Everyone has long known about the two-stage mixed hot water supply system, implemented on a type of plate heat exchanger such as a monoblock. Monoblock is a special type of plate heat exchanger for a two-stage hot water system, in which both stages are located in one housing; such a heat exchanger has six pipes.
- H1 - Input of return coolant from the heating system.
- H2 - DHW circulation water inlet.
- H3 - Output of heated hot water.
- H4 - Inlet of hot coolant from the heating network.
- F3 - Cold tap water inlet.
- F4 - Output of the general return coolant into the heating network.
The wide application of the monoblock was determined by the following factors: greater compactness compared to two separate heat exchangers, and, accordingly, lower cost. These same factors are the main and, perhaps, the only advantages of a candy bar. Let's try to decide on the disadvantages.
"Simplicity" of installation
It seems natural that it is much easier to mount a small device than two of the same ones. But what do we get as a result of installing a monoblock? The mounted monoblock looks like Spider-Man, entangled in garlands of pipelines, fittings and measuring instruments, if they are present, of course. This one is immediately lost important thing as ease of maintenance. If in a conventional plate heat exchanger all the pipes are located on a fixed plate (H1-H4) and its maintenance and repair only require turning off the heat exchanger and releasing the pressure, then disassembling the monoblock will require disconnecting the pipes from the movable rear plate. Further, if the back plate piping blocks access to the monoblock heat exchanger, then this also complicates access to it. That is, for normal operation of the monoblock, you should, firstly, make a competent project to link it to the existing coolant, cold and hot water in order to ensure normal access for maintenance and repair. And, secondly, a special option should be provided for attaching the pipelines to the back plate (through some removable elements) in order to ensure the mobility of the back plate without moving the heat exchanger from its place. Therefore, often a mounted monoblock occupies a volume no less than two separate heat exchangers.
Reliability issues
Naturally, two separate devices are more reliable than one that performs the same function. What do we have when one of the heat exchangers fails? In this case, we will be able to work with partial load of the hot water system while the second one is being repaired or serviced. If even one of the stages fails, the monoblock must be taken out of service as a whole, since there is only one housing for both stages.
Functionality, efficiency
The selection of a monoblock heat exchanger also has its own nuances. It is often difficult or practically impossible to create a monoblock arrangement of a two-stage mixed DHW circuit, with an efficiency equal to two separate heat exchangers. This is due to the fact that the type of plate used in the monoblock is the same for both stages. And within the limits of the thermophysical properties of this type, we have to solve the problem of arranging packages for both stages, while the first and second stages may differ, at least in terms of costs, especially on the coolant side. For example, the requirements for the first stage are the ability to pass the total flow rate of the heating system coolant and the second stage coolant while ensuring low hydraulic resistance and average heat removal. The requirements for the second stage are relatively low flow rates on the coolant and hot water side, higher permissible hydraulic resistances and significantly greater heat removal. That is, if these were two separate heat exchangers, then the first-stage heat exchanger should have a larger nozzle diameter and a “short” plate, and the second-stage heat exchanger should have a smaller nozzle diameter and a “longer” plate.
Let's consider a variant of the task for selecting equipment for a two-stage mixed circuit. The basic data is as follows: DHW system load 0.4 Gcal/h, heating cold water from 5°C to 60°C, heating system load 1.2 Gcal/h, temperature graph 150/70.
Dividing the load into stages, in accordance with SP 41-101-95 for given conditions, we obtain the initial data for selecting stage heat exchangers:
I stage
II stage
* NTU - number of heat transfer units. Heat engineering. V. N. Lukanin, M. G. Shatrov and others, graduate School, Moscow, 1999
In fact, the NTU value characterizes the thermal regime at which the heat exchanger will operate. The higher the NTU, the greater the thermal "length" of the heat exchanger plate must be.
In our case, it is clear that the second-stage heat exchanger must have a greater, almost 50%, heat removal capacity (thermal “length”) than the first-stage heat exchanger. In addition, the costs on the heating side of both stages differ by almost three times. This means that if for the second stage heat exchanger the pipes DN32 are sufficient, then for the heat exchanger of the first stage the pipes must be larger, not less than DN50.
Plate package
As noted above, a monoblock is, in fact, two heat exchangers placed in one frame. This means two packages of plates placed in one frame, separated by a reversal plate that has two (upper or lower) blind port holes. Typically, the second stage package is located closer to the fixed plate, and behind it the first stage package. But due to the infectious functions performed by these packages (see above), they have different layouts and number of plates. Since all these packages are located in one housing, there is a possibility that during the maintenance process an error will occur when assembling the entire package of monoblock plates. That is, if, after disassembling the monoblock, the packages are swapped or arranged incorrectly (for example, plates of the first stage with a small thermal “length” are installed for the second stage and vice versa), then, having reassembled the device, we will not get from it the characteristics that were included in it from the very beginning.
