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District Heating SubstationsPerformance, Operation and Design

Janusz Wollerstrand

Doctoral thesis

ISSN 0282-1990ISRN LUTMDN/TMVK— 1012—SE September 1997

DEPARTMENT OF HEAT AND POWER ENGINEERING DIVISION OF ENERGY ECONOMICS AND PLANNING LUND INSTITUTE OF TECHNOLOGY PO. BOX 118, S-221 00 LUND SWEDEN

District Heating SubstationsPerformance, Operation and Design

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Janusz Wollerstrand

Akademisk avhandlingsom for avlaggande av teknisk doktorsexamen vid tekniska fakulteten vid Lunds Universitet kommer att forsvaras vid offentlig disputation

fredagen den 17 oktober 1997 kl. 1013 i sal M:B, M-huset, Ole Romers vag 1, Lunds Tekniska Hogskola

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OrganizationLUND UNIVERSITY

Document nameDOCTORAL DISSERTATION

Division of Energy Economics and Planning Department of Heat and Power Engineering

Date of issueSeptember 22,1997

Lund Institute of Technology CODEN:ISBN LUTMDN/TMVK--1012--SE

Authorfs)Janusz Wollerstrand

Sponsoring organization

Title and subtitleDistrict Heating Substations. Performance, Operation and Design.

AbstractThis thesis work is concerned with the efficient layout and operation of substations, i.e. those district heating (DH) system units which connect the network and internal building heating systems, such as radiator heating systems and domestic hot water distribution networks. The ambition was to make realistic investigations closely related to practical technologies within the field.

As an introduction, the general state of the art and important results obtained by other authors are presented. Next, seven papers are included. The main topic of papers I-III is the performance of the substations during altered operation, as:

• Varying forward temperature and differential pressure of the DH water• Forced building warm-up in the morning• Reduced water flow rate in the radiator circuit• The stability of the domestic hot water temperature control at varying load• Varying circulation water flow rate in the domestic hot water circuit.Paper IV is a report dealing with the optimum choice of the temperature level in domestic hot water heaters, involving topics

like water quality, corrosion, fouling and bacterial growth. The report also presents the results of advanced laboratory tests and field experiments. Various types of fouling mechanisms as well as preventive steps are discussed, mainly applied to plate heat exchangers in DH substations. General recommendations concerning factors affecting particle fouling, scaling and microbial fouling are given.

Paper VI describes laboratory tests on heat transfer reduction in a small plate heat exchanger while using a new type of drag reducing additive, a zwitterionic surfactant. A static mixer situated immediately before a heat exchanger inlet was found to significantly increase the overall heat transfer coefficient in the plate heat exchangers tested.Papers V and VII describe the question of extreme loads in DH substations, and of design criteria for the substations. A sophisticated method for cutting the total load peaks in the substation at minimum inconvenience to the consumer is addressed. It is shown that extreme-value Gumbel distribution theory could be used for the sizing of new hot water heaters, or for the validation of the sizing of heaters already in operation. Furthermore, simulation of domestic hot water consumption, and the design criteria based on quantile approach are also presented. Simulated design flows obtained according to the criteria are compared with empirical values, and with flows recommended by the Swedish District Heating Association.

KeywordsDistrict heating, substation, performance, design, simulation, heat exchanger fouling, microbial fouling, drag reducing additives, domestic hot water, sizing, heater, extreme value

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Supplementary bibliographical information LanguageEnglish/Swedish

ISSN and key title |ggN 0282-i 990 ISBN

Recipient's notes Number of pages g-j 4 Price

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Distribution by (name and address)Division of Energy Economics and Planning, Lund Institute of Technology Box 118, S-221 00 LUND, SwedenI, the undersigned, being the copyright owner of the abstract of the above-mentioned dissertation, hereby grant to all reference sources permission to publish and disseminate the abstract of the above-mentioned dissertation.

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U(Ljl.Date September 22,1997

District Heating SubstationsPerformance, Operation and Design

Janusz Wollerstrand

Doctoral thesis September 1997

DEPARTMENT OF HEAT AND POWER ENGINEERINGLUND INSTITUTE OF TECHNOLOGYSWEDENhttp:/www.vok.lth.se

©Janusz Wollerstrand 1997

ISSN 0282-1990ISRN LUTMDN/TM VK-1012- SE Printed in Sweden KFS AB Lund 1997

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SummaryThis thesis work is concerned with the efficient layout and operation of substations, i.e. those district heating (DH) system units which connect the network and internal building heating systems, such as radiator heating systems and domestic hot water distribution networks. The ambition was to make realistic investigations closely related to practical technologies within the field. The themes of the papers included were chosen according to their importance from a scientific point of view, but also by taking into account preferences expressed by the research partners.

As an introduction, the general state of the art and important results obtained by other authors are presented. Next, seven papers are included. The main topic of papers im is the performance of the substations during altered operation, as:

• Varying forward temperature and differential pressure of the DH water• Forced building warm-up in the morning• Reduced water flow rate in the radiator circuit• The stability of the domestic hot water temperature control at varying load• Varying circulation water flow rate in the domestic hot water circuit.

Paper IV is a report dealing with the optimum choice of the temperature level in domestic hot water heaters, involving topics like water quality, corrosion, fouling and bacterial growth. Starting with an international literature survey, the report also presents the results of advanced laboratory tests and field experiments. Based on the findings, various types of fouling mechanisms as well as preventive steps are discussed, mainly applied to plate heat exchangers in DH substations. General recommendations concerning factors affecting particle fouling, scaling and microbial fouling are given, bearing in mind medium parameters such as temperature, flow velocity, pH and the concentration of the matter forming the deposit.

