Abstract: A novel tube heat exchanger is provided for a gas powered fryer where the cross-sectional area for flame passage decreases along the length of the tube. An exemplary tube heat exchanger has a spiral corrugated surface to have increase surface area, wherein a tapered spiral insert improves the interaction of the flame with the tube wall to improve the heat transfer. A split tube having flame passing cross-sectional area reduced along the direction of the flame flow improves interaction of the flame with the walls of the tubes and increases the surface area for better heat transfer.