With two separate devices the situation is simpler. In this case, even if we assemble the entire package incorrectly, we will not get such a fatal reduction in thermal power, costs and changes in hydraulic resistance as in the case of a monoblock.
To summarize, let's summarize all the pros and cons of a plate heat exchanger with a monoblock layout in one table.
Advantages and disadvantages
pros
- Lower initial cost.
- Separately, a monoblock is more compact than two heat exchangers.
Minuses
- More complex installation and inconvenient maintenance due to sockets on the pressure plate.
- Less reliability.
- Less efficient operation.
- Demanding when assembling a package of plates.
Result
Everyone decides for themselves what is more important to them - cost savings or more reliable operation of the equipment.
Plate heat exchangers are specialized devices that transfer heat from a hot medium to a medium that needs to be heated through corrugated plates. The plates themselves can be made of different materials, for example, graphite, copper, steel and so on. In this case, the cold and hot layers are placed one after the other.
Advantages and disadvantages
The plate heat exchanger appeared relatively recently, but has already gained popularity among consumers due to its outstanding qualities. One of the greatest advantages is that collapsible heat exchangers are very compact and save installation space.
If there is a need to increase the number of plates, then the equipment does not necessarily need to be dismantled, since the required number of plates is added or reduced during operation, and this is a big plus.
In addition, all plate units are easy to clean, and their degree of contamination is the lowest. Operating costs and cash costs for energy supply are low. And the equipment is able to fully function at very low temperatures.
The application is quite effective if there is a need for low-grade heat and its transmission. Thanks to all of the above, plate heat exchangers remain the most technologically advanced and durable, and therefore the most in demand. You can get acquainted with different types of plate heat exchangers at http://www.teploprofi.com/.
The disadvantages of these heat exchangers include the fact that when using low-quality coolant, the heat exchanger will quickly become clogged. And in this case, you will have to systematically clean it using a special product. There are other types of heat exchangers, for example, shell-and-tube or brazed heat exchangers, but their specialization is too narrow, so they are not as popular as plate heat exchangers.
Scope of application of plate heat exchangers
Heat exchangers of this type are used for heating, cooling and condensation:
- In heating, ventilation and air conditioning systems, including heating points.
- In swimming pools, in hot water supply systems.
- In case of separation of energy systems.
- During heat extraction and recovery in the municipal sector.
- In specialized refrigeration equipment, evaporators and condensers in refrigeration systems.
- For cooling purposes different environments for technological needs.
- In food, automotive, steel, textile and many other areas of modern industry.
Currently, many companies are producing heat exchangers. These units are special devices within which heat exchange occurs between two or more coolants. In addition, in some cases, heat exchange can be carried out using a surface solid and coolant.
There are many different models of heat exchangers. These include spiral, shell-and-tube, twisted, and ribbed devices, etc. Among this wide variety, special attention should be paid to plate ones, as they are the most popular. Their popularity is explained by a considerable number of advantages.
First of all, it should be noted such an important advantage as ease of maintenance. In cases where this unit becomes clogged, it is necessary to disassemble the device and rinse it thoroughly. After this, it should be dried and collected. Moreover, this procedure does not require any large physical or time costs.
The second advantage is due to the fact that when using this type of heat exchanger it is possible to observe low level contamination of the heat exchange surface. This is achieved due to the high turbulence of the fluid flow, which is formed by corrugation. In addition, this factor is also influenced by the fact that the heat exchange plates have high-quality polishing.
The third important advantage is cost-effectiveness. This unit can last more than 20 years. Moreover, if during the process it is necessary to replace the plates, then this can be easily done, while saving considerable money. So, for example, when repairing a shell-and-tube unit, you will have to say goodbye to a larger amount of money.
Since we are talking about plates, it should also be said that the volume of this type of heat exchanger can be increased or decreased at any time. All that is required for this is just to add the required number of plates or, conversely, remove them. This is also a very significant advantage of this type of equipment.
However, despite a large number of advantages, it is also necessary to note the disadvantages of this unit. The most important disadvantage of a plate heat exchanger is that if you use a low-quality coolant, the device will soon become dirty.
You can currently order a plate heat exchanger or purchase a device of another type using the Internet. In addition, it will also allow, for example, to order the installation of ITP or use any other service to create comfortable living conditions.
