UOL Project Plate Heat Exchanger
Transcript of UOL Project Plate Heat Exchanger
CCB 2092 Unit Operation Lab 1Jan 2014Group Project Plate Heat Exchanger Group:1
Group Members:1. ALI ABDULLAH SALEHAL-YAFEAI(17036)
2.AMIRA NURAIN BINTI RAMDAN(18624)
3.AZLAN BIN RAMALI (18645)
4. CHEN SWEE KEAT(18604)
Lab Instructor :Dr. Maziyar Sabet
Date of experiment:6th April 2014
INTRODUCTIONA plate heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two fluids. This has a major advantage over a conventional heat exchanger in that the fluids are exposed to a much larger surface area because the fluids spread out over the plates. This facilitates the transfer of heat, and greatly increases the speed of the temperature change. Plate heat exchangers are now common and very small brazed versions are used in the hot-water sections of millions of combination boilers. The high heat transfer efficiency for such a small physical size has increased the domestic hot water (DHW) flowrate of combination boilers. The small plate heat exchanger has made a great impact in domestic heating and hot-water. Larger commercial versions use gaskets between the plates, smaller version tend to be brazed.The plate heat exchanger (PHE) is a specialized design well suited to transferring heat between medium- and low-pressure fluids. Welded, semi-welded and brazed heat exchangers are used for heat exchange between high-pressure fluids or where a more compact product is required. In place of a pipe passing through a chamber, there are instead two alternating chambers, usually thin in depth, separated at their largest surface by a corrugated metal plate. The plates used in a plate and frame heat exchanger are obtained by one piece pressing of metal plates. Stainless steel is a commonly used metal for the plates because of its ability to withstand high temperatures, its strength, and its corrosion resistance. The plates are often spaced by rubber sealing gaskets which are cemented into a section around the edge of the plates. The plates are pressed to form troughs at right angles to the direction of flow of the liquid which runs through the channels in the heat exchanger. These troughs are arranged so that they interlink with the other plates which forms the channel with gaps of 1.31.5 mm between the plates.
Figure 1: Schematic conceptual diagram of a plate heat exchanger
The plates produce an extremely large surface area, which allows for the fastest possible transfer. Making each chamber thin ensures that the majority of the volume of the liquid contacts the plate, again aiding exchange. The troughs also create and maintain a turbulent flow in the liquid to maximize heat transfer in the exchanger. A high degree of turbulence can be obtained at low flow rates and high heat transfer coefficient can then be achieved.A plate heat exchanger consists of a series of thin, corrugated plates which are mentioned above. These plates are gasketed, welded or brazed together depending on the application of the heat exchanger. The plates are compressed together in a rigid frame to form an arrangement of parallel flow channels with alternating hot and cold fluids.
Figure 2: An individual plate for a heat exchangerAs compared to shell and tube heat exchangers, the temperature approach in a plate heat exchangers may be as low as 1 C whereas shell and tube heat exchangers require an approach of 5 C or more. For the same amount of heat exchanged, the size of the plate heat exchanger is smaller, because of the large heat transfer area afforded by the plates (the large area through which heat can travel). Increase and reduction of the heat transfer area is simple in a plate heat-exchanger, through the addition or removal of plates from the stack.All plate heat exchangers look similar on the outside. The difference lies on the inside, in the details of the plate design and the sealing technologies used. Hence, when evaluating a plate heat exchanger, it is very important not only to explore the details of the product being supplied, but also to analyze the level of research and development carried out by the manufacturer and the post-commissioning service and spare parts availability.A very important aspect to take into account when evaluating a heat exchanger are the forms of corrugation within the heat exchanger itself. There are two types of corrugations; intermating and chevron corrugations. In general, greater heat transfer enhancement is produced from chevrons for a given increase in pressure drop and is more commonly used than intermating corrugations.
