Almost 60 people attended the 14^th HEXAG meeting, which returned to Birmingham University after a break of about 4 years at the invitation of Reg Bott. There were two principal themes in the programme, ‘fouling minimisation’ and ‘improved heat exchangers and performance enhancement’.

After a welcome by Peter Fryer, Head of the School, and of course well known for his own work in fouling, and the update on HEXAG activities by David Reay, Reg Bott described the work done at Birmingham on the use of polymer fibres (‘Microfloss’) to minimise biological fouling in cooling water systems. Interestingly, this work, involving partners Applied Coolant Technologies, Birmingham University and ETSU, arose out of seeds sown by an impromptu presentation at the HEXAG meeting at BP, Sunbury in late 1998. Previous studies showed that the fibres could control inorganic fouling, and this new work, carried out at Birmingham on a pilot scale plant simulates cooling water systems with a single species pseudomonas fluorescens as the contaminating bacterium, fed by nutrients. The bio-accumulation was measured using infra-red absorbance.

Control experiments were done without fibres present, and then the fibres, 250 microns long x 12 microns diameter, were added. Concentration of fibres was 100 and 200 ppm. Generally similar results were obtained regardless of fibre concentration, with 100 ppm being adequate when the water velocity was >1 m/s. The effectiveness is a function of water velocity, with complete control occurring at a velocity of 1.6 m/s. It was also found that the quality of the surface was important.

Some sloughing was noted at 1 m/s and 0.6 m/s velocities, with 100 ppm of fibres, but there was virtually zero accumulation at 1.6 m/s. With 200 ppm, a similar trend was noticed. At the highest velocity, after 15 days of running the fibres were removed and the biofilm started to grow.

In future, work will examine the effect of fibre concentration and velocities in more detail, and also study the effect on other species. Reg believed that the method could be effective against scaling. With regard to polymer surfaces, Reg pointed out that these tended to be less hospitable to biofouling than metals. Cooling towers were another area, but one can have organisms left over, in spite f continuous feed and bleed. An interesting point made was that these were adiabatic tests, and the effect of heat transfer on behaviour was queried. Optimum growth occurred at 30-40^oC, so heat transfer where this temp. range occurred might give potential for film growth, but it was felt that the fibre method could cope. Contact Reg Bott on tel: 0121 414 5280; or Applied Coolant Technologies on fax: 01527 547941, Email: act@barclays.net

David McDowell of NEL then discussed the NEL fouling monitor and the interpretation of data, leading to fouling mitigation strategies. This is a research project which David is doing for his Ph.D. degree. The fouling monitor takes a side stream of the fluid, to which a heat load is applied to simulate the internal conditions of the heat exchanger – so the same conditions are seen by the monitor as those received by the main exchanger. The fouling resistance Rf is measured to 10^-5 m²/kW.

By using the Rf data, one can evaluate fouling mitigation strategies – for operations this can help identify the problem and allow real time analysis, while for design one can select a more representative Rf and understand the effect of the choice of this parameter. David showed examples, including a power plant condenser where the analysis of Rf data had allowed identification of the factors affecting deposition fouling, and the direction of a more detailed study. An open evaporative cooling tower with biofouling was also studied. The Rf measurements helped to identify features which correlated strongly with the growth of fouling, and changes in nutrient effects were shown to be the dominant mechanism, helping the operators to understand and thus overcome the problem.

David stressed the need to improve the usefulness of Rf or find a more meaningful term – he suggested ‘economic cleaning factor’. Future work he world be doing will include further improving of the understanding of the economics of fouling, and more analysis of real time data from the monitor. Contact David on tel: 013552 272536.

The third ‘fouling’ contribution was from Hans Zettler of Surrey University, updating us on their activities on low fouling alloys/coatings on heat exchangers. He started by reporting on calcium sulphate fouling, where a BASF pilot plant with bio. and particulate fouling was taking place, and at the University, where crystallisation fouling was being studied, based on evaluating low fouling surfaces and new hx plate designs, starting with Alfa Laval M3 s/s plates. Treatments include ion implantation magnetron sputtering, vacuum arc sputtering, composite coating and carbon-nitriding. An important aspect is determination of the appropriate surface energies, and roughness seemed to have little effect on this parameter.

