There are two operation configurations in the spray dryer, open-loop and closed-loop. The open-loop uses air as the drying gas that is not recirculated. The open-loop configuration is usually preferred because it is more cost effective and stable. The closed-loop, on the other hand, uses an inert gas that is recycled in the drying chamber. In addition, the closed-loop is used to prevent the mixing of explosive gases and to manipulate the oxygen sensitive substances. There are two directions in which the drying gas can flow with respect in the direction of the liquid atomization. It can flow in the same direction (co-current flow) or it can flow in the opposite direction (counter-current flow). The figure below shows the two different directions. In case A, the final product is in contact with the coldest air. As a result, case A is preferred for drying heat sensitive materials. In case B, however, the dry product is in contact with the hottest air. Therefore, case B cannot be used with temperature sensitive materials. But, in terms of higher thermal efficiency, case B is preferred. The direction does not only have to be with or against the liquid atomization. There are intermediate configurations containing mixed flow between co-current and counter-current.