The application and analysis of plate heat exchanger in fluid system
Hydraulic and lubrication system of hydraulic pump, lubrication pump, hydraulic motors and other power’s transmission efficiency loss, fluid control components and pressure loss in the process of transmission and so on are in the form of energy into heat energy, system oil temperature rise high viscosity decreased, inside leaking increase wear components, all kinds of frequent failure. Medium plate heat exchanger Have the advantages of high thermal efficiency and small temperature difference, which has been widely applied in the temperature control of fluid transmission.
Plate heat exchanger and tube type heat exchanger has more obvious advantages, but in terms of structural strength, and pipe in the structure strength is better than that of plate heat exchanger, tubular heat exchanger leakage accident has obvious gradient feature, and is easier to found in the inspection and control in extended period, in comparison, the plate heat exchanger leakage accident Is burst strongly and more dangerous, in a short period of time can lead to large amounts of medium leakage or channeling.
Conventional large industrial cooling medium is soft water,which adopt central filtering and the certain chemical additive, the media has a certain function of remove contaminant l in the process of circulation features, adopt centralized circulation of plate heat exchanger’s leakage will cause more chain system problems, especially the leaking oil medium will be attached on the inner wall of the pipeline system and related equipment, decaying forming colloidal material after the plug valve body, pump body and so on, the remedies trival, repair cost is higher. For this type of accident control and prevention has good practical significance.
2.1 plate heat exchanger is composed of upper and lower guide beam, fixed plate, pressure plate, pipeline interface, heat transfer plate, plate of the seals, hold-down bolts and other spare parts. Such as graphic:
In hot plate according to the process to make a combination process of assembly, fixed clamp platen and activities around the hole with seal. Assembly time, so the heat exchange plate with seal side toward the platen, alternate combination of plate group formed on both sides of cold and hot flow channel. This passage through the external pipe connected to interface with the corresponding medium, two medium of heat transfer, to the expected temperature control requirements.
working principle Heat transfer plate is the key component of the heat exchanger, the plate forming process and material properties of sealing and thermal efficiency in direct effects. Heat exchanger, usually with water as cooling medium plate most made of stainless steel sheet on a plate of suppressing ripple chute, adjacent to the space between the two plates is the medium flow channel, cold and hot fluid flow on both sides of plate, and through the plate heat exchange.
Special flow, which is formed by the ripple in the fluid under the condition of low velocity turbulence (about 200) Re Renault coefficient, low coefficient of Renault's turbulence has its descaling effect, effectively destroy the thermal boundary layer, reduce the liquid film thermal resistance on the interface. In general plate heat exchanger heat transfer coefficient K value in 3000 ~ 6000 w / ㎡ ℃ range, at the same time, two kinds of medium is almost complete counter-current flow, heat transfer efficiency is higher. Under the same in thermal efficiency, plate heat exchanger need only half of the tube and shell heat exchanger area ~ 1/4 can achieve the same effect of heat transfer.
3.The combination of process
Plate heat exchanger after used 1 ~ 2 years period (according to actual working condition and decide) is necessary after overhaul, clean, down test, etc. For plate which exist problems of deformation or perforation need to change in time, in the process of the cooling plate assembly must be strictly according to the flow chart. Flow chart is designed according to the cooling process, adopt the way of parallel or serial connect each plate, common have a single process and double flow heat exchanger (or process), medium access and out of the single flow heat exchanger pipe are usually fixed clamp side, heat and cold medium media respectively in a fixed clamp vertical axis of the unilateral arrangement, the same medium at the same time, on the left or on the right side at the same time. Here is (2) the pareto diagram for a single process, pareto diagram icon (3) for the process.
Common problem analysis
4.1 staggered board piece two medium caused by short circuit or leakage.
Single process of plates from the gasket side observation, by the right flow always flowed from the right: by flow always flowed from the left on the left. To herringbone corrugated board piece, if the flow of fluid from the left, and type A herringbone pointing towards the board, the board will be A perpendicular to the axis of rotation of the panel 180 ° as type B plate, fluid from the right in and out, as shown in the graphical representation (4) :
Assumed with sealing gasket for each hole of the plate with A: a1, a2, a3, a4, blind hole for nought; B plate for b1, b2, b3, b4 blind hole is b0. Layout from one end fixed clamp plate elastic seal face toward the activity linking piece on one side, single flow heat exchanger of four hole on the fixed clamp, namely the first piece of plate four hole hole, two or more processes activities platen in heat exchanger and the fixed clamp on each have two hole, the first piece of plate and the last piece of plate corresponds to the two hole and two blind holes, board piece, in turn, staggered, debugging board will occur two short-circuit or leakage of medium, can't normal use.
