Today, energy conservation is a priority for the development of the global economy. The depletion of natural energy reserves, the increase in the cost of thermal and electric energy inevitably leads us to the need to develop a whole system of measures aimed at improving the efficiency of energy-consuming plants. In this context, the reduction of losses and the secondary use of expended heat energy becomes an effective tool in solving the problem.

In the context of an active search for reserves of fuel and energy resources, more and more attention is attracted to the problem of further improving air conditioning systems as large consumers of thermal and electric energy. An important role in solving this problem is called upon to play measures to improve the efficiency of heat and mass transfer apparatus, which form the basis of the polytropic air processing subsystem, the operating costs of which reach 50% of the total cost of operating hard currency.

Utilization of thermal energy from ventilation emissions is one of the key methods of saving energy resources in air conditioning and ventilation systems of buildings and structures for various purposes. In fig. Figure 1 shows the main schemes for utilizing the heat of exhaust air, which are implemented on the market of modern ventilation equipment.

An analysis of the state of production and use of heat recovery equipment abroad indicates a tendency to primarily use recirculation and four types of exhaust heat heat exchangers: 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 the ventilation and air conditioning systems, economic considerations, the relative position of the supply and exhaust centers, and operational capabilities.

In the table. Figure 1 shows a comparative analysis of various heat recovery schemes for exhaust air. Among the main requirements on the part of the investor for heat recovery plants, it should be noted: price, operating costs and operational 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 a unit with a cross-flow recuperator (Fig. 2) as the most suitable for the climatic conditions of the European part of Russia and Poland.

Recent studies in the field of creating new and improving existing heat recovery units of air conditioning systems indicate a distinct trend in the development of new design solutions for plate heat exchangers (Fig. 3), the decisive factor in choosing which is the ability to ensure trouble-free operation of the unit in conditions of moisture condensation at low temperatures outside air.

The temperature of the outside air, starting from which frost is formed in the exhaust air ducts, depends on the following factors: temperature and humidity of the exhaust air, the ratio of the supply and exhaust air flow rates, and design characteristics. We note the peculiarity of the heat exchangers at low outside temperatures: the higher the heat transfer efficiency, the greater the risk of frost on the surface of the exhaust air channels.

In this regard, low heat transfer efficiency in a cross-flow heat exchanger can be an advantage in terms of reducing the risk of icing of the surfaces of the exhaust air channels. Ensuring safe conditions is usually associated with the implementation of the following traditional measures to prevent nozzle freezing: periodic shutdown of the outdoor air supply, its bypassing or preheating, the implementation of which certainly reduces the efficiency of exhaust heat recovery.

One way to solve this problem is to create heat exchangers in which plate freezing is either absent or occurs at lower air temperatures. A feature of the operation of air-to-air heat recovery units is the possibility of realizing heat and mass transfer processes in the “dry” heat transfer modes, simultaneous cooling and drying of the removed air with condensation in the form of dew and frost on all or part of the heat exchange surface (Fig. 4).

The rational use of the heat of condensation, the value of which under certain operating modes of heat exchangers reaches 30%, can significantly increase the range of changes in the parameters of the outdoor air at which icing of the heat exchange surfaces of the plates does not occur. However, the solution of the problem of determining the optimal operating modes of the heat exchangers under consideration, corresponding to certain operational and climatic conditions, and the field of its appropriate application, requires detailed studies of heat and mass transfer in the nozzle channels taking into account condensation and frost formation.

As the main research method, numerical analysis is chosen. It has the least laboriousness, and allows you to determine the characteristics and identify patterns of the process based on processing information about the influence of the initial parameters. Therefore, experimental studies of heat and mass transfer processes in the devices under consideration were carried out in a much smaller volume and, mainly, to verify and adjust the dependences obtained as a result of mathematical modeling.

In the physical and mathematical description of heat and mass transfer in the studied recuperator, a one-dimensional transfer model (ε-NTU model) was preferred. In this case, the air flow in the nozzle channels is considered as a fluid flow with constant velocity, temperature, and mass transfer potential over its cross section equal to the mass-average values. In order to increase the efficiency of heat recovery in modern heat exchangers, fin surface finning is used.

