Calculation of the heat exchanger. Types and principle of operation of heat exchangers

You can make a coil with your own hands from round or shaped pipes. For different operating conditions, one or another material is selected. Such products are used for heat transfer in water heating systems. They can even be built into fireplaces or stoves, allowing them to be used as a boiler room to heat all the rooms in the house.

Types of coil heat exchangers

The heated towel rail is also a serpentine heat exchanger.

You can make a coil with your own hands of various designs and from several types of metal (steel, copper, aluminum, cast iron). Aluminum and cast iron products are stamped in factories, since the required conditions for working with these metals can only be achieved in production conditions. Without this, it will only work with steel or copper. It is best to use copper, as it is malleable and has a high degree of thermal conductivity. There are two schemes for making a coil:

• screw;
• parallel.

The helical scheme means the arrangement of the turns of the spiral along a helix. The coolant in such heat exchangers moves in one direction. If necessary, to increase the thermal power, several spirals can be combined according to the "pipe in pipe" principle.

To minimize heat loss, you need to choose. It also depends on the material of the walls.

You need to do it based on the vapor permeability of thermal insulation.

In a parallel circuit, the coolant constantly changes its direction of movement. Such a heat exchanger is made of straight pipes connected by a 180 degree elbow. In some cases, for example, for the manufacture of a heating register, swivel elbows may not be used. Instead, a direct bypass is installed, which can be located on one or both ends of the pipe.

Heat Transfer Methods

The principle of operation of a coil heat exchanger is to heat one substance at the expense of the heat of another. So, the water in the heat exchanger can be heated by an open flame. In this case, it will act as a heat sink. But also the coil itself can act as a heat source. For example, when a coolant flows through the pipes, heated in a boiler or by means of a built-in electric heating element, and its heat is transferred to water from the heating system. Essentially, the ultimate goal of heat transfer is to heat the air in the room.

Where are coil heat exchangers installed?

The method of heat exchange depends on where the coil is installed:

• boiler;

The boiler has finned coils.

In the boiler, the flame heats the water in the coil, and then it diverges throughout the system, giving off thermal energy to the room by the convective method through. Some of them also fall into the category of coil heat exchangers. For example, heated towel rails and from a round or profile pipe.

Contact with an open flame imposes some requirements on the performance of the metal that was used in production. The emphasis is on reliability and durability. Therefore, steel and cast iron are most often used. The latter is considered the best option.

In the boiler and heat accumulator, the heat exchange rate and corrosion resistance are of priority. In this case, there is nothing better than copper. The main thing is that it does not come into contact with aluminum. A reaction takes place between these metals, which leads to chemical corrosion.

How to calculate a heat exchanger

It is imperative to calculate the coil heat exchanger, otherwise its thermal power may not be enough to heat the room. The heating system is designed to compensate for heat loss. Accordingly, we can find out the exact amount of required thermal energy only based on the heat loss of the building. It is rather difficult to make a calculation, therefore, on average, they take 100 W per 1 square meter with a ceiling height of 2.7 m.

There should be a gap between the turns.

The following values ​​will also be required for the calculation:

• Pi;
• the diameter of the pipe that is available (take 10 mm);
• metal thermal conductivity lambda (for copper 401 W/m*K);
• delta of the temperature of the supply and return of the coolant (20 degrees).

To determine the length of the pipe, you need to divide the total thermal power in W by the product of the above multipliers. Consider the example of a copper heat exchanger with a required thermal power of 3 kW - this is 3000 W.

3000/ 3.14 (Pi) * 401 (lambda thermal conductivity) * 20 (temperature delta) * 0.01 (pipe diameter in meters)

From this calculation, it turns out that you need 11.91 m of copper pipe with a diameter of 10 mm, so that the thermal power of the coil is 3 kW.

How to make a screw coil

After you have made the calculation of the heat exchanger coil, you can proceed directly to manufacturing. It is quite easy to make a screw design. The diameter of the loop must be selected based on the size of the tank into which the installation will be carried out. It is necessary that the pipes do not touch the body.

You need to wind the turns on a round blank. Copper bends easily, so no additional tool is needed. It is advisable to observe a small indent between the turns so that the coolant is in contact with the pipe from all sides. This will increase the heat exchange area, which will allow us to achieve the maximum thermal power that we expected.

How to make a heat exchanger from straight pipes

To make a coil in a parallel circuit, you need to have the skills of welding metals. For such work, steel pipes are used, which are very difficult to bend, although having a good pipe bender is still possible. But in most cases, you have to resort to welding.

