How to Clamp the Plate Heat Exchanger after Disassembly and Repair?
The cooling medium commonly used by major industries is soft water, which has been collectively filtered and added with chemical additives at suitable ratio and has the function of descaling in the cyclic process. A large deal of leakage in plate heat exchanger used in centralized cycle will lead to many system chain reactions--especially the oil medium will attach to the pipeline and the interior wall of the related equipment, form gelatinoid and block the valve and the pump, with cumbersome remedial measures and high repair costs. Thus, the control and prevention of such accidents have practical implications
The heat transfer plate is the core component of heat exchanger, the sealing and heat transfer efficiency of which is directly affected by the forming process and the material characteristics. The heat exchanger usually uses water as cooling medium, uses stainless steel sheets as plates with corrugated channels pressed on and the space between the adjacent two plates is the flow channel. The cooling and heating fluids flow on the both sides of the plate and exchange heat through it.
The special channels formed by the corrugations make the fluid generate turbulence at a very low flow rate (Reynolds number, Re, is about 200). The turbulence under the low Reynolds number has self-descaling effect, destroying the thermal-protective boundary coating effectively and reducing the thermal resistance of the liquid film on the interface. Generally, the heat transfer coefficient (K value) of the plate heat exchanger is in the range of 3 000-6 000 W/(m2·K）and meanwhile the two kinds of medium nearly flow in opposite directions with high heat conduction efficiency. Under the same heat exchange efficiency, the plate heat exchanger can achieve the same heat transfer effect only with 1/4-1/2 of the area of the shell-and-tube heat exchanger.
It is necessary of overhaul, wash and pressure test when the plate heat exchanger runs for a period of 1~2 years (according to the actual operating conditions). Any problematic plate, such as the deformed or performed one, needs to be replaced in time and in this process the cooling plates must be assembled strictly according to the drawing which is designed according to the cooling process—connect the plates in parallel or in series. The common types include single-pass and double-pass (or multi-pass) heat exchanger. For the former, the inlet and outlet orifices are on one side of the pressing plate, the cooling and heating mediums are arranged respectively on the both sides of the vertical axis of the pressing plate, with the same medium on the same side.
The staggered plates lead to short circuit or leakage of two mediums. From the perspective of the gasket, the fluid flowing in from the right side always flows out from the right and so does the fluid flowing from the left side. For the herringbone corrugated plate, if the fluid flows in from the left, rotate the Plate A (the herringbone points upward) 180 degrees along the axis perpendicular to the plate and make it become Plate B, then the fluid will flow out from the right side.
After the overhaul of the plate heat exchanger, it is necessary to clamp the plates as required. When adding plates in order to further improve the heat transfer capacity, the deformation strength of the fixed and movable plate should be fully taken into account. Using the same level of experimental pressure, the preload force of the bolts also needs to be increased with the number of plates increasing. When the elastic deformation of the plates on both sides exceeds the range of permission, the plane compression of the sealing element slides in radial direction, creates dislocation and leads to the sealing failure---outside leakage or inside fluid channeling, then the exchanger cannot work properly.
For the heat exchanger that has not been used for a long time, in general, the screw should be loosened to lower the tension. After being compressed for a long time the plates and gaskets lose the necessary elasticity, the sealing is easy to fail and the service life decreases. The hold-down bolts of the heat exchanger are uniformly forced. Before the assembly, the plates should be clamped diagonally and the inner distance between two compressing plates should be measured in real-time, ensuring that the plates are basically parallel to each other with the parallel misalignment of less than 2%. Preload each bolt according to the order as illustrated.
In order to make heat transfer performance effective, the circulation mode of the two kinds of medium in the plate heat exchanger adopts complete countercurrent flow. In practical use, consider the matching of the off-line spare parts in hydraulic pressure system, the exchanger with larger heat transfer area is used as common parts, which have different connection directions. The connections of the cooling and the heating medium or the inlet and the outlet of the same medium interchange with each other, which may not have negative effect on the sealing performance but will have a certain impact on heat exchange efficiency. As it is shown, in the countercurrent the temperature difference between two medium and the heat loss is smaller while in the concurrent the temperature difference is high and the heat transfer efficiency is low.
(1) The plate heat exchanger has strict requirements on the horizontal pull of the connecting screw. Especially to hoist the high-power heat exchanger with heavy weight must use the specialized hoisting position, otherwise the unconventional hoisting method does certain damage to the forced structure of the screw and the plates prone to dislocation. In addition, when the large-scale plate heat exchanger is filled up with the cooling and heating medium, its weight will rise sharply, so its installation should also be placed on the horizontal plane.
(2) When the heat is transferred between the cooling and heating medium, there is also heat exchange between the exposed heat transfer plate and the ambient air. Thus, there should be the necessary space (over 1m) set aside around the heat exchanger for smooth air circulation, conducive to bring heat transfer capacity into full play.