Paper VI describes laboratory tests on heat transfer reduction in a small plate heat exchanger while using a new type of drag reducing additive, a zwitterionic surfactant. An important property of the surfactant is said to be that its mycelle chains, which reduce the turbulence of the solution stream near the pipe wall, are very stable. A static mixer situated immediately before a heat exchanger inlet was found to significantly increase the overall heat transfer coefficient in the plate heat exchangers tested.

Papers V and VII describe the question of extreme loads in DH substations, and of design criteria for the substations. A sophisticated method for cutting the total load peaks in the substation at minimum inconvenience to the consumer is addressed. Extreme-value theory is introduced as a tool to handle the prediction of large hot water loads occurring at given, low probability. It is shown that extreme-value Gumbel distribution theory could be used for the sizing of new hot water heaters, or for the validation of the sizing of heaters already in operation. Furthermore, simulation of domestic hot water consumption, and the design criteria based on quantile approach are also presented. Simulated design flows obtained according to the criteria are compared with empirical values, and with flows recommended by the Swedish District Heating Association.

AcknowledgementsMany people active at the Department of Heat and Power Engineering contributed to the results presented in this thesis. I would like to specially thank my supervisor, Docent Svend Frederiksen, for his guidance and for many stimulating discussions throughout the course of this work, and my examiner, Professor Lennart Thomqvist, for valuable advice and support. My thanks should also be addressed to the other members of the Department, both researchers and technical staff, who contributed with scientific criticism, workshop competence, administrative help and social life, or by simply never asking ’’why are you taking so long?”. Especially Dragutin Nikolic, MSc, who provided valuable assistance in laboratory investigations should not be forgotten here.

The studies included in this thesis were mainly financed by NUTEK (The Swedish Board for Industrial and Technical Development) and the Swedish District Heating Association. The work also involved fruitful co-operation with the companies Danfoss A/S, Denmark, AKZO Nobel AB and Cetetherm AB, Sweden, the Malmo Energy Utility, Sweden, and with Department of Mathematical Statistics at Lund Institute of Technology. I would like to express my gratitude to all the people from the organisations mentioned above who promoted carrying out of the thesis.

Finally, I would like to thank my family for accepting me ’’having the thesis as my only hobby” for quite a long time.

Contents

IntroductionPapers included in the thesis.................................................................................................... 7Background............................................................................................................................. 8Scandinavian theses concerning district heating substations.......................................................9District heating substations as the subject of other publications............................................... 10Static and dynamic performance of district heating substations................................................ 12Fouling in plate heat exchangers operating in substations........................................................ 14Effect of drag-reducing additives on heat exchangers.............................................................. 15Sizing of hot water heaters and primary flow limitation in substations..................................... 16References..............................................................................................................................17

Paper IPerformance of District Heating House Station in Altered Operational Modes.15 pp.

Paper IIDistrict Heating House Stations for Optimum Operation.15 pp.

Paper IIIThermostatic Control of Instantaneous Water Heaters in District Heating Substations. 10 pp.

Paper IVFouling in plate heat exchangers for district heating substations.90 pp (in Swedish).

Paper VMaximum and Design Hot Water Loads in District Heating Substations.12 pp.

Paper VIEffect of static mixer on the performance of compact plate heat exchanger operating with zwitterionic type of Drag-reducing additives. 13 pp.

PaperVIIOn Sizing of Domestic Hot Water Heaters of Instantaneous Type.17 pp.

Introduction

Papers included in the thesis

The thesis comprises the following 7 papers:

I. S. Frederiksen, J. Wollerstrand: Performance of District Heating House Station in Altered Operational Modes. 23:rd UNICHAL-congress, Berlin 17-19.6.1987, 15 pp.

n. S. Frederiksen, D. Nikolic, J. Wollerstrand: District Heating House Stations for Optimum Operation. 24:th UNICHAL-congress, Budapest 4-6.6.1991, 15 pp .

ITT. S. Frederiksen, J, Wollerstrand: Thermostatic Control of Instantaneous Water Heaters in District Heating Substations. 5-th International Symposium on Automa­tion of District Heating Systems, 20-23.8.1995, Otaniemi, Espoo, Finland. 10 pp.

IV. S. Frederiksen, J. Wollerstrand: Fouling in plate heat exchangers for district HEATING SUBSTATIONS (FORSMUTSNINGSFORLOPP I PLATTVARMEVAXLARE FOR FJARR- varmeabonnentcentraler). Report, Stiftelsen for varmeteknisk forskning, Stockholm, August 1995, ISSN 0282-3772, 90 pp (in Swedish).

V. L. Arvastson, S. Frederiksen, T.I. Hoel, J. Holst, A. Holtsberg, B. Svensson, J. Woller­strand: Maximum and Design Hot Water Loads in District Heating Substations. 5-th International Symposium on Automation of District Heating Systems, 20-23.8.1995, Otaniemi, Espoo, Finland. 12 pp.

VI. C. Blais, J. Wollerstrand: Effect of static mixer on the performance of compact plate heat exchanger operating with zwitterionic type of Drag-reducing ad­ditives. Report, ISRN LUTMDN/TMVK- -3177- -SE, Dept of Heat & Power Engi­neering, Lund Institute of Technology, June 1997, Lund, Sweden. 13 pp.

