Hey, thought I would share a few tips on compressed air and humidity after reading some users comments on humidity in their systems and controlling it for dry processes like sand blasting.
Water traps are great, I use a home built water trap that pulls air from the compressor via the drain outlet and uses a mobile piston to force incoming air into the already trapped water and blows this water up along the inside of the collection tank, which rapidly cools it to ambient temperature. This has the funny effect of decreasing the relative and total humidity by decreasing temperature from the compressor.
When it comes to compressed air, the humidity in the tank is always going to be at 100% relative humidity because air's capacity to hold water is *pretty much* flat across the pressures used in home systems. This means that if I use a compressor to take air that is at only 25% relative humidity and compresses it to half it's volume, the humidity is now 50%.
In an air compressor the pressure increase is more likely in the range of14PSIa to over 120PSIg. The head temperature is high enough, due to adiabatic compression, that water does not condense in the cylinders, but excess water does come out in the tank and where the air is cooled.
The point of all this is that there are only four readily available ways for us to decrease the humidity of compressed air; isothermal expansion, decrease in temperature, deliquescents and desiccants.
When air is expanded isothermally, say a slow flow through a regulator where the temperature does not change, the relative humidity is reduced even though the total water volume remains the same.
When air is cooled to below the dew point, the relative humidity and total humidity is reduced to the dew point, this is how heat pump air driers work.
Deliquescents like magnesium or calcium chloride both chemically absorb the water to produce hydrated forms of their respective salts.
Desiccants like silica gel or Alumina rely on the generally reversible process of adsorption where the water is captured and held in pores.
Anytime air is rapidly expanded there will be a decrease in air temperature and a rapid spike in relative humidity, if the temperature hits the dew point of held humidity, you will get liquid water. This clogs air blasting nozzles, shorts plasma cutters etc.
So long story short, water traps are great but are only for liquid water. To actually get dry air you need to physically remove it from the systems air with a dryer of sorts.
Water traps are great, I use a home built water trap that pulls air from the compressor via the drain outlet and uses a mobile piston to force incoming air into the already trapped water and blows this water up along the inside of the collection tank, which rapidly cools it to ambient temperature. This has the funny effect of decreasing the relative and total humidity by decreasing temperature from the compressor.
When it comes to compressed air, the humidity in the tank is always going to be at 100% relative humidity because air's capacity to hold water is *pretty much* flat across the pressures used in home systems. This means that if I use a compressor to take air that is at only 25% relative humidity and compresses it to half it's volume, the humidity is now 50%.
In an air compressor the pressure increase is more likely in the range of14PSIa to over 120PSIg. The head temperature is high enough, due to adiabatic compression, that water does not condense in the cylinders, but excess water does come out in the tank and where the air is cooled.
The point of all this is that there are only four readily available ways for us to decrease the humidity of compressed air; isothermal expansion, decrease in temperature, deliquescents and desiccants.
When air is expanded isothermally, say a slow flow through a regulator where the temperature does not change, the relative humidity is reduced even though the total water volume remains the same.
When air is cooled to below the dew point, the relative humidity and total humidity is reduced to the dew point, this is how heat pump air driers work.
Deliquescents like magnesium or calcium chloride both chemically absorb the water to produce hydrated forms of their respective salts.
Desiccants like silica gel or Alumina rely on the generally reversible process of adsorption where the water is captured and held in pores.
Anytime air is rapidly expanded there will be a decrease in air temperature and a rapid spike in relative humidity, if the temperature hits the dew point of held humidity, you will get liquid water. This clogs air blasting nozzles, shorts plasma cutters etc.
So long story short, water traps are great but are only for liquid water. To actually get dry air you need to physically remove it from the systems air with a dryer of sorts.
