CERAMIC AND GLASS FAQ

 

Warning:

 

Some of these discussions deal with electricity and electrical connections.  It cannot be stressed enough – you must always disconnect the power to equipment that you need to service before opening up any boxes or panels.  If you have any questions or are not qualified to work on the equipment – hire a licensed electrician.

 

Disclaimer:

 

The contents of this document are provided “AS IS”.  The information could contain technical inaccuracies, typographical errors and out-of-date information. This document may be updated or changed without notice at any time. Use of the information is therefore at your own risk. In no event shall Duralite be liable for special, indirect, incidental or consequential damages resulting from or related to the use of this document.

 

1.       How do I determine the voltage coming into my building?

 

The best way to measure voltage is with your own meter.  Alternatively, if you rent space, have the building maintenance person provide this information.  If you use an electrician, have them measure it.  In rural areas, or at the end of the line, large equipment sharing your line may influence the voltage.  If possible, measure voltage with your kilns and ovens in operation. Old buildings often have 208V, you may also have 220, 230, or 240 Volts.

 

2.       What are ohms and resistance?

 

Ohms are the units of resistance to the flow of electrical current.  Ohms determine the power rating of a heating element at a given voltage.

 

3.       How much power do I need?

 

The power you need is identified as Watts.  Power requirements for a kiln is often listed on a plate mounted to the side.   If you are designing your own equipment or have equipment that does not have any information, guidelines are presented below for estimating your power requirements.

 

There are relationships of Watts to Amps and Volts using variations on Ohm’s Law.  Ohm’s law can be stated as the voltage across a resistor is directly proportional to the current flowing through it.  Here the resistor is your heating element.  Written as equations that is

 

Volts = Ohms * Amps

rearranging this equation gives us these

Amps = Volts / Ohms

Ohms = Volts / Amps. 

 

Power supplied to the resistor (heating element) is absorbed by the heating element and dissipated as heat.  The rate of energy dissipation is described as these equations.  Here are variations that can be made using the equations above:

 

Watts   = Volts * Amps

            = Volts * Volts /Ohms       = Volts² / Ohms

                                    =  Amps * Ohms * Amps  = Amps² * Ohms.

 

Variations on equipment construction, materials and usage can make power calculations complex.  Over time we have developed guidelines for quick estimates of  power.  These rules of thumb do not replace a careful examination of a design and rigorous power calculations, but offer some guidance for best and worst case power scenarios.

 

For design purposes small general purpose kilns (~2250°F) of 5  to 10 ft3 using 2300°F insulating brick plus minimal (1”) fiber insulation often require 850-1200 Watts per cubic foot of interior capacity.   Smaller units require higher wattage per cubic foot.  Larger units often can use lower Watts per cubic foot. 

 

Low temperature furnaces (metal tempering, annealers or burnout ovens) of  8-20 cubic feet require about 300-800 watts per cubic foot depending on construction.  If it will be used occasionally to reach higher temperatures such as bisque firing or glass casting, slumping, enameling, or fusing a furnace of this size may require as much as 1250 Watts per cubic foot. 

 

Glass melting furnaces or equipment designed for high temperatures (2300°F+) may require 2100 Watts per cubic foot or more.

 

4.       How can I determine what Amperage I have available?

 

Amperage can usually be determined by checking the fuse or circuit breaker box in your building.  Check the amperage draw of each of your pieces of equipment in your shop when in operation.  Add these operating amperages together and subtract from the panel capacity to come up with available

amperage.  You may need the help of an electrician to make this determination.

 

5.       What type of thermocouple do I need?

 

Many kilns and annealers use type K thermocouples.  For very high (2300°F and higher) temperatures kilns and furnaces will often use a type S or R thermocouple.  Leads on a type K thermocouple are usually marked – one lead is yellow (-), one is red (+)

 

6.       What does the stretch length of a coil mean?

 

Heating elements are usually made close wound and stretched to the final length.  The final length is equal to the length of the groove in your kiln. 

 

7.       How do I measure for the stretch length?  How can I stretch my own coils?

 

Use a string or rope to measure the groove length.  You can stretch your own element but it requires gripping one end at a stationary point like a vise.  You’ll need to mark out the distance on a bench or board, grab the other end with heavy pliers or locking pliers, and stretch the coil 10% beyond your mark.  The coil will spring back to a point less than you are looking for.  It may take several attempts to reach the correct length.  Over-stretched coils are difficult to deal with.  Sometimes you can shorten an over-stretched coil by inserting a metal rod into it and compressing the coil.  We recommend you have coils stretched to length by the manufacturer as they are experienced with the procedure. 

 

8.       How do I measure the OD (outside diameter) of the coil?

 

Coil diameter can be estimated with a ruler or metal scale but it is best measured with a micrometer or caliper.  Digital calipers can be purchased for as little as $35.00 - $40.00.  Element diameters are usually given in thousandths of an inch or millimeters. 

 

In a pinch – you can wrap a piece of paper around an element – then mark the point where the paper begins to overlap itself.  Unwrap, and measure the distance from the edge of the paper to the mark – then divide by 3.1416.  This gives you a reasonable estimate of OD.  If you can measure down to 1/32 with care – you can often come within 0.01 in. of the actual diameter.

