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.
Modern ventilation and air conditioning systems use a wide variety of equipment: 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. 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. Recovery of waste heat for heating fresh air (or cooling of incoming fresh air with waste air after the air conditioning system in summer) is the simplest form recycling. 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 is different from the 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. A heat pipe heat exchanger can be considered as logical development the design of the heat exchanger described above, in which the two air flows into the chambers remain absolutely 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:
· Source medium thermal energy;
· The environment is a consumer of thermal energy;
· Heat receiver - a heat exchanger that receives heat from a source;
The heat exchanger is a heat exchanger that transmits thermal energy 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 is required and maintaining a comfortable temperature only in working time(at the same time, as a rule, the ovens 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. Flue gas heat recovery has great importance in technological processes, since the heat returned to the furnace or boiler in the form of heated blast air reduces the consumption of fuel natural gas 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 environmental 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 V. G., Moskovko Yu. G. Some features effective use 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
2006-02-08The need for energy saving in the design, construction and operation of buildings of any purpose is beyond doubt and is associated primarily with the depletion of fossil fuel reserves and, as a result, its continuous rise in price. Special attention at the same time, it is necessary to pay attention to reducing heat costs specifically for ventilation and air conditioning systems, since the share of these costs in the overall energy balance can be even higher than transmission heat losses, primarily in public and industrial buildings and after increasing the thermal protection of external fences.
One of the most promising, low-cost and fast-payback energy-saving measures in mechanical ventilation and air conditioning systems is the utilization of exhaust air heat for partial heating of the inflow into cold period of the year. For the implementation of heat recovery, devices of various designs are used, incl. plate cross-flow recuperative heat exchangers and regenerators with a rotating rotor, as well as devices with so-called heat pipes (thermosyphons).
However, it can be shown that under the conditions of the price level for ventilation equipment prevailing in the Russian Federation and, mainly, due to the practical absence of in-house production of the listed types of devices, from a technical and economic point of view, it is advisable to consider heat recovery only on the basis of devices with an intermediate coolant. This design is known to have a number of advantages.
Firstly, serial equipment is used for its implementation, since here the supply unit is supplemented only with a heat exchanger, and the exhaust unit is supplemented with a heat exchanger, which are structurally similar to conventional heaters and coolers. This is especially significant, since in the Russian Federation there are a number of enterprises that conduct their own production of the products in question, incl. such large ones as Veza LLC.
In addition, this type of heat recovery equipment is very compact, and the connection of the supply and exhaust units only through a circulation circuit with an intermediate heat carrier allows you to choose a place for their placement almost independently of each other. As a coolant, low-freezing liquids such as antifreezes are usually used, and the small volume of the circulation circuit makes it possible to neglect the cost of antifreeze, and the tightness of the circuit and the non-volatility of antifreeze make the question of its toxicity secondary.
Finally, the absence of direct contact between the flows of supplied and exhausted air does not impose restrictions on the cleanliness of the extract, which practically unlimitedly expands the group of buildings and premises where heat recovery can be used. As a disadvantage, they usually indicate a not too high temperature efficiency, not exceeding 50-55%.
But this is just the case when the question of the advisability of using heat recovery should be decided by a technical and economic calculation, which we will discuss later in our article. It can be shown that the payback period for additional capital costs for a heat recovery device with an intermediate coolant does not exceed three to four years.
This is especially significant in an unstable market economy with a markedly changing level of prices for equipment and tariffs for energy resources, which does not allow the use of capital-intensive engineering solutions. However, the question of the most economically feasible temperature efficiency of such heat recovery equipment k eff remains open, i.e. the share of heat spent on heating the supply air at the expense of the heat of the exhaust air in relation to the total heat load. Commonly used values for this parameter are between 0.4 and 0.5. Now we will show on what basis these values are taken.
This problem will be considered on the example of a supply and exhaust ventilation unit with a capacity of 10,000 m 3 / h, using the equipment of Veza LLC. This task is an optimization one, since it comes down to identifying the value of k eff, which provides a minimum of the total discounted costs of the SDZ for the installation and operation of ventilation equipment.
