In Northern Europe and Scandinavia, ventilation systems of multi-storey residential buildings with supply air heating due to the heat removed with the help of heat recovery units have become widespread. Heat exchangers in ventilation systems were developed in the 1970s during the energy crisis.

To date, heat recovery units have been widely used: - recuperative type based on plate air-to-air heat exchangers (Fig. 41); - regenerative with a rotating heat exchange nozzle (Fig. 42); - with an intermediate heat carrier with "liquid-air" heat exchangers (Fig. 43).

According to their execution in multi-storey residential buildings, heat exchangers can be central to all buildings or a group of apartments and individual, apartment-by-apartment.

Rice. 42. Heat exchanger with a rotating heat exchange nozzle

Rice. 41. Heat exchanger of recuperative type (ventilation air heat recovery unit)

With similar weight and size indicators, regenerative heat exchangers (80-95%) have the highest energy efficiency, followed by recuperative ones (up to 65%), and heat exchangers with an intermediate coolant (45-55%) are in last place.

According to their design features, heat exchangers with an intermediate heat carrier are not very suitable for individual apartment ventilation, and therefore, in practice, they are used for central systems.

Rice. 43. Ventilation air heat recovery unit with an intermediate heat carrier: 1 - supply ventilation unit; 2 - exhaust ventilation unit; 3 - heat exchanger; 4 - circulation pump; 5 - filter; 6 – body of the utilizer

Regenerative heat exchangers have a significant drawback - the likelihood of mixing a certain part of the exhaust air with the supply air in the unit body, which, in turn, can lead to the transfer of unpleasant odors and pathogenic bacteria. The volume of overflowing air in modern devices is reduced to fractions of a percent, but, nevertheless, most experts recommend limiting their scope to the limits of one apartment, cottage or one room in public buildings.

Recuperative heat exchangers, as a rule, include two fans (supply and exhaust), a plate heat exchanger, filters (Fig. 41). In modern designs, two water or electric heaters are built into the heat exchanger. One serves to protect the exhaust tract of the heat exchanger from freezing, the second - to reheat the supply air temperature to a predetermined value.

These systems, in comparison with traditional ones, have a number of advantages, which include significant savings in thermal energy spent on heating ventilation air - from 50 to 90%, depending on the type of heat exchanger used; as well as a high level of air-thermal comfort, due to the aerodynamic stability of the ventilation system and the balance of supply and exhaust air flow rates.

When installing recuperative heat exchangers for apartments, the following appear: - the ability to flexibly regulate the air-thermal regime depending on the mode of operation of the apartment, including using recirculated air; - the possibility of protection from urban, external noise (when using sealed translucent fences); – the possibility of cleaning the supply air with the help of highly efficient filters.

The implementation of these advantages is associated with the solution of a number of problems: - it is necessary to provide appropriate space-planning solutions for the apartment and allocate space for the placement of heat recovery units and additional air ducts; – it is necessary to provide protection against freezing of heat exchangers at low outdoor temperatures (-10 °C and below); – heat exchangers must be of low noise design and, if necessary, equipped with additional silencers; – it is necessary to ensure qualified maintenance of heat recovery units (replacement or cleaning of filters, flushing of the heat exchanger).

Various modifications of exhaust air heat exchangers are produced by a total of more than 20 companies. In addition, the production of energy-saving equipment begins at domestic enterprises.

The sound power level is given without duct network, without silencers for an open heat exchanger.

The widespread use of mechanical ventilation systems in residential multi-storey buildings with exhaust air heat recovery is constrained by a number of factors: - there is practically no material incentive for energy saving among consumers - owners of apartments; - Investors-developers are not interested in additional costs for engineering equipment in economy and business class houses, believing that the quality of ventilation is a secondary indicator in the formation of the market value of housing; – “scars away” the need for maintenance of mechanical ventilation; - the population is not sufficiently informed about the criteria for the air-thermal comfort of a dwelling, its impact on health and performance.

At the same time, there has been a positive trend towards overcoming the noted problems, and both investors and apartment buyers have a practical interest in modern technical solutions for ventilation systems.

Let's compare the effectiveness of traditional ventilation and new technical solutions in relation to residential multi-storey buildings of mass development.

