recovery process. Air recuperator: what is it and how it works

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 countercurrents of heat-exchanging media can be organized. In the case of counter currents, which determine the high values ​​of the recovery coefficients, the practical laws 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 irretrievably discharged into the external environment, then for the efficient use of this heat it is desirable to use a device with a high recovery factor. Examples include the use of heat or cold of 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 heat from 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 exchange 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). (one)

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 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 designate the temperature of the air around the air curtain T 0, heated by the air curtain - T, and the temperature of the hot water entering the heat exchanger T g, (Fig. 3), for we get:

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

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

The notation (cg) min is smallest value from air cg and water with W G thermal equivalents, with W - heat capacity of water, G - second mass flow rate of water: (cg) min = 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 media as water, air and others, produce negligible small job compared with the energies of heat exchange, we can assume that with such heat exchange, the Clausius postulate is fulfilled with a high degree 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 change in entropy 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.

CHANGES IN 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. (eight)

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". On the examples of real heat exchange units, we will find the values ​​of the recovery coefficients and show how 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. Increasingly used for residential ventilation forced ventilation with heat recovery.

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 efficient 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 high level 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, based on chamber recuperators, 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%.

Currently issued compact units for recovery air masses. One of the main advantages of mobile heat exchangers is the absence of the need for air ducts.

Main objectives of heat recovery

1. Ventilation based on heat recovery is used to maintain the required level of humidity and temperature indoors.
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.

Correctly selected supply and exhaust ventilation with heat recovery for your home will allow you to significantly save on heating in winter and air conditioning in 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.

Recovery is the process of returning the maximum amount of energy. In ventilation, recuperation is the process of transferring heat energy from exhaust air to supply air. There are many various kinds recuperators and in this article we will talk about each of them. Each type of heat exchanger is good in its own way and has unique advantages, but any of them will allow you to save at least 50%, and more often up to 95%, on supply air heating in winter.

The process of heat transfer from the exhaust air to the supply air is very interesting. Next, we will begin to disassemble each type of air recuperator so that you can more easily understand what it is and what kind of recuperator you need.

The most popular type of recuperators, or rather air handling units with a plate heat exchanger. It gained its popularity due to the simplicity and reliability of the design of the heat exchanger itself.

The principle of operation is simple - two air flows (exhaust and supply) intersect in the heat exchanger of the heat exchanger, but in such a way that they are separated by walls. As a result, these streams do not mix. Warm air heats the walls of the heat exchanger, and the walls heat the supply air. The efficiency of plate heat exchangers (efficiency of a plate heat exchanger) is measured as a percentage and corresponds to:

45-78% for metal and plastic heat exchangers of recuperators.

60-92% for plate heat exchangers with cellulose hygroscopic heat exchangers.

Such a jump in efficiency in the direction of cellulose recuperators is due, firstly, to the return of moisture through the walls of the recuperator from the exhaust air to the supply air, and secondly, to the transfer of latent heat in the same moisture. Indeed, in recuperators, the role is played not by the heat of the air itself, but by the heat of the moisture contained in it. Air without moisture has a very low heat capacity, and moisture is water ... with a known high heat capacity.

For all recuperators, except for cellulose, it is obligatory to remove the drainage. Those. when planning the installation of a heat exchanger, you need to remember that a sewerage supply is also required.

So the pros:

1. Simplicity of design and reliability.

2. High efficiency.

3. Lack of additional electricity consumers.

And, of course, the cons:

1. For the operation of such a heat exchanger, both supply and exhaust must be supplied to it. If the system is designed from scratch, then this is not a minus at all. But if the system is already available and the inflow and exhaust are at a distance, it is better to apply.

2. When sub-zero temperatures the heat exchanger of the heat exchanger may freeze up. To defrost it, either the cessation or reduction of the air supply from the street is required, or the use of a bypass valve that allows the supply air to bypass the heat exchanger while it is defrosted by the exhaust air. With this defrosting mode, all cold air enters the system bypassing the heat exchanger and a lot of electricity is required to heat it up. The exception is cellulose plate heat exchangers.

