Up until recently, I had been content using a homemade air dryer to dry my first-stage air before it went into my Shoebox F10 booster pump, and then again between the Shoebox and the SCBA tank. My air dryer is a bucket of ice/water with the air lines under water, and a bleed valve incorporated at the low points to drain the moisture every once in a while. I recently decided to do some studying and learn more about how pressure affects relative humidity and dew points, and it opened my eyes to the fact that my setup was inadequate. I was concerned about the effect colder temps might have on my air rifles' performance because I do a lot more cold weather hunting now. I was casing my guns before bringing them back inside from the cold to prevent external condensation, and wondered if internal condensation was occuring. Turns out it was.
Compressing air not only increases the temperature of the air, but also the dew point and relative humidity. As the pressure increases, the air can no longer hold the same amount of water, and the amount of water over that limit condenses into a liquid. Some people mistakenly believe that compression "creates" moisture. It does not. It simply lowers the capacity of the air to hold water as a vapor, and some of the water must condense into a liquid. When our air comes out of our compressors, headed towards our rifles or tanks, it is ALWAYS going to be at 100 % relative humidity. As it starts to cool down, it immediately begins to condense water. The bigger the temperature drop, the more water will condense, and the biggest part of the temperature drop will happen in your SCBA tank or your rifle's tank.
So, why desiccant and not bleed valves, cotton filters, or refrigerated dryers (like mine)? It's about limitations. My system only lowered the dew point of my compressed air to around 35 degrees because that was the temp of the water I was cooling the air with. As long as I stored my tank, and only used my rifles above 35 degrees, I was good. The moisture left in the air remained a vapor. But if I went hunting at temps below that, liquid moisture was forming inside my rifles' tanks. If you have been relying on that cotton filter on the output of a Yong Heng, then your equipment is only protected against liquid water inside it as long as it remains at or above the temp it was filled at. Desiccant, on the other hand, doesn't rely on condensation to remove moisture from the air. Two primary factors control how much moisture desiccant removes. One is relative humidity. The higher the relative humidity, the more water it can adsorb from the air. Remember, our air is coming out of the compressor saturated, at 100% relative humidity, so this works in our favor. The second controlling factor is temperature. Silica gel desiccant quickly loses its ability to remove moisture from the air at around 90 degrees and higher. This means that heat of compression is working against us, and we may need to cool the air off a little before it gets to the desiccant.
How much of a difference does it make? A lot. If I used the desiccant between my shop compressor and the Shoebox, which is 100 PSI, it will drop the dew point to around -40 degrees. It's even lower when used at the output of the Shoebox, but I couldn't find a calculator that would accept 4500 PSI as an input, so I don't know the exact number. What about molecular sieve, you ask? Well, it removes moisture even better than silica gel desiccant, especially at temps above 100 degrees and at lower relative humidity. The drawbacks are molecular sieve is more expensive and it's takes a lot higher temp to dry it back out for re-use.
So in summary, there is no other method available to us airgunners that will provide condensation-free air at home, other than post-compression desiccant filtration. For those of you that already use it, I hope this helped you understand why it's better. For those of you that don't use desiccant, I hope this helps you understand why you should.
Shoot straight, speak the truth, and be kind to one another!
James
Compressing air not only increases the temperature of the air, but also the dew point and relative humidity. As the pressure increases, the air can no longer hold the same amount of water, and the amount of water over that limit condenses into a liquid. Some people mistakenly believe that compression "creates" moisture. It does not. It simply lowers the capacity of the air to hold water as a vapor, and some of the water must condense into a liquid. When our air comes out of our compressors, headed towards our rifles or tanks, it is ALWAYS going to be at 100 % relative humidity. As it starts to cool down, it immediately begins to condense water. The bigger the temperature drop, the more water will condense, and the biggest part of the temperature drop will happen in your SCBA tank or your rifle's tank.
So, why desiccant and not bleed valves, cotton filters, or refrigerated dryers (like mine)? It's about limitations. My system only lowered the dew point of my compressed air to around 35 degrees because that was the temp of the water I was cooling the air with. As long as I stored my tank, and only used my rifles above 35 degrees, I was good. The moisture left in the air remained a vapor. But if I went hunting at temps below that, liquid moisture was forming inside my rifles' tanks. If you have been relying on that cotton filter on the output of a Yong Heng, then your equipment is only protected against liquid water inside it as long as it remains at or above the temp it was filled at. Desiccant, on the other hand, doesn't rely on condensation to remove moisture from the air. Two primary factors control how much moisture desiccant removes. One is relative humidity. The higher the relative humidity, the more water it can adsorb from the air. Remember, our air is coming out of the compressor saturated, at 100% relative humidity, so this works in our favor. The second controlling factor is temperature. Silica gel desiccant quickly loses its ability to remove moisture from the air at around 90 degrees and higher. This means that heat of compression is working against us, and we may need to cool the air off a little before it gets to the desiccant.
How much of a difference does it make? A lot. If I used the desiccant between my shop compressor and the Shoebox, which is 100 PSI, it will drop the dew point to around -40 degrees. It's even lower when used at the output of the Shoebox, but I couldn't find a calculator that would accept 4500 PSI as an input, so I don't know the exact number. What about molecular sieve, you ask? Well, it removes moisture even better than silica gel desiccant, especially at temps above 100 degrees and at lower relative humidity. The drawbacks are molecular sieve is more expensive and it's takes a lot higher temp to dry it back out for re-use.
So in summary, there is no other method available to us airgunners that will provide condensation-free air at home, other than post-compression desiccant filtration. For those of you that already use it, I hope this helped you understand why it's better. For those of you that don't use desiccant, I hope this helps you understand why you should.
Shoot straight, speak the truth, and be kind to one another!
James
