Jacketed Vessel Design 57351t

Jacketed Vessel Design Nov 08 2010 01:20 PM | Guest in Heat Transfer

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|More Jacketing a process vessel provided excellent heat transfer in of efficiency, control and product quality. All liquids can be used as well as steam and other high temperature vapor circulation. The temperature and velocity of the heat transfer media can be accurately controlled. The various types of jackets used in process industry are : 1. 2. 3. 4.

Spirally baffled jackets/ conventional jackets Dimple jackets Partial-pipe coil /limpet jacket type/ plate type coil jackets Commonly used heat transfer medias include water, steam (various pressures), hot oil (such as Therminol™), and Dowtherm™ vapor.

Matching Jacket Types to Heat Transfer Media Water: Depending on the process temperature, stress corrosion cracking can sometimes be a concern due to the chlorides usually found in water. In some cases, dimple jackets may requires the use of high-nickel alloys which are very expensive. The half-pipe coil can use 1/4'' thick carbon steel for the jacketing but their economy versus conventional jackets must to be considered. With services involving large volumes of water (used to maintain a high temperature difference) the conventional jacket usually offers the best solution. Steam: Both dimple and half coil jackets are well suited use with high pressure steam. The dimple jackets are generally limited to 300 psig design pressure while half-coil jackets can be used up to a design pressure of 750 psig. For half-pipe coil jacket, the higher heat flux rate may require multiple sections of jackets to avoid having condensate covering too much of the heat transfer area. For low pressure steam services convention jackets are a much more economical choice. Hot Oils and Heat Transfer Fluids: Although pressures are usually low when using oils or heat transfer fluids, the temperatures are usually high. The result is low allowable stress values for the inner-vessel material. Therefore both half-pipe jackets and dimple jackets can provide good solutions. Conventional jackets require a greater shell thickness along with expansion ts to eliminate stresses induced by the difference in thermal expansion when the jacket is not manufacturered from the same material as that of shell. Dowtherm™ Vapors:The ability to vary the distance between the outer and innver vessel walls makes conventional jackets ideally suited to handle Dowtherm™ vapors. Also since Dowtherm vapor has a low enthalpy (1/10 that of steam) a large jacket space is needed for given heat flux. The jacket must be designed in accordance with ASME Code specifications. The maximum allowable space is limited by section UA-104 Paragraph c and s.

Conventional Jackets "Conventional jackets" can be divided into two (2) main categories: baffled and non-baffled. Baffled jackets Figure 1: Conventional often utilize Jacket what is known as a spirally wound baffle. The baffle consist of a metal strip wound around the inner vessel wall from the jacket utility inlet to the utility outlet. The baffle directs the flow in a spiral path with a fluid velocity of 1-4 ft/s. The fabrication methods does allow for small internal leakage or by around the baffle. Generally, by flows can exceed 1/3 to 1/2 of the total circulating flow. Conventional baffled jackets are usually applied with small vessels using high temperatures where the internal pressure in more than twice the jacket pressure. Spirally baffled jackets are limited to a pressure of 100 psig because vessel wall thickness becomes large and the heat transfer is greatly reduced. In the case of an alloy reactor, a very costly vessel can result. For high temperature applications, the thermal expansion differential must be considered when choosing materials for the vessel and jacket. Design and

construction details are given in Division 1 of the ASME Code, Section VIII, Appendix IX, "Jacketed Vessel". Heat Transfer Coefficients: Conventional Jackets without Baffles (hj De / k) = 1.02 (NRe) 0.45 (NPr) 0.33 (De/ L) 0.4 (Djo/ Dji) 0.8 (NGr) 0.05

Figure 2: Schematic of Conventional Jacket

Eq. (1)

Where: hj = Local heat transfer coefficient on the jacket side De = Equivalent hydraulic diameter NRe = Reynolds Number

NPr = Prandtl Number L = Length of jacket age Djo = Outer diameter of jacket Dji = Inner diameter of jacket NGr = Graetz number The Reynolds Number is defined as: NRe = DVρ/μ Where D is the equivalent diameter, V is the fluid velocity, ρ is the fluid density, μ and is the fluid viscosity. The Prandtl Number is defined as: NPr = μ / k Where is the specific heat, μ is the viscosity, and k is the thermal conducitivity of the fluid. The Graetz Number is defined as: NGr = (m ) / (k L) Where m is the mass flow rate, is the specific heat, k is the thermal conducitivity, and L is the jacket age length. The equivalent diameter is defined as follows: De = Djo-Dji for laminar flow De = ((Djo)2 - (Dji)2)/Dji for turbulent flow Conventional Jackets with Baffles

For conventional jackets with baffles, the following can be used to calculate the heat transfer coefficient: hj De/k= 0.027(NRe)0.8 (NPr)0.33 (µ/µw)0.14 (1+3.5 (De/Dc) ) ( For NRe > 10,000)

Eq. (2)

hj De/k = 1.86 [ (NRe) (NPr) (Dc/De) ]

Eq. (3)

0.33

(µ/µw)

0.14

( For NRe < 2100 )

Two new variables are introduced. Dc is defined as the centerline diameter of the jacket age. It is calculated as Dji + ((DjoDji)/2). The viscosity at the jacket wall is now defined as µw. When calculating the heat transfer cofficients, an effective mass flow rate should be Figure 3: Schematic of Conventional Jacket with Baffle

taken as 0.60 x feed mass flow rate to for the substantial bying that will be expected. De is defined at 4 x jacket spacing. The flow cross sectional area is defined as the baffle pitch x jacket spacing.

