Air Dryers and Dew Points

A Google search for "compressed air+dew point+dryer" brought up many hits that looked like they may have complete answer to your question.

What's the weather like? How cold does it get? Based on how high these dew points are, I would say the dew points are based at the discharge pressure of the compressor. Not atmospheric. Usually you dry air down below -20 deg C for tools. Instrument air is even drier.

I suspect your (refrigerant) dryer is set up so that two compressors work together when the dew point is above 6 degrees C. Refrigerant (commonly freon) that has not returned to a gaseous state before the compressor intake may "slug" the compressor as a liquid. It appears that your unit is set up to reduce effort as it approaches the lowest dew point and then cuts out to prevent slugging. This also protects the compressor from very low ambient temperatures or unusually cold inlet air.

Someone said "If it ain't broke don't fix it." It ain't broke.

Tom Kreher

I depends on how the dewpoint is sensed.

If it is a simple fridge dryer then the dewpoint could be a thermostatic sensor inside the chiller to measure the actual temperature of the air leaving the chiller.

Or it could be mounted on the outside of the chiller outlet.

Or it could be a sensor to measure the refrigerant gas pressure (that equates to the temperature) in he chiller.

Each of these relate to the air temperature at working pressure that equates to a pressure-dewpoint. Each method has a characteristic benefit or disadvantage - but generally gives a reasonably acceptable pressure-dewpoint - as far as tools are concerned - ie, no liquid water in the airlines and tools.

The ambient temperature of the weather influences the load on the air compressor and hence the temperature of the air then entering the dryer (make sure the after-coolers are working properly).

The dryer tries to pull the air temperature down to 6C and/or 3C and obviously has to work harder in hot weather (especially in high humidities). At the same time the hot weather reduces the need for a low pressure-dewpoint and thus the dryer settles down (when not overloaded) to provide air dry enough for tools.

That is, the pressure-dewpoint needs to be below the ambient temperature (where the tools are used) by at least 5C for protection.

This might not be possible in the cold weather because the dryer must cut-out at 3C to prevent internal freezing - otherwise the ice will block the chiller, separator and auto-drain.

For more critical applications check the dewpoint (or relative humidity) of the air itself when expanded to atmospheric pressure with a hygrometer.

The dewpoint at atmospheric pressure will be much lower than the dewpoint at pressure - thus convert the readings to take account of higher working pressure.

I am confuse some one talking about dew point of compressed air going to pneu. tools and someone is saying about chiller air. Please explain start from what is dew point and how its affect.

Reply to sandeep lokhande

It is too complicated to explain the science fully. Please look at 'Water Molecules' in Wikipedia.

But to give some idea of how to apply the science to air compressors in practice just consider everyday experience. The atmosphere cools as it rises and water vapour condenses to form clouds. On dry days with low Relative Humidity (RH) the clouds are very high and any droplets falling as mist/rain re-evaporate before reaching the ground. On wet days with high RH, the clouds are much lower - and the heavier droplets reach the ground before they can evaporate.

The temperature at which clouds start forming is the dewpoint.

At any given pressure and temperature a fixed volume of air can only hold a maximum amount of water vapour. There are numerous ways of expressing this, but the easiest one to follow when dealing with air compressors is to consider the weight of water per unit volume of air (density if you like).

In round figures, at atmospheric pressure and 20C the weight of water (as vapour) in 1 cubic metre of saturated air (RH=100%) is 17 grammes - or 17g/m3. If not saturated, and say the air only held 8.5g/m3 (half) - the RH would be 50%, or say 1.7g/m3 (a tenth) the RH would be 10%, and so on.

Now increase the pressure (double the absolute pressure) so that the air is half the volume. The weight of water stays the same (ie a gramme of water weighs a gramme regardless of pressure and temperature). Thus the original 8.5g/m3 (at 50%RH) now becomes 8.5g/0.5m3 or 17g/m3, at which the RH becomes 100%. Or 1.7g/0.5m3 (10%RH) becomes 3.4g/m3 and 20%RH. An increase of absolute pressure by a factor of ten, such that the volume is 1 tenth of the original, brings the RH back to 100%.

In other words, the act of compression raises the RH (until 100% is reached).

An RH above 100% is not possible, so vapour condenses into liquid - and being a liquid occupies a volume of negligible amount - enough to restore the equilibrium.

The original 17g/m3 at 10 times the absolute pressure gives rise to a density of 170g/m3. This is not possible, so of the original 1m3 of air with 17g of vapour, only 1.7g remains as vapour, and the rest condenses to 15.3g of liquid water to restore equilibrium. The air is 100%RH.

In order to lower the dewpoint, we need to reduce the temperature of the compressed air (in the chiller). Water vapour condenses to liquid. The water must be removed - usually by a separator - and dumped. The air needs to be reheated to lower the RH but the dewpoint is equal to the chiller temperature. The air must then be cooled back to the chiller temp for water to condense. Thus if the chiller runs at a temp below ambient it is not likely the air will cool in the system.

I hope this helps.

why sometimes chiller parts started sweating?

Sandeep,

Chiller parts are colder than atmospheric temperatures (hence their chilling capabilities). Lower temperatures bring lower satuation limits (amount of water vapour that can stay dissolved in air at a given temperature). As the temperature decreases, water comes out of the 'solution' (solution = air + water) from the atmosphere, as seen on a foggy day. Thus, sweating chiller parts are actually pulling moisture out of the air, not really 'diffusing' moisture through the chiller part itself...

Hope that makes sense. Happy holidays.

-MechMatt

Umeshgupta,

Don't forget that the since dew point is determined by the saturation point of water in air, it also depends on air pressure. If you're above/below atmospheric pressure (such as after the first compressor, if pulling air from atmosphere initially, or perhaps even further upstream of first compressor), this will most definitely affect your saturation temperature. Check out pressure-specific volume tables/charts or psychrometric charts for these humidity calculations.

Best of luck, happy holidays.

-MechMatt