Dew Point and Water in Compressed Air: Understanding the Effects

In systems, it is important to understand the type of medium that is being used.  For most EXAIR products, this will be compressed air.  As the air compressor draws in ambient air, it also brings in dust, contamination, and moisture into the system.  If untreated, the pneumatic system will have to contend with these foreign “invaders” that will affect the performance of your pneumatic devices.  One of the most common problems is water.

Water enters the compressed air system from the water vapor already present in the ambient air, which is referred to as the dew point or relative humidity.  When you take ambient air and compress it, the amount of “elbow room” for the water vapor decreases.  This causes the water vapor to condense and create liquid water.  It would be similar to a water-soaked sponge.   As you compress it with your hands, the sponge will not be able to hold on to the water.  Similarly, as the air is compressed, water will start to form and fall out into the compressed air system.  Water is a by-product of a compressed air system.

Visual depiction of the impact of water vapor contained amongst air particles and how this reduces available volume during compression.

The definition for determining if liquid water is present in your system is called the pressure dew point.  Dew point is the temperature at which water vapor will condense and form water droplets.  If the dew point temperature and the air temperature are equal, then the air is considered 100% saturated (water vapor will start to condense to form water droplets).  In compressed air systems, air dryers are used to reduce the dew point temperature.  This means that unless the ambient temperature falls below the dew point temperature, water vapor will not condense into a liquid state.

There are two major types of compressed air dryers; refrigerated and desiccant.  The refrigerated air dryers are the most common, and the dew point is measured at about 39oF (4oC).  So, unless the air temperature gets close to freezing, i.e., the piping system that goes outside in cold weather, water should not be present.  Desiccant air dryers can achieve dew points as low as -40oF (-40oC).  This compressed air is very dry and can be used for medical systems, food and beverage processing, and instrument air.  The reason is that bacteria cannot survive in compressed air that is that dry.  The other types are dewpoint reducing systems, which include membrane and deliquescent dryers. 

Good engineering practice calls for point of use filtration and moisture removal, such as that provided by EXAIR Filter Separators.

For most pneumatic devices, a Filter Separator with an auto-drain should be used as a minimum amount of protection.  Even with systems that have compressed air dryers as described above, they are mechanical devices.  So, failures can occur.  You should review your compressed air system to ensure that your pneumatic system, including EXAIR products, is operating at peak efficiency.  This will include your supply system, compressed air leaks, and blow-off devices. 

Moisture-laden compressed air can cause issues such as increased wear on the pneumatic tools, the formation of rust in piping and equipment, quality defects in painting processes, and frozen pipes in colder climates.  Regardless of what products you’re using at the point-of-use, a compressed air dryer is undoubtedly a critical component of the compressed air system.  Providing clean, dry air to EXAIR Products or other pneumatic devices will help to extend the life of your equipment.  If you wish to discuss more about your compressed air system or how EXAIR can provide a more efficient way to use that compressed air, an Application Engineer will be happy to assist you.

John Ball
International Application Engineer

Twitter: @EXAIR_jb

Refrigerated Air Dryers

Whenever air gets compressed, it reduces the space for the water molecules to remain as a vapor; which causes condensation.  For this, compressed air dryers are an important part of a compressed air system.  They are designed to remove moisture to prevent condensation further downstream in the system.  The three main types of dryers are refrigerated, desiccant, and membrane. For this blog, I will cover the refrigerant-type compressed air dryers.

Compressed air dryers are rated with a dew point rating.  A dew point is the temperature at which the air has a relative humidity of 100%.  Since the air cannot become more saturated with water than 100%RH, water will condense and fall out like “rain”.  You can see this effect during the cool mornings when dew forms on the grass.  Compressed air dryers are designed to reduce the dew point temperature of your compressed air.  For a refrigerant type, they are near the dew point temperature of 38oF (3oC).  Like a refrigerator, they use refrigerant to cool the compressed air.  We cannot go below this temperature as it could form ice inside the dryer.  But, as long as the ambient temperature does not go below 38oF (3oC), liquid water will not be present in the pneumatic system. 

