How does a chamber-type air recuperator work? Recovery coefficient during heat exchange. Change in the quality of heat transfer at different flow rates of heat carriers

A special type of forced ventilation system is forced ventilation with heating and heat recirculation, which provides partial heating of the inlet air flow due to the warm air removed from the room using a special device - a heat exchanger. In this case, the main heating of the outside air is carried out by a conventional air heater.

Heat recovery in supply and exhaust ventilation- the phenomenon is not new, but it is still uncommon in our country. From a technical point of view, recuperation is the most common heat exchange process. The word “recovery” itself is of Latin origin and means “return of what has been spent”. Ventilated heat recuperators return part of it back to the room through heat exchange between the incoming and outgoing flow. The reverse process occurs in hot weather, when the outgoing cold conditioned air cools the oncoming warm air flow. In this case, it should be called cold recovery.

Why is recovery needed? Obviously, to save energy in the first place. The recuperator is a device in which heat exchange of incoming and outgoing air masses takes place. With normal ventilation, the temperature difference between the incoming and outgoing air in the cold and hot seasons is significant. If, for example, it is -20°C outside and +24°C indoors, then the difference is more than 40°C. This difference will need to be covered by the heating system. In summer, the difference is smaller, but it will also add a load on the air conditioner. The recuperator allows you to reduce this difference to a minimum. Properly selected equipment provides at 0°C outdoor air and +20°C indoors the difference between the incoming and outgoing flow is within 4°C, i.e. cut it five times. The recovery efficiency drops as the outside temperature drops, but the savings are still significant. Moreover, when there is a significant difference between indoor and outdoor temperatures, recovery is especially useful.

Many modern building technologies involve airtight and vapor-tight enclosing structures. For effective ventilation and removal of water vapor from rooms with sealed walls and double-glazed windows, forced supply and exhaust ventilation is necessary. Heat recovery in this case is the key to comfortable air exchange with minimal heat loss.

In the USA and Canada, long before the advent of heat recovery equipment, in order to get not too cold air into the room in winter, and too warm in summer, they came up with the idea of ​​using a ground heat exchanger, which later became known as the “Canadian well”. his idea

It consists in the fact that, before entering the premises, the outside air passes through the supply air ducts buried in the ground, acquiring a temperature value close to + 10 ° C - a constant temperature of the soil at a depth of 2 m or more. The Canadian well, in fact, is not a recuperator, but it reduces energy costs for heating and air conditioning. The ventilation of the premises in the traditional scheme with a Canadian well is natural, but can also be forced.

Recuperators as an element of ventilation equipment are actively used in European countries. The reason for their popularity is the economic benefits provided by the return of heat. There are two types of recuperators: plate and rotary. Rotary ones are more efficient, but also more expensive. They are able to return 70-90% of heat. Lamellar ones are cheaper, but they save less, within 50-80%.

One of the factors affecting the efficiency of recovery is the type of room. If the temperature in it is maintained above 23°C, then the heat exchanger definitely pays for itself. And the more expensive the cost of energy, the shorter the payback period. The service life of recuperators is quite long, and with timely maintenance and replacement of inexpensive consumables, it is theoretically unlimited. Recuperators can be supplied as a monoblock or several separate modules.

The heat exchanger is a special type of heat exchanger, to which the inlets and outlets of the supply and exhaust channels of the ventilation system are connected. The polluted air removed from the room, passing through the heat exchanger, gives off its heat to the incoming outside air, without directly mixing with it. Such additional heating of supply ventilation can significantly reduce energy costs for heating the inlet air, especially in winter period.

Plate heat exchangers

Plate heat exchangers are designed in such a way that the air flows in them do not mix, but contact each other through the walls of the heat exchange cassette. This cassette consists of many plates that separate cold air from warm air. Most often, the plates are made of aluminum foil, which has excellent heat-conducting properties. Plates can also be made of special plastic. These are more expensive than aluminum, but increase the efficiency of the equipment.

Plate heat exchangers have a significant drawback: as a result of the temperature difference, condensate forms on cold surfaces, which turns into frost. An iced heat exchanger stops working efficiently. To defrost it, the incoming flow is automatically transferred to bypass the heat exchanger and is heated by a heater. The outgoing warm air, meanwhile, melts the frost on the plates. In this mode, of course, there is no energy saving, and the defrost period can take from 5 to 25 minutes per hour. To heat the incoming air during the defrosting phase, heaters with a power of 1-5 kW are used.

Some plate heat exchangers preheat the incoming air to a temperature that prevents ice formation. This reduces the efficiency of the heat exchanger by about 20%.

Another solution to the icing problem is hygroscopic cellulose cassettes. This material absorbs moisture from the exhaust air stream and transfers it to the incoming one, thereby returning moisture back. Such recuperators are justified only in buildings where there is no problem of waterlogging. The undoubted advantage of hygrocellulose recuperators is that they do not need electric air heating, which means that they are more economical. For recuperators with a double plate heat exchanger, the efficiency reaches 90%. Ice does not form in them, due to the transfer of heat through the intermediate zone.

