design of heating coil

[rocky01]

Hi

I need to design a heating element to prevent the cryostat from frosting. The frosting occurs due to the flow of helium. I need to maintain the temperature of the cryostat upto 50° C. I have a 30 volt 10 amp power supply. Please guide me how to make the appropriate heating element.

 

[Reloadron]

I would just think about buying some heat trace tape designed for 12 or 24 volt use or I would think about the use of Silicon Rubber Enclosure Heaters designed for low voltage DC. That assumes you do not wish to use line voltage mains power.

You could also start working with nichrome wire to design and build a heating element or pad but when all is said and done, including insulating I see buying as more practical.

Ron

 

[dr pepper]

The above is the professional way of doing it, if you wanted a cheaper way then you could use a couple of power resistors connected to the power supply, and thermally connected to the stat/vaporiser.

 

[rocky01]

thank you for your replies. But as this is the part of the project, i am not allowed to buy anything readily made. I forgot to mention that the part to be heated is circular in shape and its diameter is 7 cm. Now if i take a wire of length around 20 cm., the resistance of the wire is very low and hence the current required will be more. how can i solve this?

 

[dr pepper]

Lookup nichrome wire, this is otherwise called 'resistance wire', the same stuff used in power resistors and electric heaters.
You can buy this on the roll, or if your making a prototype you might be able to rob some from a scrap appliance with a heater element, you'll need to make a former to fit around the valve that is heat conductive and resistant, paxolin tube works if you can get some.

Heating with ordinary cable isnt reccomended, very inefficient, unless you build an induction heater, if your not talking more than a couple of hundred watts an induction heater is still an option.

 

[rocky01]

thank you for your response.View attachment 64260
I am attaching the image of the cryostat head and the part which is to be heated. clearly induction heating is not possible in this case due to space crunch. so i need to use the wire.
how do i calculate the wattage required if i want to heat it upto 50°C ?

 

[tcmtech]

Have you calculated the amount of energy that will be required to heat the part up to 50C and keep it there?

If it needs more than 300 watts/hr to do it you will need a different power source to make it work.

 

[dr pepper]

Looking at that induction heating is impractical yes, you'd need some fancy set ups.
I'm not much help on heat energy calculations, once it gets out of component heatsinking its into the very different realm of mechanic engineering to me.
If I was faced with the task I'd look at commercially available units and make an educated guess as to heater power and make a test jig, then go from there, but this approach isnt any good for a educational project.

 

[cachehiker]

Have you calculated the amount of energy that will be required to heat the part up to 50C and keep it there?

I agree. How much power do you need?

I have no trouble getting coiled nichrome wire for next to free from the appliance store (think electric clothes dryer) at the bottom of the hill from where I live.

The only problem is matching it to the voltage I'm using.

 

[ronv]

Give this stuff a look. It is insulated but only good to 425 F. It will take some expermenting with the power applied to get the heat you want without stressing the wire. Do you just need to heat the larger tube or the whole center section. Is it just to prevent condesation or are you trying to maintain a certain temperature.

 

[rocky01]

i need to heat up the center seciton just to prevent condensation

 

[rocky01]

thats true. but how do i calculate the ampount of energy that will be required to heat the part up to 50 C and keep it there

 

[rocky01]

may be i can use the temp. controller to maintain the temp at 50C. but i dont have any clue how to calculate the energy that you have mentioned

 

[KeepItSimpleStupid]

You might find this useful: **broken link removed**

 

[rocky01]

exactly. even i am also having the same problem. how do i match it with the voltage i am using

 

[jpanhalt]

It is a little hard to make out exactly what needs to be heated and how big it is from that picture.

As alternatives, have you considered using infrared heaters?

You might also consider putting a box or bag around the whole thing and filling it with dry nitrogen/air or argon. Dry nitrogen is easily obtained from evaporation of liquid nitrogen. Dry argon has the advantage the it is heavy, so the top of the box, if small, can probably be left open.

I have used the dry nitrogen approach when doing low-temperature spectroscopy.

John

edit: I also used the I2R bags -- not related to Apple -- for keeping water and oxygen out of low-temperature work. I2R was a re-usable or disposable, plastic glove bag. It appears Instruments for Research and Inductry (i.e., I2R) was acquired by Glas-Col. Here is a link to the current product: https://www.glascol.com/product/subproduct/id/58

 

[dr pepper]

I cant help you with finding what power you'd need, however I can help you a little with the heater calculations:

To work out the wattage of a length of nichrome, say you want 120w, your power supply is 120v, you need to set the resistance of the coil to 120 ohms, so that 120v/120ohms = 1amp current draw, 1amp x 120v = 120watts.

The datasheet for the nichrome will tell you the ohms per foot (or you could measure a 10 foot length), and the fusing current rating (you could deloberately blow a piece of wire on a high current supply, then divide this current by 1.5 or so as a safety margin).

Theres a practical limit to the power you can make a heating element, one is the maximum current the wire can carry without fusing, and the other is the maximum temp the heater coil gets to has to less than the melting point of the nichrome (this can be releived somewhat by your mentioned use of a thermostat).

 

[KeepItSimpleStupid]

To make DPs post a bit more understandable, replace "fusing" with "melting".

 

[dr pepper]

Yes sorrys, practical speak meets textbook speak again.

 

[ronv]

I have a rule of thumb that I use for heatsinks:
Anyway maybe you can get a ballpark using 50/sq rt of the surface area in cm. This will give the temp rise in deg. C / watt.
Stainless won't be as good as copper but it may at least give you a target without a class in thermodynamics.

Sorry, I didn't attach the link in my previous post:

https://www.briskheat.com/resistancewire.aspx