3. Basically, these recuperators do not return moisture and the air supplied to the premises is too dry. The exception is cellulose plate heat exchangers.

The second most popular type of recuperators. Still ... High efficiency, does not freeze, more compact than lamellar, and even returns moisture. Some pluses.

The rotary heat exchanger is made of aluminum, wound in layers on the rotor, with one sheet being flat and the other being zigzag. For air to pass. It is driven by an electric drive through a belt. This “drum” rotates and each part of it heats up when passing through the exhaust zone, and then, moving to the inflow zone, it cools, thereby transferring heat to the supply air.

A purge sector is used to protect against air overflows.

A new and not very well-known type of air recuperators. In fact, roof heat exchangers use plate heat exchangers and sometimes rotary ones, but we decided to make them a separate type of heat exchangers, because. roof heat exchanger is a specific separate view air handling units with a heat exchanger.

Roof heat exchangers are suitable for large one-volume rooms and are the pinnacle of ease of design, installation and operation. To install it, it is enough to make the necessary window in the roof of the building, put a special "glass" that distributes the load, and put a roof heat exchanger in it. Everything is simple. Air is taken from under the ceiling in the room, and according to the wishes of the customer, either from under the ceiling or into the breathing zone of workers or visitors shopping centers.

Recuperator with intermediate heat carrier:

And this type of recuperators is suitable for existing ventilation systems "inflow separately - exhaust separately".

Well, or if it is impossible to build a new ventilation system with any type of heat exchanger, which involves supplying inflow and exhaust to one room. But it is worth remembering that both plate and rotary heat exchangers have a higher efficiency than glycol.

Any enclosed space needs daily ventilation, but sometimes this is not enough to create a comfortable and pleasant microclimate. In the cold season, when the windows are open in the ventilation mode, heat quickly leaves, and this leads to extra costs for heating. In the summer, many people use air conditioners, but along with the cooled air, hot air from the street also penetrates.

To balance the temperature and make the air fresher, a device such as an air recuperator has been invented. AT winter time it allows you not to lose room heat, and in the summer heat does not allow hot air to enter the room.

What is a recuperator?

Translated from Latin, the word recuperator means - return receipt or return, with regard to air, it means the return of thermal energy, which is carried away with the air through the ventilation system. Such a device as an air recuperator copes with the task of ventilation, balancing two air flows.

The principle of operation of the device is very simple, due to the temperature difference, heat exchange occurs, due to this, the air temperature equalizes. The heat exchanger has a heat exchanger with two chambers, they pass exhaust and supply air flows through them. The accumulated condensate, which is formed due to the temperature difference, is automatically removed from the heat exchanger.

The recovery system allows not only to ventilate the air in the room, it significantly saves heating costs, as it effectively reduces heat loss. The recuperator is capable save more than 2/3 heat leaving the room, which means that the device reuses thermal energy in one technological cycle.

Device classification

Recuperators differ in the schemes of movement of heat carriers and in design, as well as in their purpose. Are there several types of recuperators?

1. lamellar
2. Rotary
3. Aquatic
4. Devices that can be placed on the roof.

Plate heat exchangers

They are considered the most common, because their price is low, but they are quite effective. The heat exchanger located inside the device consists of one or more copper or aluminum plates, plastic, very durable cellulose, they are in a stationary state. Air entering the device passes through a series of cassettes and does not mix; during operation, a simultaneous cooling and heating process takes place.

The device is very compact and reliable, it practically does not fail. Plate-type recuperators operate without power consumption, which is an important advantage. Among the disadvantages of the device - in frosty weather, the lamellar model cannot work, moisture exchange is impossible due to freezing of the exhaust device. Its exhaust channels collect condensate, which freezes at sub-zero temperatures.

Rotary heat exchangers

Such a device is powered by electricity, its blades from one or two rotors must rotate during operation followed by air movement. Usually they have a cylindrical shape with plates tightly installed and a drum inside. They are forced to rotate by air flows, first the room air comes out, and then, changing direction, the air comes back from the street.

