The need for energy saving in the design, construction and operation of buildings for any purpose is beyond doubt and is associated primarily with the depletion of fossil fuel reserves and, as a consequence, its continuous rise in price. Particular attention must be paid 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 enclosures.
One of the most promising, low-cost and quick-payback energy-saving measures in mechanical ventilation and air conditioning systems is the recovery of exhaust air heat to partially heat the influent air during the cold season. To carry out 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 (thermosiphons).
However, it can be shown that in the conditions of the prevailing price level for ventilation equipment in the Russian Federation and, mainly, due to the practical absence of domestic 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 recovery heater, and the exhaust unit with a heat recovery cooler, 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, heat recovery equipment of this type is very compact, and the connection of the supply and exhaust units only through a circulation circuit with an intermediate coolant allows you to choose a place for their placement almost independently of each other. Low-freezing liquids such as antifreeze are usually used as a coolant, and the small volume of the circulation circuit allows the costs of antifreeze to be neglected, and the tightness of the circuit and the non-volatility of antifreeze make the issue of its toxicity secondary.
Finally, the absence of direct contact between the supply and exhaust air flows does not impose restrictions on the cleanliness of the hood, which practically unlimitedly expands the group of buildings and premises where heat recovery can be used. As a disadvantage, they usually indicate that the temperature efficiency is not very high, not exceeding 50-55%.
But this is exactly the case when the question of the feasibility 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 important in unstable conditions market economy with a noticeably changing level of prices for equipment and tariffs for energy resources, which does not allow the use of capital-intensive engineering solutions. However, it remains open question about the economically most feasible temperature efficiency of such heat recovery equipment keff, i.e. the share of heat spent on heating the inflow due to the heat of the exhaust air, in relation to the total heat load. Typically used values for this parameter range from 0.4 to 0.5. We will now show on what basis these values are adopted.
This problem will be considered using the example of a supply and exhaust ventilation unit with a capacity of 10,000 m 3 /h, using equipment from Veza LLC. This problem is an optimization one, since it boils down to identifying the value of keff that 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 is the total capital costs, rub; E — total annual operating costs, rub/year; p — discount rate, %. When calculating, 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 a fairly complete manner using relatively elementary means, since all cost components are easily taken into account and calculated quite simply.
The solution to this problem was first published by the author in a paper for the level of prices and tariffs in force at that time. However, as will be easy to see, when recalculated to 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 select the optimal option for an engineering solution, since all other tasks will differ only in determining the value of K.
But this is easily done using the catalogs and price lists of manufacturers of the relevant equipment. In our example, capital costs were determined according to data from the Veza company, based on the performance and the accepted set of sections of the supply and exhaust units: front panel with one vertical valve, G3 class cellular filter, fan unit; In addition, in the supply unit there is also an additional air heater for the heat recovery system and a reheating air heater with heat supply from the heating network, and in the exhaust unit there is an air cooler for the heat recovery system, as well as a circulation pump. The diagram of such an installation is shown in Fig. 1. The costs of installation and commissioning of ventilation units were assumed to be 50% of the main capital investment.
The costs of heat recovery equipment and a preheating heater were calculated based on the results of computer calculations using Veza programs, depending on the efficiency of the heat exchanger. At the same time, with increasing efficiency, the value of K increases, since the number of rows of heat exchanger tubes of the recovery 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 under the same conditions) .
Operating costs consist of annual costs for thermal and electrical energy and depreciation charges. When calculating them, the duration of operation of the installation during the day in the calculations was taken equal to 12 hours, the air temperature behind the reheating heater + 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 +24.7°C by default (veza LLC selection program for heat exchangers). The tariff for thermal energy was adopted according to data from Mosenergo OJSC for mid-2004 in the amount of 325 rubles/Gcal (for budget consumers). It is obvious that as keff increases, the cost of thermal energy decreases, which, generally speaking, is the goal of heat recovery.
Energy costs are calculated through the electrical power required for the drive circulation pump heat recovery systems and fans for 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 coolant, as well as the aerodynamic resistance of ventilation units and networks. All of the listed values, except for the density of the coolant, assumed to be 1200 kg/m 3, are calculated according to the programs for selecting heat recovery and ventilation equipment of Veza LLC. In addition, the efficiency coefficients of the pumps and fans used are also included in the power expressions.
Average values were used in the calculations: 0.35 for GRUNDFOS type pumps with wet rotor and 0.7 for RDN type fans. The electricity tariff was taken into account according to data from Mosenergo OJSC as of mid-2004 in the amount of 1.17 rubles/(kW ֹh). As keff increases, the level of energy costs increases, since with an increase in the number of rows of recovery heat exchangers, their resistance to air flow increases, as well as pressure loss in the circulation circuit of the intermediate coolant.