The type and location of the ribs significantly affects the nature of the flow of heat and mass transfer. Changing the temperature along the height of the rib leads to the implementation of various variants of heat and mass transfer processes (Fig. 5) in the channels of the removed air, which significantly complicates the 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 realized in an orthogonal coordinate system with axes OX and OY directed parallel to the flows of cold and warm air, respectively, and axes Z1 and Z2, perpendicular to the surface of the nozzle 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 utilizer under study is described by differential equations of heat and material balances compiled for interacting air and nozzle flows taking into account the heat of phase transition and thermal resistance of the frost layer formed. To obtain an unambiguous solution, the system of differential equations is supplemented by boundary conditions that establish the values \u200b\u200bof the parameters of the exchanging media at the inputs to the corresponding channels of the recuperator.

The formulated nonlinear problem cannot be solved analytically; therefore, the integration of the system of differential equations was carried out by numerical methods. A sufficiently large amount of numerical experiments performed on the ε-NTU model made it possible to obtain an array of data that was used to analyze the characteristics of the process and identify its general laws.

In accordance with the objectives of studying the operation of the heat exchanger, the selection of the studied modes and ranges of variation of the parameters of the exchanging flows was carried out in such a way that the real processes of heat and mass transfer in the nozzle were most fully simulated at negative values \u200b\u200bof the outdoor temperature, as well as the flow conditions of the most hazardous operating modes of the heat recovery equipment .

Presented on fig. 7-9, the results of the calculation of the operating modes of the apparatus under study, characteristic of climatic conditions with a low calculated outdoor temperature in the winter season, allow us to judge about the qualitatively expected possibility of the formation of three zones of active heat and mass transfer in the channels of the removed air (Fig. 6), which differ in character processes occurring in them.

An analysis of the heat and mass transfer processes occurring in these zones makes it possible to assess possible ways of realizing the effective capture of the heat of the removed ventilation air and reducing the risk of frost formation in the channels of the heat exchanger nozzle based on the rational use of phase transition heat. Based on the analysis, the boundary temperatures of the outside air are established (Table 2), below which frost is observed in the channels of the exhaust air.

conclusions

The analysis of various heat recovery schemes for ventilation emissions is presented. The advantages and disadvantages of the considered (existing) schemes for utilizing the heat of exhaust air in ventilation and air conditioning units are noted. Based on the analysis, a scheme with a plate cross-flow recuperator is proposed:

• on the basis of a mathematical model, an algorithm and a program for calculating on a computer the basic parameters of heat and mass transfer processes in the studied heat recovery unit were developed;
• the possibility of the formation of various zones of moisture condensation in the channels of the nozzle of the utilizer, within which the nature of the heat and mass transfer processes varies significantly;
• analysis of the obtained patterns allows us to establish rational operating modes of the studied devices and the areas of their rational use for various climatic conditions of the Russian territory.

CONVENTIONS AND INDICES

Legend: h reb - rib height, m; l reb is the length of the rib, m; t is the temperature, ° C; d is the moisture content of air, kg / kg; ϕ - relative humidity,%; δ reb - rib thickness, m; δ in - the thickness of the layer of frost, m

Indices: 1 - outside air; 2 - removed air; e - at the entrance to the nozzle channels; p eb - rib; in - hoarfrost, o - at the exit from the nozzle channels; ros - dew point; sat is the state of saturation; w is the wall of the channel.

In this article, we propose to consider an example of the use of modern heat recovery units (recuperators) in ventilation installations, in particular rotary ones.

a) condensation rotor - utilizes mainly apparent heat. Moisture transfer is carried out if the exhaust air is cooled on the rotor to a temperature below the “dew point”.
b) enthalpy rotor - has a hygroscopic foil coating that promotes moisture transfer. In this way, total heat is utilized.
Consider a ventilation system in which both types of heat exchanger (recuperator) will work.

We assume that the calculation object is a group of premises in a certain building, for example, in Sochi or Baku, we will calculate only for the warm period:

Outside air parameters:
outdoor temperature in the warm period, with a security of 0.98 - 32 ° C;
the enthalpy of outdoor air in the warm season is 69 kJ / kg;
Indoor Air Parameters:
internal air temperature - 21 ° С;
relative humidity of internal air - 40-60%.

