Heating Wire Selection Criteria
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|The criteria for determining the heating wire of SAH type heater are established as follows.
|The current value of the heater to be designed is determined. I = P / E [A] current [A] = power [w] ÷ voltage [v]
|The heating wire diameter is determined from the current value determined above using the following graphs.
|At first, the surface load for the heating wire is assumed as 10 w/cm2.
|The higher the surface load, the shorter will be the heater. However, reducing the wire length can cause problems such as overheating, and the service life will also reduce, and the limit is 12.5 w/cm2.
|In some cases, the wires are designed with a large surface load when lower hot air temperature is allowed. These are special products limited to certain applications and customers.
|The smaller the surface load, longer the service life when used at higher temperatures. Problems are not caused when used in abnormal conditions such as heating without air flow as this gives the heating wire additional margin.
|However, the heater becomes longer when the surface load is decreased, and the surface load cannot be reduced excessively based on this point. In reality, the surface load can be reduced up to 7.5 w/cm2.
|In the past we were designing with 11 w/cm2, but recently we consider 10 w/cm2 as the standard. However, we do not select the heating wire diameter to get the standard wire diameter. There are restrictions on the heating wire such as standard sizes, and available size in stock, and we will have to select from the sizes that can be obtained, and we may not be able to set the surface load freely.
|In addition, if the current value is very low (around 1 A), the heating wire will become very thin and can be cut very easily, in such cases, the surface load will be set to a lower value. A surface load of 7.5 w/cm2 for a current value around 1 A is considered. On the other hand, the surface load can be increased for products with high current if the design service life is the same.
|This graph has been calculated with the performance DS curve, the Cantal AF curve can also be used as the difference is not significant. (Cantal AF curve results in a slightly lower surface load)
|The length of the heating wire (UCL) is determined from the resistance value Rm [O] of the heater and conductor resistance value Rm [O/m] displayed on the bobbin of the heating wire, using the following formula.
|Heater resistance value R = voltage × voltage ÷ power [O] (R = E^2/P)
|Heating wire length UCL = 0.97 × R / Rm [m] *(0.97 is a constant to compensate for the resistance temperature coefficient)
|Note: Units of the length is [m], therefore the value has to be multiplied by 1000 for conversion to [mm].
|In actual practice, the value can be finalized with UCL = R/Rm without multiplying by 0.97. The length of the lead portion is reducing by following this method, and the value will be closer to the resistance value. In addition, a higher difference in the resistance value is better than a lower difference. (This is the safer side since the service life increases in this direction)
|The coil winding should be prepared keeping the resistance value errors in the +3%, -1% range.
Heater coil length
1. Designing from the surface load of the heating element
|First, determine the current value of the heater. The relation is Current = Power/voltage, therefore the current value can be determined by dividing the heater power by the heater voltage.
|Next, determine the surface load of the heating element. In case of hot air heaters, 10 - 11 w/cm2 for current 3A or more, 8 - 9 w/cm2 for current 2A or more, and 7 - 8 w/cm2 for current 1A or more. In addition to these, the surface load is further reduced for high temperature hot air applications and design with high safety. On the contrary, in case of low temperature hot air, the surface load can be increased and the heater size can be reduced.
|Once the target surface load is determined, the diameter of the heating wire to be used is determined from the heating wire selection criteria of the above figure. An ideal heating wire diameter cannot be always chosen. The appropriate heating wire diameter is determined from the standard dimensions of the manufacturer, and based on the wire availability in stock.
|Determining the length when a coil is required. The relationship of the electrical resistance value of the heater is Electrical resistance = Voltage x Voltage / Power, the electrical resistance value of the heater is determined using this equation. From the “Electrical resistance per 1 mm (O/mm) of coil” of the following graph, find out the electric resistance value per 1 mm of coil for the heating wire diameter selected, and determine the required coil length by dividing the electrical resistance value with the per 1 mm (O/mm) value. If the coil length is appropriate, finalize with this value. If the length is too long or short, change the heating wire diameter, and repeat the above calculation again.
2. If test results are available, determine the dimensions using proportional calculation
|The coil length of the heater is proportional to the power and inversely proportional to the surface load. If this relationship is used and dimension data of the heater serving as a reference is available, the total length of the new design coil can be determined with the following equation.
|Heater length L = L0 (P/P0) (Sp0 / Sp)
|L0: Coil length of the standard heater
P: Power of new design heater
P0: Power of standard heater
Sp: Heating wire surface load of new design heater
Sp0: Heating wire surface load of standard heater
|Note: If you add intermediate fixing for the coil as measures for heater service life, this dimension has to be added.