Heat loss from underground pipe (insulated versus non insulated) 5

[mechanical3010]

The question is about how much energy saving would result by insulating with HDPE the 3 inch dia CPVC Pipe buried in ground at a depth of 3 feet that transports domestic hot water at 60 degrees celcius from a central plant to the various buildings of the project and the water is then recirculated back to the calorifiers for reheating. The supply and return legs add up to nearly 1500 feet and the flow rate is in the order of 75 GPM, and the ambient (above ground) temperature is nearly 5 degrees celcius.
If the energy saving is minimal, the high cost of insulating the pipe might not be justified. Feedback about this will be appreciated.

 

[chicopee]

It all depends on climate, soil condition, steady or intermittent flows thru the piping. Also Is the HDPE in a foam state?

 

[IRstuff]

Can't you measure the temperature of the water at its distributions points? That'll be a simple way to determine how much heat is lost.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[mechanical3010]

Re: Comments from Chicopee
To simplify the analysis, the flow of 75 GPM is assumed to be steady state. The ambient temperature of 5 degrees celcius assumed all around the year without any snow defines the climatic conditions and the soil is the usual non rocky soil with the pipe laid on prepared bedding of sand. The pipes are preinsulated with High density polyethelene non foam insulation.
Re: Comments from IRstuff
The work is still not executed so physical field measurements are not possible. The energy saving of insulating versus not insulating must be therefore be calculated to decide whether to insulate or not to insulate.
Many thanks............And still waiting for the needed tips.

 

[IRstuff]

Well, what do you want the answer to be?

Seems to me that you've stacked the deck by choosing a material that's got a thermal conductivity about 10x what is readily available: 0.46 W/m-K HDPE vs. 0.036 W/m-K for PE foam.

If you're really interested in getting a meaningful answer, you could start with these articles:

http://webwormcpt.blogspot.com/2009/08/estimating-heat-loss-from-buried-pipe.html

http://www.osti.gov/bridge/purl.cov...6BBCFEFDAA4C?purl=/629429-7Q2Vec/webviewable/

Or the various threads in this site that have already discussed this subject:

http://www.eng-tips.com/viewthread.cfm?qid=102246

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[StrykerTECH]

I would suggest approaching it from the negative aspect. How much does the insulation cost? and what type of heat delta would you need to justify that cost. Start with that and give us some feedback. What does 1deg C in heat loss cost you? I could do a thermal calculation based off the information you gave me, but before I invest time in running that, find out if you are even in the ball park for justification. If you need to save 10 deg C or (x)btu's that should shed some light in to what efficiency gains you would need. I.E. 99% efficiency gain. probably not worth it. If you only need a 5% gain, probably worth it, lets run the test.

StrykerTECH Engineering Staff

http://www.stryker-tech.com/

 

[mechanical3010]

To StrykerTECH
Any step by step calculation procedure with assumed thermal conductivity, insulation and power costs, etc., would be your useful contribution to the forum, given that all its members are not experts in all the fields. I have read some very useful references in other forums which take the reader from zero to results. Much like when we started calculus in the Engineering School, the teacher assumed no prior knowledge of it (although in high school we covered the subject fairly thoroughly) and started from first principles by drawing curves of equations showing the meaning of the differentiation and the hand drew slices of areas under the curves to explain integration.
This approach is needed in Seminars and Forums also. Since the person has the basics in the back of the mind, they will be able to put two and two together whereas non engineers would never be able to. So it will be appreciated if you bear with us novices. Many thanks.

 

[ione]

Good stuff above. You can find useful graphs in the link
below

http://geoheat.oit.edu/bulletin/bull22-3/art2.pdf

I've never used the software (QPIPE) in the link below

http://geoheat.oit.edu/software.htm

but for only 7.50 $ it could deserve a shot

 

[StrykerTECH]

Well if you want a simple calculator that will give you a good ballpark. Use this . .

http://tierling.home.texas.net/PipeHeatLossShell.htm

StrykerTECH Engineering Staff
Milwaukee, WI

http://www.stryker-tech.com/

 

[mechanical3010]

The calculator StrykerTECH has generously provided appears to be for above ground pipes. To evaluate heat loss for underground pipes we have the matter of thermal conductivity of the PVC pipe itself then that of the HDPE foam insulation, and then of the ground cover of 3 feet.
The calculator does not seem to have provision for all that?

 

[vpl]

For a first cut, you can assume that the soil is an additional layer of insulation. Some heat transfer books have k values for different types of soils (at least my Kreith does).)

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[StrykerTECH]

Step by Step just as you asked.
now be happy

The formula that IRstuff provided is a good start.
Q= S x Km x (T1-T2)
S= 2piL/Ln(4zD)

S = shape factor of pipe
L = Length of pipe
z = depth of pipe
D = diameter of pipe
All have to be in Meters
In your case
L= 457.2m
z = 1m
D = .0762m
which then
S = -2417.87 <-- don't worry about the negative heat loss is negative.

Q = S x Km x (T1-T2)

Here is where we make a whole of assumptions

Km is the heat transfer coefficient of the soil, from workign with one of our customers in the power plant business the varies tremendously especially if the soil is wet. I am gonig to use 0.9 as the formula recommends but I have seen in vary from 0.6 to 1.2, usually this is empirically determined based off field measurements

Next is the assumptions on temperature. I am going ot assume that the ground is air temperature. This is a conservative assumption generally speaking if you bury a hot pipe in the ground during winter you may see the snow melt where the pipe is buried. That means the ground is actually warmer then air temp. From a practical standpoint this is not good, but from an equation stand point it actually will yield less heat transfer because the delta between T1 and T2 is less. all other things being equal, which is another assumption entirely.
Anyway. That gives you this
Q= -2418 x 0.9 x (60-5)
Q= -120,000 watts

That is assuming the pipe outer sking temp is 60 degrees, that is also assuming the ground temp is air temp which is 5 degrees.
So how does the insulation play in? Well I ran a simple thermal simulation using foamed insulation in my heat transfer software and found that you'll pick up roughly 10 degrees in outer pipe surface temperature. Again assuming the insulation doesn't degrade in some manner. So instead of 55 degree delta your looking at 45. So that 10 degrees buys you roughly the following.
Q = -2418 x 0.9 x (50-5)
Q = -97,929 watts
How much is 22kW watts worth to you?

Keep in mind, look at the variability of your soil in the equation and also the temp delta, we are assuming best case justification for the insulation.

As a design reference, steam pipes for the power industry run a pipe in a pipe with mineral wool insulation in between. granted they are at 500 deg steam.

Hopefully I have been of assistance

StrykerTECH Engineering Staff
Milwaukee, WI

http://www.stryker-tech.com/

 

[mechanical3010]

Thank you.