Since a large number Money can be saved by recycling the heat of condensate, the owner of any enterprise that consumes steam sooner or later faces the question:
How can I utilize the heat of condensate in the steam-condensate system of my enterprise?
This section will discuss typical methods for recovering condensate heat, which, to one degree or another, can be implemented in almost any steam-condensate system.
But without a detailed and comprehensive examination of the existing steam-condensate system, it is impossible to say unambiguously whether any of the considered methods can be applied in this particular case or not.
What is meant by the phrase “recovery of condensate heat”?
Let's start with a few fundamental principles:
- To heat any product in a heat exchanger to a certain temperature, saturated steam should be used.
- The temperature of the saturated steam must be higher than the temperature of the heated product at the outlet of the heat exchanger.
- Steam pressure and steam temperature are interrelated, i.e. The temperature in the heat exchanger depends on the steam pressure.
- The enthalpy of saturated steam is the sum of the enthalpy of water (heat of condensate) and the heat of vaporization (latent heat).
- In the vast majority of cases, heat exchangers are designed to transfer only latent heat to the product, while the resulting condensate must be removed immediately from the heat exchanger.
Condensate and its heat are lost irretrievably if the condensate is simply released into the atmosphere and not reused. Even if the condensate is collected in an open tank and then used as feed water for the boiler, some of the heat of the condensate is still lost along with the flash steam that is formed after the steam traps and then escapes into the atmosphere from the open condensate tank. We will consider this phenomenon below.
Utilization of condensate heat in this context means the most efficient use of the heat carried away along with the condensate from the heat exchanger.
To remove condensate from heat exchange equipment, condensate drains are used, which at the same time act as a throttling device, i.e. There is a pressure drop at the steam traps, i.e. the pressure difference between the steam pressure in the heat exchanger and the condensate pressure in the condensate system.
Point 1: Steam entry into the heat exchanger
Point 2: Condensate at saturation temperature or with slight subcooling at the outlet of the heat exchanger or in front of the condensate trap.
Section 1 2: Transfer of latent heat of vaporization in a heat exchanger at constant pressure and temperature.
Point 3: Condition of condensate after the condensate drain.
Section 2 3: Pressure drop - at constant enthalpy - from the pressure before the steam trap (Pv) to the pressure after the steam trap (Pg) or from the temperature before the steam trap to the saturation temperature.
Point 4: Condensate at saturation temperature after the steam trap.
Section 3 4: Energy released by a drop in pressure in the form of flash vapor.
Section 4 5: Residual heat of condensate.
The amount of flash steam generated can be calculated using the following formula:
m condensate flow [kg/h]; h"2 enthalpy of condensate before boiling [Kcal/kg or kJ/kg]; h"4 enthalpy of condensate after boiling [Kcal/kg or kJ/kg]; r heat of vaporization at pressure behind the steam trap [Kcal/kg or kJ/kg].
An alternative way to calculate the amount of flash steam is to use the diagram in Fig. 69, showing the dependence of the amount of flash steam (in kg) formed from 1 kg of condensate on the pressure in front of the steam trap (in the heat exchanger) and the pressure after the steam trap.
For example: excess pressure before the condensate trap is 5 bar, excess pressure after the condensate trap is 0 bar, the amount of flash steam is 0.11 kg/kg, i.e. eleven%.
As we can see, the amount of flash steam depends on the pressure drop across the steam trap and the amount of condensate. This fact also explains why “clouds” of steam form after a properly operating steam trap (they are especially visible when the condensate after the steam trap is discharged into the atmosphere).
If condensate is discharged into an open tank, then it is easy to observe how flash steam escapes from the tank into the atmosphere. In this case, the “clouds” of steam are even larger, since condensate enters the tank from several condensate traps simultaneously.
At low pressures the specific volume of steam is quite high. It is impossible to distinguish live steam from flash steam, so sometimes even experts confuse flash steam with live steam and make the erroneous conclusion that steam traps are passing live steam, when in fact these steam traps are working normally.
In Fig. Figure 70 shows an example of the formation of a large volume of flash steam after a steam trap: 100 kg/h of condensate (from steam with a pressure of 8 barg) produces 24 m3/h of flash steam, while the volume of water after the steam trap is only 0.086 m3/h.
This example shows that steam trap monitoring equipment should only be installed upstream of steam traps, not downstream of steam traps.
However, if high-quality steam traps are used, which guarantee excellent and trouble-free operation, then monitoring their condition in most cases is not required. From our widest range of GESTRA steam traps, we can offer you reliable and high-quality steam traps to solve any problem.
From the above, it becomes clear that the heat contained in the condensate before the steam trap is divided into flash steam and residual heat of the condensate after the steam trap.
Since the residual condensate and, consequently, its heat is almost always reused (the condensate is returned back to the boiler room and goes to replenish the boiler), then in this context, by condensate heat recovery we mean only the effective use of flash steam.
There are 4 main ways to effectively utilize flash steam:
- flooding of heat exchange surfaces with condensate;
- the use of special vessels (separators) for separating and recycling flash steam;
- installation of a heat exchanger on a common condensate pipeline;
- installation of a preheater in front of the main heat exchanger.
Method No. 1:
Flooding of heat exchange surfaces with condensate
To prevent the formation of flash steam after the steam trap, it is necessary to retain the condensate in the heat exchanger, i.e. It is necessary to heat the heat exchange surfaces. This means that part of the heat of the condensate will be transferred to the heated product and, thus, the condensate will cool. The condensate temperature must be reduced inside the heat exchanger to a saturation temperature (or lower) corresponding to the pressure in the condensate line after the trap.
