Enclosure Panel Material and Heat Transfer

[Scott Chapel]

Referring to the attached, the bottom right is an electronics compartment. The external panels are 20GA steel powder coated yellow. There is 1-inch of R-6 foam board insulation on the inside of all external walls. The skid is exposed to ambient heat and direct sunlight when on site. If we replace the 20GA steel with aluminum panels, will this reduce the heat transfer into the compartment or is the powder coating & color the deciding factors?

 

[hutchphd]

How much power do the electronics use (worst case)? Where is the foam located?

 

[Scott Chapel]

Foam is on all six sides with the foil reflective side against the 20-gauge steel panels, so the compartment is encapsulated in the insulation and completely sealed. For the power consumption, we run the equipment on shore power right at 15A on 120VAC, which does not include the HVAC that is on a separate shore power circuit. The PLC uses PWM to manage the power load. As part of the overall heat load management, I am considering replacing the 20-gauge steel enclosure side panels with 18-gauge aluminum panels as well as using a lighter powder coating color. What I am unsure of is whether either or both of these modifications will make enough difference to warrant the changes.

 

[Scott Chapel]

Here are a couple of photos to give you an idea of the compartment. The external photo shows the two main sides exposed to the sun and reflective heat from the gravel pad.

 

[Scott Chapel]

I have a call with a powder coating manufacturer tomorrow to see what they have to say about their colors and textures WRT reflectivity in the IR spectrum. Hopefully they will have some answers.

 

[hutchphd]

Most of the energy (heat) from the sun is in the visible not the IR. If you want to minimize solar heating add a mirrored shield with an air space behind on the outside sunny side. The box walls can be either Al or steel...it will matter little.

 

[Tom.G]

This turned into a post that is probably more than you wanted to know - but here goes.

At 1800 Watts dissipated inside that enclosure, much of the heat is from the electrical dissipation, about 6100BTU. Get rid of that R-6 insulation, you want that heat to escape thru the sheet metal, which is about R-1.

To get the problem in perspective, if the only cooling is conduction thru the enclosure, limiting to a 10°F temperature rise above ambient requires 610 sq.ft. of enclosure surface area, in the shade - 7 times that area if using the R-6. (estimates assume convection of still air)

Solar energy is about 95 Watts (300BTU) per sq.ft. (clear day, Sun overhead)

Being in contact with the enclosure, that foil effectively becomes part of the enclosure and serves no useful purpose. If you must have the insulation for sound deadening, space it from the enclosure, that way the foil reflective layer can radiate some of the solar heat back out.

Then as @hutchphd suggested, add that mirrored shield with an air space behind.

If the horizon is visible from the enclosure, you might want to tilt the mirror down a little to avoid blinding people around Sun rise & set.Cheers,
Tom

 

[Scott Chapel]

Thanks Tom and this is exactly the conversation for which I have been looking! In addition to the PLC, computer, modem, and 120VAC-24VDC power supply putting out heat, there is a heated head pump, a heated filter, and a heated valve. This train is connected with 1/4" stainless tubing with Swagelok fittings. All three devices maintain 375F and are insulated. The tubing connections are wrapped in felt insulation to maintain the same temperature. With all of the insulation, the heat does not immediately radiate into the compartment and some of it is expelled through an exhaust line. Once the PLC stops heating the equipment, it takes about 1.5 hours for the HVAC to cool the compartment down to ambient, depending on the ambient temperature at the time. Obviously this is because the heated devices are radiating through the insulation over that period of time. I had thought about removing the insulation, but wasn't sure if that would work considering the interior temperature is maintained at 76F, so wouldn't the higher ambient temperature heat the compartment?

 

[Tom.G]

I had thought about removing the insulation, but wasn't sure if that would work considering the interior temperature is maintained at 76F, so wouldn't the higher ambient temperature heat the compartment?

Since thermal energy will flow from a hotter area to a colder area, that depends on the internal temperature of the enclosure. Unless you are in the tropics, I expect the ambient temperature + insolation to be well below the interior temp with the HVAC turned off.

You have not said if the HVAC is ON or OFF during initial warmup. Your wording seems to imply HVAC is OFF during warmup. Statements 1 & 2 below are based on that assumption.

How hot does the interior currently get before the HVAC cools it down?

1. If your desired 76°F is above ambient, removing insulation will reduce the HVAC energy needed - but will also slightly increase the energy needed to heat the internal devices..
2. If your desired 76°F is below ambient, removing insulation will increase the HVAC energy needed during operation.
​

Remotely getting all the details of your configuration here, and doing the detailed calcs would probably take longer than you just trying the options and collecting the data.

Overall, a somewhat better solution is to eliminate the problem by having the hot devices and cold devices in separate enclosures. If this is not practical for your situation, try one of these approaches:

3. Put a double-walled barrier (a plenum chamber) between the hot and cold devices, isolating them, and blow ambient air thru the plenum.

4. Put as much insulation as possible between the hot and cold regions of the enclosure.

Either will effectively thermally separate the hot and cold equipment and reduce the HVAC load. Insulate the hot chamber (and maybe the cold chamber) as needed by your environment.

Please keep us updated on the project.

Cheers,
Tom

 

[Scott Chapel]

When the system powers up, the recirculating fans in HVAC units come on for three minutes. This gives them time to go through pre-start checks as well as start moving air about the enclosure to get a proper initial temperature reading. We also have recirculating fans in the compartment that also come on to move the air around and reduce hot spots. If the internal temperature of the compartment is below 50F, the heating unit in the HVAC comes on to raise the temperature to 60F, which is the minimum operating temperature of the analyzer. Once it reaches 60F, the HVACs will go into fan only mode. While this is going on, the PLC will be powering up the heated equipment that will also assist in raising the temperature in this situation. On the other hand, if ambient temperature in the compartment at startup is above 70F, the HVAC units will begin cooling after the three minute pre-start checks. Either way, the system is setup to keep the ambient temperature in the compartment between 70-80F, which is the best operating temperature for the analyzer. If the ambient temperature is below the operating temperature of the equipment in the compartment, I am not concerned with raising the internal temperature since we have the heat mode on the HVAC units and the heated equipment raising the internal temperature.

We did look at the option of putting the heated equipment in a separate compartment, but do not have enough space to do this, so we increased the HVAC capacity as well as added additional insulation to the heated devices to contain the heat and reduce the amount of power and heat loss into the compartment. This really helped! We also repositioned the heated equipment to the top-rear of the compartment and added the recirculating fans to move the hot air from this equipment toward the HVAC intake. This also helped. It is not feasible to separate the heated equipment from the other equipment, other than we have put them on different sides of the compartment, since it is all one compartment and we cannot split it. Also, the heated head pump, which radiates the most heat, has a max operating temperature of 110F for the pump portion, so we have to have it in the HVAC portion. We also looked at ventilating the compartment, but it has to be IP66 rated and ventilation system that meet IP66 are too big. We also run into a humidity/condensation problem in the compartment, as you can imagine.

Based on your #2, it appears that we want to keep the insulation in place since we are concerned with a higher ambient temperature than a lower.

From what I can tell, we have two additional options. One is materials, color and texture and the second is some sort of heat sink to transfer heat from the heated area of the compartment to the outside. We are currently looking into the heat sink. It is the materials, color and texture option that has me stumped. At first I thought that using aluminum for the compartment side panels painted white would reduce the heat transfer, but then I started reading that for our higher ambient temperature scenario, steel would transfer heat slower than the aluminum, but I cannot wrap my head around the heat/thermal conductivity formula in my application. Been too much beer and time since I took thermodynamics!