COMPRESSED AIR DRYERS WHITEPAPER WATER LEAKAGE IN COMPRESSORS
Atmospheric air always contains water, mostly in the form of vaporized moisture and sometimes in the form of actual water (when it rains). Even though vaporized water in the atmosphere is usually neither visible nor noticeable, there can be quite a lot of it.
At a temperature of 35°C, air can contain a maximum of 39 grams of water in the form of vapor per m³. If the air reaches that level, it has a relative humidity of 100%. If there is more water in the air, anything above those 39 g/m³, it will condense and form droplets.
When the temperature decreases, the maximum amount of water vapor that the air can hold decreases as well. For example, air at a temperature of 20°C can only contain a maximum of 17 grams of water per m³ in the form of vapor. This means that when we take 1m³ of air at 35°C with 100% relative humidity and cool it down to 20°C, 22 grams of vapor will condense, as the air is not able to hold this amount of vapor anymore.
The dew point is a parameter that is often used to indicate the dryness of the air. It is the temperature at which the air is fully saturated with water vapor. In our example, air at 35°C that contains 39 g of water vapor per m³, i.e. air with a relative humidity of 100% has a dew point of 35°C. If the air only contains 22 g of water per m³, which means that its relative humidity is 56%, then it has a dew point of 20°C.
Note: If these values are measured at an atmospheric temperature pressure, then we speak of the atmospheric dew point. If they are measured under pressure, then we refer to it as pressure dew point. But what happens when atmospheric air is compressed? During compression, the concentration of the water vapor increases together with the pressure ratio, which means that more water vapor will be fitted into the same space.
An example: Under normal ambient conditions, a temperature of 35°C and a relative humidity of 60%, 1 cubic meter of air contains about 23 grams of water vapor. If the ambient air is compressed from the atmospheric pressure to a pressure of 7 bar(g), the water vapor concentration increases 8-fold. As a result, a cubic meter of that compressed air now contains 184 grams of water. That means that 161 grams of additional water enter the compressed air system for each cubic meter of compressed air produced at 35°C.
If that doesn’t seem like a lot, let’s look at what this would mean for a 90 kW compressor that runs 8 hours a day. During that time, it would deliver 970m³ of compressed air containing a total of 140 liters of excess water, which could fill a bath tube, and 37 liters of water vapor.
Watch the wiki video for a clear explanation of why water exists in compressed air and how to treat it properly to avoid any potential risks.
This brings us to the key question:
Fortunately, there are multiple ways to remove the water and/or moisture from compressed air and protect downstream equipment and products. The first is called “over-compression.” Here, the pressure is increased above the required level. As we have just learned, that would result in additional water in the compressed air and the formation of more droplets, which are then removed. Afterwards, the pressure is reduced to the actually required level. The result is that the air is much dryer because only water vapor remains in the compressed air, resulting in a relative humidity below 100%.
A second method is cooling. As we explained before, the amount of water vapor that the air can hold decreases together with the temperature of the air. During cooling, the compressed air is cooled to a lower temperature. When the temperature decreases, the relative humidity exceeds 100% and water droplets are formed, which can then be collected and removed. Afterwards, the temperature of the compressed air is increased again and, once again, only water vapor remains in the compressed air. At this point, the relative humidity will be lower than 100%.
The last technology is called ‘chemical drying’. With this often used method, moisture is removed by absorption or adsorption by an external substance. In case of absorption, the moisture is captured by a hygroscopic liquid or powder. The moisture is absorbed by this material. Since the chemical composition of the absorbing material changes, it cannot be regenerated anymore. Consequently, the absorbing material must be removed and replaced after being saturated to start the process again.
In the case of adsorption, the moisture is captured by hygroscopic beads. By means of diffusion, moisture molecules are transported into the pores of the beads, where they accumulate. When the beads are saturated, they have to be regenerated in order to begin the process anew. Regenerating the beads can be done in ways: Either a heated or a very dry air stream can be sent over de desiccant beads. In either case, the forces retaining the water are being disrupted, leading to the removal of the water molecules.
Let’s take a brief look at the popularity and benefits of each method, how they can be used and any potential drawbacks.
Over-compression: This is perhaps the simplest method of drying compressed air, but it is only suitable for very small airflows due to its high energy consumption.
Cooling: Cooling is a popular drying method. In some cases, the compressed air is cooled using a heat exchanger with chilled cooling water. At this low temperature, the moisture is condensed into water droplets, which are collected and drained. This system is limited by the temperature of the cooling water and it also requires the use of a watercooler.
Absorption: This is an expensive method as the dryer’s liquid waste must be treated as chemical waste. The absorption material has to be replaced constantly, which makes this system very costly. In addition, the dew point of this type of dryer can only be lowered down to 15°C.
Refrigeration drying: A more commonly used drying method that relies on cooling compressed air can be carried out by refrigerant dryers. In these dryers, a refrigerant circuit cools the compressed air. However, if the temperature in these dryers falls below 0°C, the formed water droplets freeze and block the flow of compressed air in the heat exchanger, causing failure of the refrigerant dryer.
Adsorption drying: Adsorption dryers are mainly used when the required pressure dew point must be below 0°C. Most of these applications require dew point temperatures as low as -40°C or even -70°C.
Process of moisture removal within a compressed air system
Next, let’s look at how dryers are changing over time.
