Heat-exchange equipment
1. General information about heat exchangers
Heat-transfer apparatus, heat exchangers, are called apparatus for transferring heat from a heated coolant to another less heated coolant. Heat exchangers as independent units or parts of other apparatuses and machines are widely used in chemical plants, because the carrying out of technological processes in most cases is accompanied by heat release or heat absorption.
For long-term performance in the process of operation, when processing media contaminated or emitting deposits on the walls of the apparatus, periodic inspections and cleaning of surfaces must be carried out.
Apparatus should have sufficient strength and have small overall dimensions. In the process of construction, it is necessary to find optimal solutions that take into account the requirements for ensuring the possibility of dismantling the working part of the apparatus and the tightness of the channel system, possibly high heat transfer coefficients due to an increase in speed of the working medium with minimal hydraulic losses in the apparatus.
In chemical plants, up to 70% of heat exchangers are used for liquid-liquid and vapor-liquid media at pressure up to 1 MPa and temperature up to 200 ° C. For these conditions, general-purpose heat exchangers of shell-and-tube and spiral types have been developed and mass-produced. Recently, plate heat exchangers of general purpose have become widespread. One of the advantages of tubular heat exchangers is the simplicity of the сonstruction. The specific metal capacity of shell-and-tube apparatus is 2-3 times more than the metal capacity of new plate apparatus.
In the process of construction, the question of the direction of the heat carriers to the piped or intertubular space should be reasonably resolved. For example, heat carriers, contaminated and under pressure, are usually directed to the tube space. Saturated steam is better to supply to the intertubular space, from which it is easier to remove condensate. Cleaning of the pipe space (in which the contamination is most likely to fall out) is easier, and the living cross section for the passage of the coolant is smaller. Consequently, in the tube space, it is possible to provide the heat carrier with higher speeds and, consequently, higher heat transfer coefficients.
2.1. Ribbed tubular heat exchangers
Ribbed tubular heat exchangers (Fig.) are used to increase the heat exchange surface by ribbing from the side that is characterized by the greatest thermal resistances. Ribbed heat exchangers (air heaters) are used, for example, when heated by steam or water with air or gases. An important condition for the effective use of ribs is their close contact with the main pipe (without air gap), as well as the rational placement of ribs. In addition to compactness and low weight, these heat exchangers have other advantages. The constructions allow the use of a multi-pass flow of the heat transfer medium in a single-pass heat exchanger.
Ribbed tubular heat exchangers are most effective in conditions where the heat transfer coefficients on both sides of the wall vary considerably.
Ribbed tubular heat exchangers are most effective in conditions where the heat transfer coefficients on both sides of the wall vary considerably (air cooler) the coefficient of heat transfer from the hot air to the wall does not exceed 100 W / (m? • K), while from the wall to the cooling water it is 1000-3000 W / (m? •TO).
Improving of the heat transfer conditions is achieved by artificially increasing of the surface of heat transfer by placing plates on the tube or by making ribs, protrusions or needles that are monolithic with the tube body on the side of the wall where the heat transfer coefficient is small (Fig.).
At present, bimetallic tubes (heat exchanging tubes) with finning are widely used (Fig.). On the inner smooth steel (brass) tube is put the tube made of aluminum alloy, then on its outer surface the ribs are rolled. The rolling of the fins increases the heat exchange surface and enhances the contact between the tube surfaces.
Ribbed heat exchangers are widely used in heating systems (air heaters), turbogenerators (gas coolers) and hydrogenerators (air coolers), electric motors, exciters, etc.
2.2. Shell-and-tube heat exchangers
They are fairly easy to manufacture, characterized by the possibility to develop a large heat transfer surface in one unit, reliable in operation.
Shell-and-tube heat exchangers with fixed tube grilles and with transverse divide walls in the intertubular space, used in the chemical, oil and other industries, are designated by indices and classified:
• by designation (the first letter of the index): T - heat exchangers; X - refrigerators; K - capacitors; И - evaporators;
• by construction (the second letter of the index) - Н - with fixed tube grids; K - with a temperature compensator on the housing; П - with a floating head; U - with U-shaped pipes; PC - with floating head and
compensator on it;
• by location (the third letter of the index): Г - horizontal; B - vertical.