The calculation should be carried out subject to the use of borrowed funds for the construction of ventilation units and bringing the SDZ to the end of the considered time interval T according to the following formula:
where K - total capital costs, rub; E — total annual operating costs, rub/year; p is the discount rate, %. In calculations, it can be taken equal to the refinancing rate of the Central Bank of the Russian Federation. Since January 15, 2004, this value has been equal to 14% per annum. In this case, it is possible to study the problem in sufficient in full relatively elementary means, since all components of the costs are easily taken into account and quite simply calculated.
For the first time the solution of this problem was published by the author in the work for the level of prices and tariffs in force at that time. However, as it will be easy to see, when recalculated for later data, the main conclusions remain valid. At the same time, we will show how the technical and economic calculation itself should be carried out if it is necessary to choose the best option engineering solution, since all other tasks will differ only in the definition of the value of K.
But this is easily done according to the catalogs and price lists of the manufacturers of the corresponding equipment. In our example, capital costs were determined according to the data of the Veza company, based on the performance and the accepted set of sections of the supply and exhaust units: front panel with one vertical damper, cell filter class G3, fan unit; Besides, in air handling unit also additionally an air heater of the heat recovery system and a reheating heater with heat supply from the heating network, and in the exhaust air cooler of the heat recovery system, as well as a circulation pump. A diagram of such an installation is shown in fig. 1. Expenses for installation and adjustment of ventilation units were taken in the amount of 50% of the main capital investments.
The costs for heat recovery equipment and the reheating heater were calculated based on the results of calculations on a computer using the programs of the Veza company, depending on the efficiency of the heat exchanger. At the same time, with an increase in efficiency, the value of K increases, since the number of rows of tubes of heat exchangers of the utilization system increases faster (for k eff = 0.52 - up to 12 in each installation), than the number of rows of the reheating heater decreases (from 3 to 1 in the same conditions) .
Operating costs are made up of annual costs for heat and electricity, respectively, and depreciation. When calculating them, the duration of operation of the installation during the day in the calculations was assumed to be 12 hours, the air temperature behind the reheating air heater was +18°C, and after the heat exchanger, depending on k eff through the average outside temperature for the heating period and the temperature of the exhaust air.
The latter is equal to +24.7°C by default (program for selection of heat recovery units by Veza LLC). The tariff for thermal energy was taken according to the data of OAO Mosenergo for the middle of 2004 in the amount of 325 rubles/Gcal (for budget consumers). Obviously, with an increase in k eff, the cost of thermal energy decreases, which, generally speaking, is the goal of heat recovery.
Energy costs are calculated in terms of the electrical power required for the drive circulation pump heat recovery systems and fans of supply and exhaust units. This power is determined based on the pressure loss in the circulation circuit, the density and flow rate of the intermediate heat carrier, as well as the aerodynamic resistance of ventilation installations and networks. All of the above values, except for the density of the coolant, assumed to be 1200 kg/m 3 , are calculated according to the selection programs for heat recovery and ventilation equipment of Veza LLC. In addition, the efficiency of the applied pumps and fans also participate in the expressions for power.
The calculations used average values: 0.35 for GRUNDFOS pumps with wet rotor and 0.7 for RDH type fans. The tariff for electric power was taken into account according to the data of OAO Mosenergo as of the middle of 2004 in the amount of 1.17 rubles/(kWh). With an increase in k eff, the level of electricity costs increases, since with an increase in the number of rows of utilization heat exchangers, their resistance to air flow increases, as well as pressure losses in the circulation circuit of the intermediate heat carrier.
However, in general, this component of costs is significantly less than the cost of thermal energy. Depreciation deductions also increase with increasing k eff insofar as this increases capital costs. The calculation of these deductions is carried out based on the provision of costs for full recovery, overhaul and current repairs of equipment, taking into account the estimated service life of the TAM equipment, taken in the calculations equal to 15 years.
In general, however, the total operating costs decrease with increasing utilization efficiency. Therefore, the existence of a minimum of SDZ is possible at one or another level of k eff and a fixed value of T. The results of the corresponding calculations are shown in Figs. 2. On the graphs, one can easily see that the minimum on the SDZ curve appears for almost any calculation horizon, which, according to the meaning of the problem, is equal to the required payback period.
