The Best Ways to Dry Compressed Air
There’s water in the air all around us. The moisture level in air changes as the temperature changes. For example, cold winter air can feel drying to our skin, but a hot humid day can create moisture in all kinds of unassuming places. Your hair may show signs of unruliness on a hot day due to the increased moisture in the air. Many common practices to remove moisture from compressed air involve cooling the air.
Pressurized air needs some type of system or device to remove its naturally occurring moisture. Several options exist to remove water from your compressed air, which can be as simple as a drain valve on the tank or as complex as a four-stage air drying system. You may be using compressed air for purposes that require moisture-free output air. In this case, any existence of moisture will negatively impact your operations.
There are applications that can tolerate a low moisture content and won’t need an elaborate air-drying setup. Either way, moisture present in the compressor tank and lines is not ideal, so drying the air and releasing the water is absolutely necessary in some form.
For a compressor that delivers 20 liters of air per second, it also supplies 24 liters of water a day. Most of that water needs to be removed, and it’s usually done in several stages. For example, an industrial air compressor that produces about 24 liters of water a day will move the wet and hot compressed air to an aftercooler, which removes 15 liters, then to a refrigerated air dryer, which removes another 7 liters. There may be another stage with a desiccant air dryer for the driest air possible.
Air compressors are used for a wide range of applications, so there isn’t one perfect solution to every dry compressed air application. Cooler air holds less water, so it uses many systems to cool the compressed air. This allows water to drop out of the air, so it can be collected and drained.
Draining the Tank
The first stage of keeping moisture out of your air lines is to drain your air compressor tank frequently. There’s a drain at bottom of your air compressor pressure tank, which releases the water and oil mixture that collects at the bottom. This should be done every time you use your air compressor to prevent rust building up on the inside of your tank and to keep moisture out of your lines and tools.
If you avoid draining your tank because it’s difficult to reach the drain valve, you should install a drain extension kit so it’s easily accessible. You could also install an automatic tank drain with a timer that you can set to periodically drain the tank for you. Either way, it’s a matter of safety and essential.
Water Trap and Filter Regulator
Designed to work with incoming cool air, a water trap is a simple method of removing water. The air enters one side, and then it circulates around the bowl where the water collects at the bottom of the trap and drains out. The air will exit through a filter, which traps other impurities in the air.
A water trap is not generally used on its own, but it’s part of a multi-step system to remove as much moisture from the air as possible, especially for applications that require extremely dry air, such as painting, sandblasting or powder coating.
Refrigerated Air Dryer
Working in a similar manner as an air conditioning unit, a refrigerated air dryer is connected to the air compressor and cools the air to a specified temperature, usually between 35˚F to 50˚F (1.5˚C to 10˚C). This results in a pressure dew point (PDP) of 33˚F to 39˚F (0.5˚C to 3.8˚C).
The water drops out of the air and is separated, then the air is heated up and sent through the lines for its end use. Some applications require a lower PDP, which would require a desiccant air dryer because the condensate will freeze at 32˚F (0˚C) and not be able to be removed.
This is not one of the cheapest ways to dry compressed air, but it is one of the most efficient and has a complete drying system in one unit.
Understanding Pressure Dew Point
We measure moisture in pressure dew point (PDP), which refers to the temperature that the air would have to be at to achieve the same amount of dryness. In many industrial applications, a PDP of at least -40˚F (-40˚C) is desired, so the air is required to have the same moisture level that it would have at -40˚F.
Two Configurations of Refrigerated Air Dryers
There are two configurations of refrigerated air dyers: non-cycling and cycling.
Non-cycling refrigerated air dryers:
- Cool the compressed air in a heat exchanger
- Warm compressed air as it travels in one side, while low pressure liquid refrigerant is metered on the other side of the heat exchanger
- Reduce the temperature of the compressed air as refrigerant is heated
- Regulate the flow of refrigerant as the heat load from the compressed air changes
Cycling refrigerated air dryers:
- Cool the compressed air through a heat exchanger medium like sand, metal or a fluid
- Contain two heat exchangers fitted inside a tank that is filled with a thermal conducting fluid, such as water with propylene glycol added to prevent freezing and corrosion
- Remove heat from the thermal conducting medium using the refrigeration system, then the compressed air is cooled by the thermal conducting medium
- Cycle on when the fluid rises to its upper limit
- Are designed to be more energy efficient than non-cycling designs because refrigeration is only used to cool the heat exchanger medium and not the constant flow of hot air
- Use a simpler refrigeration circuitry than non-cycling because hot gas bypass valves are not required
Desiccant Air Dryer
A multitude of absorbent tiny beads — called desiccant beads — remove water from the air in a desiccant air dryer. These units are similar in design than the water trap filter. The air goes through the center of the pressure vessel, removing water as it circulates. Once the specified moisture content is reached, the air is moved out of the dryer to the lines for its end use. A high-efficiency coalescing prefilter is needed to prevent damage from water and oil sludge.
Most desiccant dryers have two vertical pressure tanks that continuously operate through a drying-regeneration cycle, referred to as a NEMA cycle. An example setting may be a 10-minute NEMA cycle, where the unit is drying for 5 minutes, then regenerating for 5 minutes.
While the drying time cycles, the fully pressurized compressed air flows through the online desiccant vessel through a bed of desiccant beads that strip the water vapor and hydrocarbon molecules from the air. The compressed air is released from the vessel when the specified PDP is reached.