There are three options for organizing ventilation in residential 17-storey buildings of the P-44 series for Moscow conditions:
A. Ventilation according to a typical design (natural ducted exhaust from the kitchen, bathroom and toilet rooms and inflow due to infiltration and from
window transom covers).
B. Mechanical exhaust, central ventilation system with the installation in the apartments of supply and exhaust valves with a constant air flow.
B. Mechanical supply and exhaust ventilation system with heat recovery of the removed air in recuperative heat exchangers.

The comparison was carried out according to three criteria: – air quality; - consumption of thermal energy in ventilation systems; – acoustic mode.

For the conditions of Moscow, according to meteorological observations, the following climatic conditions were adopted.

The following values ​​of heat transfer resistance were taken into account in the calculations: - walls - 3.2 m2 °C/W; – windows – 0.62 m2 °С/W; - coatings - 4.04 m2 °C / W.

Heating system with traditional convectors for coolant parameters 95/70 °С.

In each entrance on the floor there are two 2-room, one 1-room and one 3-room apartments. Each apartment has a kitchen with an electric stove, a bathroom and a toilet.

The extract is produced in accordance with the standards: - from the kitchen - 60 m3 / h; – from the bathroom - 25 m3/h; - from the toilet - 25 m3 / h.

For the analysis, it is assumed that in option A, due to ventilation by opening the transoms of the windows, the average daily volume of inflow corresponds to the volume of exhaust from the apartment.

Rice. 44. Recuperator with installation of air heaters in the apartments of the experimental building: 1 - exhaust air fan; 2 – supply air fan; 3 - plate heat exchanger; 4 – electric heater; 5 – heat exchanger heater; 6 - filter for outside air (class EU5); 7 - filter for exhaust air (class EU5); 8 - sensor against freezing of the heat exchanger; 9, 10 - automatic reset of thermal protection; 11, 12 - manual reset of thermal protection; 13 - supply air temperature sensor

In option B, constant air exchange is ensured by the operation of a central exhaust fan connected to each apartment by a network of air ducts. Consistency of air exchange is ensured by the use of constant flow inlet valves installed in the window sashes and self-regulating exhaust valves in the kitchen, bathroom and toilet.

In option B, a mechanical supply and exhaust ventilation system is used with heat recovery from the exhaust air to heat the supply air in a plate heat exchanger. When comparing, the condition of constancy of air exchange was also accepted.

According to the air quality criterion, option A is significantly inferior to options B and C. Ventilation is carried out periodically during a time arbitrarily chosen by residents, that is, it is subjective and therefore far from always effective. In winter, ventilation is associated with the need for residents to leave the ventilated premises. Attempts to adjust the opening of the transoms for constant ventilation most often lead to ventilation instability, drafts, and thermal discomfort. With periodic ventilation, the air quality deteriorates after the vents are closed, and residents spend most of their time in a polluted air environment (Fig. 45).

Rice. 45. Change in air exchange and concentration of harmful substances during periodic ventilation of premises:
1 - air exchange;
2 - concentration of harmful substances;
3 - standard level of concentration of harmful substances

A special ventilation mode is provided for the kitchen. When cooking, an over-stove umbrella equipped with a high-performance multi-speed fan is included in the work. The air capacity of modern over-slab umbrellas reaches 600-1000 m3 / h, which is many times higher than the calculated air exchange in the apartment. To remove air from above-plate umbrellas, as a rule, separate air ducts are provided that are not connected with the general exhaust ventilation system from the kitchen. Compensatory flow of supply air is provided by a supply valve in the wall, which is opened during the operation of the umbrella. The general conclusion on the compared options can be drawn as follows: option B with exhaust air heat recovery has the highest efficiency in terms of air-thermal comfort and thermal energy saving; To normalize the acoustic regime, additional noise protection measures for the fan installation are required.

Constantly operating ventilation of apartments using supply valves (option B) built into window sashes or external walls at low outdoor temperatures can lead to thermal discomfort associated with uneven distribution of temperature and air velocity in the premises. Despite the fact that it is recommended to place supply valves above or behind heating appliances, experts in Western Europe limit the effective scope of such ventilation systems to areas with an outdoor air temperature of at least -10 ° C. Of greatest interest is the ventilation option B, i.e. mechanical supply and exhaust ventilation with heat recovery of the exhaust air in recuperative heat exchangers. It is this system that was used to design and build the experimental system.

