These coolers are generally called direct expansion coils and consist of a series of tubes through which refrigerant flows (Figure 3.21). The tubes, which are finned to increase the heat transfer rate from the medium to be cooled (e.g. air) to the boiling point, are normally arranged into a number of parallel circuits fed from a single throttling valve. The hot refrigerant vapor is accumulated in the outlet (suction) gas header. These direct expansion coils are used only in the positive displacement compressor systems, owing to quite lowpressure ratios. Like liquid coolers, these coolers are also classified as flooded and dry types. In a flooded coil, a float valve is used to maintain the preset level in the coil, keeping the evaporator coil close to full of the liquid refrigerant. This full contact of the liquid with the tube walls provides a high heat transfer rate. In practical applications, flooded type evaporators are not preferable, because they require large amounts of refrigerant. A dry coil requires only a small amount of refrigerant and this reduces the cost of the refrigerant charge. Sometimes a metering device (thermal expansion valve) regulates the amount of the liquid entering the coil to maintain a predetermined amount of superheat in the refrigerant at the coil outlet. The dry expansion coil contains mostly liquid at the inlet and only superheated vapor at the outlet, after absorbing heat from the medium to be cooled. In the air coolers, when the surface temperatures fall below 0°C, frosting occurs. Thick layers of frost act as insulation and reduce the air flow rate (in the forced convection coils) and the available inner space.
Several methods are used for defrosting, e.g. hot-gas defrost and water defrost. But recently frost-free refrigeration systems have become popular because of the problems mentioned above.