The main purpose of exhaust ventilation is to remove exhaust air from the serviced premises. Exhaust ventilation, as a rule, works in conjunction with supply air, which, in turn, is responsible for supplying clean air.
In order for the room to have a favorable and healthy microclimate, it is necessary to draw up a competent design of the air exchange system, perform the appropriate calculation and install the necessary units in accordance with all the rules. When planning, you need to remember that the condition of the entire building and the health of the people who are in it depend on it.
The slightest mistakes lead to the fact that ventilation ceases to cope with its function as it should, fungus appears in the rooms, decoration and building materials are destroyed, and people start to get sick. Therefore, the importance of the correct calculation of ventilation cannot be underestimated in any case.
The main parameters of exhaust ventilation
Depending on what functions the ventilation system performs, existing installations are usually divided into:
- Exhaust. Required for the intake of exhaust air and its removal from the room.
- Supply. Provide supply of fresh clean air from the street.
- Supply and exhaust. At the same time, old stale air is removed and new air is introduced into the room.
Exhaust units are mainly used in production, offices, warehouses and other similar premises. The disadvantage of exhaust ventilation is that without the simultaneous installation of a supply system, it will work very poorly.
If more air is drawn out of the room than it enters, drafts are formed. Therefore, the supply and exhaust system is the most efficient. It provides maximum comfortable conditions both in residential premises, and in industrial and working type premises.
Modern systems are equipped with various additional devices, which purify the air, heat or cool it, humidify and evenly distribute it throughout the premises. The old air is expelled through the hood without any difficulty.
Before proceeding with the arrangement of the ventilation system, you need to seriously approach the process of its calculation. Direct calculation of ventilation is aimed at determining the main parameters of the main components of the system. Only by determining the most suitable characteristics, you can make such ventilation that will fully fulfill all the tasks assigned to it.
During the calculation of ventilation, parameters such as:
- Consumption.
- Operating pressure.
- Heater power.
- Cross-sectional area of air ducts.
If desired, you can additionally calculate the energy consumption for the operation and maintenance of the system.
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Step-by-step instructions for determining system performance
The calculation of ventilation begins with the determination of its main parameter - performance. The dimensional unit of ventilation performance is m³/h. In order for the air flow calculation to be carried out correctly, you need to know the following information:
- The height of the premises and their area.
- The main purpose of each room.
- The average number of people who will be in the room at the same time.
To make the calculation, you will need the following devices:
- Roulette for measurements.
- Paper and pencil for notes.
- Calculator for calculations.
To perform the calculation, you need to know such a parameter as the frequency of air exchange per unit of time. This value is set by SNiP in accordance with the type of premises. For residential, industrial and administrative premises, the parameter will vary. You also need to take into account such points as the number of heaters and their power, the average number of people.
For domestic premises, the air exchange rate used in the calculation process is 1. When calculating ventilation for administrative premises, use the air exchange value equal to 2-3, depending on specific conditions. Directly, the frequency of air exchange indicates that, for example, in a domestic room, the air will be completely updated 1 time in 1 hour, which is more than enough in most cases.
Performance calculation requires the availability of data such as the amount of air exchange by frequency and number of people. It will be necessary to take great importance and, already starting from it, select the appropriate power of exhaust ventilation. The calculation of the air exchange rate is performed using a simple formula. It is enough to multiply the area of \u200b\u200bthe room by the height of the ceiling and the multiplicity value (1 for household, 2 for administrative, etc.).
To calculate the air exchange by the number of people, the amount of air consumed by 1 person is multiplied by the number of people in the room. As for the volume of air consumed, on average, at a minimum physical activity 1 person consumes 20 m³/h, with medium activity this figure rises to 40 m³/h, and with high activity it is already 60 m³/h.
To make it clearer, we can give an example of a calculation for an ordinary bedroom with an area of 14 m². There are 2 people in the bedroom. The ceiling has a height of 2.5 m. Quite standard conditions for a simple city apartment. In the first case, the calculation will show that the air exchange is 14x2.5x1=35 m³/h. When performing the calculation according to the second scheme, you will see that it is already equal to 2x20 = 40 m³ / h. It is necessary, as already noted, to take a larger value. Therefore, specifically in this example, the calculation will be performed by the number of people.
The same formulas are used to calculate the oxygen consumption for all other rooms. In conclusion, it remains to add up all the values, get the overall performance and choose ventilation equipment based on these data.
The standard values for the performance of ventilation systems are:
- From 100 to 500 m³/h for ordinary residential apartments.
- From 1000 to 2000 m³/h for private houses.
- From 1000 to 10000 m³/h for industrial premises.
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Determination of heater power
In order for the calculation of the ventilation system to be carried out in accordance with all the rules, it is necessary to take into account the power of the air heater. This is done if, in combination with exhaust ventilation, supply ventilation is organized. A heater is installed so that the air coming from the street is heated and enters the room already warm. Essential in cold weather.
The calculation of the power of the air heater is determined taking into account such values as air flow, required temperature outlet and minimum inlet air temperature. The last 2 values are approved in SNiP. Regarding this normative document, the air temperature at the heater outlet must be at least 18°. The minimum outside air temperature should be specified in accordance with the region of residence.
Modern ventilation systems include performance regulators. Such devices are designed specifically so that you can reduce the rate of air circulation. In cold weather, this will reduce the amount of energy consumed by the air heater.
To determine the temperature at which the device can heat the air, a simple formula is used. According to her, you need to take the value of the power of the unit, divide it by the air flow, and then multiply the resulting value by 2.98.