This means that the section of the pipe in which such condensate cooling occurs must be long enough, i.e. the heat exchanger will be flooded with condensate to a greater or lesser extent.
In standard heat exchangers, such a scheme for recycling condensate heat is used relatively rarely, since flooding of the heat exchange surfaces reduces the power and, therefore, the efficiency of the heat exchanger, and can also lead to water hammer.
However, in the case of satellite heating, this method of utilizing condensate heat can be implemented through the use of appropriate condensate traps (see section 4.26 “Steam satellites”).
Heat exchangers with condensate control in most cases operate with partial flooding of the heat exchange surfaces with condensate. In this case, flooding of surfaces with condensate is required to maintain the product temperature constant. However, such a control scheme is quite inertial and is recommended for use only on heat exchangers with vertical heating surfaces and with permanent mode work.
In Fig. 71 shows a fuel heater equipped with a direct-acting temperature controller, which regulates the condensate flow depending on the temperature of the product at the outlet of the heater. The condensate trap prevents the passage of live steam in cases where the temperature regulator is in the fully open position (during start-up modes or during a breakdown).
Method number 2:
The use of special vessels (separators) for separating and recycling flash steam
If the plant’s steam-condensate system uses steam of different pressures, then this method of utilizing condensate heat is optimal.
If, nevertheless, steam of the same pressure is used in the steam-condensate system of the plant, then it is necessary to conduct a detailed examination of this system to look for one or two heat exchangers that could consume steam of a lower pressure. In the vast majority of cases, there is such a heat exchanger or heat exchangers in the system. The only reason why all heat exchangers in a system consume steam at the same pressure is very often because that is the only steam available for use in the system.
It is obvious that feedwater deaerators in steam boiler houses are consumers of low pressure steam. In most cases, these deaerators consume live steam at a pressure of 0.2-0.5 bar(g).
For example, low pressure flash steam can be used in space heating systems.
In Fig. Figure 72 shows a schematic diagram of a steam-condensate system with several heat exchangers consuming steam at different pressures.
In practice, naturally, there can be much more steam consumers.
In this case, a so-called open condensate system is shown, in which flash steam escapes from the condensate tank into the atmosphere.
This system can be optimized by installing flash steam separation vessels between different groups of heat exchangers, as well as by replacing the open-type condensate tank with a closed-type condensate tank.
In Fig. 73 shown closed system with three flash steam separators. The condensate from the “16 bar” heat exchanger is discharged to the “5 bar” flash steam separator. The flash steam from this separator goes to the “5 bar” heat exchanger. If this steam from the separator is not enough for the heat exchange process, the pressure regulator will begin to open automatically and supply the missing amount of live steam to the heat exchanger, thereby maintaining a constant pressure in the heat exchanger and in the separator. The condensate from the “5 bar” separator is discharged through a float trap to the “2 bar” flash steam separator. Condensate from the 5 bar heat exchanger is also discharged into this separator. The flash steam from the “2 bar” separator goes into the “2 bar” heat exchanger. The pressure regulator automatically supplies the missing amount of live steam to the heat exchanger, maintaining a constant pressure behind it.
Condensate from the “2 bar” heat exchanger and condensate from the “2 bar” separator are discharged to the “0.2-0.5 bar” separator. The flash steam generated in this separator is used to feed the atmospheric deaerator. The remaining condensate in the separator is pumped into the feedwater tank.
It is necessary to install automatic air vents on flash steam separators “5 bar” and “2 bar”, since non-condensable gases (for example, air) in the steam can significantly worsen heat exchange processes.
In the case of reconstruction of an existing steam and condensate system, for example, when moving from an open condensate system to a closed condensate system, it is necessary to ensure whether the capacity of the existing steam traps will be sufficient to operate in the new mode. The fact is that in the case of a closed condensate system, the back pressure on the condensate traps increases. As a result, the pressure drop across these steam traps decreases and therefore their capacity decreases.
Of course, the use of three flash steam separators is not always required. In most cases, one or two will be enough. In Fig. 74 and 75 show such systems.
If all the flash steam generated in the system can be completely used in one heat exchanger, then it makes sense to apply the thermosiphon principle. See fig. 75. The only requirement is that the heat exchanger must be located above the flash steam separator.
In accordance with gas laws, flash steam will rise upward into the “2 bar” heat exchanger. The condensate, under the influence of gravity, will flow down into the flash steam separator.
In this case, the condensate should enter the separator below the water level so as not to interfere with the rise of steam to the top.
To ensure normal thermosyphon circulation, it is necessary to effectively remove air and other non-condensable gases from this circulation circuit. The thermosyphon principle can only be implemented if the heat exchanger operates at constant pressure.
Any regulation of the operation of the heat exchanger on the “steam side” is impossible.
Method number 3:
Utilization of condensate heat by installing a heat exchanger on a common condensate pipeline.
The schematic diagram is shown in Fig. 76.
The optimal product temperature is maintained by a 3-way temperature controller. This valve prevents excessive pressure build-up in the common condensate line. For normal operation of this system, it is necessary that the amount of heat in the steam-condensate mixture be greater than the amount of heat required to heat the product in the heat exchanger. The excess amount of steam-condensate mixture is discharged into the condensate tank below the water level. This steam-condensate mixture is used to heat softened water. To prevent water hammer in the condensate tank, the steam-condensate mixture must be supplied to the tank below the water level and always through a bubble pipe. The total area of all holes in the bubble pipe must be equal to the cross-sectional area of this pipe.
