ROLLS-ROYCE, ANSTY, 30TH SEPTEMBER 1999.
Approximately 65 members attended the HEXAG meeting at Rolls-Royce, Ansty, where industrial & marine gas turbines are made. Colin Adderley (fax: 01332 622948) welcomed members and described the RR activities at Ansty and elsewhere. The Ansty set-up was divided into 3 oil and gas power systems, marine power, and power generation. Overall, RR was 50% civil aerospace, 30% power generation and the balance defence, the latter shrinking in size. There were of course many applications of heat transfer/heat exchanger technology. Colin cited the high bypass turbofan engine, the Trent 900. Heat transfer problems were several internal air was used to cool the blades and a range of heat exchangers were used inside the engine. Gas turbines were used in power stations, where heat exchangers were needed for heat recovery etc. The WR21 intercooled and recuperated GT was used on ships, with plate-fin heat exchangers (PFHEs) located between compressor stages. Reciprocating engines which of course use heat exchangers, were made by the Group, and the company was also in nuclear power. RR also makes heat exchangers, the Rolls-Laval titanium unit, and the spiral heat exchanger, the subject of talks later at this HEXAG meeting.
David Reay then welcomed new HEXAG members and gave a profile of HEXAG and what benefits it brought to members. He also highlighted the HEXAGWeb Site, on which members can freely register their areas of interest, (contact DAReay@aol.com or P.A.Kew@hw.ac.uk for this).
Tim Moore of RR then talked about heat exchanger needs on the intercooled and recuperated WR21 GT. He gave some historical background to start with 40 years ago the RM60, a 2.5 MW GT, used shell & tube heat exchangers in these roles. There were limits in terms of efficiency, corrosion and response time, and control technology, in terms of response, was inadequate. In the 1980s, the use of digital electronic control in conjunction with PFHEs allowed the idea to become workable again.
The requirements arose initially in 1991, when the US Navy funded the RR GT and an AlliedSignal recuperator & intercooler. An engine was run at DERA Pyestock in 1994, and the UK, US and French navies funded a development programme which is almost finished, to culminate in a 500 h test and 3000 h endurance test, with shock qualification tests, too. The GT has an efficiency of 37% without recuperation, increasing by 25% and increased mass flow through the engine with recuperation. Fuel savings are 10% at full power and 50% at idle, (off-peak running is important in navy ships, where they run for 25-33% full power for most of the time).. The power turbine has variable area nozzles which keep the temperature high at the back end of the turbine, thus allowing maximum heat recovery.
The recuperator is a full counterflow PFHE, tension brazed, and is rated at 16 MW (the engines ISO rating is 25 MW). It uses a high grade stainless steel. It operates in an environment where corrosion and creep can occur. Typical duty is cooling 70 kg/s gas from 575oC to 275oC. The temperature difference between the top and bottom of the recuperator is 300oC and a single block weighs about 8 t. The WR21 GT needs to run even if a shell or missile goes through the recuperator. Gas-side fouling is a concern (not from missiles!) this arises from particulates in combustion gases (0.01 ppm by weight), which although filtered, some fouling still gets through. There are 8 cores/recuperator, and sections can be removed.
In discussion, an interesting point was made by Colin Adderley concerning flying engines with recuperators. In order to keep extra weight to a minimum, the recuperator should perform another extra function, e.g. in Russian military engines the recuperator also forms part of the structure.
Ian Wilson of Cambridge University (fax: 01223 334796) talked about his continuing work on scheduling and cleaning of refinery heat exchanger systems, under EPSRC Grant Ref. GR/L33658. A network, such as that at Esso Fawley which has 67 heat exchangers, needs a special approach when looking at fouling mitigation strategies. Mitigation can involve modifications to equipment, regular cleaning, or use of an anti-fouling strategy such as using chemicals. Comparing the cost of these requires accurate estimates of the optimised operating costs. One can also have only one of the above, or a combination of all three! Ian pointed out also that there may be no new refineries built in the next decades, therefore we have to make the best of what we have got! A typical scenario would be to look at the best cleaning schedule for a 5-7 year refinery operating period.