Reporting some of the results, Hans said that ion implantation with Si and F had led to Rf’s less than 50% of those of untreated surfaces, and amorphous carbon sputtering had reduced CaSO₄ fouling, as had carbon nitriding. With a Ni-P-PTFE coating, it was hypothesised that the NI present may increase the CaSO₄ solubility. Electropolishing reduced the scale formation, suggesting a surface roughness effect in this case.

Corrosion tests using BP equipment were carried out, showing that several coatings met the target of <0.1 mm/annum attrition. Future studies will include looking at possible applications, in-tube treatment possibilities, costs, durability and studies of milk fouling and wax deposition. Contact Hans Zettler on fax: 01483 876581, Email: H.Zettler@surrey.ac.uk

Stephen Morris then described his patented vortex flow inducer design, which has been evaluated on a power plant condenser. The challenge was to design a device which was compatible with existing tube cleaning methods, and which provided passive augmentation. Current methods, such as helical tube inserts, are not compatible with internal tube cleaning and can be costly to install. The vortex flow inducer is located upstream of the heat exchanger tube, and can be used in conjunction with sponge ball cleaning methods.

Stephen reported on tests on tubes on a 500 MW power plant condenser, with 20,000 tubes, steam condensing on the shell-side. Inlet and outlet temperatures were measured on two adjacent tubes, with flows of 16 m³/s of water. A ‘U’ increase of >22% was calculated for the tube with the vortex flow inducer. Integrated over the plant, this would be worth 0.5% on heat rate or 0.2% on efficiency, valued at £1.3 million p.a. at the station.

Further work is needed to measure pressure differentials, the degree of forced vortex generated and the length it extends along the tube, and the effect on Nu for a range of fluids. Contact Stephen on tel: 01777 706952.

Another form of vortex generator was described by Martin Fisher of Warwick University. These were used on gas-side fins, as delta winglets punched out of the fin surface. The work is part of a European Union project and Martin’s input involved using liquid crystal methods for examining the winglet behaviour experimentally, numerical modelling via the finite volume technique and validation of the CFD predictions. Good agreement was achieved between experiment and CFD when the isotherms on the fins were examined. Flow conditions were: Re range: 65 – 653 (characteristic dimension is the channel height, with the velocity being that at inlet); velocity: 0.25 – 2.5 m/s; vortex generator angle of attack: 15, 45 & 60^o.

Tests were made in plain channels and heat exchanger sections. In the former case, using a vortex generator gives local boundary layer thinning, with the delta winglet pair giving the best performance (average heat transfer coefficient (h) increased by 30%, pressure drop (D p) increased by 7%). For in-line tubes in a finned heat exchanger section, h rose by 7.1% and D p by 2.7%. With staggered tubes, and studying performance over a range of attack angles from 15^o to 60^o, at the higher angles it was noted that the D p reduced to be less than that of a plain channel, while h increased. It was hypothesised that this might be due to reduced separation, giving a lower wake area behind the first tube row.

In discussion, it was suggested that there might be cross flow from the other side of the fin. Although this could not be tested in the rig as a glass fin was used, it would have the effect of reducing the vortex strength. One could use an embossed vortex generator in a gas-gas unit, where mixing of streams did not matter. Contact Martin via Dr. Bob Critoph at Warwick, tel: 01203 523137; email: esrec@eng.warwick.ac.uk

As well as exhibiting an interesting selection of heat exchangers involving extended surfaces both inside and outside tubes, Dave Ellis of Britannia Heat Transfer described Britannia’s finning processes and the new, more compact, fin-fan coolers using the ELFIN block developed by the company. The key is the use of a finning machine which employs linear motor drives. The interlock/indexing of the motor guarantees the fin pitch to 10 microns. Units can be made in any appropriate metals, and no thinning occurs at the fin tips. The resulting block is extremely strong.

Following the report by Martin above, Dave said that the ELFIN concept provided a platform for introducing vortex generators.

