Industrial Oven Heat Gain 1

[mech5]

I'm designing a ventilation system for an oven room and need to know what is the best way to account for the heat gain into the room from the ovens.
There are 5 ovens that are rated at 47kW and one gas fired at 250,000 btu/hr. I think that using the rated input will yield too much cfm's, so I'm thinking there is some divesity factor or "rule of thumb" to use for my estimate. In addition, I'm using a delta T of 15 degree F for my CFM calculations. Can anyone offer some insight. Thanks.

 

[IRstuff]

Frankly, I'd rather be in an oven room that's too cold than one that's marginally warm.

TTFN

Eng-Tips Policies FAQ731-376

 

[MintJulep]

Can the heat from the ovens go anywhere else?

The gas-fired oven probably has an exhaust. Is combustion air taken from inside the room, or outside?

 

[mech5]

all ovens (electric and gas-fired) have a flue that I assume is exhausting to the outside.

 

[tomwalz]

What will be the external temp of the ovens in operation? Can you take the surface area and treat it as a source of thermal radiation at that temperature?

What will be the internal temperature when the door is opened for unloading?

What temperature will the material be when it is unloaded and how much material will there be? In my experience, ovens are almost always unloaded at a temperature above fully ambient in order to reduce cycle time.

What it the internal temperature of the oven when the door is open.

We used to run a process where we ran the ovens up to 2,000 F, opened the door and loaded parts. When the ovens came back up to 1500F, we reopened the doors and unloaded about a pound of parts at 1500 F and let them cool in the room.

As the rest pointed out, most to the heat generated will be handled as part of the original design.

While you are doing this consider operator safety. When you open a door that is 12” x 12” at arms length on an oven at 2000 F the heat hits you in the chest with a force you can feel as a force as well as heat.

tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[MintJulep]

For a pretty good ball-park, measure the cfm and temp of the flue gas (confirm that it is actually going outside).

Depending on the temperatures you are running, don't forget to correct for density.

You could then assume that the difference between heat input - and here I mean real input, not nameplate rating - and heat in the exhaust is going into the room. At least under steady-state operation.

As others have pointed out, the material in the ovens will have heat when removed, so you need to consider where those parts go.

 

[mech5]

There are several ovens that have different operating temperatures, however, I don't think that any one will exceed 850 degree F.
Sometimes the operators leave the doors open to cool down the ovens faster. In some cases, the material is left in the oven with the door open or ajar to cool everything down at a gradual pace rather than subject it to thermal shock.

 

[civilperson]

Add make-up air supply at 150% of the flue flow and also provide 10 changes of air per hour, (volume of room/6 minutes). The temperature in the room will approximate the temperature of the make-up air or higher.

 

[tomwalz]

Sounds like you need measurements as well as some idea of the processes.

Tell the operators you need their help to keep them safe and comfortable. If you are lucky you work in a place where the company truly believes that operator comfort increases productivity. And the operators believe the company believes it. Check with the manager, foreman and unofficial team leader.

Spend a lot of time there making measurements. Ask the team when it gets hottest.

Maybe put in high capacity ducting with a thermostat. Be sure to give the operators the ability to override the thermostat.

Remember a room is hotter for guys working than for an engineer standing.

Check clothing. Recommend all cotton if they get hot. JC Penney Big Mac work shirts – all cotton, reasonable price, long lasting, catalog only. Get the blue chambray, not the hickory (mattress pattern fabric.)

Remember to allow for what the operator feel based on outside temperature. People seem to like rooms warmer in winter and cooler in summer.

Can you just ignore the ovens and go with the room in you calculations? Ignore why the room is warm and just calculate how much air you need to change to cool it.

Where does make up air come from?

tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[zekeman]

If,as you say, you can expect oven temperatures of about 850, I would make the conservative assumption that the walls will reach that temperature for the sake of the calculation. If you can get the surface area, i. e. the outside area of all of the surfaces, then from my calculation of radiative and convective heat transfer assuming an emmisivity of unity, the effective overall coefficient is about 7.5 BTU/hr/ft^2/deg F.
Note that the radiation coefficient is
hr=e*0.173[(T1/100)^4-(T2/100)^4]/(T1-T2)
T1,T2 in deg Rankine
e= emmisivity assumed = 1
For example if your area is 50 ft^2, you would get
Q=50*(850-70)*7.5=292,000 BTUH.
You then size the ventilation accordingly, to remove this heat.
If you can get better data on surface temps, then of course, these numbers would be smaller.