h_j  D_e/k = 1.86 [ (N_Re) (N_Pr) (D_c/D_e) ] ^0.33 (µ/µ_w)⁰.14               ( For N_Re < 2100 )

Two new variables are introduced.  Dc is defined as the centerline diameter of the jacket passage.  It is calculated as D_ji + ((D_jo-D_ji)/2).   The viscosity at the jacket wall is now defined as µ_w. 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.   D_e is defined at 4 x jacket spacing.  The flow cross sectional area is defined as the baffle pitch x jacket spacing.

Hydrualic Radius: Conventional Jacket with Baffles

Referring to the graphic above, the hydrualic radius is calculated as follows:

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

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 (D_ci):

For a 180° central angle:

Equivalent Heat Transfer Diameter, De = P / (4 D_ci)

Cross Section Area of Flow, Ax = P / (8 (D_ci²))

For a 120° central angle:

Equivalent Heat Transfer Diameter, De = 0.708 D_ci

Cross Section Area of Flow, Ax = 0.154 (D_ci²)

Using the same nomenclature as previous, the heat transfer coefficients are calculated as follows:

Do not confuse D_ci with D_c.  D_c is defined as D_ji + ((D_jo-D_ji)/2).

Hydrualic Radius: Half-Pipe Coil Jackets

Referring to the figure on the right:

< previous page  next page >

By: Santosh Singh, Guest Author

Please direct inquiries to: