What do you think???

[BottleFed70]

Originally Posted by ed_denu

Here's another way of looking at the size versus wattage question. The oven you're proposing is 600 cu. ft. with 24,000 watts. This computes to 40 watts per cu. ft.

I might be making a false assumption that wattage requirements can be computed in a linear fashion based on oven size, but if other members would like to share their oven measurements and element wattage it might make for interesting comparisons.

My oven is 27 cu. ft. and has 5,200 watts, or 192 watts per cu. ft. For my needs the heat-up cycle time is acceptable at around 25-30 minutes.

I think a better way to look at this would be to evaluate wattage per square foot (surface area of the inside of the oven).

A 5x5x5 oven and a 10x10x1.25 (extreme example) oven both have 125cubic feet of internal volume, but the 5x5x5 oven would be much more efficient due to the much smaller amount of surface area.

[ed_denu]

Originally Posted by BottleFed70

I think a better way to look at this would be to evaluate wattage per square foot (surface area of the inside of the oven).

A 5x5x5 oven and a 10x10x1.25 (extreme example) oven both have 125cubic feet of internal volume, but the 5x5x5 oven would be much more efficient due to the much smaller amount of surface area.

You make a valid point. It takes energy to heat the inner skin, so surface area would be a consideration. I guess no two oven heat patterns would be identical due to placement of the elements, construction material, etc. I was just looking to determine if a general observation could be made as to a minimum wattage requirement for a given size. Ed

[BottleFed70]

Originally Posted by ed_denu

You make a valid point. It takes energy to heat the inner skin, so surface area would be a consideration. I guess no two oven heat patterns would be identical due to placement of the elements, construction material, etc. I was just looking to determine if a general observation could be made as to a minimum wattage requirement for a given size. Ed

Its not so much the heating of the inner skin. Rather, the heat lost is transfered through the oven walls. The more surface area (amount of wall) the more heat that will be transfered. Thats why the online calculators ask for the dimensions of your oven rather than the cubic footage.

[ed_denu]

Originally Posted by BottleFed70

Its not so much the heating of the inner skin. Rather, the heat lost is transfered through the oven walls. The more surface area (amount of wall) the more heat that will be transfered. Thats why the online calculators ask for the dimensions of your oven rather than the cubic footage.

The only online calculator I've seen mentioned on this site is in an Excel spreadsheet format. While it does ask for oven dimensions, they are only used to compute internal volume. There is no calculation within it that references surface area. What other oven calculators are available?

[CarWiz]

The calculators are all over the web. Google "BTU calculator" and you'll get hundreds. Some convert that to watts needed to raise the temperature to X degrees over ambient. Which is exactly what you need. You can add "heat" to the end and probably narrow-down the hits.

I looked in my Favorites but could find the one I've been using. I forgot where I put it or I'd give you the link.

[CarWiz]

Also keep in mind that the BTU calculators arrive at a BTU and/or watts based on per hour. If you want your oven to heat faster than that then adjust your heat (watts) accordingly. I found this in my oven notes to convert BTU/watts.

The Handbook of Chemistry and Physics lists conversion factors; many
calculators also have the ability to convert watts to BTUs. One watt of
energy equals 3.412 BTUs per hour. So if you equipment is generating 1000
watts of heat, it is putting off 3412 BTUs of heat per hour.

[ed_denu]

Originally Posted by CarWiz

The calculators are all over the web. Google "BTU calculator" and you'll get hundreds. Some convert that to watts needed to raise the temperature to X degrees over ambient. Which is exactly what you need. You can add "heat" to the end and probably narrow-down the hits.

I looked in my Favorites but could find the one I've been using. I forgot where I put it or I'd give you the link.

I've seen some of the calculators on the web, but most appear to be aimed at other applications than oven building, such as computing btu requirements to heat a room or for sizing a fireplace/furnace, etc. I've not seen many that are specific to oven building.

[BottleFed70]

Originally Posted by ed_denu

The only online calculator I've seen mentioned on this site is in an Excel spreadsheet format. While it does ask for oven dimensions, they are only used to compute internal volume. There is no calculation within it that references surface area. What other oven calculators are available?

A good one to start with is the one linked to from the sticky thread in this forum.
http://www.heatershop.com/btu_calculator.htm

It really comes down to the fact that heat is lost through the walls. More walls = more lost heat.