4.2 plate uneven sealing dislocation caused by leakage
Plate heat exchanger after overhaul need to clamp plate as required, if in order to further improve the heat transfer ability need to add board, should fully consider the fixed clamp platen of the activity and deformation intensity, the same level of experimental stress, increase in the number of plate of prestressing force of bolt at the same time also need to increase, when the clamp on both sides of the elastic deformation beyond the scope of the license, the seals of radial sliding plane compression, form the dislocation, at this point, the seal failure, two medium leakage or internal fluid, can't normal use.
For long-term not put into use of the heat exchanger is usually appropriate loosen screw pull, after long-term compression plate and sealing washer lose the necessary flexibility, seal failure easily, reduce the service life. Heat exchanger of the hold-down bolts uniformly distributed force, need in place before the board, in each diagonal clamping, and the inside of the real-time measuring two hold-down plate distance, two compression plate in parallel condition, basic guarantee the corners of the parallel degree of deviation less than 2%, each bolt pre-tightening as shown (5) marked by order.
General, clamping to the maximum size of the given first, such as leak, found in use may continue to clamp to a given minimal size. Equipment are in a state of work under the pressure, we should stop again after pressure relief can be preloaded bolt. Tighten bolts with pressure seal extrusion or damaged, appear easily cause an accident.
4.3 in thermal efficiency degradation
For efficient heat exchange efficiency, the two kinds of medium of plate heat exchanger cycle used the complete counter-current flow, in be used actually offline form a complete set of spare parts of each hydraulic system into consideration, with large heat transfer area of exchanger as interchangeable parts reserves, medium interface direction difference, cold and hot medium swap interface or the same switch in and out of the use of medium etc, although this kind of problem won't be adverse to the seal form but can produce certain effect on the heat exchange efficiency, such as graphic (6) : upstream of two kinds of medium temperature difference is small, small heat loss, in high thermal efficiency; Downstream of the big temperature difference in the thermal efficiency is low.
4.4. The heat exchanger of environmental conditions
4.4.1 plate heat exchanger on the level of the connecting screw rod tension has strict requirements, particularly in its weight lifting large hoisting position set up must be used in high power heat exchanger, non-standard hoisting ways to structure stress of the screw has certain damage, heat exchanger plate prone to dislocation. In addition, a large plate heat exchange equipment after full of cold and hot medium, medium weight will rise sharply, the installation of the equipment must also be placed in a horizontal plane.
4.4.2 of heat exchanger heat exchange with the cold and hot medium at the same time, the formation of the leakage of the cooling plate also exist with ambient air heat exchange process, often need to reserve the necessary space around the heat exchanger (more than 1 m), to form a smooth flow of air environment, conducive to give full play to the heat transfer.
4.5 prevention of plate heat exchanger leakage
4.5.1, in the process of continuous running of heat exchanger leakage accident probability is low, usually this is because the hot and cold at the same time for the two kinds of medium pressure and differential pressure equilibrium, the temperature is controlled range, heat exchanger heat deformation of the space is lesser, dislocation of the seal or plate impact deformation of the accident possibility is very small.
The most trouble is emergency rev. Stop, stop the pump pressure shock and equipment cooling after the operation of equipment, under the condition of the heat exchanger of seal lost part due to long-term compression elastic memory function, when the temperature drops to form cold, plate gap increased. Open after the pressure and temperature rise does not have synchronicity, temperature recovery is relatively lag, plate hot shrinkage clearance between supplement is relatively slow, seal extrusion caused leakage accident emergency.
Based on the above analysis, should be open before start the pump heat exchanger of all valve and exhaust valve, inlet valve closed heat exchanger. Start the pump, slowly open the pump outlet valve, the pressure gradually increase. To prevent unilateral overpressure, into the heat exchanger of two fluid inlet valve should be open at the same time, or slowly injected fluid on the low voltage side first, and then slowly into the high side fluid, avoid high pressure impact.
Board piece of hot shrinkage clearance leakage problem, should be in the rev. Stop the pump to during thermal recovery well real-time inspection and monitoring, the necessary periodic overhaul the, pressure can be used according to specific conditions and decide, plate heat exchanger if no in place for more than six months, before use should be hydrostatic test. Experimental pressure is usually set to 1.25 times the design pressure, respectively, for cold and hot medium inside the cavity unilateral pressure maintaining 30 minutes for qualified without leakage.
4.5.2 of aging sealing gasket needs to be changed in time.
In the replacement of plate heat exchanger, plate sealing groove daub assembly adhesive curing cycle usually takes 24 hours. If there are more than half a year when not in use heat exchanger, heat exchanger clamping bolt should be relaxed 1 ~ 2 pitch, preloaded again when using, to prevent the loss of elasticity of sealing gasket, affect the service life.