This means that at the existing prices for equipment and tariffs for energy resources, any, even the smallest investment in heat recovery pays off, and quite quickly. Therefore, the utilization of heat with an intermediate heat carrier is almost always justified. With an increase in the expected payback period, the minimum on the SDZ curve quickly shifts to the region of higher efficiency, reaching 0.47 at T = T AM = 15 years.
It is clear that the optimal value of k eff for the accepted payback period will be the one at which a minimum of SDZ is observed. A graph of the dependence of such an optimal value of k eff on T is shown in Fig. 3. Since a longer payback period exceeding the estimated service life of the equipment is hardly justified, it should, apparently, stop at the level k eff = 0.4-0.5, especially since when further growth The increase in optimal efficiency slows down sharply.
In addition, it should be taken into account that the method of heat recovery under consideration for any heat exchange surface and coolant flow rate cannot in principle provide a value of k eff higher than 0.52-0.55, which is confirmed by the calculation according to the program of the Veza company. If we accept the tariff for thermal energy as for commercial consumers in the amount of 547 rubles / Gcal, the reduction in annual costs due to heat recovery will be higher, so the graph in Fig. 3 shows the upper limit of the possible payback period.
Thus, the specified range of values k eff from 0.4 to 0.5 finds a complete feasibility study. Therefore, the main practical advice according to the results of the study, it is possible to use the exhaust air heat recovery with an intermediate heat carrier in any buildings where mechanical supply and exhaust ventilation and air conditioning are provided, with the choice of a temperature efficiency coefficient close to the maximum possible for this type of installation. Another recommendation is that it is obligatory for a market economy to take into account the discounting of capital and operating costs in the technical and economic comparison of engineering solutions according to formula (1).
Moreover, if only two options are compared, as is most often the case, it is convenient to compare only additional costs and assume that in the first case K = 0, and in the second, on the contrary, E = 0, and K is equal to additional investment into activities, the expediency of which is justified. Then instead of E in the first option, you need to use the difference in annual costs for the options. After that, graphs of the dependence of SDZ on T are constructed, and at the point of their intersection, the estimated payback period is determined.
If it turns out to be higher than T AM, or the schedules do not intersect at all, the measures are not economically justified. The results obtained are of a very general nature, since the dependence of the change in capital costs on the degree of heat recovery in the current market situation has little to do with a specific manufacturer of ventilation equipment, and the main impact on operating costs is generally only the cost of heat and electrical energy.
Therefore, the proposed recommendations can be used in making economically sound decisions on energy saving in any mechanical ventilation and air conditioning systems. In addition, these results have a simple and engineering form and can easily be refined when the current prices and tariffs change.
It should also be noted that the payback period obtained in the above calculations, depending on the accepted k eff, reaches 15 years, i.e. up to TAM, is in some respects the marginal, arising when all capital costs are taken into account. If we take into account only additional investments directly into heat recovery, the payback period is indeed reduced to 3-4 years, as mentioned above.
Therefore, exhaust air heat recovery with an intermediate coolant is indeed a low-cost and fast-payback measure and deserves the widest application in a market economy.
- O.D. Samarin. About regulation of thermal protection of buildings. S.O.K. Magazine, No. 6/2004.
- O.Ya. Kokorin. Modern systems air conditioning. - M .: Fizmatlit, 2003.
- V.G. Gagarin. On the insufficient justification of the increased requirements for thermal protection of the outer walls of buildings. (Changes No. 3 of SNiP II-3-79). Sat. report 3rd conf. RNTOS April 23–25, 1998
- O.D. Samarin. Economically expedient efficiency of heat exchangers with an intermediate heat carrier. Assembly and special works in construction, No. 1/2003.
- SNiP 23-01-99 * "Construction climatology" .- M: GUP TsPP, 2004.
The cost of heat for heating the sanitary norm of the supply air with modern methods of thermal protection of building envelopes in residential buildings is 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 designs buildings can only be created if
exhaust air heat utilization for heating the sanitary standard of the supply air.
Also successful experience in administrative building in Moscow, recycling plants with pump circulation of the intermediate coolant - antifreeze.
