Heating Sources/Devices

Huxley · member

I am curious to get feedback from those more experienced than me as to which means/method of heating is preferred. I am in need of a heating source, but also, want to keep costs down.

Since I will be using reflux and distillation setups (among others), I want something that will heat RB flasks efficiently and want to also be able to use a magnetic stirrer.

My first consideration was to pick up combination hotplate/stirrer. With this, I thought I could either use an oil bath heated by the hotplate (which I presume would still work with the stirrer function of the hotplate), or else use a florence flask directly on the hotplate. [BTW, since I have not used an oil bath before, what is an optimum container/receptacle for the oil?]

The other option is a heating mantle, however, I am unfamiliar with the various types. Can anyone recommend (or comment on the merts of) a cost-efficient variable size (e.g. up to 1000ml) heating mantle/power supply which would work with a stirrer plate?

I am guessing though that the second option would be considerably more expensive than the first.

Any comments, further information, or general help is greatly appreciated.

Huxley

IndoleAmine · Dreamreader Deluxe

Heating mantles are expensive and require external heat control devices which can become very expensive too when a brand new "variac" for a big mantle is to be bought..

And they have the disadvantage that a breaking flask sometimes ruins them in a matter of seconds (anybody like a nice 500bucks mantle with included variac(!), saturated with a protocatechualdehyde/pyridine.hcl mixture hardened to a VERY stinky crystalline mess all between the glass fibre insulation?) Evil or Very Mad

Oil baths are cheap and can be discarded in case of an accident, but can't go higher than 160-190°C depending on the oil used. Higher temps. require other, expensive bath fluids (silicone damper oil, or various PEGs).

Mag. stirring works fine though a bit slower than usual if you put a second stirbar inside the oil bath. Steel bowls are fine as a container, the magnet works through them in all cases I've seen so far.

i_a

nyarlothotep · Thu Apr 21, 2005 4:45 am

Swin prefers a large glass container for the oil bath, as his hotplate once cautioned (upon arrival with instructions) that the ceramic surface could be damaged by heating metal.

Also, smoke points for some oils go as high as 250 C....What you'd want to heat that high at home, he has no clue.

Only possible disadvantage to the hotplate stirrer is the nasty little "clicking" that happens upon heating....but one would think that source of combustibility could be extracted, the relevant wires spliced and extended, and removed from the immediate proximity to protect against accidental ignitions.

MargaretThatcher · Thu Apr 21, 2005 5:03 am

Fresh, refined avocado oil reputedly has the highest smoke point of 255-270 Celsius (490-520 F) but is expensive stuff. Safflower oil is almost as good - 265 Celsius (510 F) and much cheaper.

Edit: the oils need to be refined and fresh (they rapidly oxidise and smoke at a lower temperature with use).

The smoke point of an oil is the temperature at which the oil begins to decompose and visible fumes (smoke) are given off. Keep an oil below its smoke-point and it will decompose much more slowly.

The flash-point of most vegetable oils is around 320 C and the fire-point is around 400 C.

Huxley · member

Thanks for the suggestions, I think I will go with the hotplate/stirrer combo option...any minimum recommended stirring power (rpm)? Is there significant benefits of ceramic over aluminum hotplates?

IndoleAmine · Dreamreader Deluxe

Keep sawflower oil at 200°C for 20 minutes and you smell like a french fries maker........
(it oxidizes very fast when heated and stirred, and the hotter the faster it starts to stink. Sawflower oil IME is only good for up to 180°C if new)

Next thing to try: avocado oil.... Laughing

MargaretThatcher · Fri Apr 22, 2005 4:25 am

Was it refined safflower oil? Lots of cheap oils aren't refined and have lots of fatty acids and stuff.

This has some good info:

http://www.cookingforengineers.com/article.php?id=50

IndoleAmine · Dreamreader Deluxe

OK then the more expensive brands will be checked... Very Happy

I like IKAMAG stirrer/hotplates, powerful motor (1500rpm) and fast heating (600W). Together with a digital thermocouple, this makes for exact heating up to +-1°C accuracy....

They're NOT explosion proof, and the aluminum hotplate - well, oxidizes a bit fast, but ceramic ones are too expensive IMO... Very Happy

I usually don't use open rxn vessels when heating something flammable/volatile/corrosive, so this is not important to me. Wink

The second stirbar is for even heat distribution, exactly. Also it enables faster stirring without the flask's stirbar start going to "walk about" (maybe the distance magnet-magnet is simply shorter?)

Hope this helps.

cheers

bio · Working Bee

The best way I have found to heat an oil bath by far is a 100-400 watt immersion heater controlled by a dimmer.

Bend the coils carefully for best fit so get the aluminum ones. The coil can be expanded to circle the bowl which works fine w/o stirring the oil. Heating the bowl with the 1600W 12inch hotplate puts more heat into the room than into the oil.

Advantage; takes way less power, tunes in closer and very little lag compared to the hotplate. Just place a sheet of asbestos under the bowl and place directly on the hotplate/stirrer. A 300W unit heats an oil bath for a 3L flask to 200deg no problem yet still does fine with a 500ml. A common mistake is to use way too much oil so match the bowl to the flask for about an inch max clearance. If you want to use 2liters oil instead 500ml don't expect a very fast response.

MargaretThatcher · Fri Apr 29, 2005 5:59 am

asbestos!

bio · Working Bee

learner · Tue May 03, 2005 11:13 am

bio · Working Bee

The kitchen coffee cup types work fine. !00-175w IIRC usually stainless steel which is a little hard to bend but it's OK to just flatten the coils a little and place in one side. You might need to insulate the bowl if using higher temps with a low wattage unit. Hardware stores in my area have the aluminum ones in all sizes and cheap too. You might try maybe a camping store. Yes the industrial ones are quite costly it's true. Even two of the coffee cup type paralleled should handle damn near anything you will do. Just don't use too much oil.

Also get a triac type ceiling fan speed controller instead of an el cheapo dimmer. They have much less hysteresis (means won't drop out at low settings) and also a trim pot adjustment that is used to set the minimum speed (heat) in the case of a fan. If you have an RMS voltmeter then about 53V is 20% of power so you shouldn't have to go lower than this. By taking the voltage you can mark the dial in percent power which is real handy. A decent fan controller will hold a couple of degrees tolerance no problem.

Download AN1003 at this link for more info.

http://www.littelfuse.com/cgi-bin/r.cgi/know_content.html?LFSESSION=lOfryh1i3O&ContentID=86&WhichApp=4

A symetrical firing unit is easy to make from a cheap dimmer with 2 resistors and 4 diodes if you want the ultimate repeatable calibratable dimmer. Most dimmers use an 8 amp triac and with a real heat sink will easily handle 1 KW load. I have a 40ampere 480V 3phase one I built that works like a charm, but I doubt you need 33KW for your oil bath. Shocked