VII. L. Arvastson, J. Wollerstrand: On Sizing of Domestic Hot Water Heaters of In­stantaneous Type. 6-th International Symposium on Automation of District Heating Systems, 28-30.8.1997, Reykjavik, Island. 17 pp.

As is indicated by the authorships, each of the papers listed above was the result of the work of more then one person. My co-authors have been my supervisor, Docent S Frederik­sen, and/or other research workers.

In Papers I-TV Frederiksen and I contributed equally with own elements and mutually at numerous discussions. The general thermodynamic framework and the systematic exploration of connecting schemes are mainly derived from Frederiksen, while I carried out most of the computer modelling and experimental verification. In developing new technological solutions the two authors contributed roughly equally. In Paper II the contribution made by D. Nikolic was mainly practical, i.e. carrying out experiments according to a schedule and making graphical presentations of results.

Paper V reports results partly derived by T. Hoel, then a Nordic post-graduate student at Lund. I here co-supervised Hoel and primarily contributed to sections 4 and 5 of the paper.

Paper VI reports the results of experiments carried out in laboratory in co-operation with a large chemical industrial corporation. While C. Blaise mainly contributed with chemical aspects, I designed the test-rig and covered most of the thermo-hydraulical aspects of the

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work. The report was presented at the IDHCA congress, June 17-19, 1996 in the USA as a paper entitled Drag Reduction in District Heating and Cooling Circuits - Temporary Disrup­tion of Micelles to Preserve the Heat Exchanger Capacity. Main results from the investiga­tion were also presented in a paper entitled Drag Reduction by N-Alkylbetaines - A Type of Zwitterionic Surfactants presented at the Asme Fluids Engineering Division Summer Meeting, July 7-11, 1996, also in the USA. Both papers were authored by Caroline Blais, Ian Harwigsson, Martin Hellsten and myself.

Finally, Paper VII was the result of equal contributions from L. Arvastson and myself. However, I would like to give full credit to L. Arvastson for ideas described in appendices A.2 and C.

BackgroundDistrict Heating (DH) systems connect many buildings to a single or a few large heat- producing plants through a network, usually installed underground and employing pressurised hot water as the heat-carrying medium. Provided a favourable organisational framework ex­ists, DH systems allow the economical use of a number heat-production technologies which are beneficial in terms of rational use of primary energy and low environmental impact. One important technology of this kind is co-generation of heat end electricity.

DH technology is employed to greatly varying degrees in different countries, due not only to obvious reasons related to climatic differences, but also because of differences in en­ergy policies. In several Scandinavian countries, among them Sweden, DH enjoys a high share of the total building heating market. Therefore, a common organisation co-ordinating research in the field of DH was established in 1985 by the Nordic Council of Ministers.

Some of the central issues in the development of DH technology are:• Cheaper distribution technology. DH systems distribute rather low-grade energy and

therefore inherently represents a rather expensive technology, compared with competing en­ergy distribution technologies, such as natural gas or electricity.

• Lower network distribution temperatures. Lowering of network distribution tempera­tures may in itself facilitate the employment of cheaper distribution mains. Also, lower tem­peratures are associated with thermodynamic benefits, both in the sense of the first and the second laws of thermodynamics.

• More reliable operation. Generally, DH is today a reliable technology. Nevertheless, substations and other parts of the system may operate less reliably. This is the case, not least, when technological goals are pushed to their limits.

Several topics concerning subscriber substations (house stations) and related subjects have been investigated at the Department of Heat and Power Engineering at Lund Institute of Technology (LTH), Sweden, in co-operation with the Swedish District Heating Association, selected members of the association, as well as industrial partners.

This thesis work, which was carried out at the Department, is concerned with the effi­cient layout and operation of substations, i.e. those DH system units which connect the net­work and internal building heating systems, such as radiator heating systems and domestic hot water distribution networks. The ambition was to make realistic investigations closely related to practical technologies within the field. However, the subject comprises too many topics to be covered in detail by a single researcher. Only a subset of topics was therefore treated here. The themes of the papers included in the thesis were chosen according to their

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importance from a scientific point of view, but also by taking into account preferences ex­pressed by our research partners.

Before presenting the papers, the general state of the art and important results obtained by other authors are presented. This account is divided into two parts. Firstly, Scandinavian theses are presented, followed by other publications.

Scandinavian theses concerning district heating substationsDuring the last decade, a number of theses dealing with DH system technology and economy, as well as other closely related topics, have been published. Some of them concentrate on DH substations as their main subject. An important goal in such investigations is to study differ­ent connecting schemes for the substations and to compare their performance. A common attribute of these studies is that dynamic simulation of the substations is employed. The simulation models of the components considered, are mathematical descriptions based on known physical relations, while the lumping technique is used to simulate heat exchange. The models are verified by comparison with field and/or laboratory measurements.