Well-known manufacturers of plate heat exchangers:

• SCHRAG (Germany),
• MITSUBISHI (Japan),
• ELECTROLUX,
• SYSTEMAIR (Sweden),
• SHUFT (Denmark),
• REMAK, 2W (Czech Republic),
• MIDEA (China).

Rotary heat exchangers

Unlike lamellar ones, in them there is a partial mixing of incoming and outgoing air. Their main element is a rotor mounted in the body, which is a cylinder filled with layers profiled metal (aluminum, steel). Heat transfer occurs during the rotation of the rotor, the blades of which are heated by the outgoing flow and give off heat to the incoming flow, moving in a circle. The heat transfer efficiency depends on the rotor speed and is adjustable.

In a rotary heat exchanger, it is technically impossible to completely eliminate the mixing of incoming and outgoing air. In addition, this type of equipment, due to the presence of moving parts, needs more frequent and more serious maintenance. Nevertheless, rotary models are quite popular due to their high heat recovery rates (up to 90%).

Manufacturers of rotary heat exchangers:

• DAIKIN (Japan),
• KLINGENBURG (Germany),
• SHUFT (Denmark),
• SYSTEMAIR (Sweden),
• REMAK (Czech Republic),
• GENERAL CLIMATE (Russia-Great Britain).

From an economic point of view, heat recuperators will sooner or later justify themselves, but much depends on how efficiently the recovery itself will be organized. The equipment is highly reliable, and the consumer can count on a long period of operation. Many companies produce a wide range of supply heat exchangers designed specifically for apartments. So a supply unit with heat recovery for a 2-3-room apartment can cost about 17,000 rubles. The performance of the ventilation system in apartments is in the range of 100-800 m³ / h. For country cottages, this figure is about 1000-2000 m³ / h.

Recuperators with intermediate heat carrier

This heat exchanger consists of two parts. One part is located in the exhaust duct, the other in the supply duct. Water or a water-glycol solution circulates between them. The exhaust air heats the coolant, which, in turn, transfers heat to the supply air. In this heat exchanger, there is no risk of transferring contaminants from the exhaust air to the supply air. Changing the circulation rate of the coolant can control the heat transfer. These recuperators have no moving parts, but they have low efficiency (45-60%). Mainly used for industrial facilities.

Chamber recuperators

The shutter divides the chamber into two parts by a shutter. One part is heated by the exhaust air, then the damper changes the direction of the air flow. Due to this, the supply air is heated from the warm walls of the chamber. Pollution and odors can be transferred from the exhaust air to the supply air. The damper is the only moving part of this heat exchanger. Its efficiency is quite high (70-80%).

heat pipes

This heat exchanger consists of a sealed tube system. They are filled freon or other easily evaporating component. These substances evaporate from heating by the removed air. Vapors condense in another part of the tube and again turn into a liquid state. In this heat exchanger, the transfer of contaminants is excluded, there are no moving parts, the efficiency is quite low (50-70%).

Many people think that RECUPERATORS are expensive, cumbersome devices with a short service life that are difficult to integrate into technological processes, and their repair stops production for a long period, making the use of a heat exchanger ineffective. These shortcomings allow skeptics to put up with colossal losses of thermal energy and environmental issues. As a result, recuperators are far from being installed at all enterprises, where it is expedient.

The solution can be the installation of Finned Plate Heat Exchangers (OPT™ type recuperators)

Technical features of OPT type recuperators

• reduce the cost of its purchase by up to 40% due to the return of thermal energy;
• reduce fuel consumption by increasing the combustion temperature of exhaust gases (heating scheme for boiler houses, furnaces, etc.);
• improve the quality characteristics of fuel combustion through the use of previously heated air, reduce the mechanical underburning of fuel in the furnace heating cycle in boiler houses and other facilities;
• cool flue gases to comply with environmental requirements and sanitary standards;
• use the heat of waste gases for space heating, heating the outdoor air;
• for technological processes requiring low temperatures, to cool the flue gases;
• reduce the temperature of the flue gases, thereby reducing the cost of gas cleaning;
• replace recuperators that require complex repairs with more reliable ones;
• successfully comply with the requirements of Law No. 261 FZ “On Energy Saving”;

Advantages of Finned Plate Heat Exchangers over traditional plate, rotary and shell-and-tube models

• the possibility of using in aggressive and abrasive environments, in environments with strong gas contamination and dusting;
• increased operating temperature limits - up to 1250 C, while the service life of analog recuperators is reduced even at 800 C;
• optimized dimensions and weight - 4-8 times lighter than analog recuperators;
• significantly lower cost;
• shortened payback periods;
• low indicators of resistance during the passage of air flows along the tracts;
• improved design preventing accumulation of slags;
• extended service life;
• an extended working period before preventive measures;
• improved weight and size characteristics, facilitating the installation and transportation of recuperators

Why this type of heat exchanger can be considered a competent choice?