Steel coil from round pipes.

Work algorithm:

• cut even lengths of steel pipes;
• lay them parallel on a flat surface;
• connect them with knees with a 180 degree turn - if there are no such knees, then you can weld two corners of 90 degrees each;
• weld plugs with a pipe for connection to the heating system into the lower and upper ends.

In addition, a plug can be installed in the lower part, in the center of which a hole is cut. Then a nut is welded into this hole. Its inner diameter must fit a standard electric heating element. In this case, it will be possible to use a home-made heat exchanger as an electric heater.

heat exchanger called an apparatus designed to communicate heat to one of the coolants as a result of its removal from water from another coolant. The process of supply and removal of heat in a heat exchanger can pursue various technological goals: heating (cooling) of a liquid or gas, turning a liquid into steam, condensing steam, etc.

According to the principle of operation, heat exchangers are divided into recuperative, regenerative and mixing.

Recuperative are called heat exchangers, in which the transfer of heat from one coolant to another is carried out through a solid wall separating them. In automotive internal combustion engines, mainly recuperative heat exchangers are used, which are used to cool engine oil, cooling system fluid, air entering the engine cylinders, and other purposes. Figure 14 shows a diagram of a water-oil heat exchanger, which is often implemented in the design of oil coolers for diesel lubrication systems.

Rice. 14. Scheme of the simplest shell-and-tube recuperative heat exchanger for transferring heat from one coolant (I) to another (II).

Regenerative called heat exchangers, in which a hot coolant comes into contact with a solid body (ceramic or metal nozzle) and gives it heat, in the subsequent period, a “cold” coolant comes into contact with the solid body, which perceives the heat accumulated by the body.

In the metallurgical industry, regenerative heat exchangers have long been used to heat air and combustible gases. The accumulating nozzle in the heat exchanger is made of red brick. A feature of regenerators is that the heat transfer process in them is non-stationary. Therefore, technical calculations of regenerative heat exchangers are performed on averaged temperatures over time.

Mixing called heat exchangers, in which the transfer of heat from one coolant to another is carried out by their direct contact, therefore, is accompanied by a complete or partial exchange of matter. Such devices are used for cooling and heating gases with water or for cooling water with air in gas production, air conditioning, steam condensation, etc.

Despite the wide variety of heat exchangers, the basic provisions for their calculation remain common.

When calculating heat exchangers, two cases usually occur:

1) constructive calculation, when the parameters of the heat carriers at the inlet and outlet and the flow rates of the heat carriers (or the heat flow rate) are known. Having previously chosen the design of the heat exchanger, the calculation determines the heat exchange surface;

2) verification calculation, when the heat exchange surface and the design of the apparatus are known, and their parameters at the inlet are partially known. The calculation finds unknown parameters (for example, output parameters), coolant flow rates or other characteristics of the apparatus (for example, efficiency).

In both cases, the main calculation equations are: heat balance equation:

Q= m 1 s 1 (t" 1 - t"" 1) = m 2 s 2 (t" 2 - t"" 2) (40)

and the heat transfer equation:

Q = kF(t 1 - t 2).

In these equations and below, the index 1 means that the quantities refer to a hot liquid, and the index 2 - to cold. The temperature at the inlet is indicated by one stroke, and at the outlet by two; T— mass flow rate of liquid; With is the heat capacity of the liquid.

When deriving the calculation formulas for heat transfer, the change in the temperature of heat carriers was not taken into account. In heat exchangers, the hot medium is cooled, and the cold one is heated, and therefore the temperature difference also changes. Δt. Under such conditions, the heat transfer equation can only be applied to the surface element dF, i.e.:

dQ = kΔtdF. (41)

In addition, it is necessary to take into account the dependence of the heat transfer coefficient k from changes in the temperature of the working fluids. For the most part, such accounting comes down to referring the heat transfer coefficient to the average temperatures of the heat carriers, sometimes the heat transfer coefficient is found from the temperatures of the heat carriers at the beginning and at the end of the heating surface. If the received values k" And k"" slightly differ from one another, then the arithmetic mean value is taken as the average value of the heat transfer coefficient: k = (k"+k"")/2.

With a significant difference in values k" And k"" the heating surface is divided into separate sections, within which the values k change little, and the heat transfer coefficient is determined for each section.

The total amount of heat transferred through the entire surface F, is determined by integrating expression (41):

Where Δt m is the average logarithmic value of the temperature difference over the surface:

If the temperature of the heat carriers along the heating surface changes slightly, then the arithmetic mean head can be used in the calculation:

Δt m = Δt avg. = 0,5(t"+ t"")

Arithmetic mean head Δt avg. always greater than the mean logarithmic Δt m, but at ∆t"/∆t""> 0.5 they differ from each other by less than 3%.