It should be noted that rotary devices are larger, but They are much more efficient than the plate ones. They are great for large rooms - halls, shopping centers, hospitals, restaurants, so it is not practical to buy them for the home. Among the disadvantages, it is worth noting the expensive maintenance of such devices, since they consume a lot of electricity, they are not easy to install due to bulkiness, and they are expensive. For installation, a ventilation chamber is required due to the large size of the rotary heat exchanger.

Heat exchanger water and placed on the roof

Recirculation devices transfer thermal energy to the supply heat exchanger with the help of several heat carriers - water, antifreeze, etc. This device very similar in performance to plate heat exchangers, but differs in that it is very similar to water system heating. The disadvantage is low efficiency and frequent maintenance.

The heat exchanger, which can be placed on the roof, saves space in the room. Its efficiency is a maximum of 68%, it does not need operating costs, all these qualities can be attributed to the advantages of this type. The downside is that such a heat exchanger is difficult to mount, it requires a special mounting system. Most often this type is used for objects industrial use.

Natural ventilation must be designed and installed in any residential building, but it is always influenced by weather conditions, depending on the time of year, the strength of ventilation depends on this. If in winter the ventilation system works efficiently in frost, then in summer it practically does not function.

The tightness of a residential building can be reduced by improving natural ventilation, but it will give a tangible result only in the cold season. There is also negative side, for example, heat will leave a residential building, and the incoming cold air will require additional heating.

In order for such a ventilation process not to be too costly for the owners of the house, it is necessary to use the heat of the air removed from the room. It is necessary to make forced air circulation. To do this, a network of supply and exhaust air ducts is laid out, then fans are installed. Air will be supplied through them to separate rooms and such a process will not be associated with weather conditions. Especially for this, a heat exchanger is installed at the intersection of fresh and polluted air masses.

What does an air recuperator provide?

The recuperation system allows to minimize the percentage of mixing of incoming and exhaust air. The separators that are in the device carry out this process. Due to the transfer of flow energy to the boundary, heat exchange occurs, the jets will pass in parallel or cross. The recovery system has many positive features.

1. A special type of grille at the air inlet keeps dust, insects, pollen and even bacteria from the outside.
2. Purified air enters the room.
3. Polluted air leaves the room, which may contain harmful components.
4. In addition to circulation, the supply jets are cleaned and warmed.
5. Promotes better and healthier sleep.

The positive properties of the system make it possible to use it indoors various types to create more comfortable temperature conditions. Very often they are used in industrial premises where ventilation of a large space is necessary. In such places it is necessary to maintain a constant air temperature, this task is handled by rotary heat exchangers that can work at temperatures up to +650 ° C.

Conclusion

The necessary balance of fresh and clean air with normal humidity can be provided by the supply and exhaust ventilation. By installing a recuperator, you can solve many problems associated with saving energy resources.

When choosing an air recuperator for your home, you must take into account the area of ​​\u200b\u200bthe living space, the degree of humidity in it and the purpose of the device. You should definitely pay attention to the cost of the device and the possibility of installation, its efficiency, on which the quality of ventilation of the whole house will depend.

When operating ventilation units in residential buildings or industrial premises in order to save money spent, it is necessary to provide for the installation of energy-saving equipment, called supply and exhaust ventilation systems using heat energy recovery processes, even at the design stages.

The device itself called "recuperator" is a certain type of heat exchanger, consisting of double walls, passing both cold supply and exhaust warm air. The main characteristics of recuperators include its efficiency, which in most cases depends on some important parameters:

• metal composition of the heat exchanger structure;
• total area of ​​contact with air currents;
• the ratio of the volume of passable air masses (supply to exhaust).

In general, the differences between ventilation heat exchangers are also determined by many other factors that are included in specific types of recuperators.

Species classification of recuperators

Air recuperators are quite often equipped not only with a heat exchanger, but also with two fans for separate removal of clean and exhaust air. In addition, various technical devices can be included in these devices in order to improve the quality of the supplied air. Based on this, heat exchangers are classified according to the coolant used, design or flow pattern of coolants into the following types:

Plate heat exchanger (also called cross-point) is the most popular type of heat exchangers due to its compact design simplicity, relatively low cost and reliability. This type of equipment consists of a set of cassettes separated by channels of supply and exhaust air flows made of galvanized metal. The efficiency of these devices can reach an average of up to 70%. and do not need to use electrical energy. The main advantages of such ventilation systems include:

• increased efficiency (performance level);
• lack of consumers of electrical energy;
• convenient and easy installation;
• noiseless operation.