 

9.       How do I use the pinning wire and the connectors?

 

The wire we provide for pins should typically be cut into ½-1+ inch lengths and bent with pliers into  “U” or “V” shape.  Trial and error will provide the best length/shape for your groove and element configuration.  These pins are placed between the individual coils of an element and forced into the brick to prevent the element from jumping out of the grooves.  Most elements connect to the kiln wires using crimp connectors, which Duralite also provides.

 

10.    Most kiln or furnace elements have its own connector that is made of high temperature resistance wire.  It serves as a transition between the heating element and the conductors. What is this called?

 

This can be called a lead, twisted lead, tail, twisted tail, pigtail, or connector. 

11.    Are there different types of leads?  How do I know which type I need?

 

Industrial furnaces typically employ either a heavy rod or strip welded to the element.  This is  fabricated to provide a cool and gas tight junction between the element and a high temperature conductor wire.  Typically a rod is  threaded to accept nuts.  Often strip has a hole used for a bolt to pass through.  Rod terminals are easy to design for – only a small hole in the refractory is required for electrical connections.  Rod terminals are subject to rotation that places torque on the weld between the terminal and the element that can be a source of damage during installation.  Strip terminals are easier to stabilize, resulting in fewer failures due to undue torque on the terminal to element weld – but require a larger hole through the furnace wall.

 

Kilns and glass annealers typically use twisted leads.  This simply means two wires are twisted together where the ends of the element pass through the wall of the kiln and are connected to the kiln wires.  Twisting the wires together essentially lowers the temperature at the connector.  A few kilns and glass melting furnaces use welded rod terminal of 5/16” or 3/8” diameter. 

 

12.    How can I determine how many coils are in my kiln? 

 

Many kilns use 1 to 12 elements but there are large units that use 48 or more.  In most kilns, elements go around once or twice.  The best way to determine the number of coils in your kiln is to read your owners’ manual.  Alternatively, disconnect your power and open the box on the side of your kiln and count the connections; each element has two ends that connect in the box.  Sometimes this can be more complicated than it might appear.  When in doubt, consult an electrician.  ALWAYS disconnect the power before proceeding!

 

13.    What is the difference between coils wired in series and coils wired in parallel?

 

Series-connected elements connect one element into the next.  If all the elements in the series are identical, each element sees a voltage equal to the total supplied voltage divided by the number in series.  This is most effective for high temperature systems where a heavy wire diameter is desired.  Heavy wire has lower resistance per foot than smaller wire.  It generally forces you to use lower voltages in space-limited designs.  The series connection then makes the voltage for each element low enough to keep power demands manageable.  The disadvantage of this configuration is if one element fails, it shuts down all the elements that are in the series.

 

Parallel-connected elements are arranged so each element is connected to the total supplied voltage.  This is very common in small to medium size kilns, furnaces, and ovens.  Where smaller wire diameters are useful this is the best method for multiple-element systems.  An advantage with parallel elements is if one burns out, the rest of the elements still function.

 

14.    What type of switch does my kiln use?

 

There are two types of switches typically used on kilns. Both are rated at 15 amps.  One is a step switch rated at high, medium and low.  The other is an infinite switch such as used on electric ranges 0-100%.  Voltages can be 120 or 240.  The 240V unit is used for 208V, 220V, 230V, or 240V.

 

15.    Can I cut the coil if it is too long?

 

If the element is the correct wattage but stretched too long, it may be possible to shorten it  by placing it over a rod and compress it, then re-stretch it to the correct length.  The amount of space there is between each loop will ultimately determine if you can compress the part or not.  Generally if there is one wire diameter or more between each loop, you will be able to shorten it by compressing and stretching the element to the correct length.

 

NEVER cut a coil to shorten it.  Shortening a coil in this manner will up the wattage and increase the amperage and the temperature.

16.    What is a silicon controlled rectifier (SCR)?

 

Silicon controlled rectifier is a solid state device, or thyristor, with no moving parts.  SCRs control power to the load by switching the AC voltage on and off within one cycle (phase angle firing).  There are variations of SCRs known as a solid state relay (SSR).  The latest versions of these usually only switch on and off when the AC waveform crosses zero volts.  This version of switching is often referred to as zero-cross firing.

 

17.    What is meant by burst fire?

 

Burst firing is a power control method that repeatedly turns on and off full or half AC cycles.  Also called zero cross fire, it switches close to zero-voltage point of the AC sine wave.  Variable – time – base burst  fire selectively inhibits or passes AC cycles to achieve the desired power levels.  Burst firing is a preferred mode to control elements.  An SSR coupled with an appropriately optioned Watlow Series 982 provides the most even distribution of power with the lowest level of noise generation (RFI).

 

18.    What is an SSR Relay?

 

It is a solid state relay.  See also SCR above.

 

19.    What kind of power controller should I use?

 

We recommend the solid state relay (SSR).  Duralite carries both mercury displacement relays (MDRs) and SSRs, but Duralite encourages use of  SSRs since they do not contain liquid mercury.

 

20.    When would I need to use quartz tubes?

 

Quartz tube-enclosed heating elements are recommended for ovens and kilns whose interiors are made of ceramic fiber blanket where there in no way to provide grooves in which to mount elements.  Quartz has very good electrically-insulating qualities even when hot, and is virtually immune to thermal shock.