24TH OCTOBER, 2000.


This HEXAG meeting featured talks from Keith Symonds and Paul Shields of Chart Heat Exchangers, whose factory is located close to the meeting venue, and 40 attended the meeting.

Announcements: David Reay welcomed members and gave information on the conference 'UK Heat Transfer 2001' on 11-12 September 2001 at Nottingham University. A 'call for papers' was available - see also www.heat-transfer.org.uk He also mentioned the process intensification network, PIN, and the next meeting, 30 November in York.

A new proposal had been submitted to the DTI to allow funding for HEXAG from April 2001 to March 2003, and this is under review at the Department. David refered to other funding opportunities, including the new EC call - see the HEXAG web site for a link to this - University funding (EPSRC), ETSU, and DTI support. Later in the day, Fiona Porter of ETSU outlined the newly announced opportunities for support for low carbon technologies, (see below).

The HEXAG web site has been reorganised and we have a new address - www.hexag.hw.ac.uk - and Peter Kew discussed some plans for the site later (see below).

An invitation to host the next meeting was put out, and later in the meeting Prof. Peter Heggs of UMIST offered to hold the next meeting in his Dept. Chemical Engineering, where there is a major facility dealing with energy-efficient distillation.

Technical Presentations: Keith Symonds of Chart Heat Exchangers (keith.symonds@chartmarston.co.uk) kicked off the technical session with his novel method for manufacturing tubular heat exchangers. After explaining the background to Chart, Keith described conventional practice in tubular exchanger assembly, listing some disadvantages, such as limits on pressure caused by header plate thickness, (the thicker the plates the greater the difficulty of drilling small diameter holes at close pitch). The time to weld or braze joints (if expanded joints cannot be used) can also be long. As an example, Keith showed the aerospace shell & tube unit with 2 mm i.d. tubes for fuel preheating/oil cooling.

The composite tube & shell construction proposed by Keith involves vertical assembly of stacks of 0.5 mm thick shims etched to form the tube passages and headers and, if necessary, integral tube-side enhancement features. The stack is diffusion bonded and has a pressure rating of 10,000 psi (approaching 700 bar) - making it strong and heavy! A 6-pass example was shown to the audience.

The advantages of this construction are several - the tubes may be smaller and positioned much closer together than with conventional construction; tube side performance may be varied by introducing features in the shims that can promote turbulence, spiral flow etc.; and bypassing between tubes and baffles and tube bundle and shell can be eliminated. The units can be made in most materials, including copper. If aluminium was used, they would be vacuum brazed rather than diffusion bonded. The disadvantages are that small passages can become blocked, the scale of shim photochemical etching may limit the small size, and costs may be higher.

Keith mentioned the interest of Chart in chemical reactors, including the Marbond unit. Process intensification was receiving DTI and ETSU support, and under an EU JOULE 3 project Keith said that 'staggering' results had been achieved on rectors.

The next talk, by Peter Kew of Heriot-Watt University (p.a.kew@hw.ac.uk) covered boiling outside small diameter tubes/bundles, 'small' being 1-6 mm o.d. The study was to examine the effect of tube diameter on nucleate pool boiling of water. In the fully developed boiling regime, the boiling curve was steeper than expected, (i.e. compared with the Cooper correlation). With water as the fluid, Heriot-Watt data gives the correlation for heat transfer coefficient (HTC) as a function of heat flux q as:

HTC = 0.35q0.88 While Cooper gives: HTC = 4.25q0.67 There was also a variation between tubes in the HW data, the constant in the HTC equation varying between 0.30 and 0.39. With R141b as the working fluid, the exponent in the equation dropped to be nearer the conventional 0.7.

Peter concluded that the test facility was effective and provided accurate data. Tube diameter may influence nucleate pool boiling heat transfer at all heat fluxes. Other factors, such as externally induced flow and surface finish on the tubes may have a more significant effect. The heat fluxes covered were from 50 kW/m2 to 800 kW/m2. It was also suggested that flow visualisation might be of interest. The validity of the conclusions was questioned, given the small quantity of data and the effect of the somewhat subjective choice of the fully developed region of the boiling curve on the resulting correlation.