The required air consumption for the assimilation of hazards in this group of premises is 35,000 m³ / h. The beam of the room process is 6800 kJ / kg.
The air distribution pattern in the rooms is “bottom-up” by low-speed air distributors. In this regard (we will not apply the calculation, since it is voluminous and goes beyond the scope of the topic of the article, we have everything we need), the parameters of the supply and exhaust air are as follows:

1. Supply:
temperature - 20 ° С;
relative humidity - 42%.
2. Removable:
temperature - 25 ° С;
relative humidity - 37%

Let's build the process on the I-d diagram (Fig. 1).
First, we denote the point with the parameters of the internal air (B), then draw a process beam through it (note that for this diagram design, the initial point of the beam is the parameters t \u003d 0 ° C, d \u003d 0 g / kg, and the direction is indicated by the calculated value (6800 kJ / kg) indicated on the edge, then the resulting beam is transferred to the parameters of the internal air, while maintaining the angle of inclination).
Now, knowing the temperatures of the supply and exhaust air, we determine their points, finding the intersections of the isotherms with the process beam, respectively. We build the process from the reverse, in order to obtain the set parameters of the supply air we omit the segment - heating - along the line of constant moisture content to the curve of relative humidity φ \u003d 95% (segment P-P1).
We select a condensation rotor that utilizes the heat of the removed air for heating P-P1. We get the efficiency (calculated by temperature) of the rotor about 78% and calculate the temperature of the removed air U1. Now, we select an enthalpy rotor working to cool the outside air (H) with the obtained parameters U1.
We get the efficiency (calculated by enthalpy) of the order of 81%, the parameters of the treated air on the influx of H1, and exhaust hood U2. Knowing the parameters H1 and P1, you can choose an air cooler with a power of 332,500 watts.

Let us depict the ventilation unit schematically with recuperators (Fig. 2).

Now, for comparison, we will choose another system, for the same parameters, but with a different configuration, namely: we will install one condensation rotor.

Now (Fig. 3), the P-P1 is heated by an electric air heater, and the condensation rotor will provide the following: efficiency of about 83%, the temperature of the treated supply air (H1) - 26 ° C. We select an air cooler for the required power of 478,340 W.

It should be noted that system 1 requires less cooling power and, in addition to this, does not require additional energy costs (in this case, alternating current) for the second air heating. Let's make a comparison table:

Summarizing

We clearly see the differences in the operation of condensation and enthalpy rotors, and the savings in energy costs associated with this. However, it is worth noting that the principle of system 1 can be organized only for southern, hot cities, because during heat recovery in the cold period, the enthalpy rotor performance does not differ much from the condensation one.

Production of ventilation units with rotary recuperators

The Aircat Klimatekhnik company has been successfully developing, designing, manufacturing, and installing air handling units with rotary heat exchangers for many years. We offer modern and innovative technical solutions that work even with the most complex operation algorithm and extreme conditions.

In order to get an offer for ventilation or air conditioning, just contact any of

The main purpose of exhaust ventilation is the elimination of exhaust air from the served premises. Exhaust ventilation, as a rule, works in conjunction with the supply, which, in turn, is responsible for the supply of clean air.

In order for the room to have a favorable and healthy microclimate, you need to draw up a competent design of the air exchange system, perform the appropriate calculation and make the installation of 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 errors lead to the fact that ventilation ceases to cope with its function as it should, a fungus appears in the rooms, decoration and building materials are destroyed, and people begin to get sick. Therefore, the importance of a proper ventilation calculation 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:

1. Exhaust. Necessary for the intake of exhaust air and its removal from the premises.
2. Supply Provide fresh clean air from the street.
3. Supply and exhaust. At the same time, the old musty air is removed and a new one is fed into the room.

Exhaust plants are mainly used in production, in offices, warehouses and other similar premises. A drawback of exhaust ventilation is that without a simultaneous supply system, it will work very poorly.

If more air is drawn from the room than it enters, drafts are formed. Therefore, the supply and exhaust system is the most effective. It provides the most comfortable conditions both in premises, and in rooms of industrial and working type.

Modern systems are equipped with various additional devices that purify the air, heat or cool it, moisturize and evenly distribute throughout the premises. Old air without any difficulties is discharged through the hood.

Before proceeding with the arrangement of the ventilation system, one must 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 a ventilation that will fully fulfill all the tasks assigned to it.