For example, if the air flow at the facility is 200 m³ / h, and the heater has a power of 3 kW, then by substituting these values in the above formula, you will get that the device will heat the air by a maximum of 44 °. That is, if in winter time it will be -20° outside, then the selected air heater will be able to heat oxygen up to 44-20=24°.
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Operating pressure and duct cross section
Calculation of ventilation involves the mandatory determination of parameters such as operating pressure and cross-section of air ducts. An efficient and complete system includes air distributors, air ducts and shaped products. When determining the working pressure, the following indicators must be taken into account:
- The form ventilation pipes and their section.
- Fan settings.
- The number of transitions.
The calculation of a suitable diameter can be performed using the following ratios:
- For a residential building, a pipe with a cross-sectional area of 5.4 cm² will be enough for 1 m of space.
- For private garages - a pipe with a cross section of 17.6 cm² per 1 m² of area.
Such a parameter as the speed of the air flow is directly related to the cross section of the pipe: in most cases, the speed is selected in the range of 2.4-4.2 m / s.
Thus, when calculating ventilation, whether it is an exhaust, supply or supply and exhaust system, a number of important parameters must be taken into account. The efficiency of the entire system depends on the correctness of this stage, so be careful and patient. If desired, you can additionally determine the power consumption for the operation of the system being arranged.
Today, energy conservation is a priority in the development of the world economy. Depletion of natural energy reserves, increase in the cost of heat and electrical energy inevitably leads us to the need to develop whole system measures aimed at improving the efficiency of energy-consuming installations. In this context, the reduction of losses and the reuse of the consumed thermal energy becomes an effective tool in solving the problem.
In the context of an active search for reserves to save fuel and energy resources, the problem of further improvement of air conditioning systems as large consumers of thermal and electrical energy is attracting more and more attention. An important role in solving this problem is to be played by measures to improve the efficiency of heat and mass exchangers, which form the basis of the polytropic air treatment subsystem, the operating costs of which reach 50% of all costs for the operation of SCR.
Utilization of thermal energy from ventilation emissions is one of the key methods for saving energy resources in air conditioning and ventilation systems for buildings and structures for various purposes. On fig. 1 shows the main heat recovery schemes extract air sold on the market of modern ventilation equipment.
An analysis of the state of production and use of heat recovery equipment abroad indicates a trend towards the predominant use of recirculation and four types of exhaust air heat utilizers: rotating regenerative, plate recuperative, based on heat pipes and with an intermediate heat carrier. The use of these devices depends on the operating conditions of ventilation and air conditioning systems, economic considerations, relative position supply and exhaust centers, operational capabilities.
In table. 1 shown comparative analysis various schemes for heat recovery of exhaust air. Among the main requirements on the part of the investor for heat recovery plants, it should be noted: price, operating costs and efficiency. The cheapest solutions are characterized by simplicity of design and the absence of moving parts, which makes it possible to distinguish among the presented schemes the installation with a cross-flow heat exchanger (Fig. 2) as the most suitable for climatic conditions European part of Russia and Poland.
Recent studies in the field of creating new and improving existing heat recovery units for air conditioning systems indicate a clear trend in the development of new constructive solutions plate heat exchangers(Fig. 3), the decisive moment in the choice of which is the possibility of ensuring trouble-free operation of the installation in conditions of moisture condensation at negative outdoor temperatures.
The outdoor air temperature, starting from which frost formation is observed in the exhaust air ducts, depends on the following factors: the temperature and humidity of the exhaust air, the ratio of the supply and exhaust air flow rates, and design characteristics. Let us note the peculiarity of heat exchangers operation at negative outdoor air temperatures: the higher the heat exchange efficiency, the greater the risk of frost formation on the surface of the exhaust air channels.
In this regard, the low efficiency of heat exchange in a cross-flow heat exchanger can be an advantage in terms of reducing the risk of icing on the surfaces of the exhaust air channels. Security safe modes usually associated with the implementation of the following traditional measures to prevent freezing of the nozzle: periodically turning off the supply of outside air, bypassing it or preheating, the implementation of which certainly reduces the efficiency of exhaust air heat recovery.
One of the ways to solve this problem is the creation of heat exchangers in which freezing of plates is either absent or occurs at more low temperatures air. A feature of the operation of air-to-air heat exchangers is the possibility of implementing heat and mass transfer processes in “dry” heat transfer modes, simultaneous cooling and drying of the removed air with condensation in the form of dew and frost on the entire or part of the heat exchange surface (Fig. 4).
The rational use of the heat of condensation, the value of which reaches 30% under certain operating modes of the heat exchangers, makes it possible to significantly increase the range of changes in the parameters of the outside air, in which icing of the heat exchange surfaces of the plates does not occur. However, the solution to the problem of determining optimal modes operation of the heat exchangers under consideration, corresponding to certain operating and climatic conditions, and the area of its expedient application, requires detailed studies of heat and mass transfer in the packing channels, taking into account the processes of condensation and frost formation.
Numerical analysis was chosen as the main research method. It also has the least laboriousness, and allows you to determine the characteristics and identify the patterns of the process based on the processing of information about the influence of the initial parameters. Therefore, experimental studies of heat and mass transfer processes in the considered devices were carried out in a much smaller volume and, mainly, to verify and correct the dependencies obtained as a result of mathematical modeling.