Monoblock type plate heat exchanger is the basis of a two-stage mixed hot water supply system (hot water supply)
“Monoblock” is a type of plate heat exchanger designed to work in a two-stage hot water system, in which both stages are combined in one heat exchanger; such a heat exchanger has six pipes. (see picture).
The main and perhaps the only advantages of a monoblock are its compactness, in comparison with two heat exchangers separately and, accordingly, lower cost, which determines the breadth of application of heat exchangers of the “Monoblock” type.
Now let's try to determine its disadvantages.
Rice. Monoblock for two-stage DHW system. Location of pipes: H1 - Return coolant inlet from the heating system, H2 - DHW circulating water inlet, H3 - Heated DHW water outlet, H4 - Hot coolant inlet from the heating network, F3 - Cold tap water inlet, F4 - General return coolant outlet into the heating network .
"Simplicity" of installation.
It is believed that it is easier to install one device than several of the same ones. But the installed monoblock looks like a spider, entangled in a web of pipelines, various shut-off valves and instruments. Thus, the main advantage is lost - ease of maintenance and repair. If in a single-pass plate heat exchanger all the pipes are located on the front plate H1-H4 and for its maintenance and repair it is only necessary to use shut-off valves and vents, then to disassemble the monoblock system, dismantling the pipes of the movable rear plate is inevitable. Also, the back plate piping may block access to the monoblock heat exchanger. For normal operation of the monoblock, it is first necessary to make a competent design for connecting it to the coolant, cold and hot water pipelines in order to ensure easy access for maintenance and repair. A correctly mounted monoblock takes up no less space than two separate heat exchangers.
Reliability.
It is important to remember that two separate heat exchangers are more reliable than one that performs the same functions. What do we get when one of the heat exchangers fails? In this case, the system can operate at partial load while the second heat exchanger is repaired or serviced. If even one stage is inoperative, the entire monoblock must be taken out of operation, because there is only one housing for both stages.
Efficiency.
When calculating a monoblock heat exchanger, there are also some nuances. Quite often it is difficult to create a monoblock system of a two-stage mixed DHW circuit, comparable in efficiency to two separate plate heat exchangers. This is due to the fact that the established type of plate in the monoblock for two stages is the same. And within the limits of the thermophysical properties of this type, we need to solve the problem of arranging plate packages for both stages, while one stage may differ in flow rates from each other, especially on the coolant side. Here, for example, the requirements for the first stage are the ability to pass the total flow rate of the heating system coolant and the second stage coolant while ensuring low hydraulic resistance and average heat removal. For the second stage, these are relatively low flow rates on the coolant side and hot water supply side, higher permissible hydraulic resistances and significantly greater heat removal. That is, if these were two separate heat exchangers, then the first-stage heat exchanger should have a larger nozzle diameter and a “short” plate, and the second-stage heat exchanger should have a smaller nozzle diameter and a “longer” plate.
There is a task option for selecting equipment for a two-stage mixed circuit. The initial data are as follows: the load of the hot water supply system is 0.4 Gcal/h, heating of cold water from 5 ° C to 60 ° C, the total load of the heating system is 1.2 Gcal/h, the temperature curve is 150-70.
By dividing the load into stages, in accordance with (SP 41-101-95), for given conditions we obtain the initial data for selecting stage heat exchangers (see table).
In fact, the NTU value characterizes the thermal regime at which the heat exchanger will operate. The higher the NTU, the greater the thermal "length" of the heat exchanger plate must be.
In this case, it is clear that the heat exchanger of the second stage must have a greater, almost 50%, heat removal capacity (thermal “length”) than the heat exchanger of the other stage. In addition, the costs on the heating side of both stages differ by almost 3 times. This means that if for the second stage heat exchanger the pipes DN32 are sufficient, then for the heat exchanger of the first stage the pipes must be larger, not less than DN50.
Plate package
As stated above, a monoblock is essentially two heat exchangers located in one frame. This means two packages of plates placed in one frame, separated by a reversal plate that has two (upper or lower) blind port holes. Often the second stage package is located closer to the fixed plate, and behind it the first stage package. Due to the different functions performed by these packages (see above), they have different layouts and numbers of plates. And since all these packages are located in one frame, there is a possibility that during the service process an error will occur when assembling the entire package of monoblock plates. That is, if, after disassembling the candy bar, the packages are swapped or arranged incorrectly, then when we reassemble the device, we will not receive from it the inherent characteristics that were originally included in it.
Table. Data for the selection of heat exchangers.
With two heat exchangers the situation is simpler. In this case, even if we assemble the entire package incorrectly, we will not get such a colossal reduction in power, costs and changes in hydraulic resistance as with a monoblock.