The end of the bubble pipe must be plugged. It is necessary to provide a small hole in the pipe above the water level (inside the tank), which prevents condensate from being sucked into the bubble pipe when the system is stopped. This system ensures maximum utilization of flash steam.
Method No. 4:
Utilization of condensate heat by installing a preheater with a pre-main heat exchanger.
If utilization of flash steam directly in the main heat exchanger is not possible, then a preheater can be installed in front of this heat exchanger.
A heat exchanger is used to heat the product from the initial temperature to the final temperature.
This heat exchange process requires a certain amount of steam. However, if "secondary heat" is used to preheat the product, then less steam will be required in the main heat exchanger to reach the final product temperature.
Pre-heating of the product can be carried out by uncontrolled supply of flash steam to the pre-heater (if possible, using a thermosiphon, see Fig. 75) or, for example, in small systems by supplying the steam-condensate mixture directly to the pre-heater (Fig. 77)
The main heat exchanger heats the product - in our example water - to the required final temperature. If the steam-condensate system is large enough and extensive, then, naturally, several preheaters can be used at different points in the system to sequentially heat the product.
In the case of large heat exchangers, it is recommended to utilize flash steam and part of the condensate heat in preheaters, which may be integral elements of these heat exchangers, or can be installed in close proximity to these heat exchangers (on the side or below).
In Fig. 78 schematically shows a heater with a preheater installed at the air inlet into the heater.
The mixture of condensate and flash steam from the various heating sections goes into the condensate tank through the preheater. The latent heat of vaporization of the secondary steam and part of the heat of the condensate are transferred to the cold air entering the heater. The condensate after the preheater flows into the condensate tank relatively cold and without flash steam.
In the example in Fig. 79 shows a preheater installed under the main heat exchanger.
The condensate from the main heat exchanger flows by gravity into the preheater and transfers its heat to the product. The cooled condensate is removed from the preheater by means of a float-type condensate drain. There must be a bend in the pipeline between the preheater and the float steam trap, and the upper point of the bend must be above the preheater.
To maintain a constant level before and after the preheater, a pressure equalization tube must be installed. This tube should connect the highest point of the piping section between the preheater and the float trap and the steam supply piping to the main heat exchanger. In this case, the preheater will always be flooded with condensate. The pressure in the main heat exchanger and in the preheater will be the same (in this case we neglect the static pressure of the liquid column between the main heat exchanger and the preheater).
An automatic air vent must be installed at the outlet of the main heat exchanger.
This method relative position main heat exchanger and preheater has some advantages over the method shown in Fig. 78 (preheater is located on the side of the main heat exchanger): only water is used as a heating medium in the preheater; product inlet temperature is higher; pipeline diameters can be reduced; Problems associated with water hammer, cavitation and erosion in pipelines are almost completely eliminated (these problems are typical for two-phase steam/condensate flows).
The area of the heating surfaces of the preheater is calculated based on the amount of “secondary heat” available for utilization and the required outlet temperature of the condensate.
If you want to improve the heat balance of your enterprise by reducing heat losses, then GESTRA specialists are always ready to discuss existing problems with you and develop a detailed action plan that specifically satisfies your needs. Your requirements. Naturally, we will also supply you with all the necessary equipment and carry out installation supervision and commissioning work.
All over the world and especially in countries Western Europe and the USA are widely used technical solutions, allowing to reduce the life cycle cost of the refrigeration unit. This includes the use of electronic expansion valves, and optimization of condensation pressure depending on the outside air temperature, and setting the suction pressure of the refrigeration machine depending on the load on it, and controlling compressors and condenser fans using frequency converters, which can significantly reduce energy consumption. In Russia, the active implementation of such solutions for a long time was held back due to noticeably lower energy prices than in the West, which did not make it possible to recoup additional capital investments in relatively short term. However, in last years Energy saving technologies are becoming more and more relevant in our country.
Refrigeration machine condensation heat recovery systems stand apart from the solutions listed above because they do not save electricity consumed directly by the refrigeration system, but make it possible to reduce the costs of other systems used at the facility.
If we consider the thermodynamics of the cycle, we can see that there are two main possibilities for removing heat. The first is to use superheating of the gas compressed in the compressor. The second is to utilize the heat of condensation of the refrigerant.
When superheating compressed gas is used, an additional heat exchanger is installed in the refrigeration circuit. In this case, it is possible to utilize up to 20% of the total heat discharged by the installation. Since the temperature of the refrigerant at the end of the compression process can exceed 100 °C, the medium (air or water) is heated to 80-90 °C.
When utilizing the heat of condensation, much more heat can be removed, but low-grade heat, which allows you to heat water or air only up to 30 degrees.
What can recovered heat be used for? The most obvious application is air heating in winter. In the simplest version, the installation has two parallel installed condensers, one outdoors (it works in the warm season), and the second indoors (it heats the air in cold weather). In an inexpensive version, such a solution does not have any control automation. Switching from winter to summer mode is done manually by turning off the corresponding condenser using shut-off valves. More complex options have one condenser installed indoors and a system that directs the air flow either outside or inside the room. Flow distribution control can be either manual or automatic.
Currently, the use of recovered heat to heat water used for various technical needs is gaining popularity.