In the course of calculating ventilation, such parameters are determined as:

1. Consumption.
2. Operating pressure.
3. The power of the heater.
4. The cross-sectional area of \u200b\u200bthe ducts.

If desired, you can additionally calculate the energy consumption for 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 calculation of air flow to be performed correctly, you need to know the following information:

1. The height of the premises and their area.
2. The main purpose of each room.
3. The average number of people who will simultaneously stay in the room.

To make the calculation, you will need the following devices:

1. Roulette for measurements.
2. Paper and pencil for notes.
3. Calculator for calculations.

To perform the calculation, you need to know such a parameter as the rate of air exchange per unit time. This value is set by SNiPom in accordance with the type of room. For residential, industrial and administrative premises, the parameter will vary. You also need to take into account such moments as the number of heating appliances and their power, the average number of people.

For domestic premises, the air exchange rate used in the calculation process is 1. When calculating the ventilation for administrative buildings, use an air exchange value of 2-3 - depending on specific conditions. Directly, the multiplicity of air exchange indicates that, for example, in a domestic building the air will be completely renewed once in 1 hour, which is more than enough in most cases.

The calculation of productivity requires the availability of data such as the amount of air exchange in the multiplicity and number of people. It will be necessary to take the greatest importance and, already starting from it, choose the appropriate exhaust ventilation power. The calculation of the rate of air exchange is carried out according to a simple formula. It is enough to multiply the area of \u200b\u200bthe room by the ceiling height and the multiplicity value (1 for household, 2 for administrative, etc.).

To calculate the air exchange according to the number of people, the amount of air that 1 person consumes is multiplied by the number of people in the room. As for the volume of air consumed, on average, with a minimum physical activity, 1 person consumes 20 m³ / h, with an average activity this indicator rises to 40 m³ / h, and at high it is already 60 m³ / h.

To make it clearer, you can give an example of calculation for an ordinary bedroom with an area of \u200b\u200b14 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 \u003d 35 m³ / h. When performing the calculation according to the second scheme, you will see that it is already 2x20 \u003d 40 m³ / h. It is necessary, as already noted, to take greater importance. Therefore, specifically in this example, the calculation will be performed according to the number of people.

Using the same formulas, the oxygen consumption for all other rooms is calculated. In conclusion, it remains to add up all the values, get the overall performance and select ventilation equipment based on these data.

Standard values \u200b\u200bfor the performance of ventilation systems are:

1. From 100 to 500 m³ / h for ordinary residential apartments.
2. From 1000 to 2000 m³ / h for private homes.
3. From 1000 to 10000 m³ / h for industrial premises.

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Determining the power of the heater

In order to calculate the ventilation system in accordance with all the rules, it is necessary to take into account the power of the heater. This is done in the event that in the complex with exhaust ventilation a supply will be organized. A heater is installed so that the air coming from the street is heated and enters the room already warm. Actual in cold weather.

The calculation of the power of the heater is determined taking into account such values \u200b\u200bas air flow, the required temperature at the outlet and the minimum temperature of the incoming air. The last 2 values \u200b\u200bare approved in SNiP. In accordance with this regulatory document, the air temperature at the outlet of the heater must be at least 18 °. The minimum outside temperature should be specified in accordance with the region of residence.

Modern ventilation systems incorporate performance controllers. Such devices are designed specifically to reduce the speed 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 it, 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 air heater has a power of 3 kW, then substituting these values \u200b\u200bin the above formula, you will find that the device heats 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 to 44-20 \u003d 24 °.

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Working pressure and duct section

The calculation of ventilation requires the mandatory determination of parameters such as operating pressure and the cross section of the ducts. An effective and complete system includes air distributors, ducts and fittings. When determining the working pressure, the following indicators should be taken into account:

1. The shape of the ventilation pipes and their cross-section.
2. Fan parameters.
3. The number of transitions.

The calculation of a suitable diameter can be performed using the following ratios:

1. A pipe with a cross-sectional area of \u200b\u200b5.4 cm² will be sufficient for a residential type building on 1 m of space.
2. For private garages - a pipe with a cross section of 17.6 cm² per 1 m² of area.

