To obtain an adequate "heat flux" value (radiant coil(s) temperature)... a general rule is that for a given wattage, thicker wire requires a lower resistance and thinner wire a higher resistance... to stay within a particular heat flux range and APV current output values.
You can use the Steam Engine coil modelling software (see sigline hyperlink below - adjustable heat flux value is in the sixth line down in the results box) to 'build' your RDA for a 'desired' wattage output value... using the heat flux (radiant temperature) as one of the constants.
I'll provide you with 2 examples.
- At 30 watts of output, a 28 gauge, dual parallel build consisting of a 8/7 wrap (2 X 67mm of wire - terminal to terminal) on a 2mm mandrel with a generous 4mm of leg length, will provide a 0.6Ω net resistance and a heat flux value of 222 mW/mm² (milliwatts per square mm)... which is in the yellow color code zone - slightly warmer than the median 'green' zone.
- To 'prove' the statement made in my first paragraph... all else being the same, but using 30 gauge Kanthal (2 X 84.3mm of wire - terminal to terminal) to produce a 11/10 wrap coil with a net resistance of 1.2Ω - will provide the same heat flux as the thicker, lower resistance coil(s) of the first example... 222 mW/mm².
The 'appropriate' heat flux for an application can vary greatly... the greatest of variables being the personal tastes of the individual user. Some of the less subjective elements that affect what one might consider an optimal coil temperature are the juice blend, RBA air flow potential... including air inlet/outlet adjustments and duration/frequency of the draw.
As you didn't provide sufficient build specifics for me to develop what you might consider a satisfactory heat flux, the example I provided may be too cool or too warm for your personal tastes... so you'll end up playing with it.