When the supply and exhaust units are located at a distance of more than 30 m from each other, the disposal system with pump circulation of antifreeze is the most rational and economical. If they are located nearby, even more effective solution. So in climatic regions with mild winters, when the outdoor temperature does not fall below -7 ° C, plate heat exchangers are widely used.
On fig. 1 shows a structural diagram of a plate recuperative (heat transfer is carried out through a separating wall) heat recovery heat exchanger. Shown here (Fig. 1, a) is an “air-to-air” heat exchanger assembled from plate channels, which can be made of thin sheet galvanized steel, aluminum, etc.
Picture 1.a - lamellar channels, in which exhaust air L y enters from above the dividing walls of the channels, and horizontal supply air outdoor air L b.s.; 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. 1b 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-turbulizing 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 exhaust air being removed.
The resulting condensate, under the influence of air flow L y, will intensively drain into the pan 7, from where it is discharged into the sewer (or storage tank) through the pipeline connected to the branch pipe 8.
The plate heat exchanger is characterized by the following equation for the heat balance of the transferred heat to the outside supply air:
where Q tu is the heat energy utilized by the supply air; L y, L p.n - costs of heated exhaust and outdoor supply air, m 3 / h; ρ y, ρ p.n - specific densities of heated exhaust and outdoor supply air, kg / m 3; I y 1 and I y 2 - initial and final enthalpy of heated exhaust air, kJ/kg; t n1 and t n2, s p - initial and final temperatures, ° С, and heat capacity, kJ / (kg · ° С), of the external supply air.
At low initial temperatures of the outside air t n.x ≈ t n1 on the dividing walls of the channels, the condensate falling out of the exhaust air does not have time to drain into the pan 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 showed 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, 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 a bypass or electric preheating significantly reduces the thermal efficiency of plate heat exchangers in air handling systems. exhaust ventilation 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. 3 shows a schematic diagram of the plant for heat recovery of exhaust air X for heating supply outdoor air L p.n.s. rapid elimination freezing channels 2 to improve the passage of exhaust air through the plate heat exchanger 1.
Air ducts 3 heat exchanger 1 is connected to the path of supply outdoor air L p.n, and air ducts 4 to the path of passage of exhaust air removed L y.
Figure 3. Schematic diagram of the use 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 corresponding to the heat balance equation [see. formula (1)]. A decrease in the temperature of the exhaust air occurs with abundant moisture condensation on the walls of the lamellar channels. Part of the condensate has time to drain from channels 2 and is removed through pipeline 9 to the sewer (or storage tank). However, most of the condensate freezes on the walls of the channels 2. This causes an increase in the pressure drop ∆Р у in the exhaust air flow measured by sensor 8.
When ∆Р y increases to the set value, a command will be sent 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 exhaust air with high temperature there will be a rapid thawing from the walls of the ice channels 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 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 h.x = -26 °С) industrial and administrative building, a heat recovery unit (HTU) based on a recuperative plate heat exchanger (with an indicator θ t p.n = 0.7) was installed in the supply and exhaust ventilation system ). 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 the aerodynamic drag ∆Р у. 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. It can be seen from the construction on the I-d diagram (Fig. 4) 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 through exhaust air cooled to negative temperatures. It is in this part of the plate heat exchanger that intensive formations of frost and frost are observed, which will block the channels for the passage of exhaust air. This will cause an increase in aerodynamic drag.
At the same time, the control sensor will give a command to open the automatic valve for hot water supply to the tubes of the heat exchanger, mounted in the exhaust air duct up to the plate heat exchanger, which will ensure the heating of the exhaust air to a temperature of t.sub.1 = +50 °C.
The flow of hot air into the lamellar channels ensured the defrosting of frozen condensate in 10 minutes, which is removed in liquid form to the sewer (to the storage tank). For 10 minutes of heating the exhaust air, the following amount of heat was spent:
or by formula (5) we get:
7. The heat supplied in the heater 5 (Fig. 3) is partially spent on 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).