In thermal calculations, the concept of the so-called water equivalent of coolant W, which determines the amount of water equivalent in heat capacity to the second flow rate of the liquid in question, i.e.

W = mc p .(44)

Taking into account the water equivalent, the equation (40) of the heat balance is converted to the form:

Thus, the ratio of the change in the temperature of heat carriers is inversely proportional to the ratio of their water equivalents.

The nature of the change in the temperatures of heat carriers along the heating surface depends on the scheme of their movement and the ratio of the values ​​of water equivalents. If in a heat exchanger hot and cold liquids flow in parallel and in the same direction, then such a movement scheme is called once-through(Fig. 15, A).

Fig.15. Schemes of movement of working fluids in heat exchangers.

With counterflow, liquids move in parallel, but in opposite directions (Fig. 15, b). In the cross-flow circuit, fluids move in crossing directions (Fig. 15, V). In addition to the listed simple schemes for the movement of fluids, there can be complex ones that combine various combinations of elements of simple schemes (Fig. 15, G And e).

On fig. 16, where the value of the heating surface is plotted along the abscissa axis F, and the temperature along the ordinate axis, four characteristic pairs of temperature change curves along the heating surface are shown depending on the flow pattern (forward flow, counterflow) and the values ​​of water equivalents of heat carriers W 1 And W2.

As can be seen from the graphs, a larger change in temperature Δt" = t" - t" has a liquid whose water equivalent is less, which corresponds to equation (45).

Rice. 16. The nature of the change in the temperatures of heat carriers in the schemes of co-current and counter-current.

From the examination of the graphs, the following conclusions can be drawn:

1. For co-current, the end temperature of the cold liquid is always lower than the end temperature of the hot liquid;

2. The temperature difference along the surface with forward flow changes more significantly, and its average value is less than with counterflow, therefore, as follows from formula (42), a smaller amount of heat is transferred with forward flow than with counterflow.

3. Schemes of direct flow and counterflow can be considered equivalent if the temperature of at least one of the heat carriers is constant. This happens when liquids boil and when vapors condense, or when the value of the water equivalent of one of the heat carriers is so large that its temperature changes slightly.

4. With countercurrent, the final temperature of the cold liquid t"" 2 can be higher than the final temperature of the hot, i.e., at the same initial temperature of the cold liquid with counterflow, it can be heated to a higher temperature.

Thus, from a thermotechnical point of view, counterflow should always be preferred, unless some other reasons (for example, constructive ones) force the use of a cocurrent circuit.

Perhaps the only drawback of the counterflow scheme is the more difficult temperature conditions for the material of the heat exchanger walls, since individual sections from the side of the hot liquid inlet are washed from both sides by liquids with a maximum temperature.

As mentioned above, when verification calculation it is necessary to calculate the final temperatures of the coolants t"" 1 And t"" 2 and the amount of transferred heat. In this case, for an approximate estimate, you can use the dependencies:

heat exchanger efficiency

The efficiency of the process in the heat exchanger is estimated by the efficiency factor η characterizing the share of the heat of the hot liquid used to heat the cold liquid:

Where Q1- the amount of heat perceived by the cold liquid;

Qpacn. - the available amount of heat of a hot liquid.

For heat exchangers of vehicles, the weight and overall characteristics of the apparatus are important. The compact design of the heat exchanger can be estimated specific heating surface β, which is the area of ​​the working surface per unit volume of the apparatus: β beats = F slave /V cool . .

The efficiency of the heat exchanger depends on the structural structure of the cooling surface, which is evaluated ribbing coefficient ξ op.= F cool /F liquid, Where F cool- surface area cooled by air; F kike- area of ​​the cooling surface washed by water.

When choosing the type of coolant, its thermophysical properties, cost, the possibility of wall corrosion, etc. should be taken into account. For example, when choosing antifreeze or water, it should be borne in mind that with ease of use of antifreeze (low freezing point), it has lower thermophysical properties than water, which reduces the efficiency of the heat exchanger (radiator).

To increase the compactness and reduce the weight of heat exchangers, various means of intensifying heat transfer are used.

An effective means of increasing the compactness of the heat exchanger is the installation of ribs on its surfaces, which can be used both in plate and tubular heat exchangers. On fig. 17, A a plate heat exchanger with flat continuous fins is shown, and in fig. 17, b- heat exchanger with finned tubes of oval section.