In the physico-mathematical description of heat and mass transfer in the recuperator under study, preference was given to the one-dimensional transfer model (ε-NTU model). In this case, the air flow in the packing channels is considered as a liquid flow with constant velocity, temperature and mass transfer potential over its cross section, equal to the average mass values . In order to improve the efficiency of heat recovery in modern heat exchangers finning of the nozzle surface is used.
The type and location of the ribs significantly affects the nature of the heat and mass transfer processes. The change in temperature along the height of the rib leads to the implementation of various options for heat and mass transfer processes (Fig. 5) in the channels of the exhaust air, which significantly complicates mathematical modeling and the algorithm for solving the system of differential equations.
The equations of the mathematical model of heat and mass transfer processes in a cross-flow heat exchanger are implemented in an orthogonal coordinate system with the OX and OY axes directed parallel to the cold and warm air flows, respectively, and the Z1 and Z2 axes, perpendicular to the surface of the packing plates in the supply and exhaust air channels (Fig. 6 ), respectively.
In accordance with the assumptions of this ε-NTU model, heat and mass transfer in the heat exchanger under study is described by differential equations of heat and material balances, compiled for interacting air flows and nozzles, taking into account the heat of the phase transition and the thermal resistance of the resulting frost layer. To obtain an unambiguous solution, the system of differential equations is supplemented with boundary conditions that establish the values of the parameters of the exchanged media at the inputs to the corresponding channels of the recuperator.
The formulated nonlinear problem cannot be solved analytically, so the integration of the system of differential equations was carried out by numerical methods. A fairly large amount of numerical experiments carried out on the ε-NTU model made it possible to obtain a data array that was used to analyze the characteristics of the process and identify its general patterns.
In accordance with the tasks of studying the operation of the heat exchanger, the choice of the studied modes and the ranges of variation of the parameters of the exchanging flows was carried out so that the real processes of heat and mass transfer in the packing at negative values outdoor air temperature, as well as the flow conditions of the most dangerous operating modes of heat recovery equipment from the point of view of operation.
Presented in fig. 7-9 the results of calculating the operating modes of the test apparatus, characteristic for climatic conditions with a low calculated outdoor air temperature in winter period seasons, allow us to judge the qualitatively expected possibility of the formation of three zones of active heat and mass transfer in the channels of the exhaust air (Fig. 6), which differ in the nature of the processes occurring in them.
An analysis of the heat and mass transfer processes occurring in these zones makes it possible to evaluate possible ways to effectively capture the heat of the removed ventilation air and reduce the risk of frost formation in the channels of the heat exchanger packing based on rational use heat of phase transition. Based on the analysis performed, the boundary temperatures of the outside air were established (Table 2), below which frost formation is observed in the exhaust air ducts.
findings
An analysis of various schemes for the utilization of heat from ventilation emissions is presented. The advantages and disadvantages of the considered (existing) schemes for utilizing the exhaust air heat in ventilation and air conditioning installations are noted. Based on the analysis carried out, a scheme with a plate cross-flow heat exchanger is proposed:
- on the basis of a mathematical model, an algorithm and a computer calculation program for the main parameters of heat and mass transfer processes in the heat exchanger under study were developed;
- the possibility of formation of various zones of moisture condensation in the channels of the heat exchanger nozzle, within which the nature of heat and mass transfer processes changes significantly, has been established;
- the analysis of the regularities obtained makes it possible to establish the rational modes of operation of the studied devices and the areas of their rational use for various climatic conditions of the Russian territory.
SYMBOLS AND INDICES
Legend: h reb — rib height, m; l rib - length of the rib, m; t is temperature, °C; d is the moisture content of the air, kg/kg; ϕ—relative air humidity, %; δ rib is the thickness of the rib, m; δ in is the thickness of the frost layer, m.
Indices: 1 — outside air; 2 - removed air; e - at the entrance to the nozzle channels; rb - rib; in - frost, o - at the outlet of the nozzle channels; dew - dew point; sat is the state of saturation; w is the channel wall.
Heat consumption for heating sanitary standard supply air at modern methods thermal protection of enclosing structures are in residential buildings up to 80% heat load on heating appliances, and in public and administrative buildings - more than 90%. Therefore, energy-saving heating systems in modern designs buildings can only be created if
exhaust air heat utilization for heating the sanitary standard of the supply air.
Also successful experience in administrative building in Moscow, recycling plants with pump circulation of the intermediate coolant - antifreeze.
When the supply and exhaust units are located at a distance of more than 30 m from each other, the disposal system with pump circulation of antifreeze is the most rational and economical. If they are located nearby, even more effective solution. So in climatic regions with mild winters, when the outdoor temperature does not fall below -7 ° C, plate heat exchangers are widely used.
On fig. 1 shows a structural diagram of a plate recuperative (heat transfer is carried out through a separating wall) heat recovery heat exchanger. Shown here (Fig. 1, a) is an “air-to-air” heat exchanger assembled from plate channels, which can be made of thin sheet galvanized steel, aluminum, etc.
Picture 1.a - plate channels, in which exhaust air L y enters from above the dividing walls of the channels, and horizontal supply air L p.n.; b - tubular channels, in which the exhaust air L y passes from above in the tubes, and the supply air passes horizontally in the annular space L p.n.
Lamellar channels are enclosed in a casing with flanges for connection to supply and exhaust air ducts.
On fig. 1, b shows an “air-to-air” heat exchanger made of tubular elements, which can also be made of aluminum, galvanized steel, plastic, glass, etc. The pipes are fixed in the upper and lower tube sheets, which forms channels for the passage of exhaust air. The side walls and tube sheets form the frame of the heat exchanger, with open facade sections, which are connected to the supply air duct L a.s.
