Jacketed Vessel Design
Jacketing a process vessel provided excellent heat transfer in terms 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 :
- Spirally baffled jackets/ conventional jackets
- Dimple jackets
- Partial-pipe coil /limpet jacket
- Panel 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 the Heat Transfer Media
Water: Depending on the process temperature, stress corrosion *****ing 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 havingcondensate 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 joints 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 often utilize what is known as a spirally wound baffle. The baffle
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Figure 1: Conventional Jacket
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 bypass around the baffle. Generally, bypass 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 Jacket without Baffles
(hj De / k) = 1.02 (NRe) 0.45 (NPr) 0.33 (De/ L) 0.4 (Djo/ Dji) 0.8 (NGr) 0.05
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hj = Local heat transfer coefficient on the jacket side
De = Equivalent hydraulic diameter
NRe = Reynolds Number
NPr = Prandtl Number
L = Length of jacket passage
Djo = Outer diameter of jacket
Dji = Inner diameter of jacket
NGr = Graetz number
The Reynolds Number is defined as:
NRe = DVr/m
Where D is the equivalent diameter, V is the fluid velocity, r is the fluid density, m and is the fluid viscosity.
The Prandtl Number is defined as:
NPr = Cp m / k
Where Cp is the specific heat, m is the viscosity, and k is the thermal conducitivity of the fluid.
The Graetz Number is defined as:
NGr = (m Cp) / (k L)
Where m is the mass flow rate, Cp is the specific heat, k is the thermal conducitivity, and L is the jacket passage length.
The equivalent diameter is defined as follows:
De = Djo-Dji for laminar flow
De = ((Djo)2 - (Dji)2)/Dji for turbulent flow
Heat Transfer Coefficients: Conventional Jacket 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)
hj De/k = 1.86 [ (NRe) (NPr) (Dc/De) ] 0.33 (µ/µw)0.14 ( For NRe < 2100 )
Two
new variables are introduced. Dc is defined as the centerline diameter of the jacket passage. It is calculated as Dji + ((Djo-Dji)/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 taken as 0.60 x feed mass flow rate to account for the substantial bypassing 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.
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Hydrualic Radius: Conventional Jacket with Baffles
Referring to the graphic above, the hydrualic radius is calculated as follows:
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Half-Pipe Coil Jackets
Half pipe coils provide high velocity and 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 maximum
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Figure 2: Half-Pipe Coil Jacket
spacing 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 multipass 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 (Dci):
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For a 180° central angle:
Equivalent Heat Transfer Diameter, De = P / (4 Dci)
Cross Section Area of Flow, Ax = P / (8 (Dci2))
For a 120° central angle:
Equivalent Heat Transfer Diameter, De = 0.708 Dci
Cross Section Area of Flow, Ax = 0.154 (Dci2)
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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)
Do not confuse Dci with Dc. Dc is defined as Dji + ((Djo-Dji)/2).
Hydrualic Radius: Half-Pipe Coil Jackets
Referring to the figure on the right:
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