A parameter such as the air flow rate is directly related to the cross-section of the pipe: in most cases, a speed is selected within 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 effectiveness of the entire system depends on the correctness of this stage, so be careful and patient. If desired, you can additionally determine the energy consumption for the operation of the system being set up.

Part 1. Heat recovery devices

Flue gas heat utilization
technological furnaces.

Technological furnaces are the largest consumers of energy in oil refining and petrochemical enterprises, in metallurgy, as well as in many other industries. At refineries, they burn 3-4% of all refined 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 the flue gases of process furnaces is of utmost importance.

At a temperature of flue gases above 500 ° C, heat recovery boilers - KU should be used.

At a temperature of flue gases less than 500 ° C it is recommended to use air heaters - VP.

The greatest economic effect is achieved when there is a two-unit plant consisting of KU and VP (in KU gases are cooled to 400 ° C and enter the air heater for further cooling) - it is most often used at petrochemical plants at high flue gas temperatures.

Heat recovery boilers.

IN KU flue gas heat is used to produce water vapor. The furnace efficiency is increased by 10 - 15.

Recovery boilers can be built-in in the convection chamber of the furnace, or remote.

External waste heat boilers are divided into two types:

1) gas-tube type boilers;

2) boilers package convection type.

The choice of the required type is carried out depending on the required pressure of the resulting steam. The former are used in the production of steam of relatively low pressure - 14-16 atm., The latter - for the generation of 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 steam is consumed in the installation itself or if there is an excess that needs to be output to the factory-wide network. In the latter case, the vapor pressure in the boiler drum must be taken in accordance with the vapor pressure in the plant-wide network in order to output excess steam to the network and to avoid uneconomical throttling when it is output to the low-pressure network.

Gas-tube recovery boilers are structurally reminiscent of 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.

Package convection-type waste heat boilers have a more complex design. A schematic diagram of the operation of the KU of this type is shown in Fig. 5.4.

It uses the natural circulation of water and presents the most complete configuration of KU with an economizer and superheater.

Schematic diagram of the operation of the recovery boiler

batch convection type

Chemically purified water (HOW) enters the deaerator column to remove gases dissolved in it (mainly oxygen and carbon dioxide). Water flows down the plates, and a small amount of water vapor is passed countercurrently towards it. 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 to the atmosphere. Steam, giving its heat to water, condenses. Deaerated water from the bottom of the column is drawn in by the pump and the required pressure is pumped. Water is passed through an 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. Water circulates through the steam production coils due to the difference in densities (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 overheats to the desired temperature and is discharged to the consumer. Part of the resulting steam is used to deaerate the feed water.

Reliability and profitability of KU operation largely depends on the proper organization of the water regime. In case of improper operation, scale is intensively formed, corrosion of the heating surfaces proceeds, and steam pollution occurs.

Limescale is a dense deposit formed by heating and evaporating water. Water contains bicarbonates, sulfates and other salts of calcium and magnesium (hardness salts), which when heated are converted to bicarbonates and precipitate. Scale, which is several orders of magnitude smaller than metal, thermal conductivity, leads to a decrease in heat transfer coefficient. Due to this, the power of the heat flux through the heat exchange surface decreases and, of course, the efficiency of the KU decreases (the amount of generated steam decreases). The temperature of the flue gases discharged from the boiler increases. In addition, overheating of the coils and their damage due to a decrease in the bearing capacity of steel occurs.

To prevent scale formation, pre-chemically purified water (can be taken at thermal power plants) is used as feed water. In addition, continuous and periodic purging of the system (removal of part of the water) is performed. A purge prevents an increase in the concentration of salts in the system (water constantly evaporates, but the salts it contains do not, therefore, the concentration of salts increases). Continuous purge of the boiler is usually 3-5% and depends on the quality of the feed water (should not exceed 10%, since heat loss is associated with the purge). In the operation of high pressure KU working with forced circulation of water, internal boiler phosphating is additionally used. At the same time, the cations of calcium and magnesium, which are part of the sulfate-forming scale, bind to phosphate anions, forming compounds that are slightly soluble in water and precipitate in the thickness of the boiler’s water volume, in the form of sludge easily removed by blowing.