Four doctoral theses are most relevant. In his thesis published 1989, Gummerus [1] de­veloped models of DH substation configurations common in Sweden. He took into account different types of components available on the market (such as heat exchangers, control valves), and designed component models according to the geometric and thermodynamic knowledge available about them. He also analysed typical load variations in radiator circuits and in domestic hot water circuits of the substations utilised in residential buildings, and de­veloped a simulation model for domestic hot water consumption based on the work of Holm- berg [2], In the next step, he defined the performance of a DH substation as the flow-rate- weighted cooling of primary water flowing through the substation, and investigated the per­formance of different connecting schemes (parallel, 2-stage and 3-stage connection) on an average annual basis. The domestic hot water circuit employed was of the instantaneous type. The simulation package developed within the scope of Gummerus’ thesis was later improved and completed. Today, a user-friendly version of the package is available on a commercial basis.

Another doctoral thesis in this field was published by Hjorthol in 1990 [3]. Using similar modelling methods to those of Gummerus, Hjorthol focused his work on the behaviour of control circuits in DH substations at different loads and, more important, upon sudden changes of the load. This kind of operation is typical for domestic hot water circuits, and Hjorthol performed detailed investigations of the dynamics of such a circuit, starting with its components. Different types of temperature sensors, control valves, valve actuators and con­trollers were investigated, as well as the influence of thermal parameter variation of the pri­mary water on the stability of the control loop. He found that hot water circuits were difficult to control and recommended the use of PID controllers, with carefully designed control algo­rithms in such systems.

Domestic hot water heaters with storage tanks intended to serve multi-family houses (e.g. apartment blocks) are uncommon in most Scandinavian countries, and were therefore not of primary interest to Gummerus and Hjorthol. Devices of this type are however in frequent use in other countries, including Denmark, and are the topic of a thesis by Libing Yang [4] from 1994. In her work, the modelling of DH substations was extended by component models of two types of domestic hot water storage tanks - one with an internal heating coil and one with an external heat exchanger. Using these models, the hot water capacity of a given storage tank as a function of system parameters, heat losses, hot water circulation flow rate, fouling of the

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heating coil, etc. was investigated as well as the average cooling of primary water. Four types of DH substations with hot water storage were simulated. The substations were of parallel and 2-stage type, while the hot water storage modules were externally loaded or of once- through type. As a result of simulations and field measurements it was found that 2-stage substations with an externally loaded storage tank performed best, but the substation with the internal coil was still thermodynamically acceptable and preferable from an economical point of view.

In the theses mentioned above the heating of ventilation air was not taken into consid­eration. In his doctoral thesis published in 1996, Volla [5] further developed a simulation model presented by Hjorthol, by adding modules simulating air heating circuits. The new model was used to investigate how new connecting schemes involving air heating modules could improve the cooling of DH water. In particular, a substation characterised by serial connection of radiator heating and air heating circuits was studied. Simulations showed that the performance of this substation was better than that of a conventional solution where cir­cuits mentioned above were connected in parallel. Volla also stated that the advantage of 2- stage substations, in terms of primary water cooling, strongly depends on temperature levels in the system and on hot water consumption. To make realistic simulations of the behaviour of DH substations serving occupational buildings possible, he also analysed field measure­ments performed in a number of hospitals and office buildings in Norway. His conclusion was that more restrictive sizing of hot water circuits would improve the stability of control loops without significant inconvenience to the consumer. Additional savings of heat ex­changer area could be made in the circuits if a peak load hot water storage tank was included.

District heating substations as the subject of other publicationsIn addition to the doctoral theses studied some licentiate* dissertations and other scientific publications should be also mentioned.

In his dissertation [6], Winberg described laboratory tests and simulations of domestic hot water heaters with a storage tank. Similar work concerning heaters of the instantaneous type was performed by the present author, Wollerstrand [7] (the contents and results of this dissertation are summarised later on).

Raberger, in her dissertation[8], developed a method of identifying possible reasons for the poor cooling of primary water in DH substations. By combining field measurements with simulations, she was also able to suggest improvements, and confirm whether the modifica­tions suggested by the results of simulation gave the expected effect or not.

Simulation of DH substations connected to various building internal circuits and new ideas regarding optimum connection schemes have been described in IEA reports by Hjorthol et al. [9] and Volla et al. [10]. A systematic presentation of connecting schemes, control strategies and components frequently used in DH substations has been published by Frederik- sen et al. [11].

Finally a great deal of information on DH substation technology and related topics can be found in handbooks published by ASHRAE [12] and Hakansson [13].

Periodicals published by the European association of heat distributors - Euroheat & Power (former: Unichal), and in co-operation with the German DH association - AGFW (Arbeitsgemeinschaft Femwarme), provides a valuable source of information about the state

* Licentiate degree results from at least 2 years’ postgraduate study and is between MSc and a PhD

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of the art in the field of DH technology. In these periodicals, titled Euroheat & Power - Fem- varme International (FWI) and Euroheat & Power Yearbook, articles on DH technology in mainly Germany, in Scandinavia, and, to some extent, in eastern Europe, can be found. The contents include articles written by single authors, as well as periodic reports published by members of Studying Committees of Euroheat & Power and summaries from congresses arranged by the association. During the years, problems concerning DH substations have fre­quently been addressed. Some facts found in the periodicals, published between 1972 and 1996, will be commented upon briefly below.

Unlike Scandinavian countries, in Germany DH substations, or house stations (German: Hausstation), are often subdivided into main modules - the supplier’s to consumer’s ’’transmission station” (German: Ubergabestation) and the ’’house substation” (German: Hauszentrale). This division separates not only different types of functionality of DH substa­tions, but also legal responsibilities. The transmission station, belonging to the heat supplier, includes devices controlling the parameters of DH water supplied to the customer, e.g. supply pressure, difference pressure and flow-rate limiting as well as accounting devices, while the house substation, belonging to the house owner, includes all devices necessary to manage space heating and domestic water heating circuits, especially heat exchangers, circulation pumps and control equipment. The domestic water heater is often, although not always, treated as a separate module served by the house station.