However, in general, this component of costs is significantly less than the cost of thermal energy. Depreciation charges also increase with increasing keff insofar as capital costs increase. The calculation of these deductions is carried out on the basis of covering the costs of complete restoration, major and current repairs of equipment, taking into account the estimated service life of TAM equipment, assumed in the calculations to be 15 years.
In general, however, total operating costs decrease with increasing recycling efficiency. Therefore, the existence of a minimum SDZ is possible at a particular level of keff and a fixed value of T. The results of the corresponding calculations are shown in Fig. 2. From the graphs you can easily see that the minimum on the SDZ curve appears for almost any calculation horizon, which, in the meaning of the problem, is equal to the required payback period.
This means that at current prices for equipment and tariffs for energy resources, any, even the most insignificant investment in heat recovery pays off, and quite quickly. Therefore, heat recovery with an intermediate coolant is almost always justified. As the expected payback period increases, the minimum on the SDZ curve quickly shifts to an area of higher efficiency, reaching 0.47 at T = T AM = 15 years.
It is clear that the optimal value of keff for the accepted payback period will be the one at which the minimum SDZ is observed. A graph of the dependence of such an optimal value of keff on T is shown in Fig. 3. Since a longer payback period, exceeding the design service life of the equipment, is hardly justified, it is apparently necessary to stop at the level of keff = 0.4-0.5, especially since with a further increase in T, the increase in optimal efficiency sharply slows down.
In addition, it should be taken into account that the heat recovery method under consideration for any heat exchange surface and coolant flow rate generally cannot, in principle, provide a keff value higher than 0.52-0.55, which is confirmed by calculations using the Veza company program. 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 keff values from 0.4 to 0.5 finds a complete feasibility study. Therefore the main practical advice Based on the results of this study, it is possible to make wider use of heat recovery from exhaust air with an intermediate coolant 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 mandatory for a market economy to take into account discounting of capital and operating costs when technical and economic comparison of options 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 investments in activities, the feasibility of which is justified. Then, instead of E in the first option, you need to use the difference in annual costs between the options. After this, 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 TAM, or the graphs do not intersect at all, the measures are not economically justified. The results obtained are very general character, since the dependence of changes in capital costs on the degree of heat recovery in the current market situation has little connection with a specific manufacturer of ventilation equipment, and the main influence on operating costs in general is exerted only by the costs of thermal and electrical energy.
Therefore, the proposed recommendations can be used when 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 clarified when current prices and tariffs change.
It should also be noted that the payback period obtained in the above calculations, depending on the accepted keff, reaches a value of 15 years, i.e. up to TAM, is in some respects the limiting one that arises when all capital costs are taken into account. If we take into account only additional capital investments directly in heat recovery, the payback period is actually reduced to 3-4 years, as indicated above.
Consequently, the recovery of heat from exhaust air with an intermediate coolant is indeed a low-cost and quick-payback measure and deserves the widest use in a market economy.
- O.D. Samarin. On standardization of thermal protection of buildings. Magazine "S.O.K.", No. 6/2004.
- O.Ya. Kokorin. Modern air conditioning systems. - M.: “Fizmatlit”, 2003.
- V.G. Gagarin. On the insufficient justification for increased requirements for thermal protection of external walls of buildings. (Changes No. 3 SNiP II-3–79). Sat. report 3rd Conf. RNTOS April 23–25, 1998
- O.D. Samarin. Economically feasible efficiency of heat exchangers with intermediate coolant. Installation and special works in construction, No. 1/2003.
- SNiP 23-01–99* “Construction climatology.” - M: State Unitary Enterprise TsPP, 2004.
In an air conditioning system, the heat of the exhaust air from the premises can be recovered in two ways:
· Using air recirculation schemes;
· Installing heat recovery units.
The latter method is usually used in direct-flow air conditioning systems. However, the use of heat recuperators is not excluded in schemes with air recirculation.
IN modern systems ventilation and air conditioning uses 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 working conditions in the room. Maintaining all this equipment requires quite a lot of energy. Heat exchangers are becoming an effective solution for saving energy in ventilation systems. The basic principle of their operation is heating the air flow supplied to the room, using the heat of the flow removed from the room. When using a heat exchanger, less heater power is required to heat the supply air, thereby reducing the amount of energy required for its operation.