There are two main types of refrigerated air dryers; cycling and non-cycling.  Cycling type refrigerant air dryers will cool a liquid mass, generally a glycol-water mixture, to a set-point and turn off.  The liquid will go through an air-to-liquid heat exchanger to remove the heat from the compressed air.  Referring to the cycling action, when the liquid mass goes above the set point, the refrigeration system will restart and cool the liquid mass again.  The cycling refrigerant air dryers are more expensive, but they are more efficient. 

Non-cycling refrigerant air dryers are more common.  The refrigeration system continues to run through an air-to-air heat exchanger to cool the compressed air.  It is similar to your AC system in your car.  With this type of system, they are more susceptible to the environment, i.e., temperature, elevation, and humidity.  So, adjustments are required for proper installation. 

With both types of refrigerant dryers, the internal compressed air section is very similar.  They will have a filter separator to remove the liquid that is created from the condensation from the cold temperatures.  They also have an additional air-to-air heat exchanger.  This will provide two important features for the refrigerated air dryers.  As the cold air leaves the refrigerant section, it helps to cool the incoming compressed air.  This will make the system more efficient.  And as the hot incoming compressed air helps to warm the cold air leaving the dryer, it will stop the condensation of liquid water on the outside of the pipes.  Like the dew forming on the grass during cool mornings, the same will occur with the compressed air piping system. 

Moisture-laden compressed air can cause issues such as increased wear on the pneumatic tools, the formation of rust in piping and equipment, quality defects in painting processes, and frozen pipes in colder climates.  Regardless of what products you’re using at the point-of-use, a compressed air dryer is undoubtedly a critical component of the compressed air system.  Delivering clean, dry air to your EXAIR Products or other pneumatic devices will help to ensure a long life out of your equipment.  If you wish to discuss more about your compressed air system or how EXAIR can provide a more efficient way to use that compressed air, an Application Engineer will be happy to assist you. 

John Ball, CCASS

Application Engineer
Twitter: @EXAIR_jb

Photo: Grass morning dew by RuslanSikunovPixabay License

Air & Water DO Mix – Why That’s A Problem for Compressed Air Systems

Wherever you go, humidity – and its effects – are an inescapable fact of life. Low humidity areas (I’m looking at you, American Southwest) make for a “dry heat” in the summer that many prefer to the wet & muggy conditions that areas with higher humidity (like much of the rest of the United States) encounter during the “dog days” of summer.

Regardless of human comfort level issues, all atmospheric air contains water vapor in some finite proportion…in fact, next to nitrogen and oxygen, it makes up a bigger percentage of our air’s makeup than the next eleven trace gases combined:

Reference: CRC Handbook of Chemistry and Physics, edited by David R. Lide, 1997.

And, because warmer air is capable of holding higher moisture concentrations (a 20°F rise in temperature doubles the potential for holding moisture), chances are good that it’ll become a bigger problem for your compressed air system in the summertime. So…how BAD of a problem is it? Let’s do some math. Consider a nice, typical summer day in the midwest, when it’s 80°F outside, with a relative humidity of 75% and we’ll use the data from the tables below to calculate how much water collects in the compressed air system:

Source: Compressed Air & Gas Institute Handbook, Chapter 3
Source: Compressed Air & Gas Institute Handbook, Chapter 3

Let’s assume:

  • An industrial air compressor is making compressed air at 100psig, and at a discharge temperature of 100°F.
  • The demand on the compressed air system (all the pneumatic loads it services) is 500 SCFM.

Table 3.3 tells us that, at 80°F and 75% RH, the air the compressor is pulling in has 0.1521 gallons per 1,000 cubic feet.

Table 3.4, tells us that, at 100°F and 100psig, the compressor is discharging air with a moisture content of 0.0478 gallons per 1,000 Standard Cubic Feet.

The difference in these two values is the amount of water that will condense in the receiver for every 1,000 SCF that passes through, or 0.1521-0.0478=0.1043 gallons. Since the demand (e.g., the air flow rate out of the receiver) is 500 SCFM, that’s:

500 SCFM X 60 min/hr X 8 hr/shift X 0.1043 gallons/1,000 SCF = 25 gallons of condensate

That’s 25 gallons that has to be drained from the receiver tank over the course of every eight hours, so a properly operating condensate drain is crucial. There are a few types to choose from, and the appropriate one is oftentimes included by the air compressor supplier.