As a rule, superheating of compressed gas is used for both heating and water heating, since the temperature that can be obtained by recycling the heat of condensation of the refrigerant is not enough. Using gas superheating allows you to heat water to 40-50 °C and higher. In the case when the refrigeration machine does not provide the required performance or cannot operate continuously, and the capacity of the storage tank is not enough to maintain the temperature, electric heaters or gas boilers are used.
An interesting variety of such systems are cascade installations with a high-temperature heat pump as an upper circuit, which heats the water to 65-80 °C. This water can be used for sanitization surfaces (at this temperature most bacteria die), in chemical production. When there is a large demand for hot water for industrial needs, it is advisable to use systems with a transcritical cycle using CO 2. They are less efficient than traditional ones, but they allow you to heat water to a higher temperature.
To use heat recovery systems, it is desirable that the operating schedules of the refrigeration machine and the demand for hot water coincide as much as possible. Therefore, it is most advisable to use these systems where cold is constantly produced. For example, in food industry enterprises where hot water is needed for cleaning premises. It seems interesting to use systems of this kind on ice skating rinks. Hot water here it can be used to protect the soil under a cooled plate from freezing, as well as for various technological needs. An article in the magazine “Climate World” No. 52 was devoted to assessing the economic efficiency of using recycling systems at industrial enterprises.
Shops and retail chains are showing increasing interest in such systems. Indeed, with relatively small additional capital costs, heat recovery systems make it possible to provide hot water a whole supermarket!
The American experience of using superheated heat from milk cooler condensers on farms is interesting. The installation diagram is shown in Fig. 1. Water coming from the water supply is heated by hot gas and enters the heater, where its temperature increases to the required value. The operation of such installations for a year made it possible to reduce energy consumption for heating water by three times. A particularly noticeable economic effect was obtained where heating was carried out with liquid fuel.
It should be noted that the heat recovery system can also be installed on an existing refrigeration machine. Thus, the Canadian energy efficiency service The Office of Energy Efficiency (OEE) published a report on the modernization of the kitchen refrigeration system of one of the large medical centers in Canada. The discharge lines of all 10 compressors were combined into one and a brazed plate heat exchanger was installed on it, in which the water was heated from 10°C to 30°C and sent to a gas boiler, where it was brought to the required temperature. Thanks to the use of recycling, annual gas consumption decreased by 40%, the payback period of the system was 2.3 years. In our country, successful experience in modernizing an existing installation was carried out by the Prostor-L company at the Lokomotiv ice arena in Yaroslavl. A heat recovery system producing hot water for technological needs was installed a year and a half after the facility was put into operation. Thanks to its use, the consumption of hot water from the city network was reduced tenfold, and the system itself paid for itself in less than two years.
It is important to note that heat recovery systems are usually made according to individual projects for a specific task. It is extremely important to correctly select all components of the system and design it without errors. The recovery heat exchanger, as a rule, has a plate design, although large installations Shell-and-tube heat exchangers are also used. If the design provides for a pre-condenser, its precise selection is necessary to prevent refrigerant condensation. When using several heat sources at the same time, for example, medium- and low-temperature central refrigeration machines, it is important to provide such a layout in the engine room that will provide convenient installation of pipelines for hot water and access to automation systems and shut-off valves.
As an example of the use of heat recovery in industry, let's consider a system used by one of the leaders in the refrigeration business - the company Termokul LLC (Moscow) (Fig. 2). Hot water is produced by the refrigeration system of the blast freezing chamber. The water produced by heating is used to defrost meat, thaw the blast freezer and clean floors after the end of the shift. It can be used for other needs as well. In this system, a pre-condenser is mounted on the discharge line in front of the main condenser (Fig. 3), which is a brazed plate heat exchanger from Danfoss. The total heat of superheated hot gas generated by the refrigeration system based on three Bitzer HSN 8571 screw compressors is 450 kW. The precondenser allows you to recover up to 400 kW of heat. Water at a temperature of 8 °C is heated to 40 °C with a productivity of 11 cubic meters per hour, which allows us to fully satisfy all technological needs. To compensate for the decrease in productivity when compressors are turned off, a storage tank with a volume of 3 cubic meters is installed in the system.
The use of such a technical solution allows you to save on electricity and laying utilities, which is very important for the enterprise.
The article was prepared by Sergey Buchin and Sergey Smagin
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Heat recovery has been widely used in heat and power engineering for many years. e - feed water heaters, economizers, air heaters, gas turbine regenerators, etc., but in refrigeration technology it is still given insufficient attention. This can be explained by the fact that heat of low potential is usually discarded (at a temperature below 100°C), so to use it it is necessary to introduce additional heat exchangers and automation devices into the refrigeration system, which complicates it. At the same time, the refrigeration system becomes more sensitive to changes in external parameters.
In connection with the energy problem, designers, including refrigeration equipment, are currently forced to more carefully analyze traditional systems in search of new schemes for the recovery of condensation heat.
If the refrigeration unit has an air condenser, you can use the heated air directly after the condenser to heat rooms. The heat of superheated refrigerant vapors after the compressor, which have a higher temperature potential, can also be usefully used.
For the first time, heat recovery schemes were developed by European companies, since in Europe there were higher prices for electricity compared to prices in the USA.
Complete refrigeration equipment from the Kostan company (Italy), developed in recent years, with a heat recovery system from air condensers, is used to heat the sales area of supermarket-type stores. Such systems can reduce overall energy consumption in a store by 20-30%.
primary goal— use of the maximum possible amount of heat generated by the refrigeration machine in environment. Heat is transferred either directly by flow warm air after the condenser into the sales area of the store during the heating season, or into an additional heat exchanger-accumulator (heat of superheated refrigerant vapors) to produce warm water, which is used for technological needs throughout the year.