Their main disadvantage is the possible freezing of the heat exchanger as a result of the formation of excess condensate on the plates. To eliminate this disadvantage as much as possible, the domestic heat exchanger is equipped with outlets for collecting condensate liquid (condensate collectors). The only exceptions are cellulose heat exchangers.

The plate heat exchanger, the principle of operation of which is quite convenient and simple, and is based on the intersection without mixing in the heat exchanger of two flows of air masses (supply and exhaust), has sufficient efficiency due to the efficiency index, measured as a percentage, and can correspond to the following values:

• 45-78% - when using plastic or metal heat exchangers;
• 60-92% - when using plate heat exchangers with a cellulose hygroscopic heat exchanger.

The duct plate heat exchanger can be used in rooms where high requirements and standards are imposed on the purity of the incoming air. For the device of the ventilation system can be purchased as finished device, and make .

Based on plate air handling units, there is also a membrane heat exchanger that allows simultaneous moisture and heat exchange in order to eliminate the need to create an additional drainage system to remove excess condensate. Membrane plates have selective permeability, in connection with which water molecules pass through and gas molecules are retained.

1. The rotary heat exchanger, the principle of operation of which is based on the rotation of the rotary heat exchanger at a certain and constant speed, is a cylindrical structure, inside which layers of corrugated metal are densely located. The built-in drum, making rotational movements, initially passes the heated air, after which the supply cold air. As a result, the corrugated layers are progressively cooled or heated, and part of the heat is transferred to the cold air flow. Such ventilation systems have a number of advantages, among which are:
   • partial return of moisture (no need for);
   • the ability to control the speed of rotation of the rotors;
   • compact design and installation.

   Along with the advantages, rotary heat exchangers have significant disadvantages - they require the use of electricity, the installation of additional filter components and have moving elements.

   The efficiency of a rotary heat exchanger can be 60-85%, so they are used in systems characterized by high air flow rates.

2. The glycol heat exchanger is one of the representatives of installations with intermediate heat carriers, which allows you to connect two separate ventilation systems. This equipment is ideal for upgrading existing ventilation systems that operate separately from each other. A glycol heat exchanger, the principle of operation of which is based on the installation of a heating heat exchanger with the supply of antifreeze (circulation of a water-glycol solution), is often calculated individually. The basic characteristics of such installations include:
   • the ability to adjust the system using built-in automation and the speed of circulation of the coolant;
   • operation of the unit at sub-zero temperatures without the need for defrosting;
   • connection of several inflows and one exhaust or vice versa;
   • lack of moving parts;
   • the gap between the exhaust and inflow can reach up to 800m.

   The main disadvantage is the low efficiency of work - 45-60%.

3. Water heat exchanger is a kind of air heat exchangers used in supply and exhaust systems. The mechanism of action of such a device is due to the transfer of heat through water. In this case, heat exchangers can be located at a remote distance using heat-insulated pipelines. This circumstance is the main purpose of the application - the connection of ventilation lines. Water recuperators are used quite rarely due to low efficiency values ​​and the need for frequent maintenance.

The main criteria for choosing recuperators

When selecting a suitable and optimal heat exchanger in terms of efficiency, the following criteria must be observed:

• recovery level (energy saving) - depending on the manufacturer and model, this parameter should be in the range of 40-85%;
• sanitary and hygienic indicators - the ability to control the degree of purification and quality of incoming air;
• energy efficiency - the importance of energy consumption;
• performance characteristics - overall service life, equipment suitability for performance repair work, the need for minimal maintenance;
• adequate cost.

Considering all these indicators, choosing the most high-quality and efficient types of recuperators in terms of performance will not be very difficult for those who want to both create and improve the existing ventilation system.