The experimental building consists of four sections; the total number of apartments is 264. Under the building there is a parking garage for 94 cars. On the 1st floor there are auxiliary non-residential premises, the two upper floors are reserved for a sports and fitness center. Residential apartments are located from the 2nd to the 16th floor. In free-plan apartments from 60 to 200 m2 of total area, in addition to living quarters, a kitchen, a bathroom with a toilet, a laundry room, a guest toilet, storage rooms, glazed loggias are provided. The building was built according to an individual project (architect P.P. Pakhomov). Structural solutions of the building are a monolith with an effective insulation with a brick cladding. The concept of energy-saving solutions for the building was developed under the guidance of the President of the Association of Engineers for Heating, Ventilation, Air Conditioning, Heat Supply and Building Thermophysics, Professor Yu. ".

The project provides for a comprehensive solution, in which energy-saving architectural and planning solutions, effective enclosing structures and new generation engineering systems are functionally linked.

Building structures have a high level of thermal protection. Thus, the heat transfer resistance of walls is 3.33 m2 °C / W, metal-plastic windows with double-glazed windows - 0.61 m2 * °C / W, top coatings - 4.78 m2 °C / W, loggias are glazed with sun-protection tinted glasses.

The internal air parameters for the cold period are taken as follows: - living rooms - 20 °С; – kitchen - 18 °С; – bathroom - 25 °С; - toilet - 18 °C.

The building is designed with a horizontal per-apartment heating system with perimeter piping throughout the apartment. Metal-plastic pipes with thermal insulation in a protective corrugation are embedded in the preparation of the "black" floor. For the entire building with a total area of ​​about 44 thousand m2 in the heating system of the residential part, there are only four pairs of risers (supply and return) according to the number of sections. On each floor in the elevator hall, distribution manifolds to the apartments are connected to the risers. The collectors are equipped with fittings, balancing valves and apartment heat meters.

The building has been designed and implemented per apartment adjustable supply and exhaust ventilation system with heat recovery of the exhaust air.

A compact air handling unit with a plate heat exchanger is located in the false ceiling of the guest toilet next to the kitchen.

The intake of supply air is carried out through a heat-insulated air duct and a hole in the outer wall facing the kitchen loggia. The exhaust air is taken from the kitchen area. The extract from the toilets and the bathroom is not heat-utilized, because at the time of the approval of the project, the standards forbade combining kitchen, bathroom and toilet extracts into one ventilation network within the apartment. Currently, according to the "Technical recommendations for the organization of air exchange in apartments of a multi-storey residential building", this restriction has been removed.

In the conditions of free planning of apartments, the unification of three or four zones by a common horizontal exhaust air duct requires special architectural and planning solutions, the installation of a horizontal air duct network in the apartment, which is difficult to implement for structural reasons.

During the heating period of 2003-2004 in a 3-room apartment on the 12th floor, preliminary tests of the apartment ventilation system were carried out with the heat recovery of the removed air. The total area of ​​the apartment is 125 m2. The tests were carried out in an apartment without finishing, without interior partitions and doors. Selective test results are given in table. 22. The outdoor air temperature 4 ranged from +4.1 to -4.5 ° C with mostly cloudy weather. The air temperature in the room tB was maintained by an apartment heating system with steel radiators equipped with thermostatic valves in the range from 22.8 to 23.7 °C. In the course of tests with the help of air humidifiers, the relative humidity φ was varied from 25 to 45%.

A recuperative heat exchanger was installed in the apartment, with a maximum capacity for supply air Lnp = 430 m3/h. The volume of the removed air Lngutl was approximately 60-70% of the supply air, which is due to the setting of the device for the utilization of only a part of the removed air.
The device is equipped with air filters for the supply and exhaust paths and two electric heaters. The first heater with a rated power of 0.6 kW is designed to protect the exhaust tract from freezing condensate, which is discharged into the sewer through a special drainage tube through a water seal. The second heater with a power of 1.5 kW is designed to reheat the supply air tw to a predetermined comfort value.

