cannot prevent this water vapor from being taken into the compressor. Instead, the water vapor is usually squeezed out of the air during compression and transformed into steam by the heat of compression. The steam condenses to form moisture droplets downstream from the compressor, as the compressed air charge is cooled. Since moisture can damage some of the machines that use compressed air (pneumatic tools, for example), the moisture must be removed from the air before it is sent to the storage tank. So, a FILTER and a MOISTURE SEPARATOR ASSEMBLY are placed between the compressor and the storage tank. The assembly removes most of the moisture, or any other impurities, entrained in the air before it is sent on to storage. The assembly is fitted with a valve or drain cock, so accumulations of water and dirt can be drained now and then by the operator.

The tank that stores the compressed air is called the AIR RECEIVER, as shown in figure 6-42. In this way, demands for compressed air are made upon the receiver, rather than directly on the compressor itself. And there is little chance of the demand for air exceeding the supply. To this end, the air receiver has with it some type of control system or device to monitor the supply of compressed air in the receiver. The control device may be a PRESSURE SWITCH that senses predetermined thresholds or levels of pressure. When the compressor has sufficiently charged the receiver with compressed air, the pressure switch automatically opens and shuts down the compressor. If and when the demand for compressed air begins to drain the receiver to a preset pressure threshold, the pressure switch closes and automatically starts the compressor.

In systems where the demand for air is more or less constant and prolonged, a type of CONSTANT- SPEED CONTROL can be used. The compressor is permitted to run continuously to keep the receiver charged with air, while the constant-speed control functions somewhat like a pressure relief valve. If the pressure of the compressed air in the receiver rises, because of a momentary drop in demand within the system, the control simply vents the excess compressed air to the atmosphere, rather than shut down the compressor.

Figure 6-42. - LP air compressor piping arrangements.