Oxygen and carbon dioxide dissolved in feed water cause corrosion of the internal walls of the boiler, and the corrosion rate increases with increasing pressure and temperature. To remove gases from the water, thermal deaeration is used. Another measure of protection against corrosion is to maintain a speed in the pipes at which air bubbles cannot be held on their surface (above 0.3 m / s).

In connection with an increase in the hydraulic resistance of the gas path and a decrease in the force of natural traction, it becomes necessary to install a smoke exhauster (artificial traction). 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 exhaust flue gases, the more powerful it is necessary to have a smoke exhaust (electricity consumption is growing).

The payback period of KU usually does not exceed one year.

Air heaters. Used to heat the air supplied to the furnace to burn fuel. Heated air can 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 compared to KU are the simplicity of their design, safe operation, no need to install additional equipment (deaerators, pumps, heat exchangers, etc.). However, air heaters with the current ratio of fuel and water prices are less economical than KUs (the price for steam is very high - 6 times higher for 1 GJ). Therefore, it is necessary to choose a method of utilizing the heat of flue gases, based on the specific situation at the given installation, enterprise, etc.

Two types of air heaters are used: 1) recuperative (heat transfer through the wall); 2) regenerative (heat storage).

Part 2. Heat recovery from ventilation emissions

A large amount of heat is consumed for heating and ventilation of industrial and domestic buildings and structures. For individual 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 the 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), production facilities, etc. For various purposes, various types of ventilation systems can be used. Usually, if the volume of air that should be replaced in the room per unit time (m 3 / h) is small, then apply natural ventilation. Such systems are implemented in every apartment and most public institutions and organizations. In this case, the convection phenomenon is used - heated air (has a reduced density) leaves through the ventilation openings and is discharged into the atmosphere, and fresh cold (higher density) air from the street is sucked in through its leaks in windows, doors, etc. . In this case, heat losses are inevitable, since the additional coolant flow rate is required to heat the cold air entering the room. Therefore, the use of even the most advanced thermal insulation structures and materials during construction cannot completely eliminate heat loss. In our apartments, 25 - 30% of heat loss is associated with the operation of ventilation, in all other cases this value is much higher.

Compulsory (artificial) ventilation systems they are used when intensive exchange of large volumes of air is necessary, which is usually associated with the prevention of an increase in the concentration of hazardous substances (harmful, toxic, fire and explosion hazardous, having an unpleasant smell) in the room. Forced ventilation is implemented in industrial premises, in warehouses, in storages of agricultural products, etc.

Are used forced ventilation systems three types:

Supply system consists of a blower forcing fresh air into the room, a supply air duct and a uniform air distribution system in the room. Excessive air volume is forced out through leaks in windows, doors, etc.

Exhaust system consists of a blower that draws air from the room into the atmosphere, an exhaust duct and a system for uniformly venting air from the volume of the room. In this case, fresh air 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, if necessary, a very intensive exchange of air in large rooms; at the same time, the heat consumption for heating fresh air is maximum.

The use of natural ventilation systems and separate exhaust and supply ventilation systems does not allow the use of heat from the exhaust air to heat the fresh air entering the room. When operating combined systems, it is possible to utilize the heat of ventilation emissions to partially heat the supply air and reduce the consumption of thermal energy. Depending on the difference in air temperature in the room and on the street, the heat consumption for heating fresh air can be reduced by 40-60%. Heating can be carried out in regenerative and regenerative heat exchangers. The former are preferable, since they have smaller dimensions, metal consumption and hydraulic resistance, have greater efficiency and a long service life (20 - 25 years).

Ducts are led to heat exchangers, and heat is transferred directly from air to air through a dividing wall or storage nozzle. But in some cases there is a need for the separation of the supply and exhaust ducts over a considerable distance. In this case, a heat exchange circuit 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.

Heat exchange scheme with an intermediate circulating coolant:

1 - exhaust duct; 2 - supply air duct; 3.4 - ribbed
tubular coils; 5 - pipelines for the circulation of the intermediate coolant
(as an intermediate heat carrier in such systems, concentrated aqueous solutions of salts, brines, are usually used); 6 - pump; 7 - coil for
additional heating of fresh air with steam or hot water

The system operates as follows. Warm air (+ 25 ° C) from the room is discharged through an exhaust duct 1 through the camera in which the finned coil is installed 3 . Air washes the outer surface of the coil and transfers heat to the cold intermediate coolant (brine) flowing inside the coil. The air is cooled to 0 ° C and discharged into the atmosphere, and brine heated to 15 ° C through the circulation pipelines 5 enters the fresh air heating chamber on the supply air duct 2 . Here, the intermediate heat transfer medium transfers heat to fresh air, heating it from - 20 ° С to + 5 ° С. In this case, the intermediate heat carrier itself is cooled from + 15 ° С to - 10 ° С. The cooled brine arrives at the pump intake and returns to the system for recycling again.

