Hydronic Heating Systems

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THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

The Effect Of Design On System Sensitivity

Anders Trüschel

Department of Building Services Engineering Chalmers University of Technology

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Hydronic Heating Systems

The Effect Of Design On System Sensitivity

Anders Trüschel ISBN: 91-7291-175-1 © Anders Trüschel 2002 Second edition

Doktorsavhandlingar vid Chalmers Tekniska Högskola Ny serie nr 1857

ISSN 0346-718X Document: D62:2002

SE-412 96 Göteborg Sweden

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Hydronic Heating Systems

The Effect Of Design On System Sensitivity

Anders Trüschel

ABSTRACT

This thesis starts from the recognition that a hydronic heating system can be optimised, but can never be totally perfect. Sooner or later, in practice, deviations - caused by one or more components having slightly different characteristics or settings than they are assumed or supposed to have - arise. The aim of this work is to show how system design affects the overall sensitivity to deviations, in terms of the effect on performance and return water temperature. The systems that have been analysed are radiator systems and air heaters controlled by valve groups, both supplied by heat from district heating. In particular, the analysis has been concentrated on differences between high-flow and low-flow systems.

Based on fundamental theory in this area, as well as on physical measurements made in test rigs, models have been developed and/or applied in order to investigate system function in the desired manner. The effect of deviations have then been shown and quantified, using results from simulations.

The simulations show that thermostatic radiator valves are most effective in low-flow systems. Low-flow systems, too, produce the lowest return temperatures. However, incorrect or changed radiator valve settings can result in substantial increased return temperature and differences in room temperatures in such systems.

A direct connection of an air heater (that is without recirculation) presents the least risk of control instability, which means that performance tends to be more stable. In this respect, there is no difference whether the system is balanced for a high flow or a low flow. However, balancing does have a considerable effect on the control performance of valve groups with a recirculation connection and with low-flow systems running a greater risk of instability.

Keywords

Hydronic Heating, District Heating, Radiator System, Air Heater, Heating Coil, Valve Group, Shunt Group, Water Return Temperature, Room Temperature, Thermal Power Output, Controllability, P-band, Deviations

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This thesis refers to the research project “Heating systems in buildings” by The Swedish District Heating Association and the research grant no. 960493-5 from BFR (Swedish Council for Building Research, now FORMAS).

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PREFACE

The work described in this thesis has been carried out at the Department of Buildings Services Systems at Chalmers University of Technology, as part of the Department's aim to improve detailed knowledge of hydronic heating and cooling systems. I would therefore like to tender my special thanks to the Department and its personnel for providing the opportunity of the work, as well as for the resulting valuable and interesting time for me as a PhD candidate.

In particular, my thanks are due to my supervisor, Stefan Aronsson, whose help during the work has been invaluable. In addition, I am most grateful for the support and interest of Associate Professor Jan-Olof Dahlenbäck, Professor Per Fahlén and Professor Emeritus Enno Abel.

Warm thanks, too, to Tommy Sundström and Josef Jarosz for all their help with the test rigs, as well as to Magnus Thordmark of IMI Indoor Climate AB and Per-Göran

Persson of TA Control AB for their assistance with the provision of equipment. I would also like to thank the Monitoring Centre for Energy Research at Chalmers, which made the measurements in the Bankogatan properties, as well, of course, as Familjebostäder and particularly Yngve Andersson for their assistance in connection with these measurements.

Another person not to be forgotten is Neil Muir, who translated the material.

Finally, I would like to thank all members of the reference group that has monitored the work, and particularly Lennart Berndtsson from HSB who has been responsible for the organisation.

Gothenburg, May 2002

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NOMENCLATURE

Capital letters

A = Area [m²]

C = Thermal capacity flow [W/K] V

Q

D_& _& = Radiator sensitivity [-] H = Valve opening [%] s K = System gain [%/°C] r K = Controller gain [%/ºC] rad K = Radiator constant [W/Kn] L = Length [m] M = Thermal capacity [J/K] N = Number (quantity) [-]

NTU = Number of Transfer Units (measure of a heat exchanger’s size related to its flow) [-]

Nu = Nusselts number [-]

Q& = Thermal power output [W]

R = Relationship between the heat capacity flows for water and air through an air heater [-]

Re = Reynolds number [-] d

T = Dead time [s] k

T = Time constant [s]

U = Coefficient of thermal transfer [W/m²K] V& = Volume flow [m³/s]

Lower-case letters

c = Velocity [m/s] p

c = Specific thermal capacity [J/kgK] d = Diameter [m]

e = Control error (difference in temperature) [°C] v

k = Valve capacity [m³/h] vs

k = Maximum valve capacity [m³/h]

k = Flow resistance [kPa/(l/h)² ; kPa/(l/h)] or Roughness of pipe [mm] m_{& = Mass}flow [kg/s]

n = Radiator exponent [-] p = Pressure [Pa]

t = Temperature [°C]

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Greek symbols

α = Coefficient of thermal transmittance [W/m²K] β = Valve authority [-]

λ = Coefficient of friction [-] or Thermal conductivity [W/mK] ϕ = Relationship between the waterflow through a control valve and

the waterflow through the corresponding air heater [-] p

∆ = Differential pressure, Pressure drop [Pa] t

∆ , T∆ = Difference in temperature, Change in temperature [°C] η = Efficiency [-]

ρ = Density [kg/m³] τ = Time [s]

ν = Kinematic viscosity [m²/s]

Index

A = Control port for a control valve B = Shunt port for a control valve BV = Check valve

C = Constant-flow port for a control valve H = Heat releasing component

R = Control valve, Controlling T = Total

0 = Reference index

w = Media index for liquid (water) a = Media index for air

s = Shunt group (i.e. valve group) rad = Radiator

i = Arbitrary index

in = Inlet, Incoming (temperature or media) out = Outgoing, Outdoor (temperature or media) intern = Internal

fully closed = Valve opening 0 % fully open = Valve opening 100 % crit = Critical nec = Necessary balanced = Balanced design = Design nom = Nominal room = Room

room, mean = Mean value for a certain number of rooms return = Return (temperature)

system = System m = Mean value

am = Arithmetic mean value lm = Logarithmic mean value min = Minimum, Least

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1 INTRODUCTION

1.1 Background

Hydronic heating systems in buildings are designed so that they can maintain a desired indoor temperature, which means that the physical conditions determining the design are more or less given. In addition, if the heating system is supplied (or will be supplied) by district heating, it is also important to ensure that the system provides a low return temperature of the district heating water.