• increase in the heat transfer surface area per unit volume and mass;
• high reliability of the heat exchanger used;
• a significant reduction in the possibility of a heat exchanger failure due to abrasive wear and thermal deformations;
• simplification of the processes of repair and maintenance of recuperators;
• Possibility of modular design and assembly of recuperators
• The most common cases of using a recuperator.

Gas-gas heat exchangers are used in many areas, which can be divided into the following categories:

Processes with a low coolant temperature:

Interval 20 to 60°C

• with small volumes of gases, for example, as a flue gas utilizer when operating gas boilers in a small room, where the heat exchanger is used in the ventilation system.
• with large volumes of gases, for example, in the ventilation system of workshops, concert halls, indoor stadiums and other large rooms.

Range 60 to 200°C

• at small volumes of gases, for example, to remove the flue product of fuel combustion, which is released as a gas during a variety of technological processes.
• with large volumes of gases, for example, the use of a gas heat exchanger is possible in the ventilation system of drying and painting shops.

Processes with an average level of coolant temperature.

The range is from 200 to 600°C, an example is the utilization of flue gas heat during the operation of boiler houses, and it is also possible to save coal by redirecting excess heat to warm the air supplied to the furnace.

Processes with a high level of coolant temperature.

• The range is from 600 to 800°C, for example in the plastics industry, a heat exchanger can be useful for cooling gas or for recovering heat carried by flue gases.
• The interval is up to 1000°C and above, which are observed in the production of glass, in metallurgy, oil and gas processing and other areas of production, where the heat exchanger will become the basis for solving problems such as saving coal, or act as a utilizer of the resulting flue gases.

It should be noted that the use of a gas-gas heat exchanger at a flue gas temperature of 45-50°C requires a separate efficiency calculation.

conclusions

Installations with heat recovery can reduce energy costs for space heating by half. Their installation often pays off in the first heating season. The installation of recuperators during construction and reconstruction makes it possible to partially reduce the load on the heating system of the entire building and to abandon a significant part of traditional heating equipment. The cost of installing recuperators is an investment not only in reducing heating costs, but also in ensuring optimal climatic conditions indoors and ultimately to human health.

Appliances capable of saving heat and other forms of energy are becoming more and more important, as energy prices are constantly rising. Also, we have long had no doubt about the need to breathe fresh clean indoor air. A negative role in the construction was played by the installation of popular plastic windows and sealed doors. They disrupt air exchange and lead to undesirable consequences. Against the background of all these factors, ventilation systems with heat recovery come to our aid. They not only save us money, but also protect our health.

Rename the theme. On educational program does not pull at all. Pulls only for PR.
Now I'll fix it up a bit.

Advantages of a rotary heat exchanger:
1. High heat transfer efficiency
Yes, I agree. The highest efficiency among domestic ventilation systems.
2. Dries the air in the room, as it is not hygroscopic.
No one specifically uses the rotor for drainage. Why is it counted as a plus?

Minuses:
1. Large sizes.
Disagree.
2. The rotor is a complex moving mechanism that is subject to wear and tear, and operating costs will increase accordingly.
A small stepper motor that turns a rotor costs 3 kopecks and rarely fails. You call it a "complex moving mechanism" that increases operating costs?
3. Air streams are in contact, due to which the admixture is up to 20%, according to some information up to 30%.
Who said 30? Where did you get it? Please provide us with a link. I can still believe in 10 percent of the flow, but 30 is nonsense. Some plate heat exchangers are far from hermetic in this regard, and a small overflow there is in the order of things.
4. Condensate drain needed
Dear educationalist, read at least one instruction manual for a rotary installation for apartments and cottages. It says in black and white: at standard air humidity, condensate removal is not required.
5. Fixing the PES in one position.
Why is this a minus?
6. Dries the air in the room, as it is not hygroscopic.
If you know the ventilation system market, you have already paid attention to the development of rotors made of hygroscopic material. The question of how much this is necessary and how much all this hygroscopy is needed, including in plate-type heat exchangers, is a rather controversial question and often not in favor of hygroscopicity.

Thanks for the answer.
No one even claimed to be a likbes. A topic for discussion and possible help for the user, as well as for me as a user.

"Because I'm a slightly interested person, I'll compare with what I work with." - I wrote at the very beginning. Compare with what I work with.

The rotor has larger dimensions than the lamellar one. Because I compare with what I work with.

The fact that it has the highest efficiency rates, in my opinion, is not true, for a triple lamellar they are larger and frost resistance is Higher. Again, I compare with what I work with.

It is a moving mechanism and is subject to wear and tear, so it costs three kopecks. This is good.

Mounting in one position is a minus. It is not always possible to deliver exactly as shown in the diagram.

Hygroscopy is needed to reduce the operating temperature at which the heat exchanger will not freeze.

