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Jacketed vessel design


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dimple jacket design

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Jacketed vessel design

  1. 1. Jacketed Vessel Design (design of dimple jackets) Jacketed Vessel Design (design of dimple jackets)The design of dimple jackets permits construction from light gaugemetals without sacrificingthe strength required to withstand the specified pressure. This resultsin considerable cost saving as compared to convention jackets. Designcalculation begin with an assumed flow velocity between 2 and 5 ft/s.As a rule of thumb the jacket pressure will be governing when internalpressure of vessel is less than 1.67 times the jacket pressure. At suchconditions, dimple jackets are typically more economical than otherchoices. However in small vessels (less than 10 gallons) it is notpractical to apply dimple jackets.The design of dimple jackets is governed by the National Board ofBoiler and Pressure Vessel Inspectors and can be stamped inaccordance with ASME Unfired Pressure Vessel Code. Dimple jacketsare limited to a pressure of 300 psi by Section VIII, Div.I of the ASMECode. The design temperature is limited to 700 °F. At hightemperatures, it is mandatory that jacket be fabricated from a metalhaving same thermal coefficient of expansion as that used in innervessel. Heat Transfer Coefficients: Dimple Jackets
  2. 2. All other variables are as previously defined. Garvin (CEP Magazine,April 2001) reports an average error of 9.8% with manufacturers datafor the above correlation and a maximum error of 30% over 116 data points. This results in average deviations in the heat transfer coefficient of 15-20% most of which was at velocities below 2 ft/s.Good agreement with manufacturers data was found between 3 and 6 ft/s. A recommended excess area of 15% should be used in this velocity range. Note: The correlation above is for integrally welded jackets (ie. jackets welded directly to the vessel). If a dimple jacket is clamped onto an existing vessel and adhered with heat transfer mastic, theoverall heat transfer coefficient of the system will be very low. Mastic is used to try to minimize air pocket resistances between the vesselwall and the jacket. Historically, this arrangement results in poor heat transfer. A recommended overall heat transfer coefficient of 10-15 Btu/h ft² °F should be used for such systems regardless of the utility used. Pressure Drop: Dimple Jackets The pressure loss in a dimple jacket can be estimated from the following for water or water-like fluids: Pressure Loss in Jacket = (Total Lenght of Flow, ft) x ((0.40 x Velocity, ft/s) - 0.35) Pressure Loss Across Entire Jacket (including inlets and outlets) = Pressure Loss in Jacket + (0.10)(Pressure Loss in Jacket)The above estimates should be used for velocities ranging from 1.5 to 6 ft/s. This method is based on a graph found on page 217 of the Encyclopedia of Pharmaceutical Technology by James Swarbrick. For detailed design, it is advisable to rely on manufacturers data for
  3. 3. pressure drop calculations. Heat Transfer Coefficients Inside Agitated Process Vessels In order to complete the overall heat transfer coefficient calculation, an estimate must also be made inside the process vessel. The following estimate should yield reasonable results: Calculating the Overall Heat Transfer CoefficientWhen calculating the overall heat transfer coefficient for a system, the vessel wall resistance and any jacket fouling must be taken into account: Notice that the thermal conducitivity of the vessel wall and the wallthickness are included in the calculation. A typical jacket fouling factor is around 0.001 h ft² °F/Btu. When calculating the overall heattransfer coefficient, use a "common sense" analysis of the final value. The table below will give some guidance to reasonable final values: English Units
  4. 4. Metric Units
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