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2024-03-11

The Importance of Dry Compressed Air in CNC Machine Tools

Machine tools require compressed air to perform multiple functions, including delivering oil to the spindle bearings, starting the tool change mechanism, and blowing away chips and contaminants from tools and workpieces. For example, every time an automatic tool change occurs, the tool holder and spindle taper will be exposed to chips, oil, dust and other contaminants in the cutting area. Any particles adhering to these exposed surfaces will reduce the accuracy and rigidity of the machine, and even cause permanent damage to the connection. Most CNC machines use a jet of dry compressed air to keep bare surfaces clean. If the air is not dry enough, the water in the air will condense on the surface and eventually cause rust.

Figure 1. The dew point temperature is related to the air temperature for different relative humidity values.

There is water in the workshop air, so there is water in the compressed air. The amount of water depends on many factors, but mainly on the temperature. Warm air has more water than cold air. This quantity is usually expressed in relative humidity (RH), that is, the amount of water in the air relative to the amount of water that the air can hold. This is a problem because the process of producing compressed air changes its temperature.

When the compressor compresses air, work generates heat, and the air discharged from the compressor usually exceeds 100 °F(37.8°C). When air contacts a cold surface (such as the inside of a compressed air tank) and cools, water condenses from the air into dew. Draw "sweat" on a glass with a cold drink or a foggy glass on a bathroom mirror ".

In addition, when the pressure is released, for example when the air exits the nozzle, the temperature of the air drops again and the water condenses. The temperature at which water begins to condense is the dew point.

When the air is "saturated" (relative humidity is 100%), the dew point is the same as the air temperature). When there is less water in the air, the RH is lower and the dew point is lower than the air temperature. The purpose of machine tool air drying is to remove enough water so that the dew point of the air is much lower than any temperature that the air will encounter during use. So what should we do?

The first step of air drying involves the contact of compressed air with the inside of the tank. The condensed water here is collected at the bottom of the tank and should be drained periodically or automatically.

More water can be removed using membrane air dryers, deliquescent dryers and desiccant dryers. In a membrane air dryer, air passes through bundles of hollow semi-permeable tubes. Water vapor can pass through the tube wall, but air cannot. The water is collected on the outside of the tube bundle and discharged.