Due to the developed surface of the channels and the arrangement of air-turbulent nozzles in them, in such “air-to-air” heat exchangers, a high thermal efficiency θ t bp (up to 0.75) is achieved, and this is the main advantage of such devices.
The disadvantage of these recuperators is the need to preheat the supply air in electric heaters to a temperature not lower than -7 °C (to avoid freezing of condensate on the side of the humid exhaust air).
On fig. 2 shows the structural diagram of the supply and exhaust unit with a plate exhaust air heat exchanger L y for heating the supply outside air L a.s. Supply and exhaust units are made in a single housing. Filters 1 and 4 are installed first at the inlet of the supply outdoor L p.n. and the removed exhaust L near the air. Both purified air flows from the operation of the supply 5 and exhaust 6 fans pass through the plate heat exchanger 2, where the energy of the heated exhaust air L y is transferred to the cold supply L b.s.
Figure 2. Structural diagram of the supply and exhaust units with a plate heat exchanger having a bypass air duct for the fresh air supply:1 - air filter in supply unit; 2 - plate utilization heat exchanger; 3 - flange for connecting the air path for the intake of exhaust air; 4 - pocket filter for cleaning exhaust air L y; 5 - supply fan with an electric motor on one frame; 6- exhaust fan with an electric motor on one frame; 7 - pallet collecting condensed moisture from the exhaust air passage channels; 8 - condensate drain pipeline; 9 - bypass air channel for passage supply air L b.s.; 10 - automatic drive of air valves in the bypass channel; 11 - heater for reheating supply air, fed hot water
As a rule, the exhaust air has a high moisture content and a dew point temperature of at least +4 °C. When cold outside air with a temperature below +4 °C enters the channels of the heat exchanger 2, a temperature will be established on the dividing walls at which water vapor will condense on a part of the surface of the channels from the direction of movement of the removed exhaust air.
The resulting condensate, under the influence of air flow L y, will intensively drain into the pan 7, from where it is discharged into the sewer (or storage tank) through the pipeline connected to the branch pipe 8.
The plate heat exchanger is characterized by the following equation for the heat balance of the transferred heat to the outside supply air:
where Q tu is the heat energy utilized by the supply air; L y, L p.n - costs of heated exhaust and outdoor supply air, m 3 / h; ρ y, ρ p.n - specific densities of heated exhaust and outdoor supply air, kg / m 3; I y 1 and I y 2 - initial and final enthalpy of heated exhaust air, kJ/kg; t n1 and t n2, s p - initial and final temperatures, ° С, and heat capacity, kJ / (kg · ° С), of the external supply air.
At low initial temperatures of the outside air t n.x ≈ t n1 on the dividing walls of the channels, the condensate falling out of the exhaust air does not have time to drain into the tray 7, but freezes on the walls, which leads to a narrowing of the flow area and increases the aerodynamic resistance to the passage of the exhaust air. This increase aerodynamic drag is perceived by the sensor, which sends a command to the actuator 10 to open the air valves in the bypass channel (bypass) 9.
Tests of plate heat exchangers in the Russian climate showed that when the outside air temperature drops to t n.x ≈ t n1 ≈ -15 °С, air valves in the bypass 9 are completely open and all the supply air L p.n. passes through, bypassing the plate channels of the heat exchanger 2.
Heating of fresh air L p.n. from t n.x to t p.n. In this mode, Q tu, calculated according to equation (9.10), is equal to zero, since only exhaust air passes through the connected heat exchanger 2 and I y 1 ≈ I y 2, i.e. there is no heat recovery.
The second method to prevent freezing of condensate in the channels of heat exchanger 2 is the electric preheating of the supply air from t n.x to t n1 = -7 °C. Under the design conditions of the cold period of the year in the climate of Moscow, the cold supply air in the electric heater must be heated by ∆t t.el = t n1 - t n.x = -7 + 26 = 19 °С. Heating of supply outdoor air at θ t p.n = 0.7 and t y1 = 24 °С will be t p.n = 0.7 (24 + 7) - 7 = 14.7 °С or ∆t t.u \u003d 14.7 + 7 \u003d 21.7 ° С.
The calculation shows that in this mode the heating in the heat exchanger and in the heater is practically the same. The use of bypass or electric preheating significantly reduces the thermal efficiency of plate heat exchangers in systems supply and exhaust ventilation in the Russian climate.
To eliminate this shortcoming, domestic specialists developed original method quick periodic defrosting of plate heat exchangers by heating the removed exhaust air, which ensures reliable and energy-efficient year-round operation of the units.
On fig. 3 shows a schematic diagram of the plant for heat recovery of exhaust air X for heating supply outdoor air L p.n.s. rapid elimination freezing channels 2 to improve the passage of exhaust air through the plate heat exchanger 1.
Air ducts 3 heat exchanger 1 is connected to the path of supply outdoor air L p.n, and air ducts 4 to the path of passage of exhaust air removed L y.
