Introduction: A TEMPERATURE CONTROLLED LABORATORY HEATER

A TEMPERATURE CONTROLLED LABORATORY HEATER

 

The device described is an electrically heated air bath with adjustable power input and temperature regulation via a PID digital temperature controller. It is suitable for use as a heater for chemical apparatus, such as that used in distillation and reflux operations.  An air bath is the preferred means of heating distillations flasks, because its superior uniformity of heating avoids local overheating and charring effects, which are commonly experienced with electric heating mantles. The digital temperature control is optional; the heater can be used with the energy regulator alone if the user is present to monitor the operation and manually control the temperature as necessary.

 

Parts Required ( Prices taken from eBay listings ) 

BASIC HEATER

1 Aluminium saucepan, approx 150 mm diameter with lid § $ 2.00

1 empty tin can, 100 mm diameter x 150 mm high $ 0.00

Fibreglass Electrical Heating tape, 25 mm x 1 metre, 220 V 100W $ 3.00

1 x IEC panel mount sockets with screw terminals $ 2.50

Glass wool insulation ( salvaged )*

1 x ABS plastic enclosure 70 mm x 40 mm x 25 mm $ 2.00

1 x ABS plastic enclosure 120 mm x 75 mm x 60 mm $ 9.80

1 x IEC Power Cord $ 2.75

1 x SCR Power Controller 220 V 2000 W $ 7.50

250 mm Copper braid ( salvaged from old coaxial cable )

2 x Cable Clamps & Terminal Strips $ 1.00

Assorted screws & nuts $ 2.00

( Sub-Total ) ($ 32.55 )

DIGITAL TEMPERATURE CONTROL

Type K Thermocouple probe 400 oC $ 1.00

Digital PID Temperature controller 220 V $ 13.00

1 x ABS plastic enclosure 120 mm x 75 mm x 60 mm $ 9.80

1 x 220 V Extension cord, 1 m $ 4.00

2 x Cable Clamps & Terminal Strips $ 1.00

 

( Total ) ( $ 61.35 )

§ from a second hand shop

* found in old electric ovens or ceiling insulation batts.

 
As can be seen, the most expensive items are the plastic boxes for the electronics. If you can use boxes you already have or make your own, you can save nearly 1/3 of the cost.

 

 

Step 1:

 

!!!!!    SAFETY WARNING    !!!!!

This equipment operates at mains voltages; if you intend to build this device yourself, be sure that you understand how to do mains wiring safely, or else get someone who is qualified to do it to help you. Mistakes can be FATAL !
 

Construction

I began by drilling out the rivets and removing the handle of the saucepan. A hole approx. 25 mm x 15 mm was cut for the wiring to the heating tape and a 4 mm diameter drain hole was drilled in the bottom.

Step 2:

The tin can was prepared by cutting out the bottom leaving a hollow cylinder. Around this was wrapped a layer of glass wool insulation, about 12 mm thick. I used masking tape to temporarily hold the pieces in place until the whole outer surface of the can was covered, then I wrapped the glass wool tightly with strong polyester thread to keep it in place, removing the masking tape as I went. The heating tape was then wrapped around the insulated can, forming 3 coils. The cords at the ends of the heating tape were tied together to hold it in place.

Step 3:

Next a 250 mm length of copper braid was secured with a nut and bolt to the can, and the other end was led out through the hole in the saucepan to the Earth ( E ) connector of the IEC socket. At a convenient point along the copper braid, it was also bolted to the aluminium saucepan, forming a continuous earth connection to the metal components.

The two leads to the heating tape were then secured to the Live (L ) and Neutral ( N ) terminals of the IEC socket. This was then placed inside the small plastic box, making sure there was no possibility of the wires shorting out onto any of the metal surfaces, and the box was bolted to the side of the saucepan. Some glass wool was packed into the hole in the saucepan to keep the wires from moving about.

Step 4:

The can was then located centrally inside the saucepan and the space between it and the saucepan walls was stuffed with glass wool up to the level of the top of the saucepan. A round hole slightly larger than the can diameter was cut in the saucepan lid, which was placed on top of the heater. The lid that I used was a loose fit on the base, so I secured it with some small brackets and screws.  If you have a lid that is a nice tight fit, this step may not be necessary.  After I had cut the hole in the lid, I realised that I could have made it a little smaller, so that it fitted snugly inside the opening of the can, instead of leaving a gap.  That would look better, and I will probably remake the lid that way when I find another saucepan lid the right size.

 This completed the assembly of the heater component.  As can be seen above, the coating on the inside of the can will start to char when it gets hot. This is nothing to worry about, but you will want to have the heater in a well ventilated area to let the smoke get away while it is doing so.

Step 5:

Energy Regulator

 

A 220 V SCR ( Silicon Controlled Rectifier ) energy regulator was wired as in the diagram and placed inside a suitable box, having a label showing the range of settings on its front panel. ( The regulator I used was actually a 3200 W device, but one rated at 2000 W is quite sufficient and is a few dollars cheaper . Different models may have slightly different wiring layouts, check the technical data sheet for the model you are using ) The IEC power cable was cut in half, with the mains connection end forming the input to the energy regulator and the IEC connector supplying the output to the heater.

