Cooling tunnels ensure controlled temperature reduction in both batch and continuous process systems. They are similar to air convention ovens, but have no heating capacity. Cooling tunnels use blowers to force ambient air or outside air into the tunnel while drawing heated air from the parts out the opposite end. Passing air over the parts results in significantly faster cooling times and more even cooling. There are two basic types of cooling tunnels: continuous and discontinuous. Continuous cooling tunnels are designed for applications in which all products require the same cool down period. By contrast, discontinuous cooling tunnels are designed for applications in which products need to exit from the tunnel as soon as a desired end temperature is attained. Discontinuous cooling tunnels are more expensive than continuous tunnels, but require less energy because of their greater efficiency. They can handle pallets with different heights and reduce product-handling times during cooling. Optional accessories for cooling tunnels include vibrating, lifting, or detachable tables; extra space for product handling; granulate distribution systems; and synchronization with other processing equipment. Selecting cooling tunnels requires an analysis of product specifications and features. Specifications include conveyor width, conveyor belt material, tunnel length, canopy material, humidity range, and number of cooling zones. Dimensions such as width and length are measured in English units such as inches (in) and feet (ft), or metric units such as meters (m) and centimeters (cm). Conveyor belts are made or plastic or metal and, depending on the application, have a rough or smooth surface. In the United States, the Food and Drug Administration (FDA) establishes requirements for cooling tunnel components that are used in food processing applications. Industrial applications for cooling tunnels require conveyor belts that are made of more durable materials. Typically, stainless steel is the material of choice for both the cooling tunnel canopy and side panels. The number of cooling zones varies by model and manufacturer. Products with both top cooling and bottom cooling are commonly available. In terms of features, some cooling tunnels are portable, washdown capable, or feature a modular design. Others use a programmable logic controller (PLC) and feature a digital control panel. Red and green light emitting diodes (LED) indicate whether the cooling equipment is on or off, and whether parameters such as temperature need to be adjusted.