AIMThroughout this group project, our objectives are to :- Understand how a plate heat exchanger works basically Compare the advantages of using a plate heat exchanger with other types of heat exchanger such as shell and tube heat exchanger and double pipe heat exchanger Able to give example of applications of plate heat exchanger in modern industries Study about the design of a plate heat exchanger and methods for calculating the area of heat transfer by using the NTU and LMTD methods Suggest ways to improve the performance of a plate heat exchanger
ADVANTAGESAs an alternative to the conventional method of heat transfer, the plate heat exchanger has many advantages over the other types of heat exchangers. The key features are as follows:- CompactnessThe units in a plate heat exchanger occupy less floor space and floor loading by having a large surface area that is formed from a small volume. This in turn produces a high overall heat transfer coefficient due to the heat transfer associated with the narrow passages and corrugated surfaces.
FlexibilityChanges can be made to heat exchanger performance by utilizing a wide range of fluids and conditions that can be modified to adapt to the various design specifications. These specifications can be matched with different plate corrugations. Low Fabrication CostsWelded plates are relatively more expensive than pressed plates. Plate heat exchangers are made from pressed plates, which allow greater resistance to corrosion and chemical reactions. Ease of CleaningThe heat exchanger can be easily dismantled for inspection and cleaning (especially in food processing) and the plates are also easily replaceable as they can be removed and replaced individually.
Temperature ControlThe plate heat exchanger can operate with relatively small temperature differences. This is an advantage when high temperatures must be avoided. Local overheating and possibility of stagnant zones can also be reduced by the form of the flow passage. ExpandableA significant benefit of the plate heat exchanger is that it is expandable, allowing anincrease in heat transfer capability.As your heat transfer requirements change, you cansimply addplates instead of buying an entire new frame unit, saving time and money. High EfficiencyThe pressed plate patterns and narrow gaps allow for very high turbulence at relatively low fluid velocity. Combined with counter directional flow will results in very high heat transfer coefficients. Close Approach TemperatureThe same features that give the plate heat exchanger its high efficiency also makes it possible to reach a close approach temperatures which is particularly important in heat recovery and regeneration applications. Approach temperatures of 1F are possible. Multiple Duties In a Single UnitThe plate heat exchanger can be built in sections, separated with simple divider plates or more complicated divider frames with additional connections. This makes it possible to heat, regenerate, and cool a fluid in one heat exchanger or heat or cool multiple fluids with the same cooling or heating source. Avoid Cross ContaminationEach medium is individually gasketed and as the space between the gaskets is vented to the atmosphere, cross contamination of fluids is eliminated. Less FoulingVery high turbulence is achieved as a result of the pattern of the plates, the many contact points, and the narrow gap between the plates. This combined with the smooth plate surface reduces fouling considerably compared to other types of heat exchangers. Lower CostHigh heat transfer coefficients mean less heat transfer area and smaller heat exchangers, and sometimes even less heat exchanger. This and less space requirements will reduced flow rates and smaller pumps means.
APPLICATIONSPlate heat exchangers offer the highest efficiency mechanism for heat transfer available in industry today. Many industries prefer to use plate heat exchangers compared to other types of heat exchangers because of its wide variety of applications. Below are some of the examples of applications of a plate heat exchanger in industries :- Food and BeveragesMilk or cream pasteurization, syrup pasteurization, juice pasteurization, milk reception, nectar pasteurization, cultured milk treatment (ice cream or cheese treatment), sugar dissolving and pasteurization of concentrates. Petroleum/Chemical ProcessingBrine cooling, heating/cooling of corrosive fluids, sea water coolers, crude oil cooling/interchanging, lean/rich fluid interchanger & cooler, isobutene condenser & reactor interchanger, crude oil heat treatment, acid gas condenser and treated/untreated crude interchanger. Hydrocarbon ProcessingSodium hydroxide cooling, propylene condensers & coolers, naphtha preheating, methanol preheating, formalin cooling and liquid product cooling. PolymersNylon salt cooling,glycol cooling, solvent heating, polyol product cooling, pellet, refrigerated matter cooling, heating/cooling isocyanate, heating/cooling of naoh, heating/cooling viscose & acids and heating of spin bath solution. PharmaceuticalsProduct heating/cooling, heating jacketed reactor, cooling water systems, condensers & interchangers and hot water systems. Energy and PowerAuxiliary cooling circuit isolation, co-generation applications, geothermal applications, lubrication oil cooling, diesel engine cooling and heat recovery. MarineSeawater isolation exchanger, central cooling, jacket fresh water cooling, lube oil cooling, camshaft lube and oil cooling.
