Ball of Wax 1

[Reid2006]

Hi all, I've got a bit of a challenge for you (I think).

We need to melt a large tote (big plastic cube) full of wax. A heat wrap blanket was purchased to accomplish this, but the results were not acceptable. The heating blanket took 5 days to melt the wax, when ideally, we'd like to have it melted in 24 hours.

Here are the vitals:

Volume of tote: 47.2" x 39.4" x 45.7" (1.39 Cubic Meters)
Weight of wax: 83 KG
Heat of Fusion (melting) for Wax: 34 Calories / Gram
Melting Temp of Wax: 27 to 32 degrees C
Specific Heat of Wax: 2.1 KJ/(KG*Kelvin) (@ 20 degrees C)
Moloar Mass: 720 to 780 g/mol

Heating Blanket:
Wattage: 1440W @ 120VAC
Max Temp: 71 degrees C
Time taken to melt the tote of wax: 5 days
The tote has a large metal cage attached to it, so the hot blanket is not in direct contact with the tote.

It is proposed to replace the heating blanket with a hot box (essentially a large oven). This oven would heat 2 totes at once. Here are it's vitals:

Wattage: 9000W
Max Temp: 135 degrees C, plus a circulating fan to imporve convection.

Here's the question:

Can you prove, using appropriate formulas, that the hot box will in fact melt the wax faster than the heating blanket (preferably in ~24 hours).

Thanks in advance,
Adam

 

[IRstuff]

Is this for a class?

What are the specifics for the fan?

TTFN

 

[Reid2006]

Although it sounds like it, this is actually not for a class. This is for a real life application at my plant, and the original configuration failed. We must now "prove" that the proposed new method will work before we will obtain additional funding for the project.

As for the fan, I'm not sure of the particulars, but it is safe to assume that there will be slow moving air inside the hot box.

Thanks!
Adam

 

[IRstuff]

Well, in both cases, the heat source was more than adequate to cause the wax to melt, so the main issue is that access to the heat for wax. To that end, think convection oven and determine whether the fan is moving enough air for your purposes.

Note that the heat transfer coefficient of the moving air is driving the majority of heat transfer to the wax, so slow air is BAD.

Will the new configuration still have a cage around the wax? if so, all the more reason to move as much air around as possible.

TTFN

 

[insult2injury]

I don't quite understand the utility of this system/process. There are a lot of ways to melt the wax. It all depends on what the end result needs to be... i.e. quantity, time, efficiency, etc...

I2I

 

[25362]

If the tote is full of solid wax, the weight cannot be 83 kg. Is the molecular mass indeed 720-780 g ? Or is this a density figure ? This is apparently not a mineral paraffin wax. Can you enlarge about the chemistry of the wax ?

 

[BigInch]

Apparently, from the info given, the density of the paraffin is 59.6 Kg/m3. Molar weight isn't relevant to the solution, although I think the following info is,

What's the starting temperature of the blocks and oven?
What's the heat conductivity of paraffin?
What's the maximum allowed temp of paraffin during the melt?
Is there a temperature control for the oven?
What is the oven temperature setting during the melt process?
What is the oven size?
Is it a well insulated oven?
Does the oven apply heat from the top, sides and bottom equally?
Is the liquid wax permitted to remain pooling at the bottom of the oven? (will the blocks of wax float on the liquid wax?)
What is the density of the liquid wax?
Clarify or confirm that each block of wax weighs 83 Kg and that 2 blocks of that weight will be melted simultaneously.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[25362]

BigInch, you are right. I wanted to have more information on the wax (for example, on scorching possibilities by high heat fluxes) before starting to attack the heat transfer problem.

BTW, the density of the molten wax would probably be lower than that of the solid block, thus it wouldn't accumulate at the bottom but float on top of the solid. Agree ?

 

[BigInch]

You're probably right. I don't know the density of the liquid vs the solid, but as most substances arn't like water and it is less, it would fill up any void space around the solid wax block.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Reid2006]

Thanks for the input all.
I will try to have as many asnwers for you today as possible.

 

[Reid2006]

Ok. Here we go...

