Supply and exhaust ventilation system with heat recovery of the removed air. Supply and exhaust ventilation system with exhaust air heat recovery Calculation of exhaust air heat recovery systems

The cost of heat for heating the sanitary norm of the supply air at modern methods thermal protection of enclosing structures are in residential buildings up to 80% of the heat load on heating devices, and in public and administrative buildings - more than 90%. Therefore, energy-saving heating systems in modern building designs can only be created under the condition

exhaust air heat utilization for heating the sanitary standard of the supply air.

Also successful is the experience of using a recycling unit with pump circulation of an intermediate coolant - antifreeze, in an administrative building in Moscow.

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 side by side, an even more effective solution is possible. So in climatic regions with mild winters, when the outside air 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 supply air: 1 - air filter in the 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 the passage of supply air L p.n.; 10 - automatic drive of air valves in the bypass channel; 11 - heater for reheating supply air, fed with 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 heat balance 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 in aerodynamic resistance is perceived by the sensor, which sends a command to the drive 10 to open the air valves in the bypass channel (bypass) 9.

Tests of plate heat exchangers in the climate of Russia have shown that when the outdoor air temperature drops to t n.x ≈ t n1 ≈ -15 ° С, the air valves in bypass 9 are fully 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 plate heat exchangers in supply and exhaust ventilation systems in the Russian climate.

To eliminate this shortcoming, domestic specialists have developed an original method for rapid periodic defrosting of plate heat exchangers by heating the extracted exhaust air, which ensures reliable and energy-efficient year-round operation of the units.

On fig. Fig. 3 shows a schematic diagram of the plant for recovering the heat of exhaust air X for heating supply outdoor air L p.n. with rapid removal of freezing of 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 the channels 2 of the plate heat exchanger 1.6 - automatic valve on the hot water supply pipeline 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 supply air (t n1 \u003d t n. x ≤ 7 ° С) through the walls of the plate channels 2, the heat from the exhaust air is completely transferred to the heat that corresponds 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 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 difference in channels 2 will decrease and sensor 8 will send a command to close valve 6 via connection 7 and the supply of hot water to heater 5 will stop.

Consider the process of heat recovery on the I-d diagram, shown in fig. four.

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 with the heat recycled inlet outside 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 exchanger is defrosted by periodically increasing the temperature of the exhaust 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 aerodynamic resistance ∆Р y. Let us determine the possible amount of 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 modes of heating the supply air (process H 1 - H 2) by the heat recovered from the exhaust air (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:

Using 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 inlet to the tubes of the heat exchanger, mounted in the exhaust duct up to the plate heat exchanger, which will ensure the heating of the exhaust air to a temperature t s.l.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 melting ice, which, according to 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).

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

a) condensing rotor - utilizes mainly sensible heat. Moisture transfer occurs when 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. Thus, the total heat is utilized.
Consider a ventilation system in which both types of heat exchanger (recuperator) will operate.

Let us 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 air temperature during the warm period, with a security of 0.98 - 32 ° С;
enthalpy of outdoor air during the warm period of the year - 69 kJ/kg;
Options indoor air:
indoor 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. Room process beam – 6800 kJ/kg.
Scheme of air distribution in the premises - "bottom-up" low-speed air distributors. In this regard (we will not apply the calculation, because it is voluminous and goes beyond the scope 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. Removed:
temperature - 25°С;
relative humidity - 37%

Let's build the process on the I-d diagram (Fig. 1).
First, let's designate a point with the parameters of the internal air (B), then draw a process beam through it (note that for this design of the diagrams, the starting point of the beam is the parameters t=0°C, d=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 indoor air, while maintaining the angle of inclination).
Now, knowing the supply and exhaust air temperatures, we determine their points by finding the intersections of the isotherms with the process beam, respectively. We build the process from the reverse, in order to obtain the specified parameters of the supply air, we lower the segment - heating - along the line of constant moisture content to the curve of relative humidity φ = 95% (segment P-P1).
We select a condensing rotor that utilizes the heat of the exhaust air for heating P-P1. We obtain the efficiency (calculated by temperature) of the rotor of about 78% and calculate the temperature of the removed air U1. Now, let's select an enthalpy rotor that works to cool the outside air (H) with the obtained parameters U1.
We get, the efficiency (calculated by enthalpy) is about 81%, the parameters of the treated air at the inlet H1, and at the exhaust U2. Knowing the parameters H1 and P1, you can choose an air cooler with a capacity of 332,500 watts.

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

Now, for comparison, let's select another system, for the same parameters, but with a different configuration, namely: we install one condensing rotor.

Now (Fig. 3) P-P1 is heated by an electric air heater, and the condensing rotor will provide the following: efficiency is about 83%, the temperature of the treated supply air (H1) is 26°C. We will 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, no additional energy costs (in this case, alternating current) are required for the second air heating. Let's make a comparison table:

Summarizing

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

Production of ventilation units with rotary heat exchangers

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

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

Part 1. Heat recovery devices

Waste heat utilization flue gases
technological furnaces.

