Heater Element Selection

[vtmike]

Hi,

I am bulding a simple drying oven with dimensions 6' x 2' x 2'. The oven needs to operate within a temperature range of 150 F ~ 200 F.

Construction of the oven will consist of Al or Steel walls with a 2.5" thick mineral wool insulation & the temperature will be maintained with a PID controller connected to a thermocouple.

When I used the Oven temp time to rise calculator excel sheet, I got a rise time of 30 second with a 500 Watt heater element. I am having trouble selecting a heater element. Should I go for an infrared heater light with 500 Watt capacity or a heater element that is normally used in a conventional oven with 500 Watt capacity? Any suggestions?

How can I maintain the the oven temperature at a particular value? Will the PID controller turn the element on and off too many times & in the process damage it? Would I need to put vent holes in the oven to allow circulation?

[ed_denu]

Hi,

I am bulding a simple drying oven with dimensions 6' x 2' x 2'. The oven needs to operate within a temperature range of 150 F ~ 200 F.

Construction of the oven will consist of Al or Steel walls with a 2.5" thick mineral wool insulation & the temperature will be maintained with a PID controller connected to a thermocouple.

When I used the Oven temp time to rise calculator excel sheet, I got a rise time of 30 second with a 500 Watt heater element. You might want to double check your input values. I ran the calculator with a 24X24X72 oven for a temp rise from 65 to 175 and with 500 watts it calculates 42 minutes? Using a 2600 watt element the heat up time is calculated at 8 minutes. I am having trouble selecting a heater element. Should I go for an infrared heater light with 500 Watt capacity or a heater element that is normally used in a conventional oven with 500 Watt capacity? Any suggestions? Use an oven element rated around 2000-3000 watts and 240 volts. Alternately if you want to run this on a 20 amp 120 volt circuit look for an element around 1500 watts at 120 volts. The later would approximately double the heat-up time.

How can I maintain the the oven temperature at a particular value? That's the function of the pid, to reach and maintain the set temperature. Will the PID controller turn the element on and off too many times & in the process damage it? No Would I need to put vent holes in the oven to allow circulation? yes, most all ovens have a vent hole.

You will also need an ssr or contactor to drive the element load.

If you decide to go with the 120vac solution and have problems finding 120 volt elements you could use 240 volt elements, you just get reduced output. (2) 2600 watt elements would get around 1300 watts total output with 120 volts.

[piv1]

Has any one heard of these Powder coat ovens from Habor Freight?

230 V Powder coat oven, 500W, 60hz, Interior 18" W x 18"H x 18-1/4 D- (one 500W heating block) or the

Voltage: 110V; Amps: 13.8 amps (running), 14.8 amps (start up); Heat range: 0-480° Fahrenheit, 0-250° Celsius; Oven dimensions: 18" W x 18" H x 18" D; Overall dimensions: 31-1/2" x 24" x 23-1/4" (Three 500 watt heating elements )

How long will these ovens take to heat up?
Which one is more likely to heat up quicker?
Are these ovens worth $300 bucks?

[ed_denu]

Has any one heard of these Powder coat ovens from Habor Freight?

230 V Powder coat oven, 500W, 60hz, Interior 18" W x 18"H x 18-1/4 D- (one 500W heating block) or the

Voltage: 110V; Amps: 13.8 amps (running), 14.8 amps (start up); Heat range: 0-480° Fahrenheit, 0-250° Celsius; Oven dimensions: 18" W x 18" H x 18" D; Overall dimensions: 31-1/2" x 24" x 23-1/4" (Three 500 watt heating elements )

How long will these ovens take to heat up? Obviously, the 1500 watt will heat up much quicker than the 500 watt. Heat up time for the higher wattage would probably be around 5-7 minutes. I'm not sure why they make a 500 watt, 240 volt oven unless it was marketed for Europe which uses 240 volts as a standard.
Which one is more likely to heat up quicker?
Are these ovens worth $300 bucks?Not imo. See comment below.

Personally I would look for an old kitchen oven which can often be found for free and would be approximately the same size.

[piv1]

Thanks Ed - Appreciate it.