In this article, we will consider such a heat transfer characteristic as the recovery coefficient. It shows the degree of use by one heat carrier of another during heat exchange. The recovery factor may be referred to as heat recovery factor, heat exchange efficiency or thermal efficiency.

In the first part of the article, we will try to find universal relations for heat transfer. They can be derived from the most general physical principles and do not require any measurements. In the second part, we will present the dependences of the real recovery coefficients on the main characteristics of heat transfer for real air curtains or separately for heat exchange units “water-air”, which have already been considered in the articles “Heat curtain power at arbitrary flow rates of the coolant and air. Interpretation of experimental data” and “Heat curtain power at arbitrary coolant and air flow rates. Invariants of the heat transfer process”, published by the journal “Climate World” in issues 80 and 83, respectively. It will be shown how the coefficients depend on the characteristics of the heat exchanger, as well as how they are affected by the flow rates of the heat carriers. Some paradoxes of heat transfer will be explained, in particular the paradox of a high value of the recovery coefficient with a large difference in the flow rates of heat carriers. To simplify, the very concept of recuperation and the meaning of its quantitative definition (coefficient) will be considered using the example of air-to-air heat exchangers. This will allow us to define an approach to the meaning of the phenomenon, which can then be extended to any exchange, including "water - air". It should be noted that in the air-air heat exchange units, both cross currents, which are fundamentally close to the water-air heat exchangers, and counter currents of heat-exchanging media can be organized. In the case of counter currents, which determine the high values ​​of the recovery coefficients, the practical patterns of heat transfer may differ somewhat from those discussed earlier. It is important that the universal laws of heat transfer are generally valid for any type of heat exchange unit. In the reasoning of the article, we will assume that energy is conserved during heat transfer. This is equivalent to the statement that the radiation power and heat convection from the body of thermal equipment, due to the value of the temperature of the body, are small compared to the power of useful heat transfer. We also assume that the heat capacity of carriers does not depend on their temperatures.

WHEN IS A HIGH RECOVERY COEFFICIENT IMPORTANT?

We can assume that the ability to transfer a certain amount of thermal power is one of the main characteristics of any thermal equipment. The higher this ability, the more expensive the equipment. The recovery factor in theory can vary from 0 to 100%, and in practice often from 25 to 95%. Intuitively, it can be assumed that a high recovery factor, as well as the ability to transmit high power, implies high consumer qualities of the equipment. However, in reality, such a direct relationship is not observed, everything depends on the conditions for using heat transfer. When is a high degree of heat recovery important, and when is it secondary? If the coolant from which heat or cold is taken is used only once, that is, it is not looped, and immediately after use it is irrevocably discharged into external environment, then for the efficient use of this heat, it is desirable to use an apparatus with a high recovery coefficient. Examples include the use of heat or cold from a part of geothermal installations, open reservoirs, sources of technological excess heat, where it is impossible to close the heat carrier circuit. High recovery is important when in the heating network the calculation is carried out only on the water flow and the value of the temperature of the direct water. For air-to-air heat exchangers, this is the use of the heat of the exhaust air, which immediately after the heat exchange goes into the external environment. Another limiting case is realized when the coolant is paid strictly according to the energy taken from it. This can be called an ideal option for a heat supply network. Then it can be stated that such a parameter as the recovery coefficient does not matter at all. Although, with restrictions on the return temperature of the carrier, the recovery coefficient also makes sense. Note that, under certain conditions, a lower equipment recovery factor is desirable.

DETERMINATION OF RECOVERY COEFFICIENT

The definition of the recovery factor is given in many reference manuals (for example, , ). If heat is exchanged between two media 1 and 2 (Fig. 1),

which have heat capacities c 1 and c 2 (in J / kgxK) and mass flow rates g 1 and g 2 (in kg / s), respectively, the heat transfer recovery coefficient can be represented as two equivalent ratios:

\u003d (c 1 g 1) (T 1 - T 1 0) / (cg) min (T 2 0 - T 1 0) \u003d (c 2 g 2) (T 2 0 - T 2) / (cg) min ( T 2 0 - T 1 0). (1)

In this expression, T 1 and T 2 are the final temperatures of these two media, T 1 0 and T 2 0 are the initial ones, and (cg) min is the minimum of the two values ​​​​of the so-called thermal equivalent of these media (W / K) at flow rates g 1 and g 2 , (cg) min = min((s 1 g 1), (s 2 g 2)). To calculate the coefficient, any of the expressions can be used, since their numerators, each of which expresses the total heat transfer power (2), are equal.

W \u003d (c 1 g 1) (T 1 - T 1 0) \u003d (c 2 g 2) (T 2 0 - T 2). (2)

The second equality in (2) can be considered as an expression of the law of conservation of energy during heat transfer, which for thermal processes is called the first law of thermodynamics. It can be seen that in any of the two equivalent definitions in (1), only three of the four exchange temperatures are present. As stated, the value becomes significant when one of the coolants is discarded after use. It follows from this that the choice from the two expressions in (1) can always be made in such a way that it is the final temperature of this carrier that is excluded from the calculation expression. Let's give examples.

a) Extract air heat recovery

A well-known example of a heat exchanger with a high required value is the extract air heat exchanger for heating the supply air (Fig. 2).

