Today, organizing processes for providing water is one of the main conditions for creating a comfortable life for citizens. There are a few in various ways how to ensure water supply, including the creation of hot water supply network systems, but one of the effective ways today is to heat water through a heating network.
Heat exchangers must be selected based on installation and placement conditions, as well as according to user requests and general capabilities for the installation and operation of heating equipment. In most cases only correct installation and competent calculation allow citizens to forget about what interruptions or complete absence hot water supply.
Using plate-type heat exchangers to provide domestic hot water
Heating water through heating networks is useful in economic terms, since heat exchangers, when compared with classic boilers using electric or gas energy, work only for the heating system, and nothing else. As a result, the cost hot water per liter will be much lower.Plate-type heat exchangers use heat energy in heating networks to heat ordinary tap water. Heated by heat exchange plates, hot water penetrates all water distribution points, including faucets, taps, and showers.
It is also important to take into account that the water being heated and the water, which is a heat carrier, do not interact with each other in any way within the heat exchanger. The media for water flow are separated from each other by plates placed in heat exchanger, so heat exchange passes through them.
It is impossible to use water in heating systems to meet domestic needs; it is harmful and irrational. This is explained by the following reasons:
- 1. The processes of water preparation for equipment and boilers is an expensive and, most often, complex procedure that requires special knowledge, experience and skills.
- 2. In order to soften the water and make it less hard for heating system, reagents and chemicals are used that have a negative impact on human health.
- 3. Over many years, heating pipes accumulate a large amount of deposits, which are also harmful to humans and their health.
Types of heat exchangers for hot water systems
Today there are many of them, but among all, the two most popular for everyday use are shell-and-tube and plate-type systems. It should be noted that shell and tube systems have almost disappeared from the markets due to low efficiency and large sizes.
A plate-type heat exchanger for hot water supply consists of several corrugated plates located on a rigid frame. They are identical to each other in design and dimensions, however they follow each other, but according to the principle of mirror reflection, and are divided among themselves by specialized gaskets. Gaskets can be either steel or rubber.
Due to the alternation of plates in pairs, cavities appear that, during operation, are filled with either a heating liquid or a heat carrier. It is due to this design and principle of operation that the displacement of media between themselves is completely eliminated.
Through the guide channels, the liquids in the heat exchanger move towards each other, filling the even-numbered cavities, and then exit the structure, receiving or giving up some of the heat energy.
Scheme and principle of operation of a hot water supply plate heat exchanger
The more plates in number and size there are in one heat exchanger, the larger the area it can cover, and the greater its productivity and useful action at work.
For some models, there is space on the direction beam between the locking plate and the frame. It is enough to install a couple of slabs of the same type and size. In this case, the tiles installed additionally will be installed in pairs.
All plate-type heat exchangers can be divided into several categories:
- 1. Soldered, that is, non-separable and having a sealed main body.
- 2. Collapsible, that is, consisting of several individual tiles.
The main advantage and plus of working with collapsible structures is that they can be modified, modernized and improved, from removing unnecessary ones or adding new plates. As for soldered designs, they do not have such a function.
However, plate-brazed heat supply systems are more popular today, and their popularity is based on the absence of clamping elements. Thanks to this, they are distinguished by compact sizes, which do not in any way affect their usefulness and performance.
Connection diagrams
A heat exchanger operating on the water-water principle has several various schemes connections, however, the primary type circuits are mounted to the distribution pipes of the heating network (it can be private or implemented by city services), and the secondary type circuits are mounted to the water supply pipeline.Most often, it depends only on design decisions what type of connection is allowed to be used. Also, the installation diagram and its choice are based on the norms of “Design of Heating Stations” and in the SP standard under number 41-101-95. If the ratio and difference of the maximum possible water heat flow for hot water supply to the heat flow for heating is determined in the range from ≤0.2 to ≥1, then the basis is a connection diagram in one stage, and if from 0.2≤ to ≤1, then from two degrees .
Standard
The easiest to implement and most cost-effective scheme is parallel. With this scheme, heat exchangers are mounted in series with respect to the control valves, that is shut-off valve, as well as parallel to the entire heating network. In order to achieve maximum heat exchange within the system, high heat carrier flow rates are required.