The fins are usually made of copper or aluminum thin sheets and are securely soldered to the base surface. They can be smooth or grooved. The fins can be made in the form of separate plates, which are located in the channel of the plate heat exchanger in a checkerboard or in-line order .

Rice. 17. Fragments of a plate heat exchanger with flat continuous fins (a) and a heat exchanger with finned oval tubes (b).

Currently, for car engines, tubular-lamellar and tubular-tape radiator designs are most widely used (Fig. 18).

Fig.18. Radiator grille cores:

A- tubular-lamellar; b- tubular-tape.

In the manufacture of cooling grilles for tubular-plate radiators, tubes are used (seam or seamless, which are made of aluminum alloy, brass copper L-68 or L-90 with a thickness of up to 0.15 mm) (Fig. 19). The finning plates are made flat or wavy from the same material as the tubes. In tubular-tape structures, the tape is made of M-3 copper with a thickness of 0.05 ... 0.1 mm.

IN tubular-lamellar radiators cooling tubes can be arranged in a row, staggered and staggered at an angle with respect to the cooling air flow (Fig. 20).

Fig.19. Radiator tubes:

A- copper brazed; b- Welded aluminum alloy.

Rice. 20. Cooling elements for grilles of tubular-plate radiators:

A- row arrangement of tubes; b- chess arrangement; V- the same at an angle to the air flow; G- cooling plate with bent notches.

In tubular-ribbon radiators (Fig. 21), the cooling tubes practically do not differ in their design from the tubes used in tubular-lamellar radiators, but they are located only in a row. To increase the turbulence of the air flow on the tapes, either figured stamping is performed (Fig. 21, b), or bent notches.

The compactness of the design of modern automotive heat exchangers, estimated by the value specific heating surface βsp, corresponds to 440…850 m 2 / m 3. The fin coefficient for these heat exchangers varies in the range: ξ op.= 5…11,5.

Rice. 21. Elements of a tubular-tape radiator:

A- cooling grille; b- cooling tape with figured stamping; 1 - cooling tape; 2 - liquid cooling tube.

Example. In the heat exchanger, a liquid with a water equivalent W 1= 116 W/deg cools off t" 1= 120°С up to t"" 1= 50°С with water at a temperature t" 2= 10°C, for which W2= 584 W/deg. Determine the required heating surface for co-current and counter-current schemes, if the heat transfer coefficient k :

0,6 m 2;

b) with countercurrent.

Calculation of the heat exchanger currently takes no more than five minutes. Any organization that manufactures and sells such equipment, as a rule, provides everyone with their own selection program. It can be downloaded for free from the company's website, or their technician will come to your office and install it for free. However, how correct is the result of such calculations, can it be trusted and is the manufacturer not being cunning when fighting in a tender with his competitors? Checking an electronic calculator requires knowledge or at least an understanding of the methodology for calculating modern heat exchangers. Let's try to figure out the details.

What is a heat exchanger

Before performing the calculation of the heat exchanger, let's remember what kind of device this is? A heat and mass transfer apparatus (aka a heat exchanger, or a TOA) is a device for transferring heat from one coolant to another. In the process of changing the temperatures of heat carriers, their densities and, accordingly, the mass indicators of substances also change. That is why such processes are called heat and mass transfer.

Types of heat transfer

Now let's talk about - there are only three of them. Radiation - heat transfer due to radiation. As an example, consider sunbathing on the beach on a warm summer day. And such heat exchangers can even be found on the market (tube air heaters). However, most often for heating residential premises, rooms in an apartment, we buy oil or electric radiators. This is an example of a different type of heat transfer - it can be natural, forced (hood, and there is a heat exchanger in the box) or mechanically driven (with a fan, for example). The latter type is much more efficient.

However, the most efficient way to transfer heat is conduction, or, as it is also called, conduction (from the English. Conduction - "conductivity"). Any engineer who is going to conduct a thermal calculation of a heat exchanger, first of all, thinks about how to select efficient equipment in minimum dimensions. And it is possible to achieve this precisely due to thermal conductivity. An example of this is the most efficient TOA today - plate heat exchangers. A plate heat exchanger, according to the definition, is a heat exchanger that transfers heat from one coolant to another through a wall separating them. The maximum possible contact area between the two media, together with correctly selected materials, plate profile and thickness, allows minimizing the size of the selected equipment while maintaining the original technical characteristics required in the technological process.