Figure 3 circuit diagram applications of a plate heat exchanger in the climate of Russia: 1 - plate heat exchanger; 2 - lamellar channels for the passage of cold supply outside air L p.n. and warm exhaust air L y; 3 - connecting air ducts for the passage of fresh air L p.n.; 4 - connecting air ducts for the passage of the removed exhaust air L y; 5 - heater in the exhaust air flow L y at the inlet to channels 2 plate heat exchanger 1.6 - automatic valve on the supply pipeline hot water G w g; 7 - electrical connection; 8 - sensor for controlling the resistance of the air flow in the channels 2 for the passage of exhaust air L y; 9 - condensate drain
At low temperatures of the supply outside air (t n1 = t n. x ≤ 7 °C), through the walls of the lamellar channels 2, the heat from the exhaust air is completely transferred to the heat corresponding to the heat balance equation [see. formula (1)]. A decrease in the temperature of the exhaust air occurs with abundant moisture condensation on the walls of the lamellar channels. Part of the condensate has time to drain from channels 2 and is removed through pipeline 9 to the sewer (or storage tank). However, most of the condensate freezes on the walls of the channels 2. This causes an increase in the pressure drop ∆Р у in the exhaust air flow, measured by the sensor 8.
When ∆Р y increases to the set value, a command will follow from the sensor 8 through a wire connection 7 to open the automatic valve 6 on the pipeline for supplying hot water G w g to the tubes of the heater 5 installed in the air duct 4 for the intake of the removed exhaust air into the plate heat exchanger 1. When open automatic valve 6 hot water G w g will enter the tubes of the heater 5, which will cause an increase in the temperature of the exhaust air t y 1 to 45-60 ° С.
When passing through the channels 2 of the removed air with a high temperature, there will be a rapid thawing from the walls of the channels of frost and the resulting condensate will drain through the pipeline 9 into the sewer (or into the condensate storage tank).
After the icing is defrosted, the pressure drop in channels 2 will decrease and sensor 8 will send a command to close valve 6 via connection 7 and the hot water supply to heater 5 will stop.
Consider the process of heat recovery on the I-d diagram, shown in fig. 4.
Figure 4 Construction on the I-d-diagram of the operating mode in the climate of Moscow of a utilization plant with a plate heat exchanger and its defrosting according to a new method (according to the scheme in Fig. 3). U 1 -U 2 - design mode of heat extraction from the exhaust air removed; H 1 - H 2 - heating by heat recovery of supply air in the design mode; U 1 - U under 1 - heating of the exhaust air in the defrosting mode from the icing of the lamellar channels for the passage of the removed air; Y 1. time - the initial parameters of the removed air after the release of heat to thaw the ice on the walls of the lamellar channels; H 1 -H 2 - heating of the supply air in the defrost mode of the plate heat exchanger
Let us evaluate the influence of the method of defrosting plate heat exchangers (according to the scheme in Fig. 3) on the thermal efficiency of exhaust air heat recovery modes using the following example.
EXAMPLE 1. Initial conditions: In a large Moscow (t n.x = -26 °С) industrial and administrative building, a heat recovery unit (TUU) was installed in the supply and exhaust ventilation system based on a recuperative plate heat exchanger (with an indicator θ t p.n = 0.7 ). The volume and parameters of the exhaust air removed during the cooling process are: L y \u003d 9000 m 3 / h, t y1 \u003d 24 ° C, I y 1 \u003d 40 kJ / kg, t r. y1 \u003d 7 ° C, d y1 \u003d 6, 2 g/kg (see construction on the I-d diagram in Fig. 4). The flow rate of supply outdoor air L p.n = 10,000 m 3 / h. The heat recovery unit is defrosted by periodically increasing the temperature of the removed air, as shown in the diagram in fig. 3.
Required: To establish the thermal efficiency of heat recovery modes using a new method of periodic defrosting of the apparatus plates.
Solution: 1. Calculate the temperature of the supply air heated by the utilizable heat in the design conditions of the cold period of the year at t n.x = t n1 = -26 °С:
2. We calculate the amount of utilized heat for the first hour of operation of the recovery unit, when the freezing of the plate channels did not affect the thermal efficiency, but increased the aerodynamic resistance in the channels for passing the exhaust air:
3. After an hour of operation of the TUU in the calculated winter conditions, a layer of frost accumulated on the walls of the channels, which caused an increase in the aerodynamic drag ∆Р y. Let's define possible number ice on the walls of the channels for the passage of exhaust air through the plate heat exchanger, formed within an hour. From the heat balance equation (1) we calculate the enthalpy of the cooled and dried exhaust air:
For the example under consideration, according to formula (2), we obtain:
On fig. 4 shows the construction on the I-d-diagram of the heating modes of the supply air (process H 1 - H 2) with the heat of the exhaust air utilized (process Y 1 - Y 2). By plotting on the I-d-diagram, the remaining parameters of the cooled and dried exhaust air were obtained (see point U 2): t y2 \u003d -6.5 ° C, d y2 \u003d 2.2 g / kg.
4. The amount of condensate that has fallen out of the exhaust air is calculated by the formula:
According to formula (4), we calculate the amount of cold spent to lower the ice temperature: Q = 45 4.2 6.5 / 3.6 = 341 W h. The following amount of cold is spent on ice formation:
The total amount of energy spent on the formation of ice on the separating surface of plate heat exchangers will be:
6. From the construction on the I-d diagram (Fig. 4), it can be seen that during countercurrent movement along the plate channels of the supply L p.n. and exhaust L at the air flows at the inlet to the plate heat exchanger, the coldest outside air passes exhaust air cooled to negative temperatures. It is in this part of the plate heat exchanger that intensive formations of frost and frost are observed, which will block the channels for the passage of exhaust air. This will cause an increase in aerodynamic drag.
At the same time, the control sensor will give a command to open the automatic valve for hot water supply to the tubes of the heat exchanger, mounted in the exhaust air duct up to the plate heat exchanger, which will ensure the heating of the exhaust air to a temperature of t.sub.1 = +50 °C.