Background of development
The heat of the air that is removed into the atmosphere is a source of energy savings. It is no secret that 40…80% of heat consumption is spent on heating the air that enters the building. Therefore, the idea of heating fresh air at the expense of exhaust air is not new. Even in the Soviet Union, work was continuously carried out to create installations that would make it possible to use the thermal energy of exhaust air. But unfortunately, the results of these studies were used only in special projects (industrial, defense, scientific).
Abroad, the first energy crisis became the reason for the application, which caused the beginning of the use of such installations. At the same time, the devices for utilizing the thermal energy of the removed air were originally designed for use in multi-apartment residential buildings and cottages. As a result of this, today air heating It is widely used in Canada and the neighboring states of the USA. So in Canada, water heating systems are not used at all.
In Russia, heat recovery units began to be used en masse with the start of active low-rise construction when private developers began to show interest in energy-efficient, energy-saving equipment.
The use of electricity for heating
The use of ventilation heating technology involves the use of electricity for heating. Until recently, the use of electricity for heating was prohibited by law. This is due to the energy saving policy pursued in the Soviet Union. Since the breakup Soviet Union a lot has changed.
At present, when new materials are being used and new technologies are being mastered, the opinion of experts on the admissibility of using electricity for heating is beginning to change. The introduction of new norms in 2000, which require the improvement of the thermal protection of residential buildings, contributes to this. According to the new standards, normalized heat losses through external walls are reduced by 2.5–3.0 times compared to the 1995 standards. 
In the future, the norms for thermal protection and energy efficiency will only become tougher. Under these conditions, the very concept of air infiltration will disappear, the premises will be airtight. In such conditions, the use of heat recovery devices will open up the widest prospects.
Existing types of recuperators
The real nomenclature of heat recovery units is very diverse. But all the variety can be reduced to the following types: a) shell-and-tube and plate heat exchangers, including cross-current; b) rotary (regenerative); c) heat pumps with an intermediate working fluid. The capabilities of most modern devices make it possible to utilize and use only 60% of the exhaust air heat for heating the air supplied to the premises. For objects with a small building volume, in order for the installation of a heat exchanger to pay off, this figure must be 90%. 
A promising direction for the development of heat recovery units
To increase the efficiency of heat recovery units allows the use of the method described below. As you know, the heat capacity of water is the highest compared to other liquids. The heat capacity of air is 4.5 times lower than the heat capacity of water. The technology of ultra-dispersion of the removed air in water is based on the use of water. In order to increase the rate of heat transfer from the removed air, this air is passed through the water in a special way, creating micron-sized bubbles.
The rate of heat transfer increases as micron-sized bubbles destroy the thermal resistance of the surface layer of water. The application of the technology of ultra-dispersion of the removed air in water will make it possible to use 90-95% of the heat of the removed air. It is important that the heat exchanger built according to this technology has a minimum number of parts, minimum dimensions, it is easy to operate. 
Ways to use heat exchangers
- The first way is to use a heat exchanger of a recuperative type. At the same time, partial heating of the air supplied to the room takes place.
- The second way is heat recovery with the help of heat pumps.
- The third way is to use the heat of the outgoing air to heat the incoming water. The system includes large water heaters and hot water accumulators.
The current state of affairs in Russia on the issue under consideration
Federal Law No. 261-FZ “On Energy Saving and Increasing Energy Efficiency ...” prescribes to reduce the energy intensity of building engineering systems. The goal is to reduce the energy intensity of GDP by 40% by 2020 compared to 2007 levels. This tendency to increase energy efficiency, improve thermal protection is ubiquitous.
Decree of the Government of Moscow No. 900 dated October 5, 2010 “On Improving the Energy Efficiency of Residential, Social and Public-Business Buildings in the City of Moscow…” established the level of energy consumption, which cannot be ensured without heat recovery.
The Russian Federation, having joined the WTO, undertook to bring energy prices for domestic consumers to the level of world prices. All over the world, energy efficiency issues, and as a result, heat recovery issues are very acute. National governments put in place and enforce programs to improve energy efficiency. Therefore, with the growth of domestic energy prices, interest in heat recovery plants will inevitably grow.
In the "Russian stove" the supply air was heated, with the help of this the living room was heated. In Europe, the heating system, where channels were provided, as in a Russian stove, was called "Russian". This recognized the great efficiency of the Russian stove in comparison with European heating. Currently, we can talk about the need to return to the roots in matters of heating.
