# Minimizing heat transfer to the contents of a non-airtight pipe

Consider the following simple (half)pipe setup:

• The grey part is a galvanized-steel half-pipe.

• The green part consists of the ABS plastic non-airtight top lid and an internal layer of ABS plastic covering the inside of the metal pipe. Lid is removable, and has a number of evenly-spaced through-holes covered with snug fitting, but non-airtight removable ABS smaller plastic lids

• The Blue part is the liquid contents of the pipe. Liquid comes from the HVAC chiller plant,has a temperature ~10 Degrees C lower than ambient temperature and has thermal properties very similar to water. Flow rate is slow enough to be considered negligible (i.e. liquid is almost stagnant).

• The Red part is the proposed thermal insulation around the half-pipe to reduce the heat exchange rate between the metal and the outside.

The pipe will run under the transparent roof, meaning it is shielded from the rain, but is fully exposed to the sun. Ambient air temperature is expected to be in the range of 30-35 degrees, 60-70% RH with the luminance ratings of about 80000-100000Lux

I understand that without more information about the setup it is impossible to do even the back-of-the envelope estimations. Therefore, I'd like to ask you to answer the questions below based on your practical experience:

1. How good (in general) is the commonly used pipe thermal insulation (fiberglass foam or spray foam) at keeping the heat out relative to how good it is at keeping the heat in? I.e is insulation efficiency the same "both ways" as opposed to screening from the cold environment only?

2. What insulator material/brand works best for keeping the contents of the pipe cooler than the ambient as opposed to keeping the contents warmer than the ambient?

3. Will adding the reflective foil layer to the surface of the top cover help minimize heat transfer to the liquid in any meaningful way? Meaningful meaning at least 2 degrees increase in ambient temperature required for the liquid inside to heat up by 1 degree.

4. If you wanted to do a thermodynamic simulation of the heat transfer for this system, what software would you use?

Hot and cold insulation may be different materials, but generally the same foam or fiberglass insulation is used. That the choices are minimal indicate to me that they work fairly well. I don't remember the equations, but I do remember that at some point adding insulation thickness actually results in increased heat transfer (God, that 7 am class we took to get the easy & cool heat transfer prof). You will need sufficient thickness to prevent condensation in your high-humidity environment.

I suppose that reflective foil would have some value, though in practice you see that on steam pipes, not chiller pipes. I'm guessing it is because the temperature differential isn't enough for chiller pipes to warrant it. More insulation on top of the pipe would be a better choice in my opinion.

I would hire a local industrial mechanical contractor to spec it out, they would know what installers are familiar with in your region and what works.

1. It is possible to combine the three modes of heat transfer — conduction, convection, radiation — into an equivalent heat transfer coefficient. The contribution from each would depend on parameters relevant to the specific mechanism — for conduction, the geometry and property of the insulation.
2. Once the heat transfer is known, total heat flux (rate per unit area) will depend only on surface area and temperature difference between fluid and ambient — i.e. only temperature difference is relevant, not whether heating or cooling. Having said that, heating or cooling may be different because the convective heat transfer is slightly different (via fluid property and flow of that fluid inside). For back of the envelope calculations, this may be ignored and the actual difference may not be significant for most cases.
3. Any additional layer will reduce heat transfer. A reflective layer will have a large influence on the radiation conponent.
4. EES is a useful tool (Ive never used it) but may already be an overkill. You may find similar toolboxes/codes in python.
5. For more details you should browse through any basic heat transfer book. One by Lienhard and Lienhard is available freely.