The flow of hot air into the lamellar channels ensured the defrosting of frozen condensate in 10 minutes, which is removed in liquid form to the sewer (to the storage tank). For 10 minutes of heating the exhaust air, the following amount of heat was spent:
or by formula (5) we get:
7. The heat supplied in the heater 5 (Fig. 3) is partially spent on the melting of ice, which, according to the calculations in paragraph 5, will require Q t.ras = 4.53 kWh of heat. For the transfer of heat to the supply air from the heat expended in the heater 5 for heating the exhaust air, the following heat will remain:
8. The temperature of the heated extract air after the consumption of part of the heat for defrosting is calculated by the formula:
For the example under consideration, according to formula (6), we obtain:
9. Exhaust air heated in heater 5 (see Fig. 3) will contribute not only to the defrosting of condensate icings, but also to an increase in heat transfer to the supply air through the dividing walls of the lamellar channels. Calculate the temperature of the heated supply air:
10. The amount of heat transferred to heat the supply air during 10 minutes of defrosting is calculated by the formula:
For the considered mode, according to formula (8), we obtain:
The calculation shows that in the defrosting mode under consideration there are no heat losses, since part of the heating heat from the exhaust air Q t.u = 12.57 kW h is transferred to additional heating of the supply air L p.n. to a temperature t n2.raz = 20 ,8 °С, instead of t н2 = +9 °С when using only the heat of exhaust air with a temperature t у1 = +24 °С (see item 1).
Part 1. Heat recovery devices
Waste heat utilization flue gases
technological furnaces.
Process furnaces are the largest consumers of energy in oil refining and petrochemical plants, in metallurgy, as well as in many other industries. At refineries, they burn 3–4% of all processed oil.
The average temperature of the flue gases at the outlet of the furnace, as a rule, exceeds 400 °C. The amount of heat carried away with flue gases is 25–30% of the total heat released during fuel combustion. Therefore, the utilization of heat from flue gases from process furnaces is extremely important.
At flue gas temperatures above 500 °C, waste heat boilers - KU should be used.
At a flue gas temperature of less than 500 °C, it is recommended to use air heaters - VP.
largest economic effect is achieved in the presence of a two-unit unit consisting of a CHP and an VP (gases are cooled in the CHU to 400 °C and enter the air heater for further cooling) - it is more often used at petrochemical enterprises when high temperature flue gases.
Waste boilers.
AT KU flue gas heat is used to produce water vapor. The efficiency of the furnace increases by 10 - 15.
Waste-heat boilers can be built into the convection chamber of the furnace, or remote.
Remote boilers Recyclers are divided into two types:
1) gas-tube type boilers;
2) boilers of batch-convective type.
The choice of the required type is made depending on the required pressure of the resulting steam. The former are used in the production of steam relatively low pressure- 14 - 16 atm., the second - to generate steam with a pressure of up to 40 atm. (however, they are designed for an initial flue gas temperature of about 850 °C).
The pressure of the generated steam must be selected taking into account whether all the steam is consumed at the plant itself or whether there is an excess that must be output to the general plant network. In the latter case, the steam pressure in the boiler drum must be taken in accordance with the steam pressure in the general plant network in order to discharge excess steam into the network and avoid uneconomical throttling when outputting it to the low pressure network.
Waste heat boilers of the gas-tube type are structurally similar to "pipe-in-pipe" heat exchangers. Flue gases are passed through the inner pipe, and water vapor is generated in the annulus. Several of these devices are located in parallel.
Waste heat boilers of batch-convective type have a more complex design. A schematic diagram of the operation of a KU of this type is shown in fig. 5.4.
It uses natural water circulation and presents the most complete CHP configuration with an economizer and a superheater.
Schematic diagram of the operation of the waste heat boiler
packet-convective type
Chemically purified water (CPW) enters the deaerator column to remove gases dissolved in it (mainly oxygen and carbon dioxide). Water flows down the plates, and countercurrently flows towards it. a large number of water vapor. Water is heated by steam to 97 - 99 °C and due to the decrease in the solubility of gases with increasing temperature, most of them are released and discharged from the top of the deaerator into the atmosphere. The steam, giving off its heat to the water, condenses. Deaerated water from the bottom of the column is taken by the pump and pumped required pressure. Water is passed through the economizer coil, in which it is heated almost to the boiling point of water at a given pressure, and enters the drum (steam separator). The water in the steam separator has a temperature equal to the boiling point of water at a given pressure. Through the steam generation coils, water circulates due to the density difference (natural circulation). In these coils, part of the water evaporates and the vapor-liquid mixture returns to the drum. Saturated water vapor is separated from the liquid phase and discharged from the top of the drum into the superheater coil. In the superheater, saturated steam is superheated to the desired temperature and discharged to the consumer. Part of the resulting steam is used to deaerate the feed water.
The reliability and efficiency of the CU operation largely depends on proper organization water regime. In case of improper operation, scale is intensively formed, corrosion of heating surfaces proceeds, steam pollution occurs.
Scale is a dense deposit formed when water is heated and evaporated. Water contains bicarbonates, sulfates and other calcium and magnesium salts (hardness salts), which, when heated, are converted into bicarbonates and precipitate. Scale, which has several orders of magnitude lower thermal conductivity than metal, leads to a decrease in the heat transfer coefficient. Due to this, the power of the heat flow through the heat exchange surface is reduced and, of course, the efficiency of the KU operation is reduced (the amount of generated steam is reduced). The temperature of the flue gases removed from the boiler increases. In addition, overheating of the coils occurs and they are damaged due to a decrease in bearing capacity become.