Two-stage scheme
Two-stage mixed system
If you use a two-stage scheme, then water is heated either in a pair of independent devices or in a monoblock installation. It is important to remember that the installation scheme and its complexity will depend on the overall network configuration. On the other hand, with a two-stage design, the efficiency level of the entire system increases and the consumption of coolants decreases (by about 40 percent).
With this scheme, water preparation occurs in two steps. During the first step, apply thermal energy, heating the water to 40 degrees, and during the second step the water is heated to 60 degrees.
Serial connection
Two-stage sequential circuit
This scheme is implemented within one of the devices for heat exchange of hot water supply, and this type of heat exchanger is much more complex in design when compared with standard schemes. It will also cost much more.
Calculation of heat exchangers
When determining a heat exchanger, it is necessary to take into account such parameters as:- 1. number of users or residents;
- 2. consumption and consumption rate of warm water per day for each consumer;
- 3. the maximum possible temperature of coolants for a certain time period;
- 4. temperature and other indicators tap water for a certain time period;
- 5. permissible heat loss rates (according to standards, this figure should not exceed 5 percent);
- 6. the total number of places for water intake (this can be taps, mixers or showers);
- 7. mode and operation of equipment (continuous or periodic).
The performance and efficiency of the heat exchange system for apartments in the city (in particular, when connected to the heating network) is calculated based on performance indicators in winter period. In winter, the temperature of heat carriers can reach 120/80 degrees.
At the same time, indicators during spring or autumn can drop to the level of 70/40 degrees, and the temperature will remain very low until the critical level. That is why it is important to carry out calculations and indicators of the heat exchanger simultaneously both for spring and autumn, and for operation during winter.
It is also important that no one can guarantee that these calculations will be 100 percent correct. The thing is that in the housing and communal services sector they often prefer to ignore or neglect standards for servicing the end consumer.
In the private sector, these indicators are much more accurate, because the user is always confident in the efficiency and performance of the boiler and the entire heating system.
Hot water supply (HW) networks have much in common with cold water supply networks. The hot water supply network comes with lower and upper wiring. The hot water supply network can be dead-end and looped, but, unlike cold water supply networks, looping the network is necessary to maintain a high water temperature.
Simple (dead-end) hot water networks are used in small low-rise buildings, in household premises industrial buildings and in buildings with stable hot water consumption (baths, laundries).
Schemes of hot water supply networks with a circulation pipeline should be used in residential buildings, hotels, dormitories, medical institutions, sanatoriums and rest homes, in preschool institutions, as well as in all cases where uneven and short-term water withdrawal is possible.
Typically, a hot water supply network consists of horizontal supply lines and vertical distribution pipelines-risers, from which apartment distribution lines are arranged. Hot water supply risers are laid as close to the appliances as possible.
Figure 1. Diagram with the upper distribution of the supply line: 1 - water heater; 2 - supply riser; 3 - distribution risers; 4 - circulation network
In addition, hot water supply networks are divided into two-pipe (with looped risers) and single-pipe (with dead-end risers).
Let's consider some of the large number of possible hot water supply network schemes.
When the lines are routed from the top, the prefabricated circulation pipeline is closed in the form of a ring. The circulation of water in the pipeline ring in the absence of water intake is carried out under the influence of gravitational pressure that arises in the system due to the difference in the density of cooled and hot water. The water cooled in the risers falls down into the water heater and displaces water with a higher temperature from it. Thus, continuous water exchange occurs in the system.
Dead-end network diagram(Fig. 2) has the lowest metal consumption, but due to significant cooling and irrational discharge of cooled water, it is used in residential buildings up to 4 floors high, if the risers are not equipped with a heated towel rail and the length of the main pipes is small.
Figure 2. Dead-end hot water supply circuit: 1 - water heater; 2 – distribution risers
If the length of the main pipes is large and the height of the risers is limited, use circuit with looped supply and circulation lines with installation of a circulation pump on them (Fig. 3).
