Heat Exchanger Action Group Hexag

HEAT EXCHANGER ACTION GROUP – HEXAG

MINUTES OF THE 22ND MEETING HELD AT WARWICKUNIVERSITY, 12 MAY 2004.

Introduction

The 22nd HEXAG meeting was hosted by Professor Bob Critoph () in the School of Engineering, WarwickUniversity. Thirty four members attended.

Bob welcomed HEXAG members to the School, and highlighted the rating in Engineering of Warwick (5 for both teaching and research at the last assessment). He then described some of the research in adsorption cycles, used for heat pumping and/or cooling, more details on which can be found in the minutes of the 21st HEXAG meeting (UMIST). After discussing advantages and disadvantages of adsorption cycles, Bob moved on to illustrate the four main projects for which he is responsible. These included the convective thermal wave unit, and the monolithic carbon generator. The third project, originally a multiple bed regenerative cycle, is now a fixed bed tri-generator system, ‘SOCOOL’, and is a collaborative EU project. The final concept is based upon plate heat exchanger-type structures, with the adsorbent being bonded to the plates in thin layers. With a cooling duty of 6.5 kW/kg mass of adsorbent, the unit may also be configured like a ‘plate-and-shell’ heat exchanger.

HEXAG Update

David Reay () then updated those present with HEXAG announcements. Articles were invited for inclusion in the next issue of PIN News, due in July 2004. David mentioned the possibility of closer links with the Heat Transfer Society (the subject of a separate section later in these minutes), and also said that it was hoped to seek funding from the DTI under the Innovation Programme support for networks, possibly in conjunction with PIN, the Process Intensification Network.

Technical Presentations

The first technical talk was by Cosimo Buffone () on the subject of using thermochromic liquid crystals (TLCs) to measure the temperature of an evaporating meniscus in a capillary tube. The basis of the concept is the fact that TLCs reflect light when the temperature falls within a certain range. The liquid crystals themselves are an intermediate between a liquid and a slid, but all their properties are those of a liquid.

Cosimo showed how light interacted with layers of TLC – as the temperature rises the colour changed, but one also saw different colours from different angles – an undesirable feature.

The work is attempting to characterise thermocapillary Marangoni convection. To date measurements of temperature have not been made. The convection is self-induced by evaporation, and the cooling effect of the evaporation has only recently been studied. It was shown that most evaporation takes place near the edge of the meniscus, within only 1-2% of the meniscus length. This observation is important in boiling, heat transfer in porous media, and in thin film heat transfer. The TLC method was used to indicate where peak cooling – i.e. maximum evaporation – takes place at the meniscus. Cosimo used the observations to explain where the heat input to sustain evaporation came from. He measured axial heat flux and evaluated the meniscus heat flux profile.

Robin Young () then described the work of the Faraday Partnership, EPPIC – (Electronics & Photonics Packaging and Interconnections – see – which has now been extended to micro-systems, and in which thermal problems are important. Within this context, a network related to electronics thermal management is being set up.

The role of the activity is to facilitate technology transfer and technology translation. The subset of research priorities which may be of interest to HEXAG members include:

• Thermal transfer in photonic systems (where there are very high heat fluxes and power densities)
• Non-hermetic packaging
• Packaging of high power devices
• Integrated thermoelectric coolers

Robin said that there was a trend towards integrating passive (cooling) devices in the thermal modules themselves. Application examples include electric vehicles, where the power modules have aggressive requirements, and electronics, where 55% of failures are caused by temperature excursions, and failure is also caused by vibration and humidity.

Passive cooling methods being studied or available include printed circuit boards, heat pipes, (which are perceived by Robin to have a low penetration into industry), Peltier coolers, and improved (minimisation of) interface resistances. Modelling was regarded as important, in order to reduce lead times.

The research themes identified include interfaces, thyristor thermal management, development of low thermal resistance joints and low bonding temperature materials. Materials will have an important role to play here, including advanced carbons/graphites, carbon tapes (LeedsUniv.), and composites with low expansion coefficients and high thermal conductivity.

Eric Smith (tel: 01334 472753) introduced us to a theory giving an unambiguous performance comparison of heat exchanger configurations, to be detailed in his new book due out soon.

Eric described specific performance (kW/m3K) or what he called an exergy loss number. He pointed out that exergy and entropy analyses led to different locations of the pinch point, and said that for the design of heat exchangers operating in the cryogenic region, the temperature selection in the heat exchanger is vitally important.