Heat recovery in air-conditioned buildings can be achieved through heat recovery from ventilation emissions. Disposal waste heat for heating fresh air (or cooling incoming fresh air with waste air from the air conditioning system in the summer) is the simplest form of recycling. In this case, four types of recycling systems can be noted, which have already been mentioned: rotating regenerators; heat exchangers with intermediate coolant; simple air heat exchangers; tubular heat exchangers. A rotating regenerator in an air conditioning system can increase the supply air temperature in winter by 15 °C, and in summer it can reduce the supply air temperature by 4-8 °C (6.3). As with other recovery systems, with the exception of the intermediate heat exchanger, the rotary regenerator can only function if the exhaust and suction ducts are adjacent to each other at some point in the system.
A heat exchanger with an intermediate coolant is less efficient than a rotating regenerator. In the presented system, water circulates through two heat exchange coils, and since a pump is used, the two coils can be located at some distance from each other. Both this heat exchanger and the rotating regenerator have moving parts (the pump and electric motor are driven and this distinguishes them from air and tube heat exchangers. One of the disadvantages of the regenerator is that contamination can occur in the channels. Dirt can settle on the wheel, which then carries it into the suction channel.Most wheels now have a purge feature, which reduces the transfer of contaminants to a minimum.
A simple air heat exchanger is a stationary device for exchanging heat between 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 chamber-type channels. 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 into the heat exchanger, and since a significant surface area is contained in a compact space, relatively high efficiency is achieved. The heat pipe heat exchanger can be considered 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 the other. Although the pipe wall can be considered as an additional thermal resistance, the efficiency of heat transfer within the pipe itself, in which the evaporation-condensation cycle occurs, is so great that up to 70% of the waste heat can be recovered in these heat exchangers. One of the main advantages of these heat exchangers compared to a heat exchanger with an intermediate coolant and a rotating 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 recovery system.
With all the variety of design solutions for heat recovery devices from secondary energy resources, each of them contains the following elements:
· The environment is a source of thermal energy;
· The environment is a consumer of thermal energy;
· Heat receiver - heat exchanger that receives heat from the source;
· Heat transferr - 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-to-air (air-liquid) recuperative heat exchangers, the working substance is the heat exchange media themselves.
Application examples.
1. Air heating in systems air heating.
Heaters are designed to quickly heat air using a water coolant and distribute it evenly using a fan and guide blinds. This is a good solution for construction and production workshops where rapid heating and maintenance is required. comfortable temperature only during working hours (at the same time, as a rule, the furnaces also work).
2. Heating of water in the hot water supply system.
The use of heat exchangers makes it possible to smooth out peaks in energy consumption, since maximum water consumption occurs at the beginning and end of the shift.
3. Heating water in the heating system.
Closed system
The coolant circulates in a closed circuit. Thus, there is no risk of contamination.
Open system. The coolant is heated by hot gas and then transfers heat to the consumer.
4. Heating of the blast air going to combustion. Allows you to reduce fuel consumption by 10%–15%.
It has been calculated that the main reserve for saving fuel when operating burners for boilers, furnaces and dryers is the utilization of heat from waste gases by heating the burned fuel with air. Heat recovery from flue gases has great importance V technological processes, since the heat returned to the furnace or boiler in the form of heated blast air allows reducing the consumption of fuel natural gas by up to 30%.
5. Heating of fuel going into combustion using liquid-liquid heat exchangers. (Example – heating fuel oil to 100˚–120˚ C.)
6. Heating of process fluid using liquid-liquid heat exchangers. (An example is heating a galvanic solution.)
Thus, a heat exchanger is:
Solving the problem of energy efficiency of production;
Normalization of the environmental situation;
Availability of comfortable conditions in your production – heat, hot water in administrative and utility premises;
Reducing energy costs.
Picture 1.
Structure of energy consumption and energy saving potential in residential buildings: 1 – transmission heat loss; 2 – heat consumption for ventilation; 3 – heat consumption for hot water supply; 4– energy saving
List of used literature.
1. Karadzhi V.G., Moskovko Yu.G. Some features of the effective use of ventilation and heating equipment. Management - 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 ASV M., 2008
The main purpose of exhaust ventilation is to remove exhaust air from the serviced premises. Exhaust ventilation, as a rule, works in conjunction with supply ventilation, which, in turn, is responsible for supplying clean air.
In order to have a favorable and healthy microclimate in the room, you need to draw up a competent design of the air exchange system, perform the appropriate calculations and install the necessary units according to all the rules. When planning, you need to remember that the condition of the entire building and the health of the people who are in it depend on it.