This configuration dominated in new DH substations in Germany at the beginning of the 1970s, and was not the subject of further development as such. However, improvements and integration of components used in the transmission station were expected, such as hybrid pressure reduction and safety valves, and integrated difference pressure controllers and flow rate limiters [14]. The question of improved cooling of primary water was raised, and differ­ent connecting schemes for domestic hot water heaters were investigated. Apart from solu­tions frequently utilised in Scandinavia, such as 3-stage, 2-stage and parallel connection with instantaneous-type water heaters, several schemes with storage tanks were analysed. Ac­cording to Hollander [15], only the schemes employing preheating of domestic hot water by primary return water from space heating circuits were characterised as "DH friendly”. Several schemes of this type, both with and without storage tanks were then approved and recom­mended by Unichal.

About ten years later, higher energy prices, together with new manufacturing technolo­gies paved the way for new ideas in the design of DH substations. Frank [16], estimated that integration of transmission and house station modules into compact, industrially prefabri­cated units, could, in the case of small substations manufactured in Germany (design load < 200 kW), result in a 50% lowering of its costs. It was assumed that domestic hot water heat­ers of the instantaneous type, were fully integrated into the substation. Moreover, it was pointed out that prefabricated compact substations were, at this time, already well established on the Scandinavian market. It was also suggested that domestic hot water heaters with stor­age tanks, then being dominant on the German market, should be restricted only to large in­stallations in the future, due to the higher specific cost of such heaters, and when high pri­mary water cooling was demanded. Reduced temperature levels in space heating circuits were also forecasted.

At the beginning of the 1990s the upper limit for compact substations was increased to design load < 500 kW. In a typical DH system in Germany, 70-80% of the substations in­stalled was of the compact type. Combined difference pressure controllers and flow-rate lim­iters/controllers became established products [17], and the integration of remote control and accounting data_acquisition systems with local control equipment was introduced [18]. The

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new design of the substations, and the reliability of the components included meant that, in most of cases, the use of separate ’’transmission stations” in the substations became expensive and obsolete [19]. The development above was accentuated by the need to renovate DH sys­tems in the eastern part of Germany.

The progress towards more compact and sophisticated DH substations is continuing to­day. The common use of compact plate heat exchangers together with increased domestic hot water temperatures of 55-60°C (because of the risk of bacterial growth), has called for faster and more accurate temperature control in the water heaters. According to considerations of Braunig, Zschemig and Brachetti [20, 21], money and space savings could be gained, not only by avoiding the use of storage tanks in the substations, but also by appropriate sizing of the heaters, which are often oversized by up to 80-90%, or in some cases up to 200%. Sig­nificantly decreased space heating load to domestic hot water load ratio, has became another important concern today. This implies that the specific size of the substation related to the size of the building served will decrease.

The trend towards a decrease in the size of DH substations is illustrated in Fig. 1. The specific weight and space demand of four generations of prefabricated substations, as a func­tion of the number of apartments connected, is exemplified in the figure. The diagrams show that the weight of the substations has, in general, decreased much more than the correspond­ing space demand. The curves were compiled from data sheets obtained from a large manu­facturer of plate heat exchangers [22]. The substations compared were all of the 2-stage type, available on the Swedish market from 1970-1996. Text in the diagrams indicates which year the respective design of substation first appeared on the market.

0 100 150No. of apartments

0 100 1£

No. of apartments

Figure 1 Development of the specific space demand and weight of prefabricated DH substations of 2-stage type, during the period 1970-1996. Based on reference [22].

Static and dynamic performance of district heating substationsAn important factor affecting the temperature levels in the DH system is the performance of its substations. The performance of a particular substation depends on its design as well as on the thermal parameters of the working medium, on the DH and consumer sides of the substa­tion. In Sweden, three main connecting schemes for substations are used: parallel-, 2-stage and 3-stage. The substations are usually equipped with domestic hot water heaters of the in­stantaneous type. The ability of the substations to cool the primary water at varying loads was extensively discussed during 1980s. Both the stationary and the dynamic behaviour of the substations was considered.

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The ability to carry out research on substations has been greatly extended by the rapid development of microprocessors and of data communication technology during recent dec­ades. New tools for monitoring and controlling DH plants, networks, substations and their components have become available. The widespread usage of computers has made simulation a common tool in the design and monitoring of DH systems. At the same time, new, cheap and accurate sensors have simplified data acquisition, both in the field and in test-rigs. This has extended the potential for on-line monitoring of systems, and of validation of results of calculations and simulations.

The tools mentioned above have been extensively used by the author in earlier studies which formed his licentiate thesis, ”District heating substations with tap water heaters of instantaneous type” [7],

In my licentiate dissertation, the substations of parallel- and 2-stage types were investi­gated both theoretically and through field measurements. Sizing of the substations'and their performance, in terms of primary water cooling, at different load conditions was considered. In a field test, a DH substation of 2-stage type operating in a residential building, adapted so that a temporary change-over to parallel connection was possible, was investigated. The re­sults of calculations and field measurements were mainly in good agreement, after an unin­tended connection and suboptimal operation of the field substation had been identified and corrected. The study was later complemented by comprehensive laboratory tests of substa­tions, including connecting schemes of the 3-stage type. The substations were tested and compared within their full operating range with regard to both stationary and dynamic per­formance. A substation of 2-stage type was found to perform slightly better than one of 3- stage type, and significantly better than a parallel type.