To prevent the formation of scale, pre-treated water is used as feed water (it can be taken at thermal power plants). In addition, continuous and periodic purging of the system (removal of part of the water) is carried out. Purging prevents the increase in salt concentration in the system (water constantly evaporates, but the salts contained in it do not, so the salt concentration increases). The continuous blowdown of the boiler is usually 3 - 5% and depends on the quality of the feed water (should not exceed 10%, as heat loss is associated with the blowdown). During the operation of the CU high pressure working with forced circulation of water, in addition, intra-boiler phosphating is used. At the same time, calcium and magnesium cations, which are part of the sulfates that form scale, bind with phosphate anions, forming compounds that are poorly soluble in water and precipitate in the thickness of the water volume of the boiler, in the form of sludge that can be easily removed when blowing.
Oxygen dissolved in feed water carbon dioxide cause corrosion of the internal walls of the boiler, and the corrosion rate increases with increasing pressure and temperature. Thermal deaeration is used to remove gases from water. Also, a measure of protection against corrosion is to maintain such a speed in the pipes at which air bubbles cannot be retained on their surface (above 0.3 m / s).
In connection with the increase in the hydraulic resistance of the gas path and the decrease in the natural draft force, it becomes necessary to install a smoke exhauster (artificial draft). In this case, the temperature of the flue gases should not exceed 250 ° C in order to avoid the destruction of this apparatus. But the lower the temperature of the flue gases, the more powerful it is necessary to have a smoke exhauster (electricity consumption increases).
The payback period of CU usually does not exceed one year.
Air heaters. They are used to heat the air supplied to the furnace for fuel combustion. Air heating allows to reduce fuel consumption in the furnace (efficiency increases by 10 - 15%).
The air temperature after the air heater can reach 300 - 350 °C. This helps to improve the combustion process, increase the completeness of fuel combustion, which is a very important advantage when using high-viscosity liquid fuels.
Also, the advantages of air heaters in comparison with CHP are the simplicity of their design, operational safety, no need to install additional equipment (deaerators, pumps, heat exchangers, etc.). However, air heaters, with the current ratio of prices for fuel and steam, turn out to be less economical than CHP (our price for steam is very high - 6 times higher per 1 GJ). Therefore, it is necessary to choose a method for utilizing the heat of flue gases, based on specific situation at a given plant, plant, etc.
Two types of air heaters are used: 1) recuperative(heat transfer through the wall); 2) regenerative(heat storage).
Part 2. Utilization of heat from ventilation emissions
A large amount of heat is consumed for heating and ventilation of industrial and municipal buildings and structures. For certain industries (mainly light industry) these costs reach 70 - 80% or more of the total heat demand. At most enterprises and organizations, the heat of the removed air from ventilation and air conditioning systems is not used.
In general, ventilation is used very widely. Ventilation systems are built in apartments, public institutions(schools, hospitals, sports clubs, swimming pools, restaurants), industrial premises etc. For various purposes, can be used Various types ventilation systems. Usually, if the volume of air that must be replaced in the room per unit time (m 3 / h) is small, then natural ventilation. Such systems are implemented in every apartment and most public institutions and organizations. In this case, the phenomenon of convection is used - heated air (has a reduced density) leaves through ventilation holes and is discharged into the atmosphere, and in its place, through leaks in windows, doors, etc., fresh cold (more high density) air from the street. In this case, heat losses are inevitable, since it is necessary to heat the cold air entering the room. additional expense coolant. Therefore, the use of even the most modern heat-insulating structures and materials in construction cannot completely eliminate heat loss. In our apartments, 25 - 30% of heat losses are associated with the operation of ventilation, in all other cases this value is much higher.
Forced (artificial) ventilation systems are used when intensive exchange of large volumes of air is required, which is usually associated with the prevention of an increase in concentration hazardous substances(harmful, toxic, fire and explosion hazardous, having an unpleasant odor) in the room. Forced ventilation is implemented in industrial premises, warehouses, storage facilities for agricultural products, etc.
Are used systems forced ventilation three types:
supply system consists of a blower that blows fresh air into the room, a supply air duct and a system for even distribution of air in the volume of the room. Excess air volume is displaced through leaks in windows, doors, etc.
Exhaust system consists of a blower that pumps air from the room into the atmosphere, an exhaust duct and a system for uniform air removal from the volume of the room. Fresh air in this case is sucked into the room through various leaks or special supply systems.
Combined systems are combined supply and exhaust ventilation systems. They are used, as a rule, when a very intensive air exchange is required in large rooms; while the heat consumption for heating fresh air maximum.
The use of natural ventilation systems and separate exhaust and supply ventilation does not allow the heat of the exhaust air to be used to heat the fresh air entering the room. While operating combined systems it is possible to utilize the heat of ventilation emissions for partial heating of the supply air and reduce the consumption of thermal energy. Depending on the temperature difference between the indoor and outdoor air, the heat consumption for heating fresh air can be reduced by 40-60%. Heating can be carried out in regenerative and recuperative heat exchangers. The former are preferable, since they have smaller dimensions, metal consumption and hydraulic resistance, have greater efficiency and long service life (20 - 25 years).
Air ducts are connected to heat exchangers, and the heat is transferred directly from air to air through a separating wall or accumulating nozzle. But in some cases there is a need to separate the supply and exhaust air ducts over a considerable distance. In this case, a heat exchange scheme with an intermediate circulating coolant can be implemented. An example of the operation of such a system at a room temperature of 25 °C and an ambient temperature of 20 °C is shown in fig. 5.5.