We returned to Chart Heat Exchangers for the next talk, by Paul Shields. Paul (Paul.Shields@chartmarston.co.uk or www.chartmarston.co.uk) took us through the construction and general application areas of the plate-fin heat exchanger (PFHE), before concentrating on two aspects - the 'core in kettle' unit and mercury-tolerant aluminium PFHEs. Paul cited the main advantage of the aluminium PFHE as being its capacity to handle more than 2 streams. The 'in-house' record was 23 individual process streams in a single core!

The 'core in kettle' unit, a PFHE core in a shell, has significant advantages over the shell & tube unit - 50% of the size and 20% of the weight of the latter. Secondary cost savings, e.g. plot size, also exist, and reduced temperature approaches, typically 1-3oC. With up to ten times more surface area/m3, it is better than units with high flux tubing. As well as retaining the multi-stream capability, one can have independent multiple cores within the same shell.

A case study given by Paul related to a 10,000 hp compressor for propylene refrigerant, where running costs (electricity) were 5c/hp.h. By reducing the approach temperatures from 80C (shell & tube unit) to 30C (core in kettle), a 15% reduction in compressor power resulted, saving $657,000 pa or $75/h. Later, when compressor replacement was needed, one could purchase a smaller compressor, saving on capital cost. There are limitations on pressure for core in kettle units - 100 bar for the core and 65 bar for the kettle. Chemical cleaning can be used if the exchanger suffers oil contamination.

With regard to PFHEs resistant to mercury, Paul said that a two-stage protection system was employed. Liquid mercury is a threat to aluminium PFHEs at surface temperatures above 400C, and for damage to occur, it must be in contact for some time. The aluminium prone to attack is type 5083. The heat exchangers offered by Chart for duty in such environments have fully drainable construction.

Eric Smith of Energy Systems (fax: 01334 476470) presented an aspect of the detail design of compact heat exchangers which was an extension of the work of Shah & London, covering laminar flow forced convection in ducts. Arguing that surface area/volume ratio was not the best way of comparing compact surfaces, one reason being that this approach neglected identification of whether this was based upon primary or secondary surfaces, or a combination of both, he proceeded to show that in the duct design, the best geometry was one of high aspect ratio.

Eric also commented on offset strip fin configurations, and recommended that one should not go as narrow as other types. He also said that one should not aim for higher than 90% fin efficiency on the hot low pressure side of the exchanger.

The new call for proposals for studies into low carbon technologies was announced at a meeting in London on 13 October. Fiona Porter of ETSU (Fiona.Porter@aeat.co.uk) told us of the reasons for this initiative - climate change and the need to minimise CO2 emissions - and the new funding available to cover feasibility studies and larger projects. In particular, the Government's Energy Efficiency Best Practice Programme is seeking ideas for feasibility studies to investigate the potential for longer term, innovative low carbon technologies for industry. Such studies can attract typically £20k funding from the DETR, and up to £50k in some cases. Fiona emphasised that of particular importance was the need to 'think outside the box' in the search for innovative solutions to greenhouse gas emissions. The contact for discussing R&D opportunities is Dr. Cherry Tweed, cherry.tweed@aeat.co.uk

David Reay then briefly described the outcome of some of the workshops at the London meeting, where he chaired the group on low temperature process industries. Subjects such as heat recovery, ways for encouraging OEMs to put energy-efficient systems in before plant leaves the factory gate, heat pumps and heat transformers were among topics highlighted for effort. The combining of heat recovery and effluent treatment in a single piece of equipment was also recommended, particularly for textiles and similar sectors.