If we designate the temperature of the exhaust air T room, street T st, and the supply air after heating in the heat exchanger T pr, then, given the same value of heat capacities from two air flows (they are almost the same, if we neglect the small dependences on humidity and air temperature), you can get a good known expression for:

G pr (T pr - T st) / g min (T room - T st). (3)

In this formula, gmin denotes the smallest g min \u003d min (g in, g out) of the two second flow rates g in the supply air and g out in the exhaust air. When the supply air flow does not exceed the exhaust air flow, formula (3) is simplified and reduced to the form = (T pr - T st) / (T room - T st). The temperature that is not taken into account in formula (3) is the temperature T' of the exhaust air after passing through the heat exchanger.

b) Recuperation in an air curtain or an arbitrary water-air heater

Because for all options the only temperature, the value of which may not be significant, is the return water temperature T x, it should be excluded from the expression for the recovery factor. If we denote the ambient air temperature air curtain T 0 , heated by a curtain of air - T, and the temperature of the air entering the heat exchanger hot water T g, (Fig. 3), for we get:

Cg (T - T 0) / (cg) min (T g - T 0). (4)

In this formula, c is the heat capacity of air, g is the second mass air flow.

The designation (cg) min is the smallest value of air cg and water with W G thermal equivalents, c W is the heat capacity of water, G is the second mass flow rate of water: (cg) min \u003d min ((cg), (c W G)). If the air flow is relatively small and the air equivalent does not exceed the water equivalent, the formula is also simplified: \u003d (T - T 0) / (T g - T 0).

PHYSICAL MEANING OF THE RECOVERY COEFFICIENT

It can be assumed that the value of the heat recovery coefficient is a quantitative expression of the thermodynamic efficiency of power transfer. It is known that for heat transfer this efficiency is limited by the second law of thermodynamics, which is also known as the law of non-decreasing entropy.

However, it can be shown that - this is really the thermodynamic efficiency in the sense of non-decreasing entropy only in the case of equality of the thermal equivalents of two heat-exchanging media. In the general case of inequality of equivalents, the maximum possible theoretical value = 1 is due to the postulate of Clausius, which is formulated as follows: "Heat cannot be transferred from a colder to a warmer body without other changes at the same time associated with this transfer." In this definition, other changes are the work that is done on the system, for example, in the reverse Carnot cycle, on the basis of which air conditioners operate. Considering that pumps and fans during heat exchange with such carriers as water, air and others, produce negligible work on them compared to heat exchange energies, we can assume that with such heat exchange the Clausius postulate is fulfilled with a high degree of accuracy.

Although it is commonly believed that both the postulate of Clausius and the principle of non-decreasing entropy are just formulations of the second law of thermodynamics for closed systems that are different in form, this is not so. To refute their equivalence, we will show that they can generally lead to various restrictions on heat transfer. Consider an air-to-air recuperator in the case of equal thermal equivalents of two exchanging media, which, if the heat capacities are equal, implies the equality of the mass flow rates of two air flows, and = (T pr - T st) / (T room - T st). Let, for definiteness, the room temperature T room \u003d 20 ° C, and the street temperature T street \u003d 0 ° C. If we completely ignore the latent heat of the air, which is due to its humidity, then, as follows from (3), the supply air temperature T pr \u003d 16 o C corresponds to a recovery coefficient = 0.8, and at T pr = 20 o C it will reach a value of 1. (The temperatures of the air thrown out into the street in these cases T' will be 4 o C and 0 o C, respectively). Let us show that exactly = 1 is the maximum for this case. After all, even if the supply air had a temperature of T pr \u003d 24 ° C, and thrown out into the street T ' = -4 ° C, then the first law of thermodynamics (the law of conservation of energy) would not be violated. Every second, E = cg 24 o C Joules of energy will be transmitted to the street air and the same amount will be taken from the room air, and in this case it will be equal to 1.2, or 120%. However, such heat transfer is impossible precisely because the entropy of the system will decrease in this case, which is prohibited by the second law of thermodynamics.