Other than chemical industries, plate heat exchangers are widely used in automobile, aerospace, and cryogenic due to high effectiveness, compactness (high surface area density), low weight and affordable cost. Heat exchanger design is one of the problem that are commonly faced by engineers. The fluid inlet temperatures and flow rates, as well as a desired hot or cold fluid outlet temperature, are described in the heat exchanger design problem. Normally, design problem is faced when the heat exchanger is custom-built for a specific application. Unlike heat exchanger performance calculation, heat exchanger design problem is responsible in specifying a specific heat exchanger type, and determining the size or area of the heat exchanger that are essential to achieve the desired outlet temperature. Meanwhile, performance calculation analyzed the existing heat exchanger in order to determine the heat transfer rate and fluid outlet temperatures, for prescribed flow rates and inlet temperatures.The plate component of plate heat exchanger are described in the Figure below :Figure 3: Plate heat exchanger components
Plate heat exchanger is also called a plate and frame heat exchanger. A plate heat exchanger is a form of compact heat exchanger consisting of layers of plates which are gasketed or copper, Cu brazed sandwiched between two frames. One frame is fixed, and the other one is adjustable. The adjustable frame allows the number of plate to be modified, either increased or decreased, depends on the usage of the heat exchanger itself. This criteria made plate heat exchanger as a compact heat exchanger because the number of plates are changeable, without the need of buying a new heat exchanger. The plates are commonlymade of aluminum or stainless steel, due to the reason of low weight and do not rust easily. While the frames are made of iron.Hence, it can be said that the compact construction of the plate heat exchangerallows for an expansion in capacity without difficulty. Meanwhile, on the plate itself, it has an enhanced surface called fins. There are various types and sizes of fins, either tubular or plate in shape. However, the main purpose of fins is to increase the surface area of the heat exchanger. According to the equation , it is obviously stated that area, A is directly proportional to heat transfer rate, Q. Therefore, with the presence of fins, the surface area of heat exchanger is increased, and the heat transfer will be increased.Plate heat exchanger is made of a stack of corrugated fins alternating with nearly equal numbers of flat separators known as parting sheets, bonded together to form a monolithic block. Appropriate headers are welded to provide the necessary interface with the inlet and the exit fluid streams. The schematic view of such an exchanger is given in Figure below. The corrugations serve both as secondary heat transfer surface and as mechanical support against the internal pressure between layers.
Figure 4: Plate fin heat exchanger assembly and details of Side bars, Plates or Parting Sheets, Fins, Fluid 1, Fluid 2, Cap Sheet HeaderIn addition, cost is one of the most crucial aspect for the heat exchanger design. Few factors need to be taken into consideration to design the heat exchanger, so that the cost required is affordable. The important factors to minimize the heat exchanger cost are Pressure drop and Log Mean Temperature Difference, LMTD. The higher pressure drop the smaller the heat exchanger and the higherthe temperature difference, the smaller the heat exchanger will be.For heat exchanger design problem, basically there are two ways to calculate the area, A. First is by using the NTU (Number of Transfer Units) method. Few calculations are needed to find the heat exchanger surface area, as explained below : When the outlet and inlet temperatures are known, calculate .
Calculate the capacity ratio Cr = Cmin/Cmax Calculate the overall heat transfer coefficient, U. When and Cr and the flow arrangement are known, determine NTU by using appropriate -NTU equations. When NTU is known, calculate the total heat transfer surface area, A.
For this method, in order to find NTU, different -NTU equation is used for different flow. However, if , regardless of the flow pattern, the -NTU equation used is :
This equation shows that the heat exchanger behavior is independent on flow arrangement. Besides that, Area can also be calculated by using the LMTD method. The steps are explained below : Calculate Q and the unknown outlet temperature by using the equation and
Calculate LMTD and obtain the correction factor (F) if necessary. Calculate the overall heat transfer coefficient, U using the equation Determine Area.