So, you’ve got a condensate drain on your compressor’s receiver, and it’s working properly. Crisis averted, right? Well, not so fast…that 100°F compressed air is very likely going to cool down as it flows through the distribution header. Remember all that moisture that the hot air holds? Assuming the compressed air cools to 70°F in the header (a reasonable assumption in most industrial settings), a bunch of it is going to condense, and make its way to your air tools, cylinders, blow off devices, etc., which can cause a host of problems.

Reversible Drum Vacs have tight passages where contaminants (like pipe rust) can accumulate and hamper performance. Fortunately, they are designed to be easy to clean and returned to peak performance.

And…I trust you saw this coming…we’re going to calculate just how much condensation we have to worry about. Using table 3.4 again, we see that the header’s air (at 100psig & 70°F) can only hold 0.0182 gallons per 1,000 SCF. So, after cooling down from 100°F (where the air holds 0.0478 gallons per 1,000 SCF) to 70°F, that means 0.0296 gallons per 1,000 SCF will condense. So:

500 SCFM X 60 min/hr X 8 hr/shift X 0.0296 gal/1,000 SCF = 7.1 gallons of condensate

Qualified installers will have sloped the piping away from the compressor, with drip legs strategically placed at low points, so that condensate can drain, collect, and be disposed of…oftentimes via similar devices to the condensate drains you’ll find on the compressor’s main receiver. Good engineering practice, of course, dictates point-of-use filtration – EXAIR Automatic Drain Filter Separators, with 5-micron particulate elements, and centrifugal elements for moisture removal, are also essential to prevent water problems for your compressed air operated products.

Good engineering practice calls for point of use filtration and moisture removal, such as that provided by EXAIR Filter Separators.

EXAIR Corporation remains dedicated to helping you get the most out of your compressed air system. If you have questions, give me a call.

Russ Bowman, CCASS

Application Engineer
EXAIR Corporation
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Humidification Calculations Related To Atomizing Spray Nozzles

I had an application where a customer needed to have a room at 75% relative humidity (RH).  They produced a nylon backing for carpet, and they needed the high RH to reduce the “stickiness” in the gluing process.  Currently they were at 40% RH in a room that was measured at 40ft long by 20ft wide by 20ft high (12.2m long X 6.1m wide X 6.1m high).  They wondered if our Atomizing Nozzles could help him to increase the relative humidity in the room.  I decided to put on my engineering hat to calculate the amount of water that he would need to increase the humidity.

Relative humidity (RH) is the percentage of water vapor as compared to the saturation level at the same temperature.  So, at 100% RH, the ambient air is saturated and cannot hold any more water vapor. You can feel the difference in the Amazon versus Arizona at the same temperature.  With dryer conditions, water can be added to increase the relative humidity; like a humidifier.  With the EXAIR Atomizing Nozzles, we can break liquid water into very small droplets to help increase the humidification rate.  For the customer above, I will have to determine what size and how many Atomizing Nozzles are required. 

Equation 1

H = V * ACH * (Wf – Wi) / (v * 7000)                                        


H – mass flow rate of water, Lbs/hr                                         

V – Volume of room, ft3                                                                

ACH – Air changes per hour                                                        

Wf – Final Water Content, Grains/lb of dry air                     

Wi – Initial Water Content, Grains/lb of dry air                   

v – Specific Volume of Air, ft3/lb                                               

Conversion Constant – 7000 Grains/lb                   

The customer stated that the room was set to 68oF (20oC), and they used an air handling unit (AHU) that produced 1,600 cfm (44.5 M3/min) of air into the room.  From these factors, we can determine some of the variables above.  For the Air Changes per Hour (ACH), we can use Equation 2. 

Equation 2

ACH = 60 * Q / V


ACH – Air changes per hour

Q – Volumetric flow rate, CFM

V – Volume of room, ft3                

The volume of the room is V = 40ft X 20ft X 20ft = 16,000 ft3.  The volumetric flow rate by the AHU is 1,600 ft3/min.  From Equation 2,

ACH = 60 * (1600 ft3/min) / 16,000 ft3

ACH = 6/hr. 