When the system is started up, its function and performance will depend on its actual design and on the actual conditions, which often introduce greater or lesser deviations from the parameters that were assumed when designing the system. As used here, “deviations” mean that one or more components have characteristics other than as were originally assumed. Examples of such deviations are oversizing of radiators, poor system balancing, incorrectly operated control valves and so on. These deviations can exist even before the system has been taken into use. In addition, there can be constant changes to the system due to such effects as modifications by the building owners, adjustments or damage caused by users and natural wear and tear, all of which gradually (or otherwise) give rise to deviations.

This research project was initiated in order to investigate how an hydronic heating system should and should not be designed in order to ensure its proper function and performance when subjected to a range of deviations.

1.2 Purpose

The purpose of this work has been to show how the function and performance of a hydronic heating system is affected by its design. The intention has been to bring forward basis and information from which, in the extent, working methods can be developed for use during the system design stage in order to ensure subsequent stable operational function. The material should show the advantages and drawbacks of various design and balancing principles, in respect of their ability to reduce the effects of deviations. The work has therefore been intended to contribute to improving the ability to forecast the effects and characteristics of a selected design.

1.3 Working

method

The work has been carried out in four stages, of which the first has been to identify and describe typical examples of system types. From this, based on fundamental theory in the field and on measurements of actual systems, appropriate models have then been developed and/or applied that have made it possible to investigate system function in the desired manner. This has been followed by identification and description of typical examples of deviations found in these systems. Finally, the last stage of the work has involved system simulation to identify and quantify the effects of system deviations on various performance parameters.

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1.3.1 Systems investigated

The purpose of a heating system in a building is to maintain the desired indoor air temperature. How well the system works depends on its ability to handle

“disturbances”, i.e. parameter changes. This ability depends primarily on the design of the heating system, and secondarily on how well the control system meets the needs of the heating system. The main emphasis of this thesis is on the design of the heating system, rather than on the control system.

There is considerable potential for improving heating systems when the supply of heat to the buildings is temperature-sensitive. This is the case when systems are supplied by district heating, where it is highly desirable to keep both the supply and return

temperatures down and to limit the water flow rate. For this reason, it is primarily hydronic heating systems supplied by district heating on which this work has been concentrated.

Today's hydronic heating systems supply heat primarily by means of radiators, although to some extent also by means of air heaters. This work has therefore concentrated on systems having radiators (slow thermal response) or air heaters (rapid thermal

response), primarily supplied by district heating.

Radiator system

The interaction between radiators and rooms is a slow process, which means that the dynamic in such a system is not particularly apparent. It is therefore the equilibrium condition in the system that says more about the system's function and performance than does the process between these static levels.

It is important to obtain as good an overall picture of the system to be analysed as possible. In the case of a radiator system, a complete, although relatively small, distribution system in a building is therefore analysed. This system is shown in Figure 1 below, and described in detail in Chapter 4. The system starts and finishes at the connections to the heat exchangers in the district heating substation unit, which is therefore not included in the analysis. It has thus been assumed that the supply temperature in the building's heating system can always reach the required levels.

Figure 1. The 2-pipe radiator system considered in the analysis.

Heat ex.

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1 INTRODUCTION

Air heater with valve group

The heat release process from an air heater in a ventilation duct, on the other hand, is considerably faster, which means that the dynamics of the process/equipment are important in determining the function of the equipment when in use. Of course, as for radiator systems, the static properties are also very important.

In the case involving analysis of systems incorporating air heaters, both the dynamic and the static characteristics depend largely on the design of the air heater and of its local valve group. In this respect, the difference between it and a radiator system is the considerably faster and more advanced local control of air heaters. In this case, the work is therefore concentrated solely on the air heater and its local valve group. The system boundary conditions are expressed in the form of available pressure drop and of the supply temperature.

The flow in heating systems supplied by district heating is always variable, as this provides the most effective cooling of the district heating water. However, it is not always desirable to have a variable flow rate through an air heater, and so valve groups are therefore often used, to maintain a reasonably constant flow rate through the air heater itself. The arrangement of these valve groups can vary, and so three different types have therefore been analysed:

• Direct connection; without shunt, which means variable flow through the air heater, controlled by a two-way control valve.

• District heating connection; with shunt, which means constant flow through the air heater, controlled by means of a two-way control valve.

• The “SABO” connection; with shunt, which means constant flow through the air heater, controlled by means of a three-way control valve.

These different types of connections has, in this work, been given the names mentioned in the listing above (instead of different numbers) for instant clarity whenever discussed in the text.

Balancing

Although the system analyses have compared high-flow and low-flow balancing (and other system arrangements), it is not desirable to make physical changes to the system, such as by using larger or smaller radiators, as this affects the comparison. The

analyses for the radiator system have therefore been based on two different basic cases, without deviations: one for each balancing method, with pipe sizes and arrangements, radiators and heat output being the same. The same applies for the system using air heaters.

The difference between the two basic cases is that the flow has been assumed to be twice as great in the high-flow case as in the low-flow case. The required condition for these two arrangements to work is that the design supply temperature in the low-flow case should be higher than in the high-flow case.

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1.3.2 Measurement and simulation

After the selected systems were defined, a number of measurements were made in test rigs in order to investigate the systems' properties and to verify any design relationships that would be required in the next stages, i.e. of simulation of selected cases. When making actual physical measurements, the test conditions may be more or less fixed, depending on the design of the test rig and the measurement system. This means, that in certain cases, it can be time-consuming or even impossible to obtain the required results. This constraint does not exist in the case of simulations, as the conditions can be altered as required. However, the initial physical measurements are important, as they enable the simulation models to be verified.

The test rigs that were used consisted of a radiator rig, having two radiators on one branch, and an air heater rig with associated valve group. The design of this valve group is such that it can easily be changed to suit any particular required measurement case.

Two simulation programs were used: a commercial program (Flowmaster) for

simulation of the air heater and its valve group, and an Excel spreadsheet, designed for simulation of a radiator system.

