I am wanting to build a low powered resistor heater to heat a box about 10 cu/ft to 100 degrees and hold that temperature Constant. i will use a thermostat to regulate the heat. Here is my theory of operation. I wish to use a few 5w ceramic resistors in parallel/series using a 120v supply rectified ( either full or half bridge rectification). The idea is to build a long lasting power efficient heater for a egg incubator. My idea is to parallel and/or series as many resistors that it would take to hold the 100 degrees in the sealed enclosure. The questions that i have are first, how many resistors and what ohms could i use on a continuous basis and in what configuration. I don't need instant heat it can have warm up time. typically we turn on the incubator 2 or 3 hours before setting in the eggs. It will however have to be able to sustain the heat in the box at all times and from time to time the box must be opened to refill the water tray for humidity. So it will have to recover about 20 degrees in about 5 minutes max. There are two small lovers allowing fresh air into the box so that will have to be accounted for and there is a fan in the box that will circulate the air. I would probably set the resistors in front of the fan. The fan constantly runs. I was hoping to get away with 50 watts. however i can go higher if need be. But the lower the wattage to better the unit in my opinion. i like efficiency. Any help or suggestions would be most appreciated. Thanks!

 

Assuming the eggs take up ~20% of the 9000cc space, to raise the temperature of eggs and air 20 degrees C in 5 minutes it will need 984 watts (assumed: heat capacity of eggs ~3.8, humid air ~1.1). If it's just the air that needs reheating, then it will need 530 watts.

As an aside, efficiency is not garnered by decreasing the heater power (the same amount of energy is required to do the same work - e.g. heating a mass - if it slowly or quickly); efficiency may be increased by improving insulation or by using a different heat generation technique.

I can attach a spreadsheet to work this out if that would be of any use.

EDIT: As you were probably speaking in imperial units, the above power requirements for heating 12.5 degrees C (20 degrees F) is 616W (eggs & air) and 330W (air only).

 

You might want to consider a set of two relays ... There would be a separate set of resistors to be energized for each respective relay. One relay would control the heat necessary to recover the set temperature when the access door was opened. Another relay would control the heat required to maintain the constant temperature when there is no open door ... i.e. when the only heat out of the container would be due to ventilation.

Another consideration would be to obtain a temperature sensor that would cut off the active resistor heating current when the maximum temperature has been reached, and turn it back on when the minimum acceptable temperature occurred.

 

thank you both for your input . As for the rateing of the temperature, i am referring to the air temp only. The eggs will retain the temperature for the minute or so that the box is opened. All i really need is to figure out how many resistors would be required to maintain the temp through out the incubation process. One of the major incubator company's run a 60 ohm resistance coil on 120v, i had previously tested this with my ohm meter. they claim 110v @ 225 watts usage on the heater however there sq/in is near double of my cabinet design. Therefore i was thinking 1/2 would have to account for near 100 watts. i was hoping that using the resistors instead of the resistance wire would not be only cheaper but also more efficient.
As for the thermostat i already have one designed and built to handle what specs you are proposing, however i don't want to have a secondary heater in the mix. The idea is to keep the power consumption low. When the door is open only air is displaced. there is a tray of water, metal egg turner racks and trays, the eggs themselves, and a bottom catch tray to radiate and recover the heat lost in the short time that the door would be open.
So with that in mind, if i were to build a bread board with 20@5watt 5.6 ohm ceramic resistor circuit, say 10@ (2) 5 watt resistors in series and wire the sets in parellel and connect a 120v V+ to one bus and the V- to the opposing bus after rectification to dc. will this sustain the 100 degree temp of the box?

 

Firstly, you don't need any rectifiers.
Secondly you want to run 5watt resistors at 3watts to prevent scorching and fire.
If you want 50 watts of dissipation, you can use 10R or 15R resistors.
You will need 22 x 10R or 15 x 15R

 

Are you saying that15 @ 15 ohm 3 watt resistors will serve my purpose? like these Digi-Key - P15W-3BK-ND (Manufacturer - ERG-3SJ150) ? would i wire them all in parellel?

 

No. They would be in series. To calculate the power through the resistors, P = V*V/R. So 110*110/15 = 806W - so your 3W resistor will not appreciate it. In series, P = 3.6W per resistor, which is still too much for a 3W resistor. You could use 17 in series for 2.8W each (47.5W total).

The efficiency will be the same; they both create heat the same way. The only difference is that the resistor has a larger thermal mass than the plain wire. I would think that they'd be more expensive also.

Also note that using 50W of heating will take a while for you to reach your target temperature.

 

Here's 30 resistors for $5.00 on eBay:

15 Ohm 5W Wire Wound Resistors. 30pc box. 0 Bids Buy It Now $4.95
$5.55 6h 59m

 

100W will really struggle to warm 10 cubic feet. Even with a good insulated enclosure (like a styrofoam cooler) 100W would probably not be enough. For like a wooden box figure about 30W per cubic foot, running at about a 60% duty cycle to maintain temperature.

For 10 cubic feet I would use a minimum of 300W and maybe more, especially if it is a metal container or has a glass window both of which will cause big heat losses. And that is for indoor use. If it used in a cold barn etc you may need more power still.

One approach is to use light bulbs, either as an assist to the heaters or as the only heaters. My small incubator has a 100W lightbulb in a 2 cubic foot wooden box, with some resistors in series with the light bulb to extend bulb life through millions of switching cycles. You can also run 240v light bulbs on 120v as heaters, they will last millions of switching cycles too but you'll get mainly heat and not much light.

 

You can use an electronic dimmer on the bulbs to avoid switching cycles, control the dimmer via the thermal monitoring circuit and you have an instantly expandable system, just by adding more bulbs, a couple fans and you're all set.

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Nichrome wire looks like a reasonable choice .....
Wire: Nichrome (tm) & Other Resistance Alloys - Tech Data
At 2oo watts, 115 VAC, 1.74 amps, and about 67 hot ohms, you would only need about 1 foot of AWG 40 nichrome wire.
Nichrome wire available here ... 10 ft. for US $2.
Nichrome Wire

Without more complete specifications regarding insulation and other construction details, it would be difficult to estimate the heating and cooling transients, for a given heat input level.

You will probably have to build a test heater .... see what results you get, and modify accordingly.

 

using wire was my first idea. i have this: 32 ga, 100 ft, Nichrome Resistance Wire, Nikrothal 80. i figured 6' would give me 64 ohms at 120v. but when i got it it looks like sowing thread. I am afraid that it will burn up on the first use. Are you saying that it won't? can i just straight wire to 120 and it won't burn up?
# resistance: 10.66 Ohm/ft (35.0 Ohm/m)
# maximum temperature: 2190 F (1200 C)

 

6' of that stuff will dissipate 225W @ 120VAC. Which is just 1.25W/cm --> which is bugger all. There's a really easy way to find out

 

well i'll l go and get out the alligator clips and see what it will do at 6'. i'll try not to damage my self lol

 

I don't know how much you have to invest in doing this properly, but I would suggest a cartridge heater.