How to calculate spiral ratio of tracer and distance between spiral for Pipeline?

Spiral Ratio 

Spiral Ratio Definition:

A ratio between heat loss and cable output is called spiral ratio. i.e. how much heating cable is sufficient on the one-meter pipe for heat loss compensation. Please note in this case spiral ratio must be equal & greater than 1. 

 S.R = Total Heat Loss / Heating Cable Output 

Example: 

Pipe size = 4"

Insulation thickness = 40mm

Thermal Conductivity of Insulation = 0.043 W/m⁰C @ 50⁰C mean

Maintain Temperature  = 74⁰C 

Minimum Ambient Temperature  = 24⁰C 

Design Margin = 10%

Now we calculated heat loss as per IEEE-515:

We get heat loss = 27.9W/m

 

Let we have considered heating cable output is 25W/m.

Spiral Ratio = 27.9/25 = 1.116

Distance between Spiral

π x (Pipe outer dimeter + heating cable or tracer diameter) / sqrt(α^2-1) 

α is spiral ratio.

Example:

Pipe outer diameter is 114.3mm

Heating cable or tracer diameter is 10mm

α is 1.116

Putting all value in the formula 

= π x (114.3 + 10) / √ (1.116^2-1)

= 788.2mm

i.e.  distance between two spiral or round is 788.2mm.