What’s a heating substation?
The compact heating substation is used for controlled heat exchange in district heating systems. The heat exchange is ensured between district heating pipeline system (primary system) and house and building heating pipeline network (secondary system). A heating substation is connected to a primary and secondary system of the pipework, measurement devices, domestic water inlet, expansion line, and to the ventilation system. Heating substation consists of heat exchangers, primary and secondary regulating devices, pumps, measurement devices, valves, pipework and pipe armature.
Two-way motorised control valve controlled by a microprocessor controller provides optimal heat exchange according to outside temperature. The maximum flow rate can be limited to the specified value programmed with a relevant control function. The plate heat exchanger, specially designed for district heating use, is available as a compact brazed type or gasketed plate. Both types have different range sizes. Ultrasonic heat meter provides stable and accurate measurement and reading of all measured values. The compact heat substation offers a high comfort level and optimum energy utilization for customers. They could be easily connected to the supervisory control and data acquisition systems (SCADA). Compact heat substations are produced like standard products according to specified customers requirements and all technical conditions of local heat energy distributors.
It is an independent i.e. closed heating system, which means that the district heating water is not used in the heating system of the building. Two different water systems are separated by the heat exchanger. An independent system has several advantages over the dependent i.e. an open system where district heating water is used directly in the building’s heating systems. Firstly, any leak in the system is limited to its own circuitry. Secondly, the radiators are not dimensioned to the pressures that occur in the district heating system. With the independent heating system, the risk of building’s heating system leak is much lower. Thirdly, district heating companies – boiler houses do not want the dirty water of poorly maintained buildings’ heating systems to enter their system.
Radiator heating system
Purpose of the radiator heating system is to heat the rooms using heat radiation from the radiators. The system consists of:
- Heat exchanger – provides hot water for building’s heating system, i.e. hot water for the secondary system.
- Circulation pump – controls water circulation in the system
- Valves – limit the flow rate in order for every part of the heating system to have a right flow rate which ensures the required room temperatures.
- Expansion tank – compensates for the alternations in system’s volume that are caused by the temperature changes in the system.
- Piping – carries the water from the substation to the radiators and back
In addition to these, regulating system bears the utmost importance in the work balance of the system, regulating the hot water temperature going to the radiators, according to the outside temperature, i.e. automatic regulator regulates the work of the actuator valve in accordance with the outside temperature.
If the hot water temperature is too high/low compared to the outside temperature, hot water temperature sensor will send a close/open signal to the actuator valve.
Domestic hot water system
Purpose of the hot water system is to ensure the hot water supply, in conditions of varying consumption and on pre-set temperatures(usually 55℃). On higher temperatures, the scale starts to build up in the heat exchanger which in turn lowers the efficiency of the heat exchanger. Lower temperatures(35 – 40℃) should be avoided since these are good for bacterial growth. To avoid Legionella bacteria, domestic water should be circulating in the system at all times, i.e. a hot water circulation pump should be used.
Components of the hot water system are as follows:
- Heat exchanger
- Regulated valve
- Actuator valve
- Automatic regulator
- Temperature sensors
Domestic hot water system’s work is regulated with an automatic regulator that keeps the prescribed hot water temperature. Hot water temperature is measured on the secondary side of the heat exchanger, on the outgoing pipe( the pipe that provides domestic hot water to the house). Regulator calculates the temperature difference between the real and the prescribed temperatures. If the difference is positive, water temperature is too low. In this case, a signal is sent to the actuator valve and the valve is opened, i.e. hot water flow is increased, which in turn increases the heat exchange and therefore the water temperature.
Heating substation components
Heat exchangers
In heating substations, brazed plate heat exchangers are generally used. Brazed plate heat exchangers provide efficient heat transfer with a small footprint. They are maintenance free, provide a long service life and can withstand high temperatures and extremely high design pressures. They are used in a range of duties including cooling, heating, evaporation and condensing. In Copper Brazed heat exchangers, the surfaces used to provide heat transfer from one medium to the other are thin, corrugated stainless steel plates stacked on top of each other. The two media are sealed within the plate pack by a brazed copper seal around the edge of the plates. This results in a heat exchanger that – unlike traditional designs – consists solely of surfaces which actively contribute to heat transfer, resulting in significant increases in overall efficiency. The channels formed between the plates and corner ports are arranged so that the media flow through alternate channels – always in a counter-current flow pattern in order to achieve the most efficient heat transfer.
Automatic regulator
The automatic regulator is an essential part of the substation’s control system. Regulator receives signals from the sensors, then formulates the signals into a comparable value and compares the value to the prescribed value. The regulator controls the actuator valve, which in turn controls the control valve position, measuring heat exchanger’s incoming and outgoing water temperatures with temperature sensors.
Temperature sensors
Sensors are key to precision management of the heating substation. If the sensors are not working correctly, the substation can’t work correctly either. Temperature sensors are used to measure primary and secondary side’s water temperatures and the outside temperature.
Actuator valve
An actuator valve automatically regulates the position of a control valve.
Valves
Valves are important for controlling and regulating the flow rate of the steam and water in the district heating system. Commonly used valves are a control valve, balancing valve, shut-off valve and safety valve.
The control valve is used for flow rate control. Balancing valve uses the latest flow technology to ensure that the design flow rate is achieved at all times irrespective of any pressure changes within the system, they can also work as shut-off valves. Safety valves protect the system from any damage that might occur when the system’s pressure rises due to the increase in water temperature and flow rate.
Circulation pumps
Circulation pumps are used on the secondary side of the substation to ensure the water circulation in the system. Wet rotor motors are commonly used for the application.
Filters
In order to protect the heat exchangers, control valves and other components, inputs of the heating substation need to be equipped with filters. The inputs are district heating’s feed line, return lines of the radiator and ventilation contours and the cold water inlet. Holes of the filter’s sieve have to measure 0,6 mm and the sieves have to be easily cleanable.
Expansion tank
An expansion tank or expansion vessel is a small tank used to protect closed (not open to atmospheric pressure) water heating systems and domestic hot water systems from excessive pressure. The tank is partially filled with air, whose compressibility cushions shock caused by water hammer and absorbs excess water pressure caused by thermal expansion. The modern vessel is a small container or tank divided in two by a rubber diaphragm. One side is connected to the pipework of the heating system and therefore contains water. The other, the dry side, contains air under pressure, and normally a Schrader valve (car-tire type valve stem) for checking pressures and adding air. When the heating system is empty or at the low end of the normal range of working pressure, the diaphragm is pushed against the water inlet; as the water pressure increases, the diaphragm moves, compressing the air on its other side.