The slightest mistakes lead to the fact that ventilation ceases to cope with its function as it should, fungus appears in the rooms, finishing and building materials are destroyed, and people begin to get sick. Therefore, the importance of correct calculation of ventilation should not be underestimated in any case.
Main parameters of exhaust ventilation
Depending on what functions the ventilation system performs, existing installations are usually divided into:
- Exhaust. Necessary for the intake of exhaust air and its removal from the room.
- Inlet. Provides fresh, clean air from the street.
- Supply and exhaust. At the same time, old musty air is removed and new air is introduced into the room.
Exhaust units are mainly used in production, offices, warehouses and other similar premises. The disadvantage of exhaust ventilation is that without a simultaneous installation of a supply system, it will work very poorly.
If more air is drawn out of a room than is supplied, drafts will form. Therefore, the supply and exhaust system is the most effective. It provides the most comfortable conditions both in residential premises and in industrial and working premises.
Modern systems are equipped with various additional devices that purify the air, heat or cool it, humidify it and distribute it evenly throughout the premises. The old air is removed through the hood without any difficulty.
Before you begin arranging a ventilation system, you need to take the process of calculating it very seriously. The ventilation calculation itself is aimed at determining the main parameters of the main components of the system. Only by determining the most suitable characteristics can you make ventilation that will fully fulfill all its tasks.
During the ventilation calculation, the following parameters are determined:
- Consumption.
- Operating pressure.
- Heater power.
- Cross-sectional area of air ducts.
If desired, you can additionally calculate the energy consumption for operation and maintenance of the system.
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Step-by-step instructions for determining system performance
The calculation of ventilation begins with determining its main parameter - productivity. The dimensional unit of ventilation performance is m³/h. In order for the air flow calculation to be performed correctly, you need to know the following information:
- The height of the premises and their area.
- The main purpose of each room.
- The average number of people who will be in the room at the same time.
To make the calculation, you will need the following equipment:
- Tape measure for measurements.
- Paper and pencil for notes.
- Calculator for calculations.
To perform the calculation, you need to find out such a parameter as the rate of air exchange per unit of time. This value is set by SNiP in accordance with the type of room. For residential, industrial and administrative premises the setting will vary. You also need to take into account such points as the number of heating devices and their power, the average number of people.
For domestic premises, the air exchange rate used in the calculation process is 1. When calculating ventilation for administrative premises, use an air exchange value of 2-3, depending on the specific conditions. The frequency of air exchange directly indicates that, for example, in a domestic room the air will be completely renewed once every 1 hour, which is more than enough in most cases.
Calculation of productivity requires the availability of data such as the amount of air exchange by multiplicity and the number of people. It will be necessary to take the largest value and, starting from it, select the appropriate exhaust ventilation power. The air exchange rate is calculated using a simple formula. It is enough to multiply the area of the room by the ceiling height and the multiplicity value (1 for household, 2 for administrative, etc.).
To calculate air exchange by number of people, multiply the amount of air consumed by 1 person by the number of people in the room. As for the volume of air consumed, on average, with minimal physical activity, 1 person consumes 20 m³/h, with average activity this figure rises to 40 m³/h, and with high activity it is already 60 m³/h.
To make it clearer, we can give an example of a calculation for an ordinary bedroom with an area of 14 m². There are 2 people in the bedroom. The ceiling has a height of 2.5 m. Quite standard conditions for a simple city apartment. In the first case, the calculation will show that the air exchange is 14x2.5x1=35 m³/h. When performing the calculation according to the second scheme, you will see that it is already equal to 2x20 = 40 m³/h. It is necessary, as already noted, to take a larger value. Therefore, specifically in in this example The calculation will be performed based on the number of people.
Using the same formulas, oxygen consumption is calculated for all other rooms. In conclusion, all that remains is to add up all the values, obtain the overall performance and select ventilation equipment based on these data.
Standard performance values for ventilation systems are:
- From 100 to 500 m³/h for ordinary residential apartments.
- From 1000 to 2000 m³/h for private houses.
- From 1000 to 10000 m³/h for industrial premises.
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Determining the power of the air heater
In order for the calculation of the ventilation system to be carried out in accordance with all the rules, it is necessary to take into account the power of the air heater. This is done if, in combination with exhaust ventilation an inflow will be organized. A heater is installed so that the air coming from the street is heated and enters the room already warm. Relevant in cold weather.
The calculation of the power of the air heater is determined taking into account such values as air flow, the required outlet temperature and the minimum temperature of incoming air. The last 2 values are approved in SNiP. In accordance with this regulatory document, the air temperature at the heater outlet must be at least 18°. The minimum outside air temperature should be specified in accordance with the region of residence.