Indeed, according to the definition of entropy S, its change is associated with a change in the total energy of the gas Q by the relation dS = dQ / T (temperature is measured in Kelvins), and given that at a constant gas pressure dQ = mcdT, m is the gas mass, s (or as it is often denoted with p) - heat capacity at constant pressure, dS \u003d mc dT / T. Thus, S = mc ln(T 2 / T 1), where T 1 and T 2 are the initial and final gas temperatures. In the notation of formula (3), for a second change in the entropy of the supply air, we obtain Spr = cg ln(Tpr / Tul), if the street air heats up, it is positive. To change the entropy of the exhaust air Sout = c g · ln(T / Troom). The change in the entropy of the entire system in 1 second:

S \u003d S pr + S vyt \u003d cg (ln (T pr / T st) + ln (T ' / T room)). (5)

For all cases, we will consider T st \u003d 273K, T room \u003d 293K. For = 0.8 from (3), T pr = 289K and from (2) T’ = 277K, which will allow us to calculate the total entropy change S = 0.8 = 8 10 –4 cg. At = 1, we similarly obtain T pr = 293K and T' = 273K, and the entropy, as expected, remains S = 1 = 0. The hypothetical case = 1.2 corresponds to T pr = 297K and T' = 269K, and the calculation shows decrease in entropy: S = 1.2 = –1.2 10 –4 cg. This calculation can be considered a justification for the impossibility of this process c = 1,2 in particular, and in general for any > 1 also due to S< 0.

So, at flow rates that provide equal thermal equivalents of two media (for identical media, this corresponds to equal flow rates), the recuperation coefficient determines the exchange efficiency in the sense that = 1 determines the limiting case of entropy conservation. The postulate of Clausius and the principle of non-decreasing entropy are equivalent for such a case.

Now consider unequal air flow rates for air-to-air heat exchange. Let, for example, the mass flow rate of the supply air be 2g, and that of the exhaust air be g. To change the entropy at such costs, we obtain:

S \u003d S pr + S vyt \u003d 2s g ln (T pr / T st) + s g ln (T ' / T room). (6)

For = 1 at the same initial temperatures T st = 273 K and T room = 293 K, using (3), we get T pr = 283 K, since g pr / g min = 2. Then from the law of conservation of energy (2) we obtain the value T ' = 273K. If we substitute these temperature values ​​in (6), then for a complete change in entropy we obtain S = 0.00125cg > 0. That is, even in the most favorable case c = 1, the process becomes thermodynamically non-optimal, it occurs with an increase in entropy and, as a consequence of this, unlike the subcase with equal costs, is always irreversible.

To estimate the scale of this increase, let us find the recuperation coefficient for the exchange of equal costs already considered above, so that as a result of this exchange the same entropy value is produced as for costs that differ by a factor of 2 at = 1. In other words, we estimate the thermodynamic non-optimality of the exchange of different costs under ideal conditions. First of all, the change in entropy itself says little, it is much more informative to consider the ratio S / E of the change in entropy to the energy transferred by heat exchange. Considering that in the above example, when the entropy increases by S = 0.00125cg, the transferred energy is E = cg pr (T pr - T ul) = 2c g 10K. Thus, the ratio S / E = 6.25 10 -5 K -1. It is easy to see that the recovery coefficient = 0.75026 leads to the same “quality” of exchange at equal flows ... Indeed, at the same initial temperatures T ul = 273K and T room = 293K and equal flows, this coefficient corresponds to temperatures T pr = 288K and T' = 278K. Using (5), we obtain the change in entropy S = 0.000937сg and taking into account that E = сg(T pr - T ul) = сg 15K, we obtain S / Е = 6.25 10 –5 K -1 . So, in terms of thermodynamic quality, heat transfer at = 1 and at twice different flows corresponds to heat transfer at = 0.75026 ... with identical flows.

One more question can be asked: what should be the hypothetical exchange temperatures with different flow rates for this imaginary process to occur without an increase in entropy?

For = 1.32 at the same initial temperatures T st = 273 K and T room = 293 K, using (3), we obtain T pr = 286.2 K and from the energy conservation law (2) T’ = 266.6 K. If we substitute these values ​​in (6), then for a complete change in entropy we get cg(2ln(286.2 / 273) + ln(266.6 / 293)) 0. The law of conservation of energy and the law of non-decreasing entropy for these temperatures are satisfied, and yet the exchange is impossible because T' = 266.6 K does not belong to the initial temperature range. This would directly violate the postulate of Clausius, transferring energy from a colder environment to a heated one. Consequently, this process is impossible, just as others are impossible not only with the conservation of entropy, but even with its increase, when the final temperatures of any of the media go beyond the initial temperature range (T st, T room).

At costs that provide unequal thermal equivalents of the exchange media, the heat transfer process is fundamentally irreversible and proceeds with an increase in the entropy of the system, even in the case of the most efficient heat transfer. These considerations are also valid for two media of different heat capacities; the only important thing is whether or not the thermal equivalents of these media coincide.