The wax is a Polyethylene Glycol Monomethyl Ether (In the polymer family) Trade Name: PEG M750

Website for heating blanket:

http://www.briskheat.com/products/tote-wrap-around-heater.aspx

Website for Hot Box:

http://www.benkoproducts.com/hotboxes.html

The Hot Box:
(Qty - 6) 1.5 KW Electric Heating Elements - 3 on the right and left sides of the box
Blower can be 1020 or 2000 CFM located on the top of the box
There is an oven temp control - heat capacity up to 275F
Dimensions 111 3/4" Legnth, Height 66 3/4" and 63" Wide
Hot box will hold 2 totes at once.


The tote:
WIll still be caged
The mass was given from manufacturer of wax.

Density of wax 1.09 g/cm3 @ 122F
Viscosity of wax 32mm2/s @ 122F

If you need any more info. Just let me know.

Thanks again!

 

[BigInch]

Reid, Still missing these.

What's the starting temperature of the blocks and oven? Should it start at something like 70ºF, 20ºC or what?

What's the heat conductivity of wax? As the edges of the wax heat up, some heat will be transferred into the wax block itself and begin to raise the temperature of the interior of the wax.

Assuming you don't want to smoke the wax, what is the maximum oven temperature setting you can have during the melt process?


Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[25362]

Upon making a rough estimation on about 1,350 (not 83) kg of waxy stuff one may arrive to the conclusion that your new arrangement would do the job as planned.

See please,

With the blanket, there was heat lost to the surroundings. Say, 1/3. Also assume (I feel biginch is averse to assumptions) 70 kcal/kg to cover for latent and sensible heat. Then,

[(1350)(70)][÷][(1.44)(0.65)(860)(24)] ~ 4.9 days​

With the hot box and no heat losses:

[(1350)(2)(70)][÷][(6)(1.5)(860)(24)] ~ 1 day​

Any comments ?

 

[IRstuff]

I get 40 kcal/kg, assuming a 20ºC base temp and 32ºC melting point.

But, the big question is the heat transfer coefficient of the oven, given a best case of 2000 cfm and assuming a 6" clearance around the blocks, the air speed is 13 mph. That results in a best case heat transfer coefficient of 20W/(m²ºC).

Total starting area is about 7.5 m². Assuming a deltaT of 100ºC and an average surface area of 2.5m² to account for shrinkage, I get about 1.4 days. To get a better area, you'd need to do the derivative and then average.

This all rather hand-wavy, since the melt has to go somewhere. Is there a catch basin in the oven for this? That'll affect the air volume, which affects the air velocity.


The max power is somewhat irrelevant, since the controller monitors temperature, not power. Obviously, running the oven at a delta temperature of 200ºC would cut the time down to 0.7 days.

TTFN

 

[Reid2006]

The correct mass is approximately 1518 Kg.

The whole application is indoors and starts at Room Temperature.

The max heating oven capacity is 275F.

Sorry, can't seem to find the heat conductivity.

 

[Reid2006]

25362: What is the formula you are using?

 

[Reid2006]

There is no catch basin. The tote is a plastic container (approx. melting temp of 350F) caged in by a metal frame w/ fork truck entry on the bottom of the cage.

 

[IRstuff]

Oh, that means that the max air temp can't be much about 300F, so the max delta temp is only about 130ºC. I get about 1.1 days at max air temp.

TTFN

 

[25362]

IRstuff, the molten wax transfering heat to the solid must perforce be warmer than the melting point. Reid2006 said up to 71 Celsius in his first posting.
The Cp of the liquid is usually higher than that of the solid, although polymeric solids may need some heats of crystalline transition. If I'm right, my assumption of 70 kcal/kg may be real. Do you agree ?

 

[IRstuff]

Sure, but the Cp of the liquid is irrelevant to the melting of the solid. The thermal conductivity might be more germane, but even then, the original heat blanket approach presumably had better thermal contact against the walls of the tote and took 5 days. Doubling the delta T would only get you to 2.5 days.

That makes the thermal conductivity of the liquid rather poor, so the forced convection might be the only thing that gets you close.

Another approach might be to dig a few heater stakes into the bulk of the wax while it's in the oven. This might even be done passively, e.g., with copper stakes or blades that have exposed fins in the air stream.

TTFN