The Best Ways to Dry Compressed AirLast updated on: June 10th, 2020 at: 10:24 am
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. But what happens when this moisture builds up and your air compressor is full of water? Common practices to remove moisture from compressed air include:
- Draining the tank
- Using a water trap and filter regulator
- Using a refrigerated air dryer
- Using a desiccant air dryer
- Using a deliquescent air dryer
- Through piping system air drying
- With the storage tank cooling method
- Through absorption drying
Why Is There Water in My Compressor?
The two leading causes for water collecting in your air compressor are natural moisture that occurs during the compression process and the quality of the air being used. Your compressor’s filtration system works to separate pure air from other pollutants, including excess moisture. The compressor filter traps that moisture and stores it away until it is dried or drained, leading to a natural build-up of water.
Both outdoor and indoor air contains a certain level of moisture or humidity. This percentage is influenced by factors such as:
- Air conditioning
- Running water
- Leaking roofs or windows
- Poor ventilation
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.
The Danger of Having Too Much Moisture in Your Compressed Air
Although some level of moisture is normal, too much water can:
- Damage your air compressor: When water builds up and remains stagnant in the same location, it can cause rust to form, which will wear down your air compressor and contaminate your final product. The pressure from excessive moisture in your tank will also put unnecessary strain on the compressor’s parts, including the bearings, and prevent your tank filters from working correctly. When these parts become damaged, you risk costly repairs or an entire machine replacement.
- Lower air quality: If you are working in a heavily regulated industry, then you may no longer meet set standards or quality codes if your compressed air is tainted with excess moisture.
- Result in a poor final product: Overly moist compressed air can create an inferior final product. For example, wet compressed air can warp woodworking projects, water down powder coating and could lead to rust formation on metal surfaces.
Removing excess water and moisture from your air compressor is crucial for the longevity of your machine, as well as the quality of your compressed air.
How to Remove Water From Compressed Air
Pressurized air needs some type of system or device to remove its naturally occurring moisture. Several options exist to remove water from your air compressor, 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, air compressor moisture in your 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 seven 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 the 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 moisture from compressed air. 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 as part of a multi-step system to remove as much moisture from the air as possible. It is especially used 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 most economical ways to dry compressed air, but it is one of the most efficient and has a complete drying system in one unit.
There are two configurations of refrigerated air dryers: 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
Alternatively, 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 tiny absorbent 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 five minutes, then regenerating for five 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
Quincy Compressor manufactures 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 environments.
Quincy offers four models of desiccant dryers to accommodate the various applications of air compressor systems.
The 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
The 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 percent with the optional Quincy MicroBurst Regeneration system
The QDBP Blower Purge Desiccant Dryer:
- Regenerates the off-line tower by combining heat with ambient air
- Offers relatively low maintenance costs and operating costs
- Has a high initial cost
The 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, which consists of an amplified compound of activated alumina. This formula facilitates absorption and a lower pressure drop, which helps to improve the energy efficiency of your air compressor over time.
If you’re deciding between a heated or heatless desiccant dryer, remember this general rule: heatless dryers are best for applications below two thousand scfm, and heated dryers are best for applications above two thousand scfm.
Deliquescent Air Dryers
Deliquescent air compressor 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 most cost-efficient 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 or 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.
Storage Tank Cooling Method
Storage tanks, also known as air receiver tanks, act as a temporary storage reservoir to store compressed air as it exits the air compressor. They are used in high-demand projects that require a lot of compressed air with little time to spare, including many industrial applications. The air storage tank pressurizes the compressed air to keep it usable. Receiver tanks can be used for either “wet storage” or “dry storage:”
- Wet storage: Wet storage is when compressed air is held post-compression, but before going through the air-drying process. This allows additional moisture to condense out of the compressed air before it ever reaches the air dryer, which greatly minimizes the amount of water in the final result.
- Dry storage: Dry storage is when the compressed air is stored in the receiver tank post-air drying, which protects the compressed air from gathering any additional moisture.