Figure 3. Scheme with looped main pipelines: 1 - water heater; 2 - distribution risers; 3 - diaphragm (additional hydraulic resistance); 4 - circulation pump; 5 - check valve
The most widespread two-pipe scheme(Fig. 4), in which circulation through risers and lines is carried out using a pump that takes water from the return line and supplies it to the water heater. A system with one-sided connection of water points to the supply riser and installation of heated towel rails on the return riser is the most common version of such a scheme. The two-pipe scheme turned out to be reliable in operation and convenient for consumers, but it is characterized by high metal consumption.
Figure 4. Two-pipe hot water supply scheme: 1 - water heater; 2 - supply line; 3 - circulation line; 4 - circulation pump; 5 - supply riser; 6 - circulation riser; 7 - water intake; 8 - heated towel rails
To reduce metal consumption, in recent years they have begun to use a scheme in which several supply risers are combined by a jumper with one circulation riser(Fig. 5).
Figure 5. Scheme with one connecting circulation riser: 1 - water heater; 2 - supply line; 3 - circulation line; 4 - circulation pump; 5 - water risers; 6 - circulation riser; 7 - check valve
Recently appeared diagrams of a single-pipe hot water supply system with one idle supply riser per group of water risers(Fig. 6). The idle riser is isolated and installed in pairs with one water riser or in a sectional unit consisting of 2-3 looped water risers. The main purpose of the idle riser is to transport hot water from the main to the upper lintel and then to the water risers. In each riser, independent additional circulation occurs due to the gravitational pressure that arises in the circuit of the sectional unit due to the cooling of water in the water risers. The idle riser helps the correct distribution of flows within the sectional unit.
Figure 6. Sectional single-pipe hot water supply diagram: 1 - supply line; 2 - circulation line; 3 - idle supply riser; 4 - water riser; 5 - ring jumper; 6 - shut-off valves; 7 - heated towel rail.
In order for any residential building to function normally, it is necessary to install a water supply system. Its proper design will ensure timely supply and sufficient water pressure. This article will discuss in detail the hot water supply scheme, types of connection and its features in an apartment building.
What is special about the water supply of an apartment building?
It is very difficult to provide water to a building with a large number of storeys. After all, the house consists of many apartments with separate bathrooms and plumbing fixtures. In other words, water supply schemes in apartment buildings- this is a kind of complex with separate pipe distribution, pressure regulators, filters and metering equipment.
Most often, residents of high-rise buildings use water from the central water supply. With the help of a water supply, it is supplied to individual plumbing fixtures under a certain pressure. Often water is purified using chlorination.
Composition of the central water supply system
Centralized water supply schemes in multi-storey buildings consist of a distribution network, water intake structures and treatment plants. Before entering the apartment, the water passes long haul from pumping station to the pond. Only after purification and disinfection is water sent to the distribution network. With the help of the latter, water is supplied to appliances and equipment. Pipes of the central hot water supply circuit multi-storey building can be made of copper, metal-plastic and steel.
The latter type of material is practically not used in modern buildings.
Types of water supply schemes
There are three types of water supply system:
- collector;
- sequential;
- combined (mixed).
Recently, when a large number of plumbing equipment is increasingly found in apartments, they use collector wiring diagram . She happens to be the best option normal functioning of all devices. The collector-type hot water supply scheme eliminates pressure drops at different connection points. This is the main advantage of this system.
If we consider the diagram in more detail, we can conclude that there will be no problems with using the plumbing equipment for its intended purpose at the same time. The essence of the connection is that each individual water consumer is connected to the cold and hot water supply riser collectors separately. The pipes do not have many branches, so the likelihood of leakage is very low. Such water supply schemes in multi-storey buildings are easy to maintain, but the cost of the equipment is quite high.
According to experts, the collector hot water supply system requires the installation of a more complex installation of plumbing fixtures. However, these negative aspects are not so critical, especially considering the fact that the collector circuit has many advantages, for example, hidden installation of pipes and accounting individual characteristics equipment.
Sequential hot water supply circuit multi-storey building - this is the easiest way to wire. This system is time-tested; it was put into operation during the Soviet era. The essence of its device is that the cold and hot water supply pipelines are parallel to each other. Engineers advise using this system in apartments with one bathroom and a small amount of plumbing equipment.