Types of heat exchangers

Before calculating the heat exchanger, it is determined with its type. All TOA can be divided into two large groups: recuperative and regenerative heat exchangers. The main difference between them is as follows: in regenerative TOAs, heat exchange occurs through a wall separating two coolants, while in regenerative ones, two media have direct contact with each other, often mixing and requiring subsequent separation in special separators. are subdivided into mixing and into heat exchangers with a nozzle (stationary, falling or intermediate). Roughly speaking, a bucket of hot water, exposed to frost, or a glass of hot tea, set to cool in the refrigerator (never do this!) - this is an example of such a mixing TOA. And pouring tea into a saucer and cooling it in this way, we get an example of a regenerative heat exchanger with a nozzle (the saucer in this example plays the role of a nozzle), which first contacts the surrounding air and takes its temperature, and then takes away part of the heat from the hot tea poured into it , seeking to bring both media into thermal equilibrium. However, as we have already found out earlier, it is more efficient to use thermal conductivity to transfer heat from one medium to another, therefore, the most useful (and widely used) TOAs in terms of heat transfer today are, of course, regenerative ones.

Thermal and structural design

Any calculation of a recuperative heat exchanger can be carried out on the basis of the results of thermal, hydraulic and strength calculations. They are fundamental, obligatory in the design of new equipment and form the basis of the methodology for calculating subsequent models of a line of similar devices. The main task of the thermal calculation of TOA is to determine the required area of ​​the heat exchange surface for the stable operation of the heat exchanger and maintaining the required parameters of the media at the outlet. Quite often, in such calculations, engineers are given arbitrary values ​​of the weight and size characteristics of the future equipment (material, pipe diameter, plate dimensions, bundle geometry, type and material of fins, etc.), therefore, after the thermal calculation, they usually carry out a constructive calculation of the heat exchanger. After all, if at the first stage the engineer calculated the required surface area for a given pipe diameter, for example, 60 mm, and the length of the heat exchanger turned out to be about sixty meters, then it would be more logical to assume a transition to a multi-pass heat exchanger, or to a shell-and-tube type, or to increase the diameter of the tubes.

Hydraulic calculation

Hydraulic or hydromechanical, as well as aerodynamic calculations are carried out in order to determine and optimize the hydraulic (aerodynamic) pressure losses in the heat exchanger, as well as calculate the energy costs to overcome them. The calculation of any path, channel or pipe for the passage of the coolant poses a primary task for a person - to intensify the heat transfer process in this area. That is, one medium must transfer, and the other receive as much heat as possible in the minimum period of its flow. For this, an additional heat exchange surface is often used, in the form of a developed surface ribbing (to separate the boundary laminar sublayer and enhance flow turbulence). The optimal balance ratio of hydraulic losses, heat exchange surface area, weight and size characteristics and removed thermal power is the result of a combination of thermal, hydraulic and structural calculation of TOA.

Research calculations

TOA research calculations are carried out on the basis of the obtained results of thermal and verification calculations. They are necessary, as a rule, to make the last amendments to the design of the designed apparatus. They are also carried out in order to correct any equations that are embedded in the implemented calculation model of TOA, obtained empirically (according to experimental data). Performing research calculations involves tens and sometimes hundreds of calculations according to a special plan developed and implemented in production according to the mathematical theory of experiment planning. Based on the results, the influence of various conditions and physical quantities on the TOA efficiency indicators is revealed.

Other calculations

When calculating the heat exchanger area, do not forget about the resistance of materials. TOA strength calculations include checking the designed unit for stress, for torsion, for applying the maximum allowable working moments to the parts and assemblies of the future heat exchanger. With minimum dimensions, the product must be strong, stable and guarantee safe operation in various, even the most demanding operating conditions.

Dynamic calculation is carried out in order to determine the various characteristics of the heat exchanger in variable modes of its operation.

Design types of heat exchangers

Recuperative TOAs can be divided into quite a large number of groups according to their design. The most famous and widely used are plate heat exchangers, air (tubular finned), shell-and-tube, tube-in-pipe heat exchangers, shell-and-plate and others. There are also more exotic and highly specialized types, such as spiral (coil heat exchanger) or scraped type, which work with viscous or as well as many other types.

Heat exchangers "pipe in pipe"

Consider the simplest calculation of the "pipe in pipe" heat exchanger. Structurally, this type of TOA is maximally simplified. As a rule, a hot coolant is let into the inner pipe of the apparatus to minimize losses, and a cooling coolant is started into the casing, or into the outer pipe. The engineer's task in this case is reduced to determining the length of such a heat exchanger based on the calculated area of ​​the heat exchange surface and the given diameters.

