Plate Heat Exchangers
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Purpose of the Equipment: Plate heat exchangers (PHE), often called plate-and-frame heat exchangers, are used to change the temperature of a liquid, vapor or gas media. A thin, corrugated plate is used to transfer the heat from the media on one side of the plate to the media on the other side. 2/26/201 Alok Garg 6
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The plate heat exchanger consists of a frame with end plates which squeeze the corrugated heat transfer plates. Figure shows a plate pack of corrugated plates with portholes for the media to flow. 2/26/201 Alok Garg
How the Equipment Works? Plate heat exchangers use the thin plates to keep two media of different temperatures apart while allowing heat energy to flow between them through the plate. The heat energy transfer across the plate acts to change the temperatures of the two media. The hotter one becomes cooler, and the colder one becomes hotter. 2/26/201 Alok Garg 9
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Gasketed plate heat exchangers: Plate thickness vary from 0.5 and 3 mm. Gap between two plates 1.5 to 5 mm. Plate surface areas range from 0.03 to 1.5 m 2, with a plate width : length ratio from 2.0 to 3.0. The size of plate heat exchangers can vary from 0.03 m 2 to 1500 m 2. The maximum flow-rate of fluid is limited to around 2500 m 3 /h. Plates are available in a wide range of metals and alloys; including stainless steel aluminum and titanium. A variety of gasket materials is also used. 2/26/201 Alok Garg 13
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Advantages Plate heat exchangers are easier to maintain. Low approach temps can be used, as low as 1 C, compared with 5 to 10 "C for shell and tube exchangers. Plate heat exchangers are more flexible, it is easy to add extra plates. Plate heat exchangers are more suitable for highly viscous materials. The temperature correction factor, F t, will normally be higher with plate heat exchangers, as the flow is closer to true counter-current flow. Fouling tends to be significantly less in plate heat exchangers. 2/26/201 Alok Garg 15
Disadvantages: A plate is not a good shape to resist pressure and plate heat exchangers are not suitable for pressures greater than about 30 bar. The selection of a suitable gasket is critical. The maximum operating temperature is limited to about 250 C, due to the performance of the available gasket materials. 2/26/201 Alok Garg 16
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PLATE HEAT EXCHANGER DESIGN 2/26/201 Alok Garg 1
PROCEDURE The design procedure is similar to that for shell and tube exchangers. 1. Calculate duty, the rate of heat transfer required. 2. If the specification is incomplete, determine the unknown fluid temperature or fluid flow-rate from a heat balance. 3. Calculate the log mean temperature difference, T LM. 4. Determine the log mean temperature correction factor, F t. 5. Calculate the corrected mean temperature difference T M = F t x T LM. 6. Estimate the overall heat transfer coefficient. 7. Calculate the surface area required.. Determine the number of plates required = total surface area/area of oneplate. 2/26/201 Alok Garg 19
PROCEDURE 9. Decide the flow arrangement and number of passes. 10. Calculate the film heat transfer coefficients for each stream. 11. Calculate the overall coefficient, allowing for fouling factors. 12. Compare the calculated with the assumed overall coefficient. If satisfactory, say 0% to + 10% error, proceed. Ifunsatisfactory return to step and increase or decrease the number of plates. 13. Check the pressure drop for each stream. 2/26/201 Alok Garg 20
FLOW ARRANGEMENTS 2/26/201 Alok Garg 21
Estimation of the temperature correction factor: For PHE it is convenient to express the LMTD correction factor, F t, as a function of the NTU, and the flow arrangement (number of passes). The correction will normally be higher for a PHE than for a STE operating with the same temperatures. For rough sizing purposes, the factor can be taken as 0.95 for series flow. The number of transfer units is given by: NTU = (t 0 -t i )/ T LM where t i = stream inlet temperature, C, t o = stream outlet temperature, C, T LM = log mean temperature difference, C. Typically, the NTU will range from 0.5 to 4.0, and for most applications will lie between 2.0 to 3.0. 2/26/201 Alok Garg 22
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Heat Transfer Coefficient: 2/26/201 Alok Garg 24
The corrugations on the plates will increase the projected plate area, and reduce the effective gap between the plates. The channel width equals the plate pitch minus the plate thickness. There is no heat transfer across the end plates, so the number of effective plates will be the total number of plates less two. 2/26/201 Alok Garg 25
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