District heating (DH) transmission systems are built up of a wide variety of mechanical and electronic components, none of which will be described in detail here. This chapter will be limited to components which, because of their relatively high construction costs, should be chosen with special care.
In this context it is important to be aware that making purchasing decisions solely on the basis of construction costs is not sufficient. For a wide range of components in a transmission system, the choices that are made can influence operating costs markedly. especially when these costs are seen over the dimensioned lifetime of a network, which for transmission systems Is in the range of 50 years.
Furthermore, is it important to realise that on the basis of cumulative experience, comprehensive training of the whole project team is mandatory. In this respect, the project team includes all persons involved in planing and construction, the latter including reestablishing all affected site items, assembling and insulating the pipe system, and establishing overall quality control. Training of this kind is not without its cost penalties. but these can be rapidly regained in terms of eventual savings in the overall operation.
The main components described In the following are:
transmission pipelines.
highpressure heat exchangers. and
heat meters.
Transmission pipelines for DH are analogous to hightension power lines that transmit electricity. Both types of network systems move large amounts of energy over long distances, and both supply systems are held separate from their respective distribution systems. i.e. the interface in an electricity system is a transformer, in a heat transmission system it is a heat exchanger.
Regarding system distances in Denmark. networks extend up to 50 km from the production unit to the furthest heat exchanger station.
Transmission pipelines are employed whenever hot water from a production plant is to be carried to very distant localities. In Denmark, this is normally accomplished at high pressure up to 25 bar but, contrary to practices in other countries, the supply temperature rarely exceeds 1250C. There are a two main reasons for limiting this temperature.
First, it ensures that the widest possible use can be made of preinsulated DH pipes. The higher the temperature in the carrier piping the more rapidly these pipes age. Therefore, in order to gain a longer life, temperatures in excess of 1351400C must be the exception. By using preinsulated pipes the total construction costs of transmission pipelines can be substantially reduced.
Second, by operating DH systems at moderate temperatures in a given combined heat and power system the efficiency can be Improved. This is because the use of a low feed temperature and a sufficient cooling of the DH water supply enables much more heat to be delivered per kilogram of fuel consumed .
Two main types of preinsulated pipes are used for transmission pipelines:
steelinsteel pipe
-and steelinplastic pipe.
Both types comprise a steel carrier pipe in side a protective casing pipe with the cavity between them insulated with polyurethane foam (PUR). The casing pipe is of steel or polyethylene (PEH).
Steel casing is used in exceptional cases where transmission lines are placed above ground level. Here cartontype steel is usually used because of its characteristic, maintenancefree surface properties. The total costs of construction and operation are thus minimised.
Steel casing is, as a rule, also used when underground transmission pipelines are located in areas with high ground water levels or when they are actually submerged in lakes. In both situations it is necessary to protect the steel casing with a polyethylene coating in the same way as is done in natural gas and oil pipelines.
However, by far the greater number of preinsulated transmission pipelines are constructed of steelinplastic piping with steel piping diameters up to 1200 mm.
These pipes are produced in 6 to 24m lengths. The carrier pipe sections are joined by electrowelding; to prevent ingress of water the PEcasing pipe sections are then joined with the aid of socalled welding muffs, i.e. PEpipe flanges with inlaid copper wires. When an electric charge is applied the muff is fusionwelded, the muff and casing thereby forming a continuous whole. As a final phase of the assembly process the cavity between the carrier pipe and welding muff is filled with PUR insulation under a carefully monitored process.
The discovery of atmospheric ozone depletion in recent years has motivated pipe manufacturers to engage in extensive research and development to produce blown foams for preinsulated pipes marketed today that do not involve the use of gases which contribute to this depletion.
Many pipe manufacturers offer all members of project teams the appropriate training for a full understanding of the systems and the necessary work routines. This saves time and money and heightens the quality of the project.
Preinsulated pipes are produced following the principles used in the two main systems:
sliding systems and
bonded systems.
The sliding systems are designed to allow the various materials to move in relation to each other without the need to provide extra longitudinal space in the excavations of the system. Expansion is taken up with the aid of compensators and loop expansion joints. i.e within the casing of the system. In this type of system, excavations can be filled in again immediately after the muffs are fitted, and prestressing is not required. Also, these systems are especially suited to pipelines placed above ground level.
The main characteristic of the bonded system is its ability to prevent movement of the various materials in relation to each other, by fabricating it in a socalled sandwich construction. This means that the pipe structure will move as a whole in the ground under the influence of the stresses created by temperature variations in the DH water. At the same time attention must be given to keeping a long portion of the piperun excavation open the socalled friction length until banding Is completed. This may entail a long elapsed time from the start of the excavation until the pipeline trench can again be filled in with ensuing longlasting traffic and other problems.
Banding is carried out by preheating the pipe with hot air or by means of an electric charge, and maintaining the preheating temperature while the pipeline is covered again with infill. The pipes are held in position by the surrounding infill. which must be care fully compacted, safeguards must then be established to ensure that no excavation will be carried out at a later date in longer stretches of the pipeline.
For all preinsulated types and brands of pipe in Denmark a complete range of fittings is produced, viz, bends, clamps, compensators, reduction fittings. tees, etc.
Changes in direction can be made in various ways: Some manufacturers market precurved pipe lengths. For pipe dimensions up to 315 mm diam., bend fittings are avail able with angles of 7.5. 15, 45, and 90 deg. For largerdimension pipes, minor changes in direction can be made by mitering straight welding flanges, and more extensive changes by the use of prefabricated bends.
