Heat Exchanger | Water | Steam | Gas | Liquid | Tube | Brazed Plate | Plate Frame
Introduction. Heat exchangers are devices used to transfer energy between two fluids at different temperatures. They improve energy efficiency, because the. With a plate type heat exchanger, the heat penetrates Temperature program. Heat load The temperature program is shown in the diagram . Heat transfer coefficient and design margin. The total .. Up-to-date Alfa Laval contact details. Heat Exchanger Sizing PROJECT DATA. Job Number: Client: Equipment Number: Date: Would you like to change how the exchanger is specified?.
Modeling and Design of Plate Heat Exchanger
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 requirementsease of cleaning to maintain strict hygiene conditions, good temperature control necessary in cryogenic applicationsand better heat transfer performance. Typical plate heat exchangers . 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 platea pressure plate movable plateupper and lower bars and screws for compressing the pack of plates Figure 2. An individual plate heat exchanger can hold up to 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.
Exploded View of a Plate Heat Exchanger . 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 3although 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. Typical cathegories of plate corrugations. Turbulent flow in PHE channels .
Design characteristics This section presents some of the main advantages and disadvantages of a PHE, compared to shell-and-tube heat exchangers. 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.
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. As the plates are only pressed or glued together, rather than welded, PHE production can be relatively inexpensive. 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. The high thermal effectiveness of PHEs means that they have a very small footprint.
Illustration of the typical size difference between a PHE and a shell-and-tube heat exchanger for a given heat load . 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 shell-and-tube heat exchangers.
Ease of inspection and cleaning: With an outside diameter tubes of 15 mm and a thickness of 1. A simple calculation shows that for two tubes, Ac is 0. Outside the tubes, the calculations are a bit more complicated. One of the quantities to be determined is the number of fins Nf, and the other the overall number of passes. They cannot be determined by iteration, including the first, which directly influences the free flow area devoted to the air.
To avoid long explanations, we will retain immediately the result obtained, namely 90 fins. Determination of free flow area for air The flow area per tube is equal to: In addition, the length of the exchanger is: When you simply want to make the technological design of a project that implements components of the Thermoptim core, it is possible to automatically create technological screens using the generic driver, which avoids having to program one.
Open the Thermoptim project, and load the generic driver by choosing from among the list of drivers, the one whose title is "generic techno design driver", then click "Set the technological design screens". A line appears corresponding to the heat exchanger, as in Figure below. Since it is a simple heat exchanger, they are perfect.
Double-clicking on this line would change the type of TechnoDesign, but this is not necessary. To display the technological design screen, the simplest here is to open the exchanger and click on "tech.
The default technological design screen is displayed as shown below. We will now see how to set it. Exchanger default technological design screen The technological design screen of the heat exchanger contains in the lower left two areas for each "exchange" process. It includes, for each of the two fluids, the four parameters Ac, dh, f and eta.
They exchanger length is added for some calculations, including pressure drops. It allows one to take into account the fouling resistance if desired. In this example we neglect i. The values that were determined above must be entered in the technological design screen of the heat exchanger. For now, the lengths of the heat exchanger are not known, since the number of passes is not determined.
Ultimately, we will see that it is equal to 3.
Heat Exchanger Sizing
The values entered in the screen below correspond to this. Set technological design screen Once the settings done, return to the generic driver screen, then click "Design the selected components". The results are displayed in the technological screen: The number of passes can be deduced from these results.
It is the ratio of the surface of the heat exchanger to the inner surface of the two water tubes, close to 3 in view of the selected geometry values. We said above that the number of passes and number of fins could be found by iteration. Indeed, the first being an integer, and the second sets the free flow area devoted to air.
You may therefore need to test several sets of values before a finding one that is consistent. The lengths to be considered for the calculation of pressure drops can be determined now: The heat exchanger will be made up of two coil tubes each 90 cm long arranged on three layers crossing a total of square iron plates of 2 cm side separated from each other by 3 mm. The pressure drop values in bar appear on the right side of the screen.
They are negligible in our case.
The files for this example are available for download. Off-design calculations In off-design mode, we generally know the inlet temperatures and flow rates of fluids through the exchanger. An initial estimate of U can be performed, even if it can be refined when the outlet temperatures are known. We can then calculate e from the appropriate relationship NTU, e. One of the two outlet temperatures is obtained from the definition of e and the enthalpy balance provides another.
In practical terms, there are two main ways of operating: This is particularly the case of two-phase heat exchangers, whose thermal equilibrium establishes the change of state temperature and thus the pressure, which has the effect of changing both the inlet and outlet conditions.
The first way, the simplest is to use the off-design calculation mode of the exchanger screen. This can be done directly by hand, entering the new value of UA in the corresponding field, or by programming.
Once the heat exchanger sized, its off-design behavior can be studied directly from the simulator screen. The off-design calculation mode makes it possible to calculate the heat exchanger by the NTU method, considering that the two inlet temperatures and two flow rates are set. Thermoptim performs an update of the exchanger upstream links from the processes, then calculates the outlet temperatures and balances it in terms of enthalpy, with the UA value entered in the screen field.
Aspen Exchanger Design & Rating
Points and processes associated are updated based on the results. However, no correction is made automatically on UA to account for the evolution of exchange coefficients based on flow rates and temperatures. Make therefore a new calculation of the exchanger, estimate the value of the resulting U, and change UA accordingly, and then restart the calculation, by repeating the operation until values become stable. For example, look at how would behave the exchanger if the flow and temperature of the incoming air were changed, rising from 0.
The new value of U is calculated: Multiplying this value by the exchanger surface, you get a new value of UA, equal to 0. Repeat steps 1 to 3: Amend UA accordingly again, which provides exchanger and TechnoDesign screens below.