1.3.3 The effect of deviations

As described earlier, this work has been concerned with investigation of what happens in a system when conditions depart from the design conditions. Recapitulating, a deviation means that something in the system is not what it should be. This could be either “software” or “hardware”, i.e. such as an incorrectly selected valve characteristic, an improperly performed balancing operation, a radiator valve that has been turned to a different setting etc. The sensitivity of the system function, depending on its design, can be analysed by regarding different systems in this way.

The analyses have been concentrated on investigation of two result parameters: • Room temperature or supply air temperature, which constitute the criterion of

assessment of system function as seen by the occupants of the building. In this investigation, the supply air temperature has been assumed to be the same as the temperature of the air leaving the air heater.

• The return water temperature, which is the main criterion of system function as seen from the district heating supplier's perspective.

The room temperature and the return temperature are two very important parameters, which provide one way of describing how well the heating system is working. A

system deviation affects both the system function and performance, which in turn means that either the room temperature or the supply temperature, or both, is/are affected. The magnitude of the effect depends on the type of deviation and on the design of the

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1 INTRODUCTION

1.4 Work in this field

As far as hydronic heating systems are concerned, it is no exaggeration to say that the number of publications concerned with such systems is almost impossible to count. This means that the articles, reports and books mentioned or used in this work represent only a small fraction of everything that has been written. Of necessity, they have been selected on a subjective basis, although with the intention of constituting a firm basis upon which to build. Much has been written, and it could be thought that this is a working area about which little new remains to be said. However, the investigation of various systems in terms of their sensitivity to various deviations, instead of in terms of optimising their performance, does not seem to have been all that common, despite the fact that investigation of the sensitivity of function to system design is the most

interesting aspect. Such work is essentially concerned with ensuring the proper function of systems, which should really be an important objective.

As previously mentioned, the analysis aspects of the work described here have been concentrated primarily on hydronic heating systems supplied by district heating. In turn, the heating systems have been restricted to radiator systems and air heater systems, which make up a large proportion of existing systems in countries such as Sweden, Denmark, Finland, Germany and Russia. In 1996, district heating supplied about 36 % of the total space heating requirements of buildings in Sweden1, in which radiator systems provided the main form of space heating. There has therefore always been a relatively high level of research into systems of this type in Sweden, which is also reflected in the reference list at the end of this thesis. Conditions in the USA, for example, are different, as district heating and radiator systems are in a minority: instead, it is air-conditioning systems that are most commonly used. It is therefore not surprising that there are relatively few publications from the USA: at least, as far as radiator systems are concerned.

A few aspects that have been of interest in this work are briefly described below (1.4.1-1.4.4). Each section describes only one or a few important articles, reports or books.

1.4.1 Buildings supplied by district heating systems

Those involved in the district heating sector have conducted various types of research into different heat production units, distribution piping, substation in buildings and other aspects over many years. However, this work has not always considered the onward link from the district heating system to internal heating systems in buildings. In fact, such consideration has often been totally omitted, or only partly recognised as the major heat sink for the district heating system which, despite everything, it actually is.

However, this is a heat sink with very varying properties, as shown by Sven Werner in his PhD thesis entitled “The heat load in district heating systems”2 (1984), which is specifically concerned with the load characteristic on district heating systems. Nevertheless, some university-level research into district heating, and involving the hydronic heating systems in buildings in one way or another, has been carried out in

1_{Statistic from the Swedish District Heating Association.}

2_{Although this particular thesis is written in English, not all of the material cited in the rest of this}

chapter or in other chapters of this thesis is necessarily in English. However, for the sake of clarity in this thesis, those titles have been translated into English (placed in clampers) where necessary.

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recent years. Examples of this include Jochen Dahm's thesis “Small district heating systems” (1999), Gunnar Larsson's “Dynamik i fjärrvärmesystem” (Dynamics of district heating systems) (1999) and Lena Olsson's thesis “Lokala fjärrvärmesystem” (Local district heating systems) (2001).

It is quite clear today that the supply and return temperatures of district heating systems affect their overall efficiency and thus their economic viability. It is also equally clear that there is a close link between these temperature levels and the design and balancing not only of the consumer service units but also of the downstream domestic hot water, air heating and radiator systems. A paper by Sven Werner and Stefan Petersson in 2000, entitled “Samband mellan produktion och vältrimmade radiatorsystem” (The relationship between production and properly balanced radiator systems) showed, for example, that district heating utilities in Sweden could save SEK 800 million/year if their systems could be operated at maximum efficiency. A substantial proportion of this potential is accounted for by improvements in the efficiency of building heating

systems. In this perspective, the importance of analysing building heating systems connected to district heating supplies is patently apparent.

1.4.2 Low-flow and high-flow balancing

Low-flow versus high-flow balancing has been a subject of fierce debate for many years. In Sweden it started in the 1960s, when Östen Sandberg balanced a difficult radiator system in a school in Kiruna by drastically reducing the flow rate and increasing the supply temperature. This reduced the pressure drop in the pipes in the system, enabling all the radiators to receive the correct flow. This approach was subsequently given the name of the Kiruna method, or the low-flow method, and is much used today by a number of property companies, such as SABO (a swedish municipal housing organisation). It is described in the book by Torkel Andersson, Per Göransson, Gunnar Wiberg and Bebs Reybekiel, “Kirunametoden – för god

energihushållning” (The Kiruna method - for good energy conservation) (1998). Torkel Andersson had previously published a substantial two-part article entitled “Konsten att styra radiatorsystem” (The art of controlling radiator systems) (1993), setting out the benefits of low-flow systems.

A long debate occurred at the end of the 1970s and the beginning of the 1980s between Östen Sandberg, who naturally supported the low-flow method, and Sven Mandorff who “defended” the hitherto generally used high-flow method. This debate was

conducted on the pages of various HVAC magazines. In it, Sven Mandorff showed the sensitivity of systems to system deviations, e.g. in his article entitled “Kirunametoden – bara fördelar?” (The Kiruna method - nothing but benefits?) (1982). The debate never really reached a firm conclusion in favour of one system or the other, and will probably never do so.

