BTU requirements for a Jacketed Pipe

[joedirt9]

So there is an exhaust pipe that releases steam at 326C. This pipe needs to have a jacket placed on it to keep the steam at an elevated temp. so that particles to do not clog the pipe as they cool. This is an 8" pipe that is 10' long. I am trying to figure out how to set up an equation to determine how many BTU's are required to keep this pipe 326-331C. I will also need to set this up to determines the BTU/hr requirements for flow of ambient air flowing through the pipe. I am not sure of the flow rate of the steam at this point. Any tips of how to set this up? Obviously this not pure stiseam but that is not of importance at this time.

 

[BigInch]

How does the air fit into this problem.

Sounds like the pipe is short enough that you could just insulate to keep it and the steam hot.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[joedirt9]

I threw air on there as this pipe needs to be evaluated using different flows, not just steam. This pipe is already insulated and that is not enough. Need to keep the high temp until exiting the pipe

 

[BigInch]

You need the flowrates and temperatures of both the steam and air, outside air temp, insulation type & thickness.

What's the exit temp now.

Calculate the BTU transferred to the pipe from the internal flow of steam/air across the boundary film, through the pipe and insulation and the outer boundary layer with the ambient air by finding the overall heat transfer coefficient for the first foot of the 10 foot pipe. That will be the largest BTU loss per foot length of pipe. Just use that for the remaining 9 feet to get a conservative end temp.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[joedirt9]

guess i was looking at it differently. my thinking was to look at this as a heat exchanger with heat coming from the vapor in the jacket to heat the steam. then add in the heat lost through the insulation from the vapor in jacket. what you are saying is to only deal with the heat loss from the steam.

i don't know the exit temp now. the pipe is only used a few times a year.

 

[BigInch]

No point heating up the atmosphere, right?

As long as you have some temperature differential to work with, insulation will be far and away the most efficient solution. Then if you still need temperature to be increased, however you wind up doing it, it will probably be a much smaller capacity system. Maybe just simple electric tracing would work.



If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[joedirt9]

well it is steam traced right now and it is not working
need to keep the pipe as hot as possible to keep it from becoming fouled

think i am making progress though

basicly breaking it into two parts. heat lost thru insulation + {q=UA(dtm)}

still working on it but seem to be moving forward
hell, even learing something while doing it

 

[BigInch]

Great. You must evaluate the film coefficient, the heat transfer across the boundary layer between steam and pipe inside wall, then the convection film outside of the insulation against the free air, as well as through the pipe and insulation. The film coefficients can get complicated.

Once you have the radial heat loss, you can apply the log temperature drop rule to get a more accurate loss in temperature along the length of pipe, or you can leave it quick and dirty like I suggested before.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[Latexman]

Why do you think the pipe will stay clean if it stays hot?

Good luck,
Latexman

 

[joedirt9]

this pipe is not unique. there are others that are jacketed that have not clogged

question on the film coefficients. not sure which one to use for the steam. still looking into that one

 

[IRstuff]

Everything seems a bit diffuse here. My approach:
> identify allowable external surface temp; 60°C would minimize burn hazard
> identify ambient temp; say 25°C
> guess at an insulation thickness; say 1-in Styrofoam equiv. material.

Simultaneous solve for thickness

sigma*area.cond*(T.gas-T.surf)/thk = htc.conv*area.conv*(T.surf-T.amb)

where:
sigma --> thermal conductivity of insulator
area.cond --> effective area of insulator
T.gas --> gas temp
T.surf --> exterior surface temp of insulation, assume that the pipe has negligible resistance
thk --> thickness of insulator
htc.conv --> convection coefficient
area.conv --> convection area
T.amb --> ambient temp

I get something like 1.6-in of Styrofoam for htc = 7.5W/m^2*K, which results in heat flow of 981W

TTFN
faq731-376

 

[joedirt9]

pipe is going to have 4 inches of insulation. that should have been included. i can solve for how much heat is lost thru the insulation. the btu/hr requred to keep the steam at at the given temp is where i am stuck

 

[IRstuff]

Huh? Isn't that essentially the same? The only thing you have to ensure is that the steam is sufficiently hot enough that the losses through the insulation don't drop the steam temperature below your limit. There are no other losses in the system.

TTFN
faq731-376

 

[zekeman]

To find the gas temperature drop in a length of pipe you must know the flow rate and is done as follows

rho*A*c*dT/dt=U*p*(T0-T)

solution:

Tf-T0=(Ti-T0)e^-zt
since t=L/v, this becomes

Tf=T0-(Ti-T0)*e^(-zL/v)

z=Up/(rho*A*c)
L= length of pipe
rho=density
A= cross sectional area pipe


p= perimeter of pipe
c= specific heat
t=transit time (time it takes fluid to reach end of pipe
Ti= entering temperature
Tf temperature at end of pipe
T0= ambient temperature

 

[TD2K]

You said your existing tracing isn't effective, what are you using? If you are trying to maintain 326C to 331C, steam isn't a very practical method, you need high pressure steam to have a condensing temperature that high.

I have a hard time seeing there's much of a temperature drop over 10' of 8" pipe with 4" of insulation.

 

[joedirt9]

Guess I am stuck on U now.

 

[zekeman]

"Guess I am stuck on U now. "

the overall conductance from the gas to ambient.

I think you had better review the subject of heat transfer to get a better understanding of your problem.
I don't have a reference but others on this forum could point you to online sources if you don't have one.

 

[BigInch]

TD2K "I have a hard time seeing there's much of a temperature drop over 10' of 8" pipe with 4" of insulation."
Me too. I almost thought it was a homework problem because of that.

If it ain't broke, don't fix it. If it's not safe ... make it that way.