2.2.1. Heat exchangers with fixed tube grids.
Heat exchangers are designed for heating and cooling, and refrigerators for cooling (by water or other by non-toxic, non-flammable and non-explosive refrigerant) of liquid and gaseous media. Heat exchangers and refrigerators can be installed horizontally or vertically, be one-, two-, four- and six-way over the pipe space. Pipes, casing and other elements of a construction can be made of carbonaceous or stainless steel, and pipes of refrigerators - also from a brass. The distribution chambers and refrigerant covers are made of carbon steel. The diagram of a heat exchanger with fixed tube grids is shown in Fig. 1. A tube bundle is disposed in the casing 1, the heat exchange tubes 2 of which are expanded in tube sheets 3. The tube grid is rigidly connected to the casing. From the ends the housing of the device is closed by the distribution boxes 4 and 5. The casing and chambers are connected by flanges.
For the supply and removal of working media (coolants), the device is equipped with fittings. One of the coolants in these apparatuses moves along the tubes, the other in the intertubular space, bounded by the casing and the outer surface of the pipes.
The peculiarity of the type H apparatus is that the pipes are rigidly connected to the tube grids, and the grids are welded to the casing. In this connection, the possibility of mutual movement of pipes and casing is excluded; therefore devices of this type are also called heat exchangers of rigid construction.
2.2.2. Heat exchangers with temperature compensator on the casing
If the temperature stresses occurring in the walls of the heat exchanger or tubes are high, then it is necessary to provide temperature compensation.
For round elements of apparatuses which diameter exceeds 100 mm, lens compensators consisting of one or more lenses are usually used. The lenses are made of stamped or of a ring torus, made with a slot, cut or welded in a wave form. One lens compensates small temperature deformations (4-5 mm), a set of lenses (no more than four) allows to compensate deformations up to 15 mm.
Lens compensators are used in vertical and horizontal apparatuses and pipelines at an excess pressure at most 1.6 MPa.
2.2.3.Floating head heat exchangers
In Fig. 3 shows a shell-and-tube heat exchanger with a floating head for cooling (heating) liquid or gaseous media without changing the aggregate state. The second tube grid, which is not fixed to the casing, together with the inner lid separating the tube space from the intertube, forms a so-called floating head. This design excludes the temperature stresses in the casing and in the pipes. These heat exchangers, normalized in accordance to GOST 14246-79, can be two- or four-way, horizontal length of 3, 6 and 9 m or vertical height of 3 m.
Shell-and-tube condensers with a floating head (GOST 14247-79) differ from similar heat exchangers with a large diameter of the screw for the supply of steam into the shell side. The permissible pressure of the cooling medium in the pipes is up to 1.0 MPa, in the intertube space - from 1.0 to 2.5 MPa. These devices can be two-, four- and six-way over the pipe space. The diameter of the casing is from 600 to 1400 mm, the height of the pipes is 6.0 m.
2.2.4. Heat exchangers with U- shaped tubes
Heat exchangers with U-shaped tubes (type У). In shell and tube apparatuses of this design, a free extension of the tubes is ensured, which excludes the possibility of occurrence of temperature stresses.
Such apparatuses (Figure 4) consist of a casing 2 and a tube bundle having one tube grid 3 and U-shaped tubes 1. The tube grid together with the distribution chamber 4 is fixed to the apparatus casing on the flange.
To ensure separate input and output of the heat carrier circulating through the pipes, a divide wall 5 is provided in the distribution chamber.
Heat exchangers of type У are two-way in the pipe space and one- or two-way in the intertubular space. In the latter case, the apparatus has a longitudinal partition, which is removed from the housing together with the tube bundle. To prevent the flow of coolant into the gaps between the casing of the apparatus and the partition, flexible metal plates or a gasket of rubberized asbestos cord are installed near the wall of the casing, the gasket is laid in the groove of the partition.
2.2.5. Heat exchangers with gaskets
At significantly higher pressures in the heat exchange equipment, sliding expansion joints are used. However, the sliding expansion joints can pass the working medium, which requires their periodic adjustment, and therefore the sliding expansion joints are used for apparatus with small diameters. The sliding expansion joints (fig.5) are made with soft packing made of nonmetallic materials in the form of a cord or rings of the corresponding profile, as well as in standard gland made of leather, rubber, plastic and other materials.
