Heat loss and insulation

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JR.
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Heat loss and insulation

Post by JR. »

As I have often mentioned reducing heat loss with better insulation can reduce energy consumption in heating applications.

My latest experiment related to my coffee roaster. I have been roasting coffee for 15-20 years .

I use a premium drum roaster called a hottop because it is metal and gets hot. The beans get up to 420'F while roasting so the roaster has guards to prevent accidentally touching the hot top in use.

Image

I have wedged some loose fiberglass insulation up into the air space behind the hand guards and it is making a definite difference in shorter roast times.

This is not automatically good if it alters the roast profile, and causing the outside of the roaster to rise to higher temperatures might affect the innards. In hindsight I will remove the insulation from the rear few inches where the drum stops and the motor is... the drum only goes back as far as that black handle on the top where the chute is for feeding green beans into the drum. Running the motor hotter would not be good. :oops:

An experiment still in process.

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Re: Heat loss and insulation

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hottop.JPG
hottop.JPG (59.49 KiB) Viewed 10608 times
Not a great picture because of very low light level but you can see the loose insulation under front half of the hand guard... I left the back half open so motor and other innards don't get too much hotter.

You can see a fresh roast (Brazilian) in the cooling tray... With the insulation the roast now takes a few minutes less than before. I'll need to see if that changes the coffee flavor, while this is my first time roasting this particular Brazilian green bean so I do not have a reference to compare it to.

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Re: Heat loss and insulation

Post by mediatechnology »

An egg needs a temperature of 158°F to become firm. In order to cook, proteins in the egg must denature (modify), then coagulate, and that won’t happen until the temperature rises enough to start and maintain the process.
Source: http://www.loc.gov/rr/scitech/mysteries/friedegg.html

OK.
The heatsink temperature on the 10W class-A power amp is 195 degrees F.
As long as we're using Peltier thermoelectrics for a crock pot and insulating the roaster I'm beginning to think I should entertain a power amp egg-fryer.
The sidewalk presents several challenges to (frying an egg.)
But there's hope for using a heatsink:
Something closer to the conditions of a frying pan would be the hood of a car. Metal conducts heat better and gets hotter, so people actually have been able to cook an egg on a car hood's surface.
So I'm encouraged.
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Re: Heat loss and insulation

Post by JR. »

mediatechnology wrote:
An egg needs a temperature of 158°F to become firm. In order to cook, proteins in the egg must denature (modify), then coagulate, and that won’t happen until the temperature rises enough to start and maintain the process.
Source: http://www.loc.gov/rr/scitech/mysteries/friedegg.html

OK.
The heatsink temperature on the 10W class-A power amp is 195 degrees F.
That's uncomfortably warm for a heat sink.... perhaps hot enough to coagulate an egg but "frying" is more of a 350'F exercise.

As long as we're using Peltier thermoelectrics for a crock pot and insulating the roaster I'm beginning to think I should entertain a power amp egg-fryer.
My Peltier cooker experiment was pretty much a wash out, for Peltier specific limitations. The technology is great for cooling down small masses, but for cooking not so much. I bailed and shifted to power resistors.... I used the cooker yesterday to cook my once a week big meal.
The sidewalk presents several challenges to (frying an egg.)
But there's hope for using a heatsink:
Something closer to the conditions of a frying pan would be the hood of a car. Metal conducts heat better and gets hotter, so people actually have been able to cook an egg on a car hood's surface.
So I'm encouraged.
The classic automobile cooker was to wrap the food item in aluminum foil and stick it on the exhaust manifold, while if you only want <200' you could put it on the intake manifold.

While this may be contrary to your mission if you want to cook with your class A amp, restrict the airflow and use something like my thermal insulated box to hold the heat in. I suspect the eggs won't consume that much heat to congeal. Frying temps may be hotter than comfortable for the devices. One of the limitations with the Peltier assemblies for cooking is the solder temp used to connect the sub assemblies together (Peltier use a bunch of cells in series sort of). Hard to use semiconductor heat above solder melting temps. :oops:

JR

{edit] Note: I removed the insulation for my last batch roast of coffee. I may just keep using it stock... Letting the drum motor and other parts get even hotter is not a good idea for reliability... inside the drum gets well over 400''F, [/edit]
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Re: Heat loss and insulation

Post by mediatechnology »

I think the laser-pointer-assisted IR thermometer I have is inaccurate at close range. (inches)

I backed away about 2 feet with the laser spot on the HS and it's more like 175 degrees F.
I need to use the contact temp probe.
Still, I'm hopeful about the egg cooker...
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Re: Heat loss and insulation

Post by JR. »

I looked into temp sensors before and don't see how the IR could be accurate at all.. I appreciate that for extremely hot stuff you cant use a simple junction or thermocouple. but for modest temps I like using a device pn or np junction.

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Re: Heat loss and insulation

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I backed away about 2 feet with the laser spot on the HS and it's more like 175 degrees F.
I checked it with the junction-based probe and it also reads about 175 F.
I've thought all along I wanted to sandwich a third heatsink into the middle of the stack.
I looked into temp sensors before and don't see how the IR could be accurate at all.
Wel,l its part of a Ryobi Ultrasonic measuring device and I never trust its distance measurements.
I think they had to put in the IR to calibrate the distance for temperature.
I've used it before for temp though and compared it to the probe and they were close then.
But at short or long range - even with the laser spot - it's hard to know what the IR actually "sees."
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Re: Heat loss and insulation

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IIRC the non-contact temperature sensors respond to the thermal radiation heating up the chip sensor, so it seems to me distance would be a variable factor. Probably fine for relative temp measurements.

years ago I wanted to combine a thermal video capture with digital frame store to use in production testing of power amp PCBs or assemblies. The thermal image should help identify loose power transistors or even wrong values if the operating current was affected. The thermal image would be a very quick go/no-go test to confirm the power amp was set up and operating properly.

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Re: Heat loss and insulation

Post by mediatechnology »

I saw a YouTube demo of a Fluke IR camera the other day that was really cool.
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Re: Heat loss and insulation

Post by mediatechnology »

So guys - What would you recommend for an absolute maximum power device temperature?

The rule of thumb I've always heard is that if it's too hot to touch it's too hot.

I'm reading 80 deg C at the heatsink.
80 deg C is not something you want to grab.
The devices have a 150 degree C maximum junction temperature.
They also have a 100 deg C beta curve which is pretty hot.
I haven't calculated the thermal resistance to arrive at junction temp but I do ultimately want a bigger heatsink.

HeatsinkUSA looks interesting: http://www.heatsinkusa.com/
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