Modern ventilation systems include performance regulators. Such devices are designed specifically to reduce the speed of air circulation. In cold weather, this will reduce the amount of energy consumed by the air heater.
To determine the temperature at which the device can heat the air, a simple formula is used. According to it, you need to take the power value of the unit, divide it by the air flow, and then multiply the resulting value by 2.98.
For example, if the air flow at the facility is 200 m³/h, and the heater has a power of 3 kW, then by substituting these values into the above formula, you will get that the device will heat the air by a maximum of 44°. That is, if in winter time It will be -20° outside, then the selected air heater will be able to heat the oxygen to 44-20 = 24°.
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Operating pressure and duct cross-section
Calculation of ventilation involves the mandatory determination of parameters such as operating pressure and cross-section of air ducts. An efficient and complete system includes air distributors, air ducts and fittings. When determining working pressure, the following indicators must be taken into account:
- The shape of ventilation pipes and their cross-section.
- Fan parameters.
- Number of transitions.
Calculation of the appropriate diameter can be done using the following relationships:
- For a residential building, a pipe with a cross-sectional area of 5.4 cm² will be sufficient for 1 m of space.
- For private garages - a pipe with a cross-section of 17.6 cm² per 1 m² of area.
A parameter such as air flow speed is directly related to the cross-section of the pipe: in most cases, the speed is selected within the range of 2.4-4.2 m/s.
Thus, when calculating ventilation, be it an exhaust, supply or supply and exhaust system, you need to take into account a number of important parameters. The effectiveness of the entire system depends on the correctness of this stage, so be careful and patient. If desired, you can additionally determine the energy consumption for the operation of the system being installed.
Part 1. Heat recovery devices
Use of waste heat from flue gases
technological furnaces.
Process furnaces are the largest consumers of energy in oil refining and petrochemical plants, in metallurgy, as well as in many other industries. At refineries, 3–4% of all processed oil is burned in them.
average temperature Flue gases leaving the furnace usually exceed 400 °C. The amount of heat carried away with flue gases is 25–30% of the total heat released during fuel combustion. Therefore, the recovery of heat from exhaust flue gases from process furnaces is of exceptional importance.
At flue gas temperatures above 500 °C, waste heat boilers (HRB) should be used.
When the flue gas temperature is 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 installation consisting of a HRSG and an VP (in the HRSG the gases are cooled to 400 °C and enter the air heater for further cooling) - it is more often used at petrochemical enterprises with high temperature flue gases.
Waste heat boilers.
IN The heat from flue gases is used to produce water vapor. The efficiency of the furnace increases by 10 - 15.
Waste heat boilers can be built-in in the convection chamber of the furnace, or remote.
Remote waste heat boilers are divided into two types:
1) gas-tube boilers;
2) boilers of batch-convective type.
The choice of the required type is carried out depending on the required pressure of the resulting steam. The former are used to generate steam at relatively low pressure - 14 - 16 atm, the latter - to generate steam at 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 installation itself or whether there is an excess that must be discharged 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 remove excess steam into the network and avoid uneconomical throttling when discharging it into the low pressure network.
Gas-tube waste heat boilers are structurally similar to “pipe-in-pipe” heat exchangers. Flue gases are passed through the inner pipe, and water vapor is generated in the inter-tube space. Several such devices are located in parallel.
Batch-convective type waste heat boilers have more complex design. A schematic diagram of the operation of a CU of this type is shown in Fig. 5.4.
Here it is used natural circulation water and presents the most complete configuration of the HRSG with an economizer and a superheater.
Schematic diagram of the operation of a 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). The water flows down the plates and is not passed towards it in a countercurrent. a large number of water vapor. The water is heated by steam to 97 - 99 °C and due to the decrease in solubility of gases with increasing temperature, the main part of them is released and discharged from above the deaerator into the atmosphere. The steam gives up its heat to the water and condenses. Deaerated water from the bottom of the column is taken by a pump and the required pressure is pumped up. Water is passed through an economizer coil, in which it is heated almost to the boiling point of water at a given pressure, and enters a drum (steam separator). The water in the steam separator has a temperature equal to the boiling point of water at a given pressure. Water circulates through the steam generation coils due to the difference in density (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 required temperature and discharged to the consumer. Part of the resulting steam is used to deaerate the feedwater.
Reliability and economical operation of the HRSG largely depends on the proper organization of the water regime. If used incorrectly, scale forms intensively, heating surfaces corrode, and steam becomes contaminated.
