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Modular Radial Flow Gas-Gas Exchanger

T. WACKER Aker Solutions

Vancouver, BC, Canada

The current trend in sulphuric acid plant construction has been to progressively increase the plant capacity. Gone are the days when the upper limit was 3,000 t/d plant. We have seen the successful operation of a 4,400 t/d plant and a 5,000 t/d plant is due on line in the near future. Not to be outdone, a single-train 6,000 t/d plant is on the drawing board. This presents a tremendous challenge with regard to fabrication and shipping of individual pieces of equipment. One of the challenges is the gas-gas exchanger. The sheer size and weight of this equipment leads to problems, especially in remote plant locations where transportation and infrastructure are limited.

The traditional solution for oversized equipment has been to field-erect, but that can lead to schedule and reliability issues. Field work is slower than the equivalent work in the shop and often the quality of work is not as good as in a shop environment.

This paper presents a novel approach in meeting the shipping problem through modularization of the gas-gas heat exchangers. The radial symmetry of Aker Solutions gas-gas heat exchangers allows them to be easily divided into segments, which are sized to fit into standard shipping containers. The critical tube-to-tube sheet joints are done in the shop for optimum control of welding quality, thereby minimizing field work. The size of the segments makes them easy to handle and assemble at site, thereby often eliminating the need for an expensive heavy lift crane.

INTRODUCTION The current trend in sulphuric acid plant construction has been to progressively increase the plant capacity. As plant capacity increases, so does the size of the gas-gas exchangers. As these units approach 6m in diameter and 200 tonnes in weight, this presents a tremendous challenge with regard to fabrication and shipping. This paper presents a novel approach in meeting the shipping problem through modularization of the heat exchanger tube bundle. By dividing the tube bundle into segments, which are sized to fit into standard shipping containers, the shipping issue is solved. Furthermore, field fabrication is greatly simplified because the critical tube to tube sheet welds are done in the shop.

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RADIAL FLOW GAS-GAS EXCHANGER The radial flow gas-gas exchanger was developed by Aker Solutions in the late 1970s. A typical unit is shown in Fig. 1. For the purpose of this discussion, the two important features of this design are the radial tube pattern and the use of disc and donut baffles.

The disc and donut baffles ensure only radial flow of gas across the tube bundle while the patented radial tube pattern (Fig. 2) produces the same radial flow area and layout angle in all flow directions. By comparison, a similar arrangement using a conventional triangular pattern would produce flow areas and layout angles varying from triangular to rotated triangular depending on flow direction. Also, note that the tubes do not fill the entire interior of the exchanger, as in a conventional segmental baffled exchanger, rather, they are restricted to an annular area near the shell (no tubes in window).

FIG. 1: RADIAL-FLOW GAS-GAS EXCHANGER

FIG. 2: RADIAL TUBE PATTERN (PATENTED)

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Another important feature is the tube to tube sheet joint. Because of the high operating temperatures of these units, an expanded joint is prone to leakage. Aker Solutions employs a two-pass welded joint for superior joint integrity at high operating temperatures.

RECENT LARGE PROJECTS Increasing plant capacity has led to some very large gas-gas exchangers. Some recent examples delivered by Aker Solutions are:

Codelco (El Teniente) 6.1m diameter 11.4m tube length 110 tonnes Vale Inco (Sudbury) 5.1m diameter 15.2m tube length 126 tonnes (Fig. 3) BHP (Ravensthorpe) 6.0m diameter 14.5m tube length 133 tonnes WMC (Kalgoorlie) 5.8m diameter 18.3m tube length 198 tonnes (Fig. 4)

FIG. 3: GAS EXCHANGER EN ROUTE TO SITE

FIG. 4: GAS EXCHANGER LEAVING FABRICATION FACILITY (Note special support cradle and number of wheels)

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These units were all completely shop assembled and shipped in one piece. As can be appreciated, the sheer size and weight of these units coupled with the remote locations of some of the installations presented quite a challenge for transportation to site and installation. Figure 5 shows the WMC exchanger being lifted into place. The main crane is 600 tonne capacity, the largest available in Western Australia at the time.

THE CASE FOR MODULARIZATION In the past, there were two ways to get around the size limitation. First, one could field-erect the exchanger. This has the disadvantage of longer construction schedules as well as higher costs. Field labour is typically less efficient than shop labour. Also, welding quality can be an issue owing to the variable environment at a construction site vs. the controlled environment in a shop. The second approach would be to redesign the exchanger as two smaller units in parallel. This is clearly more expensive than a single large unit. There are also costs associated with additional ducting, duct supports, control dampers and equipment foundations. Plant layout may also be an issue where space is at a premium. The challenge, then, is to find an economic means to modularize the gas-gas exchangers so they can be shipped to site in standard containers without the need to perform the critical and time-consuming tube-to-tube sheet welds on site. The solution involved a bit of serendipity. The high degree of symmetry of the radial tube pattern has the advantages of uniform flow distribution, low pressure drop and high heat transfer coefficient. However, the high degree of uniformity also makes the units (especially larger ones) prone to acoustic resonance. This issue has been successfully resolved by adding acoustic baffles to the tube bundle (Fig. 6). The resulting segmental pattern is still radially symmetrical, but is divided into distinct equal segments. By simply widening the gap at the acoustic baffles, it is apparent that tube bundles utilizing this pattern can be easily split into segments (Fig. 7). The number of segments can be adjusted to allow individual pieces to fit in a standard container.

FIG. 5: 200t GAS EXCHANGER BEING LIFTED INTO POSITION

FIG. 6: RADIAL SEGMENTAL TUBE PATTERN WITH ACOUSTIC BAFFLES

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TWO SUCCESS STORIES BALKHASH AND SOUTHERN PERU These are two very large (3,850 t/d for Balkhash and 3,740 t/d for SPCC) metallurgical acid plants. Both are in very remote locations with limited transport infrastructure. Modularization was clearly the only practical solution. Figures 8 and 9 show completed modules being loaded into containers. The modules were manufactured at the Aker Solutions manufacturing facility in Toronto, Canada.

FIG. 7: MODULAR TUBE BUNDLE

FIG. 8: MODULES BEING LOADED

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FIG. 9: MODULES READY FOR TRANSPORT

Balkhash is located in central Kazakhstan. Rail is the only practical means of transport. This limits the shipping envelope to 3.5m x 3.5m. The largest gas-gas exchanger is 5.4m in diameter and weighs 210 tonnes fully assembled. SPCC is located on the Pacific coast of Peru, close to the Chilean border. Access to site is by road or rail. Figures 10 and 11 show tube bundle segments being assembled. The top and bottom vestibules of these exchangers were shop fabricated in Lima and shipped in one piece to site.

FIG. 10: TUBE BUNDLE SEGMENT BEING LIFTED INTO POSITION

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FIG. 11: TUBE BUNDLE SEGMENT BEING LIFTED INTO POSITION

CONCLUSIONS The radial symmetry of the Aker Solutions gas-gas exchanger makes modularization of the tube bundle relatively straight forward. This option offers owners all the advantages of radial-flow gas-gas exchangers coupled with the opportunity to reduce the total freight and construction costs. Exchanger reliability is maintained by performing the critical tube-to-tube sheet welds in the shop. The modular radial flow exchanger is particularly well suited to remote plant locations. Of course, this design can also be considered for retrofits in order to reduce cost and schedule impact.