Eric then addressed the pressure loss in headers. He cited Dow’s theory, which is used to achieve uniform burner flames when gas is going down a pipe with holes in it for the flames. This allows one to get equal core pressure loss in the heat exchanger equivalent. However, Eric had a major reservation, and suggested that the heat exchanger design method fro cross flow is wrong to date (based on equal mass follow rates along each channel). Eric suggested that the investigation of this would make a good PhD project.

A third topic Eric made reference to was the approach to optimisation, where he compared the performance of square and triangular ducts. Finally he explored the subject of transients in contra-flow exchangers.

David Etheridge () updated us on the outcome of the project he first reported at the HEXAG meeting at NottinghamUniversity some 18 months ago. This is concerned with a more effective and controlling way of using night cooling to provide cooling of buildings during the day, (current systems are not easy to retrofit, are heavy and/or expensive and can lead to condensation problems). The latent heat of phase change materials (PCMs) used in the new unit was 200 MJ/m3 for a constant temperature, while in concrete, used in other systems, such as chilled beams, it is 4 MJ/m3. The project started with DTI funding, and then proceeded to Carbon Trust (CT) support.

The original ceiling-mounted system was changed to a floor-mounted unit at the start of the CT project. This did cause problems and allowed only two field trials.

David described the basic system, which uses a reversible heat pipe system in a PCM, with a low power fan for air distribution. After discussing the cooling unit, the control system and the operation strategy, David noted that the system also enhanced night cooling of the building fabric. He showed the module operation, starting with the original ceiling-mounted unit. Storage capacity was 7.2 kWh, based upon 7 PCM/heat pipe modules.

The results of the trials in the Summer of 2003 showed that it was crucial to freeze the PCM at night, and with 450W of artificial heating in the test room in the eco-house at NottinghamUniversity, cooling was effective on the hottest day of the year (290C). The second field trial at Pinxton, north of Nottingham, on a moderate temperature summer’s day in an occupied office, produced positive responses from the person in the test room. It was concluded that the system provides a degree of control, over temperature comparable to air conditioning.

Future work would be directed at increasing the heat transfer of the exchanger and reducing the pressure drop across the fins, and to carry out a large scale demonstration.

For the demonstration, the partners are seeking a company to construct the cooling units – please contact David if you are interested.

The possibility of obtaining Enhanced Capital allowances (ECAs) for heat exchangers has been discussed at previous HEXAG meetings. The good news, as reported by Bob Tucker () and David Reay (), is that at the 2004 Budget, it was announced that several heat exchanger types, relevant to buildings and industry, could be included on the approved list.

Firstly, Bob ran through the air-air energy recovery systems for buildings that could be included (subject to Ministerial approval). These comprised plate heat exchangers, thermal wheels, and run-around coils. As well as recovering heat, the equipment could be included on the List if it was used for pre-cooling incoming air, for example for air conditioning pre-cooling.

The reasons for including such heat exchangers on the List are their high potential carbon saving, the lack of awareness of the technologies, and the fact that they are widely applied on the Continent. Criteria for inclusion on the List include the following efficiency minima:

Plate recuperators – 50% minimum effectiveness

Heat wheels – 70% minimum effectiveness

Run-around coils – 45% minimum effectiveness.

The List will give values of the claim that may be made in terms of £/(m3/s).

David then went through a similar exercise for compact heat exchangers. The types of CHE included on the Energy Technology List are:

Plate heat exchangers (gasketed, brazed, partially welded and welded units)

Plate-fin heat exchangers (brazed and welded units)

Compact heat exchangers with precision-formed surfaces (brazed, welded and diffusion bonded units). The precision formed surfaces will included folded, etched, punched and machined forms.

Criteria to be met include a 90% design efficiency – as defined in the specification available on the ECA web site – see later – and a surface area to volume ratio greater than 200 m2/m3.

Applications to be includedon the above lists can be prepared in advance, and data on how to do this are given in the ECA web site – For CHEs contact Richard Shock for further information – or tel 0870 190 6250.

HEXAG-HTS Links: David then moved on to discuss the possibility of establishing closer links with the Heat Transfer Society, of which he is President for the current year. In the past HTS forum evenings have preceded HEXAG meetings outside London, but collaboration has not proceeded beyond this point. HEXAG and HTS memberships are similar in number, and HTS members pay an annual modest membership subscription – something that the HEXAG members at the Warwick meeting would be agreeable to, following a show of hands.