Here it is worth adding that in thermodynamics the concept of an ideal heat exchanger is introduced, that is, an apparatus of infinite length, where the heat carriers work in countercurrent, and the temperature difference is completely worked out between them. The pipe-in-pipe design is the closest to meeting these requirements. And if you run the coolants in countercurrent, then it will be the so-called "real counterflow" (and not cross, as in plate TOAs). The temperature head is most effectively worked out with such an organization of movement. However, when calculating the “pipe in pipe” heat exchanger, one should be realistic and not forget about the logistics component, as well as ease of installation. The length of the eurotruck is 13.5 meters, and not all technical premises are adapted to the skidding and installation of equipment of this length.

Shell and tube heat exchangers

Therefore, very often the calculation of such an apparatus smoothly flows into the calculation of a shell-and-tube heat exchanger. This is an apparatus in which a bundle of pipes is located in a single housing (casing), washed by various coolants, depending on the purpose of the equipment. In condensers, for example, the refrigerant is run into the casing, and the water is run into the tubes. With this method of media movement, it is more convenient and efficient to control the operation of the apparatus. In evaporators, on the contrary, the refrigerant boils in the tubes, while they are washed by the cooled liquid (water, brines, glycols, etc.). Therefore, the calculation of a shell-and-tube heat exchanger is reduced to minimizing the dimensions of the equipment. Playing with the shell diameter, the diameter and number of internal pipes and the length of the apparatus, the engineer reaches the calculated value of the heat exchange surface area.

Air heat exchangers

One of the most common heat exchangers today is tubular finned heat exchangers. They are also called snakes. Wherever they are installed, starting from fan coil units (from the English fan + coil, i.e. "fan" + "coil") in the indoor units of split systems and ending with giant flue gas recuperators (heat extraction from hot flue gas and transmission for heating needs) in boiler plants at CHP. That is why the calculation of a coil heat exchanger depends on the application where this heat exchanger will go into operation. Industrial air coolers (HOPs) installed in meat blast freezers, low-temperature freezers and other food refrigeration facilities require certain design features in their design. The spacing between the lamellas (fins) should be as large as possible in order to increase the time of continuous operation between defrost cycles. Evaporators for data centers (data processing centers), on the contrary, are made as compact as possible, clamping the interlamellar distances to a minimum. Such heat exchangers operate in "clean zones" surrounded by fine filters (up to HEPA class), so this calculation is carried out with an emphasis on minimizing dimensions.

Plate heat exchangers

Currently, plate heat exchangers are in stable demand. According to their design, they are completely collapsible and semi-welded, copper-soldered and nickel-soldered, welded and soldered by diffusion (without solder). The thermal calculation of a plate heat exchanger is quite flexible and does not present any particular difficulty for an engineer. In the selection process, you can play with the type of plates, the depth of forging channels, the type of fins, the thickness of steel, different materials, and most importantly, numerous standard-size models of devices of different sizes. Such heat exchangers are low and wide (for steam heating of water) or high and narrow (separating heat exchangers for air conditioning systems). They are also often used for phase change media, i.e. as condensers, evaporators, desuperheaters, precondensers, etc. The thermal calculation of a two-phase heat exchanger is slightly more difficult than a liquid-liquid heat exchanger, however, for experienced engineer, this task is solvable and does not present any particular difficulty. To facilitate such calculations, modern designers use engineering computer databases, where you can find a lot of necessary information, including state diagrams of any refrigerant in any deployment, for example, the CoolPack program.

Example of heat exchanger calculation

The main purpose of the calculation is to calculate the required area of ​​the heat exchange surface. Thermal (refrigeration) power is usually specified in the terms of reference, however, in our example, we will calculate it, so to speak, to check the terms of reference itself. Sometimes it also happens that an error can creep into the source data. One of the tasks of a competent engineer is to find and correct this error. As an example, let's calculate a plate heat exchanger of the "liquid-liquid" type. Let this be a pressure breaker in a tall building. In order to unload equipment by pressure, this approach is very often used in the construction of skyscrapers. On one side of the heat exchanger, we have water with an inlet temperature Tin1 = 14 ᵒС and an outlet temperature Тout1 = 9 ᵒС, and with a flow rate G1 = 14,500 kg / h, and on the other - also water, but only with the following parameters: Тin2 = 8 ᵒС, Тout2 = 12 ᵒС, G2 = 18 125 kg/h.