Rice. 46. ​​Plan of the apartment with a ventilation system: 1 - air handling unit with a heat exchanger; 2 - air intake from the loggia; 3 - extract from the kitchen; 4 - extract from the guest toilet; 5 - extract from the dressing room; 6 - extract from the bathroom; 7 - ceiling perforated air diffuser

For ease of installation, it is also made electric.

During the test, measurements were made of the temperature and humidity of the outdoor, indoor and exhaust air, the flow rate of the supply and exhaust air, the heat consumption of the apartment heating system Qm according to the readings of the heat meter, and the electricity consumption.

The heat exchanger is equipped with an automation system with a controller and a control panel. The automation system provides for turning on the first heater when the temperature of the heat exchanger wall reaches below +1 °C, the second heater can be turned on and off, ensuring the constancy of the set supply air temperature, which was in the range from 15 to 18.3 °C during testing. The fan control system allows you to select three fixed air flow rates corresponding to the air exchange rate from 0.48 to 1.15 1/h.

Control and setting of temperature and air flow is carried out from a remote wired control panel.

Tests have shown the stable operation of the apartment ventilation system and the energy efficiency of utilizing the heat of the removed air.

It should be noted a number of features in the conduct of research, which cannot be ignored when assessing the indicators of the air-thermal regime of an apartment.

1. In new buildings, fresh concrete and mortar release a significant amount of moisture into the premises. The period during which moisture in building structures comes to an equilibrium state reaches 1.5-2 years. So, as a result of tests, approximately six months after filling the monolith and laying the screed, the moisture content of the indoor air in the presence of ventilation was 4-4.5 g/kg of dry air, while the moisture content of the outside air did not exceed 1-1.5 g/kg of dry air. air.

According to our estimates, in a monolithic building, in order to bring structures into an equilibrium moisture state, it is necessary to assimilate up to 200 kg of moisture per square meter. meter of floor area. The amount of heat required to evaporate this moisture in the initial period is 10-15 W/m2, and during the test period - 5-7 W/m2, which is a significant part of the heat balance of the apartment during the cold season. It is reckless not to take this factor into account in the implementation of heating and ventilation, especially in monolithic housing construction.

2. In the process of testing, there were no so-called internal household heat emissions, the size of which in the standards is proposed to be 10 W/m2.
It seems that this indicator should be differentiated depending on the area of ​​the apartment per inhabitant.

In large apartments (more than 100 m2) with an area per person of 30-50 m2, the probable value of this indicator should decrease to 5-8 W/m2. Otherwise, the design heat output of heating and ventilation systems of buildings may be underestimated by 10-30%.

However, during construction, in particular buildings with monolithic structures that release a lot of moisture into the premises, it is more expedient to dry them out with the help of powerful electric heaters at the disposal of the builders before commissioning the buildings and especially before they are occupied. Unfortunately, such drying before testing was not carried out.

As noted, the experimental building under consideration was designed and built as energy-saving. Based on the results of the tests, adjusted for the predicted domestic heat release and the heat of evaporation of moisture in building structures, the specific heat and energy characteristics of a 3-room apartment were calculated per 1 m2 of area while maintaining a temperature of 20 °C in the apartment.

The results of the calculations showed that after finishing the apartments and settling in the building, the specific estimated annual heat consumption for heating and ventilation is almost halved from 132 to 70 kWh/(m2 year), and with the use of heat recovery to 44 kWh/(m2 year).

Further operation of the building will allow checking the assumptions made in the preliminary calculations.

Studies of the experimental system should cover all the factors that characterize its operation, including the psychological attitude of the residents using devices that are new to them.

Electric heating of air in the experimental system, compared with the use of heat from the heating system to which the building is connected, is not economically justified. This decision was made for the convenience of the experiment, in particular, for measurements related to heat consumption. However, according to the authors, over time, humanity will begin to switch to full electrical and heat supply of residential urban buildings. Therefore, an experimental study of a system in which apartment ventilation operates using electric air heaters is of interest for the future.

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 high 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 use recycling schemes that provide protection against freezing of the surface of the air heaters in the exhaust duct.

Part 1. Heat recovery devices

Utilization of flue gas heat
technological furnaces.