Fresh supply air, heated to + 5 ° C, can be immediately introduced into the room and heated to the required temperature (+ 25 ° C) using conventional heating radiators, and can be heated directly in the ventilation system. For this, an additional section is installed on the supply air duct, in which a finned coil is placed. Hot coolant flows through the tubes (heating water or steam), 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 room.

The use of this method has several advantages. Firstly, due to the high air velocity in the heating section, the heat transfer coefficient significantly (several times) increases compared to conventional heating radiators. This leads to a significant reduction in the total metal consumption of the heating system - a reduction 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, air can have a temperature significantly higher or lower than normal.

The only drawback is that the hydraulic resistance of the air path and the energy consumption for the supply air blower drive are slightly increased. But the advantages are so significant and obvious that air pre-heating 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 systems of the supply or exhaust ventilation systems separately, it is necessary to organize a corresponding centralized air exhaust or 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 leaks.

Heat exchange systems between the air removed from the room and fresh can be used not only for heating the supply air in the cold season, but also for cooling 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 room temperature in the hot season by pre-cooling fresh air, discharged by cold air.

Thermal VER.

The thermal VER includes the physical heat of the exhaust gases from boiler plants and industrial furnaces, main or intermediate products, other waste from the main production, as well as the heat of working fluids, steam and hot water spent in technological and energy units. Heat exchangers, heat recovery boilers or heat agents are used to utilize thermal VER. The heat recovery of the spent process streams in heat exchangers can pass through the surface separating them or in direct contact. Thermal VER can come in the form of concentrated heat fluxes or in the form of heat dissipated into the environment. In industry, concentrated flows account for 41%, and heat dissipation is 59%. Concentrated streams include the heat of the flue gases from furnaces and boilers, waste water from process plants and the housing and communal sector. Thermal VER are divided into high-temperature (with a carrier temperature above 500 ° С), medium-temperature (at temperatures from 150 to 500 ° С) and low-temperature (at temperatures below 150 ° С). When using installations, systems, devices of low power, the heat flux removed from them is small and dispersed in space, which makes it difficult to dispose of them due to low profitability.

In an air conditioning system, the heat of the removed air from the premises can be disposed of in two ways:

· Applying circuits with air recirculation;

· Installing heat recovery units.

The latter method, as a rule, is used in once-through circuits of air conditioning systems. However, the use of heat utilizers is not excluded in circuits with air recirculation.

In modern ventilation and air conditioning systems, a wide variety of equipment is used: heaters, humidifiers, various types of filters, adjustable grilles and much more. All this is necessary to achieve the required air parameters, maintain or create comfortable conditions for working indoors. Serving all this equipment requires a lot of energy. Heat recovery systems become an effective solution for energy conservation in ventilation systems. The basic principle of their operation is heating the air flow supplied to the room using the heat of the flow removed from the room. When using a heat exchanger, less power is required for the heater to heat the supply air, thereby reducing the amount of energy required for its operation.

Heat recovery in air-conditioned buildings can be done through heat recovery from ventilation emissions. Utilization of waste heat to heat fresh air (or cooling incoming fresh air with waste air after an air conditioning system in summer) is the simplest form of disposal. At the same time, four types of recycling systems that have already been mentioned can be noted: rotating regenerators; heat exchangers with an intermediate heat carrier; simple air heat exchangers; tubular heat exchangers. A rotating regenerator in the air conditioning system can increase the supply air temperature by 15 ° C in winter, and in summer it can reduce the incoming air temperature by 4-8 ° C (6.3). As in other disposal systems, with the exception of the heat exchanger with an intermediate coolant, a rotating regenerator can only function if the exhaust and suction channels are adjacent to each other at some point in the system.