PERFORMANCETo maximize the performance of a Plate Heat Exchanger means saving money, especially if the process is built for a long term project. Here are some ways to improve the performance of a heat exchanger: Heat transfer area Fluid flow velocity Temperature gradientThese suggested ways of improvements are based on the equation for heat transfer rate of a heat exchanger, which is:
Where,Q: Heat transfer rate between the fluidsU: Overall heat transfer coefficientA: Heat transfer area: Logarithmic mean temperature difference of the system
Heat Transfer Area
As the equation shown above, the heat transfer area (or contact area) is directly proportional to the heat transfer rate. If the heat transfer area increases, heat transfer rate increases as well.A common way to increase heat transfer area is addingfins to the surface. It is cheap to put fins to the heat transfer area but fins also increase fouling, especially in bio-process.
Overall Heat Transfer Coefficient
Overall EfficiencyIn order to predict or design the performance of a plate heat exchanger, it is essential to determine the heat lost to the surrounding for the analyzed configuration. A parameter can be defined to quantify the percentage gains or losses. Such parameter may be obtained readily by applying the concept of overall energy balances for hot and cold fluids.
Where, : Heat power emitted from hot fluid. : Heat power absorbed by cold fluid. : Mass flow rate of hot and cold fluids, respectively., : Specific heats of hot and cold fluids, respectively. : Inlet and outlet temperatures of hot fluid, respectively. : Inlet and outlet temperatures of cold fluid, respectively.
and should be equal if the plate heat exchanger is well insulated. In practice, these will not be the same due to heat gains or losses from/to the environment.
The above formulae were deducted taking into account that hot fluid is being surrounded by cold fluid. If the average cold fluid temperature is below the atmospheric temperature, heat will be gained, resulting > 100%. However, if the average cold fluid temperature is above the atmospheric temperature, then heat will be lost to the surroundings thus resulting < 100%.
Temperature efficiency of each fluid stream is a very useful measure of the heat exchanger performance. The temperature change in each fluid stream is being compared with the maximum temperature difference between two fluid streams giving a comparison with a heat exchanger of infinite size.
Figure 5: Co-current and counter-current operation for a Plate Heat ExchangerWhere,
Plate Film Coefficient
Where,plate film coefficient
Where,mass flow rate per unit cross sectional area = cross-sectional area for flowlengthfluid specific heat, heat capacity
Overall heat transfer coefficient UDue to that the temperature difference between the hot and cold fluid streams varies along the length of the heat exchanger, it is absolutely essential to derive an average temperature difference from which heat transfer calculations can be performed. This average temperature difference is known as Logarithmic Mean Temperature Difference (LMTD), Calculation of LMTD in co-current flow:
Calculation of LMTD in counter-current flow:
Hence, we can now define an overall heat transfer coefficient U as:
Where, : Heat power emitted from hot fluidA: Heat transmission areaTemperature GradientTemperature gradient is certainly a important part of heat transfer. It is the driving force for heat transfer. If we can introduce fluids with greater temperature difference into the heat exchanger, the heat transfer rate (Q) will be greater. If we go back to the temperature profiles of the co-current and counter-current flow as shown in Diagram 1.1, we can see that the driving force is great for co-current at the beginning but decreases drastically as it moves along the heat exchanger. The counter-current flow provides relatively consistent driving force and therefore performs better than co-current flow.
CONCLUSIONPlate heat exchangers are widely used in warming, heating, cooling applications food and cosmetics and chemistry. The plate heat exchanger is widely recognized today as the most economical and efficient type of heat exchangers on the market, and that could be noticed through the advantages of the plate heat exchanger such as compactness, ease of cleaning where its plates are easy to remove and replaced which make it more expandable as we can add more plates instead of buying an entire new frame. Those advantages make it more useful in many fields such as food and beverages, petroleum/chemical processing, polymers and pharmaceuticals. Plate fin heat exchangers, because of their compactness, low weight and high effectiveness are widely used in aerospace and cryogenic applications. This device is made of a stack of corrugated fins alternating with nearly equal number of flat separators known as parting sheets, bonded together to form a monolithic block. Appropriate headers are welded to provide the necessary interface with the inlet and the exit streams. While aluminum is the most commonly used material, stainless steel construction is employed in high pressure and high temperature applications.
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