SESSICN II - HEAT EXCHANGERS - HEAT RECOVERY. Olairman: D.A. REAY

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1 SESSICN II - HEAT EXCHANGERS - HEAT RECOVERY Olairman: D.A. REAY HEAT EXCHANGERS SlU'Illllacy of Session II Development of a high temperature heat pipe heat exchanger for recovery of residual heat Heat transfer, vapor-liquid flow interaction and materials canpatibility in t\\10--phase thernosyiflons Design, building and testing of a novel IOOdular gas:gas ceramic heat exchanger for exhaust heat recovery around 1200 C Compact gas-gas heat exchanger The evaporation of viscose process liquors by flow energy means HEAT REOOVERY Aluminium melting furnace heat regeneration project Design of waste heat boilers for the recovery of energy from arc furnace waste gases Energy recovery from cast iron melting furnaces -45-

2 SUMMARY OF SESSION II HEAT EXCHANGERS - HEAT RECOVERY O.A. Reay, P.A. Pilavachi A large number of industrial processes, covering most industrial sectors, use significant quantities of energy in the form of heat. The efficiency of heat utilization remains such that there is considerable scope for the use of heat exchangers and other forms of heat recovery equipment to enable waste heat to be recycled. As in the 1st Energy R & 0 Programme, heat exchangers and heat recovery remained a high priority during the formulation of the 2nd R & 0 Programme by the Commission, and this is reflected in the scale of the projects now under way, some of which are discussed below. (Note that heat recovery also comes within the project classifications of heat pumps, fluidized beds and Rankine cycle engines>. The emphasis within the 2nd R & D Programme is on improved heat exchangers, new materials, and the widening of the operating temperature range of heat exchangers and heat recovery equipment. successful completion of the projects within these areas will, it is hoped, lead to use of heat recovery equipment in new areas of application. Of particular importance is the growing appreciation by contractors of the need for a system approach to heat recovery, and this is evident from the attention given by contractors to the "peripherals" associated with the heat exchangers under development. * The success of any research and development project, or programme, may be directly measured by the degree to which the results, if successful, are exploited. This exploitation depends upon many factors, not least of which is the committment of the organisation<s> involved in the work to persue the work. Exploitation can be aided, however, by guaranteeing that the results are given wide publicity, and publishing the proceedings of these contractors' meetings is directed to this end. The meeting discussed here, as mentioned above, considered papers on heat exchangers and heat recovery. In the context of the Community R & 0 Programme, the distinction between the two is not great, as all the heat exchanger projects are related to waste heat recovery applications. Some of the projects highlighted one of the major problems facing potential installers of waste heat recovery equipment, namely the identification of a use for the recovered energy. This will be shown to be particularly true in industries where there is an excess of low and medium grade heat, and electricity generation may be the most satisfactory route to follow. * In most cases of the research projects, the progress of work has not reached a stage for economical assessment. This would be made at a later stage and economic results would be presented in a future contractor's meeting. -46-

3 Of possible greater current significance is the fact that changing economic climates affect decisions on investment in waste heat recovery equipment no Less than in other areas. The situation persisting at present does not assist the programme in this respect. Heat exchangers Of the five papers presented in this session, a majority deal with gas-gas heat exchangers, the exception being an evaporator used in a mechanical vapour recompression (MVR) system. Two papers discuss heat pipes and thermosyphons as components in heat exchangers, CUKAEA HARWELL and the INSTITUT FUR KERNTECHNIK UNO ENERGIEWANDLUNG (IKE), Stuttgart), and BERTIN & CIE describe a ceramic recuperator for use at high temperatures. INSTITUT FRANCAIS DU PETROLE are developing a plate recuperator having the advantage of compactness. COURTAULDS are investigating a complete process, of which the heat exchanger, in this case made of plastic, is only a part. tl~2!-~i~~~-2~9-!h~r~q~l~hq~~ - IKE in Stuttgart reported on the three main aspects of its study of two-phase thermosyphons : heat transfer, vapour Liquid flow interaction and materials compatibility. In stating that while heat pipes were no Longer novel, and several forms of heat pipe heat exchanger currently exist, it was pointed out that optimum design and manufacturing procedures were needed in order to enable such heat exchangers to compete effectively with other systems. Furthermore, the complex evaporation and condensation process within heat pipes/thermosyphons was not fully understood, and units operate well below their Limits. The final problem area highlighted was that of materials compatibility-this could be demonstrated for vapour temperatures of up to 280 c, but above this the selection of suitable working fluids and container materials becomes more difficult. Heat transfer coefficient measurements made in the evaporator and condenser showed that while values of 10,000 Wtm2K were achieved in the former, coefficients one order of magnitude greater than this could be measured in the condenser. While the overall conductance of heat pipes was greater than thermosyphons, the throughput of heat pipes was Less. Typical axial throughputs achievable, however, are in excess of 1 kw/cm2. It was found that high performance thermosyphons were performance Limited by Liquid/vapour flow interaction, not by boiling. A major part of the IKE programme is directed at identifying suitable working fluid/material combinations for use in heat pipes and thermosyphons at high temperatures (typically 400 C vapour temperature). While cheap carbon steels may be used as containers for fluids such as naphthalene and toluene at temperatures of up to 270 C and 28oo c respectively, more expensive materials are Likely to be needed if organics are to be successfully used at higher ( C) temperatures. An alternative solution to the problem of working fluids for use at temperatures in the range C is to select an inorganic fluid. Work -47-