During the regeneration cycle, the vessel depressurizes and the regeneration process begins to heat the moisture out of the desiccant. After regeneration, the tank is pressurized again and ready for its next drying cycle.
There are three common desiccants used:
- Silica gel – an amorphous form of silica with superior water-vapor-absorbing qualities that provide a PDP of -40˚F to -85˚F
- Activated Alumina – a porous form of aluminum oxide with silicon dioxide that provides a PDP of -40˚F to -100˚F; the best option for heatless air dryers
- Molecular sieve – a porous form of zeolites formulated to absorb specific vapor and gas molecules that provide a PDP of -100˚F and lower
Desiccant dryers are commonly found in industrial environments, where the highest amount of moisture needs to be removed from the compressed air.
Quincy Desiccant Dryers
We manufacture a full line of desiccant air dryer systems with dual towers that allow for purging — regeneration of the desiccant — in one tower while the other tower dries compressed air. This feature provides customers with longer use of the desiccant and continuous operation for extended periods, as in common in industrial settings.
Quincy offers four models of desiccant dryers to accommodate the various applications of air compressor systems.
QHD Heatless Desiccant Dryer
- Regenerates the off-line tower by using a small fraction of the dried compressed air
- Offers low maintenance costs and low initial cost
- Has an average-to-high operating cost
QDHP Heated Purge Desiccant Dryer
- Regenerates the off-line tower with half the amount of the compressed air as compared to the heatless series
- Offers low maintenance costs and fairly low operating costs
- Has an average initial cost
- Cuts energy use by 50% with the optional Quincy MicroBurst Regeneration system
QDBP Blower Purge Desiccant Dryer
- Regenerates the off-line tower by combining heat with ambient air
- Offers fairly low maintenance costs and operating costs
- Has a high initial cost
QMOD Heatless Modular Desiccant Dryer
- Regenerates the off-line tower with a small fraction of the dried compressed air
- Offers low maintenance costs and initial cost
- Runs with moderate-to-high operating costs
Quincy air dryers purify your compressed air with our proprietary Q-Sorb desiccant product. This formula is an enhanced activated alumina compound, which provides improved absorption and a lower pressure drop that will improve the energy efficiency of your air compressor system.
If you’re deciding between a heated or heatless desiccant dryer, remember this general rule: heatless dryers are best for applications below 2000 scfm and heated dryers are best for applications above 2000 scfm.
Deliquescent Air Dryers
Not the most popular option, deliquescent dryers have a single tank and also use a desiccant, but they can only provide a PDP of 20˚F to 25˚F lower than the air temperature entering the dryer. When the compressed air enters the tank, it reacts with the desiccant material to produce a liquid effluent that needs to be drained out of the dryer and disposed of according to hazardous waste regulations.
These types of dryers are not used in industrial applications because the dried compressed air can contain small particles of the affluent, which would be corrosive to equipment down the line.
Piping System Air Drying
You can plumb your air lines to remove water from your compressed air. By using long lengths of metal piping built into two or more up-and-down patterns, the air is cooled as the piping absorbs the heat. Reacting to gravity, the water drops down to the bottom of the piping system into a ‘drip leg’ or ‘drop leg’ and is trapped, either by a water trap or a T-fitting and a ball valve.
The air continues on its path in the up-and-down piping, releasing more moisture through each segment. Each drip leg will have less water. By the time the air moves through the last segment, there should be no water in the drip leg. This can be used as a stand-alone method of drying compressed air and is the most common and cheapest way to dry compressed air in a shop for business or personal use.
The most important elements of compressed air drying systems are as follows:
- Have all the horizontal piping angling downwards, so the water moves along with the air as opposed to settling in the pipe; a minimum slope of 1 in 100 is recommended
- Add a drop leg whenever the piping needs to be elevated or go vertical, so gravity will move the water down to the dip leg to be removed
- Add a water trap/filter regulator at the end of the piping to remove any remaining water
- Keep the drop legs away from electrical sockets
- Be aware of how much piping you use and how much it will affect the pressure drop (use the Compressed Air & Gas Institute Installation Pipework calculator to determine pressure drop and find other useful calculations for your compressed air system)
What to Consider When Buying a Compressed Air Dryer
The following factors will affect which type of air dryer is best for you:
- Available utilities
- Dew point requirement
- Operating pressure
- Inlet air temperature
- Ambient air temperature
The dew point requirement is the first thing to consider. You may be able to eliminate some air dryers simply by examining how much or little water needs to be removed for your application. You may need to consult a professional to determine the PDP for your applications. The ranges below indicate what type of dryer is best to achieve specific dew points:
- 0 F to 80 F – Deliquescent air dryer
- 0 F to 32 F – Refrigerated air dryer
- -40 F to 0 F – Desiccant air dryer with a silica desiccant
- -100 F to 0 F – Desiccant air dryer with activated alumina desiccant
Remember to consider the capacity (CFM rating) and the pressure (PSIG) of your air compressor. You must choose the right size for your compressed air drying system. It should be the correct size for your air compressor and application.
The purchase price is certainly one consideration, but the energy use and maintenance costs are also major deciding factors, as well as the additional cost of the pressure drop that will occur as your compressed air travels through another system. You will need to run your compressor at a slightly higher pressure with an air dryer to compensate for the loss of pressure through piping, or run it with a dryer unit to achieve the same pressure for end use.
Desiccant air dryers with two towers will regenerate the desiccant in various ways, so this is also something to analyze as your operating costs will be greatly affected depending on from where the energy is coming.