Ian likened it to a Tripos problem: First there are fouling models and data collection plant studies, then network simulation, and then the application of robust optimisation codes, with emphasis on the best route for optimisation. MINLP (Mixed Integer Non Linear Programs) are used. Fouling is non linear, cleaning is the mixed integer part, while constraints are due to operation or optimisation. Cost is the objective function.
Achievements so far include benchmarking agreeing with the analytical results. The methods do work and have been used to solve large hx network in reasonable times (hours). The solutions also take into account pressure drops. Studies have been made at British Sugar, Bury St. Edmunds, where the cleaning schedules of the preheat train were optimised during the 120 day campaign (a term used when sugar beet is processed at a specific time of year). A preheat train at an oil refinery, a 25 hx crude preheat train, was also examined. Schedules for cleaning over 2 years were worked out, the best solution saving £100K (about 20% of costs) over the two year period.
In the next 9 months of the project, consideration will be given to the stability of pump-arounds, stand-by unit use and the operability/feasibility of the approach.
Simon Pugh of ESDU Ltd. (fax: 0171 490 2701) then gave information on the work they are doing to put together an ESDU Guide to fouling. Simon said that in crude distillation, in the US alone the cost of fouling was $1300 million, while world-wide it was estimated to be $4500 million. ESDU had established a crude oil fouling Working Party, involving refinery operators and mitigation technology suppliers. To date 4 meetings had been held. The User Guide structure would cover: Refineries & types of fouling; factors influencing fouling; mitigation & control; prediction of fouling; design & retrofit options. Fouling has a bearing on economic & environmental factors; capital costs; energy costs; maintenance and production losses. A 100,000 bpd refinery, for example, could save $100,000 per annum if there was a 1oC increase in furnace temperature.
The aim of the ESDU work is to develop heat exchanger design methods that do not rely upon fouling factors. With regard to mitigation of fouling, Simon said that Italy was keen on using additives, while mechanical in-tube devices such as HiTran were being examined. The ABB Lummus helixchanger was an option for shell side mitigation. Scheduling the crudes to self-cleaning systems, e.g. aromatics can clean a heat exchanger, was a possibility. One could also look at plant control with fouling in mind, although turndown becomes a concern. Simon started a list of the Top 10 approaches to fouling prevention: In effluent desalting, care was needed when mixing crudes etc.; in new plant design, process integration can take account of fouling; in new exchanger design, examine the fouling threshold.
David Reay then updated members on European Union calls for proposals. Calls were closing imminently in the areas of gas turbines and combined heat & power, but should be repeated in some form next year, as would rational use of energy tender opportunities. (Details will be posted in HEXAG News).
The Energy Efficiency Best Practice Programme (EEBPP) update was given by Fiona Porter of ETSU. Fiona highlighted the Energy & Environmental Helpline (tel. 0800 585794), whereby SMEs can obtain a free visit from a specialist to advise on energy/environmental efficiency. After Kyoto, subsequent to which the UK agreed to reduce CO2 emissions by 20% from 1990 levels, the DETR is ken to develop new energy saving technologies, within a new R&D programme. Grants to 35% of the cost of R&D are available, and single company funding is possible for SMEs, who can receive up to 35% of the costs of product development as well. Grants are also provided for feasibility studies. Areas of particular interest to Fiona are high and low temperature process technologies, including heat exchangers, and process intensification. Contact Fiona on 01235 433012 for more information. For marketing data, contact Carl Williams on 01235 433867.
Fiona is also involved in a study by ETSU to assess the impact of the EEBPP. In particular, she is collecting data on the use of compact heat exchangers (CHEs), and the influence the promotion of the technology on take-up of CHEs may have had. A questionnaire concerning this is attached. If you are able to answer any of the questions, please return the sheet to David Reay (fax 0191 252 2229).
The piece de resistance of the meeting was the RR spiral recuperator. Lee Clawley, backed up by Jim Oswald, both of RR, described this new recuperator, examples of which were able to be viewed outside the meeting rooms. The advantages of the spiral configuration were much reduced stresses and thermal gradients compared to flat plate configurations. Fins were used between plates, on the air side the fin height being 0.8 mm, while 14 fins/inch on the gas side had a height of 3.96 mm. Fin thickness was 0.2 mm. Air is introduced into the centre of the heat exchanger core, while the gas is counterflow. The manufacture will be a continuous process, two roles of metal dimpled and fins then put in between, and the edges welded. Unit 1 had 25 wraps, to show that the unit could be fabricated. Wrapping took a long time, and the target for production is to wrap the unit in ten days.