Eventually
Pros and cons of a plate heat exchanger with a monoblock layout:
Pros:
1. Low cost.
2. The monoblock is slightly more compact than two heat exchangers.
Minuses:
1. Difficult installation and inconvenience in maintenance due to the pipeline on the pressure plate.
2. Less reliability.
3. Less productive work.
4. Demanding requirements for assembling a package of heat exchanger plates.
The task of this unit is to transfer energy from the primary source to the cold working fluid: the plate heat exchanger distributes heat using corrugated plates as walls, which protects the system from mixing media.
When calculating a plate heat exchanger, it is necessary to take into account that the basis of the device is laid on:
- fixed and pressure plates;
- pipes (inlet and outlet) with various connections;
- mounting stand;
- guides;
- threaded hardware.
Energy is transferred between coolants through plates made of rust-resistant inert materials. The latter are processed by stamping, their thickness varies between 0.4-1 mm. When assembled, the unit consists of tightly fitting thin panels with slotted channels. All elements have a contour recess on the front side into which a rubber seal is placed (this ensures a tight fit of the elements).
The plates are uniform in shape and material; they can be made of stainless steel, titanium, refractory alloys (selected depending on the application). For the production of seals, complex polymers based on synthetic rubber are used; they can be used with glycol and non-aggressive media, steam and high-temperature liquids, oil-containing and oil-based coolants.
Operating principle and unit diagram
The design, calculation and flushing of plate heat exchangers for heating are based on the fact that the unit functions due to the presence of 4 holes:
- 2 openings for inflow and outlet of hot working fluid;
- 2 holes to ensure hermetically sealed joining of the plates and prevent mixing of coolants - this task is performed by seals.
The movement of liquid in the unit is carried out according to the principle of flow vortex. As a result, due to the relatively low resistance to the movement of the working medium, the intensity of thermal energy transfer increases. Also, due to the low resistance during the passage of liquid, the amount of scale in the internal cavities is reduced.
The operating principle of a plate heat exchanger, based on loops and vortices, promotes multiple energy exchanges. As a result, maximum efficiency of the unit is achieved, which is influenced by positive influence and the output of pipes to both types of panels - clamping and fixed.
The heat exchanger design is ideally suited to the operating conditions: the number of plates increases in proportion to the potential power requirements of the system. The number of operating elements has a direct impact on the efficiency and performance of heating or cooling equipment.
Technical parameters of models
When studying the assortment, rely on the following specifications:
- the material from which the panels are made - these can be refractory compounds, thin sheet steel, pure titanium;
- the maximum permissible medium pressure in the unit usually does not exceed 25 kgf/cm²;
- in each node the number of plates used starts from 7-10, their number is determined by the future area of application;
- devices can withstand coolant temperatures of no higher than 180°C.
One working unit is capable of providing a heat exchange area in the range of 0.1-2100 square meters. m.
Types of plate heat exchangers
According to the specific design and application possibilities, devices are divided into soldered, welded and dismountable.
Soldered models
They are one-piece devices; their design does not include rubber seals. The plates are joined together by soldering. Advantages of the solution:
- budget cost of the kit;
- high efficiency and reliability;
- compact dimensions;
- ease of installation.
Brazed heat exchangers are common in ventilation and air conditioning systems, they are used in turbine and compressor technology, and are introduced into refrigeration units.
Collapsible
They are formed from a set of panels and polymer seals. Reasons for the widespread use of gasketed plate heat exchangers:
- low cost and ease of installation;
- the ability to regulate the level of productivity;
- ease of use, lack of significant operating costs;
- minimal downtime;
- low energy intensity;
- possibility of further processing during disposal.
The units have found wide application in heating systems for houses and maintenance of swimming pools, hot water supply, climate control and refrigeration equipment, and heating points.
Semi-welded and welded
Here, the working elements are connected by welds; there are no sealing gaskets in the design. Model characteristics:
- there are conditions for regulating the flow and flushing the heat exchanger;
- high resistance to aggressive environments;
- ability to work under conditions of large differences in operating temperatures;
- Maximum temperature the carrier can reach 300°C, the permissible pressure is not higher than 4.0 MPa;
- compactness of the unit, ease of installation;
- immunity to aggressive substances and abrasives;
- long service life.
Welded and semi-welded models are common in the food, pharmaceutical, chemical industry, ventilation, air conditioning, recovery systems, heat pumps. The devices provide cooling of equipment and allow you to coordinate the temperature of water in the hot water supply of baths and similar public facilities.
