Review of existing heat supply schemes
Heating systems play a key role in providing comfortable and safe living and working conditions for people, as well as in maintaining the optimal functioning of industrial enterprises and public buildings. In the modern world, where energy efficiency and environmental requirements are becoming increasingly stringent, the role of these systems goes beyond simply providing heat. They are becoming an important element of sustainable development, helping to reduce energy costs and minimize the impact on the environment. This material was prepared by us, a research team from the Moscow Power Engineering Institute. In this article, we will consider the main functions of heating systems, their types and features.
What types of heating systems are there?
According to the method of producing thermal energy, the following heat supply systems are distinguished:
Decentralized heat supply systems that provide consumers with thermal energy close to the source of its production.
Such systems use individual local heat sources distributed over a certain area. Germany, like other European countries, actively uses decentralized heat supply, especially within the framework of the Energiewende (energy transition) program aimed at switching to renewable energy sources. Decentralized heat supply is especially important in remote and northern regions, such as Canada or Finland. Such systems have high autonomy and are more environmentally friendly than centralized ones.
Their peculiarity is the absence of a single central heat source, which gives them a number of advantages over centralized systems:
reduction of losses during heat transportation due to the closer location of the source to the subscriber;
more efficient use of released heat due to a more accurate load consumption schedule;
the possibility of self-sufficiency in thermal energy through the use of innovative heating methods, the use of solar energy resources, and low-potential heat sources.
In centralized systems, heat production is carried out in a combined manner together with electricity at thermal power plants and large industrial boiler houses.
In this case, it is necessary to prepare the heat carrier and transport it for use. Centralized heat supply is the most popular type of heat supply in the CIS at the moment. During the Soviet Union, a huge number of thermal power plants and boiler houses were built, as well as extensive heat supply networks. This historical legacy allowed the creation of a powerful and extensive infrastructure that is still in use.
This method of providing the consumer with heat has the following advantages:
high economic efficiency due to large scale production;
greater reliability thanks to backup systems and the ability to switch the subscriber to another heat source in the event of an emergency;
reduced environmental impact due to high altitude combustion emissions;
convenience for consumers, since they do not need to independently provide themselves with heat and incur additional costs for heating equipment;
The system is controlled by qualified personnel, which reduces the risk of accidents directly near the consumer.
The main significant drawback of the system is the emergency condition of the pipelines, which can lead to breakthroughs in the heating network, as well as significant heat losses during its transportation to the subscriber, especially in cities with above-ground pipelines.
According to the method of connecting the hot water supply system, heating systems are divided into:
Open heating systems
In such systems, water circulating in the heating network is partially or completely removed from the system by heat consumers.
Main advantages:
the possibility of using a single-pipe heat transport system;
absence of an expensive element – a heat exchanger;
the possibility of using large quantities of waste heat water available at power plants, which allows for fuel savings and reduces the cost of hot water supply;
increasing the durability of hot water supply systems.
Main disadvantages:
direct connection between heating and hot water supply systems, which can lead to a deterioration in water quality at the time of commissioning of buildings, after connecting new systems to heating networks;
instability of hydraulic regimes caused by variable water flow in the return line;
the difficulty of detecting water leaks from the heating network in the event of an accident;
increasing complexity and cost of water treatment, increased sanitary control of the heating system.
Closed heating systems
In such systems, water circulating in the heating network is used only as a heat carrier and is not taken from the network.
Main advantages:
sanitary reliability, ensured by a short pipeline line from the building entry to the water tap;
hydraulic isolation, which ensures stable water quality in the hot water supply system;
minor leaks of coolant in the network;
rapid detection of damage to the heating network by increasing the amount of feed.
Main disadvantages:
the need to install surface heat exchangers in subscriber hot water supply units;
the appearance of scale in heaters and hot water supply system pipelines in the case of using hard water;
the need in many cases to protect hot water pipelines from internal corrosion;
the need to compensate for underheating to the temperature of the heating medium in surface heaters;
complexity of operation and repair due to the presence of a heat exchanger.
According to the method of connecting the heating system, heat supply systems are divided into:
Dependent local heating systems are applicable in conditions where the pressure in heating networks does not exceed the strength of heating devices (0.6 MPa for cast iron radiators; 1.0 MPa for steel convectors).
a) Dependent without mixing
This scheme is applicable if the temperature in the supply pipeline does not exceed the permissible temperature of 95 °C, established in SNiP 41-01-2003. In centralized heat supply systems from a combined heat and power plant, heat is supplied according to a temperature schedule of 150/70 °C or 130/70 °C, therefore, at the subscriber's input, the heating water exceeds the standard temperature value and it is necessary to use other heating connection schemes.
b) Dependent with mixing
In such schemes, return water is mixed with water from the supply line to reduce its temperature and ensure acceptable pressure in the radiators of the heating system. Such schemes are completely dependent on external heat supply.
The advantages are:
low cost of necessary equipment;
ability to withstand large temperature changes;
reduction of pipeline diameter;
lower operating costs;
lower coolant consumption;
the possibility of using a single-pipe heating system.
Flaws:
If the heating system provides the required pressure drop, then an elevator is used for mixing, which is necessary to normalize the pressure and temperature before entering the internal heating network.
The main advantage of using such a scheme is the simplicity and reliability of operation, low cost of equipment. A special feature is maintaining a stable operating mode with pressure changes, since its value is determined based on the selected nozzle diameter. The main disadvantage of using a mechanical elevator is the lack of the ability to regulate it. When covering the peaks of the temperature graph, this disadvantage becomes especially significant, since this leads to an overconsumption of heat for heating and, as a result, to additional financial costs. The use of elevators with a variable nozzle cross-section does not completely solve this problem, since in this case the nozzle resistance increases, which, at a given pressure drop at the inlet, leads to a decrease in water flow through the circuit. In addition, the system does not have independent water circulation in the heating system, which, in the event of damage to the heating network, can lead to freezing of water in the pipes and additional accidents near the subscriber.
The principle of operation of the elevator is as follows: water from the supply pipeline with pressure P1 enters the nozzle, where its static pressure becomes less than the pressure in the return pipeline P2. Thus, a vacuum is created at the outlet of the nozzle, and the return water will be mixed with the direct one. In the mixing chamber, the speed of the total flow is equalized, the pressure remains constant. In the diffuser, as its cross-section increases, the static pressure of the water increases to P3> P2, and the speed decreases. Then the flow with the required parameters goes to the consumer.
If the system does not provide the required pressure drop, then an additional centrifugal mixing pump is installed.
It can be used to regulate the water flow supplied to the subscriber, as well as regulate the pressure. The disadvantages of using a pump include energy costs, noise, and the need to install a backup pump to ensure safety in the event of an accident.
Independent heating system
In this scheme, consumers are connected to the heating network through a recuperative heat exchanger, water from the supply line heats the coolant circulating in the heating system and returns to the network through the return pipeline.
The advantages of this scheme include:
the absence of a rigid hydraulic connection between the heating network and the heating installations of consumers, which increases the reliability of the operation of heat supply systems;
reduction of water leaks in the system;
easier detection of damage in the system;
easier regulation of the coolant temperature in the heating circuit;
increased efficiency;
the possibility of using water purified from impurities in the heating system;
In the event of an accident, the temperature in the heating circuit will decrease more slowly.
The disadvantages include significant costs for installing the necessary additional equipment, and the complexity of maintenance and repair.
Authors of the material: Temrina D.N., Guzhov S.V.