Scale is dense deposits that form when water heats and evaporates. Water contains bicarbonates, sulfates and other salts of calcium and magnesium (hardness salts), which, when heated, are converted to bicarbonates and precipitate. Scale, which has thermal conductivity several orders of magnitude lower 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, naturally, the efficiency of the heat exchanger decreases (the amount of steam generated decreases). The temperature of the flue gases removed from the HRSG increases. In addition, the coils overheat and become damaged due to a decrease in the load-bearing capacity of the steel.
To prevent scale formation, pre-chemically purified water is used as feed water (can be taken from thermal power plants). In addition, the system is continuously and periodically purged (removing part of the water). Blowing prevents an increase in the concentration of salts in the system (water constantly evaporates, but the salts it contains do not, so the concentration of salts increases). Continuous boiler blowdown is usually 3 - 5% and depends on the quality of the feed water (should not exceed 10%, since heat loss is associated with blowdown). When operating the CU high pressure, working with forced water circulation, additionally use intra-boiler phosphating. In this case, calcium and magnesium cations, which are part of the scale-forming sulfates, bind with phosphate anions, forming compounds that are poorly soluble in water and precipitate in the water volume of the boiler, in the form of sludge that is easily removed during blowing.
Oxygen and carbon dioxide dissolved in feed water cause corrosion of the internal walls of the boiler, and the corrosion rate increases with increasing pressure and temperature. Thermal deaeration is used to remove gases from water. Also, a measure of protection against corrosion is to maintain a speed in the pipes at which air bubbles cannot be retained on their surface (above 0.3 m/s).
Due to an increase in the hydraulic resistance of the gas path and a decrease in the force of natural draft, it becomes necessary to install a smoke exhauster (artificial draft). In this case, the temperature of the flue gases should not exceed 250 °C to avoid destruction of this apparatus. But the lower the temperature of the exhaust flue gases, the more powerful it is necessary to have a smoke exhauster (electricity consumption increases).
The payback period for a CP usually does not exceed one year.
Air heaters. They are used to heat the air supplied to the furnace for fuel combustion. Heating the air allows you 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 and increase the completeness of fuel combustion, which is a very important advantage when using high-viscosity liquid fuels.
Also, the advantages of air heaters compared to HRSGs are the simplicity of their design, safe operation, and the absence of the need to install optional equipment(deaerators, pumps, heat exchangers, etc.). However, at the current price ratio for fuel and water steam, air heaters turn out to be less economical than HRSGs (our steam price is very high - 6 times higher per 1 GJ). Therefore, it is necessary to choose a method for utilizing heat from 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. Heat recovery from ventilation emissions
A large amount of heat is consumed for heating and ventilation of industrial and public buildings and structures. For certain industries (mainly light industry) these costs reach 70–80% or more of the total need for thermal energy. In most enterprises and organizations, the heat of exhaust 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. They can be used for various purposes Various types ventilation systems. Usually, if the volume of air that must be replaced in the room per unit time (m 3 / h) is small, then natural ventilation. Such systems are implemented in every apartment and most public institutions and organizations. In this case, the phenomenon of convection is used - heated air (has a reduced density) leaves through 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 loss is inevitable, since heating the cold air entering the room requires additional coolant consumption. Therefore, the use of even the most modern thermal insulation structures and materials during construction cannot completely eliminate heat losses. In our apartments, 25–30% of heat losses are associated precisely 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 necessary, which is usually associated with preventing an increase in the concentration of hazardous substances (harmful, toxic, fire-explosive, having an unpleasant odor) in the room. Forced ventilation is implemented in production facilities, warehouses, agricultural product storage facilities, etc.
Are used forced ventilation systems three types:
Supply system consists of a blower that forces fresh air into the room, a supply air duct and a system for uniform air distribution throughout the room. Excess air volume is forced out 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 removal of air from the volume of the room. In this case, fresh air is sucked into the room through various leaks or special supply systems.
Combined systems are combined supply and exhaust ventilation systems. They are used, as a rule, when very intensive air exchange is required in large rooms; at the same time, heat consumption for heating fresh air is maximum.
The use of natural ventilation systems and separate exhaust and supply ventilation systems does not allow the heat of the exhaust air to be used to heat the fresh air entering the room. When operating combined systems, it is possible to recover heat from ventilation emissions to partially heat the supply air and reduce thermal energy consumption. Depending on the difference in air temperatures indoors and outdoors, heat consumption for heating fresh air can be reduced by 40–60%. Heating can be carried out in regenerative and recuperative heat exchangers. The former are preferable because they have smaller dimensions, metal consumption and hydraulic resistance, are more efficient and have 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 dividing wall or storage 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.