Process furnaces are the largest consumers of energy in refineries 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 economical 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 waste heat boilers 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 to generate steam of relatively low pressure - 14 - 16 atm., the latter - 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 more than complex structure. 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 the necessary pressure is pumped up. 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.

Reliability and cost-effectiveness of KU operation largely depends on the correct organization of the 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 and carbon dioxide dissolved in the feed water cause corrosion of the inner 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 very important advantage when using high-viscosity liquid fuels.

Also, the advantages of air heaters in comparison with KU are the simplicity of their design, operational safety, no need to install optional 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 the specific situation at a given installation, enterprise, 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 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 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 different 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 (higher density) air is sucked in from the street. In this case, heat losses are inevitable, since additional heat carrier consumption is required to heat the cold air entering the room. 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 it is necessary to exchange large volumes of air intensively, which is usually associated with the prevention of an increase in the concentration of hazardous substances (harmful, toxic, flammable, explosive, having bad smell) in room. Forced ventilation is implemented in industrial premises, warehouses, storage facilities for agricultural products, etc.

Are used forced ventilation systems 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; at the same time, the heat consumption for heating fresh air is maximum.

The use of natural ventilation systems and separate systems of exhaust and supply ventilation does not allow using the heat of 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 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 first ones are preferable, since they have smaller dimensions, metal consumption and hydraulic resistance, they have greater efficiency and a long service life (20–25 years).

Air ducts are connected to heat exchangers and heat is transferred directly from air to air through a separating wall or an 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
(as an intermediate coolant in such systems, concentrated aqueous solutions salts - brines); 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 outer surface 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, the intermediate heat carrier gives off heat to the fresh air, heating it from -20 °C to + 5 °C. 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 ventilation systems separately, it is necessary to organize a centralized air outlet or air supply, respectively, 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.

One of the sources of secondary energy resources in the building is the thermal energy of the air removed into the atmosphere. The consumption of thermal energy for heating the incoming air is 40 ... 80% of heat consumption, most of it can be saved in the case of the use of so-called waste heat exchangers.

There are various types of waste heat exchangers.

Recuperative plate heat exchangers are made in the form of a package of plates installed in such a way that they form two adjacent channels, one of which moves the removed air, and the other - the supply air. In the manufacture of plate heat exchangers of this design with a large air capacity, significant technological difficulties arise, therefore, the designs of shell-and-tube waste heat exchangers TKT, which are a bundle of pipes arranged in a checkerboard pattern and enclosed in a casing, have been developed. The removed air moves in the annular space, the outer one - inside the tubes. Cross flow.

Rice. Heat exchangers:
a - plate heat exchanger;
b - TKT utilizer;
in - rotating;
g - recuperative;
1 - body; 2 - supply air; 3 - rotor; 4 - blowing sector; 5 - exhaust air; 6 - drive.

In order to protect against icing, the heat exchangers are equipped with an additional line along the outside air flow, through which, at a temperature of the tube bundle walls below the critical temperature (-20°C), part of the cold outside air is bypassed.

Extract air heat recovery units with an intermediate heat carrier can be used in mechanical supply and exhaust ventilation systems, as well as in air conditioning systems. The unit consists of an air heater located in the supply and exhaust ducts, connected by a closed circulation circuit filled with an intermediate carrier. The circulation of the coolant is carried out by means of pumps. The exhaust air, being cooled in the air heater of the exhaust duct, transfers heat to an intermediate heat carrier that heats the supply air. When the exhaust air is cooled below the dew point temperature, water vapor condenses on a part of the heat exchange surface of the exhaust duct air heaters, which leads to the possibility of frost formation at negative initial temperatures of the supply air.

Heat recovery units with an intermediate heat carrier can operate either in a mode that allows the formation of frost on the heat exchange surface of the exhaust air heater during the day with subsequent shutdown and defrosting, or, if the shutdown of the unit is unacceptable, using one of the following measures to protect the exhaust duct air heater from frost formation :

• preheating of supply air to a positive temperature;
• creating a bypass for the coolant or supply air;
• increase in coolant flow in the circulation circuit;
• heating of the intermediate coolant.

The choice of the type of regenerative heat exchanger is made depending on the design parameters of the removed and supply air and moisture release inside the room. Regenerative heat exchangers can be installed in buildings for various purposes in systems of mechanical supply and exhaust ventilation, air heating and air conditioning. The installation of a regenerative heat exchanger must provide countercurrent air flow.

The ventilation and air conditioning system with a regenerative heat exchanger must be equipped with control and automatic control means, which must provide operating modes with periodic frost thawing or frost formation prevention, as well as maintain the required supply air parameters. To prevent frost formation in the supply air:

• arrange a bypass channel;
• preheat the supply air;
• change the frequency of rotation of the regenerator nozzle.