Experiences from the field and laboratory tests mentioned above, as well as topics dis­cussed at that time among professional engineers, indicated that the problem of accurate, fast, stable and cheap control of domestic hot water heating circuits in DH substations had not been solved satisfactorily. Therefore, an extensive investigation of the design and properties of a thermostatic control valve, type AVTQ, was carried out by a research team at the De­partment in co-operation with the manufacturer, Danfoss A/S. This work was described in the second part of my licentiate thesis. Theoretical investigations and laboratory tests resulted in a number of recommendations on how to improve the design of the valve, especially consid­ering the implementation of a feed-forward control loop. I contributed to the work of the team by developing a detailed dynamic model of the valve. In addition, I developed dynamic models for plate heat exchangers. The valve model, combined with the heat exchanger model, was then used by me to simulate the behaviour of a domestic water heater. A comparison of laboratory tests and computer simulations of the heater showed good agreement. This work was reported as the last part in my licentiate dissertation, together with a description of the method of heat exchanger model design. A description of the procedure of optimising the parameters of the models was also included.

In my licentiate dissertation, the performance of DH substations during normal operation was analysed. The main topic of the first three papers included in this doctoral thesis is the performance of the substations during altered operation. The following types of changes in operation were considered in the first paper [Paper I]:

• Lowered forward temperature of the DH water

• Forced building warm-up in the morning

• Reduced water flow rate in the radiator circuit

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In all the cases studied, the primary water return temperature increased, assuming un­modified heat exchangers in the DH substation. Therefore, the possible need to increase the heat transfer area, as well as ways to avoid forced building warm-up, are pointed out in the paper. In the case of the reduced flow rate in the radiator circuit, the optimisation of the mag­nitude of the flow rate at different loads resulting in minimum primary return temperature is discussed. The proper choice of heat transfer coefficients in tire design of the heat exchangers used is also considered to be of great importance in the latter case.

In the second paper [Paper II] a theoretical investigation of the performance of the DH substations, in terms of primary water cooling, combined with systematic laboratory tests of the substations, is described. The tests include extensive investigation of the stability of do­mestic hot water temperature control at varying load. The tests were performed at varying conditions, both on the primary and secondary water side - primary water forward tempera­ture and differential pressure were varied, as well as domestic hot water circulation flow rate. The conclusion drawn from the tests is that 2- and 3-stage connecting schemes allow a high degree of cooling, but great care is necessary in designing control systems.

The results of a comparison between the dynamics of two generations of a self-acting thermostatic control valve are described in the third paper [Paper ID]. The valve, called AVTQ, is widely used in small instantaneous hot water heaters. A main feature of the valve is considered to be the supplementary use of a feed-forward loop, in addition to the feedback loop, to speed up the control response to rapid changes of the load. It is also stated that the latest generation of the valve, equipped with adjustable gain in the feed-forward loop, signifi­cantly reduces the steady-state error of the valve. The laboratory tests also show improved control stability at moderate loads, thanks to a split-range gain facility in the proportional loop. Apart from a description of the laboratory tests, the paper contains comments on the improved performance of the valve. An important result of the development of the valve is that overheating of heat exchange surfaces in the heat exchanger controlled by the valve, is avoided. The work was carried out in co-operation with the manufacturer of the valve, Dan- foss A/S.

Fouling in plate heat exchangers operating in substationsThe temperature levels found in DH substations depend mainly on the design of internal con­sumer installations in the buildings served. In general, the design temperatures of space heat­ing circuits in newly constructed buildings tends to decrease. A similar trend has been ob­served in the design of domestic hot water systems in Scandinavia which, in Sweden, resulted in the demand on forward temperature of the water decreasing from 55 to 45 °C during the 1980s. However, results of research concerning bacterial growth in hot water systems caused a subsequent revision of this practice. According to the recommendations of the Swedish District Heating Association, the set-point for domestic hot water temperature is currently 55- 60°C. This illustrates that the optimum choice of temperature, from all points of view, is a complicated matter, involving, as well as heat loss reduction and optimum conditions for co­generation, issues concerned with water quality, corrosion, fouling and bacterial growth.

The fourth publication included in the thesis, [Paper IV], is a report dealing with the topics mentioned above. Starting with an international literature survey, the report also pres­ents the results of advanced laboratory tests and field experiments. Based on the findings, various types of fouling mechanisms as well as preventive steps are discussed, mainly applied to plate heat exchangers in DH substations. General recommendations concerning factors affecting particle fouling, scaling and microbial fouling are given, bearing in mind medium

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parameters such as temperature, flow velocity, pH and the concentration of the matter form­ing the deposit.

The report is focused on plate heat exchangers, as their fouling resistance in the field has still not been widely verified. This applies especially to small, brazed exchangers as they are relatively new in DH applications. Results of endoscopic inspection and performance testing involving chemical cleaning of a number of brazed heat exchangers, described in the report, show only small or moderate fouling. The heat exchangers tested were collected from various Swedish DH networks.