Popularly, such a hot water supply scheme for a multi-storey building is called a tee scheme. That is, from the main highways there are branches that are connected to each other by tees. Despite the ease of installation and savings consumables, this scheme has several main disadvantages:
- In the event of a leak, it is difficult to look for damaged areas.
- Inability to supply water to a separate plumbing fixture.
- Difficulty in accessing pipes in case of breakdown.
Hot water supply for an apartment building. Scheme
Pipe connections are divided into two types: to the hot and cold water supply riser. Briefly they are called cold water and hot water. Special attention hot water system deserves apartment building. Scheme DHW networks consists of two types of wiring - lower and upper. To save high temperature Looped wires are often used in pipelines. The gravitational pressure forces the water to circulate in the ring, despite the absence of water intake. In the riser it cools and enters the heater. Water with a higher temperature is supplied to the pipes. This is how continuous circulation of the coolant occurs.
Dead-end highways are also not uncommon, but most often they can be found in utility rooms industrial facilities and in small residential buildings with low number of floors. If water selection is planned intermittently, then a circulation pipeline is used. Engineers advise using hot water supply in apartment buildings (the diagram was discussed above) with a number of floors of no more than 4. A pipeline with a dead-end riser is also found in dormitories, sanatoriums and hotels. Dead-end network pipes have a lower metal consumption and therefore cool faster.
DHW networks include a horizontal main pipeline and distribution risers. The latter provide pipe distribution to individual objects - apartments. DHW is installed as close as possible to plumbing equipment.
For buildings with a large length of main pipes, schemes with circulation and looped supply pipelines are used. Required condition is to install a pump to maintain circulation and constant water exchange.
Two-pipe DHW circuit - Photo 07
Modern builders and engineers are increasingly resorting to the use of two-pipe hot water systems. The principle of operation is that the pump takes water from the return line and supplies it to the heater. This pipeline has a higher metal consumption and is considered the most reliable for consumers.
The schematic diagram of a hot water supply system includes an installation for heating cold water to a temperature not exceeding 75 ° C and a network of distribution pipelines. For this purpose, high-speed buses are used. instantaneous water heaters. In such water heaters, water flows at a significant speed through the heating tubes, which in turn are heated by water from the heating network passing inside the body of the water heater and washing them.
When preparing hot water in the central heating station according to closed scheme use high-speed water heaters OCT 34-588-68 (coolant - water), OCT 34-531-68 and OCT 34-532-68 (coolant - steam).
Rice. 174. High-speed water heaters: a - sectional OST-34-588-68, b - steam; 1 - body, 2 - lens compensator, 3 - grille, 4 - brass tubes, 5 - pipe system, 6 - rear water chamber, 7 - cap, 8 - front water chamber
Water heaters OST 34-588-68 ( , a) are designed for a pressure of 1 MPa and a coolant temperature of 150 ° C. They are produced in separate sections with an outer diameter of 57 to 325 mm with a heating surface of each section from 0.37 to 28 m2. The required heating surface of the water heater is made up of similar sections connected to each other by rolls. The section consists of a body 1 with steel tube sheets 3 welded to it and a bundle of brass tubes 4 with a diameter of 16X1 mm. Nozzles with flanges are welded to the body to connect the sections in the interpipe space. Hot water from the heating network is directed into the interpipe space, and the heated water moves through the tubes of the water heater.
Steam water heaters (OST 34-531-68 and OST 34-532-68) (,6) are designed for heating water with steam in heating and hot water supply systems. Maximum operating steam pressure 1 MPa. Water heaters are produced as two-pass (OST 34-531-68) and four-pass (OST 34-532-68). The heating surface can be from 6.3 to 224 m2.
The water heater consists of housing 1, pipe system 5, front 8 and rear 6 water chambers. The pipe system includes steel gratings and a bundle of brass tubes with a diameter of 16X1 mm. Heated water enters through the lower pipe of the front inlet chamber, passes through brass tubes, is heated and goes into the network through the upper pipe. The steam that heats the water enters the interpipe space.
The water heated in the water heater enters the hot water supply system through the supply pipeline, from which consumers use it for domestic and industrial purposes. The water taken from the system is replenished from the water supply.
To heat the water that has cooled in the system, a circulation pipeline is laid that connects the hot water supply system to the water heater.