Process furnaces are the largest consumers of energy in oil refining and petrochemical plants, in metallurgy, as well as in many other industries. At refineries, they burn 3–4% of all processed oil.

The average temperature of the flue gases at the outlet of the furnace, as a rule, exceeds 400 °C. The amount of heat carried away with flue gases is 25–30% of the total heat released during fuel combustion. Therefore, the utilization of heat from flue gases from process furnaces is extremely important.

At flue gas temperatures above 500 °C, waste heat boilers - KU should be used.

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

The greatest economic effect is achieved in the presence of a two-unit plant 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 at high flue gas temperatures.

Waste boilers.

V 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 a more complex design. A schematic diagram of the operation of a KU of this type is shown in fig. 5.4.

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

Schematic diagram of the operation of the waste heat boiler

packet-convective type

Chemically purified water (CPW) enters the deaerator column to remove gases dissolved in it (mainly oxygen and carbon dioxide). Water flows down the plates, and 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 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 and their damage occurs due to a decrease in the bearing capacity of steel.

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). When operating high-pressure boilers operating with forced water circulation, intra-boiler phosphating is additionally 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 a very important advantage when using high-viscosity liquid fuels.

Also, the advantages of air heaters in comparison with CHP are the simplicity of their design, safety of operation, no need to install additional equipment (deaerators, pumps, heat exchangers, etc.). However, air heaters, with the current ratio of prices for fuel and steam, turn out to be less economical than CHP (our price for steam is very high - 6 times higher per 1 GJ). Therefore, it is necessary to choose a method for utilizing the heat of flue gases, based on 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. Various types of ventilation systems can be used for various purposes. 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 the 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 losses. In our apartments, 25 - 30% of heat losses are associated with the operation of ventilation, in all other cases this value is much higher.

Forced (artificial) ventilation systems are used when intensive exchange of large volumes of air is required, which is usually associated with the prevention of an increase in the concentration of hazardous substances (harmful, toxic, fire-explosive, having an unpleasant odor) in the 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
(concentrated aqueous solutions of salts - brines are usually used as an intermediate heat carrier in such systems); 6 - pump; 7 - coil for
additional heating of fresh air with steam or hot water

The system works as follows. Warm air (+ 25 °C) is removed from the room through the exhaust duct 1 through the chamber in which the finned coil is installed 3 . The air washes the outer surface of the coil and transfers heat to the cold intermediate heat carrier (brine) flowing inside the coil. The air is cooled to 0 °C and released into the atmosphere, and the brine heated to 15 °C through circulation pipelines 5 enters the fresh air heating chamber on the supply air duct 2 . Here, 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 the 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 flows include the heat of flue gases from furnaces and boilers, wastewater from process plants and the 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.

Description:

Currently, the indicators of thermal protection of multi-storey residential buildings have reached quite high
values, therefore, the search for reserves for saving thermal energy is in the field of improving the energy efficiency of engineering systems. One of the key energy-saving measures with a rather high potential for saving thermal energy is the use of exhaust air heat exchangers 1 in ventilation systems.

At present, the thermal protection indicators of multi-storey residential buildings have reached quite high values, so the search for reserves for saving thermal energy is in the field of improving the energy efficiency of engineering systems. One of the key energy-saving measures with a rather high potential for saving thermal energy is the use of exhaust air heat exchangers 1 in ventilation systems.

Supply and exhaust ventilation units with exhaust air heat recovery have a number of advantages compared to traditional supply ventilation systems, which include significant savings in thermal energy spent on heating ventilation air (from 50 to 90% depending on the type of heat exchanger used). It should also be noted the high level of air-thermal comfort, due to the aerodynamic stability of the ventilation system and the balance of supply and exhaust air flow rates.

Types of utilizers

The most widely used:

1. Regenerative heat recovery units s. In regenerators, the heat from the extract air is transferred to the supply air through a nozzle that alternately heats up and cools down. Despite the high energy efficiency, regenerative heat exchangers have a significant drawback - the likelihood of mixing a certain part of the exhaust air with the supply air in the apparatus case. This, in turn, can lead to the transfer of unpleasant odors and disease-causing bacteria. Therefore, they are usually used within one apartment, cottage or one room in public buildings.