Highly relevant to the earlier discussion on greenhouse gas emissions was the talk by Fred Brotherton of BEEDES on heat pumps - Fred.Brotherton@beedes.co.uk - in which he firstly offered a definition of the industrial variant as 'a process which makes thermal energy more available'. There is little growth in the UK industrial heat pump market, and applications tend to be 'one-off'.

Fred described a number of applications. In distillation, he cited xylene extraction, with heat recovery between the overhead condenser and bottom reboiler. The system resulted in 60% cooling water savings, 30% steam savings and a small extra electricity use - 19 kW. Here one is not changing the fundamental nature of the process, but one is changing the nature of the energy input. The control parameters can change, but are readily addressed. As an alternative, Fred described how a heat pump could be used to modify the operating conditions on a propylene-propane splitter. By lowering the condensing pressure one can remove cooling water constraints. This reduces the reflux ratio and the size of the column - thus here the heat pump is operating as a process. In another example, using a steam turbine to drive the hp compressor, with pass out steam for reboiler heating, led to a 50% steam saving and 40% cooling water reduction. The availability of hp steam allowed a positive change in the balance of energy use within the system.

At Pure Malt Products near Edinburgh an MVR evaporator is used to concentrate protein and carbohydrates for animal feed. The heat pump on the evaporator allows energy efficient concentration, with product recovery, and the boiled-off water is sent to a reverse osmosis plant for reuse in the process, resulting in less water purchased. In this and other systems Fred has used plate heat exchangers. As well as the usual benefits, the fact that they can tolerate big differences in flow rates on the two sides is an advantage.

Further demonstrations of the technologies discussed by Fred are needed. It is also important to recognise that although the UK heat pump market may not be large at present, experience gained will allow UK contractors to sell plant into more buoyant Continental markets.

During the lunch break there was an opportunity to examine the Chart units, including examples of the shims etched to form the compact tubular heat exchanger. The heat exchanger foam, introduced later by Porvair (see below) was also on show.

Impromptu Presentations: After lunch, Tom McDonald of Chart chaired the impromptu session, where speakers are given 5 minutes to present a topic of their choice. Bob Tucker of Advantica Technologies (was BG Technology) gave the first talk on a small gas engine driven heat pump (bob.tucker@advanticatech.com). (Full details of the talk and illustrations can be found on the HEXAG Web Site, in pdf format - go to 'meetings' and choose 'papers presented'). Summarising Bob's data, he said that the Yamaha engine-driven unit could provide 11 kW of heating and sufficient air conditioning for a house. Noise was less than 50 dBa, service intervals annually and the cooling coefficient of performance was 1 at 350C ambient, heating COP 1.2 at -100C ambient. Comparing CO2 emissions with a gas boiler/electric air conditioner combination, the heat pump gave 26 kg/m2 room area, while the gas boiler/electric a/c gave 40 kg/m2. With Government plans for 360,000 homes in the East Midlands alone by 2021, new technologies for domestic heating etc. are needed to reduce CO2 emissions.

Peter Kew then proposed a new role for the HEXAG Web Site (which is based at Heriot-Watt University). The 'Transactions of HEXAG' would be an archival record of meetings and talks, and would help to encourage the dissemination of ideas and discussion outside the meetings themselves. The advantages would be several: Free, easily accessible data, and available to members and non-members; there would be minimal author effort, and there would be spaces allocated for case studies, new product data, etc. It was proposed by Peter that authors could submit papers in electronic format or on paper for scanning in. The only perceived disadvantages were the fact that documents published on the HEXAG Web Site would not be regarded as a recognised 'academic paper', and it would be unreliable as an archive (lack of refereeing, uncertain long term future of electronic media, etc. except for checking by the Web Site manager).