To maintain a constant flow of water coming from the heating network, a flow regulator is installed, and on the supply pipeline cold water in the water heater - a water meter that takes into account water consumption. At the control unit of water heaters, valves are installed to shut off the pipeline of the hot water supply and heating system and individual parts of the unit. The pressure and temperature of water at individual points of the control unit are measured with pressure gauges and thermometers.
Depending on the purpose, hot water supply systems are made with two-pipe risers, one of which is circulation, and single-pipe.
Two-pipe hot water supply systems with circulation risers() are used where cooling of water in pipes is not allowed, for example in multi-storey residential buildings, hotels, hospitals and other buildings.
Rice. 175. Two-pipe hot water supply system with circulation risers
Rice. 176. Single-pipe hot water supply circuit: 1 - diaphragm, 2 - plug valve, 3 - supply transit line, 4 - circulation transit line
In single-pipe centralized hot water supply systems used in residential buildings(), the risers within one section at the top are connected to each other, and all risers, except one, are connected to the supply line 3, and one idle riser is connected to the circulation line 4. To ensure uniform circulation of water in the hot water supply systems of buildings connected to one central heating point, a diaphragm is installed on the idle riser.
For better water distribution to individual points of water consumption, as well as in order to maintain the same diameters along the entire height of the building in single-pipe hot water supply systems, the risers are looped. With a ring scheme, for buildings up to 5 floors high inclusive, the diameters of the risers are 25 mm, and for buildings from 6 floors and above - 32 mm in diameter. Temperature extensions in the risers of hot water supply systems of high-rise buildings are compensated by installing single-turn heated towel rails, and in double-pipe hot water supply systems by installing U-shaped compensators on the risers.
Heated towel rails made of galvanized pipes are connected to the hot water supply system using a flow-through circuit. Hot water supply pipelines, in order to protect against corrosion, should be made of galvanized steel pipes.
To ensure air removal from the system, pipes are laid with a slope to the input of at least 0.002. In systems with bottom wiring, air is removed through the top tap. With overhead wiring, air is removed through automatic air vents installed at the highest points of the systems.
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Types and advantages of flow-through DHW circuits
DHW using a flow circuit and plate heat exchangers is the most effective and hygienic way to prepare hot water. Compared to battery circuits, it has significant advantages.
For flow-through DHW, a parallel single-stage scheme, sequential and mixed two-stage schemes are used.
Parallel single-stage circuit with one heat exchanger connected to the supply pipeline of the heating network parallel to the heating system ( rice. 1), is simple and inexpensive.
A two-stage DHW scheme is used to reduce the water temperature in the return pipeline and the total water flow from the heating network. To do this, the heat exchange surface of the DHW heat exchanger is divided into two sections called stages. In the first stage it is cold tap water heated by water leaving the heating system. Then the water heated in the first stage of the heat exchanger is heated together with the recirculation water to the required temperature (55-60 °C) network water from the supply pipeline of the heating network.
At sequential circuit The second stage DHW is connected in front of the heating system to the supply pipeline ( rice. 2). Hot first network water passes through the second stage of the hot water supply, then enters the heating system. Thus, it may turn out that the temperature of the coolant will be insufficient to cover the heat losses of the building. Then during the selection large quantity hot water during peak hours, the building connected to the ITP may not heat up enough. Due to storage capacity building structure this does not affect the comfort in the premises if the period of insufficient heat supply does not exceed approximately 20 minutes. For the summer non-heating period, there is a switchable bypass through which network water after the second stage enters the first stage of the hot water supply, bypassing the heating system.
The mixed two-stage DHW scheme is distinguished by the fact that its second stage is connected to the supply pipeline of the heating network in parallel to the heating system, and the first stage is connected in series ( rice. 3). The network water leaving the second stage of the hot water supply is mixed with the return water from the heating system and also passes through the first stage.
Thus, the comfort in the premises of a building with a mixed two-stage DHW scheme is not reduced, but more network water is consumed than with a sequential DHW scheme ( rice. 4).
* Based on the book by N.M. Zinger et al. “Increasing the efficiency of heating points.” M., 1990.