In determining the water content values, you can find a chart online to determine the amount of water vapor that is contained in the air at a specific temperature and RH.  At 68oF (20oC), I was able to find the following information:

Wi = 40.58 Grains/lb of dry air at 40% RH                              

Wf = 76.71 Grains/lb of dry air at 75% RH                             

v = 14.286 ft3/lb @ 68 deg. F, 1 atm                        

V = 16,000 ft3                    

If we plug in the numbers that we have into Equation 1, we can determine how much water that we will need to spray into the air to increase the RH from 40% to 75%.

H = V * ACH * (Wf – Wi) / (v * 7000)                                        

H = 16,000 ft^3 * 6/hr * (76.71 – 40.58 Grains/lb) / (14.286 ft^3/lb * 7000 Grains/lb)

H = 34.68 lb/hr                 

With my prior line of work in room humidification, we know that there is a lead/lag time between measuring and humidifying.  This may seem complicated, but it is important to get a steady state condition for the Relative Humidity.  To help this customer, I recommend a cycle time of 15 second to turn on and wait 105 seconds to re-measure the RH.  This will help to not over-saturate the room.  As for the location of the Atomizing Nozzles, we want to be near the ceiling to get the most “air” time to vaporize.  We also have to be careful to not allow the water spray to hit any objects or each other as this will cause the water to condense. 

To start, I suggested our model AT2010SS No Drip Internal Mix 360o Hollow Circular Pattern.  This type of nozzle helps to extend the settling time of the water droplets; the amount of time that the droplets are suspended in the air.  The orientation of the spray is outward in all direction to increase coverage.  With the No Drip option, it is controlled by the air pressure to open and close the liquid side for spraying.  When the compressed air is turned off, a valve will seal the liquid side to not allow any drips.  It also helps to eliminate the need for any liquid valves next to the Atomizing Nozzles.  When it comes to cycle spraying, the No Drip option works wonderful. 

In taking into consideration the flow rate required during operation time, we can calculate the amount of liquid flow required for the Atomizing Nozzle in Equation 3.

Equation 3:                                                                                        

Flow rate: Q = H / (D * T * f)                                                       


Q – Liquid flow rate (gal/hr or GPH)

H – Mass Flow Rate (lbs/hr)                                         

D – Density of Water (8.34 lbs/gal)                                           

T – Span division (no scale)                                                          

f – Intermittent Factor (no scale)

To determine the number of Atomizing Nozzles, we want to look at the time determination with the controller and the intermittence of operation.  With the ACH = 6/hour, the air in the room will change over every 10 minutes.  We want to have a balance between the new air and the existing air.  So, with the time measurement of 15 seconds on and 105 seconds off (2 minutes), we will have 5 humidity checks over the 10 minutes.  We can divide the amount of water to be injected into the room by the span division, T, to cover the time span for check and atomization.  Thus, T = 5.  We will also have to adjust the amount for only running 15 second intervals.  So, the intermittent factor, f, will be 0.0042 (the 15 seconds portion of the hour).

With these values, we get:

Q = (34.68 lbs/hr) / (8.34 lbs/gal * 5 * 0.0042)                     

Q = 198 gal/hr (GPH)                                                                     

In the catalog, the model AT2010SS will flow 14.7 GPH (55.7 LPH) of water at 60 PSIG (4.1 Bar) liquid pressure.  If we divide these out, it will tell us how many atomizing nozzles that is needed to humidify the room.  

Number of Nozzles: 198 GPH/14.7 GPH = 13.5 or 14 Atomizing Nozzles.

With the above Atomizing Nozzles, the company was able to control the RH at a high level for his manufacturing process.  In turn, he was able to increase productivity and reduce downtime.  If you need to increase the level of humidity in your area, you can contact an Application Engineer at EXAIR for help.  We can make it feel like the Amazon.

John Ball
Application Engineer
Twitter: @EXAIR_jb

Photo: Forest by janeb13Pixabay License