[vtmike]

Thanks Ed. The rise time with a 1500 Watt element would be about 16 minutes which is what I am planning to use. Why do you suggest using an oven element over an infrared light heater element? If the wattage is the same for both, it shouldn't really matter right? Also why would the heat time double with a 1500 Watt element at 120V?

[Grumple]

The wattage rating on the heater element is for a certain voltage. So a 1500 watt heating element rated at 240 volts will have a certain amount of resistance that can be checked with an ohm meter. Or 1500 / 240 = 6.25 amps. Then voltage divided by amps = ohms: 240 / 6.25 = 38.4 ohms. So with 120v you would get 1500 / 120 = 12.5 amps and 120 / 12.5 = 9.6 ohms. So you would need a heater element rated at 120v with 9.6 ohms to do the same thing.

[ed_denu]

Thanks Ed. The rise time with a 1500 Watt element would be about 16 minutes which is what I am planning to use. Why do you suggest using an oven element over an infrared light heater element? If the wattage is the same for both, it shouldn't really matter right? Also why would the heat time double with a 1500 Watt element at 120V?

I'm not sure what type of infrared heater you were proposing to use, but I believe there are a couple of issues. First, most of the IR heaters I've seen were not meant to be placed within an enclosure. Some ambient air is expected to cool the heaters case and any electronics within. Placing the whole heater within an oven enclosure may lead to a shortened life.

Secondly conventional ovens heat two ways, convection where the air is heated and radiant where the heat from the elements 'reflects' on the part. My understanding of IR is that it primarily heats objects, not air. If the parts you intend to process are flat, then you may get balanced heating across the part with the IR. All heat sources, including IR heating will produce some convection heat as the objects it heats, including the oven walls will then heat the enclosed air.

I'm not sure what your application is as you are only looking to heat to 200 degrees maximum, so you may have other alternatives versus an oven built for powder coating that requires 400 degree heat.

As for the wattage issue, Dave pointed out the ohms law calculations to determine wattage, volts, ohms and power. My comment about doubling the heat up time referred to using a 1500 watt element versus a higher wattage(3000). The voltage isn't an issue, just the wattage. Sorry if I confused the issue. Ed

[vtmike]

I'm not sure what type of infrared heater you were proposing to use, but I believe there are a couple of issues. First, most of the IR heaters I've seen were not meant to be placed within an enclosure. Some ambient air is expected to cool the heaters case and any electronics within. Placing the whole heater within an oven enclosure may lead to a shortened life.

Secondly conventional ovens heat two ways, convection where the air is heated and radiant where the heat from the elements 'reflects' on the part. My understanding of IR is that it primarily heats objects, not air. If the parts you intend to process are flat, then you may get balanced heating across the part with the IR. All heat sources, including IR heating will produce some convection heat as the objects it heats, including the oven walls will then heat the enclosed air.

I'm not sure what your application is as you are only looking to heat to 200 degrees maximum, so you may have other alternatives versus an oven built for powder coating that requires 400 degree heat.

As for the wattage issue, Dave pointed out the ohms law calculations to determine wattage, volts, ohms and power. My comment about doubling the heat up time referred to using a 1500 watt element versus a higher wattage(3000). The voltage isn't an issue, just the wattage. Sorry if I confused the issue. Ed

Ed,

You are right, an infrared heater will provide more localized heating. There were a couple of options that I was not sure about. Take for example using a heat blower (Master Appliance MasterFlow Heat Gun or Hot Air Tool). But I am having difficulty in finding one that is continuous duty and with variable temp. An alternative would be to fit adjustable slats in the box and adjust the air flow until the right temperature is achieved.

[ed_denu]

Ed,

You are right, an infrared heater will provide more localized heating. There were a couple of options that I was not sure about. Take for example using a heat blower (Master Appliance MasterFlow Heat Gun or Hot Air Tool). But I am having difficulty in finding one that is continuous duty and with variable temp. An alternative would be to fit adjustable slats in the box and adjust the air flow until the right temperature is achieved.

For the price of these blowers, why not just buy a pid, ssr/contactor and an element. You could get the whole mix for less than half what you would pay for these blowers, plus the pid will hold the temp to your set point. You could also add a small blower for a few $$$. It probably wouldn't need to be hi-temp.