One of the sources of secondary energy resources in a building is the thermal energy of air removed 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.
Exist Various types 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. In the manufacture of plate heat exchangers of this design with high air capacity, significant technological difficulties arise, therefore, designs of shell-and-tube heat exchangers TKT have been developed, 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. Heat exchangers:
a - plate heat exchanger;
b - TKT utilizer;
c - 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 dew point temperature, condensation of water vapor occurs on part of the heat exchange surface of the exhaust duct air heaters, 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 the supply air to a 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 systems, air heating and air conditioning. 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.
In this article, we propose to consider an example of the use of modern heat recuperators (recuperators) in ventilation units, in particular rotary ones.
The main types of rotary heat exchangers (recuperators) used in ventilation units:a) condensing rotor – utilizes predominantly sensible heat. Moisture transfer occurs if the exhaust air is cooled on the rotor to a temperature below the “dew point”.
b) enthalpy rotor - has a hygroscopic foil coating that promotes moisture transfer. Thus, complete heat is utilized.
Let's consider a ventilation system in which both types of recuperator (recuperator) will operate.
Let us assume that the object of calculation is a group of premises in a certain building, for example, in Sochi or Baku, we will make the calculation only for the warm period:
Outdoor air parameters:
outside air temperature during the warm period, with probability 0.98 – 32°C;
enthalpy of outside air in the warm season – 69 kJ/kg;
Options internal air:
internal air temperature – 21°C;
relative humidity of indoor air – 40-60%.
The required air flow for the assimilation of harmful substances in this group of premises is 35,000 m³/h. Room process beam – 6800 kJ/kg.
The air distribution scheme in the premises is “bottom-up” using low-speed air distributors. In this regard (we will not attach the calculation, since it is voluminous and goes beyond the scope of the article, we have everything we need), the parameters of the supply and exhaust air are as follows:
1. Supply:
temperature – 20°C;
relative humidity – 42%.
2. Removable:
temperature – 25°C;
relative humidity – 37%
Let's build a process on I-d diagram(Fig. 1).
First, let’s designate the point with the parameters of the internal air (B), then draw the process ray through it (note that for this design diagrams, the starting point of the beam is the parameters t=0°C, d=0 g/kg, and the direction is indicated by the calculated value (6800 kJ/kg) indicated on the edge, then the resulting beam is transferred to the parameters of the internal air, maintaining the angle of inclination).
Now, knowing the temperatures of the supply and exhaust air, we determine their points by finding the intersections of the isotherms with the process ray, respectively. We build the process from the reverse, in order to obtain the specified parameters of the supply air, we lower the segment - heating - along the line of constant moisture content to the curve relative humidityφ=95% (segment P-P1).
We select a condensation rotor that utilizes the heat of the removed air to heat P-P1. We obtain an efficiency coefficient (calculated by temperature) of the rotor of about 78% and calculate the temperature of the exhaust air U1. Now, let’s select an enthalpy rotor that works to cool the outside air (H) using the obtained parameters U1.
We get an efficiency coefficient (calculated by enthalpy) of the order of 81%, parameters of the treated air at the inlet H1, and at the exhaust U2. Knowing the parameters H1 and P1, you can select an air cooler with a power of 332,500 W.
Let us schematically depict the ventilation installation with recuperators (Fig. 2).
Now, for comparison, let’s select another system with the same parameters, but with a different configuration, namely: we’ll install one condensing rotor.
Now (Fig. 3) heating of P-P1 is carried out by an electric air heater, and the condensing rotor will provide the following: efficiency of about 83%, temperature of the treated supply air (H1) – 26°C. Let's select an air cooler for the required power of 478-340 W.
It should be noted that system 1 requires less power for cooling and, in addition to this, does not require additional energy costs (in this case, alternating current) for the second heating of the air. Let's make a comparison table:
| Comparable items | System 1 (with two recuperators) | System 2 (with one recuperator) | Difference |
| Rotor motor consumption | 320+320 W | 320 W | 320 W |
| Required cooling capacity | 332,500 W | 478 340 W | 145,840 W |
| Power consumption for second heating | 0 W | 151,670 W | 151,670 W |
| Fan motor power consumption | 11+11 kW | 11+11 kW | 0 |
To summarize
We clearly see the differences in the operation of the condensing and enthalpy rotors, and the savings in energy costs associated with this. However, it is worth noting that the principle of system 1 can only be organized for southern, hot cities, because when recovering heat during the cold period, the performance of the enthalpy rotor does not differ much from the condensing rotor.
