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

§ 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.

!!!!!    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 !
 

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.

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.

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.

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.

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.

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.

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.