In systems with positive initial supply air temperatures during heat recovery, there is no danger of condensate freezing on the surface of the heat exchanger in the exhaust duct. In systems with negative initial supply air temperatures, it is necessary to apply recycling schemes that provide protection against freezing of the surface of the air heaters in the exhaust duct.

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

· Applying schemes with air recirculation;

· Installing heat exchangers.

The latter method, as a rule, is used in direct-flow circuits of air conditioning systems. However, the use of heat recovery units is not excluded in schemes with air recirculation.

AT modern systems ventilation and air conditioning, a wide variety of equipment is used: heaters, humidifiers, different kinds filters, adjustable grilles and much more. All this is necessary to achieve the required air parameters, maintain or create comfortable conditions for indoor work. A lot of energy is required to maintain all this equipment. An effective solution energy savings in ventilation systems are heat recovery units. The basic principle of their operation is the heating of 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 heating the supply air, thereby reducing the amount of energy required for its operation.

Heat recovery in air-conditioned buildings can be done by recovering the heat from ventilation emissions. Waste heat recovery for fresh air heating (or cooling of incoming fresh air with waste air from an air conditioning system in summer) is the simplest form of recovery. In this case, four types of disposal systems can be noted, which have already been mentioned: rotating regenerators; heat exchangers with an intermediate coolant; simple air heat exchangers; tubular heat exchangers. A rotary heat exchanger in an air conditioning system can increase the supply air temperature by 15°C in winter and can reduce the supply air temperature by 4-8°C in summer (6.3). As with other recovery systems, with the exception of the intermediate heat exchanger, the rotary heat exchanger can only function if the exhaust and suction ducts are adjacent to each other at some point in the system.

An intermediate heat exchanger is less efficient than a rotary heat exchanger. In the system shown, 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 this heat exchanger and the rotary regenerator have moving parts (the pump and the electric motor are driven and this distinguishes them from air and tube heat exchangers. One of the disadvantages of the regenerator is that fouling can occur in the channels. Dirt can be deposited on the wheel, which then transfers it to the suction channel.Most wheels are now equipped with scavenging, which reduces the transfer of contaminants to a minimum.

A simple air heat exchanger is a stationary device for heat exchange between the exhaust and incoming air flows, passing through it in countercurrent. This heat exchanger resembles a rectangular steel box with open ends, divided into many narrow channels like chambers. Exhaust and fresh air flows through alternating channels, and heat is transferred from one air stream to another simply through the walls of the channels. There is no transfer of contaminants in the heat exchanger, and since a significant surface area is enclosed in a compact space, a relatively high efficiency is achieved. The heat pipe heat exchanger can be seen as a logical development of the heat exchanger design described above, in which the two air flows into the chambers remain completely separate, connected by a bundle of finned heat pipes that transfer heat from one channel to another. Although the pipe wall can be considered as additional thermal resistance, the efficiency of heat transfer within the pipe itself, in which the evaporation-condensation cycle takes place, is so high that up to 70% of waste heat can be recovered in these heat exchangers. One of the main advantages of these heat exchangers compared to the intermediate heat exchanger and rotary regenerator is their reliability. The 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 diversity constructive solutions heat recovery units of secondary energy resources, each of them has the following elements:

· The environment is a source of thermal energy;

· The environment is a consumer of thermal energy;

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

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

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

In regenerative and air-air (air-liquid) recuperative heat exchangers, the heat exchange media themselves are the working substance.

Application examples.

1. Air heating in air heating systems.
Air heaters are designed for rapid heating of air with the help of a water coolant and its uniform distribution with the help of a fan and guide blinds. it good decision for construction and production shops, where rapid 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 the hot water supply system.
The use of heat recovery units allows you to smooth out peaks in energy consumption, since the maximum water consumption occurs at the beginning and end of the shift.

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

4. Heating of blast air for combustion. Allows you to reduce fuel consumption by 10%–15%.

It has been calculated that the main reserve for saving fuel during the operation of burners for boilers, furnaces and dryers is the utilization of the heat of exhaust gases by heating the combusted fuel with air. The 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 preheated blast air makes it possible to reduce the consumption of fuel natural gas by up to 30%.
5. Heating of the fuel going to combustion using "liquid-liquid" heat exchangers. (Example - heating fuel oil to 100˚–120˚ С.)

6. Process fluid heating using "liquid-liquid" heat exchangers. (Example - heating a galvanic solution.)

Thus, the heat exchanger is:

Solving the problem of energy efficiency of production;

Normalization of the ecological situation;

Availability of comfortable conditions in your production - heat, hot water in administrative and amenity premises;

Reducing energy costs.

Picture 1.

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

List of used literature.

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

2. Eremkin A.I., Byzeev V.V. 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 DIA M., 2008