Recommendation for a heater blower

[vtmike]

Hi,

My application involves heating an enclosed area of 6'X 2'X 3'(36 ft^3) to a temperature range of 150 F ~ 200 F for a period of 12 hours. This enclosure will be made out of Al sheets & insulated with foam.
(This setup is used to dry out an assembly for further testing)

Can anyone suggest a heater blower for my application? The BTU requirement comes out to be 5400 btu/hr. I came across some heater blowers through a google search, but I am not sure what kind of heater I need to use for such a small enclosure. Any ideas?

Thanks,
Mike

 

[itsmoked]

Is this a one time test setup? I'd rent a trailer for it.

If it will be used often the solutions would be much different.

There are many types of "box heaters". They are just metal box with a bunch of nichrome coils weaved around inside that you blow air thru.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[vtmike]

Yes this will be used often. I've been having problems finding a box heater with the correct temperature rating. I couldn't seem to find one that will blow air between 15 & 200 F. The temp rating is either too high or too low.

 

[btrueblood]

Vt, you don't care if the output air is 350F, as long as the average air temp. is between 150 and 200, or more importantly, the hardware is at that temp. So just control the box heat on/off to maintain the hardware temp. within limits.

 

[vtmike]

btrueblood,

The application involves maintaining the hardware in that temp range for a period of 12 hours. So this will have to work without supervision.

 

[IRstuff]

Your BTUs seem to be high. Are you in a wind tunnel?

(2W/m^2-K)*(5m^2)*(200R-70R)=2464 BTU/hr, which would be the worst-case, i.e., an uninsulated box.



TTFN

FAQ731-376

 

[itsmoked]

You use one, two, or four box heaters.

Each one should have a small blower mounted to it to force air at a high velocity thru it.

They each should have included on them, from the factory, a manual-reset over temperature switch to trip them off when a blower fails.

They should be mounted vertically in corners.

They should have ducting to make them draw air from the floor and blow it out at or near the ceiling. This is how you homogenize the air/temperature.

You control it all with a single panel-mount PID controller. You need to control the actual power with relays, possibly solid state relays, depending on the accuracy desired.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[vtmike]

IRstuff,

The btu is calculated using a factor for average insulation which is 1.25 in this case. So for a 120 rise in temp (worst case), I get 6 X 2 X 3 X 120 X 1.25 = 5400 btu/hr

Infact I get an even higher btu/hr value from this online calculator below,

http://www.heatershop.com/btu_calculator.htm

 

[vtmike]

itsmoked,

That might work but I want to keep it simple by just installing a single heater. There must be something out there that blows air between 150 F & 200 F right???

Thanks,
Mike

 

[itsmoked]

If you want to blow air at 200F how are you going to keep a large space at 200F? You have to have a hotter source than the controlled space temperature.

It's all about mixing! You need to have the air mixed. If you don't then you will get major stratification.

It doesn't matter how hot the air is out of the heater because it is supposed to be blowing all over and rapidly mixing.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[vtmike]

The application is intented to be used to dry the hardware between a temp range of 150 ~ 200 F, & there is less emphasis on maintaining the enclosure at a constant temperature.

 

[IRstuff]

The 120°F rise only occurs ONCE, at the start. After that, you're ostensibly keeping the change to less than 50°C. But, a reasonable closed loop control would have less than 20°F swing, and then, all you're doing is replacing the convective heat lost to the environment.

However, that raises the question of how long, how fast, i.e., how fast does it need to reach temperature, and how long are you doing this?

TTFN

FAQ731-376

 

[MintJulep]

Has everyone forgotten how to calculate the conductive heat load out of a box?

First, figure out the surface area:

For a 6x2x3 box it's:

6x2x2+6x3x2+2x3x2=72 square feet or 6.7 square meters.

Then look up the conductivity of the chosen insulation. Since the OP hasn't told us what type of "foam" he intends to use, let's just go for expanded polystyrene which has a thermal conductivity of 0.03 W/mK.

Then, let's not forget that we need to know the thickness of the insulation. Again, the OP hasn't told us, so let's guess an inch (0.0254 meters)

Then let's remember that the delta-T term is the difference of temperature across the insulation. We know that the desired inside temp is 200 F (93 C). We'll assume a 23 C ambient to give a nice round 70 K delta-T.

Now we can plug it all together:

q = k A dT / s

.03 x 6.7 x 70 /.0254 = 554 watts or 1890 BTU/hr.

 

[btrueblood]

"You control it all with a single panel-mount PID controller. You need to control the actual power with relays, possibly solid state relays, depending on the accuracy desired. "

Oh, smoked. Just like an EE. Personally, I'd use a more complex device, called a THERMOSTAT. Temp drops, switch goes on, temp rises, switch goes off. They've only been around about a million years, and even us dumb mech types can figure them out.

Yeah, I know, "how are you going to tune the loop with a dumb thermostat".

 

[IRstuff]

Of course, that ignores convection, so the actual heat loss is even lower than that.

TTFN

FAQ731-376

 

[itsmoked]

Thermostat. pfffft!

Drying requires air turnover. Which certainly implies more heating capacity than just the enclosure loss. Probably means no insulation too. Now we are getting to the crux of the problem. Essentially the OP just wants to blow 200F air all over something.

There are many ways to dry paint, including just lights, like a paint drying booth.

Look in

http://www.grainger.com

for heaters.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

What is the assembly wetted with? Water? Gasoline?

Are there any limits on air velocity?

The power input required will be much much higher than merely meeting the thermal losses through the enclosure because if this rig is going to dry the equipment then it has to carry the liquid out in the vapour state, so there is the latent heat of vapourisation to deliver, plus whatever the entrained heat in the exhaust stream turns out to be.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[vtmike]

ScottyUK,

The assembly will be soaked in water & no limits on air velocity.

itsmoked,

The paint drying booth is an interesting idea. It didn't occur to me to use something like a quartz lamp to provide infrared heat. I guess I would need to connect a blower of some sort to circulate air though?
I could also connect it to a thermostat to turn it off once the temp exceeds 200F & turn it back on once the temp drops below 150F like bluetrueblood pointed out.

MintJulep,

You are right. The BTU/hr requirement after I recalculated it came out to be ~ 2000 BTU/hr or 593 Watts. A Quartz lamp can provide 600 Watts. Take this one for example,

http://www.mcmaster.com/#3343k17/=2xwqq3

The only questions I am concerned with are,
1. Will this Quartz bulb provide enough energy to dry this assembly? (I mean would I need to install a blower to circulate air?)
or do I need to go for something like a blower heater,

http://www.grainger.com/Grainger/items/4Z717

2. How many vent holes would I need to punch in this enclosure?

Thanks for you replies,
Mike

 

[itsmoked]

You could try this.

Put in a few heat lamps of the 250W variety. Spread them around to try to evenly "light" your fixture.

You would want a little ventilation. So little, that convection would probably be fine. That could be a large central ceiling opening and a few low openings around the periphery.

Run your lights and measure the temperature one your fixture with a temp gun to make sure you don't exceed the allowed temp on any spots. If some spot seems too cold add or move the lights. Since it is metal you will automatically get some heat spreading about so that light pointing shouldn't be too finicky.

If you get the air flow and lighting correct you can probably skip the temp control all together.

Keith Cress
kcress -

http://www.flaminsystems.com