Martin Gough and Hans Zettler, as HEXAG members and HTS officials, gave information on the HTS. The HTS is perceived as a social club – it is a registered charity – and was formed 30 years ago. It is centred on London, where most, if not all, meetings take place – generally evening events – and appealed particularly in its early years to engineering contractors in the process industries. Its web site is

A number of proposals were made at the meeting. It was suggested that there should be one (annual) combined HTS/HEXAG meeting held away from London. It was also suggested that our respective databases of members be exchanged so that the degree of duplication of membership, and different scopes, could be ascertained.

The possibility of joint approaches to the DTI and/or EC for network funding, together with Continental equivalents such as NOVEM and the VDI Heat & Mass Transfer Committee was also suggested.

David Reay is to discuss this more fully with the HTS Committee this Summer, and will report back to HEXAG members.

Martin Gough of Cal Gavin () updated us on some of the work of al Gavin Ltd. Martin stated that the company would be 25 years old in 2005, that 90% plus of its products are for export, and it has over the years processed 6500 orders, which represents between 15,000 and 20,000 heat exchangers world-wide with Cal Gavin tube inserts, the company product.

Martin explained the role of the inserts in enhancing heat transfer, via videos of the behaviour in mixing at different Reynolds numbers. The velocity profile across a tube with an insert at Re = 500 showed an almost zero velocity at the centreline and almost turbulent characteristics near the wall. Dual Laser Doppler Velocimetry was used to provide velocity measurements very close to the wall over an Re range of 100-10000. Interestingly, Martin said that the void volume in the presence of the inserts was still as great as 92.9-97.3%

Measure velocities 0.6 mm from the tube wall showed the following;

Laminar theory gives U/Umax = 0.1

Insert with Re = 3000 – U/Umax = 0.6

Insert with Re = 10000 – U/Umax = 0.85

The effect of inserts on heat exchanger design was shown – in one example Martin showed that 8 passes could be reduced using inserts to 2, with overall HTC increasing from 180 to 525 W/m2K

Cal Gavin has software for designing units, and comparing the base case with one using HiTran inserts.

Automotive air conditioning systems were the subject of the talk by Tim Cowell of VTS Consult (). Tim had worked for several years for Delphi, makers of engine cooling and car air conditioning systems, and he described some work on air conditioning which had been prompted by EC legislation on refrigerants, in particular the European Climate Change Programme in June 2000.

R134a in automotive air conditioning systems contributes up to 0.4% of total greenhouse gas emissions in Europe, (see Powerpoint Presentation for further statistics). It is anticipated that R134a will be banned from use in new car models from 2010 and in all car production from 2014[1]. Reduced leakage systems are proposed and it is possible that car fuel consumption data will be given when the air conditioning is in operation.

As an alternative, CO2 could be of interest. The cycle operates in the transcritical region at pressures of 35-130 bar, and one can use head pressure to control the performance. An internal heat exchanger is needed to aid effectiveness, and an oil separator is required, but the compressor would be smaller than that for R134a as lower mass flows are needed. Heat exchangers would also be smaller. Tim showed us examples of the aluminium extrusions which formed the basis of some of the exchangers, using 0.5-0.8 mm diameter tubes.

Tests were carried out on a Alfa Romeo 147 car, with both R134a and CO2. The latter performed better at low ambient temperatures, but ambients of 35-40oC cause the CO2 system to struggle.

Impromptu

There was one impromptu presentation, from Jim Oswald (). Jim worked on the spiral heat exchanger/recuperator at Rolls Royce (see minutes of Ansty HEXAG meeting) before setting up his own company – see Powerpoint presentation – and said that his main activities as a consultant were energy management, design and product development. Jim updated us on what had happened to the recuperator – it was designed to operate at 650oC – and the conclusion was that the technology was OK but the market was not. A Mark 3 design – the Mark 2 was shown at the HEXAG meeting – was more compact, however.

DATE FOR YOUR DIARY

HTS President’s Night, London, 21 October 2003. (Where the HEXAG Co-ordinator will give a talk on a subject of his choice – yet to be decided!)

HEXAG NEWS – ARTICLES FOR THE NEXT ISSUE ASAP, PLEASE.

1

[1] The Second Reading of the legislation is scheduled for the Autumn of 2004.