Heat Transfer in Process Piping

[pharoofthermo]

Do any of you guys have a recommendation or know of a calculator for deteming total heat loss in a length of process piping?

Basically, I have Natural Gas flowing from a regulating station to a gas turbine. Some of the pipe is above ground, while some of it is below ground. I have all the state information on the Natural Gas as it leaves the regulating station and know pipe / insulation thickness, ambient temp., gas flow rate, etc.

After reviewing Incropera and DeWitt, it looks like I have to determine the applicable Nusselt numbers for the above ground and underground cases and then determine:

Hinner, Kpipe, and Houter for the above ground piping

and

Hinner, Kpipe, and Kdirt for the under ground piping

With these I can set up a resistance network to determine overall heat transfer and the theoretical temperature at the gas turbine.

Is my logic correct? What am I missing? Please let there be a calculator available.....I haven't calculated Re, Nu, and Ra values since college. I'm bound to screw up!

 

[pmover]

there are many posts in this forum regarding the subject matter of piping insulation - previously been addressed.

may i suggest that you conduct a search (use forum search criteria) within this forum and advise of further action needed.
-pmover

 

[davefitz]

an acruate number is more complicated then one might think. I assume you are trying to avoid condensation of hydrates or minimizing the sizing of the fuel gas heater. For this case the design case is the design winter day zas defined by ASHREA for the locale of the installaton.

You should consider the following:

a) temperature drop due to throttling thru the Press red valve

b) above ground loss can be easily minimized by adding insulation, but this is rarely done for nat gas piping. Ignore thermal resistance of pipe, and consider as primary resistances the outside convective in a strong wind condition ( h,out = 6 btu/hr /ft2/F ; R,out = 1/6. ), the inside film coefficient is from :
Nu/Re/Pr = 0.023*Re^-0.2*Pr^-0.67

c) underground pipe heat loss caan either be calculated usign a finite element model or using the old plotted curves by Heisler, or one can use the ASHRAE tables ( as doen for calculating heat loss from basements).

 

[MortenA]

when you say "process piping" its my normal procedure to disregard heat transfer in the piping. This assumes that there is a reasonable relationship between flowrate and pipelength (large flow short pipes).

Need for insulation is then based on requirementens for personal protection.

If you have a need for this calculation anyway most process simulations (HYSIM, Winsim, PROII etc) has a "pipe model" that includes heat loss to environment.

Best Regards

Morten

 

[timbones]

If you know the insulation thickness, than presumably whoever selected the insulation knows what the heat loss is going to be (otherwise how would they make the selection).

I would phone a pipe insulation manufacturer and get them to provide you with a heat loss table as a function of insulation thickness, process temperature and ambient conditions.

Or you could skip all of this and put in a line heater. I was looking at one of these the other day for one of our GE LM6000 installations. It controls to 30°C above the inlet gas temperature to ensure adequate superheat. This installation is in Alberta where the ambient temperatures can get very cold <-30°C and the gas can also be very cold <-5°C.

tim