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Low voltage high heat needed

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Joel Costonis

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Good afternoon , I saw a post on the design of a low Voltage heat source.

I design lab equipment and am seeking a way to raise the temperature of our test chamber (20 inch diameter by 20 inch hieght) that's under hi vacuum to 100+/- degree f. I have a number of different power supplies at varying voltages and can add a dedicated for the purpose of heat if needed

I would appreciate any input you may offer. I am willing to offer compensation for the same.

Kindly revert at your soonest.

If you are not interested a referral would be greatly appreciated.
 

Pommie

Well-Known Member
Most Helpful Member
Isn't raising the temperature of a vacuum going to be difficult?

If it helps, I recently built a biltong drying box and as heating elements I used 6 x 50W 3 ohm aluminium clad resistors of ebay with a 360W 12V power supply.

Mike.
 

MikeMl

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Seems like you need to raise the temperature of a small shelf/object inside the chamber, not the entire chamber itself.
 

Joel Costonis

New Member
When we start the test, the chamber is not under vacuum yet it is sealed. The effect in and if it's self drops the chamber environment about 20 degrees f. I am looking for a device or arrangement of devices that will generate heat in a 24 inch by 24 inch cylinder raising the temperature to approx 100 f before we turn on vacuum
 

Pommie

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You could use the resistors I mentioned above but make sure to turn them off before turning on the vacuum.

Mike.
 

MikeMl

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Most Helpful Member
When we start the test, the chamber is not under vacuum yet it is sealed. The effect in and if it's self drops the chamber environment about 20 degrees f. I am looking for a device or arrangement of devices that will generate heat in a 24 inch by 24 inch cylinder raising the temperature to approx 100 f before we turn on vacuum
To figure the heating requirements (Heater Watts), you need to know the mass of the cylinder, the specific heat of the material that the cylinder is made out of, and how long you are willing to wait for the cylinder temperature to increase from room ambient to the final desired temperature. The heating time will be primarily due to heating the mass; a bit of heat will be lost by convection to surrounding air prior to evacuting the chamber. The heat lost by radiation will be negligable. As soon as you evacuate, the heating requirements will drop to almost zero...

Simplified reference
 

Pommie

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Thinking about this again, I assume it will be difficult to get wires into a vacuum chamber without causing leaks. Maybe a simpler solution would be a chunk of copper that's heated and placed in the chamber to warm it.

Mike.
 

Tony Stewart

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Most Helpful Member
The temp rise is determined by Power x Thermal Insulation Rth ['C/W]. The rate depends on heat mass.,
do you have an estimate for Rth and m?

The next issue is thermal gradient across the chamber and rising from heaters to maintain temperature in a vacuum from external Insulation. What is your tolerance to thermal gradient?
 

spec

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Most Helpful Member
Hi Joel,

The volume of your chamber is 3.6 cubic feet and assuming that the ambient is 25 Deg C and you wish to raise the temperature to 38 Deg C (100 Deg F) that give a temperature change of 13 Deg C.

I haven't worked out the energy required but at a guess I would say that 1KW would give a reasonable heating response.

Another variable that we would need to know is how big can the heater be and can a three core cable be routed into the vacuum chamber?

In theory what you want is dead simple to do but there are practical aspects to consider, including safety.

At its simplest you would have a IKW mains heater in the chamber and a cable coming out of the chamber, which connects to the mains supply.

If a cable cannot exit the power supply that complicates the issue greatly, so could you advise on this aspect.:) Also can you post a picture of the vacuum chamber.

spec

PS: I am assuming that the chamber cannot be heated from the outside, either by infra red through a transparent window or by heating the body of the vacuum chamber.
 

Joel Costonis

New Member
There are a myriad of pass through that we have access to. at this point we are using both electrical and rotary pass throughs. I am not opposed to another. the target design hieght would be 25mm or smaller. 1 kw is higher than we want as I don't want to tax the syatem from existing demands.

I would like to do this, send me your best suggestions with designs - hand drawn is fine - indicating parts, sources, and prices. We will try all submitted. the one that we chose will be most effective then cost. I will send the designer of the selected design $500. email to [email protected]
 

Tony Stewart

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Most Helpful Member
  1. Is this like a 10kg metal vacuum chamber or 1kg glass bell jar? or like a Tenney Oven?
  2. Does it have any thermal insulation?
  3. How stable does it have to be at 100'F and for how long ? +/-2'F continuous? or 100'F min and 120'F or ?? or do you mean Celcius?
  4. Does it matter if one side is hotter than the other?
  5. What shape is it?
  6. Can it be heated from the bottom sides top with radiant heat or inside out is preferred?
  7. What time constant do you need? For tall cabinets, I have designed laminar flow 40'C burn-in chambers with 1'C gradient top to bottom with unit dissipating power and external heater.
  8. Most important , where can temperature be sensed reliably ? external 1 location 2 averaged? or feedthru to internal wall , immersion to object?? This depends on chamber Thermal resistance Tca and insulation of same.
  9. Is there extra material that needs to be heated just before vacuum begins or during vacuum draw? if so, What mass? in what time?
I have 2 yrs experience in Sieve analysis, moisture content, density tests, slump tests and modified Proctors plus 40 yrs experience in electronics.
 