The two-stage scheme is most widespread in residential buildings with significant DHW loads relative to heating. In buildings with very low or high thermal DHW loads, compared to heating (1< Q ГВС /Q О < 5), по current standards, a parallel single-stage DHW scheme is used.
In Western countries, recently they are increasingly thinking about using the flow-through method of hot water supply, especially after recognizing the serious danger of infection with legionella - bacteria that multiply in stagnant water. warm water. Strict regulations already adopted in European countries require regular thermal disinfection of storage tanks and the hot water pipes connected to them, including recirculation pipes. Disinfection is carried out by raising the temperature in the entire system for a certain time to 70 °C and above. The complexity of battery circuits required for this especially reveals the advantages of flow-through DHW systems with plate heat exchangers. They are simple and compact, require less investment, while providing lower return temperatures and lower network water consumption.
Lower water temperature in the return pipeline of heating networks reduces heat losses and increases the efficiency of electricity generation at thermal power plants. Lower consumption of network water requires smaller diameters of heating network pipelines and lower energy consumption for pumping it.
Regulatory options
Currently, many companies are working hard on automatic regulators, which would provide a comfortable hot water temperature with an accuracy of 1-2 °C or less. In battery tanks, uniform heating is achieved by natural or artificial mixing of the incoming water with the water in the tank.
For this purpose in flow-through DHW systems, especially with low and sharply changing flow rates, when regulating the temperature of hot water, it is necessary to take into account, in addition to temperature, flow rate as a second value. Leading manufacturing companies have developed regulators for small - for one consumer - flow, operating without auxiliary energy. These regulators take into account both the flow and temperature of hot water. Unlike conventional thermostatic regulators, in the absence of hot water flow, these devices can completely stop the supply of heating fluid, which protects the DHW heat exchanger from the formation of lime deposits.
In flow-through hot water systems with high hot water consumption, fluctuations in flow rate, compared to its overall value, are smaller, and satisfactory temperature control accuracy can be achieved by using both thermostatic and electronic controllers. However, in electronic regulators it is necessary to smooth out the regulation curve the right choice the law of regulation and the characteristics of the control valve itself - the stroke speed of the regulator drive, the valve diameter Dn, its hydraulic resistance k VS - in order to eliminate the phenomenon of oscillation throughout the entire range of its operation. Constant opening and closing of the regulator at high frequency exposes plate heat exchanger DHW is exposed to high thermal and hydraulic loads, which will lead to its premature failure due to the occurrence of external or internal leaks.
To prevent fluctuations with large differences in hot water flow or with significant fluctuations in the heating water temperature, for example 150-70 °C, it is advisable to install two parallel regulators of different diameters, which - in themselves - optimally provide a certain range of network water flow ( rice. 5).
As noted above, in the absence of hot water supply, for example in systems without recirculation or with regular water supply shutdowns, it is necessary to protect the heat exchanger from carbonate deposits by stopping the supply of network water. At high flow rates, this can be achieved by using combined regulators with two temperature sensors - heated and heating water - at the outputs of the heat exchanger ( rice. 6). The second sensor, set, for example, at 55 °C, stops the supply of coolant to the heat exchanger even in the case when the hot water temperature sensor is installed far from the heat exchanger and is not influenced by the heating medium due to the lack of water intake. At a temperature in the heat exchanger of 55 °C, the process of deposition of hardness salts slows down significantly.
The closer the sensors are installed to the environment whose parameters are subject to regulation, the more quality regulation can be achieved. Therefore, it is advisable to install temperature sensors, if possible, deeper into the corresponding fittings of the heat exchanger. To do this, you can use plate heat exchangers with fittings on both sides of the plate pack, where a temperature sensor is inserted into one of the fittings, and the other is used to extract coolant. Then the sensor is washed by the coolant before it leaves the heat exchanger, and in the absence of coolant circulation, the sensor records the temperature of the medium under the influence of thermal conductivity and natural convection, which would not occur if it was installed outside the heat exchanger.