Production of ventilation units with rotary heat exchangers
The Airkat Klimatekhnik company has been successfully developing, designing, manufacturing and installing air handling units with rotary heat exchangers for many years. We offer modern and non-standard technical solutions, which work even under the most complex operating algorithm and extreme conditions.
To receive a quote for an HVAC system, simply contact any of
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.
Modern ventilation and air conditioning systems use a wide variety of equipment: heaters, humidifiers, different kinds filters, adjustable grilles and much more. All this is necessary to achieve the required air parameters, maintain or create comfortable 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 transfer - 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 air heating systems.
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
Do you dream of having a healthy microclimate in your home and not a single room smelling musty and damp? In order for the house to be truly comfortable, it is necessary to carry out proper ventilation calculations even at the design stage.
If this important point is missed during the construction of a house, you will have to solve a number of problems in the future: from removing mold in the bathroom to new renovations and installing an air duct system. Agree, it’s not very pleasant to see breeding grounds for black mold in the kitchen on the window sill or in the corners of the children’s room, and to plunge into renovation work all over again.
The article we presented contains useful materials on the calculation of ventilation systems and reference tables. Formulas, visual illustrations and a real example for premises of various purposes and a certain area, demonstrated in the video, are provided.
With correct calculations and proper installation, the ventilation of the house is carried out in a suitable mode. This means that the air in living areas will be fresh, with normal humidity and without unpleasant odors.
If the opposite picture is observed, for example, constant stuffiness in the bathroom or other negative phenomena, then you need to check the condition of the ventilation system.
Image gallery
Conclusions and useful video on the topic
Video #1. Useful information on the principles of operation of the ventilation system:
Video #2. Along with the exhaust air, heat also leaves the home. Calculations of heat losses associated with the operation of the ventilation system are clearly demonstrated here:
Correct calculation of ventilation is the basis for its successful functioning and the key to a favorable microclimate in a house or apartment. Knowledge of the basic parameters on which such calculations are based will allow not only to correctly design the ventilation system during construction, but also to adjust its condition if circumstances change.
Heat consumption for heating sanitary standards supply of outside air at modern methods thermal protection of enclosing structures in residential buildings accounts for up to 80% of the thermal load on heating devices, and in public and administrative buildings - more than 90%. Therefore, energy-saving heating systems in modern building designs can only be created if
recycling the heat of the exhaust air to heat the sanitary standard of the supply outside air.
Also successful was the experience of using a recovery unit with pump circulation of an intermediate coolant - antifreeze - in an administrative building in Moscow.
When the supply and exhaust units are located at a distance of more than 30 m from each other, a recycling system with pump circulation of antifreeze is the most rational and economical. If they are located nearby, an even more effective solution is possible. Thus, in climatic regions with mild winters, when the outside air temperature does not drop below -7 °C, plate heat exchangers are widely used.
In Fig. Figure 1 shows the design diagram of a plate recuperative (heat transfer is carried out through the dividing wall) heat recovery heat exchanger. Shown here (Fig. 1, a) is an “air-to-air” heat exchanger assembled from plate channels, which can be made of thin galvanized steel sheet, aluminum, etc.
Picture 1.a - plate channels in which exhaust air L y enters from above the dividing walls of the channels, and horizontal supply outside air L pn; b - tubular channels in which exhaust air L y passes through the tubes from above, and external supply air L p.n passes horizontally in the inter-tube space
Plate ducts are enclosed in a casing that has flanges for connection to supply and exhaust air ducts.
In Fig. 1, b shows an “air-to-air” heat exchanger made of tubular elements, which can also be made of aluminum, galvanized steel, plastic, glass, etc. The pipes are fixed into the upper and lower tube sheets, which forms channels for the passage of exhaust air. The side walls and tube sheets form the frame of the heat exchanger, with open front sections, which are connected to the air supply duct for the supply of external air L p.n.