Furthermore, plate heat exchangers with gaskets have been reported to be more sensitive to bacterial fouling than brazed heat exchangers. A chemical-microbiological analysis carried out on samples of gasket and steel plate surface shows that, on average, ten times higher mi­crobial activity can be found on the gaskets. This applies especially to older gaskets made of less thermostable types of rubber.

Effect of drag-reducing additives on heat exchangersTo reduce pumping costs in the networks, efforts have been made to develop Drag Reducing Additive (DRA) for use in DH water. Although this development has far to go, a number of test installations are reaping benefits of over 50% reduction in pressure drop in transit pipe­lines thanks DRA.

Apart from the cost of DRA and environmental considerations, a serious drawback of DRA has been a decrease in heat transfer in heat exchangers. Paper VI in this thesis describes laboratory tests on heat transfer reduction in a small plate heat exchanger while using a new type of DRA. The new DRA tested is reported to be a zwitterionic surfactant. The drag- reducing action of the surfactant is analogous to that of the previous DRA:s, but an important difference is said to be that its mycelle chains, which reduce the turbulence of the solution stream near the pipe wall, are very stable.

It should be borne in mind that if a DRA-containing flow is exposed to turbulence, in piping elbows, in valves or in heat exchanger channels, the mycelle chains may be disrupted and the DRA effect lost. This phenomenon counteracts the reduction in heat transfer in heat exchangers, which is convenient, but on the other hand, repetitious chain damage can cause chemical degradation of the DRA. This has previously been a problem with older types of DRA. Besides chemical stability, the DRA tested here is reported to be non-toxic and envi­ronmentally friendly, according to the manufacturer.

A heat transfer coefficient reduction of 11-21% is reported in the paper. In addition to the performance tests, an idea for increasing heat transfer when using a DRA solution is de­scribed. A static mixer situated immediately before a heat exchanger inlet was found to sig­nificantly increase the overall heat transfer coefficient in the plate heat exchangers tested. In particular, heat transfer of the same magnitude as for pure water was found at large Reynolds numbers. A heat transfer coefficient reduction of 4-17% was found at low Reynolds numbers. The tests were conducted in co-operation with the company Akzo Nobel AB.

In my opinion, regular usage of DRA:s in large DH systems seems not to be recommend- able yet because of:

• the large cost of the additive• the unpredictable effect of accidentally polluted DH water on the chemical stability of

the DRA, and

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• the unpredictable effect of DRA on heat transfer in older types of heat exchangers.However, I believe that DRA-containing water will become to be a very interesting

choice in limited applications, such as:• long-distance transfer of heat energy• specially designed, local systems, e.g. in sparsely populated areas• exceptional addition of DRA to DH water in peak load cases to avoid oversizing of

pipes and pumps; this could also apply to large systems• an agent working as both inhibitor of corrosion and as a DRA.

Sizing of hot water heaters and primary flow limitation in substationsThe variation in heat demand in DH systems depends primarily on changes in space heating load which, in turn, depends mainly on the outdoor climate conditions. However, the influ­ence of domestic hot water consumption must be taken into account in the summer-time and during peak load periods (early morning and at night). Load prediction is necessary in order to manage a particular system in an optimal way. This involves the choice of suitable set of heat sources which are available to the system, as well as the choice of the forward tempera­ture of DH water, e.g. if storage of heat energy in the network to meet peak loads is required.

Most common methods of heat load prediction for whole networks are based on time- series analysis. Prognoses are made utilising heat load statistics and climate data, in some cases combined with current parameter values measured at selected points in the network. In those cases, the accuracy in short-term prognosis is rapidly increasing.

The accurate prognosis of the heat energy transferred in a single DH substation is a rather complicated task due to the stochastic nature of domestic hot water consumption, etc. Fortu­nately, this kind of prognosis is of minor interest to heating companies. On the other hand, load profiles equivalent to the load which occurs in a specific building or set of buildings, are of great importance for the suitable design of the substation. At present, heat exchangers and control valves in substations are often oversized. Therefore, there is a need for tools permit­ting static and dynamic simulations of DH substations under realistic conditions, to obtain optimal overall performance of the substation and the appropriate sizes of the components therein. In this case, realistic simulation of the load resulting from domestic hot water con­sumption is of great interest.

Papers V and VTt in this thesis describe efforts to clarify the question of extreme loads in DH substations, and of extreme hot water loads in particular. In paper V, extreme total loads in the substations and possible design criteria for the substations are investigated. Extreme- value theory is introduced as a tool to handle the prediction of large hot water loads occurring at given, low probability. It is shown that a single field dataset describing daily peak loads of domestic hot water follows well an extreme-value Gumbel distribution. This leads to the con­clusion that Gumbel distribution theory could be used for the sizing of new hot water heaters, or for the validation of the sizing of heaters already in operation. Furthermore, the sophisti­cated interaction between space heating and domestic water heating circuits of a DH substa­tion is addressed. The goal of such an interaction is defined as cutting the total load peaks in the substation at minimum inconvenience to the consumer.

Paper VII, focuses on domestic hot water consumption in terms of consumer behaviour based on field measurements and simulations, and on design criteria for water heaters. Na­tional practice and design recommendations in different countries concerning instantaneous

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water heaters are described. A hot water consumption pattern measured in the field is shown together with a corresponding pattern obtained by simulation. A description of the simulation model is appended. A method of calculating design flow for residential buildings based on the normal approximation, used in Scandinavia, is described and analysed in detail. Draw­backs of the method are exposed and improvements are suggested. Design criteria based on a quantile approach and on the extreme value approach (Gumbel distribution) are also pro­posed. Simulated design flows obtained according to the criteria are compared with empirical values, and with flows recommended by the Swedish District Heating Association.