In recent years, Stefan Petersson has carried out a performance comparison, based on both measurements and simulation, of high-flow and low-flow systems, describing the results in his licentiate thesis “Analys av konventionella radiatorsystem” (Analysis of conventional radiator systems) (1998) and in the subsequent report published by the Swedish District Heating Association, entitled “Metoder att nå lägre returtemperatur med värmeväxlar-dimensionering och injusteringsmetoder” (Methods of achieving lower return temperatures by heat exchanger design and balancing methods) (2000).

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1 INTRODUCTION

The high-flow method is still regarded as being the generally accepted method, with the low-flow method often being used as an alternative for balancing existing (often

substantially oversized) installations. However, it is not uncommon to encounter a sort of “intermediate-flow” method, so it may be that the various balancing methods are beginning to approach each other.

The general view today seems to be that, regardless of the method chosen, it is

important that systems are properly balanced. Unfortunately, this has not always been obvious. During the 1970s, it was suggested that balancing of radiator systems was unnecessary if they were complemented with thermostatic radiator valves. However, Sven Mandorff showed, in his articles entitled “Funktionen hos värmesystem med radiatortermostatventiler utan förinställning” (Performance of heating systems with radiator thermostatic valves without presetting) (1979) and “PS om termostatventiler – erfarenheter från ett radhusområde” (A PM on thermostatic radiator valves - experience from a terrace house development) (1977), that such a procedure could result in difficult problems.

When considering balancing problems, Robert Petitjean's publication “Total hydronic balancing” (1994) cannot be overlooked. It provides a detailed description of

(primarily) high-flow adjustment of hydronic heating systems, together with a number of sensitivity analyses.

1.4.3 Radiator systems, air heaters and valves

A lot has been written about radiator systems from a general perspective: an example of this is the BFR report “Värt att veta om vattenburen värme” (Worth knowing about hydronic heating), by Sune Häggbom and Per-Olof Nylund (1989), which provides a general description, mainly of radiator systems. Another example is Lennart Örberg's report “Dimensionering och injustering av vattenburna värme- och kylsystem” (Design and balancing of hydronic heating and cooling systems) (1986).

A considerable amount of research into radiator systems was carried out in Sweden during the 1970s and 1980s, concerned primarily with the function and benefit of thermostatic radiator valves, and with discussion of which balancing principle was preferable. The Department of Heating and Ventilation Technology at the Royal Institute of Technology has been the source of much that has been written about heat release from radiators and how they affect the rooms in which they are installed. Examples include “Förenklad bestämning av operativtemperaturen i radiatorvärmda rum” (Simplified determination of the operative temperature in rooms heated by radiators) by Folke Peterson (1975), “Radiatorers yttemperatur” (The surface

temperature of radiators) by Tor-Göran Malmström (1975) and “Värmevgivning från radiatorer” (Heat release from radiators) by Stig Hammarsten (1985). All of these publications are in the Royal Institute of Technology series of Technical Notices. Lars Jensen's thesis, “Digital reglering av klimatprocesser” (Digital control of climate processes) (1978), should also be mentioned in this context, as it includes a description of how the room temperature reacts when a radiator is controlled in various ways. As far as thermostatic radiator control valves are concerned, Anders Svensson's status report entitled “Radiatortermostatventilers funktion – lägesrapport” (The function of

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radiator thermostatic control valves) (1978) must be mentioned. In addition, he and Sven Mandorff have written an interesting article entitled “Radiatortermostatventiler på gott och ont” (Radiator thermostats for better or worse) (1977), which emphasised the importance of adjusting the supply temperature to track the ambient temperature in order to ensure correct operation of the thermostatic valves. Further examples of such reports include Lars Jensen's “Analys av termostatventilers statiska egenskaper” (Analysis of the static characteristics of thermostatic radiator control valves) (1986) and the BFR report “Långtidsegenskaper hos radiatortermostatventiler” (Long-term characteristics of thermostatic radiator control valves), by Geron Johansson, Matti Kolehmainen and Lars Waldner (1989).

For natural reasons, most of what has been published about radiator systems has been concentrated on two-pipe systems, although there are exceptions. Examples include “Ett-rörs system för värme – Teori, praktik och ekonomi” (Single-pipe systems for heating - theory, practice and economics) by Ulf Järnefors (1978), and the articles “1-rörsystemet ger en annan reglerstrategi” (The single-pipe system necessitates a different control strategy) by Hugo Brännström (1987) and “Single-pipe hydronic system design and load-matched pumping” by W.C. Stethem (1994).

There are many examples of Swedish work on air heaters. As far as the development and/or study of mathematical models is concerned, Elisabeth Mundt's report “Modeller av luftvärmare för simulering av stationära och dynamiska driftsfall” (Models of air heaters for simulation of stationary and dynamic operating modes) (1988) and Per E. Blomberg's PhD thesis “Experimental validation of dynamic component models – for simulation of air handling units” (1999) should be mentioned. Two other interesting reports are Per Widén's licentiate thesis “Luftvärmare i luftbehandlingsaggregat” (Air heaters in air handling units) (1994) and Hugo Brännström's BFR report

“Frysskadesäkra vattenburna luftvärmare” (Frost-resistant hydronic air heaters - field trials and practical application) (1990). The former is concerned with the linear relationships between the temperatures in an air heater through which constant media flows are passing, while the latter is concerned with air heaters without recirculation circuit and how they can be protected from freezing.

The choice of control valves is an interesting aspect of this work. A considerable amount of research into this has been carried out in Norway, with Arvid Grindal, Bent Børresen and Einar Magne Hjorthol among those to the fore. Examples of interesting literature include Arvid Grindal's article “Ventilkarakteristikker – er det på tide at vi gjør noe med dem?” (Valve characteristics - is it time to do something about them?) (1988), Bent Børresen's article “Ventildimensjonering og ventilautoritet” (Valve sizing and valve authority) (1994) and Einar Magne Hjorthol's thesis “Optimisation of design values in district heating substations by system simulation” (1990).

There are also several examples of applications from the USA dealing with the selection of balancing and control valves, such as “Selecting control and balancing valves in a variable flow system” by Richard A. Hegberg (1997) and “The effect of sizing

mismatch on coil valve performance” by Randall J. Amerson (1998), in which the latter provides an analysis of the effects of deviations. An early, very interesting, Swedish article on the same theme is “Att undvika missanpassning av styrventiler” (To avoid mis-sizing of control valves) by Karl-Åke Lundin (1980).