Thanks to the developed surface of the channels and the installation of air turbulizing nozzles in them, in such “air-to-air” heat exchangers, high thermal efficiency θ t p.n. (up to 0.75) is achieved, and this is the main advantage of such devices.
The disadvantage of these recuperators is the need to preheat the external supply air in electric heaters to a temperature not lower than -7 ° C (to avoid condensate freezing on the side of the humid exhaust air).
In Fig. Figure 2 shows a design diagram of a supply and exhaust unit with a plate-type exhaust air heat recovery unit L y for heating the supply external air L p.n. The supply and exhaust units are made in a single housing. Filters 1 and 4 are installed first at the input of the external supply L air and the removed exhaust L air. Both purified air flows from the operation of the supply 5 and exhaust 6 fans pass through the plate heat exchanger 2, where the energy of the heated exhaust air L is transferred to the cold supply L p.n.
Figure 2. Design diagram of the supply and exhaust units with a plate heat exchanger having a bypass air channel through the supply external air:1 - air filter in the supply unit; 2 - plate recovery heat exchanger; 3 - flange for connecting the air duct for the intake of exhaust air; 4 - pocket filter for cleaning exhaust air L y; 5 - supply fan with an electric motor on one frame; 6 - exhaust fan with an electric motor on one frame; 7 - tray for collecting condensed moisture from the exhaust air passage channels; 8 - condensate drainage pipeline; 9 - bypass air channel for the passage of supply air L p.n.; 10 - automatic drive of air valves in the bypass channel; 11 - air heater for reheating supply external air, powered hot water
As a rule, the exhaust air has a high moisture content and a dew point temperature of at least +4 °C. When cold outside air with a temperature below +4 °C enters the channels of heat exchanger 2, a temperature will be established on the dividing walls at which condensation of water vapor will occur on part of the surface of the channels on the side of the movement of the removed exhaust air.
The resulting condensate, under the influence of air flow L y, will intensively flow into pan 7, from where it is discharged into the sewer (or storage tank) through a pipeline connected to pipe 8.
The plate heat exchanger is characterized by the following heat balance equation of the transferred heat to the external supply air:
where Qtu is the heat energy utilized by the supply air; L y, L p.n - flow rates of heated exhaust and external supply air, m 3 / h; ρ y, ρ p.n - specific densities of heated exhaust and external supply air, kg/m 3 ; I y 1 and I y 2 - initial and final enthalpy of heated exhaust air, kJ/kg; t n1 and t n2, c p - initial and final temperatures, °C, and heat capacity, kJ/(kg · °C), of the external supply air.
At low initial temperatures of the outside air t n.x ≈ t n1 on the dividing walls of the channels, the condensate falling from the exhaust air does not have time to drain into the pan 7, but freezes on the walls, which leads to a narrowing of the flow area and increases the aerodynamic resistance to the passage of the exhaust air. This increase in aerodynamic drag is sensed by a sensor, which transmits a command to the drive 10 to open the air valves in the bypass channel 9.
Tests of plate heat exchangers in the Russian climate have shown that when the outside air temperature drops to tn.x ≈ tn1 ≈ -15 °C, the air valves in bypass 9 are completely open and all the incoming outside air L p.n passes bypassing the plate channels of the heat exchanger 2.
Heating of the supply external air L p.n from t n.x to t p.n is carried out in the heater 11, fed with hot water from a central heat supply source. In this mode, Qtu, calculated by equation (9.10), is equal to zero, since only exhaust air passes through the attached heat exchanger 2 and I y 1 ≈ I y 2, i.e. There is no heat recovery.
The second method of preventing condensate freezing in the channels of heat exchanger 2 is electrical preheating of the supply outside air from t no.x to t no.1 = -7 °C. Under the design conditions of the cold period of the year in the climate of Moscow, the cold supply outside air in the electric heater needs to be heated by ∆t t.el = t n1 - t n.x = -7 + 26 = 19 °C. Heating of the supply external air at θ t p.n = 0.7 and t у1 = 24 °C will be t p.n = 0.7 · (24 + 7) - 7 = 14.7 °C or ∆t t.u = 14.7 + 7 = 21.7 °C.