Heat exchange diagram with intermediate circulating coolant:
1 – exhaust air duct; 2 – supply air duct; 3.4 – finned
tubular coils; 5 – intermediate coolant circulation pipelines
(concentrated aqueous solutions of salts - brines are usually used as an intermediate coolant in such systems); 6 – pump; 7 – coil for
additional heating of fresh air with water 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 coolant (brine) flowing inside the coil. The air is cooled to 0 °C and released into the atmosphere, and the brine is heated to 15 °C through circulation pipelines 5 enters the fresh air heating chamber on the supply air duct 2 . Here, the intermediate coolant transfers heat to fresh air, heating it from – 20 °C to + 5 °C. The intermediate coolant itself is cooled from + 15 °C to – 10 °C. The cooled brine is supplied to the pump and returned to the system for recirculation.
Fresh supply air, heated to + 5 °C, can be immediately introduced into the room and heated to the required temperature (+ 25 °C) using conventional heating radiators, 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 coolant (heating water or water vapor) flows inside the tubes, and the air washes the outer surface of the coil and heats up to + 25 ° C, after which warm fresh air is distributed throughout the room.
The use of this method has a number of advantages. Firstly, due to the high air speed in the heating section, the heat transfer coefficient increases significantly (several times) compared to conventional heating radiators. This leads to a significant reduction in the overall metal consumption of the heating system - a reduction in capital costs. Secondly, the room is not cluttered with heating radiators. Thirdly, a uniform distribution of air temperatures throughout 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 temperature may be significantly higher or lower than normal.
The only drawback is that the hydraulic resistance of the air path and the energy consumption for driving the supply air blower slightly increase. But the advantages are so significant and obvious that preheating the air directly in the ventilation system can be recommended in the vast majority of cases.
In order to ensure the possibility of heat recovery in the case of using supply or exhaust systems ventilation separately, it is necessary to organize a centralized air outlet or air supply, respectively, through specially mounted air ducts. In this case, it is necessary to eliminate all cracks and leaks in order to prevent uncontrolled blowing or air leaks.
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 always involves high energy (electricity) costs. Therefore, you can reduce the energy consumption to maintain a comfortable room temperature in the hot season by pre-cooling fresh air, exhausted with cold air.
Thermal VER.
Thermal HER includes the physical heat of the exhaust gases of boiler plants and industrial furnaces, main or intermediate products, other waste from the main production, as well as the heat of working fluids, steam and hot water, exhausted in technological and energy units. To utilize thermal energy resources, heat exchangers, waste heat boilers or thermal agents are used. Heat recovery from waste process streams in heat exchangers can occur through the surface separating them or through direct contact. Thermal HER can come in the form of concentrated heat flows or in the form of heat dissipated in environment. In industry, concentrated flows account for 41%, and dissipated heat – 59%. Concentrated streams include heat from flue gases from furnaces and boilers, Wastewater technological installations and housing and communal services sector. Thermal HERs 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, and devices of low power, the heat flows removed from them are small and dispersed in space, which makes their disposal difficult due to low profitability.
LECTURE
by academic discipline "Heat and mass transfer equipment of enterprises"
(for the curriculum 200__g)
Lesson No. 26. Heat exchangers - heat exchangers. Designs, principle of operation
Developed by: Ph.D., Associate Professor E.E. Kostyleva
Discussed at a department meeting
protocol No. _____
from "_____" ___________2008
Kazan - 2008
Lesson No. 26. Heat exchangers are heat exchangers. Designs, principle of operation
Learning objectives:
1. Study the designs and principles of various waste heat exchangers
Type of lesson: lecture
Time spending: 2 hours
Location: room ________
Literature:
1. Electronic resources of the Internet.
Educational and material support:
Posters illustrating educational material.
Lecture structure and timing:
One of the sources of secondary energy resources in the building is thermal energy air released into the atmosphere. Thermal energy consumption for heating the incoming air is 40...80% of heat consumption, most of it can be saved by using so-called waste heat exchangers.
There are different 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, through one of which the exhaust air moves, and through the other, the supply outside air. During production plate heat exchangers Such a design with high air productivity poses significant technological difficulties, therefore, designs have been developed for shell-and-tube heat exchangers TKT, which are a bundle of pipes arranged in a checkerboard pattern and enclosed in a casing. The removed air moves in the inter-tube space, the outside air moves inside the tubes. The movement of flows is cross.
Rice. 1 Heat exchangers:
A- plate recycler; b- TKT recycler; V- 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 flow of outside air, through which part of the cold outside air is bypassed when the temperature of the walls of the tube bundle is below critical (-20°C).
