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Associate

Join Date: Dec 2010
Posts: 34

# Energy Saving by insulating underground pipe

01/04/2011 1:39 PM

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.

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Power-User

Join Date: Jan 2006
Posts: 377
#1

### Re: Energy Saving by insulating underground pipe

01/04/2011 4:41 PM

For that 1500 feet pipe length, there is a lot of heat energy being lost. One thing to check is the temperature difference between the water at both ends of the pipe run. It is possible to calculate the energy lost using that information along with the flow rate that you gave and the specific heat capacity for water (4j/gºK).

Considering that unless there is a serious breakthrough in energy supply to replace fossil fuel, energy costs are likely to rise greatly. For that reason alone, insulating the pipe makes sense.

Guru

Join Date: Oct 2008
Posts: 42294
#2

### Re: Energy Saving by insulating underground pipe

01/04/2011 8:20 PM

Your questions leave me cold. Are you a student? Do you not have an idea how to proceed? Maybe you could take a guess so we'd know how much you know. (Or don't)

Why would you use "HDPE" as an insulator? Do you really mean polyethylene foam?

Why should we do these calculations for you?

Have you considered putting bales of hay over the pipe line? Do you think that is a joke?

Associate

Join Date: Dec 2010
Posts: 34
#4

### Re: Energy Saving by insulating underground pipe

01/04/2011 11:50 PM

Regarding the comment of lynlynch:

Yes, I am a student of heat transfer and this branch of knowledge became rusty as I had no occasion to use it for such an application since 30 years of graduation. I really did not have the idea of how to proceed and that is why I circulated the question and I know very little that borders to not knowing.

I was give to understand that HDPE insulation comes factory supplied for such applications.

The piping network is actually not as simple as I described, and the flow rate is not as steady as I described and the ambient temperature is not as constant around the year as I described. The calculation for the actual case would indeed be a task that I did not expect would be done at this forum. The figures I gave were to describe the situation in a general non complicated way to obtain a general feedback that would then be given to a specialist in the field.

The intent was not to get any figures from the forum that I would submit for the project.

Guru

Join Date: Feb 2008
Location: Australia
Posts: 2135
#3

### Re: Energy Saving by insulating underground pipe

01/04/2011 9:30 PM

I'm guessing two things before providing this feedback. Firstly that you are a student or novice in the field and secondly that English is may no be your prefered language.

The fundamentals relate to the eventual steady state heat transfer from the pipe and through the soil/compacted fill/cover material to the atmosphere.

You would need to compare the relative thermal path of 3 feet of cover (your uninsulated pipe) to 1/4 inch of HDPE covered with 2feet 11 3/4 inch of cover.

(Other assumptions: That the system reaches a steady state where only thermal loss is to atmospheric transfer.) Note that the length of the system is not important to determine which is better, it will only contribute to the magnitude of energy loss in the system.

From memory, the thermal conduction properties of plastic are similar to sand/soil and I suspect there will be little benefit. You may be better getting another foot of cover instead of the cost to insulate the pipe. (You could do the sums on this.)

The only instance that I feel there might be benefit is where there is active groundwater movement and then the insulation affects the direct energy transfer from the pipe surface and would provide significant benefit, but that's a totally different scenario than you have asked about.

Good luck.

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Associate

Join Date: Dec 2010
Posts: 34
#5

### Re: Energy Saving by insulating underground pipe

01/04/2011 11:54 PM

Your guesses about me are correct and I applaud you for your sense of judgment and intuition.

Furthermore, the engineering part of your response is meaningful and appreciated.

Power-User

Join Date: Dec 2007
Posts: 120
#6

### Re: Energy Saving by insulating underground pipe

01/05/2011 1:11 AM

"Just an Engineer" mention this before, but you should seriously consider the option of just adding additional ground cover. A few extra feet of coverage may give you everything that the insulation does without the costs of uncovering and working on the pipe. Considering you would need to dig the pipe out and refill you are probably looking at getting some kind of fill anyhow. Just adding a layer on top should be very cost effective. At roughly 5-6 feet of coverage the ground temperature tends to stabilize at roughly 50f. You may be able to achieve the extra couple of feet with some local surface scrape and a minor amount of additional fill.