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1 INTRODUCTION

1.4.4 System design and control

How can the correct function of a system be ensured? The answer is that it probably cannot be ensured, although the probability of the occurrence of problems can be reduced by favourable design of the system. But how can this be quantified? One way is to consider the control of the system or, strictly, how the necessary settings of the system regulator must be set in order to avoid instability. The less well designed the system, the greater the risk of control difficulties due to it being more likely for the system to become unstable. In turn, the more difficult it is to control the system, then the greater the risk of sub-standard performance. This approach starts with Ziegler and Nichol's well-known article “Optimum settings for automatic controllers” (1942), with guidelines concerning the setting of control systems. These guidelines have been used in Norway for the classification of the difficulty of control of a system. Although there are several persons who have worked on such aspects, most of the input to this present project has been inspired from Arvid Grindal's and Bent Børresen's many publications, e.g. their excellent joint article “Controllability - back to basics” (1990).

As far as research into system control is concerned, Lars Jensen's work must again be mentioned. His PhD thesis, and various of his other publications, have provided the basis for much of the research into this area. Not only has he developed mathematical models for both components and control procedures, but he has also carried out several instrumented investigations of various controlled items. His work has been continued in Vojislav Novakovic's “Digital control of heater coils” thesis (1982), which shows how the settings values for a digital controller can be determined, and how well such a controller operates in comparison with an analogue equivalent.

However, the main emphasis of the work described here is not on system control, but on system characteristics. Interest has therefore been concentrated on determination of the system's static and in some extent dynamic characteristics.

1.5 The structure of this thesis

This thesis consists of eight chapters. Chapter 2 is devoted to general theory, describing and defining a number of fundamental concepts concerning the design of a system. This is necessary material for understanding of the following chapters.

Chapter 3 describes the measurements; how they were made and a number of independent results, such as thermal stratification in air heaters.

Chapter 4 describes the simulation programs used. In addition, it compares the measured results with those of simulation in order to verify the programs. Chapter 5 deals with planning of the simulations. In this chapter the

systemconfigurations and deviations that are used in the simulations is being presented, as well as the methods to derive and present the results.

Chapter 6, that is rather extensive, is devoted to results from simulation of the effects of deviations in a radiator system. Each section of simulation is followed by a summary and discussion of the particular case concerned. In addition, the chapter is concluded

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with a discussion of other aspects in the field, not dealt with earlier in the thesis, such as the differences between 1-, 2- and 3-pipe systems and the effects of the performance of the district heating heat exchanger connected to the radiator system.

Chapter 7 is also devoted to results, but this time from simulation of deviations in a valve group and the associated air heater.

Chapter 8 brings together and discuss the results from Chapters 6 and 7. Any more or less general trends are considered, and the consequences of some particular system designs are described.

Appendix A describes the test rigs in detail, and considers the effects of uncertainties of measurement.

Appendix B describes the theoretical and empirical relationships underpinning the structure of the design program for analysis of deviations in a radiator system. Also the derivation of an expression to describe the sensitivity of radiators to flow deviations is described. The appendix is concluded with a description of the simplified process to derive an optimum valve characteristic.

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2 SYSTEM DESIGN

2 SYSTEM

DESIGN

This chapter describes a number of concepts and definitions that need to be clarified in order to assist understanding of the rest of this presentation. However, it is assumed that a number of fundamental relationships are known by the reader, and so they will not be further discussed here. For a more fundamental review of the concepts relating to the function of a hydronic heating system, see Trüschel (1999), from which some of the material in this chapter has been taken, or (for example) Häggbom and Nylund (1989), who describe both the theory and the practice of hydronic heating systems. The function and performance of the system depend on the interaction between the system design and its control. In this presentation, the focus is entirely on the system design, due to the fact that this must always be considered first. The philosophy is that control of the system must be tailored to the system to be controlled. A control system or control method must not be applied in the hope of compensating for a poorly

designed system, and nor can it do so, which is a well-known observation, but

nonetheless important to point out (Grindal and Børresen, 1988; Hjorthol, 1990). It is therefore important to understand how the design of a system affects its properties. The better the design, the simpler the necessary control and the better the system will

perform.

As used here, system design or design of the system refers to the components selected for use in the system, how they are arranged in relation to one another and their capacities (i.e. aspects such as size, torque, power etc.). It is the intention here to characterise system design in terms of the following three main aspects:

• Structure (architecture etc.) • Balancing

• Control valves

The inclusion of balancing as part of the design may perhaps not be regarded as a general mainstream approach. This is because the selection of balancing method represents an indirect measure of the system design, in the form of temperature levels and flow rates. This simplifies the presentation, as balancing is a very important parameter when describing a system.

2.1 Structure

2.1.1 Distribution systems

The structure of a system describes primarily how the distribution part of the system has been designed. A common feature of all distribution systems is that the heat (i.e. the hot water) is distributed around the building by means of a supply pipe. After giving up heat in the heat-releasing components, the cooled flow is returned to the heat source via a return pipe. This can be said to apply in general, although distribution systems can be arranged in various ways. The commonest form is that of a 2-pipe system, in which branches are taken off the supply pipe to each heat-releasing component, and from which the flow returns via direct connection to the return pipe, as shown in Figure 2.

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Figure 2. Schematic diagram of a 2-pipe distribution system.

Alternatively, instead of having separate supply and return pipes, they can be combined into a 1-pipe system, with the supply pipe to each component forming the return pipe from the previous component, as shown in Figure 3.

As indicated by the name, the 1-pipe system consists really of only a single pipe, forming a distribution loop that starts and finishes at the heat source, with an

appropriate flow being tapped off from the loop to each component. This means that the supply flow to each component consists partly of the cooled return flow from the previous component, so that the supply temperature is progressively reduced as the flow passes each component. In order to compensate for this, the components can either be progressively larger, or the flow tapped off to each component can be increased for components further away from the source in the direction of flow.

A further development of the 2-pipe system is what is known as the 3-pipe system or the Tischelmann connection, in which the return pipe is reverse-connected, as shown in Figure 4 below.

