Heat Gain In a Chilled Water Circuit

[NALUCAS]

Hi

I'm new to the group, work in the HVAC industry and have a 'hopefully' fairy simple question for you all!

I am assessing the temperature rise of water running at 1 deg c through a copper pipe covered with SL grade styrene, which is located within the roofspace of a building. So far i have not been able to find any help in my thermodynamic texts.

Basically what i am trying to derive is the required thickness of the insulation to avoid getting an excessive temperature gain in my water circuit.

Could somone please point me in the direction of a formula which will allow me to work this out, or towards a piece of software that will be capable of working this out.

Thanks in Advance

Nathan

 

[desertfox]

Hi NALUCAS

The formula you need for your situation is this:-


Q= T1-T3/((LOGe(r2/r1)/(2*3.142*L*K12))+..............
.......((LOGe(r2/r3)/2*3.142*L*K23))

where r1 is = inner radius of pipe
r2 is = outer radius of pipe
r3 is = inner radius of pipe insulation
r4 is = outer radius of pipe insulation

k12 and k23 are the thermal conductivities of the materials respectively
L is the length of pipe
T1 is the temp. inside the pipe
T3 is the temp. outside the pipe

Hope this helps
desertfox

 

[quark]

Here is the best free software for a download.

http://www.pipeinsulation.org/pages/home.html

 

[chicopee]

Hello!
you should reference a heat transfer text or a mechanical engineering handbook such as Mark or Kent for the appropriate formulae on insulation. Thermodynamic texts do not deal with this type of problem. Bear in mind that too little or too much insulation will be incorrect therfore particularly check on the subject of the economic thickness of insulation which will require knowledge about the thermal conductivities of the three layers of the piping and heat transfer coefficient the latter of which will require knowledge about wind effects on the insuated piping. There may also be reflection effects from nearby objects such as ducting, air handling, aluminized coated roofing that you must also evaluated.

 

[25362]

Don't take the following very seriously: there is one theoretical interesting point sometimes raised in respect of pipe thermal insulation with conductivity k, in a convection environment with h as coefficient of heat transfer. Both when the pipe fluid and the environment are at fixed temperatures, there is a critical radius of the insulation called r_o = k/h.

If the outer radius of insulation r_o is smaller than k/h, heat transfer will increase by adding more insulation .
As normally expected, for outer radii greater than the critical value an increase in insulation thickness will cause a decrease in heat transfer.

From my own experience r_o is generally small enough so that any practical insulation is well above this critical value.

 

[rjw57]

Excellent reply 23562! Most people take the critical insulation thickness thing to the extreme based on a poor understanding of the phenomena. I get the feeling that many people assume that, in any instance, you might be able to add insulation and increase heat transfer from the surface, which is just not so (as you so ably pointed out). Great treatment of this is found in "Heat Transfer, Professional Edition" by Thomas. Anyone involved in learning about insulating surfaces should read up on this topic.

 

[vpl]

25362

An interesting aside about the "critical value of insulation". I have seen a case where a company had a problem with calcium carbonate (lime) plateout in their heat exchangers. They would clean them out, and then get real good heat transfer for a couple of cycles before it dropped off sharply - because they'd exceeded the "critical value" of the calcium carbonate and it started acting like an insulator. They were having a dickens of a time trying to explain why there was such a sharp drop - they expected there would be a gradual decrease in heat transfer. Patricia Lougheed

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