Have you ever noticed or heard anyone complain about water in their compressed air system or is your compressor leaking water? Such occurrences are very common, but they should not be ignored and left unattended, as it could harm your compressed air system and jeopardize the quality of your end product. Let’s take a look at why water exists in compressed air and how to treat it properly to avoid any potential risks.
Water condensation is a natural occurrence and a byproduct of compressing air. The amount of water produced by an air compressor is largely dependent on the inlet condition, the quality of ambient air in a given environment, as well as pressure. In simpler terms, air temperature, humidity, compressor size and required pressure will determine the water content coming out of the unit and potentially into the compressed air piping. Naturally, hot and humid air has a higher moisture content than cold air, it will result in more water coming out of the compressor. For example, a 55kW (75HP) rotary screw air compressor which operates in 24 °C (75 F) ambient temperature with 75% relative humidity will produce 280 liters (75 gallons) of water per day. Below, the process of moisture removal within a compressed air system is illustrated.
The water can be separated using accessories: aftercoolers, condensation separators, refrigerant dryers and adsorption dryers. A compressor that works with 7 bar(e) overpressure compresses air to 7/8 of its volume. This also reduces the air’s ability to hold water vapor by 7/8. The quantity of water that is released is tconsiderable. For example, a 100 kW compressor that draws in air at 20°C and 60% relative humidity will give off approximately 85 liters of water during an 8 hour shift. Consequently, the amount of water that will be separated depends on the compressed air’s application area. This, in turn, determines which combination of coolers and dryers are suitable.
To further explain this, let’s take a look at how parameters such as ambient temperature, flow rate (size of compressor), inlet pressure, inlet temperature and desired pressure dew point (PDP) affect the drying process and potential water content in the compressed air system.
Flow Rate or Compressor Size
For applications that require higher flow rates (CFM or l/s) will result in greater water content in the system.
Ambient Temperature / Humidity Content
Compressors that operate in higher ambient temperature and humid environment will end up producing larger amounts of water within the compressed air system.
If the inlet temperature going into a dryer is higher, more water content will be present in the compressed air, therefore needing a bigger dryer to treat the air and condense the water out.
Unlike flow, temperature or humidity, the pressure works opposite, as higher the pressure, the less water the compressed air contains and easier it is to dry. If you take a sponge filled with water into consideration, the harder you squeeze the sponge, the less water it will contain.
Pressure Dew Point (PDP)
Pressure dew point is a common way to measure the water content in compressed air. PDP refers to the temperature point where air or gas is saturated with water and begins the process of condensation or turning into a liquid state. This can also be explained as the point at which air is not able to hold any more water vapor. In order to minimize water content in our compressed air, a lower PDP level is required, while higher PDP values refer to greater amounts of water vapor in the system. The size of the dryer will determine PDP and levels of condensation in compressed air.
Excessive moisture in compressed air can have damaging effects on the facility and jeopardize the effectiveness of operations. Untreated condensation in compressed air can damage and cause problems to pneumatic systems, air motors, valves, as well as any components or machines connected to the system and possibly contaminate the process or manufacturing of the end product. Here is a list that further explains the adverse effects of moisture:
Selecting the proper drying method for compressed air is largely dependent on the specific requirements that need to be met in order not to compromise the process and end product. One of the first steps of removing moisture out of compressed air occurs inside of the compressor, as the moisture separator or aftercooler is capable of getting rid of 40-60% of vaporized water.
Once the compressed air leaves the aftercooler it remains saturated with water and can have damaging effects on the overall system if left untreated. Utilizing an air receiver can also aid in reducing water content in compressed air, as the tank’s ambient temperature is much cooler than the hot compressed air coming out of the air compressor. It is important to keep in mind that a wet tank will collect excess moisture and therefore will need to be drained daily to avoid corrosion and excessive wear.
If the application calls for further moisture removal, the need to introduce an external or internal (integrated) dryer is necessary. Depending on the desired dew point, the two dryer options are refrigerated and desiccant air dryers. In a refrigerated air dryer, the air temperature is lowered to three degrees Celsius (37 degrees Fahrenheit), causing vapor water to condensate out of the compressed air at that temperature. If refrigerated dryer dew point is not sufficient, a desiccant bead air dryer should be used to accomplish the desired result. In a desiccant dryer, the dew point is lowered to -40 degrees Celsius/Fahrenheit, resulting in bone dry air that is critical in spray painting operations, printing and other pneumatic tool uses.