Calculations show that in this mode the heating in the heat exchanger and in the air heater is almost the same. The use of bypass or electric preheating significantly reduces the thermal efficiency of plate heat exchangers in supply and exhaust ventilation systems in the Russian climate.
To eliminate this drawback, domestic specialists have developed an original method for rapid periodic defrosting of plate heat exchangers by heating the exhaust air being removed, which ensures reliable and energy-efficient year-round operation of the units.
In Fig. Figure 3 shows a schematic diagram of an installation for recovering the heat of exhaust air X for heating the supply external air L pn with the rapid elimination of freezing of channels 2 to improve the passage of exhaust air through the plate heat exchanger 1.
By air ducts 3, the heat exchanger 1 is connected to the passage path of the supply external air L pn, and by air ducts 4 to the passage path of the removed exhaust air L y.
Figure 3. Schematic diagram of the use of a plate heat exchanger in the Russian climate: 1 - plate heat exchanger; 2 - plate channels for the passage of cold supply external air L pn and warm exhaust air L y; 3 - connecting air ducts for the passage of supply external air L p.n.; 4 - connecting air ducts for the passage of removed exhaust air L y; 5 - heater in the exhaust air flow L at the entrance to channels 2 plate heat exchanger 1.6 - automatic valve on the hot water supply pipeline G w g; 7 - electrical connection; 8 - sensor for monitoring air flow resistance in channels 2 for the passage of exhaust air L y; 9 - condensate drain
At low temperatures supply of external air (t n1 = t n. x ≤ 7 °C) through the walls of the plate channels 2, the heat from the exhaust air is completely transferred to the heat corresponding to the heat balance equation [see. formula (1)]. A decrease in the temperature of the exhaust air occurs with abundant condensation of moisture on the walls of the plate channels. Part of the condensate manages to drain from channels 2 and is removed through pipeline 9 into the sewer system (or storage tank). However, most of the condensate freezes on the walls of channels 2. This causes an increase in the pressure drop ∆Р y in the exhaust air flow, measured by sensor 8.
When ∆Р y increases to the configured value, a command will be sent from sensor 8 via wired connection 7 to open the automatic valve 6 on the hot water supply pipeline G w g into the tubes of the heater 5 installed in the air duct 4 for the supply of removed exhaust air to the plate heat exchanger 1. When open automatic valve 6, hot water G w g will flow into the heater tubes 5, which will cause an increase in the temperature of the removed air t y 1 to 45-60 ° C.
When passing through channels 2 of the removed air from high temperature There will be rapid thawing from the walls of the ice channels and the resulting condensate will flow through pipeline 9 into the sewer system (or into the condensate storage tank).
After defrosting the ice, the pressure drop in channels 2 will decrease and sensor 8, through connection 7, will send a command to close valve 6 and the supply of hot water to heater 5 will stop.
Let's consider the process of heat recovery on the I-d diagram, shown in Fig. 4.
Figure 4. Plotting on an I-d diagram the operating mode in the Moscow climate of a recovery unit with a plate heat exchanger and defrosting it using a new method (according to the diagram in Fig. 3). U 1 - U 2 - design mode for extracting heat from the exhaust air; Н 1 - Н 2 - heating with the recovered heat of the supply external air in the design mode; U 1 - U under 1 - heating of exhaust air in the mode of defrosting the lamellar channels for the passage of exhaust air from frost; In the 1st time - the initial parameters of the removed air after the transfer of heat to defrost ice on the walls of the plate channels; H 1 -H 2 - heating of supply external air in the defrosting mode of the plate recovery heat exchanger
Let us evaluate the influence of the method of defrosting plate heat exchangers (according to the diagram in Fig. 3) on the thermal efficiency of exhaust air heat recovery modes using the following example.