Step 6:

PID Temperature Controller

( PID means Proportional-Integral-Derivative, but don't worry, you don't need to know any calculus to use one ! )

This was wired according to the data sheet, using a type K thermocouple as the temperature sensor.  The wiring diagram is shown above.  I used a REX C-100 controller, made by RKC Instrument Co, Japan.  If you use another type, you will need to obtain the technical data sheet to find how to connect it.

Step 7:

The photo above shows the heater being used for the distillation of xylenes through a Claisen-Vigreux fractionating column. The operating temperature is 225 °C, with the energy regulator setting at approximately 80 %.

Performance

The heater was tested empty with the top opening covered by a loose sheet of aluminium foil. On 50 % input power the air temperature inside the can held at 185°C, and at 100% power 250 °C was easily obtained. The maximum temperature rating of the heating tape is not specified, but it is not advisable to run it at full power indefinitely, as the element will overheat and burn out.

 Precautions for use

The heater should always be used with the energy regulator connected to the input. Although it is possible to plug it directly into the mains power using an IEC cord, this will almost certainly burn out the element by overheating the wires, as will running it continuously at full power from the regulator. The energy regulator needs to be adjusted to the MINIMUM setting required to maintain the desired temperature. This setting will vary with the thermal load drawn by the other apparatus being heated, and needs to be found by trial.


When conducting chemical operations in glass apparatus, there is always the possibility of breakage resulting in chemicals being dumped into equipment that was not intended to come in contact with such materials. The design of this heater is relatively resistant to spills because the heater element is not directly exposed to this danger. If anything is spilled inside the can it will drain out of the bottom without much chance of it coming in contact with the element. However strong acids will cause damage if they contact the element wires, and a burnout is very likely in this case. Of course water is also a hazard with any electrical apparatus, and flooding from leaking hoses is a possibility. The heater should therefore ALWAYS be used with a RCD or ELCB ( Reactive Control Device or Earth Leakage Circuit Breaker ) in the mains circuit. If your house is not equipped with these, you need to buy one for your protection.

Finally, please remember that this apparatus operates at high temperatures ; don't burn yourself ! The safety sign above should be attached to the outside of the heater.

 

 

 

Comments

author
corradini (author)2013-10-20

Where on earth did you get a 1m 100w heating tape for $3 (never mind which currency that $ is! >:-) I use that kind of thing a LOT, and if I could buy 'em for $3 -- AUS, US, whatever -- it'd make me quite happy...

author
kymyst (author)corradini2013-10-21

I bought it from eBay, item 281135728130 from China. The store name is popularstar2013. They are still selling them at that price ( US $ 3.00 )

author
makincoolstuff (author)kymyst2016-11-20

Just checked ebay and its no longer there, you haven't happened to find any other souces this cheap recently?

author
corradini (author)kymyst2013-10-21

Good lord - I just checked -- WOW. That's a screamingly good price (similar deals on longer ones, too). Just wish they had a bit more info, like max temp -- did yours come with ANY literature, or model/mfg, anything? (I'm going to e-mail them, too...) This could have a HUGE effect on my startup, thanks!

author
kymyst (author)corradini2013-10-21

No, there was no information supplied. I have run this heater at 280 degrees Celsius with no ill effects. At a guess I think the tapes would be OK up to 400 degrees C, but probably the only way to find their upper temperature limit would be to test one to destruction. Most commercially made heating mantles are rated at 450 degrees C maximum, but they could have thicker resistance wires than these cheap tapes ?. Generally these tapes deteriorate faster through oxidation when operated at high temperatures for long periods. The lower the temperature the longer they will last. But at least they are cheap to replace.

author
corradini (author)kymyst2013-10-22

Generally these seem fairly standard, looking at McMaster & Omega (both resellers). Wattage generally a function of length (actually lxw), with the 480°C ones (1" wide) being about 50w/ft, the 760°C ones being 155w/ft. (Obviously amps vary linearly too as voltage is constant.) They're MUCH more expensive from either - Omega's (760°C), 1.22m is $89, the 480°C one is $50. (5 weeks lead time on most lengths, so they're no doubt getting them from China.) McMaster carries only the high-temp version, closest version is about $50 (they're usually cheaper than Omega, and again this is the high-temp). SO - my GUESS is that, at that price, it's a 480°C (900°F) heater. They're also the 'double-insulated' type, from the photo, which would tend to confirm that. I think that to get up to the higher temp, they have to be a dual-element, and of course would pull higher amps (right?), so the wattage rating, correlated to a commercial heater, should give you max temp. I think? >;-)

I've used one (from Omega, single-insulated, 480-rated) at 250-300 (intermittent peaks up to ~350), inside an outer Pyrex tube, for ages w/ no burnout, although probably in an N2 environment for part of that, now that I think about it. I've also been using one of the double-insulated ones (single- are light brown with a back-and-forth sewn 'S' pattern; double- are snow-white fiberglass flat sheathing) for a little while, in air, no outer insulation/tubing, no troubles - I don't think much air gets into the inner insulation, but I do hear the same thing about deterioration at higher temps.

