Modeling and Design of Plate Heat Exchanger
Heat exchangers are devices used to transfer energy between two fluids at different temperatures.
They improve energy efficiency because the energy already within the system can be transferred to another part of the process, instead of just being pumped out and wasted. In the new era of sustainability, the growing urgency to save energy and reduce overall environ‐ mental impacts has placed greater emphasis on the use of heat exchangers with better thermal efficiency. In this new scenario, the plate heat exchanger can play an important role. A plate heat exchanger is a compact type of heat exchanger that uses a series of thin plates to transfer heat between two fluids. There are four main types of PHE: gasketed, brazed, welded, and semi-welded. The plate-and-frame or gasketed plate heat exchanger essentially consists of a pack of thin rectangular plates sealed around the edges by gaskets and held together in a frame (Figure 1). Plate heat exchangers were first introduced in 1923 for milk pasteurization applications, but are now used in many applications in the chemical, petroleum, HVAC, refrigeration, dairy, pharmaceutical, beverage, liquid food and health care sectors. This is due to the unique advantages of PHEs, such as flexible thermal design (plates can be simply added or removed to meet different heat duty or processing requirements), ease of cleaning to maintain strict hygiene conditions, good temperature control (necessary in cryogenic appli‐ cations), and better heat transfer performance. 2. Mechanical characteristics A PHE consists of a pack of thin rectangular plates with portholes, through which two fluid streams flow, where heat transfer takes place. Other components are a frame plate (fixed plate),
a pressure plate (movable plate), upper and lower bars and screws for compressing the pack of plates (Figure 2). An individual plate heat exchanger can hold up to 700 plates. When the package of plates is compressed, the holes in the corners of the plates form continuous tunnels or manifolds through which fluids pass, traversing the plate pack and exiting the equipment. The spaces between the thin heat exchanger plates form narrow channels that are alternately traversed by hot and cold fluids, and provide little resistance to heat transfer.
Thermal plates and gaskets
The most important and most expensive part of a PHE is its thermal plates, which are made of metal, metal alloy, or even special graphite materials, depending on the application. Stainless steel, titanium, nickel, aluminum, Incoloy, Hastelloy, Monel, and tantalum are some examples commonly found in industrial applications. The plates may be flat, but in most applications have corrugations that exert a strong influence on the thermal-hydraulic performance of the device. Some of the main types of plates are shown in Figure 3, although the majority of modern PHEs employ chevron plate types. The channels formed between adjacent plates impose a swirling motion to the fluids, as can be seen in Figure 4. The chevron angle is reversed in adjacent sheets so that when the plates are tightened, the corrugations provide numerous points of contact that support the equipment. The sealing of the plates is achieved by gaskets fitted at their ends. The gaskets are typically molded elastomers, selected based on their fluid compatibility and conditions of temperature and pressure. Multi-pass arrangements can be implemented, depending on the arrangement of the gaskets between the plates. Butyl or nitrile rubbers are the materials generally used in the manufacture of the gaskets.
This section presents some of the main advantages and disadvantages of a PHE, compared to shell-and-tube heat exchangers. Advantages Flexibility: Simple disassembly enables the adaptation of PHEs to new process requirements by simply adding or removing plates, or rearranging the number of passes. Moreover, the variety of patterns of plate corrugations available, together with the possibility of using combinations of them in the same PHE, means that various conformations of the unit can be tested during optimization procedures. Good temperature control: Due to the narrow channels formed between adjacent plates, only a small volume of fluid is contained in a PHE. The device therefore responds rapidly to changes in process conditions, with short lag times, so that the temperatures are readily controllable. This is important when high temperatures must be avoided. Furthermore, the shape of the channels reduces the possibility of stagnant zones (dead space) and areas of overheating. Low manufacturing cost: As the plates are only pressed (or glued) together, rather than welded, PHE production can be relatively inexpensive. Special materials may be used to manufacture the plates in order to make them more resistant to corrosion and/or chemical reactions. Efficient heat transfer: The corrugations of the plates and the small hydraulic diameter enhance the formation of turbulent flow, so that high rates of heat transfer can be obtained for the fluids. Consequently, up to 90% of the heat can be recovered, compared to only 50% in the case of shell-and-tube heat exchangers. Compactness: The high thermal effectiveness of PHEs means that they have a very small footprint. For the same area of heat transfer, PHEs can often occupy 80% less floor space (sometimes 10 times less), compared to shell-and-tube heat exchangers (Figure 5).
Reduced fouling:
Reduced fouling results from the combination of high turbulence and a short fluid residence time. The scale factors for PHEs can be up to ten times lower than for shelland-tube heat exchangers. Ease of inspection and cleaning: Since the PHE components can be separated, it is possible to clean and inspect all the parts that are exposed to fluids. This feature is essential in the food processing and pharmaceutical industries. Easy leak detection: The gaskets have vents (Figure 6) that prevent fluids from mixing in the case of a failure, which also facilitate locating leaks
Drawbacks
Temperature and pressure limitations: An important limitation of PHEs is related to the plate gaskets. Pressures and temperatures exceeding 25 atm and 160 °C, respectively, are not tolerated because they can cause the standard gaskets to leak. However, gaskets made of special materials can withstand temperatures up to 400 °C, and it is possible to weld or braze the plates to each other in order to operate under more severe conditions. This would have the additional advantages of increasing the operational limits, as well as the possibility of working with corrosive fluids, because it would eliminate the need for gaskets. However, the PHE would lose its major advantages of flexibility and ease of cleaning, and the equipment would become more expensive. High pressure drop: Because of the corrugated plates and the small flow space between them, the pressure drop due to friction is high, which increases pumping costs. The pressure drop can be reduced by increasing the number of passages per pass and splitting the flow into a greater number of channels. This diminishes the flow velocity within the channel, hence reducing the friction factor. However, the convective heat transfer coefficient is also reduced, decreasing the effectiveness of the heat exchanger. Phase change: In special cases, PHEs can be used in condensation or evaporation operations, but are not recommended for gases and vapors due to the limited space within the channels and pressure limitations. Types of fluids: The processing of fluids that are highly viscous or contain fibrous material is not recommended because of the high associated pressure drop and flow distribution prob‐ lems within the PHE. Compatibility between the fluid and the gasket material should also be considered. Highly flammable or toxic fluids must be avoided due to the possibility of leakage. Leakage: Friction between the metal plates can cause wear and the formation of small holes that are difficult to locate. As a precaution, it is advisable to pressurize the process fluid so that there is less risk of contamination in the event of leakage from a plate.
Extract from Modeling and Design of Plate Heat Exchanger
Fábio Antônio da Silva Mota
Esdras P. Carvalho
Mauro A.S.S. Ravagnani
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RispondiEliminaA Plate Type Heat Exchanger plates are simple to maintain because they can be taken out and cleaned separately. But, since they may gather debris or smell, you should avoid them.
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