Exhaust air heat recovery units with an intermediate coolant can be used in mechanical supply and exhaust ventilation systems, as well as in air conditioning systems. The installation consists of an air heater located in the supply and exhaust ducts, connected by a closed circulation loop filled with an intermediate medium. The coolant circulates through pumps. The exhaust air, cooling in the exhaust duct air heater, transfers heat to the intermediate coolant, which heats the supply air. When the exhaust air is cooled below the temperature dew point On part of the heat exchange surface of the exhaust duct air heaters, condensation of water vapor occurs, which leads to the possibility of ice formation at negative initial temperatures of the supply air.
Heat recovery installations with an intermediate coolant can operate either in a mode that allows the formation of ice on the heat exchange surface of the exhaust air heater during the day with subsequent shutdown and thawing, or, if shutting down the installation is unacceptable, when using one of the following measures to protect the exhaust duct air heater from ice formation :
- preheating supply air to positive temperature;
- creating a bypass for coolant or supply air;
- increasing coolant flow in the circulation circuit;
- heating the intermediate coolant.
The choice of the type of regenerative heat exchanger is made depending on the calculated parameters of the exhaust and supply air and moisture releases inside the room. Regenerative heat exchangers can be installed in buildings for various purposes in mechanical supply and exhaust ventilation, air heating and air conditioning systems. The installation of a regenerative heat exchanger must ensure countercurrent movement of air flows.
The ventilation and air conditioning system with a regenerative heat exchanger must be equipped with controls and automatic regulation, which should provide operating modes with periodic thawing of frost or prevention of frost formation, as well as maintain the required parameters of the supply air. To prevent frost formation in the supply air:
- arrange a bypass channel;
- preheat the supply air;
- change the rotation speed of the regenerator nozzle.
In systems with positive initial temperatures of the supply air 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 temperatures of the supply air, it is necessary to use recovery schemes that provide protection against freezing of the surface of the air heaters in the exhaust duct.
| 2. OPERATION OF HEAT EXCHANGER - RECOVERY IN VENTILATION AND AIR CONDITIONING SYSTEMS |
Heat recovery heat exchangers can be used in ventilation and air conditioning systems to recover the heat of exhaust air removed from the room.
The flows of supply and exhaust air are supplied through the corresponding inlet pipes into the cross-flow channels of the heat exchange unit, made, for example, in the form of a package of aluminum plates. When flows move through the channels, heat is transferred through the walls from the warmer exhaust air to the colder supply air. These streams are then removed from the heat exchanger through corresponding outlet pipes.
As it passes through the heat exchanger, the temperature of the supply air decreases. At low outside air temperatures, it can reach the dew point temperature, which leads to the precipitation of droplets of moisture (condensation) on the surfaces limiting the heat exchanger channels. At negative temperatures of these surfaces, condensate turns into frost or ice, which naturally disrupts the operation of the heat exchanger. To prevent the formation of frost or ice or their removal during operation of this heat exchanger, measure the temperature in the coldest corner of the heat exchanger or (optionally) the pressure difference in the exhaust air duct before and after the heat exchanger unit. When the limiting, predetermined value of the measured parameter is reached, the heat exchange block rotates 180" around its central axis. This ensures a reduction in aerodynamic drag, time spent on preventing the formation of frost or removing it, and using the entire heat exchange surface.
The goal is to reduce the aerodynamic resistance to the flow of supply air, use the entire surface of the heat exchanger for the heat exchange process when carrying out the process of preventing the formation of frost or removing it, as well as reducing the time spent on carrying out this process.
The achievement of this technical result is facilitated by the fact that the parameter by which the possibility of formation or presence of frost on the surface of the cold zone of the heat exchanger is judged is either the temperature of its surface in the coldest corner, or the pressure difference in the exhaust air channel before and after the heat exchange unit.
Preventing the formation of frost by heating the surface supplied to the channels from their outlet side by turning the heat exchanger at an angle of 180 o with the exhaust air flow (when the measured parameter reaches the limit value) ensures constant aerodynamic resistance to the supply air flow, as well as the use of the entire surface of the heat exchanger for heat exchange during the entire time of his work.
The use of a waste heat exchanger provides significant savings on space heating costs and reduces heat losses that inevitably exist during ventilation and air conditioning. And due to a fundamentally new approach to preventing the formation of condensation with the subsequent appearance of frost or ice, and their complete removal, the operating efficiency of this heat exchanger is significantly increased, which distinguishes it from other means of exhaust air heat recovery.
| 3. HEAT EXCHANGERS FROM FINNED TUBES |