In Fig. 6 is shown the ring seal of the tube grid 4. The sealing is provided by the gland rings 1 located on both sides of the drain ring 3 and pressed by flanges 2, 5. In case of the leakage through the stuffing box, the coolant is brought out of the apparatus through the hole in the drain ring. The tube grid in the apparatus of such a design should be wide enough on the periphery for the possibility of locating the gasket and the drainage rings, taking into account the movement of the grid during the elongation of the pipes.
2.2.6. Spiral heat exchangers
These apparatus are suitable for the implementation of heat exchange between media at a pressure of up to 1 MPa and at a temperature drop of up to 200 ° C.
The main working part of such device is a heat exchange element (ТОЭ), consisting of a bundle of flexible polymer pipes, the ends of which are connected by welding to a manifold (fluoroplastic grid). The latter serves to secure the ТОЭ in the body of the apparatus and the supply of working medium to it.
Fluoroplastic heat exchangers are used in the production of sulfuric acid, organochlorine products, medical preparations.
2.3. Tubular heat exchangers without casing
2.3.1.Submersible heat exchangers (oil coolers)
Oil coolers are designed to cool the oil circulating in the oil supply system of hydro generators, steam turbines and process fluids in the systems of different hydraulic drives, hydraulic oil turbine units, transformer and compressor plants, etc. Oil coolers are heat exchangers pumped by water. They maintain the preset temperature of the lubricating oil, what is necessary to maintain the set lubricant pressure. By design, they are straight, segment and U-shaped.
Oil coolers take away the heat received by oil in bearings, gear drives and other elements. Cooling the oil is done by immersing the oil cooler in the oil bath. On the water side, oil coolers are usually performed in multi-way, here this is achieved by changing the number of partitions in the covers.
2.3.2. Spiral submersible heat exchangers
The main heat exchange element is a tubing coil, a pipe bent along a specific profile.
The coil is immersed in the liquid, which is in the body of the apparatus. The speed of movement is small due to the large section of the body of the apparatus, which causes low values of heat transfer coefficients from the outer wall of the coil to the liquid (or vice versa). To increase this coefficient of heat transfer, the speed of the fluid is increased by installing it in the body of the apparatus, inside the coil, and the glass. In this case, the liquid moves along the annular space between the walls of the apparatus and the glass at an increased speed. Often, in coil heat exchangers, coils are installed from straight pipes connected by U-tubes.
Due to the simplicity of the construction, the low cost, availability, external walls of the coil for cleaning and inspection, the possibility of operating coils at high pressures, these heat exchangers are widely used in industry. Submersible coil heat exchangers have a relatively small heat exchange surface (up to 10-15m2).
2.4. Heat exchangers with with direct heat transfer
In MIXING (contact) heat exchangers, heat exchange takes place with direct contact of heat transfer media. The mixing heat exchangers are, for example, cooling towers.
2.5. Apparatuses for heat exchange with external heating
HEAT EXCHANGERS "PIPE IN PIPE" are mainly used for cooling or heating in a liquid-liquid system, when the coolant costs are low and the latter do not change their aggregate state. Sometimes such heat exchangers are used at high pressure for liquid and gaseous media, for example, as capacitors in the production of methanol, ammonia, etc.
Double-tube heat exchangers according to GOST 9930-78 manufactured with the heat exchange surface area of 0.5 to 93 m2. Apparatuses are a set of serially connected elements consisting of concentrically arranged pipes (Figure 6).
One coolant moves along the inner pipes 1, the other - along the annular ring gap between the inner and outer 2 pipes. The internal pipes 1 are connected by means of the rollers 5 and the external ones by means of connecting pipes 3. The length of the element of the heat exchanger of the pipe-in-pipe type is usually 3-6 m, the diameter of the outer tube is 76-159 mm, the inner tube is 57 -108 mm.
2.6. Regenerative heat-exchange apparatus
In regenerative heat exchangers, the process of the heat transfer from a hot coolant to a cold coolant is divided in time into two periods and occurs with alternating heating and cooling of the nozzle. Heat exchangers of this type are often used to regenerate the heat of waste gases.
Characteristic for regenerative heat exchangers is the presence of solids, which alternately contact hot and cold coolants. When in contact with the hot coolant, the solid is heated; coming in contact with a cold coolant, it gives it its heat.