Experience in operating systems using the first method has shown that they are easy to maintain, but relatively cumbersome; their use is associated with the need to install additional fans to move large quantity air and air filters, which ultimately leads to increased costs. Taking this into account, preference is given to more complex schemes, despite the fact that their implementation complicates operation.
The simplest circuit with a heat exchanger-accumulator is a circuit with a series connection of a capacitor and a battery. This scheme works as follows. At water temperatures at the inlet to the heat exchanger-accumulator and the ambient air temperature equal to 10 ° C, the condensation temperature tK is 20 C. For a short time (for example, during the night) the water in the accumulator heats up to 50 ° C, a t rises to 30°C. This is explained by the fact that the overall performance of the capacitor and battery decreases, since when the water is heated, the initial temperature pressure in the battery decreases.
An increase of 10°C is quite acceptable, however, with unfavorable combinations of high temperature and low water consumption, a more significant increase in condensation temperature may be observed. This scheme has the following disadvantages during operation: fluctuations in condensation pressure; periodic significant decrease in pressure in the receiver, which leads to disruption of the liquid supply to the evaporator; possible reverse flow of liquid into the air condenser when the compressor is stopped, when t is significantly lower than the temperature in the receiver.
Installing a condensation pressure regulator allows you to prevent the backflow of condensate from the receiver into the air condenser, as well as maintain the required condensation pressure, for example, corresponding to 25 ° C.
When tw increases to 50°C and tok to 25°C, the pressure regulator opens completely, and the pressure drop in it does not exceed 0.001 MPa.
If t drops to 10°C, then the pressure regulator closes and the internal cavity of the air condenser, as well as part of the heat exchanger-accumulator coil, are filled with liquid. When the temperature rises to 25°C, the pressure regulator opens again and the liquid from the air condenser comes out supercooled. The pressure above the surface of the liquid in the receiver will be equal to the condensation pressure minus the pressure drop in the regulator, and the pressure in the receiver can become so low (for example, correspond to tK< 15°С), что жидкость перед подачей к регулирующему вентилю не будет переох-лажденной. В этом случае необходимо ввести в схему регенеративный теплообменник.
To maintain pressure in the receiver, a differential valve is also introduced into the circuit. At tk = 20°C and tok - 40°C, the differential valve is closed, the pressure drop in the pipelines of the air condenser, heat exchanger-accumulator and pressure regulator is insignificant.
When lowered to 0°C, and t to 10°C, the liquid in front of the pressure regulator will have a temperature of approximately 10°C. The pressure drop in the pressure regulator will become significant, differential valve 6 will open and hot steam will flow into the receiver.
However, this does not completely eliminate the problem of the lack of supercooling of the liquid in the receiver. It is necessary to install a regenerative heat exchanger or use a specially designed receiver. In this case, the cold liquid from the condenser is directed directly into the liquid line. The same effect can be achieved by installing a vertical receiver, in which colder liquid sinks to the bottom, and hot steam enters the upper part.
Location of the pressure regulator in the circuit between the heat exchanger-accumulator and the air condenser. preferable for the following reasons: in winter it may take a long time to achieve the required condensation pressure; in a compressor-condensing unit, the length of the pipeline between the condenser and the receiver is rarely sufficient; In existing installations, it is necessary to disconnect the drain pipe in order to install a heat exchanger-accumulator. A check valve is also installed according to this scheme.
Circuits with parallel connection of air capacitors have been developed to maintain a temperature of 20°C in one room, and 10°C in another, where doors are often opened in winter. Such circuits also require the installation of pressure regulators and differential valves.
Parallel connected condensers with heat recovery in summer time usually do not work, and the pressure in them is slightly lower than in the main condenser. Due to loose closure of the solenoid and check valves, liquid recirculation and filling of the recovery condenser are possible. To avoid this, the circuit provides a bypass pipeline through which the condenser is periodically turned on to recover heat based on a signal from a time relay.
Fluctuations in the thermal load of the main condenser and condensers with heat recovery are associated with the need to use in such circuits a receiver with a larger capacity than in refrigeration machines without heat recovery, or to install an additional receiver in parallel with the first one, which forces an increase in the amount of refrigerant to charge the system.
Analysis of various heat recovery schemes using standard coaxial type heat exchangers (pipe in pipe) with complete condensation in them and using only the heat of superheated vapors shows that the installation operates more economically with complete condensation in a heat regenerator only with continuous and stable use of warm water.
The refrigeration machine operates in two cycles (with a boiling point of 10°C and different condensation temperatures of 35 and 55°C). An additional counterflow water heat exchanger is used as a heat regenerator, which transfers the heat of superheated refrigerant vapor at a temperature pressure of the compressor refrigeration capacity of 10 kW and a power consumption of 2.1 kW (Tc = 35°C) in the main condenser it is possible to heat water (at its flow rate is 0.012 kg/s) from 10 to 30 °C, and then in the regenerator, increase the water temperature from 30 to 65 °C. In a cycle from 55°C with a cooling capacity of 10 kW and a power consumption of 3.5 kW, water in the main condenser (at a flow rate of 0.05 kg/s) is heated from 10 to 50°C, and then water is heated in an additional heat exchanger-regenerator ( at a flow rate of 0.017 kg/s) it heats up from 50 to 91°C. In the first case, 13.7% is usefully used, in the second - 52% of the total supplied energy.