Currently generic structural and thermal models for life prediction are under investigation, tests to validate these being in progress. CFD work on header design has highlighted areas for improvement. The hx effectiveness is about 80%, and a shock validation test covers 18 g loads. Future development includes further thermal cyclic endurance testing, vibration/corrosion studies, engine integration tests, application to various engines, and licensing into other industries/partners. (Further data are given in Ref. 1, where there are some interesting pictures, one of which is shown below).
Discussion revealed the following: The heat exchanger could also operate as a gas-liquid unit. It currently relies on some pressure differential for its form. This can be done in other ways if the deltaP is small. The hot design pressure is 14 bar. Long term creep problems are not seen as a problem. Inco 65 is currently the material being used. If higher pressures were needed, a pressure vessel could be put around the outside of the core. Internal forces are low, and with thicker material the pressure could go to 100s of bar. With regard to cost benefits, RR claim the unit is 50% of the cost of a competitor at good volume of manufacture. For low volumes, it would be 30% cheaper than the competition. (RR see a high volume as 100,000 units p.a. for a microturbine, while their own view is that up to 2000 units p.a. will be needed. It is designed like making newspapers. Performance is similar to the competition per unit volume. Life, crucial in GT recuperators, is good. If stainless steel was used instead of Inco 65, one would be pushing it to the creep limit. The GT exhaust is 6500C. It could be used with mixed materials, and could be applied to cryogenic processes.
The biggest challenge was automated manufacturing plant laser welding and robot control are required. The recuperator can be used for microturbines (e.g. 40 kWe) or up to multi-MWe sizes, where several cores would be used. On the way to lunch, members could view examples of the new recuperator.
The impromptu presentations were launched with a video by David Kenning of Oxford University (fax: 01865 273010). This showed boiling phenomena in small channels. David said that the physics in small tubes was significantly different to that in large ones, with a different form of initiation of nucleation and confined bubble growth. David is interested in further industrial input to his work contact him on the above fax.
Richard Clarke of Cal Gavin then introduced us to his ideas on multi-steam heat exchangers for non-cryogenic applications. In air separation plant one could have to 5 streams in one hx, with PFHE cores manifolded together. Where else could this concept be used, with what hx type, in what applications? Clean conditions were critical, but the benefits many more integrated processes, closer deltaTs, less piping, cheaper inventory, smaller plant. Richard is asking HEXAG members to comment on this see attached questionnaire please return to Richard at Cal Gavin, (see address on form) or fax to him on 01789 400 411. The results will be reported in HEXAG News and/or at the next meeting.
Eclipse make a range of heat exchangers for gas-gas duties. Paul Wild of Eclipse/Exothermics (fax: 01905 794419) gave us a review of the product range and applications. Many are stainless steel plate heat exchangers, used as fume preheaters in areas such as catalytic and thermal oxidisers. They can deal with hot gases to 650oC, but can tolerate 850oC if used with internal insulation and expansion bellows. Access is available for inspection. The company also manufactures tubular recuperators for gases to 870oC, cross flow and multipass configurations to 85,000 Nm3/h. A new product is a dimpled plate unit for dusty/corrosive environments, with look through inspection capability.
Colin Weil (fax: 0181 428 0369) completed the session by updating us on the work on tube bursts in shell and tube heat exchangers, showing us the effect of such an event during a 100 bar test, resulting in a water jet with a 20 tonne force. Colin reported on the implications for design, and has offered to discuss the subject in more detail at a future HEXAG meeting. (Coincidentally, the co-ordinator received an EPSRC report on work in this area carried out at Liverpool University in collaboration with an industrial consortium).
We then broke into two groups for a site tour, looking at gas turbines, etc.
Our thanks go to RR for hosting the meeting, and for contributing speakers and tour guides. In particular, David Coleman, who was responsible for much of the local organisation, deserves special mention!
1. Oswald, J.J. et al. A new durable gas turbine recuperator. Paper 99-GT-369, Presented at Int. Gas Turbine & Aeroengine Congress, Indianapolis, USA, June 7 10, 1999. ASME, New York, 1999. (return to text)
6 November 1999.
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