An intermediate heat exchanger is less efficient than a rotating regenerator. In the presented system, water circulates through two heat exchange coils, and since a pump is used, the two coils can be located at some distance from each other. Both the heat exchanger and the rotating regenerator have moving parts (the pump and electric motor are driven and this distinguishes them from air and tube heat exchangers. One of the drawbacks of the regenerator is that contamination can occur in the channels. Dirt can settle on the wheel, which it then transfers it to the suction channel.In most wheels, purging is currently provided which minimizes the transfer of contaminants.

A simple air heat exchanger is a stationary device for heat exchange between exhaust and incoming air flows passing through it counterflow. This heat exchanger resembles a rectangular steel box with open ends, divided into many narrow channels such as chambers. Exhausted and fresh air flows through alternating channels, and heat is transferred from one air stream to another simply through the walls of the channels. The transfer of contaminants in the heat exchanger does not occur, and since a significant surface area is enclosed in a compact space, a relatively high efficiency is achieved. A heat exchanger with a heat pipe can be considered as a logical development of the design of the heat exchanger described above, in which two air flows into the chambers remain completely separate, connected by a bundle of finned heat pipes, which transfer heat from one channel to another. Although the pipe wall can be considered as additional thermal resistance, the heat transfer efficiency inside the pipe itself, in which the evaporation-condensation cycle takes place, is so great that up to 70% of the waste heat can be utilized in these heat exchangers. One of the main advantages of these heat exchangers compared to a heat exchanger with an intermediate heat carrier and a rotating regenerator is their reliability. Failure of several pipes will only slightly reduce the efficiency of the heat exchanger, but will not completely stop the disposal system.

With all the variety of constructive solutions of heat utilizers of secondary energy resources, each of them has the following elements:

· Wednesday - a source of thermal energy;

· Environment - the consumer of thermal energy;

· Heat receiver - a heat exchanger that receives heat from a source;

· Heat transmitter - a heat exchanger that transfers thermal energy to the consumer;

· A working substance transporting thermal energy from a source to a consumer.

In regenerative and air-air (air-liquid) recuperative heat exchangers, the working substance is the heat-exchanging medium itself.

Application examples.

1. Air heating in air heating systems.
Heaters are designed for quick heating of air using a water coolant and its uniform distribution using a fan and guide louvers. This is a good solution for construction and production workshops, where fast heating and maintaining a comfortable temperature is required only during working hours (at the same time, as a rule, furnaces also work).

2. Water heating in a hot water supply system.
The use of heat exchangers makes it possible to smooth out the peaks of energy consumption, since the maximum water consumption is at the beginning and end of the shift.

3. Heated water in the heating system.
Closed system
The coolant circulates in a closed loop. Thus, there is no risk of contamination.
Open system. The heat carrier is heated by hot gas, and then gives off heat to the consumer.

4. Heated blast air going to combustion. Allows to reduce fuel consumption by 10% –15%.

It is estimated that the main reserve of fuel economy during the operation of burners for boilers, furnaces and dryers is the utilization of the heat of the exhaust gases by heating the fuel burned with air. Heat recovery of exhaust flue gases is of great importance in technological processes, since the heat returned to the furnace or boiler in the form of heated blast air can reduce the consumption of fuel natural gas by 30%.
5. Heated fuel used for combustion using liquid-liquid heat exchangers. (An example is heating fuel oil to 100˚ – 120˚ С.)

6. Heated process fluid using liquid-liquid heat exchangers. (An example is heating a plating solution.)

Thus, the heat recovery unit is:

Solving the problem of energy efficiency of production;

Normalization of the environmental situation;

The presence of comfortable conditions in your production - heat, hot water in office buildings;

Reduced energy costs.

Picture 1.

The structure of energy consumption and energy saving potential in residential buildings: 1 - transmission heat loss; 2 - heat consumption for ventilation; 3 - heat consumption for hot water supply; 4– energy saving

List of used literature.

1. Karadzhi V.G., Moskovko Yu.G. Some features of the effective use of ventilation and heating equipment. Leadership - M., 2004

2. Eremkin A.I., Byzeev V.V. The economics of energy supply in heating, ventilation and air conditioning systems. Publishing House of the Association of Construction Universities M., 2008.

3. Skanavi A. V., Makhov. L. M. Heating. Publishing house ASV M., 2008