References

1. P. Gummerus: Analys av konventionella abonnentcentraler i fjarrvarmesystem. PhD Thesis. ISBN 91-7032-467-0. Chalmers Institute of Technology, Dept, of Energy Technology, Gothenburg, Sweden, 1989 (in Swedish).

2. S. Holmberg: Flow Rates and Power Requirements in the Design of Water Services. Tekniska Meddelanden 316 1987:2. Phd Thesis. Dept, of Heating and Ventilation Technology, Royal Institute of Technology, Stockholm, Sweden, 1997.

3. E.M. Hjorthol: Optimization of Design Values in District Heating Substations by System Simulation. PhD Thesis. Norwegian Institute of Technology, Dept, of Heating and Ventilating, Trondheim, Norway, 1990.

4. L. Yang: District Heating House Stations with Hot Water Storage. Simulation and Evaluation of Dynamic Performance. Technical University of Denmark, Centre for District Heating Technology, Laboratory of Heating and Air Conditioning, Lyngby, Denmark, 1994.

5. R. Volla: Consumer Heating Systems for District Heating - Development by System Simulations and Service Hot Water Measurements. PhD Thesis. Norwegian University of Science and Technology, Dept, of Refrigeration and Air Conditioning, Trondheim, Norway, 1996.

6. J. Winberg: On Hot Water Stoprage in District Heating Subscriber Stations. Sys­tem Measurements. Licentiate Dissertation. Dept, of Heat and Power Engineering, Lund Institute of Technology, Lund, Sweden, 1992.

7. J. Wollerstrand: FjArrvarme-abonnentcentraler med genomstromningsberedare. Licentiate Dissertation. Dept, of Heat and Power Engineering, Lund Institute of Tech­nology, Lund, Sweden, 1993 (in Swedish).

8. L. Raberger: Effektwisering av abonnentcentraler i fjarrvarmenat. ISBN 91- 7197-229-3. Licentiate Dissertation. Chalmers Institute of Technology, Dept, of Energy Technology, Gothenburg, Sweden, 1995 (in Swedish).

9. E.M. Hjorthol, R. Ulseth: Consumer Heating System Simulation (CHESS). Interna­tional Energy Agency - District Heating and Cooling Project, Annex HI, Report 1992:P5, Novem, Sittard, 1992.

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10. R. Volla, R. Ulseth, J. Slang, S. Frederiksen, A. Johnson, R. Besant: Efficient SUB­STATIONS AND INSTALLATIONS. ISBN 90-72130-88-X. International Energy Agency - Dis­trict Heating and Cooling Project, Annex IV, Report 1996.N5, Novem, Sittard, 1996.

11. S. Frederiksen, S. Werner: District heating handbook (Fjarrvarme. Teori, teknik och funktion). ISBN 91-44-38011-9. Studentliteratur, Lund, Sweden, 1993 (in Swedish).

12. ASHRAE Handbooks: ASHRAE Handbook - Fundsamentals (1993); Heating, Ventilating and Air-Conditioning. Applications (1995). American Society for Heat­ing, Refrigerating and Air-Conditioning Engineering, Atlanta, USA.

13. K. Hakansson: Handbuch der Fernvarme Praxis. ISBN 3-8027-2514-X. Vulkan- Verlag, Essen, 1986.

14. G. Fauser: Entwicklungstendenzen bei Hausstationen. Fernvarme international - FWI, Jg. 2 (1973), vol.l, pp.10-12.

15. W. Hollander: Brauchwasserwarmung mtt Fernwarme. Fernvarme international - FWI, Jg. 2 (1973), vol.6, pp. 139-146.

16. W. Frank: Kompakt-Hausstationen fur kleinere Gebaaudeeinheiten. Fernvarme international - FWI, Jg. 10 (1981), vol. 2, pp. 52-57.

17. H. Schwaiger: Optimale massenvariable Regelung von Fernwarmeausstationen. Fernvarme international - FWI, Jg. 21 (1992), vol. 10, pp. 490-495.

18. F-J. Loch, D. Magar, R. Trautman: Kompaktstationen - Stand der Entvicklung. Jahrbuch Fernvarme international 1992, pp. 177-180.

19. F. Schmitt, H-J. Dausch: Neuere technische Entwicklungen und Optimierungs- tendenzen IM Bereich der Hausstationen und Kundenanlagen. Fernvarme interna­tional - FWI, Jg. 21 (1992), vol.4/5, pp. 159-168.

20. K-U. Braunig, J. Zschemig: Sanierung der Fernwarme in den neuen Bundeslandern - CHANCEN FUR DIE WEITERENTVICKLUNG VON Ha USANSCHLUfiSTATIONEN. EUROHEAT & Power - Fernvarme international, Jg. 25 (1996), vol.9, pp. 506-518.

21. H.E. Brachetti: Progressive Konzepte fur Hausanschlusstationen der Fernvarme- versorgung. Euroheat & Power - Fernvarme international, Jg. 25 (1996), vol. 10, pp. 572-587.

22. Technical information sheets from the Alfa Laval company, and personal communica­tion to Stefan Carlstrom, Alfa Laval Industri AB, Lund, Sweden, August 1997.

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