available with hiTRAN, with a 10-fold increase in tube side heat transfer coefficient and 2.5 times increase in heat duty for the same flow rate. The insertion of thermal systems required modification of the exchanger to have two passes. This increase in duty for the same flow rate enabled the opportunity and benefit of increased throughput through this exchanger for the same energy consumption. In conclusion, given the operating constraints, the implementation of hiTRAN offered a significant opportunity for improving exchanger performance whilst possibly offering lower rates of fouling with the potential of longer cleaning cycles.

Turbine unit revamping
A company based in New Zealand approached Calgavin to investigate the possible use of hiTRAN on their air cooler in a bid to maximise the gas turbine’s performance. The air cooler, which was used to cool lubricant oil leaving the turbine, had not performed up to expectations since installation and had been the subject of many investigations. The rise in ambient air temperatures during summer necessitated reduction of the turbine speed in order to maintain the oil temperature below the alarm limit. This required a large reduction in turbine output to effect any significant change in lube oil heat-load, so production and energy losses were quite large.
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