Supply and exhaust ventilation with recuperation
Today, energy conservation is a priority in the development of the world economy. The depletion of natural energy reserves, the increase in the cost of thermal and electrical energy inevitably leads us to the need to develop a whole system of measures aimed at improving the efficiency of energy-consuming installations. In this context, the reduction of losses and the reuse of the consumed thermal energy becomes an effective tool in solving the problem.
In the context of an active search for reserves to save fuel and energy resources, the problem of further improvement of air conditioning systems as large consumers of thermal and electrical energy is attracting more and more attention. An important role in solving this problem is to be played by measures to improve the efficiency of heat and mass transfer apparatuses, which form the basis of the polytropic air treatment subsystem, the operating costs of which reach 50% of all costs for the operation of SCR.
Utilization of thermal energy from ventilation emissions is one of the key methods for saving energy resources in air conditioning and ventilation systems of buildings and structures. for various purposes. On fig. 1 shows the main exhaust air heat recovery schemes implemented on the market of modern ventilation equipment.
An analysis of the state of production and use of heat recovery equipment abroad indicates a trend for the predominant use of recirculation and four types of exhaust air heat utilizers: rotating regenerative, plate recuperative, based on heat pipes and with an intermediate heat carrier. The use of these devices depends on the operating conditions of ventilation and air conditioning systems, economic considerations, the relative position of the supply and exhaust centers, operational capabilities.
In table. 1 shown comparative analysis various schemes for heat recovery of exhaust air. Among the main requirements on the part of the investor for heat recovery plants, it should be noted: price, operating costs and efficiency. The cheapest solutions are characterized by simplicity of design and the absence of moving parts, which makes it possible to distinguish among the presented schemes a plant with a cross-flow heat exchanger (Fig. 2) as the most appropriate for the climatic conditions of the European part of Russia and Poland.
Research recent years in the field of creating new and improving existing heat recovery units of air conditioning systems indicate a clear trend in the development of new design solutions for plate heat exchangers (Fig. 3), the decisive moment in choosing which is the possibility of ensuring trouble-free operation of the unit under conditions of moisture condensation at negative outdoor temperatures.
The outdoor air temperature, starting from which frost formation is observed in the exhaust air ducts, depends on the following factors: the temperature and humidity of the exhaust air, the ratio of the supply and exhaust air flow rates, and design characteristics. Let us note the peculiarity of the operation of heat recovery units at negative outdoor temperatures: the higher the heat exchange efficiency, the more danger the appearance of frost on the surface of the exhaust air channels.
In this regard, the low efficiency of heat exchange in a cross-flow heat exchanger can be an advantage in terms of reducing the risk of icing on the surfaces of the exhaust air channels. Security safe modes As a rule, it is associated with the implementation of the following traditional measures to prevent freezing of the nozzle: periodic shutdown of the outdoor air supply, its bypass or preheating, the implementation of which certainly reduces the efficiency of exhaust air heat recovery.
One of the ways to solve this problem is the creation of heat exchangers in which freezing of plates is either absent or occurs at lower air temperatures. A feature of the operation of air-to-air heat exchangers is the possibility of implementing heat and mass transfer processes in the “dry” heat transfer modes, simultaneous cooling and drying of the removed air with condensation in the form of dew and frost on the entire or part of the heat exchange surface (Fig. 4).
The rational use of the heat of condensation, the value of which reaches 30% under certain operating modes of the heat exchangers, makes it possible to significantly increase the range of changes in the parameters of the outside air, in which icing of the heat exchange surfaces of the plates does not occur. However, the solution to the problem of determining optimal modes operation of the heat exchangers under consideration, corresponding to certain operating and climatic conditions, and the area of its expedient application, requires detailed studies of heat and mass transfer in the packing channels, taking into account the processes of condensation and frost formation.
Numerical analysis was chosen as the main research method. It also has the least laboriousness, and allows you to determine the characteristics and identify the patterns of the process based on the processing of information about the influence of the initial parameters. That's why experimental studies heat and mass transfer processes in the considered devices were carried out in a much smaller volume and, mainly, to check and correct the dependencies obtained as a result of mathematical modeling.