A show of hands produced a positive reaction to the idea. Please send inputs to Peter at p.a.kew@hw.ac.uk

It is always encouraging to find a new idea being presented as an impromptu at HEXAG meetings. The anti-fouling fibres started as this some two years ago, and at this meeting Alan Clegg of Porvair (Aclegg@porvair.com) introduced their metal foam product, which can be used as an extended surface for heat exchangers - see Web Site www.porvair.com. One example of its use was in a gas-liquid heat exchanger, the tubes penetrating the bulk of the foam, which replaced conventional extended surfaces. The tubes were sintered into the foam, giving good contact. Foam pore size was 0.1 - 5 mm. Foam could be put inside tubes or between plates, configuring a gas-gas heat exchanger. The foam can be described as an anisotopic three-dimensional extended surface. Alan explained that a contract had been signed to characterise the heat transfer behaviour of the foam, involving some mathematical modelling. This could lead to designs for tube-in-foam heat exchangers of co/counter or cross-flow configuration.

Advantages of the foam include a choice of metals to 10000C, low weight, compact and the ability to be formed in complex shapes. Some potential applications discussed included gas turbine tail gas heat exchangers, hydrogen liquefaction plant, and partial oxidation of hydrocarbons to hydrogen. The foam can be catalysed.

Brian Withington of Occo Coolers (salessouth@occocoolers.co.uk) first gave us a blast of air by demonstrating his 'direct' heat recovery unit, a fan system for circulating warm air from high up in rooms to ground level. He then introduced the use of screw threaded connections instead of bolted flanges to reduce connection sizes on compact heat exchangers. Standards Committees need to look at this, and standards need to be changed to accommodate them. Some companies (e.g. ICI) do not accept screw connections.

The Brazilian Chemical Engineering Congress was described by Jack Howard of HH Associates (fax: 0121 705 1946). Jack has recently returned from Brazil. Highlights were increased funding for biochemical engineering, in particular from the USA. A selection of 60 papers given is presented in the J. Brazilian Chemical Engineering. Jack said that the relevance of the Congress to HEXAG members was the opportunities to enter a very large highly industrialised market in South America. He cautioned against going in alone, recommending using agents.

Peter Heggs of UMIST (p.j.heggs@umist.ac.uk) briefed us on some on-going heat transfer research in his Dept. of Chemical Engineering at UMIST. Extensive facilities include a thermosyphon reboiler of 750 kW duty, supported by HTFS and Hyprotech. The energy efficiency of distillation processes has been studied under an EU contract, and this has involved improving heat transfer, by modifying tray design and incorporating an intermediate reboiler. Wieland double-enhanced tubes are used here. Reflux condensation is also being studied.

The work on boiling at Oxford University ha regularly featured in HEXAG meetings. David Kenning was unable to attend, buy an update on a new EPSRC project there was given by Huijuan Xing (contact huijuan.xing@eng.ox.ac.uk). Huijuan said that the project covered surface characterisation relevant to nucleation. The aim is to identify the locations of nucleation and to show which are stable and which are unstable. This will allow the most effective size of nucleation site to be found. Various surfaces are to be tested, including those prepared in the lab., industrial surfaces and those both before and after use. Huijuan asks for HEXAG members to supply examples of industrial surfaces they may feel are appropriate. If you are able to help, please email: david.kenning@eng.ox.ac.uk

Workshop & HEXAG as a Forum: David Reay introduced heat pumps and other heat recovery devices as a case for action - in particular in getting OEMs to install them. Perhaps the best suggestion during discussion was the use of a legally binding BATNEC (Best Available Technologies…) definition which could be brought in by Government. The use of environment impact assessments could also be a useful 'tool of persuasion'.

With regard to HEXAG as a forum for proposals to Government, the principal difficulty seemed to be associated with the time people need to write proposals, and how best to compose them. HEXAG could help here, but its strength lay more in being able to represent a strong body of opinion in bringing forward proposals on behalf of a group of members.

More on this will follow!

THE NEXT HEXAG MEETING: Peter Heggs offered to host this at UMIST in the Spring, 2001. The date will be announced in HEXAG News, in January.

Minutes written by David Reay, based on meeting notes, 7 November 2000.