Advantages and disadvantages
Advantages of using units:
- high efficiency with small dimensions. The average efficiency of devices used in hot water supply and heating reaches 80-85%. Connection ports are located on one side, making installation easier;
- low pressure loss rates. The design provides the possibility smooth adjustment channel width, an increase in the number of the latter leads to a reduction in hydraulic losses. Reducing the resistance of the medium allows you to reduce energy consumption by pumps;
- maintainability, efficiency and ease of installation. Disassembly and washing of the equipment can be done in a few hours; minor contaminants can be removed using the in-place method. The average service life of a heat exchanger is 10 years, and the plates have a service life of 15-20 years;
- flexibility. To increase the power of the device, changing its heat transfer surface is practiced. As needs grow, it is not necessary to replace the unit with a new one, just add plates;
- low pollution. Channel profiles ensure self-cleaning due to high flow turbulence. This reduces the frequency of service;
- individuality. Specialists calculate and select the configuration based on the required temperature schedules;
- vibration stability. The products are not subject to the typical two-plane vibration that typically damages tubular heat exchangers;
- glueless seals are easy to replace with new ones, and they are firmly fixed in the channels. Low probability of leaks occurring after mechanical cleaning; they are detected immediately (without disassembly);
- the kit does not require a special reinforced base and additional thermal insulation
- The average payback period, depending on the model, is 3-5 years.
The weak point of the units is the high requirements for the quality of cleaning of the working environment. Since between the panels there remains short distance, contamination of the channels occurs faster compared to the cavities of its closest competitor - a shell-and-tube heat exchanger. Clogging leads to a decrease in heat transfer efficiency and a decrease in the efficiency of the device.
Criterias of choice
When determining the optimal model of the device, you should rely on the technical characteristics of the product:
- DHW connection diagram;
- level of thermal load;
- parameters of the heating and heated medium.
The last point takes into account information such as inlet and outlet temperatures in winter and summer, potential fluid flow and permissible pressure losses, and the percentage of power reserve. This information is used as a basis when calculating the performance of a plate heat exchanger.
Nuances of installation and connection
The heat exchanger is used only in conjunction and does not imply independent use. During installation, the unit is surrounded by auxiliary equipment, such as check valves, control and measuring devices in the form of thermometers and pressure gauges, shut-off valves(manual dampers and gate valves), circulation pumps.
The connection is made according to one of the following schemes:
- single-stage parallel (independent) method;
- two-stage mixed;
- two-stage sequential.
In the first case, there is a significant saving of usable space in the installation area. The key advantage of this method is its ease of execution (which is important in conditions of repair, maintenance, or replacement of a unit). The disadvantage of the technique is the inability to heat a cold working environment.
With a two-stage mixed method, the temperature of the incoming coolant increases due to the reverse flow, as a result, the efficiency of the bundle increases by 35-40%. But in this case, to ensure hot water supply, it will be necessary to provide two heat exchangers in the system, which increases the cost of purchasing and installing equipment.
The sequential two-stage method allows you to increase the efficiency of using the working environment and stabilize the load on the network. Compared to the parallel circuit, the coolant costs are reduced by 50%, and compared to the mixed method - by 25%. The only drawback of the solution is the impossibility of fully automating the heating unit.
Areas of equipment use
The models under consideration are used in public utilities to achieve the following goals:
- additional heating of the medium in hot water supply;
- heating water in swimming pools and boilers;
- provision of an independent heating circuit from the central heating station or combined heat and power plant;
- ventilation of premises;
- laying heated floors.
In such conditions, the maximum water temperature can be 180°C against a background of pressure in the range of 10-16 kPa. The plates are made of stainless steel with a thickness of 0.4 mm; ethylene propylene is used for seals.
In the food industry, heat exchangers are used in the production of vegetable oils, dairy products, alcohol, sugar, and beer. They are used as elements of evaporation, cooling, and pasteurizing lines. Soldered and collapsible models are relevant here.
In metallurgy, plate components are included in equipment for cooling working fluids. In this industry, melting furnaces, rolling and casting mechanisms, pickling solutions, and hydraulic lubricants require intensive cooling.
Heat exchangers in the oil and gas industry help heat and cool liquids and substances involved in cracking and technological preparation of raw materials. The units are used as components network systems, equipment for chemical water treatment, low pressure. Plates for the gas and oil industries are made on the basis of pure titanium in the form of sheets no more than 0.7 mm thick. The grades of polymer used for the production of sealing gaskets are subject to high requirements for resistance to chemical and thermal influences.
Plate heat exchangers, popular in shipbuilding, serve as coolers for the entire system and the main engine. The carriers in such conditions are motor oils that differ in viscosity, sea water, coolant. The units are also relevant as part of heating circuits and hot water supply on large sea vessels.