Scheme of heat exchange with an intermediate circulating coolant:
1 - exhaust air duct; 2 - supply air duct; 3.4 - ribbed
tubular coils; 5 - intermediate coolant circulation pipelines
(concentrated aqueous solutions of salts - brines are usually used as an intermediate heat carrier in such systems); 6 - pump; 7 - coil for
additional heating of fresh air with steam or hot water
The system works as follows. Warm air(+ 25 °C) is removed from the room through the exhaust duct 1 through the chamber in which the finned coil is installed 3 . The air washes the outer surface of the coil and transfers heat to the cold intermediate heat carrier (brine) flowing inside the coil. The air is cooled to 0 °C and released into the atmosphere, and the brine heated to 15 °C through circulation pipelines 5 enters the fresh air heating chamber on the supply air duct 2 . Here intermediate coolant gives off heat to fresh air, heating it from -20 °С to + 5 °С. The intermediate heat carrier itself is then cooled from + 15 °С to - 10 °С. The cooled brine enters the pump intake and returns to the system for recirculation.
Fresh supply air, heated up to + 5 °C, can be immediately introduced into the room and heated to the required temperature (+ 25 °C) using conventional heating radiators, or it can be heated directly in ventilation system. To do this, an additional section is installed on the supply air duct, in which a finned coil is placed. A hot heat carrier flows inside the tubes (heating water or water vapor), and the air washes the outer surface of the coil and heats up to + 25 ° C, after which warm fresh air is distributed in the volume of the room.
The use of this method has a number of advantages. Firstly, due to the high air velocity in the heating section, the heat transfer coefficient is significantly (several times) higher compared to conventional heating radiators. This leads to a significant reduction in the overall metal consumption of the heating system - a decrease in capital costs. Secondly, the room is not cluttered with heating radiators. Thirdly, a uniform distribution of air temperatures in the volume of the room is achieved. And when using heating radiators in large rooms, it is difficult to ensure uniform heating of the air. In local areas, the air may have a temperature significantly higher or lower than normal.
The only drawback is that the hydraulic resistance of the air path and the power consumption for the drive of the supply blower are slightly increased. But the advantages are so significant and obvious that air preheating directly in the ventilation system can be recommended in the vast majority of cases.
In order to ensure the possibility of heat recovery in the case of using supply or exhaust systems ventilation separately, it is necessary to organize a centralized air outlet or air supply through specially mounted air ducts. In this case, it is necessary to eliminate all cracks and leaks in order to exclude uncontrolled blowing or air leakage.
Heat exchange systems between the air removed from the room and fresh air can be used not only to heat the supply air in the cold season, but also to cool it in the summer if the room (office) is equipped with air conditioners. Cooling to temperatures below ambient temperature is always associated with high energy (electricity) costs. Therefore, it is possible to reduce the energy consumption for maintaining a comfortable temperature in the room during the hot season by pre-cooling the fresh air discharged with cold air.
Thermal WER.
Thermal WERs include the physical heat of exhaust gases from boiler plants and industrial furnaces, the main or intermediate products, other wastes of the main production, as well as the heat of working fluids, steam and hot water that have been used in technological and power units. Heat exchangers, waste heat boilers or heat agents are used to utilize thermal SERs. The heat recovery of waste process streams in heat exchangers can pass through the surface separating them or through direct contact. Thermal SERs can come in the form of concentrated heat flows or in the form of heat dissipated into the environment. In industry, concentrated flows account for 41% and dissipated heat 59%. Concentrated streams include heat from flue gases from furnaces and boilers, wastewater technological installations and housing and communal sector. Thermal WERs are divided into high-temperature (with a carrier temperature above 500 °C), medium-temperature (at temperatures from 150 to 500 °C) and low-temperature (at temperatures below 150 °C). When using installations, systems, devices of low power, the heat flows removed from them are small and dispersed in space, which makes their utilization difficult due to low profitability.
Do you dream that the house has a healthy microclimate and that no room smells musty and damp? In order for the house to be truly comfortable, even at the design stage, it is necessary to carry out a competent calculation of ventilation.
If during the construction of the house you miss this important point, in the future, you will have to solve a number of problems: from removing mold in the bathroom to new repairs and installing an air duct system. Agree, it’s not too pleasant to see black mold nurseries in the kitchen on the windowsill or in the corners of the children’s room, and to dive into it again. repair work.
In our article we have collected useful materials on the calculation of ventilation systems, reference tables. Formulas are given visual illustrations and real example for premises of various purposes and certain area shown in the video.
At correct calculations and proper installation, ventilation of the house is carried out in a suitable mode. This means that the air in the living quarters will be fresh, with normal humidity and without unpleasant odors.
If the opposite picture is observed, for example, constant stuffiness in the bathroom or other negative phenomena, then you need to check the condition of the ventilation system.
Image gallery
Conclusions and useful video on the topic
Roller #1. Useful information according to the principles of operation of the ventilation system:
Roller #2. Together with the exhaust air, heat also leaves the home. Here, the calculations of heat losses associated with the operation of the ventilation system are clearly demonstrated:
The correct calculation of ventilation is the basis for its successful functioning and the guarantee of a favorable microclimate in a house or apartment. Knowing the basic parameters on which such calculations are based will allow not only to correctly design the ventilation system during construction, but also to correct its condition if circumstances change.