The required power (Q0) is calculated using the heat balance formula (see figure above, formula 7.1), where Ср is the specific heat capacity (table value). For simplicity of calculations, we take the reduced value of the heat capacity Срв = 4.187 [kJ/kg*ᵒС]. We believe:

Q1 \u003d 14,500 * (14 - 9) * 4.187 \u003d 303557.5 [kJ / h] \u003d 84321.53 W \u003d 84.3 kW - on the first side and

Q2 \u003d 18 125 * (12 - 8) * 4.187 \u003d 303557.5 [kJ / h] \u003d 84321.53 W \u003d 84.3 kW - on the second side.

Please note that, according to formula (7.1), Q0 = Q1 = Q2, regardless of which side the calculation was made on.

Further, according to the basic heat transfer equation (7.2), we find the required surface area (7.2.1), where k is the heat transfer coefficient (taken equal to 6350 [W / m 2 ]), and ΔТav.log. - average logarithmic temperature difference, calculated according to the formula (7.3):

ΔT sr.log. = (2 - 1) / ln (2 / 1) = 1 / ln2 = 1 / 0.6931 = 1.4428;

F then \u003d 84321 / 6350 * 1.4428 \u003d 9.2 m 2.

In the case where the heat transfer coefficient is unknown, the calculation of the plate heat exchanger is slightly more complicated. According to formula (7.4), we consider the Reynolds criterion, where ρ is the density, [kg / m 3], η is the dynamic viscosity, [N * s / m 2], v is the speed of the medium in the channel, [m / s], d cm - wetted channel diameter [m].

Using the table, we look for the value of the Prandtl criterion that we need and, using formula (7.5), we obtain the Nusselt criterion, where n = 0.4 - under conditions of heating the liquid, and n = 0.3 - under conditions of cooling the liquid.

Further, according to formula (7.6), the heat transfer coefficient from each coolant to the wall is calculated, and according to formula (7.7), we calculate the heat transfer coefficient, which we substitute into formula (7.2.1) to calculate the area of ​​the heat exchange surface.

In these formulas, λ is the thermal conductivity coefficient, ϭ is the channel wall thickness, α1 and α2 are the heat transfer coefficients from each of the heat carriers to the wall.

What you need to know for the correct calculation of heat exchange equipment?

When choosing and installing heat exchange equipment, individual features and conditions of a particular facility should be taken into account. For this reason, before buying a heat exchanger, it is important to calculate the heat exchanger and find out the main characteristics of the system in which it will be installed. Based on the data obtained, you can choose the most suitable device.

To buy a suitable heat exchanger, the technical characteristics of which are suitable for a specific system, you need to know:

1. Where will the device be located and where will it be used. It can be a ventilation system, hot water supply, heating or technological processes.

2. The power of the heat exchanger and its heat load. If there is no information on the heat load, you need to know the water flow in the heat exchanger

3. When calculating the plate heat exchanger water-water, oil-water and steam-water, one should take into account the type of medium in which the device will operate. Also, heat exchange equipment is used in the food industry and in complex technological processes.

4. Of no small importance when choosing a heat exchanger is the temperature of the working medium.

Thanks to this information, you can learn how to calculate the heat exchanger and determine the material for the manufacture of plates and sealing elements. Also, these data will help you choose the layout, frame dimensions, number of plates and their thickness.

How to calculate the power of a heat exchanger?

The calculation of the power of a plate heat exchanger begins with the fact that you need to know the volume of the heated medium and the temperature difference between the liquids. The power of the heat exchanger is calculated by the formula:
P = 1.16 x ∆T / (t x V), where
P is the required power of the heat exchanger;
1.16 is a specially selected constant;
∆Т – temperature difference;
t is time;
V is the volume.

For the calculation, the water flow through the heat exchanger, the heat exchanger power, the average temperature difference between the media and the heat transfer coefficient are important. These characteristics are calculated using the heat balance equation:

Q=Q1=Q2
Q - the amount of heat transmitted or received by the coolant (W). It comes out of this:
Q1 = G1c1 (t1н – t1к) and Q2 = G2c2 (t2к – t2н)
Where
G1,2 – water consumption in the heat exchanger [kg/h];
c1,2 – heat capacities of hot and cold coolants [J/kg deg];
t1.2 n - initial temperature of hot and cold coolants [°C];
t1.2 k - final temperature of hot and cold heat carriers [°C];

Where can I get the data for the calculation?

In the TU of an enterprise that is engaged in heat supply;
in the terms of reference, which is compiled by the engineer and the chief technologist;
in the design of the heat exchange system or at the point where the device is located;
in an agreement with a company that is responsible for heat supply.

How to calculate a plate heat exchanger?

