&#;&#;&#;&#;&#;&#;PRACTICAL TECHNIQUES & KNOW-HOW FOR MAKING & MEASURING IN THE LABORATORY & WORKSHOP

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Obtaining a Uniform Temperature in Tubular Heaters

A simple tubular heater for experimental laboratory work can be realised by winding a length of resistance wire on to a borosilicate or quartz tube and passing a current through it. This is best accomplished by first wrapping a loop of 18 SWG Iron wire around the tube at each end of the intended winding, each loop being secured by twisting the wire ends tightly. The end loops can be then further secured by cementing them to the tube using a ceramic cement (for example Aremco's Ceramabond) which is cured by baking in an oven after application. It is important to attach these Iron wires as securely as possible because the tightness and stability of the subsequent resistance wire winding depends on the integrity of these terminations.

The winding of the heater coil can be achieved most conveniently by the use of a glassblowing or machining lathe. The tube is secured in the chuck and the resistance wire previously attached on one of the iron wire terminations is wound on to the tube by rotating the chuck by hand while maintaining a tension on the resistance wire. A scale on the glass or quartz tube, or a scale printed on paper rolled and inserted in the tube provides a guide to maintaining the desired pitch. When the far end of the tube is reached with the winding, the wire is secured to the iron wire termination by twisting it tightly and both connections can then be doubly secured by spot welding. A line of ceramic cement can be applied to the winding along its length to prevent any movement of coils as they are heated.

In this present study, heater coils were wound using 0.4mm (26 Gauge) Nichrome wire wound on a borosilicate tube, approximately 25mm diameter, between iron-wire connections 85 mm apart. A total of 40 turns of wire were wound using approximately 2.6 metres of wire, with a resultant resistance of about 23 Ω. Current from a Hewlett Packard B power supply was passed through the heater coil and the temperature at various points along the inside length of the tube was measured by a Comark Chromel Alumel Thermocouple meter. Typical coils used in this study are shown below.

As expected, a uniformly wound coil exhibited a non-uniform temperature profile along its length with a maximum temperature in the middle, with decreasing temperatures towards the ends. In order to make the temperature profile more uniform, it was proposed to wind the coils with a non- uniform pitch. The scheme adopted for the purposes of this study was to divide the 85 mm winding length into four equal segments, to maintain the total number of turns at 40, but to vary the distribution of these turns among the four segments. As an extreme case, a coil was prepared having 15 turns in the first segment, 5 each in the second and third segments, and 15 in the fourth segment. Such a coil is illustrated in the photograph above.

A third coil was prepared having a distribution of 11-9-9-11 turns in the four segments. In order to wind this accurately, a printed guide was inserted in the tube prior to winding, as illustrated below.

The Temperature Profile obtained from helical windings of resistance wire.

The plots below show the temperature profiles obtained from three coil configurations: a uniform winding, a 15-5-5-15 configuration and a 11-9-9-11 configuration.

The plots demonstrate the value of trimming the winding pitch of a tubular heater to minimise the temperature variation along its length. Taking the results using 1 amp and 24 volts, giving an average temperature of 181°C, the uniform winding showed a variation of 9.8 °C between 20mm and 60mm along the length, but the winding using the 11-9-9-11 configuration showed only 3.8 °C variation in same interval.

Other possibilities present themselves: It may be possible to calculate and realise a continuous variation of pitch along the length of the winding and design a pitch profile to produce any temperature profile along the heater as may be desired. It is noteworthy that only slight pitch changes are necessary to improve the temperature profile of a uniform winding and that this indicates that considerable care should be exercised in the laying of any tubular heater winding to obtain a predictable result.

Published November 9, . Updated June 19, .

Introduction

A coil winder is essential when it comes to making a beautiful coil on large forms.

It takes considerable less time to wind large coils and varnishing the coils while they are rotating gives a much better finish.

Foshan Osno contains other products and information you need, so please check it out.

Considerations

Control of speed and acceleration is key issues when building a coil winder.

The best choice is to get a Variable Frequency Drive meant for 3 phased asynchronous induction motors, especially if the motor have a gear.

With a VFD it is possible to control the speed with a potentiometer, control start and stop with a single push and hold button and control ramp up and down times.

Having both hands free to guide the wire onto the form is important

Construction

27th March

With materials at hand the motor was secured to a horse with a custom drive wheel with a rubber band to get contact to the form. A gear is attached to the motor with the ratio of 1:14, this makes it possible to run the motor at higher speeds with the VFD. Running at speeds close to its native speed makes cooling more efficient as its self cooled from the rear fan.

The form is pressed against the roller wheels which keeps the form in place.

Controlling the winders motor is done with a Siemens Micromaster Vector 0.37kW VFD, a potentiometer is used to adjust the speed and a foot pedal is used for start / stop. The VFD is programmed with a slow start ramp to avoid breaking the copper wire.

Conclusion

The machine was built without buying any new materials and is quickly put aside as the rollers are hold to the horses with welding clamps.

Further improvements could be a turn counter and automatic wire guide, but with the relatively low number of large coils I make there is other fields I like to focus on.

Demonstration

A time lapse movie of me winding 0.25 mm wire on a 160 mm diameter form.

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