2. Recuperative heat recovery units. These heat exchangers, as a rule, include two fans (supply and exhaust), filters and a plate heat exchanger of counterflow, cross and semi-cross types.

With the apartment-by-apartment installation of recuperative heat recovery units, it becomes possible to:

1. flexibly regulate the air-thermal regime depending on the type of operation of the apartment, including the use of recirculated air;
2. protection from urban, external noise (when using sealed translucent fences);
3. purification of supply air using high-performance filters.

3.Heat recovery units with intermediate heat carrier. Due to their design features, these heat exchangers are of little use for individual (apartment) ventilation, and therefore, in practice, they are used for central systems.

4. Heat recovery units with a heat exchanger on heat pipes. The use of heat pipes allows you to create compact energy-efficient heat exchange devices. However, due to the complexity of the design and high cost, they have not found application in ventilation systems for residential buildings.

In basic terms, the distribution of heat energy consumption in a typical multi-storey building is carried out almost equally between transmission heat losses (50–55%) and ventilation (45–50%). Approximate distribution of the annual heat balance for heating and ventilation: transmission heat losses - 63–65 kWh/m2 year; ventilation air heating – 58–60 kWh/m2 year; internal heat generation and insolation - 25–30 kWh/m2 year. To increase the energy efficiency of apartment buildings allows the introduction into the practice of mass construction: modern heating systems using room thermostats, balancing valves and weather-dependent automation of heating points; mechanical ventilation systems with exhaust air heat recovery.

With similar weight and size indicators, the best result in residential buildings is shown by regenerative heat recovery units (80–95%), followed by recuperative ones (up to 65%), and in last place are heat recovery units with an intermediate coolant (45–55%).

Heat recovery units should be mentioned, which, in addition to transferring thermal energy, transfer moisture from the exhaust air to the supply air. Depending on the design of the heat transfer surface, they are divided into enthalpy and sorption types and allow utilizing 15–45% of the moisture removed with the exhaust air.

One of the first implementation projects

In 2000, for a residential building at 6, Krasnostudenchesky Prospekt, one of the first systems of apartment-by-apartment mechanical supply and exhaust ventilation was designed with exhaust air heat recovery for supply air heating in a cross-flow air-to-air plate heat exchanger.

A compact, low-noise apartment air handling unit is located in each apartment in the false ceiling space of the guest bathroom, located next to the kitchen. The maximum supply air capacity is 430 m 3 /h. To reduce energy consumption, outdoor air is taken in most apartments not from the street, but from the space of a glazed loggia. In other apartments, where there is no technical possibility of air intake from the loggias, air intake grilles are located directly on the facade.

The outside air is cleaned, if necessary, preheated to prevent freezing of the heat exchanger, then heated or cooled in the heat exchanger due to the removed air, then, if necessary, finally heated to the required temperature by an electric heater, after which it is distributed throughout the premises of the apartment. The first heater with a rated power of 0.6 kW is designed to protect the exhaust tract from condensate freezing. Condensate is discharged through a special drainage tube through a water seal into the sewer. The second heater with a power of 1.5 kW is designed to heat the supply air to a predetermined comfortable value. For ease of installation, it is also made electric.

It should be noted that, according to the calculations of the designers, the need for additional air heating after the heat exchanger could arise only at very low outdoor temperatures. Nevertheless, taking into account that twice as much supply air passes through the heat exchanger of the supply and exhaust unit as the exhaust air, an electric air heater was installed on the supply. Operational practice confirmed these assumptions: additional heating is almost never used, the heat of the exhaust air is enough to heat the supply air to a temperature that does not cause discomfort to residents.

The heat exchanger is equipped with an automation system with a controller and a control panel. The automation system provides for the inclusion of the first heater when the temperature of the heat exchanger wall reaches below 1 °C, the second heater can be switched on and off, ensuring the constancy of the set supply air temperature.

The supply fan has three fixed speeds. At the first speed, the supply air volume is 120 m 3 /h, this value satisfies the requirements for a one- and two-room apartment, as well as a three-room apartment with a small number of inhabitants. At the second speed, the supply air volume is 180 m 3 /h, at the third - 240 m 3 / h. Residents rarely use the second and third speeds.