The 3-pipe system works in the same way as the 2-pipe system, except that the return connection is reversed. The purpose of this arrangement is to attempt to reduce the differences in differential pressure across the components, as occurs in a 2-pipe system with direct return. The differential pressure depends on the pressure losses in the supply and return pipes: the further away from the heat source that a component is, the longer are the supply and return connections to and from it, and thus the less is the differential pressure available across the component. It can be seen from Figure 4 that the total distance, in terms of the sum of the supply pipe length and the return pipe length, is the same for each component, which means that the differential pressure available across the components does not vary as much as in the basic 2-pipe system shown in Figure 2. The reason for the interest in attempting to achieve constant differential pressure is that

Supply pipe Return pipe Heat-releasing component Supply pipe Return pipe Supply pipe Return pipe

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2 SYSTEM DESIGN

heat released by it. Balancing, the purpose of which is to balance the flows through each component, so that the flow through the components is set to the desired values, is facilitated if the effect of the differential pressure differences on the distribution system is reduced.

1-pipe systems and (particularly) 3-pipe systems are relatively uncommon, in

comparison with the use of 2-pipe systems. As a result, most of the emphasis of this presentation will be on 2-pipe systems, with the other two types being dealt with only in general terms.

2.1.2 Valve groups

The concept of structure or architecture of distribution systems, as used here, also includes valve groups, which provide local control of air heaters. The design of such valve groups affects the control principle. The simplest case consists of no valve group at all, which means that the heat provided by the air coil is controlled only by changing the flow through the component. The arrangement is referred to here as direct

connection (see Figure 5).

Figure 5. An example of direct connection.

However, in general, control is provided in and by a valve group by mixing the cooled return water flow from the component with hot water from the supply pipe in order to achieve a required inlet temperature, which is defined as being the temperature of the water at the point of entry to a heat-releasing component. The supply temperature is defined as being the temperature of the water entering the distribution system supply pipe after the central temperature control function, which also can be controlled by another valve group, e.g. if an oil-fired boiler is being used as the heat source. The inlet temperature in a heating system will always be lower than the supply temperature, regardless of the type of local control, due to heat losses in the supply pipe.

Figure 6 shows two examples of common valve groups, referred to here as the district heating connection (as this arrangement is mostly used in systems connected to district heating supplies), and the SABO1 connection (which is an fairly accepted name for this type of valve group). The right-hand side of the shunt connection, or the bypass

connection, is referred to as the secondary side, with the left-hand side being referred to as the primary side. Both valve groups are intended to provide an almost constant flow rate through the air heater (the secondary side), with a variable flow rate in the

distribution system (the primary side).

1_{SABO is a large country-wide municipal housing association.}

Direct connection

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Figure 6. Two valve group arrangements to provide variable flow on the primary side and constant flow on the secondary side.

Figure 7 shows two further valve group arrangements, intended to produce constant flow rates on both the primary and secondary sides. These arrangements are generally known respectively as the Swedish connection and the Norwegian connection.

Figure 7. Two valve groups to maintain constant flow rates on both the primary and secondary sides.

Heating systems connected to district heating supplies use valve groups with variable flows on the primary side (Figures 5 and 6) and that is why other types of valve groups will not be further considered in this presentation.

2.2 Balancing

The purpose of balancing is to create a balanced flow in the system. However, the performance of balancing affects not only the distribution of flows through the system, but also the interaction between the heat-releasing components and their individual characteristics.

For a fluid to flow, there must be a pressure difference. At the same time, the amount of the flow is restricted by resistance. Flow resistance is due to energy losses in the form of friction, losses caused by changes of direction (e.g. pipe bends etc.) or losses due to sudden velocity changes. The relation between the flow resistance and the differential pressure determines the magnitude of the flow. This is shown by the following simple relationship (which admittedly applies only for full turbulent flow) (Abel et. al, 1997):

k p

V& = ∆ (1)

V& = Flow [m³/s] p

∆ = Differential pressure [Pa] k = Flow resistance [Pa/(m³/s)²]

The purpose of the pump in the circulation system is to create a pressure difference. This pressure difference, which is the driving force behind the flow, is progressively dissipated by pressure drops in the distribution system with increasing distance from the

District heating connection SABO connection Swedish connection Norwegian connection

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2 SYSTEM DESIGN

pump. This means that the pressure difference across a radiator close to the pump is higher than that across a radiator further away, as shown in Figure 8 below.

Figure 8. Available differential pressure in a circulation system with five radiators. It can be seen that the differential pressure across component 1 is

considerably higher than that across component 5.

If the flow through each radiator is to be the same, there needs to be a higher flow resistance through radiator 1 and a lower resistance through radiator 5. This is achieved during balancing by means of the balancing valves. Changing the openings of the valves changes their flow resistance. However, it is not the flow resistance of the valves that is specified during balancing, but their capacity.

2.2.1 Valve capacity

A measure of the valve capacity is given by its kv value, which is defined as:

ρ ρ ⋅ ∆ ∆ = 0 0 v p p V k & (2)

V& = Volume flow through the valve [m3/h] p

∆ = Pressure drop (differential pressure) across the valve [bar] 0

p

∆ = Reference pressure drop = 1 bar

ρ = Density of the liquid passing through the valve [kg/m3_]

0

ρ = Reference density = 1000 kg/m3_(water)

The larger the opening, the higher the valve capacity and the greater the kv value.

Closing the valve completely gives a kv value of zero. The liquid in hydronic heating

systems is generally water, which can be regarded as an incompressible fluid, so that the Pressure

Length ∆ppump ∆p1 ∆p2 ∆p3 ∆p4 ∆p5

Pump 1 2 3 4 5 Radiator

The kv value indicates the magnitude of the flow in [m³/h] passing through the valve

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fluid correction factor (ρ0/ρ) can be expressed as unity. In addition, as the reference

pressure drop is 1, the expression for the kv value can be simplified to the following:

p V k_v

∆

= & (3)

In many contexts, it is impractical to use the quantities [m3/h] and/or [bar]. There are therefore many different expressions for the kv value, although the differences are due

only to the use of different units. (The reference units remain unchanged.) Table 1 shows a number of examples:

Differential pressure \ Flow [m3/h] [l/h] [l/s] [bar] kv V_p ∆ = & p V 001 , 0 kv ∆ ⋅ = & p V 6 , 3 kv ∆ ⋅ = & [kPa] kv 10 _pV ∆ ⋅ = & p V 01 , 0 kv ∆ ⋅ = & p V 36 kv ∆ ⋅ = &

Table 1. Different expressions for the kv value of a valve, depending on the choice of units.