In all cases, when choosing a heat recovery system for a refrigeration machine, it is necessary to determine the following:
- compressor cooling capacity and condenser thermal load;
- operating mode of the refrigeration machine in summer and winter; possibility of using recovered heat; the relationship between the necessary heat for heating the room and heating water;
- the required temperature of warm water and its consumption over time; reliability of the refrigeration machine in cold production mode.
- Experience in operating heat recovery systems shows that the initial capital costs of such a system in large stores pay off within 5 years, so their implementation is economically feasible.
Of all types consumed in chemical industry energy, the first place belongs to thermal energy. The degree of heat utilization during a chemical technological process is determined by thermal efficiency:
where Q t and Q pr, respectively, is the amount of heat theoretically and practically expended to carry out the reaction.
The use of secondary energy resources (waste) increases efficiency. Energy waste is used in chemical and other industries for various purposes.
Especially great importance in the chemical industry there is the recovery of heat from reaction products leaving reactors for preheating materials entering the same reactors. Such heating is carried out in devices called regenerators, recuperators and waste heat boilers. They accumulate heat from waste gases or products and release it for processes.
Regenerators are periodically operating chambers filled with a nozzle. For a continuous process it is necessary to have at least 2 regenerators.
The hot gas first passes through regenerator A, heats its nozzle, and cools itself. Cold gas passes through regenerator B and is heated by a previously heated nozzle. After heating the nozzle in A and cooling in B, the dampers are closed, etc.
In recuperators, the reagents enter a heat exchanger, where they are heated by the heat of hot products leaving the reaction apparatus, and then fed into the reactor. Heat exchange occurs through the walls of the heat exchanger tubes.
In recovery boilers, the heat from waste gases and reaction products is used to produce steam.
Hot gases move through pipes located in the boiler body. There is water in the interpipe space. The resulting steam passes through the moisture separator and leaves the boiler.
Raw materials
The chemical industry is characterized by high material intensity of production. As a rule, several tons of raw materials are consumed for one ton of finished chemical products. It follows that the cost of chemical products is largely determined by the quality of raw materials, methods and costs of its production and preparation. In the chemical industry, the cost of raw materials in the cost of production is 60-70% or more.
The type and quality of raw materials significantly determines the complete use of production capacities of chemical industries, heat productivity, equipment operating time, labor costs, etc. The properties of the raw material, the content of useful and harmful components in it determine the technology used for its processing.
The types of raw materials are very diverse and can be divided into the following groups:
- mineral raw materials;
- plant and animal raw materials;
- air, water.
1. Mineral raw materials - minerals extracted from the bowels of the earth.
Minerals, in turn, are divided into:
- ore (metal production) important polymetallic ores
- nonmetallic (fertilizers, salts, H + , OH - glass, etc.)
- combustibles (coals, oil, gas, shale)
Ore raw materials are rocks from which it is environmentally beneficial to obtain metals. Metals in it are mostly in the form of oxides and sulfides. Non-ferrous metal ores quite often contain compounds of several metals - these are sulfides of Pb, Cu, Zn, Ag, Ni, etc. Such ores are called polymetallic or complex. Indispensable integral part All industrial ores are FeS 2 – pyrite. When processing some ores, other products are obtained along with metals. So, for example, simultaneously with Cu, Zn, Ni, H 2 SO 4 is also obtained during the processing of sulfide ores.
Non-metallic raw materials are rocks used in the production of non-metallic materials (except for alkali metal chlorides and Mg). This type of raw material is either directly used in the national economy (without chemical processing) or is used for one or another chemical production. These raw materials are used in the production of fertilizers, salts, acids, alkalis, cement, glass, ceramics, etc.
Non-metallic raw materials are conventionally divided into the following groups:
- Construction Materials– raw materials are used directly or after mechanical or physical-chemical processing (gravel, sand, clay, etc.)
- industrial raw materials – used in production without processing (graphite, mica, corundum)
- chemical mineral raw materials - used directly after chemical treatment (sulfur, saltpeter, phosphorite, apatite, sylvinite, rock and other salts)
- precious, semi-precious and ornamental raw materials (diamond, emerald, ruby, malachite, jasper, marble, etc.)
Combustible mineral raw materials are fossils that can serve as fuel (coal, oil, gas, oil shale, etc.)
2. Plant and animal raw materials are products of agriculture (agriculture, livestock farming, vegetable growing), as well as meat and fisheries.
According to its purpose, it is divided into food and technical. Food raw materials include potatoes, sugar beets, cereals, etc. Chemical and other industries consume plant and animal raw materials that are unsuitable for food (cotton, straw, flax, whale oil, claws, etc.). The division of raw materials into food and technical is in some cases arbitrary (potatoes → alcohol).
3. Air and water are the cheapest and most accessible raw materials. Air is a practically inexhaustible source of N 2 and O 2. H 2 O is not only a direct source of H 2 and O 2, but also participates in almost all chemical processes and is also used as a solvent.
The economic potential of any country in modern conditions is largely determined by the natural resources of minerals, the scale and qualitative characteristics of their locations, as well as the level of development of raw materials industries.
The raw materials of modern industry are very diverse, and with the development of new technology, the introduction of more effective methods production, the raw material base is constantly expanding due to the discovery of new deposits, the development of new types of raw materials and the more complete use of all its components.
The domestic industry has a powerful raw material base and has reserves of all types of mineral and organic raw materials it needs. Currently, the United States ranks first in the world in the extraction of reserves of P, rock salts, NaCl, Na 2 SO 4, asbestos, peat, wood, etc. We have one of the first places in explored oil and gas deposits. And proven reserves of raw materials are increasing from year to year.
