Help with oven comparison....

[Chiefxxx]

Hi Guys,

I have a small problem and I am really hoping that you guys may be able to help, given the huge amounts of experience that is here. So let me explain.

I am trying to make a comparison of ovens/autoclaves and i am interested in how much energy is consumed in each process. So, i have taken both of these instruments and measured the energy.

I had a thermal (industrial oven) with a volume of 3.1 m3 which required 50 kWh of electricity to carry out a task.

I made the same experiment in an autoclave. The volume was 0.23 m3 and this equipment required 9.76 kWh of electricity.

Now I was expecting that the autoclave would need more power, however it didn't. The thermal oven used a lot more power in this instance. So how can i make a comparison??

I guess i will need to scale one or the other down or up?

Can some one point me in the right direction of how i could make a comparison ? I am guessing is not as easy as obtaining power req per volume?

Any help to get me going on this is so much appreciated.

Cheers

 

[IRstuff]

What exactly are you trying to compare, though?

Shouldn't you start with the actual requirements of the "process?" and go from there?

As for your autoclave, it's 1/13th the volume of your oven, so it's performance has to be bounced against its size.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[MintJulep]

KWh is a unit of energy, not power.

So where does the energy go?

IF the "process" is identical then the difference is losses and efficiency.

Bigger oven, bigger surface more losses, more mass, more energy going to heat up the oven not the process.

 

[Chiefxxx]

Ok thanks for the replies far. There are some things i should clear up.

Starting with the requirements of the process, The process requires that the item be at 180 deg C and it should be at that for 2 hours. Yes I could calculate how much energy it needs but this is not what I am after.

The other comment was about energy and power, sorry for not being specific.

So, maybe i should again start with the explanation.

I would like to compare a process carried out in an autoclave with a process carried out in an oven. Perhaps i was not specific enough in my question. I know that my oven is not the same as my autoclave and that the performance should be "bounced against size" and it is exactly this that i am asking. also I know that i have a bigger oven with bigger losses.

Again, i have measured the "energy" for each process in the plant i have described. and because they are different sizes I cannot make a crisp comparison.

So what do i need to do?

Can i calculate per square m or what ? How do I bounce the performance against size?

How can i compare the two? its simply not possible for me to go and buy an oven of the same size just for this test.


Thanks again, and particularly for your patience

 

[MintJulep]

I am trying to make a comparison of ovens/autoclaves and i am interested in how much energy is consumed in each process. So, i have taken both of these instruments and measured the energy.

Ok. You were interested in how much energy is used by each. You measured it. Now you know. Seems that there is nothing left to do for this part.

Now I was expecting that the autoclave would need more power, however it didn't.

Apparently your expectations were wrong.

How can i compare the two?

Compare them for what?

One is big, the other small.

One is silver, the other blue.

One uses more energy than the other to do the same thing.

 

[vpl]

It depends on what you're trying to do.

Obviously, the energy/volume ratio (which is the only things that I can compare as that's the only data you've given) is higher for the industrial oven -- it's probably not as well insulated.

However, if you're heating piece parts and you can fit three of them in the oven when the autoclave will only fit one, then it's cheaper to use the oven. Then you're looking at a comparison of energy /(#of pieces* volume).

But, as a general rule, bigger areas require more energy to heat.

Your original post mentioned that you're comparing ovens to autoclaves -- why? There are some applications where an industrial oven is what is needed and others where an autoclave is necessary. That's why both are produced -- you use the one that's right for your application.

Is this a science project?

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[IRstuff]

Again, i have measured the "energy" for each process

No, you didn't, or at least, not from you've said. You measured the power consumed by the each plant, which, obviously, is only partially related to the actual energy consumed by your "process." Was you autoclave running as an autoclave, or as an oven? There's obviously a big difference there.

Since you've not said what your "process" is, there's nothing to munch on, calculation wise. Until there's some further information about the nature of the process, it's impossible to even begin to tell you how you might be able to come up with the proper measurement. The only thing one can even do at this point is to measure the temperature and mass rate transfer of heat for each of these systems, and that's about the extent of the analysis that anyone can do on the machines themselves.

The exact thermal characteristics of the "process" is integral to any sort of analysis. Is it a step function, or something characterized by an activation energy, etc.?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Chiefxxx]

ok guys thanks to all that commented.

 

[vpl]

Consider taking a heat transfer class. Or you can try reading through this free heat transfer book:

http://web.mit.edu/lienhard/

http://www/ahtt.html

What you're asking is too general to be answered easily. It is not a simple plug and chug formula. That's why there are engineers who specialize in this area.

There needs to be some "give" of information in order to get the "take" of expertise. Both IRStuff and Mint Julep are long time posters with expertise in this area who have freely given their time and experience to a lot of posters. This is your first post, you haven't helped anyone else and appear angry because Mint gave you a no bullshit answer that you didn't want to hear and no one else has given you eactly what you want.

I totally expect that you will post a "screw you and Eng-Tips too" post with a claim that you'll find a solution to your problem on another forum. Based on the hundreds of previous posters who have made similar claims (and whose lonely unanswered posts are seen on other forums), it's no loss. There's a reason that Eng-Tips has the reputation for being the best free forum to get engineering questions answered -- and it's the time and expertise of the long term posters, not those who come in with "you're not solving my inarticulately worded general question" and don't otherwise contribute.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[Compositepro]

The cost of oven versus autoclave curing of composites is a perennial topic in the composite industry. It all depends on specific details of the design of the specific equipment and how the equipment is maintained and operated. If companies were truly interested in energy costs they would not be curing 100 lb. parts on molds that weigh 5,000 lbs.

There is no inherent reason for an autoclave to use more energy than an oven. It is simply a pressurized oven. However, the capital cost of an autoclave will always be much greater than an equivalent size oven.

 

[IRstuff]

An autoclave, at least one that sterilizes, not only pressurizes, but generates steam at pressure, which is what actually does the sterilization.

So, in addition to the raw thermal change in temperature, there is the vaporization of the water, the pressurization thereof, and the venting thereof. So for the same volume, I would expect the autoclave, given its more complex functionality, to consume more power.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[zekeman]

Chief,

If you make the assumption that most of the heat is lost by convection and radiation from the walls of the oven, and the further and more dicey one, that the insulation properties of the walls are similar, then the heat loss is proportional to the 2/3 power of the volume. In your case, given 3.1@50KW, the expected value at .23m^3 is
50*(.23/3.1)^2/3=8.75 KW
which is in the ballpark of your result.

However, I wouldn't take that calculation to the bank.

BTW, the 2/3 exponent is the ratio of surface area to volume for similar configurations of volumes.