The usual construction is a 20 period exponential-moving-average with bands at 2 times atr. Because ATR is smoother than standard deviation, Keltner channels expand and contract more gradually than bollinger-bands.
That smoothness makes them better suited to trend-following use. A close outside the upper Keltner band is often treated as evidence of a directional move beginning rather than as an extreme to fade.
The comparison between the two band types is itself informative: when Bollinger Bands contract inside the Keltner channel, volatility is unusually low, which is the standard definition of a bollinger-squeeze. Neither channel forecasts direction on its own.
Related: bollinger-bands, donchian-channel, atr, bollinger-squeeze, trend-following