4 carried out at UKAEA HARWELL, following on from an earlier development programme Leading to the manufacture of a number of prototype Low temperature heat pipe exchangers, is based on the use of sulphur, with added iodine, as the working fluid. The system would operate at moderate vapour pressures, and would therefore not create containment problems in this respect. The work programme involved studies of both the thermal performance and corrosion behaviour of sulphur/iodine mixtures. Thermal tests on the working fluid in a series of silica thermosyphons showed that predominantly dropwise condensation was occuring in the condenser, but only over the Lower (closest to the evaporator) section. The system exhibited characteristics similar to those of a gas-buffered heat pipe, namely a sharp decrease in temperature as the "cold" end of the condenser is approached. Compatibility tests showed that none of the 6 candidate metals selected as containers would be recommended. Those tested included Hastalloys, Fecralloy and stainless steel. f~r~~i _9~~=9~~-h~~!-~~ h~~9~i - The development of heat exchangers to recover heat in very high temperature processes, where exhaust gases exceed 1000 c, has been attempted by many Laboratories. A novel approach has been adopted by BERTIN & CIE, who reported on the progress of their project to develop a gas-gas ceramic heat exchanger for use in exhaust gases of up to 1500 C. Based on the use of silicon carbide, the design is directed at optimising both convective and radiative contributions to heat transfer. Forced convection is maximised by using jet impingement heat transfer within the ceramic perforated plates of the heat exchanger. This permits overall heat transfer coefficients of the order of 150 Wtm2K to be achieved. As a result of a detailed market survey, prime uses have been identified in the metallurgical, ceramics and oil (reformers) industries. f~~q~!_9~~=9~~-h~~!-~~ h~~9~i- While upper operating temperature is one constraint Limiting the application of heat exchangers in heat recovery, installation costs, regardless of operating temperature, are also highly important. The project involving collaboration between the INSTITUT FRANCAIS DU PETROLE (IFP) and CREUSOT-LOIRE is directed at the development of a heat exchanger which will assist to minimise installation costs. IFP has estimated that within the EEC, gas-gas heat recovery could Lead to savings of the order of 2 x 106 toe per annum in industrial sectors (excluding glass metals and textiles). The system under development by IFP, which can accept gases at up to 400 C and could have a duty of typically 650 kw, has size advantages over competing systems and its modular construction should also Lead to Lower installation costs. ~~~Q~I~!~r_f~r-~~ h~~i ~l-~~q~~i-r~ ~~2r~~~i~~-!~Y~2-~t~!~~~ - A project in which the heat exchanger is a small part of a total process is typified by the work at COURTAULDS in the UK on the evaporation of viscose process Liquors. The process involves MVR techniques, in conjunction with the use of thin walled plastic tubes as a heat transfer surface. -48-

5 A significant part of the development programme is associated with the plastic heat exchanger. The short and Long term mechanical stability, resistance to fouling and wettability, are of particular interest. Heat transfer coefficients in the range W/m2K across the tube bundle have been obtained with the viscose Liquors. The evaporation process is directed at the removal of water from the process Liquor, and estimated energy savings are of the order of 5 x 108 kwh per annum. Heat recovery The session on heat recovery discussed three projects (CERCHAR were unable to present information on their project on heat recovery in coke plant). ALL these projects deal with metallurgical industry applications. JAMES HOWDEN are studying the use of rotating regenerators on an aluminium melting furnace, a project which originated during the 1st Energy R & D Programme. BRITISH STEEL CORPORATION and TEKSID (a member of the Fiat Group) are both studying energy conservation by heat recovery in the steel industry, where one of the main difficulties is the identification of suitable uses for the recovered energy. ~~!2!i~g_r~g~~~I2!~I-~~-2~_2l~~i~i~~-f~I~2 ~ - The concept of the rotating regenerator is not new, and it was pointed out by JAMES HOWDEN that about 50,000 such heat exchangers were in use throughout the world. The aim of this project, in which a rotating regenerator would be employed on an aluminium remelting furnace, is to extend the Lungstrom principle to higher temperatures. A maximum of 750 C is specified, giving a preheat temperature of 600 c. This project has been plagued by the recession in the aluminium industry in the UK. Alcoa, the original partner, scrapped the furnaces where the installation was first proposed. Alcan Plate, the new partner, has cut back production due to low demand from the aircraft industry, and this is delaying work on the current contract. ~2~!~-h~2!-~~il~I~-2~-~l~!Ii _2I _f~i~2 ~~ -The iron and steel industry uses a significant proportion of the energy used by industry as a whole, and has received considerable attention from the point of view of energy conservation. Gas-gas heat recovery and pressure recovery techniques are used, but a new field of study is the application of waste heat boilers on electric arc furnaces. It has been estimated that the energy rejected in the exhaust gas from such furnaces could amount to 200 kwh/tonne steel. In this project, undertaken by the BRITISH STEEL CORPORATION, a number of sites have been studied in detail. Demand for steam varies from site to site, in terms of both quantity and application, and one is also faced with an intermittent and variable heat source. A possible use for a waste heat boiler in combination with, or as an alternative to, a scrap preheater has been identified. -49-

6 f2~!-i!2d-~~i!iog_f~!d2 ~~ - TEKSID, a subsidiary of Fiat in Italy, approached a similar problem to that described above in a slightly different manner, opting directly for the generation of electricity as the end product of the recovered energy. Based on using the exhaust from a cupola, via a waste heat boiler, a turbo-generator plant can, it is predicted, generate 87.5 kwh/tonne cast iron produced. A pilot plant, to investigate the thermal aspects, has been constructed and successfully operated. -50-