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District heating is a technical system for heating a town or a part of a city. The heat is produced in a power plant, and then transported in a piping system to the consumers in the form of hot water or steam.
The heat can be obtained from an oil-fired furnace, a Combined Heat and Power plant (CHP) producing both heat and electricity or a geothermal plant.
District heating, shown in Figure 5.1, is used mostly in places where the heat demand is high and the consumers are located fairly closely. Hence, district heating networks can be found in towns in countries with a cold climate.
Due to tradition and the history of technological evolution, the greatest relative use of district heating is in the Nordic countries, Eastern Europe and Russia. However, district heating is also found in the USA, Western Europe, China, Korea and Japan (see Table 5.1).
In this chapter, the focus will be on the so-called substation, or subscriber station. This is the technical system placed at the consumer end, normally in the basement of the subscriber's building. It modifies the heat to suit the consumer, and must meet a combination of demands from consumers and the district heating company:
The systems presented are indirect, which means that the district heating water is not used directly for heating. Instead, there is a heat exchanger between the district heating water and the radiator water (see Figure 5.3).
An indirect configuration is beneficial for several reasons. First, any leakage is restricted to its own circuit. Second, the radiators are not dimensioned for such high pressure as in the primary network (up to 8 bar), and there is more corrosion in direct systems. Hence, the risk of radiator system leakage is much lower for indirect systems. Finally, the heating companies do not want dirt to enter their systems from badly maintained secondary systems.
Radiator System
The radiator system (see Figure 5.4) is a system used for space heating. The radiator is a type of heat exchanger that heats the air in a room by a combination of natural convection and radiation.
The radiator system consists of:
In most water-borne systems, there is also a control system. This uses an outdoor temperature sensor and a forward radiator pipe temperature sensor to decide the forward pipe temperature as a function of the outdoor temperature.
The radiator temperature after the heat exchanger is a function of the outdoor temperature. The temperature set point is adjusted on the controller according to the outdoor temperature. When the radiator temperature is too high or too low, according to the outdoor temperature sensor, the temperature sensor sends a signal to the actuator to close or open the control valve.
The purpose of the hot tap-water system (see Figure 5.5) is to provide hot tap water at a specified temperature, usually 40-65°C, in all load conditions. An excessively high temperature promotes scaling on the heat exchanger surface.
The risk of bacterial growth at 35-40°C must also be considered. Because hot tap water in most district heating stations is freshly prepared, the heat must be transferred at the same moment the hot tap water is consumed. To avoid the presence of Legionella, the hot water system must not be stagnant, i.e. a hot water circulation system should be installed.
The hot tap water system consists of the following components:
The temperature set point for the hot tap water is adjusted on the controller. The temperature of the hot tap water is measured by a temperature sensor (see Figure 5.5) located in the outlet pipe of the secondary side of the heat exchanger.
The difference between the desired and the actual hot tap water temperature is calculated in the controller. If the difference is positive, the hot tap water temperature is too low. The controller sends an open signal to the actuator. The actuator opens the control valve and the flow is increased. The heat transferred to the cold water is increased, which increases the hot tap water temperature.
To reduce the time to obtain hot tap water at the correct temperature, the hot tap water conduit is equipped with a return pipe with a small diameter. A pump is placed on the return pipe close to the district heating station. The pump circulates a small amount of water to keep the temperature in the hot tap water pipe at a constant high level. The purpose of the circulation system is to compensate for the heat losses from the hot tap water pipe.
Components such as valves, actuators and controllers sometimes fail. To avoid overheating of the hot tap water, the hot tap water pipe after the heat exchanger is equipped with a three-way valve that mixes the not so hot water with the hot water from the heat exchanger.
The emergency valve is normally closed. Its purpose is to send cold water to the hot water taps when the water heater is shut off so that consumers do not leave the taps open at such times.
Different substations can have various numbers of stages, various types of water heaters, etc. In this section, the following main layouts are discussed:
The classification of district heating stations depends on the number of cooling stages the primary heat carrier undergoes as it returns to the production plant.
Two-stage scheme
The most common substation principle is known as the two-stage configuration (see Figure 5.6).
The primary return flow from the radiator heat exchanger is mixed with primary flow from the so-called after-heater (see Figure 5.6).
The mixed flow then enters a third heat exchanger, the preheater. The main purpose of the preheater is to preheat the cold water before it enters the after-heater. This utilizes some heat from the radiator system and cools the primary heat carrier before it returns to the production plant. The after-heater and the preheater are often in one heat exchanger known as a two-stage heat exchanger. This is explained in greater depth in District Heating/CBE Design: Tap Water Heat Exchanger. Figure 5.7 shows an indirect two-stage connection scheme with the principal components.
One of the major problems of this type of system is the risk of high pressure-drop over the preheater if the primary return flow from the space heating exchanger is higher than the required primary flow for heating tap water.
Fouling of the heat transfer surface in the tap water circuit should also be considered if the temperatures required for the radiator system are high. For example, the temperature program 70-90°C for the radiator system will give a return temperature on the primary side of 75°C (at the lowest outdoor temperature) entering the preheater stage of the tap water system. If there is no tap water flow, the temperature in the preheater stage will increase and eventually reach the same temperature as the primary return temperature. The effect of this problem may be apparent with outdoor temperatures of -5°C or below.
Parallel scheme
By using separate heat exchangers for the space heating and the hot tap water systems, the primary heat carrier is cooled in only one stage per system (see Figure 5.8).
A district heating stations with this arrangement can be categorized as a one-stage configuration or, more commonly, a parallel configuration. The SWEP concept corresponds to the parallel configuration due to its simplicity and robustness. A district heating system with the parallel configuration is shown in Figure 5.9. This layout is simpler that the two-stage layout.
Traditionally, district heating substations were built in the two-stage configuration. However, it has been proven over the last ten years that the efficiency difference between two-stage and parallel configurations is often negligible. Parallel configuration is therefore preferred because it is simpler, more robust and cheaper.
German configuration
The German configuration is a variation of the parallel configuration in which the radiator and hot tap water systems are connected. This configuration requires more components, and the hot tap water system is often equipped with an accumulator tank, although this is not necessary. The principle of the German configuration is shown in Figure 5.10.
Figure 5.11 shows a district heating system with the German configuration.
The advantage of the German configuration is that the thermal strain on the tap water BPHE is reduced because the immediate loop has a lower temperature than the primary circuit. In fact, the primary forwarding temperature in Germany is quite high (130-140 °C), which would lead to high thermal tensions unless an intermediate loop were used.
If you would like to learn more about efficient District Heating applications, download the free digital handbook "A Technical Handbook for Heating Applications". Go to download page.