On modern stage In the development of industry, the rational use of raw materials, which involves the following activities, becomes of great importance. Rational use of raw materials allows you to increase the environmental efficiency of production, because the cost of raw materials constitutes the main share in the cost of chemical products. In this regard, they strive to use cheaper, especially local, raw materials. For example, at present, oil and gas are increasingly being used as hydrocarbon raw materials, rather than coal, and ethyl alcohol obtained from food raw materials is being replaced with hydrolyzed alcohol from wood.
State educational institution higher vocational education
"Samara State Technical University»
Department of Chemical Technology and Industrial Ecology
COURSE WORK
in the discipline "Technical thermodynamics and heat engineering"
Topic: Calculation of a heat recovery installation for the waste gases of a process furnace
Completed by: Student Ryabinina E.A.
ZF course III group 19
Checked by: Consultant Churkina A.Yu.
Samara 2010
Introduction
Most chemical enterprises generate high- and low-temperature thermal waste, which can be used as secondary energy resources (SER). These include flue gases from various boilers and process furnaces, cooled streams, cooling water and waste steam.
Thermal RES largely cover the heat needs of individual industries. Thus, in the nitrogen industry, more than 26% of the heat demand is met through renewable energy sources, and in the soda industry - more than 11%.
The number of used SERs depends on three factors: the temperature of the SERs, their thermal power and the continuity of output.
Currently, the most widespread is the recovery of heat from waste industrial gases, which for almost all fire engineering processes have a high temperature potential and can be used continuously in most industries. The heat of exhaust gases is the main component of the energy balance. It is used primarily for technological, and in some cases, for energy purposes (in waste heat boilers).
However, the widespread use of high-temperature thermal HERs is associated with the development of recycling methods, including the heat of hot slags, products, etc., new methods for recycling the heat of waste gases, as well as improving the designs of existing recycling equipment.
1. Description technological scheme
In tubular furnaces that do not have a convection chamber, or in radiant-convection furnaces, but with a relatively high initial temperature of the heated product, the temperature of the exhaust gases can be relatively high, which leads to increased heat losses, a decrease in furnace efficiency and higher fuel consumption. Therefore, it is necessary to use the heat from the exhaust gases. This can be achieved either by using an air heater, which heats the air entering the furnace for fuel combustion, or by installing waste heat boilers, which make it possible to obtain water vapor necessary for technological needs.
However, to heat the air, additional costs are required for the construction of an air heater, a blower, as well as additional electricity consumption consumed by the blower motor.
To ensure normal operation of the air heater, it is important to prevent the possibility of corrosion of its surface from the side of the flue gas flow. This phenomenon is possible when the temperature of the heat exchange surface is below the dew point temperature; in this case, part of the flue gases, in direct contact with the surface of the air heater, is significantly cooled, the water vapor contained in them partially condenses and, absorbing sulfur dioxide from the gases, forms an aggressive weak acid.
The dew point corresponds to the temperature at which the pressure of saturated water vapor is equal to the partial pressure of water vapor contained in the flue gases.
One of the most reliable methods of protection against corrosion is to preheat the air in some way (for example, in water or steam heaters) to a temperature above the dew point. Such corrosion can also occur on the surface of convection pipes if the temperature of the feed entering the furnace is below the dew point.
The source of heat to increase the temperature of saturated steam is the oxidation (combustion) reaction of the primary fuel. The flue gases formed during combustion give up their heat in the radiation and then convection chambers to the raw material flow (water vapor). Superheated water vapor is supplied to the consumer, and combustion products leave the furnace and enter the waste heat boiler. At the exit from the HRSG, saturated water vapor is fed back into the steam superheating furnace, and the flue gases, cooled by feed water, enter the air heater. From the air heater, flue gases enter the KTAN, where the water entering through the coil is heated and goes directly to the consumer, and the flue gases are released into the atmosphere.
2. Furnace calculation
2.1 Calculation of the combustion process
Let us determine the lower heat of combustion of fuel Q рн. If the fuel is an individual hydrocarbon, then its heat of combustion Q p n is equal to the standard heat of combustion minus the heat of evaporation of water contained in the combustion products. It can also be calculated using the standard thermal effects of the formation of initial and final products based on Hess’s law.
For a fuel consisting of a mixture of hydrocarbons, the heat of combustion is determined by the additivity rule:
where Q pi n is the heat of combustion of the i-th fuel component;
y i is the concentration of the i-th fuel component in fractions of unity, then:
Q р n cm = 35.84 ∙ 0.987 + 63.80 ∙ 0.0033+ 91.32 ∙ 0.0012+ 118.73 ∙ 0.0004 + 146.10 ∙ 0.0001 = 35.75 MJ/m 3 .
Molar mass of fuel:
M m = Σ M i ∙ y i ,
where M i is the molar mass of the i-th fuel component, hence:
M m =16.042 ∙ 0.987 + 30.07 ∙ 0.0033 + 44.094 ∙ 0.0012 + 58.120 ∙ 0.0004 + 72.15 ∙ 0.0001 + 44.010 ∙ 0.001+ 28.01 ∙ 0 .007 = 16.25 kg/ mole.
kg/m 3,
then Q р n cm, expressed in MJ/kg, is equal to:
MJ/kg.