There are continuously operating and periodically acting
regenerative heat exchangers. Continuously operating
regenerative heat exchangers are heating installations with a circulating granular material.
The regenerative heat exchanger of batch cooling for air cooling is shown in Fig. 7.
2.7. Sheet heat-exchange apparatus
2.7.1. Spiral heat exchangers
SPIRAL HEAT EXCHANGERS are manufactured with a heat exchange surface of 10-100 m2; they work both under vacuum and at a pressure of up to 1 MPa at a working medium temperature of 20-200 ° C. They can be used to realize heat exchange between liquid-liquid working media, gas-gas, gas-liquid, as well as condensation of vapors and vapor-gas mixtures.
The increasing prevalence of these heat exchangers has recently been explained mainly by the ease of manufacture and compact construction. In such apparatus one of the heat carriers enters the peripheral channel of the apparatus and, moving along a spiral, leaves the upper central channel. Another coolant enters the lower central channel and leaves the peripheral channel.
The cross-sectional area of the channels in this heat exchanger is constant throughout the length, so it can work with contaminated liquids (the contamination is washed off by the flow of the coolant).
2.7.2. Demountable plate heat exchangers
PLATE HEAT EXCHANGERS are apparatuses whose heat exchange surface is formed by a set of thin stamped plates with a corrugated surface. They are divided by the degree of accessibility of the heat exchange surface for mechanical cleaning and inspection for demountable, semi-dismountable and undismountable (welded).
The most widely used dismountable plate heat exchangers, in which the plates are separated from each other by gaskets. Installation and dismantling of these devices are carried out quickly enough, cleaning the heat exchange surfaces requires little labor input.
The main dimensions and parameters of the most widely used plate heat exchangers in the industry are defined by GOST 15518-83. They are manufactured with a heat exchange surface from 2 to 600 m2, depending on the size of the plates; these heat exchangers are used at pressures up to 1.6 MPa and working medium temperatures from -30 to + 180 ° C to realize heat exchange between liquids and vapors (gases) as refrigerators, heaters and condensers.
2.8. Air-cooled heat exchangers
APPARATUS OF AIR COOLING. In the chemical and especially petrochemical industry, most of the heat exchangers are condensers and refrigerators. The use for condensation and cooling of various technological products of water cooling apparatus, shell-and-tube or irrigative apparatus, is associated with significant water costs and, consequently, with high operating costs. The use of air cooling units as refrigerators-condensers has a number of advantages:
• costs for preparation and pumping of water are excluded;
• labor intensity and cost of repair work are reduced; no special cleaning is required for the external surface of the pipes, which are streamlined by the air flow; labor intensity and cost of repair work are reduced; no
special cleaning is required for the external surface of the pipes, which are streamlined by the air flow;
• easier regulation of the cooling process, etc.
The horizontal air-cooling apparatus (Fig. 9) is equipped with a welded frame 1 on which a number of heat-exchange sections 2 are arranged. They consist of a bundle of cross-finned tubes in which the condensed (cooled) medium is pumped. At the bottom to the frame are attached a diffuser 3 and a collector 6, in the center of which there is an axial fan 5. The fan together with the angular gear 9 and the electric motor 7 are mounted on a separate frame 8. The air pumped by the fan passes through the heat exchange sections, washing the outer surface of the finned tubes and providing the condensation and cooling of the medium passing through the tubes.
2.9.Block heat exchangers.
BLOCK HEAT EXCHANGERS are mainly made of artificial graphite or graphite plastics - plastics based on phenol-formaldehyde resin, in which fine-grained graphite is used as the filler. Apparatuses have a number of valuable properties: they are effective, because graphite is four times more efficient in heat conductivity than corrosion-resistant steel; they have a high resistance to aggressive media (acids, alkalis, organic and inorganic solvents); relatively cheap. The main method of joining details based on graphite is bonding with artificial resins.
Heat exchangers of this type (Figure 10) are manufactured from separate compressed blocks 1, connected together by a special putty. The blocks have horizontal and vertical channels for the passage of coolants. The units for connecting blocks can also be sealed with gaskets made of thermo- and corrosion-resistant rubber or PTFE. The unit has distribution chambers 2 fastened with blocks and with each other with lids 10 and buckles 7.