PARADOX OF MINIMUM HEAT TRANSFER QUALITY WITH RECOVERY COEFFICIENT 1/2

In this paragraph, we consider three cases of heat transfer with recovery coefficients of 0, 1/2 and 1, respectively. Let equal flows of heat-exchanging media of equal heat capacities with some different initial temperatures T 1 0 and T 2 0 be passed through the heat exchangers. With a recovery factor of 1, the two media simply exchange temperature values ​​and the final temperatures mirror the initial ones T 1 = T 2 0 and T 2 = T 1 0 . Obviously, the entropy does not change in this case S = 0, because the same media at the outlet have the same temperatures as at the inlet. With a recovery factor of 1/2, the final temperatures of both media will be equal to the arithmetic mean of the initial temperatures: T 1 = T 2 = 1/2 (T 1 0 + T 2 0). An irreversible process of temperature equalization will take place, and this is equivalent to an increase in entropy S > 0. With a recovery coefficient of 0, there is no heat transfer. That is, T 1 \u003d T 1 0 and T 2 \u003d T 2 0, and the entropy of the final state will not change, which is similar to the final state of the system with a recovery coefficient equal to 1. As the state c \u003d 1 is identical to the state c \u003d 0, also by analogy it can be shown that the state = 0.9 is identical to the state c = 0.1, etc. In this case, the state c = 0.5 will correspond to the maximum increase in entropy from all possible coefficients. Apparently, = 0.5 corresponds to heat transfer of minimum quality.

Of course, this is not true. The explanation of the paradox should begin with the fact that heat transfer is an exchange of energy. If the entropy increased by a certain amount as a result of heat transfer, then the quality of heat transfer will differ depending on whether heat was transferred 1 J or 10 J. It is more correct to consider not the absolute change in entropy S (in fact, its production in the heat exchanger), but the ratio of change entropy to the energy E transferred in this case. Obviously, for various sets of temperatures, these values ​​can be calculated for = 0.5. It is more difficult to calculate this ratio for = 0, because this is an uncertainty of the form 0/0. However, it is easy to take the redistribution of the ratio at 0, which in practical terms can be obtained by taking this ratio at very small values, for example, 0.0001. In tables 1 and 2, we present these values ​​for various initial conditions for temperature.

For any values ​​​​and at household temperature ranges T st and T br (we will assume that T br / T st x

S / E (1 / T st - 1 / T room) (1 -). (7)

Indeed, if we designate T room \u003d T street (1 + x), 0< x

On graph 1 we show this dependence for temperatures T ul = 300K T room = 380K.

This curve is not a straight line defined by approximation (7), although it is close enough to it that they are indistinguishable on the graph. Formula (7) shows that the quality of heat transfer is minimal precisely at = 0. Let's make one more estimate of the scale S / E. In the example given in , we consider the connection of two heat reservoirs with temperatures T 1 and T 2 (T 1< T 2) теплопроводящим стержнем. Показано, что в стержне на единицу переданной энергии вырабатывается энтропия 1/Т 1 –1/Т 2 . Это соответствует именно минимальному качеству теплообмена при рекуперации с = 0. Интересное наблюдение заключается в том, что по физическому смыслу приведенный пример со стержнем интуитивно подобен теплообмену с = 1/2 , поскольку в обоих случаях происходит выравнивание температуры к среднему значению. Однако формулы демонстрируют, что он эквивалентен именно случаю теплообмена с = 0, то есть теплообмену с наиболее низким качеством из всех возможных. Без вывода укажем, что это же минимальное качество теплообмена S / E = 1 / Т 1 0 –1 / Т 2 0 в точности реализуется для ->0 and at an arbitrary ratio of coolant flow rates.

CHANGING THE QUALITY OF HEAT TRANSFER UNDER DIFFERENT EXPENSES OF HEAT CARRIERS

We will assume that the flow rates of heat carriers differ by n times, and heat transfer occurs with the highest possible quality (= 1). What quality of heat exchange with equal costs will this correspond to? To answer this question, let's see how the value of S / E behaves at = 1 for various ratios of costs. For the cost difference n = 2, this correspondence has already been calculated in point 3: = 1 n=2 corresponds to = 0.75026… for the same flows. In Table 3, for a set of temperatures of 300K and 350K, we present the relative change in entropy at equal flow rates of coolants of the same heat capacity for different values.

In Table 4 we also present the relative change in entropy for different flow ratios n only at the highest possible heat transfer efficiency (= 1) and the corresponding efficiencies resulting in the same quality for equal flow rates.

Let's present the obtained dependence (n) on graph 2.

With an infinite difference in costs, it tends to a finite limit of 0.46745 ... It can be shown that this is a universal dependence. It is valid at any initial temperatures for any media, if instead of the cost ratio we mean the ratio of thermal equivalents. It can also be approximated by a hyperbola, which is indicated in the graph by line 3 of blue color:

‘(n) 0.4675+ 0.5325/n. (8)

The red line indicates the exact relationship (n):

If unequal costs are realized in an exchange with an arbitrary n>1, then the thermodynamic efficiency in the sense of the production of relative entropy decreases. We give its upper estimate without derivation:

This ratio tends to exact equality for n>1 close to 0 or 1, and for intermediate values ​​it does not exceed an absolute error of a few percent.

The end of the article will be presented in one of the next issues of the journal "CLIMATE WORLD". Using examples of real heat exchange units, we will find the values ​​of the recovery coefficients and show how much they are determined by the characteristics of the unit, and how much by the flow rates of heat carriers.