The maximum kv value of a valve, i.e. its capacity when fully open, is usually referred

to as the kvs value.

The flow resistance (k) and the valve capacity (kv) symbolise two completely different

things, and must therefore not be confused, although this can easily happen as the symbols are so similar. It is, of course, more practical to express the size or setting of a valve by means of a measure of capacity than by means of a measure of its flow

resistance (which is a somewhat “empty” expression).

2.2.2 Pump and system characteristics

A hydronic heating system consists of a circulation system, in which the water is circulated by a pump. The pump characteristic, which is determined by the design and size of the pump, shows the relationship between the pressure rise in the pump and the flow through it. The heating system has a hydraulic resistance, which expresses itself as pressure drops through/across pipes, valves, heat-releasing components etc. The total pressure drop of the system changes depending on the magnitude of the flow. The relationship between the total flow and the total pressure drop in the system is referred to as the system characteristic. The system operating point will automatically lie at the point where the total flow through the system is such that the total pressure drop in the system equals the pressure rise across the pump.

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2 SYSTEM DESIGN

Figure 9. Pump and system characteristics.

The system characteristic depends on the total flow resistance of the system. The greater the flow resistance, the greater the pressure drop in the system for any given flow. This means that whenever anything changes in the system, the position of the operating point will change, giving rise to a new total flow through the system.

Balancing, in other words, means that the system characteristic will be changed, due to the fact that the balancing valves are partially closed, as necessary. At the same time, the pressure rise across the pump changes. (Alternatively, one can say that the total pressure drop in the system changes, if preferred.)

Figure 10. Changing the operating point in a system.

It can be seen from the figure above that the change in the total flow is due both to the change in the system characteristic and to the shape of the pump characteristic curve. A steep pump characteristic results in only a slight change in the total flow, but a more substantial change in the pressure rise/pressure drop. A flat pump characteristic, on the other hand, produces the opposite results, i.e. a substantial change in the total flow but only a slight change in the pressure rise/pressure drop. This is shown schematically in Figure 11, with steep and flat pump characteristics being symbolised by a vertical and a horisontal straight line respectively.

System characteristic

Pump characteristic Operating point Differential pressure

Flow

New system characteristic

Pump characteristic

Old operating point Differential pressure

Flow

New operating point Old system characteristic

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Figure 11. Changes in the operating point, depending on the pump characteristic, in response to a change in the system characteristic.

2.2.3 The effect of pipe pressure drop

The interaction, i.e. how individual flow changes affect the flow balance of the entire system, depends on the pressure drops in the pipes of the system and on the magnitude of the pressure drops across the components. The pressure drops in the pipes depend on the size of the pipes and on the flow in them, which is a result of the system balancing. Pipe pressure drop can be described by the following simple relationship, which follows the same principle as that of equation (1):

x

pipe pipe k V p = ⋅ &

∆ (4) where ∆p_pipe = Pipe pressure drop [kPa]

k_pipe = The coefficient of flow resistance in the pipe [kPa/(m³/s)x_]

V& = Flow [m³/s]

x = An exponent, which depends on the type of flow [-]

For fully laminar flow, the exponent equals to 1, while for fully turbulent flow it equals to 2 (Abel et. al, 1997). These represent the two limiting cases of flow conditions. The exponent varies, in other words, between 1 and 2, depending on the flow conditions. From the previous equations, we can derive the following relationship, which describes how a change of flow affects the pressure drop. It is valid provided that neither the pipe parameters nor exponent x are changed. Index 1 indicates the value of the quantity before change, and index 2 indicates its value after change.

x 1 2 1 , pipe 2 , pipe V V p p       = ∆ ∆ & & (5)

The magnitude of the interaction depends on by how much the pressure drop across the heat-releasing components (i.e. its associated valves) changes in response to a change of the flow balance in the system. This is illustrated by an example with a group of

radiators, as shown in Figure 12. The figure shows the effect of two different pipe Differential pressure

Flow Steep pump characteristic

Change in the system characteristic New operating

points, depending on the pump characteristic

Flat pump characteristic Old operating point

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2 SYSTEM DESIGN

pressure drops, as could result, for example, from different flows or from different pipe sizes (or from both mechanisms).

Figure 12. The increase in pressure drop in response to a reduction in flow through the radiator group. A comparison between different pipe pressure drops (2 kPa and 8 kPa).

In the first case, the pipe pressure drop in the supply and return pipes is 2 kPa, while the lowest balanced differential pressure (across radiator 5) is 1 kPa. A reduction of 50 % in the flow through the group means that the pipe pressure drop falls to 25 % of its original value, in accordance with equation (5) above (provided that the flow is fully turbulent). The differential pressure across radiator 5 then increases to 2.5 kPa, which will result in an increase of 58 % in the flow if the radiator valve setting is not changed. In the second case, the pipe pressure drop at 8 kPa is four times higher than in case 1, while the lowest balance differential pressure remains at 1 kPa, across radiator 5. A reduction of 50 % in the flow through the group results in a new differential pressure of 7 kPa across radiator 5, resulting in a flow increase of 164 % through it.

It can be seen, therefore, that the interaction increases with the pipe pressure drop, which can be affected by balancing and, perhaps primarily, by the size of the pipes. Large pipe sizes also have the advantage that not only is the interaction reduced, but pumping costs are also reduced as there is a lower pressure drop in the system. However, large pipe sizes also involve higher investment costs. In addition, it is not always beneficial to minimise pressure drops in the system, as described in the following section.