Two-stage DHW schemes differ in that in the first stage of heating, heat is taken from the return water of the heating system. Due to the discrepancy between the thermal loads of heating and hot water supply in winter or night mode, it may turn out that hot water is heated above the required 55-60 °C. For example, with a coolant with a temperature of 70 °C (calculation point), DHW water in the first stage can be heated to 67-69 °C. To prevent overheating and intense carbonate deposits at these temperatures, it is possible to install a regulating three way valve at the inlet or outlet of the heat exchanger ( rice. 7). Its task, depending on the temperature of the coolant at the outlet of the heat exchanger, is to pass heating water through the heat exchanger or past it - through the bypass. The three-way valve sensor is installed in the return line. At the same time as regulating the temperature of the heating fluid, it indirectly limits the temperature of the hot water. At the same time, heat extraction from the return pipeline is not limited, but is optimized, increasing the reliability and comfort of the hot water supply.
In favor of a soldered heat exchanger
In Western countries, in the vast majority (over 90%) of cases, brazed plate heat exchangers are used for DHW purposes. This is due to the relative cheapness and ease of maintenance of these devices.
As a rule, Russian and Ukrainian customers who have experience in operating high-speed shell-and-tube heat exchangers, which often require cleaning, prefer gasketed plate heat exchangers. However, it must be taken into account that these devices are equipped with gaskets made of polymer (rubber) materials, which are subject to aging - they crack and become brittle. After five years of operation, when repairing a gasketed plate heat exchanger, it is often no longer possible to ensure its satisfactory density. And purchasing a new set of seals costs a price that is sometimes almost comparable to the price of a new heat exchanger.
If the seals are attached to the plates with glue, then their replacement involves work such as destroying the existing seals in liquid nitrogen and gluing new ones. To carry them out, special equipment and highly qualified personnel are required. Heat exchanger manufacturers provide relevant services to customers, but the heat exchanger often needs to be sent to a specialized facility. All this has led to the widespread use of brazed plate heat exchangers in Western countries for DHW purposes.
Note: doubts regarding the possibility of using brazed heat exchangers in the post-Soviet countries, associated with the poor quality of the coolant, are not justified - hard water is found all over the world. You just need to correctly adjust the DHW and limit the temperature of the walls of the heat exchanger, as described in the previous section.
Brazed plate heat exchangers are subjected to chemical washing. If you notice insufficient hot water heating or return cooling, and chemical composition water has a high content of hardness salts, it is necessary to regularly wash the heat exchanger with special solutions that do not destroy either the walls of the heat exchanger or the copper solder. The customer can carry out the washing on his own: this work is simple, washing units and reagents are affordable and quickly pay for themselves.
At extremely high heating water temperatures (for example, if temperature graph 150/70 °C), when it is possible that the temperature of the heat exchanger wall may exceed the temperature at which intensive scale formation occurs, a preliminary reduction in the temperature of the coolant in front of the heat exchanger is required. There are two ways to do this - pumping diagram injection or elevator circuit. In the first case, a separate sensor is required to turn on the pump, and a significant amount of electricity is consumed; the equipment used is subject to wear and tear. Elevator scheme extremely simple, with a thermostatic drive does not depend on electrical network and more economical in implementation and operation ( rice. 8). Connecting the elevator suction pipe to the return pipe of the heating system has the additional effect of reducing the temperature in the return pipe of the heating network.
Point solution
A two-stage DHW scheme requires two heat exchangers - for the first and second stages. Selecting heat exchangers by power, that is, dividing the total power into stages, is not an easy task, requiring several iterations in calculations (their responsibility is the responsibility of the supplier). The lack of mass-produced DHW units with a two-stage scheme is due to certain deadlines supplies.
Two brazed heat exchangers need to be connected with pipelines. The piping takes up space and determines a significant part of the cost of a two-stage DHW module. Therefore, manufacturers began to produce brazed heat exchangers with an intermediate dividing wall and six fittings.
Connecting heating points based on them is simplified, but there are problems with calculations and the lack of serial production remain.
In addition, during operation there are periods when the first or second stages of the system are not loaded at all. So, in summer period The second stage would be sufficient, and at the heating design point - the first.
The author of this article has developed and patented a solution for a mixed two-stage DHW circuit, including one commercially produced brazed plate heat exchanger ( rice. 9). Its essence lies in the use of a special fitting inserted into one of the serial fittings. Through this fitting both return water from the heating system and hot network water from the heating network are supplied. The heat exchange surface is fully activated in any mode.