The calculation of heat exchange equipment is a complex and lengthy process in which it is easy to make a mistake. Therefore, the calculation of the heat exchanger should be carried out exclusively by a specialist with experience. In most cases, this is done by an official dealer or a specialist from the manufacturer of heat exchange equipment. In order to minimize possible errors in calculations, professionals use special programs and formulas.

In such programs, there are special tables where the initial data are entered, after which several correct calculation options are automatically issued.

Official dealers make calculations much faster than the manufacturer's specialists. In addition to heat exchange equipment, a device calculation sheet is issued. From it it will be possible to easily determine whether the parameters of the selected device correspond to the technical conditions of the particular system in which the heat exchanger is mounted. It is important to understand that it is almost impossible to independently calculate the heat exchanger, since the data necessary for this is hidden, and not everyone can get it.

Do you have any questions?

You can always get advice on billing plate, brazed, shell-and-tube heat exchangers, as well as special heat exchange equipment from our engineers for free.

We will help determine which option is best for your object, taking into account the technical characteristics and wishes.
Contact by number 8-804-333-71-04 (toll free), or write by e-mail
You can always find the most complete information about heat exchange equipment on our website.

The main condition for a stable, efficient operation of a heat exchange system is the selection of heat exchange units, taking into account exact compliance with specific operational and technical requirements. The key factor for this selection is the calculation of the area of ​​the heat exchanger.

Of course, there are certain standards, with universal parameters, according to which you can choose equipment for your facility. However, often in this area an individual approach more than justifies itself. Conducting measurements and calculations on specific data allows you to get the most out of the heat exchange system. In addition, such calculations are simply necessary when it comes to work on the terms of reference with strictly defined parameters.

The method for calculating the heat exchanger involves several stages.

Determining the amount of heat

The heat transfer equation used for steady-state units of time and processes is as follows:

Q = KFtcp (W)

In this equation:

• K is the value of the heat transfer coefficient (expressed in W/(m2/K));
• tav is the average temperature difference between different coolants (the value can be given both in degrees Celsius (0С) and in kelvins (K));
• F is the value of the surface area for which heat exchange occurs (the value is given in m2).

The equation makes it possible to describe the process during which heat is transferred between heat carriers (from hot to cold). The equation takes into account:

• heat transfer from the coolant (hot) to the wall;
• wall thermal conductivity parameters;
• heat transfer from the wall to the coolant (cold).

Determination of the heat transfer coefficient

For preliminary calculations of heat exchange equipment and various kinds of checks, approximate values ​​​​of the coefficients are used, standardized for certain categories:

• heat transfer coefficients for the process of water vapor condensation - from 4000 to 15000 W/(m2K);
• heat transfer coefficients for water moving through pipes - from 1200 to 5800 W / (m2K);
• heat transfer coefficients from vapor condensate to water - from 800 to 3500 W/(m2K).

The exact calculation of the heat transfer coefficient (K) is made using the following formula:

In this formula:

• α1 is the heat transfer coefficient for the heating fluid (expressed in W/(m2K));
• α2 is the heat transfer coefficient for the heated coolant (expressed in W/(m2K));
• δst - pipe wall thickness parameter (expressed in meters);
• λst - coefficient of thermal conductivity of the material used for the pipe (expressed in W / (m * K)).

Such a formula gives an "ideal" result, usually inconsistent with 100% of the real state of affairs. Therefore, one more parameter is added to the formula - Rzag.

This is an indicator of the thermal resistance of various contaminants that form on the heating surfaces of the pipe (i.e., ordinary scale, etc.)

The formula for the pollution index looks like this:

R = δ1/λ1 + δ2/λ2

In this formula:

• δ1 is the thickness of the deposit layer on the inner side of the pipe (in meters);
• δ2 is the thickness of the deposit layer on the outer side of the pipe (in meters);
• λ1 and λ2 are the values ​​of the thermal conductivity coefficients for the respective layers of pollution (expressed in W/(m*K)).

Method for calculating the heat exchanger (surface area)

So, we calculated parameters such as the amount of heat (Q) and the heat transfer coefficient (K). For the final calculation, the temperature difference (tav) and the heat transfer coefficient are additionally required.

The final formula for calculating a plate heat exchanger (heat transfer surface area) looks like this:

In this formula:

• the values ​​of Q and K are described above;
• the value of tav (average temperature difference) is obtained by the formula (arithmetic mean or logarithmic mean);
• heat transfer coefficients are obtained in two ways: either using empirical formulas, or through the Nusselt number (Nu) using similarity equations.