Heat Loss in a Hot Water Pipe System

[pjgregory]

Hi

I hope you do not mind me joining this discussion, but I found this thread on a Google search. I have a related problem.

I am thinking of installing a set of Glass tube heat pipe solar pannels to help run my underfloor heating system. My only problem is that they can only be situated about 50 meters from the house, so the flow and return pipes will have to be burried underground. What I would like to know is how much heat loss I can expect in the pipes?

The pipes will be 30 - 40 mm diam Polyethylene water pipe, burried 0.75 m below ground. I am not sure how I could insulate these - I would need some type of waterproof foam plastic lagging as glass fibre would pass ground water. When the system is working (sun shining) the flow temperature will be 40-50 C with a return of 10-20 C lower. When it matters, in the winter, I assume a ground temperature of 2-10 C . This would effectively be a steady state system as the main issue is how much heat can I get during a winterday when the sun is shinning.

Any help or advice would be most welcome.

PJG

 

[Artman]

Hi

I hope you do not mind me joining this discussion, but I found this thread on a Google search. I have a related problem.

I am thinking of installing a set of Glass tube heat pipe solar pannels to help run my underfloor heating system. My only problem is that they can only be situated about 50 meters from the house, so the flow and return pipes will have to be burried underground. What I would like to know is how much heat loss I can expect in the pipes?

The pipes will be 30 - 40 mm diam Polyethylene water pipe, burried 0.75 m below ground. I am not sure how I could insulate these - I would need some type of waterproof foam plastic lagging as glass fibre would pass ground water. When the system is working (sun shining) the flow temperature will be 40-50 C with a return of 10-20 C lower. When it matters, in the winter, I assume a ground temperature of 2-10 C . This would effectively be a steady state system as the main issue is how much heat can I get during a winterday when the sun is shinning.

Any help or advice would be most welcome.

PJG

Fill the trench around the pipes with perlite insulating concrete.
provides a k factor range of 0.58 to 0.66 Btu-inch/h-ft2-F (0.085 to 0.095 W/m-k).

 

[pjgregory]

Hi Artman

Thank you for your response. Let me see if I understand this.

The heat loss in Watts is

Watts = k * Area * Temperature Differnce / Thickness Material (L)

So assume k for Perlite concrete is 0.09. Then simplifying for one pipe laid in a cylindrical tube of concrete radius 0.25m (L), the area per unit length would be 1.57 so for 50m of pipe and a 40 degree K temperature difference, the loss would be:

Loss = 0.09*1.57*50 *40/0.25 = 1130 watts.

So I will need to add about 1Kwatt to estimated load for the house to compensate for the heat loss in the underground pipes.

I have found a supplier of Perlite products in Italy. Now all I have to do is work out how to explain this to my builder in Italian.

Thanks again

PJG

 

[Artman]

Here is another possibility to consider. Preinsulated pipe. I don't know which would be cheaper or easier.

http://www.maxx-r.com/?gclid=CP-Z4IiN3YQCFRs1Swod6jZ6Ww"

 

[heatmover]

copper water interface heat transfer

Hello,

Great discussion. Any references for a similar problem?

Copper pipe is bathed in a hot material.
Copper pipe has cooler water flowing through the pipe.

How to calculate heat flow through the copper pipe to moving water?

Non laminar flow is thought to improve heat transfer to water.

 

[Artman]

Hello,

Great discussion. Any references for a similar problem?

Copper pipe is bathed in a hot material.
Copper pipe has cooler water flowing through the pipe.

How to calculate heat flow through the copper pipe to moving water?

Non laminar flow is thought to improve heat transfer to water.

What you are describing here is a unit called a "heat exchanger". The calculations for a heat exchanger can be rather complex. Dealing with the mediums, the temperature difference entering to leaving of both the transfer medium and the product being heated, the materials used in the exchanger, the insulating/transfer values of the tube side and shell sides. For references to this problem try searching for information on "Copper tube heat exchangers."