As an extra safeguard, transmission systems have a builtin electronic alarm system in the pipes. By means of this system, any damage that occurs giving rise to dampness in the pipe insulation Is registered, its geographical position pinpointed with great accuracy. The damage can then be quickly repaired .
Preinsulated pipes have now been manufactured for more than 20 years and production is carried out at a high level of quality control and quality assurance in modern, fully automated factory plants, thus ensuring a uniform, quality product with resulting long life.
In conclusion, it should be emphasised that it is decisive for the life cycle of preinsulated systems that due consideration be given both during design and in actual practice to the tensile stresses caused by temperature variations under system startup and opera tion.
The use of heat exchangers is an example of a compromise that is made between investment and operating costs, with the objective of minimising any pressure loss in the exchanger and simultaneously maximising the heat transfer coefficient. Detailed analyses of transmission systems have shown that the above demands are satisfied best by plate heat exchangers. at the temperature and pressure levels which are used in operating the systems in which they are placed. Plate heat exchangers for use in DH are built up of a large or small number of channelled stainless steel plates.
These plates are fitted with special, adhesivebonded highpressure gaskets and are mounted in a sturdy frame with the plates interlocking to form channels through which the plant water passes, as a rule. In a direction counter to that of the feed. This makes it possible for the entire system to operate at high efficiency and with a high degree of cooling of the DH water. The gaskets form an outer seal for the plates and around the corner inlets and outlets where the water has its respective primary and secondary sides. In this way, the water flows are kept apart by gasket pairs, each with an evacuated space between them and which, via drain holes. reveal any leakages that may occur.
Plate heat exchangers have the additional advantages of demanding less space than other exchanger types and being built in modular form. The capacity of a plate heat exchanger can thus be Increased by adding more plates, an operation which can proceed in parallel with an expansion of the distribution network. In this way. Investments in heat exchanger capacity can be scheduled to follow demand.
Lastly. plate heat exchangers also allow the use of just a few plate types, and many. different sizes can be built up with small graduations in capacity. This also means that changes in heating demand in relation to the original basis for dimensioning can easily be met by reducing or increasing the number of plates installed.
Under the Ministry of Energy Research Programme for 1993 it is expected that support will be given to implementing the project "Improved energy economy and optimum operating economy in plate heat exchangers in major district heating systems via condition monitoring"
The objective of this project is to develop methods for examining the condition of large plate heat exchangers without taking them out of operation. Normally, plate heat exchangers have to be dismantled to perform this sort of condition monitoring, which often leads to extensive costs for the actual separation, cleaning, and checking of the plates, replacement of gaskets, and the subsequent reassembling of the heat exchanger components. In addition, heat energy is lost (when production is rescheduled) after removing the heat exchanger
Measurements of the heat volumes in circulation and under distribution to consumers are. of course, a central function in a transmission system as these measurements form the basis for assigning the large tariff sums. There are therefore essential requirements which must be met in large heat metering devices, the most important being measuring water temperatures and flows at high
accuracy.
Two types of meters meet these requirements:
-ultrasonic flow meters and
-magnetic induction flow meters.
In transmission systems In Denmark ultrasonic meters are the principal type in use. With their aid, pipe flow speeds are meas ured and the meter calculates the flow (volume per unit of time) electronically by accounting for the sectional area of the pipe.
Flow speed is found as the difference between the duration of electronic impulses directed alternately with and against the direction of flow. For making measurements, ultrasonic sound is used at frequencies of up to several MHz. Sound at this frequency Is produced by transducers utilising a piesoelectronic crystal adhesivebonded to a metal membrane. When this crystal is subjected to an electric charge, it deforms mechanically producing compressions and rarefactions In the air as ultrasonic sound. This process is reversible in that mechanical force applied to the crystal will produce an electric charge on it. A pulsating charge on the crystal will cause It and the steel membrane to oscillate at the applied frequency.
The ultrasonic impulses spread in motionless water at a sound velocity of approx. 1480 m/s. In flowing water the ultrasonic pulse signal is sent in the downflow direction at a relatively slower rate than the upflow signal. This difference Is directly proportional to the flow speed and independent of the sound wave velocity.
The impulses are transmitted and received by two transducers placed at an angle of 45 deg to the length axis of the measured pipe. The transducers exchange roles as transmitter and receiver. One transducer is activated by an electric charge and transmits an ultrasonic wave which is converted to an electric signal by the other transducer at the opposite end of the pipe, and the signal is then passed on to a signal converter.
The signal converter computes the flow speed. It takes into account the variable flow speed in the whole pipe crosssection and calculates the median of the speeds in the sound wave path.
Heat can be measured with the aid of magnetic induction meters (MI meters) which are used in flow measurements in industrial processes to a great extent. An MI meter operates according to Faraday's law of induction. i.e. a potential is induced in an electric conductor passing through a magnetic field. The induced potential is amplified and after digitalisation the signal activates a microprocessor.
Because of adverse experience with the formation of magnetite deposits in largedimension DH pipes, there has been a reluctance to use MI meters in transmission systems. It has been found that the formation of magnetite can lead to large measurement errors when deposits form at points within the magnetic field.
In both types of meters the measurement of flow volume is combined with data on the temperature difference between feed and return flows. In practice, this difference is found by placing a welldefined number of temperature sensors at chosen points in the cross section of the pipe so as to obtain representative measurements. The measurements of the temperature difference and flow volume are registered by the microprocessor mentioned above which transforms the signals into a registration of the energy flow in the pipeline.