Acoustic measurements were carried out at all fan speeds, which showed that at the first speed the noise level does not exceed 30-35 dB (A), and this value is valid for an unfurnished apartment. In an apartment with furniture and interior items, the noise level will be even lower. At the second and third speeds, the noise level is higher, but with the guest bathroom door closed, it does not cause discomfort to residents.

Exhaust air is taken from the sanitary facilities, then, after being filtered, it is passed through a heat exchanger and discharged through a central collection exhaust air duct. Prefabricated exhaust air ducts - metal, made of galvanized steel and laid in enclosed fire shafts. On the upper technical floor, prefabricated air ducts of one section are combined and led outside the building.

At the time of the project implementation, it was forbidden by the regulations to combine the hoods of bathrooms and kitchens for disposal, so the hoods of the kitchens are separated. The heat of about half of the volume of air removed from the apartment is utilized. This ban has now been lifted, further improving the energy efficiency of the system.

During the 2008-2009 heating season, an energy audit of heat consumption systems was carried out in the building, which showed a savings in heat for heating and ventilation in the amount of 43% compared to similar houses of the same year of construction.

Project in Northern Izmailovo

Another similar project was implemented in 2011 in Northern Izmailovo. Apartment building No. 153 provides for apartment-by-apartment ventilation with mechanical stimulation and heat recovery of the exhaust air to heat the supply air. The supply and exhaust units are installed autonomously in the corridors of the apartments and are equipped with filters, a plate heat exchanger and fans. The unit is equipped with automation equipment and a control panel that allows you to adjust the air capacity of the unit.

Passing through the ventilation unit with a plate heat exchanger, the exhaust air heats the supply air up to 4°C (at an outside air temperature of -28°C). Compensation for the lack of heat for heating the supply air is carried out by heating devices.

The outside air is taken from the apartment's loggia, and the exhaust air from the baths, bathrooms and kitchens (within the same apartment), after the heat exchanger, is discharged into the exhaust duct via a satellite and removed within the technical floor. If necessary, the removal of condensate from the heat recovery unit is provided for in the sewer riser, equipped with a drip funnel with an odor-locking device. The stand is located in the bathrooms.

Supply and exhaust air flow control is carried out by means of one control panel. The unit can be switched from normal operation with heat recovery to summer operation without heat recovery. Ventilation of the technical floor occurs through deflectors.

The volume of supply air is taken to compensate for the exhaust from the premises of the bathroom, bath, kitchen. The apartment does not have an exhaust duct for connecting kitchen equipment (the exhaust hood from the stove works for recirculation). The inflow is diluted through sound-absorbing air ducts to the living rooms. It is planned to cover the ventilation unit in the apartment corridors with a building structure with service hatches and an exhaust duct from the ventilation unit to the exhaust shaft. There are four standby fans in the maintenance warehouse.

Tests of the installation with a heat recovery unit have shown that its efficiency can reach 67%.

The use of mechanical ventilation systems with exhaust air heat recovery is widespread in world practice. The energy efficiency of heat recovery units is up to 65% for plate heat exchangers and up to 85% for rotary ones. When these systems are used in Moscow conditions, the reduction of annual heat consumption to the base level can be 38–50 kWh/m2 per year. This makes it possible to reduce the overall specific heat consumption to 50–60 kWh/m2 per year without changing the basic level of thermal protection of the fences and to ensure a 40 percent reduction in the energy intensity of heating and ventilation systems, provided for from 2020.

Literature

1. Serov S. F., Milovanov A. Yu. Apartment ventilation system with heat recovery units. Pilot residential building project// ABOK. 2013. No. 2.
2. Naumov A. L., Serov S. F., Budza A. O. Apartment exhaust air heat recovery units// ABOK. 2012. No. 1.

1 This technology was originally developed in Northern Europe and Scandinavia. Today, Russian designers also have significant experience in using these systems in multi-storey residential buildings.

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.

A wide variety of equipment is used in modern ventilation and air conditioning systems: 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. A lot of energy is required to maintain all this equipment. Heat exchangers are an effective solution for saving energy in ventilation systems. 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 flow 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 in comparison with 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 variety of design solutions for heat recovery devices 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. This is a good solution for construction and production shops, where fast heating and maintaining a comfortable temperature is required only during working hours (the ovens are usually working at the same time).

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