1 2 3 4 5

Control and balance valve Radiator branch 1 kPa 7 kPa 9 kPa 1 kPa 2.5 kPa 3 kPa Case 1 Case 2 Flow: 100 % 50 %

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2.2.4 The effect of lowest balanced differential pressure

The magnitude of the system flow is not an entirely free choice, as it is linked to the supply temperature and to the sizes of the heat-releasing components. The higher the possible supply temperature, the lower the flow required. This means that other factors, such as the choice of heat source and the size of the components (see the next section) can affect adjustment of the magnitude of the flow. For this reason, the pipe pressure drop cannot be set arbitrarily in connection with balancing. However, there is more room to manoeuvre, from a technical point of view, in deciding on the lowest balanced differential pressure.

As the flow through a component depends on both the available differential pressure across it and its flow resistance, a high differential pressure can be compensated for (or offset by) a high flow resistance, and vice versa, without affecting the flow. When balancing the system, a lowest design differential pressure across the component (or, strictly, across its balancing valve) in the system that has the lowest available

differential pressure, is determined. This component (that sets the lowest available differential pressure) is usually the one that is furthest from the pump.

There is no optimum value for lowest differential pressure: some systems are designed for a lowest differential pressure of 2 kPa, while others use 10 kPa. The greater the differential pressure, the more must the balancing valve be throttled in order to achieve the correct flow. It might seem as a bad idea to choose a high differential pressure, as this will require the balancing valves to be throttled all the more, which means that, in turn, an unnecessarily large pump has to be used in order to overcome the high pressure drops in the system. However, the advantage of a high differential pressure is that a change in a valve setting, due to control, does not have such a great effect on the flow through other parts of the system as it would in a system operating with a low

differential pressure.

The interaction between the components, in other words, is less with increasing differential pressure. We can clarify this by again showing an example (Figure 13) of the pressure levels in a radiator branch. For the purposes of the example, it has been assumed that the two radiator branches in the comparison are exactly the same: the only difference between the two cases is that the lowest balanced differential pressure (across radiator 5) is 1 kPa in one branch and 10 kPa in the other. The continuous lines in the figure show the full-flow pressure drop in the pipes, which is the same in both cases. When some of the radiator valves close, e.g. as a result of insolation, the flow through the branch is reduced, which also reduces the pressure drops in the pipes. The new pressure drops in the pipes are shown by the dotted lines in the figure. It can be seen that the differential pressure across the radiators increases by the same amounts (in absolute terms) in both cases, but that when expressed in relative terms the difference is considerable, which thus affects the flows through the radiators differently.

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2 SYSTEM DESIGN

Figure 13. Increasing the differential pressure in response to a reduction in flow through the group. A comparison between a system balanced for a minimum differential pressure of 1 kPa and one balanced for a minimum differential pressure of 10 kPa.

In the first case (with a low balanced minimum differential pressure), the differential pressure across radiator 5 increases from 1 kPa to 4 kPa. This is equivalent to a 100 % increase in flow through radiator 5 if its valve setting remains unchanged, in accordance with equation (1), which applies for fully turbulent flow.

In the second case, with a high set minimum differential pressure, the differential pressure across radiator 5 also increases by 3 kPa. However, as it was originally 10 kPa, the relative increase is only 30 %, as against 300 % in the previous case. This means that the flow through the radiator increases by only about 14 %.

The higher differential pressure, in other words, helps to reduce the interaction between the heat-releasing components in a system. This means that, if the system is set up to have a high minimum differential pressure, the amount of heat released from a component does not change as much in response to flow changes caused by other components in the system. A general opinion seems to be that this also assists system control, due to the fact that the control valves have a higher valve authority. However, this is not necessarily the case, as is discussed in Section 2.3.2, which considers and explains the concept of valve authority. The drawback of a high differential pressure is that energy is lost in the form of a high pressure drop through the valves, which

increases system running costs.

10 kPa 13 kPa 18 kPa

1 kPa _{4 kPa}

9 kPa Case 1

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2.2.5 The heat-releasing components' characteristic

As previously mentioned, balancing provides an indirect measure of the design capacity of the system, in the form of temperature levels and flows. It is important, when

analysing system function and performance, to be aware of the characteristics of the heat-releasing components, in the form of the relationship between flow and heat release power (or output air temperature). This characteristic depends partly on the size and design of the component and partly on the temperature levels involved.

Determining the design/size of a heat-releasing component, such as a radiator, involves ensuring that the component is sufficiently large to be able to provide the necessary heating power. This size also depends on the design temperature levels in the heating system and on the ambient conditions in which the component works, all in accordance with the following well-known equation that describes heat transfer in a heat exchanger:

m t A U

Q& = ⋅ ⋅∆ (6) Q& = Thermal power transferred [W]

U = Coefficient of thermal transmittance [W/m²K]

A = The surface area of the heat-releasing component [m²] m

t

∆ = The mean temperature difference between the hot and cold flows [°C] In addition, if there are no heat losses in connection with transfer of the heat, the following equation applies, with index i representing either the hot or the cold medium.

(

i,in i,out

)

i i

i V c t t C t

Q& =ρ ⋅ & ⋅ ⋅ − = ⋅∆ (7) Q& = Thermal power transferred [W]

i

ρ = Density [kg/m³] i

V& = Flow [m³/s] i

c = Specific thermal capacity [J/kg°C] in , i t = Input temperature [°C] out , i t = Exit temperature [°C] i

C = Thermal capacity flow (=ρ &_i ⋅V_i⋅c_i) [W/°C] i

t

∆ = Temperature drop (=t_i_,_in −t_i_,_out) [°C]

Radiators

For radiators, the special case applies that the coefficient of thermal transmittance U consists in principle of the coefficient of surface thermal insulance on the outside of the radiator. This coefficient depends on the temperature levels, which in turn enable the following expression to be applied (Abel et. al, 1997):

n m rad

rad K t

Q& = ⋅∆ (8) Q& = Thermal power output from the radiator [W]

Hydronic Heating Systems - The Effect of Design on System Sensitivity - Anders Truschel (Book)

HYDRONIC HEATING SYSTEMS

The Effect Of Design On System Sensitivity

Hydronic Heating Systems

Hydronic Heating Systems

ABSTRACT

PREFACE

TABLE OF CONTENTS

NOMENCLATURE

1 INTRODUCTION

1.1 Background

1.2 Purpose

1.3 Working

method

1.4 Work in this field

1.5 The structure of this thesis

2 SYSTEM

DESIGN

2.1 Structure

2.2 Balancing

(

)