Dew point theory for compressed air producers

Dew points of compressed air in a cloud image.

Dew point is the temperature at which water vapor in the air condenses into water. 

What is dew point? Dew point is the temperature at which water vapor in the air condenses into water. The warmer the air, the more water can bind to it as water vapor. Dew point of 100% saturated air is the same as its temperature.
The simplest example of dew point is probably that in the warm air of the sauna washroom, water droplets begin to condense on the surface of a cold beer can. When the moisture content of the air is high and the surface of the cold can cools it, moisture begins to condense from it.
Dew point of pressurized and free air
While talking about compressed air, two concepts are easily confused – dew point under pressure and dew point of free air. In principle, the definition and idea are exactly the same: at some temperature, even compressed air begins to condense moisture. Here we sometimes go to the gray area when we talk about the dew point of free air and forget that as the compressed air expands, its relative humidity changes.
Let’s take one example, that a compressor takes in one cubic meter of air and compresses it. If the final pressure is 7 bar, we have 8 times the original air volume in the same space. In other words: if one cubic meter of free air is compressed, 1/8 cubic meter of compressed air comes out. If the humidity of the intake air is 50% and the temperature remains the same, the humidity of the compressed air is 100% and some of the water vapor it contains condenses into water. The compressed air from the compressor aftercooler is always close to saturated.
Dew point under pressure means the dew point measured from the compressed air – not at the stage when the compressed air is allowed to expand into the environment, in which case the same absolute amount of moisture is in eight times bigger volume.

Dew point table shows how much water one cubic meter (m³) of air contains when the relative humidity is 100%.
Why determining the dew point of compressed air is important?
Defining the dew point of compressed air is important because dew point is one of the factors in compressed air quality. If the compressed air is not dried any way, it cools in the piping, where water begins to form. In almost all processes, even a small amount of moisture causes problems: the pipes rust, pneumatic actuators do not function, bacteria live and do well in the piping.
Quality standards define the purity of compressed air required for different applications. One factor in the quality classification is dew point: +10 ° C, +7 ° C, +3 ° C, -20 ° C, -40 ° C, -70 ° C.
For example, Class 3 compressed air is not yet dry enough to inhibit bacterial growth (requires -26 ° C), or rusting (requires -27 ° C). With adsorption drying even the strictest first-class dew point of -70 ° C can be achieved.
Saturated tower of the adsorption dryer requires regeneration
Adsorption dryers usually have two towers, one of which alternately dries compressed air and the other regenerates the desiccant. At its simplest, air-regenerated adsorption dryers are regenerated by a pressure blow, in which case the water molecules are released from the desiccant and rinsed off, most often with compressed air.
Air-regenerated adsorption dryers operate with clock control, for example, so that 3 minutes are always used for drying and 2 ½ minutes for regeneration, and half a minute is required for pressure equalization. During use, the cycle repeats exactly the same, regardless of any changes in conditions.
Dew point control brings savings
The idea of dew point control is not to regenerate the desiccant unnecessarily according to the clock when there is still drying capacity left. If, for some reason, the drying tower has not accumulated enough moisture to saturate it, it is useless to go to regenerate it. Instead, the dew point is measured, and regeneration is started at a set dew point limit value. As the water load in the desiccant accumulates, the control logic decides to change the tower.
Here, too, there is a limit to how long it is desired to postpone the regeneration phase. In many locations, the changeover interval can be technically increased to approx. 10-fold, but for safety reasons, the changeover is started before the -40 degree dew point limit value. Under no circumstances should the desiccant collect too much moisture, as this will destroy it.
Dew point measurement has developed considerably in recent years. The use of ceramic sensors is declining because if they get wet or oiled, the calibration is immediately badly wrong. At its simplest, some dryers measure the temperature of the coldest surface and assume that it is the same as the dew point of the compressed air. The right solution can now be found for every application. A good dew point meter works well even in bad conditions.
Dew point control can generate very big energy savings. It is often said that air regenerated drying consumes a lot of compressed air.
Given information may be indefinite, without considering the whole:
  • During the regeneration period, 20% of the dryer’s capacity is momentarily required for regeneration
  • With continuous dew point control, only 3% of the dryer’s capacity is used for regeneration at its best.
Thus, if the dryer’s load is small, dew point control provides significant energy savings, especially in big dryers (up to 34 m3/min).
If electricity price is low, it favors the use of air regeneration, because the equipment is simple and therefore reliable and easy to maintain. Simple is sometimes beautiful – complex devices have more parts, functions and the risk of failure.
On the other hand, drying the compressed air to -40 degrees takes up 4-5% of the compressor station’s total energy consumption. How much should you sacrifice time and energy to optimize it, when the compressor takes the rest > 95% in any case? Reliability should not be compromised.
Know the principles and keep your head cool
Many things revolve in the mind of the person who decides about the dryer purchase; from the need for space to the choice of regeneration method, and from the investment price to operation costs.
When purchasing a drying system, it is good to know what principles are involved in the whole picture. The energy needed in drying is always taken from somewhere. Regeneration of the desiccant always needs energy. Find out which choice is reasonable in your own facility. Remember that the devices will be used for several decades.
Heat recovery is another similar trend. Of course, it is a good idea to use the warm air produced by the compressor to heat the hall if the circumstances are suitable for this. However, the argumentation can be presented in so many ways that the achieved benefit depends, after all, on the method of calculation.
Do you need extremely dry air?
The equipment should always be kept in good condition. If water slips into the compressed air network, it will take a long time to get out of there. If there is no drying in the system at all, and air flow is 1 cubic meter per minute, 1.8 kg of water per hour, i.e. 43 kg per day, enters the piping. No one should take this risk, instead, certain criteria must be set for operational reliability.
If a low dew point is not required at all end-user points of the compressed air system, there are alternative models: end-user-specific drying can provide significant savings. There are really many kinds of facilities and solutions. Finding the best solution is like acrobatics, and it is not worth assuming necessarily that you know the right one. Here are a few advise to keep in mind:
  • Energy savings are easy to calculate, but better keep a certain pragmatism in calculations.
  • Winter conditions. If we have the pipes going outside, it is obvious that drying is needed.
  • Drying compressed air always costs money.
  • Dew point is one part of compressed air quality.
  • Strive for a sufficient dew point, doesn’t need to be too good.