EXAMPLE 1. Initial conditions: In a large Moscow (t n.h = -26 °C) industrial and administrative building, a heat recovery unit (HRU) based on a recuperative plate heat exchanger (with an indicator θ t n.h = 0.7) was installed in the supply and exhaust ventilation system ). The volume and parameters of exhaust air removed during the cooling process are: Lу = 9000 m3/h, tу1 = 24 °С, Iy1 = 40 kJ/kg, tр.у1 = 7 °С, dу1 = 6, 2 g/kg (see construction on the I-d diagram in Fig. 4). Supply external air flow L p.n = 10,000 m 3 /h. The heat exchanger is defrosted by periodically increasing the temperature of the exhaust air, as shown in the diagram in Fig. 3.
Required: To establish the thermal efficiency of heat recovery modes using a new method of periodic defrosting of the apparatus plates.
Solution: 1. Calculate the temperature of the supply external air heated by the recovered heat under the design conditions of the cold period of the year at tn.x = tn1 = -26 °C:
2. We calculate the amount of recovered heat during the first hour of operation of the recovery installation, when freezing of the plate channels did not affect the thermal efficiency, but increased the aerodynamic resistance in the channels for the passage of the removed air:
3. After an hour of operation of the TUU under design winter conditions, a layer of frost accumulated on the walls of the channels, which caused an increase in aerodynamic resistance ∆Р у. Let us determine the possible amount of ice on the walls of the exhaust air passage channels through the plate heat exchanger formed within an hour. From the heat balance equation (1) we calculate the enthalpy of cooled and dried exhaust air:
For the example under consideration, using formula (2) we obtain:
In Fig. Figure 4 shows the construction on the I-d diagram of the heating modes of the supply external air (process H 1 - H 2) with the recovered heat of the exhaust air (process U 1 - U 2). By plotting the I-d diagram, we obtained the remaining parameters of the cooled and dried exhaust air (see point U 2): t у2 = -6.5 °С, d у2 = 2.2 g/kg.
4. The amount of condensate falling from the exhaust air is calculated using the formula:
Using formula (4), we calculate the amount of cold spent on lowering the ice temperature: Q = 45 4.2 6.5/3.6 = 341 W h. The following amount of cold is spent on ice formation:
The total amount of energy spent on the formation of ice on the separating surface of plate heat exchangers will be:
6. From the construction on the I-d diagram (Fig. 4) it is clear that during countercurrent movement along the plate channels of the supply L p.n and exhaust L of the air flows at the entrance to the plate heat exchanger, the coldest outside air passes on the other side of the dividing walls of the plate channels exhaust air cooled to negative temperatures. It is in this part of the plate heat exchanger that intensive formations of ice and frost are observed, which will block the channels for the passage of exhaust air. This will cause an increase in aerodynamic drag.
At the same time, the control sensor will give a command to open the automatic valve for hot water entering the heat exchanger tubes, mounted in the exhaust air duct before the plate heat exchanger, which will ensure heating of the exhaust air to a temperature t.sub.1 = +50 °C.
The entry of hot air into the plate channels ensured the defrosting of frozen condensate within 10 minutes, which is removed in liquid form into the sewer system (into the storage tank). For 10 minutes of heating the exhaust air, the following amount of heat is consumed:
or using formula (5) we get:
7. The heat supplied to heater 5 (Fig. 3) is partially spent on melting ice, which, according to calculations in paragraph 5, will require Q t.ras = 4.53 kW h of heat. To transfer heat to the supply outside air from the heat expended in heater 5 to heat the exhaust air, there will be heat left:
8. The temperature of the heated exhaust air after spending part of the heat on defrosting is calculated by the formula:
For the example under consideration, using formula (6) we obtain:
9. The exhaust air heated in heater 5 (see Fig. 3) will not only help defrost condensate ice, but also increase the transfer of heat to the supply air through the dividing walls of the plate channels. Let's calculate the temperature of the heated supply outside air:
10. The amount of heat transferred to heat the supply outside air during 10 minutes of defrosting is calculated by the formula:
For the mode under consideration, using formula (8) we obtain:
The calculation shows that in the defrosting mode under consideration there is no heat loss, since part of the heating heat from the removed air Q t.u = 12.57 kW h is transferred to additional heating of the supply external air L p.n to a temperature t n.2.times = 20 .8 °C, instead of t n2 = +9 °C when using only the heat of exhaust air with a temperature t у1 = +24 °C (see paragraph 1).