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Joel Costonis

New Member
  1. Is this like a 10kg metal vacuum chamber or 1kg glass bell jar? or like a Tenney Oven? 14 stainless
  2. Does it have any thermal insulation? yes
  3. How stable does it have to be at 100'F and for how long ? +/- 10f, ~30 minutes
  4. Does it matter if one side is hotter than the other? don't think so
  5. What shape is it? round cylinder
  6. Can it be heated from the bottom sides top with radiant heat or inside out is preferred? inside bottom
  7. What time constant do you need? For tall cabinets, I have designed laminar flow 40'C burn-in chambers with 1'C gradient top to bottom with unit dissipating power and external heater. to be honest, not really sure what that means.
 

Tony Stewart

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Most Helpful Member
1. so it is a 14kg SS chamber with a volume of 20"D * 20"H

for temp rise ΔT['C] , mass, m [kg], and specific heat , c [ joule/kilogram °C]
Q=ΔTmc [°C*kg*(J/[kg°C])=[J] = heat energy, Q

Temp Rise: 100'F+/-10 = 38'C +/-5 with ambient assumed temp=20'C+/-5 therefore simple approximate Temp Rise =ΔT = 18'C +/-10'C

Metric dimensions: 500 mmD * 500 mmH with a volume of V(cyl)=pi(D/2)² * H
Volume: V= 0.1 m³
, thus if contents are full , what is the specific heat? how much energy is needed in say 20 minutes to raise from 20'C for 0.1 m³ of material and 14kg of SS oven.
Mass: from volume and density in 0.1 m³ chamber.
Density: kg/m³
Water 1000
SS 8000
Air ~ .0001
Concrete, Gravel 2400
Crushed Stone 1600
Earth, loam dry 1440
Earth, packed 1520​

  1. Is this like a 10kg metal vacuum chamber or 1kg glass bell jar? or like a Tenney Oven? 14 stainless
  2. Does it have any thermal insulation? yes
  3. How stable does it have to be at 100'F and for how long ? +/- 10f, ~30 minutes
  4. Does it matter if one side is hotter than the other? don't think so
  5. What shape is it? round cylinder
  6. Can it be heated from the bottom sides top with radiant heat or inside out is preferred? inside bottom
  7. What time constant do you need? For tall cabinets, I have designed laminar flow 40'C burn-in chambers with 1'C gradient top to bottom with unit dissipating power and external heater. to be honest, not really sure what that means.
ok 14kg of SS
So what is mass and specific heat of ALL contents to be heated to 38'C.? including moisture content e.g. gravel , sand water and % void
What is min. ambient T?
The intent is to define if 1kW is sufficient or even feasible, What is insulation resistance 'C/W?
Also does heater remain inside , in tact, can it be protected? Assuming electrical feedthrus via vacuum seal...
 
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Tony Stewart

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ok to be clearer is the objective to preheat heat add 50# and expect it to be dry in 30 minutes with heat & agitation or heat up both oven and 50# in 20min then 10min for test or ?
Also what power is avail AC and DC?

I think Vacuum process should remove all moisture quickly.

So choice comes down to AC or DC and AC power is cheaper but needs safety insulation
 
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Joel Costonis

New Member
Indeed you are getting closer. Certainly the vacuum does remove the moisture, however, the higher the chamber temperature, the faster the evaporation, the faster the drier
 

Tony Stewart

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Material c m[kg] Q20=Q=20*mc Heat Energy = Joules = watt-second
water 4.184 1 5%? 84
sand 0.3 11 66
stone 0.8 10 160

crushed agg. 0.705 22 310
SS 0.44 17 150
total 39 460 kJ = kWs = 460/60=7.67kW-min or 1kW for 7.7 minutes

So based on 5% moisture by weight it will consume ~2kW-minutes for the duration of evaporation.
 

Les Jones

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The sample will have to be supplied with heat energy as the water in it eveporates due to the vacuum. This energy is the latent heat of evaporation. The latent heat of evaporation of water is 2264.76 KJ/Kg If this energy is not supplied externally then it will be supplied by the cooling of the sample. The temperature drop can be calculated from the mass of the sample and it's specific heat together with the energy reqired to evaporate all of the water originally in the sample. Would it not be simpler to dry the sample before putting it in the vacuum chamber ? This is not going to be easy to calculate as the mass will be changing. A full description of exactly was happening in the original question would have been a great help. More background information on the purpose of this proceedure would help.

Les.
 

Tony Stewart

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that's the idea Les. Heat in chamber then draw vacuum to accelerate. Both heat and vacuum can draw all the moisture from my estimates in 8 minutes
 

kubeek

Well-Known Member
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Watch out for what your vacuum pump can handle, oil piston pumps should be ok, but if you need deep vacuum and turbomolecular pump it could be a big problem. Pre-drying at higher temperature before going to vacuum might be a better option.
 
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