Half Pipe Coil Jackets Half pipe coils provide high velocity and

Figure 4: Half Pipe Coil Jacket

turbulence. The velocity can be closely controlled to achieve a good film coefficient. The good heat transfer rates, combined with the structural rigidity of the design, make half-pipe coils a good choice for a wide range of applications. A good design velocity for liquid utilities is 2.5 to 5 ft/s. The maximumspacing between coils should be limited to 3/4". Half-pipe coils are ideally suited for high temperature applications where the utility fluid is a liquid. There are no limitations of the number of inlet and outlet nozzles, so the jacket can be divided in multi zones for maximum flexibility. The rigidity of the half-pipe coil design can also minimize the thickness of the inner vessel wall which can be especially attractive when utilizing alloys. Half-pipe coil jackets are not covered in Section VIII, Division I of the ASME code. Generally, they are limited to 600 psig design pressure and a design temperature up to 720 °F. A carbon steel half-pipe jacket can be applied to a stainless steel vessel up to 300 °F. Over 300 °F, the jacket should be stainless steel as well.

Heat Transfer Coefficients: Half-Pipe Coil Jackets Half-pipe coil jackets are generally manufactured with either 180° or 120° central angles (D ci):

Figure 5: Depiction of Center Angles For a 180° central angle:

Figure 6: Half-Pipe Coil to Tank Details

Equivalent Heat Transfer Diameter, De = Π / (4 Dci) Cross Section Area of

Flow, Ax = Π / (8 (Dci2)) For a 120° central angle: Equivalent Heat Transfer Diameter, De = 0.708 Dci Cross Section Area of Flow, Ax = 0.154 (Dci2) Using the same nomenclature as previous, the heat transfer coefficients are calculated as follows: hj De/ k= 0.027(NRe)0.8 (NPr)0.33 (µ/µW)0.14 (1+3.5 (Dc/De) ) (For NRe>10,000) hj De/ k = 1.86 [ (NRe) (NPr) (Dc/De) ]

0.33

(µ/µW)

0.14

(For NRe<2,100)

Eq. (7) Eq. (8)

Do not confuse Dci with Dc. Dc is defined as Dji + ((Djo-Dji)/2).

Hydraulic Radius: Half-Pipe Coil Jackets Referring to Figure7:

Figure 7: Hydraulic Radius Dimensions

The design of dimple jackets permits construction from light gauge metals without sacrificing the strength required to withstand the specified pressure. This results in considerable cost saving as compared to convention jackets. Design calculation begin with an assumed flow velocity between 2 and 5 ft/s. As a rule of thumb the jacket pressure will be governing when internal pressure of vessel is less than 1.67 times the jacket pressure. At such conditions, dimple jackets are typically more economical than other choices. However in small vessels (less than 10 gallons) it is not practical to apply dimple jackets. The design of dimple jackets is governed by the National Board of Boiler and Pressure Vessel Inspectors and can be stamped in accordance with ASME Unfired Pressure Vessel Code. Dimple jackets are limited to a pressure of 300 psi by Section VIII, Div.I of the ASME Code. The design temperature is limited to 700 °F. At high temperatures, it is mandatory that jacket be fabricated from a metal having same thermal coefficient of expansion as that used in inner vessel.

Figure 8: Vessel with Dimple Jacket Installed

Figure 9: Dimple Jacket Details

Heat Transfer Coefficients: Dimple Jackets hj Do/k= j (NRe) (NPr)0.33 (For 1000 < NRe < 50,000)

Eq. (10)

Where: j = 0.0845 (w/x)0.368 (Amin/Amax)-0.383 NRe-0.305 w = center-to-center distance between dimples x = center-to-center distance between dimples parallel to flow Note: (w/x) is equal to one for square spacings as is often the case Do = (d1 + d2)/2 Amin = z (w-Do) Amax = zw All other variables are as previously defined. Garvin (CEP Magazine, April 2001) reports an average error of 9.8% with manufacturers data for 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. 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, the overall heat transfer coefficient of the system will be very low. Mastic is used to try to minimize air pocket resistances between the vessel wall 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 manufacturer's data for pressure drop calculations.

Heat Transfer Coefficients Inside Agitated 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:

Eq. (11)

Where: Ad = agitator diameter N = agitator speed, rev/s All other variables as previously defined a is defined by the table below:

Table 1: Dimension "a" for Use with Equation 11 Agitator

Surface

"a"

Turbine

Jacket

0.62

Turbine

Coil

1.50

Paddle

Jacket

0.36

Paddle

Coil

0.87

Anchor

Jacket

0.46

Propeller

Jacket

0.54

Propeller

Coil

0.83

Calculating the Overall Heat Transfer Coefficient When calculating the overall heat transfer coefficient for a system, the vessel wall resistance and any jacket fouling must be taken into :

Eq. (12)

Notice that the thermal conducitivity of the vessel wall and the wall thickness are included in the calculation. A typical jacket fouling factor is around 0.001 h ft2 °F/Btu. When calculating the overall heat transfer coefficient, use a "common sense" analysis of the final value. The tables below will give some guidance to reasonable final values: Table 2: Estimated Overall Heat Transfer Coefficients for Jacketed Tank Systems (Imperial Units)

Table 3: Estimated Overall Heat Transfer Coefficients for Jacketed Tank Systems (Metric Units)

References 1. 2. 3. 4. 5. 6. 7.

Heat Transfer Design Methods by 'John J. McKetta' Hand Book of chemical Engineering Calculation 3rd Edition by 'Micclas P. Chopey'. Applied Process Design for Chemical and Petrochemical Plants by 'Ludwig' Volume 3. Estimate Heat Transfer and Friction in Dimple Jackets, 'John Garvin', CEP Magazine, April 2001, p. 73 Heat Transfer in Agitated Jacketed Vessels, 'Robert Dream', Chemical Engineering, January 1999, p. 90 Encyclopedia of Pharmaceutical Technology, 'James Swarbrick', p. 217 Tranter Plate Coil Product Manual