In the physico-mathematical description of heat and mass transfer in the recuperator under study, preference was given to the one-dimensional transfer model (ε-NTU model). In this case, the air flow in the packing channels is considered as a liquid flow with constant velocity, temperature and mass transfer potential over its cross section, equal to the average mass values . In order to increase the efficiency of heat recovery in modern heat exchangers, finning of the packing surface is used.
The type and location of the ribs significantly affects the nature of the heat and mass transfer processes. Changing the temperature along the height of the rib leads to the realization various options heat and mass transfer processes (Fig. 5) in the exhaust air channels, which significantly complicates mathematical modeling and the algorithm for solving the system of differential equations.
The equations of the mathematical model of heat and mass transfer processes in a cross-flow heat exchanger are implemented in an orthogonal coordinate system with the OX and OY axes directed parallel to the cold and warm air flows, respectively, and the Z1 and Z2 axes, perpendicular to the surface of the packing plates in the supply and exhaust air channels (Fig. 6 ), respectively.
In accordance with the assumptions of this ε-NTU model, heat and mass transfer in the heat exchanger under study is described by differential equations of heat and material balances, compiled for interacting air flows and nozzles, taking into account heat phase transition and thermal resistance of the resulting frost layer. To obtain an unambiguous solution, the system of differential equations is supplemented with boundary conditions that establish the values of the parameters of the exchanged media at the inputs to the corresponding channels of the recuperator.
The formulated nonlinear problem cannot be solved analytically, so the integration of the system of differential equations was carried out numerical methods. A sufficiently large amount of numerical experiments carried out on the ε-NTU model made it possible to obtain an array of data that was used to analyze the characteristics of the process and identify its general patterns.
In accordance with the tasks of studying the operation of the heat exchanger, the choice of the studied modes and the ranges of variation of the parameters of the exchange flows was carried out in such a way that the real processes of heat and mass transfer in the packing at negative values of the outdoor air temperature, as well as the conditions for the flow of the most dangerous operating modes of heat recovery equipment from the point of view of operation, were most fully modeled. .
Presented in fig. 7-9 the results of calculating the operating modes of the test apparatus, characteristic for climatic conditions with a low calculated outdoor air temperature in winter period seasons, allow us to judge the qualitatively expected possibility of the formation of three zones of active heat and mass transfer in the channels of the exhaust air (Fig. 6), which differ in the nature of the processes occurring in them.
An analysis of the heat and mass transfer processes occurring in these zones makes it possible to evaluate possible ways to effectively capture the heat of the removed ventilation air and reduce the risk of frost formation in the channels of the heat exchanger packing based on rational use heat of phase transition. Based on the analysis performed, the boundary temperatures of the outside air were established (Table 2), below which frost formation is observed in the exhaust air ducts.
conclusions
An analysis of various schemes for the utilization of heat from ventilation emissions is presented. The advantages and disadvantages of the considered (existing) schemes for utilizing the exhaust air heat in ventilation and air conditioning installations are noted. Based on the analysis carried out, a scheme with a plate cross-flow heat exchanger is proposed:
- on the basis of a mathematical model, an algorithm and a computer calculation program for the main parameters of heat and mass transfer processes in the heat exchanger under study were developed;
- the possibility of formation of various moisture condensation zones in the channels of the heat exchanger nozzle, within which the nature of heat and mass transfer processes changes significantly, has been established;
- the analysis of the regularities obtained makes it possible to establish the rational modes of operation of the studied devices and the areas of their rational use for various climatic conditions of the Russian territory.
SYMBOLS AND INDICES
Legend: h reb — rib height, m; l rib - length of the rib, m; t is temperature, °C; d is the moisture content of the air, kg/kg; ϕ—relative air humidity, %; δ rib is the thickness of the rib, m; δ in is the thickness of the frost layer, m.
Indices: 1 - outside air; 2 - removed air; e - at the entrance to the nozzle channels; rb - rib; in - frost, o - at the outlet of the nozzle channels; dew - dew point; sat is the state of saturation; w is the channel wall.