He said that 45^o winglets on the inlet between tubes had given, over a 1-7 m/s velocity range, and increase in overall heat transfer of about 25%, with a 14% rise in D p. This allowed the option of easing the velocity to balance the D p, while retaining some enhancement. An overall 25% performance gain can result, allowing one to reduced the number of rows from 5 to 4 on the same plot, or to ease out the pitch and thus alleviate fouling, (interestingly, the winglets do not change the propensity for fouling to any great extent). The flexibility with regard to plot size is interesting for size-sensitive off-shore installations.

With regard to the future, NEL is looking at characterising the matrix; micro-bore units are being examined for aircraft or automotive supercharger cooling, and coated aluminium products are being studied – these are not damaged during the special fin forming process. A high temperature unit in stainless steel with Ni brazing is being studied for high temperature heat recovery units. Contact Dave Ellis, tel: 01675 466060, fax: 01675 467675.

Roshan Jachuck of Newcastle University then updated us on the polymer film compact heat exchanger (PFCHE), for which a patent has been granted. The current study is being supported under the Government’s Energy Efficiency Best Practice Programme, via ETSU. Roshan mentioned that the polymer currently being studied at Newcastle was PEEK. In aircraft applications, such as the heat exchangers within the cabin conditioning system, light weight was of importance, hence the participation of Serck Aviation. Here a 69% weight saving using polymer for this heat exchanger, with a duty of 39 kW, would be achievable. Other partners were Serck Heat Transfer and Victrex, makers of the polymer film.

Roshan explained how the superior heat transfer was obtained, with the 2 mm spaced corrugations destroying and recreating the boundary layer. Hydraulic diameter is 1.2 mm, and film thickness of the polymer 70 microns. Thermal conductivity was shown not to be limiting. As well as the cross flow plate unit, Roshan described the spiral gas-liquid variant. Contact Roshan Jachuck on tel: 0191 222 5202, email: r.j.j.jachuck@ncl.ac.uk

Song Lin of Heriot-Watt University gave us the most recent data from his work on boiling in confined spaces, such as narrow tubes. He is developing ‘boiling maps’ and correlations, the data being useful in intensified processes and compact heat exchangers. In the tube experiments, the tube i.d. is 1.1 mm, and he is using R141b fluid. Heat fluxes are in the range 18-72 kW/m², and heat transfer coefficients 1-6 kW/m²K. Experiments were also carried out in a 2.8 mm diameter tube, and revealed a transition from nucleate to convective boiling as vapour quality increases. The influence of diameter was examined, and with regard to correlations, Song said that all large tube correlations overpredict the performance of small tubes. The correlation of Lazarek & Black was the best (horizontal tubes with varying vapour quality), but still gave a higher heat transfer coefficient. New correlations are needed, as well as studies of the mechanisms involved.

As points of information, Terry Lavin mentioned work at Southampton (Inst. Cryogenics) on 0.2 mm dia. tubes, and Barry Azzopardi cited 2 mm tube work at Nottingham Univ. Contact Song Lin at Heriot-Watt Univ. on fax: 0131 451 3129.

The impromptu presentations numbered three. David Reay introduced a proposed project on gas turbine reactors – using the GT to produce chemicals with HEX-reactors, as well as heat and power - which he hopes to submit to ETSU as a feasibility study. For information contact David Reay, email: DAReay@aol.com

Yuying Yan of Nottingham Trent Univ. reported on his research into modelling of the phase change problem in dimpled plate heat exchangers, and refered us to the Proceeding of the 1999 UK National Heat Transfer Conference (IMechE publication). He then discussed work proposed on a corrugated louvred fin geometry, and ideas for a moving grid generation to examine what occurs as a bubble rises and grows in a channel. Yuying is looking for industrial partners for validation of the work on compact heat exchangers. Contact Yuying Yan on email: yuying.yan@ntu.ac.uk

The final talk was by Martin Gough of Cal Gavin, who introduced us to his new premises at Alcester, near Stratford upon Avon. He offered space to SMEs etc. who would be interested in benefiting from working with other groups of SMEs in ‘The Mill’, sharing software, meeting facilities etc.. The organisations there have a process industry bias. Contact Martin Gough, tel: 01789 400401.

David Reay,

May 2000.

Minutes also available on the HEXAG web site.