The calculation results are summarized in table. 1:
Fuel composition Table 1
| Component | Molar mass M i, | Mole fraction y i, kmol/kmol | |
| 16,042 | 0,9870 | 15,83 | |
| 30,070 | 0,0033 | 0,10 | |
| 44,094 | 0,0012 | 0,05 | |
| 58,120 | 0,0004 | 0,02 | |
| 72,150 | 0,0001 | 0,01 | |
| 44,010 | 0,0010 | 0,04 | |
| 28,010 | 0,0070 | 0,20 | |
| TOTAL: | 1,0000 | 16,25 |
Let us determine the elemental composition of the fuel, % (mass):
,
where n i C, n i H, n i N, n i O is the number of carbon, hydrogen, nitrogen and oxygen atoms in the molecules of individual components that make up the fuel;
Content of each fuel component, mass. %;
M i is the molar mass of individual fuel components;
M m is the molar mass of the fuel.
Checking the composition:
C + H + O + N = 74.0 + 24.6 + 0.2 + 1.2 = 100% (wt).
Let us determine the theoretical amount of air required to burn 1 kg of fuel; it is determined from the stoichiometric equation of the combustion reaction and the oxygen content in the atmospheric air. If the elemental composition of the fuel is known, the theoretical amount of air L0, kg/kg, is calculated by the formula:
In practice, to ensure complete combustion of fuel, an excess amount of air is introduced into the furnace; let’s find the actual air flow rate at α = 1.25:
where L is the actual air flow;
α - excess air coefficient,
L=1.25∙17.0 = 21.25 kg/kg.
Specific volume of air (no.) for combustion of 1 kg of fuel:
where ρ in = 1.293 – air density under normal conditions,
m 3 /kg.
Let's find the amount of combustion products formed when 1 kg of fuel is burned:
if the elemental composition of the fuel is known, then the mass composition of the flue gases per 1 kg of fuel during complete combustion can be determined based on the following equations:
where m CO2, m H2O, m N2, m O2 are the mass of the corresponding gases, kg.
Total amount of combustion products:
m p.c = m CO2 + m H2O + m N2 + m O2,
m p.s = 2.71 + 2.21 + 16.33 + 1.00 = 22.25 kg/kg.
We check the resulting value:
where W f - specific consumption of nozzle steam during combustion liquid fuel, kg/kg (for gas fuel W f = 0),
Since the fuel is a gas, we neglect the moisture content in the air and do not take into account the amount of water vapor.
Let us find the volume of combustion products under normal conditions formed during the combustion of 1 kg of fuel:
where m i is the mass of the corresponding gas formed during the combustion of 1 kg of fuel;
ρ i is the density of a given gas under normal conditions, kg/m 3 ;
M i is the molar mass of a given gas, kg/kmol;
22.4 - molar volume, m 3 /kmol,
m 3 /kg; 
m 3 /kg;
m 3 /kg; 
m 3 /kg.
Total volume of combustion products (no.) at actual air flow:
V = V CO2 + V H2O + V N2 + V O2,
V = 1.38 + 2.75+ 13.06 + 0.70 = 17.89 m 3 /kg.
Density of combustion products (no.):
kg/m3.
Let us find the heat capacity and enthalpy of combustion products of 1 kg of fuel in the temperature range from 100 °C (373 K) to 1500 °C (1773 K), using the data in Table. 2.
Average specific heat capacities gases with р, kJ/(kg∙K) Table 2
| Air | |||||
| 0 | 0,9148 | 1,0392 | 0,8148 | 1,8594 | 1,0036 |
| 100 | 0,9232 | 1,0404 | 0,8658 | 1,8728 | 1,0061 |
| 200 | 0,9353 | 1,0434 | 0,9102 | 1,8937 | 1,0115 |
| 300 | 0,9500 | 1,0488 | 0,9487 | 1,9292 | 1,0191 |
| 400 | 0,9651 | 1,0567 | 0,9877 | 1,9477 | 1,0283 |
| 500 | 0,9793 | 1,0660 | 1,0128 | 1,9778 | 1,0387 |
| 600 | 0,9927 | 1,0760 | 1,0396 | 2,0092 | 1,0496 |
| 700 | 1,0048 | 1,0869 | 1,0639 | 2,0419 | 1,0605 |
| 800 | 1,0157 | 1,0974 | 1,0852 | 2,0754 | 1,0710 |
| 1000 | 1,0305 | 1,1159 | 1,1225 | 2,1436 | 1,0807 |
| 1500 | 1,0990 | 1,1911 | 1,1895 | 2,4422 | 1,0903 |
Enthalpy of flue gases generated during the combustion of 1 kg of fuel:
where c CO2, c H2O, c N2, c O2 are the average specific heat capacities at constant pressure of the corresponding lawns at temperature t, kJ/(kg K);
c t is the average heat capacity of flue gases generated during the combustion of 1 kg of fuel at temperature t, kJ/(kg K);
at 100 °C: kJ/(kg∙K);
at 200 °C: kJ/(kg∙K);
at 300 °C: kJ/(kg∙K);
at 400 °C: kJ/(kg∙K);
at 500 °C: kJ/(kg∙K);
at 600 °C: kJ/(kg∙K);
at 700 °C: kJ/(kg∙K);
at 800 °C: kJ/(kg∙K);
at 1000 °C: kJ/(kg∙K);
at 1500 °C: kJ/(kg∙K);
The calculation results are summarized in table. 3.
Enthalpy of combustion products Table 3
| Temperature | Heat capacity combustion products with t, kJ/(kg∙K) | Enthalpy combustion products Ht, |
|
| °C | TO | ||
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