Dear Sirs,
I am a process engineer working with a Polyester Manufacturing Company. We are working on a project of POY transfer line. In one of the jacketted pipes of size 4″ x 6″ the dimensions are as follows (Core: Polyester Melt & Jacket : HTM Liquid ‘Dowtherm’)

Inner Pipe(Not a standard XXS pipe) : SS pipe : Hollow Bar SS 404,DIN-17458,EN-10216-5
Tolerances : Inner Diameter = 0 / -2% (min -1mm)
Outer Diameter = 0 / +2% ( max +1mm)
Wall Thickness = (c1)-0mm
ID= 100 mm / Thickness= 20mm / Outside Diameter = 140 mm
Outer Pipe : Pipe Seamless Carbon Steel ASTM 106 Gr B Butt Weld Ends, Schedule 40
Dimensions As per ASME B 36.10
ID= 154.1 mm / Thickness= 7.1mm / Outside Diameter = 168.3 mm

If we see, the annular space(gap) between inner pipe and outer pipe is 7.01 mm. I have following concerns regarding this
1) Is this gap okay for carrying out fabrication & putting spacers between jacket & core pipe?
2) At present the Cross Sectional area available for the HTM Liquid flow is equavalent to cross section area of DN 50 pipe. This area is good enough for the HTM flow rate of 3500 Kg/hr from velocity and pressure drop considerations. Am I right ?
3) Is is possible to use the 4″ x 8″ combination instead of present 4″ x 6 “? In this case the cross sectional area available for HTM flow is equal to DN 150 pipe and my concern is if we use this the HTM flow velocity through jacket will be very low & it will also get channelized.
4) I could not find the guidelines for design of jacketted pipelines in any of standard piping reference book. Can somebody suggest where can I get this information ?

Thanks in advance.
VISHAL KHADKE