LITERATURE

1. Pukhov A. air. Interpretation of experimental data. // Climate world. 2013. No. 80. P. 110.
2. Pukhov A. C. The power of the thermal curtain at arbitrary flow rates of the coolant and air. Invariants of the heat transfer process. // Climate world. 2014. No. 83. P. 202.
3. Case V. M., London A. L. Compact heat exchangers. . M.: Energy, 1967. S. 23.
4. Wang H. Basic formulas and data on heat transfer for engineers. . M.: Atomizdat, 1979. S. 138.
5. Kadomtsev B. B. Dynamics and information // Uspekhi fizicheskikh nauk. T. 164. 1994. No. May 5 S. 453.

Pukhov Alexey Vyacheslavovich,
Technical Director
Tropic Line company

Everyone knows that there is a huge variety of systems for ventilation of the room. The simplest of them are open-type systems (natural), for example, using a window or a window.

But this method of ventilation is absolutely not economical. In addition, for effective ventilation you need to have a constantly open window or the presence of a draft. Therefore, this type of ventilation will be extremely inefficient. Supply ventilation with heat recovery is increasingly used for ventilation of residential premises.

In simple words, recovery is identical to the word "preservation". Heat recovery is the process of storing thermal energy. This is due to the fact that the flow of air that leaves the room cools or heats the air entering inside. Schematically, the recovery process can be represented as follows:

The ventilation with heat recovery takes place according to the principle that the flows must be separated by the design features of the heat exchanger in order to avoid mixing. However, for example, rotary heat exchangers do not make it possible to completely isolate the supply air from the exhaust air.

The percentage of efficiency of the heat exchanger can vary from 30 to 90%. For special installations, this figure can be 96% energy savings.

What is an air recuperator

By its design, an air-to-air heat exchanger is a unit for heat recovery of the output air mass, which allows the most rational use of heat or cold.

Why choose heat recovery ventilation

Ventilation, which is based on heat recovery, has a very high efficiency. This indicator is calculated by the ratio of the heat that the heat exchanger actually produces to the maximum amount of heat that can only be stored.

What are the types of air recuperators

To date, ventilation with heat recovery can be carried out by five types of recuperators:

1. Plate, which has metal structure and has a high level of moisture permeability;
2. Rotary;
3. chamber type;
4. Recuperator with an intermediate heat carrier;
5. Heat pipes.

Ventilation of a house with heat recovery using the first type of heat exchangers allows incoming air flows from all sides to flow around a lot of metal plates with increased thermal conductivity. The efficiency of recuperators of this type ranges from 50 to 75%.

Features of the device of plate heat exchangers

• Air masses do not contact;
• All details are fixed;
• No moving structural elements;
• Does not form condensate;
• Cannot be used as a room dehumidifier.

Features of rotary heat exchangers

The rotary type of recuperators has design features, with the help of which heat transfer occurs between the supply and output channels of the rotor.

Rotary heat exchangers are covered with foil.

• Efficiency up to 85%;
• Saves electricity;
• Let's apply to dehumidification of the room;
• Mixing up to 3% of air from different streams, in connection with which odors can be transmitted;
• Complex mechanical design.

Supply and exhaust ventilation with heat recovery, which is based on chamber heat exchangers, is used extremely rarely, as it has many disadvantages:

• Efficiency up to 80%;
• Mixing of oncoming flows, in connection with which the transmission of odors increases;
• moving parts of the structure.

Recuperators based on an intermediate heat carrier have a water-glycol solution in their design. Sometimes ordinary water can act as such a coolant.

Features of recuperators with an intermediate heat carrier

• Extremely low efficiency up to 55%;
• Mixing of air streams is completely excluded;
• Scope of application - large-scale production.

Heat recovery ventilation based on heat pipes often consists of an extensive system of tubes that contain freon. Liquid evaporates when heated. In the opposite part of the heat exchanger, freon cools down, as a result of which condensate often forms.

Features of recuperators with heat pipes

• No moving parts;
• The possibility of air pollution by odors is completely excluded;
• The average efficiency index is from 50 to 70%.

To date, compact units for the recovery of air masses are being produced. One of the main advantages of mobile heat exchangers is the absence of the need for air ducts.

Main objectives of heat recovery

1. Heat recovery ventilation is used to maintain required level humidity and indoor temperature.
2. For skin health. Surprisingly, heat recovery systems have a positive effect on human skin, which is constantly moisturized and the risk of drying out is minimized.
3. To avoid drying out furniture and creaking floors.
4. To increase the likelihood of static electricity. Not everyone knows these criteria, but with increased static voltage, mold and fungi develop much more slowly.

Properly selected supply and exhaust ventilation with heat recovery for your home will allow you to significantly save on heating in the winter and air conditioning in the summer. In addition, this type of ventilation has a positive effect on human body, from which you will be less sick, and the risk of fungus in the house will be minimized.