thickness of the insulation over a pipe. 1

[skvishwa]

Hi all,
I want to know ,what is the standard heat loss that is acceptable from an insulated pipe line?
The thickness of the insulation is decided upon some value of the heat loss that is accpetable , so how much heat loss should be accpetable as a fraction of the total heat conetent in the flowing fluid?
regards,
santosh

 

[Montemayor]

skvishwa:

The thickness of the insulation is not decided upon some value of the heat loss that is “acceptable”. The pipe insulation thickness is decided by optimizing the degree of thickness with respect to the economics involved. Go to

http://www.cheresources.com/software.shtml

and there you can download a free spreadsheet program that will show you how it is done. This program, "Economic Thickness Calculator", is typical of many others and will recommend the appropriate insulation thickness based on energy savings.

This is a classical chemical engineering optimization exercise that is routinely taught in Unit Operations courses. I hope this helps you out.


Art Montemayor
Spring, TX

 

[MortenA]

I agree with Art except that in a few cases there might be certain process requirements (e.g. avoid solidification) that is paramount. Usually in these cases you would include some sort of active cooling/heating (pipe jacket). So for most application what Art writes is the methode to progress.

Best regards

Morten

 

[25362]

Kindly note that sometimes insulation is provided for process and quality control reasons, as well as for personel protection (45 deg C for metals, and 60 deg C for canvas), and not only as a heat-saving approach.

I agree with Art Montemayor in that there are various economic models to estimate the optimum economic thickness of insulation based on heat lost or saved. The estimated thicknesses using these methods are totally dependent on the input data with their inherent variability and uncertainty.

Companies I knew, doing their own (not contracted for) insulation used approaches based on cash flow present worth estimates, or payback periods. Then made tables for designers to use according to the type of insulation, the pipe diameter and the fluid temperature, considering that commercial insulation was sold in 1/2-in. increments.

Many plant owners treat insulation as another investment in equipment. But this is not entirely true, or even reasonable, since savings from insulation are quite predictable and risk-free, whereas return on investment in plant or added production is surely not so predictable.

Then there is the "engineering" -in contrast to the "accountancy"- approach based on an acceptable heat loss thickness, as asked by skvishwa. This is based on the heat losses estimated using added insulation thicknesses compared with the bare pipe, expressed as %.

It is noted that incremental thicknesses of insulation show diminishing returns below a 3% heat-loss rate. Until the end of the 20th century the reasonable heat-loss rate was 3% to 5%.

So, nowadays what would the optimum be, 2%, 1.5%? I couldn't tell in view of the high energy costs of these days. Probably the model using the minimum amount resulting from adding the cost of heat lost plus the cost of insulation (installation and maintenance), both per linear foot of pipe, with a reasonably (probably shorter than the envisaged plant life) amortization period, would be a safer approach.

I know this is a debatable item, and I'appreciate any comments that colleagues may have.