I am building a small spray dryer, 1-meter diameter by 6-meter height, drying chamber dimension. I need 400 deg centigrade air temperature in the drying chamber. The product is detergent. I got a hint that Indirect Gas Air Heater may not easily get 400 deg C air temp, and if I use Direct Gas Air Heater, the contaminants may mix with my product. So what kind of gas fired air heater will be adequate?