Depending on the local rules you may be able to amend the extra fill you are adding with some form of (non-toxic) insulating waste. Old Styrofoam or the non-biodegradable packing peanuts come to mind. The could help with the insulating factor and put the waste to some good use. Just cover the mix with at least 4 inches of additional fill and ideally add some plant cover.

One thing to note. All these ideas are worthless if your issue is flowing groundwater. Also be careful that the pipe is capable of supporting the additional weight of the fill.

As a quick test take some measurements periodically. With a little work in an excel spreadsheet you should be able to get a rough estimate of how much energy you are losing. I think it would be optimistic to think you could reduce that by any more than 25% without some seriously cost prohibitive effort. Even 25% may be a high guess. If the energy savings from 25% of your losses doesn't pay for the project within a reasonable period of time I would say this is a lost cause.

Good luck on this.

Doug

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Anonymous Poster
#7

### Re: Energy Saving by insulating underground pipe

01/05/2011 1:43 AM

You are indeed Just (a sensible) Engineer.

The scenario at hand is that its a new project altogether, therefore the piping has not been done and so there is no retrofit or lashup involved.

The top cover can only be achieved by lowering all the services deeper in the soil to avoid any coordination conflicts, as there is no chance of building up the ground level as the finished ground level in relation to the pavement over it that serves the surrounding buildings cannot be changed.

So can I infer that going 5 feet below instead of the present level of 3 feet would enable us to cancel the insulation (of course subject to the final conclusive nod by a specialist)?

Thanks for your previous and anticipated feedback.

Anonymous Poster
#8

### Re: Energy Saving by insulating underground pipe

01/05/2011 8:26 AM

You will still have 50 degree F ground drawing away the heat. That might not be so bad if you just raise the water temperature a little over 50 degF, and then use heat pumps at the destination locations to extract heat from the cold water.

Guru

Join Date: Jun 2010
Posts: 986
#9

### Re: Energy Saving by insulating underground pipe

01/05/2011 10:30 AM

All cpvc manufacturers say not to place insulation directly on cpvc. More dirt or a 4" layer of straw will work just as good.

Raising the initial temperature equal to the difference in temperature of the water at the coldest use point.

Or put some inline tankless water heaters at the use points to bring the temperature back up to desires temperature. I'm assuming the water lines into the building are downsized to 1" or so.

Guru

Join Date: Dec 2005
Posts: 1601
#10

### Re: Energy Saving by insulating underground pipe

01/05/2011 1:07 PM

If the pipe is a meter below ground, air temp is not as significant as soil temperature and soil conductivity. A link from Google on "thermal conductivity of soil" (out of 254,000 hits):

http://en.wikipedia.org/wiki/Soil_thermal_properties

Soil thermal conductivity varies with type of soil and moisture content, and varies from 0.06 to 0.30 W/m2 °C. Given a fluid temp of 60ºC and an assumed soil temp of 10°C at 1 meter deep, I would guess an economic case for 50mm of insulation is justifiable.

If possible contact a supplier of pre-insulated piping for advise and recommendations. They will have a biased opinion, since they want to sell pre-insulated pipe, but the advice could help with your decision making for this project.

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Power-User

Join Date: Dec 2010
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Posts: 104
#11

### Re: Energy Saving by insulating underground pipe

01/05/2011 3:13 PM

My idea for it came as connecting a hose up to the Plants steam vents using an

airshaft that narrows down to hose size, and then running the hose so that it runs

underground by the side of the undergound pipe in question, may need

to use recycled glass as thick as glass rock is or the vent hose could be

braided metal rope or coil pipe that lets out steam as it runs underground.

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