Thanks for the info! I like your stuff, btw. When I was starting up my company (actually in my garage!), I had to improv EVERYTHING, from Home Depot, drug store, aquarium store (!), Target, Goodwill, you name it. A few eBay things, but mostly lots of crafty repurposing and looking at, sigh, 'illicit' sites. (Oh, yeah, 'tobacco accessory' stores have lots of good stuff, esp. scales, heating, and pyrex. >;-) Fortunately I got funded and could buy 'real' stuff (2,000° lab furnace was nice), but I still kept the 'scrounge-and-repurpose' mentality. (I.e., using a $13 coffee-grinder from Rite-Aid instead of a $200 lab ball mill for catalysts; a Rival 'dorm-room hot-pot' for a water bath, Goodwill toaster coils for heaters, Pyrex measuring cup 'beakers', etc.) My investors LOVE that stuff -- and I can podge together something from my scrap-bin NOW, not pay for overnight or wait 3-5 for UPS... ;-) Plus, Freecycle and Craigslist are beautiful -- I have a fantastic full 4'w lab fume hood I think I paid $50 for, temp-controlled heating stir plate for $20, massive vacuum pump (free), full-size freezer (free), old gas chromatograph w/ columns (free (!)), etc. And huge parts of our pilot plant - which we raised $1.2mm with - were still podged together from Home Depot plumbing, etc., parts... (It's just fun, innit? >;-)
Cheers, Andrew

author
kymyst (author)corradini2013-10-22

That is all good information Andrew. I agree with your assessment of the heating tapes, they are probably the 480 degree type, which is quite adequate for organic chemistry work. I'm glad I could put you on to a less expensive source. It's probably the same situation with most lab equipment suppliers, they have an enormous mark-up which they can get away with because most of their customers are government funded in some way. When I was working in labs we built a lot of our own apparatus including weatherometers, environmental cabinets, and sundry test devices, which is where I developed the same scrounge and re-purpose attitude as yourself.

author
shriraj1 (author)2016-09-26

Well i want to vaporise the plastic for cracking which takes 650 degree Celcius.Can this type of heater(especially glass wool) withstand the temperature.

author
kymyst (author)shriraj12016-09-26

The heating tapes will definitely NOT withstand 650 degrees Celsius. I believe their upper limit is about 450 Celsius.

author
corradini (author)2013-10-22

A few thoughts:

(a) I wouldn't run the condenser coolant tubing in front of the heater; you're risking melting the tubing, and spillage on the heater. (I assume you know this; just mentioning for the kids at home.) I like to run the coolant from under the bench and behind when possible so any 'disconnection event' will point spillage *backwards* (DAMHIKT).

(b) and btw, a plastic insulated camping ('Coleman' in the US) cooler works great as a coolant reservoir, w/ water/ice/CaCl2 (sold as closet dampness remover) bath).

(c) ELCB is a great (i.e. mandatory) idea; in the U.S. you'd want a GFCI (Ground Fault Circuit Interruptor) - same thing in Americanese. And our closest equivalent to 'mains' is probably 'wall power'. (I'll not translate 'aluminium', which is actually a far better spelling.)

(d) A PID is initially more hassle -- figuring out how to wire it up can be a PITA, and you really should spend some time learning programming and esp. 'self-learning mode' -- than a simple 'turn the knob until it gets to the right temp' control, but it's WELL worth it; you'll never go back. (Oh, definitely wire it to/through an SCR (w/ heat sink!), and have THAT switch the actual 'mains' power, don't use the internal relay - trust me.)

Finally, (e) every lab worker, no matter the budget, learns that aluminium (!) foil is as indispensable as duct tape and super-glue are at home -- but BE CAREFUL what/where it touches. It *probably* won't conduct enough to short out YOUR 'circulation pump', but it can fry plenty of other things. Easy to get casual with it, but remember that it's basically a big sheet of WIRE. >;-)

author
kymyst (author)corradini2013-10-22

Thanks for the input
(a) Yes, That is good advice The hoses weren't long enough to run around the back when I set up the photo. They weren't actually touching the heater but would have got rather warm after a while.
(b) I have a couple of 3 litre wide necked Dewar flasks which I use for ice baths.
(c) Thanks, I had never heard of the term GFCI. It's always hard to know how to translate Australian into American.
(d) The PID is definitely worth the extra $ 12. Setting it up was very easy, even with the rather weirdly translated tech data sheet. The SCR is still essential, as you say.
(e) The alfoil is there to reduce heat losses through the top of the flask and the Vigreux column. It's difficult to maintain the column temperature without it. The circulation pump is 2 metres away on the end of the water supply hose.

author
rsmith79 (author)2013-09-26

What is the controller used for if you are regulating the temperature with the SCR?

author
kymyst (author)rsmith792013-09-26

As stated in the introduction, the PID temperature controller is optional. The temperature can be regulated with the SCR if the user is present to monitor the operation and make adjustments as necessary. With the PID controller it can be set to the required temperature and it will be automatically maintained. This is useful if you need to carry out reflux operations overnight, etc, and also provides a safeguard against overheating. It is a convenience, not a necessity.

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