In both wet and dry air storage, “approach temperature” plays a large part in reducing excess moisture. Approach temperature refers to the difference in temperature between the discharged air and the temperature of the interior of the storage tank. When the two different temperatures meet, the air begins the condensation process that expels moisture into the tank and out of the compressed air. It does this until the air in the storage tank reaches a steady temperature.
If you’re wondering what absorption drying is and why you might not have heard of it before, you are likely not alone. Absorption drying is a chemical process that uses absorptive materials — typically either water-soluble sodium chloride or sulfuric acid — to absorb excess moisture. This method of air compressor water removal is not used as often as other drying methods but is still a viable option for the following reasons:
- Absorption drying has a low initial cost compared to some other types of drying machines because it does not require purchasing additional air drying equipment or tools.
- Maintenance is usually more straightforward due to the lack of moving or electrical parts involved.
- It is a simple process that requires little monitoring or involvement on your part.
There are also a few disadvantages to using absorption dryers, such as:
- The absorptive materials need to be re-added every time they dissolve, which takes additional time and money.
- The absorptive materials might solidify or build-up, which could create a blockage problem in the tank.
- Residue from the materials may transfer into the other parts of your air compression or air drying machine.
- The dew point is not lowered as much as it is with other drying methods, meaning there are more effective drying methods available.
What to Consider When Buying an Air Dryer for Your Compressor
If you are unsure what air compressor dryer to buy, 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 significantly affected depending on where the energy is coming from.
How to Keep Moisture out of Your Air Compressor
Because the air compression and filtering process results in natural moisture, it is not possible to prevent all moisture from getting into your air compressor. However, there are a few steps you can take to prevent excess moisture from building up and keep your compressor as dry as possible:
Minimize Unnecessary Moisture in Your Workspace
Even when you work indoors, there is always a level of moisture present in the air around you. Luckily, there are several steps you can take to minimize unnecessary moisture in the air:
- Ensure adequate ventilation: Make sure the room and building you work in are adequately ventilated, so airflow can help expel naturally-occurring moisture.
- Use fans: Both standard fans and exhaust fans will help dry the air in your workshop or warehouse. Assuming the additional airflow will not hinder your work operations or end product, install and place them in the area where compressed air is used and stored.
- Fix leaks and structural issues: Leaks in your walls, windows, plumbing and flooring can lead to undetectable standing water and create additional moisture in the room.
- Invest in a dehumidifier: Dehumidifiers are machines you place in your room or building to reduce the amount of humidity and moisture in the air. This also helps prevent mold and mildew formation and helps keep energy costs low.
- Eliminate standing water: Immediately eliminate all sources of standing water in the room that houses your air compressor, including areas that are wet from leaks or spills.
- Clean your air conditioning filters: If your building uses an air conditioning system, clogged or broken filters can lead to excess humidity. To prevent this, have your filters checked and changed regularly by a professional.
- Install insulation: Install insulation in your building’s walls, floors and around all windows and doors. Insulation will keep your space at a more constant temperature and will prevent cool or damp air from seeping indoors and causing water build-up in air compressors.
- Check the weather and plan ahead: Humidity in the air translates to excess moisture in your air compressor lines. Pre-plan or schedule air compressor operation for a time of day that is not as humid or when the dew point is at its lowest. For example, think about the dew that you see on the grass in the early morning — it’s usually gone by the early afternoon. Take your planning a step further and check the weekly weather forecast to see which days are going to be drier than others, which includes avoiding rainy days or days with a lot of fog.
- Use a larger air compressor: If your air compressor is too small, it will likely overheat as it tries to keep up with the demand. As the temperature inside an undersized tank begins to rise, moisture will form. Eliminate this concern by investing in a larger air compressor, or using multiple smaller systems to get the job done.
- Use and maintain compressor air filters: Air compressors require a filtration system to separate pollutants and contaminants from pure air, including water and moisture. Make sure you are using a high-quality air filtration system with your compressor, and that it is fitted correctly to your machine. Once installed, you must regularly inspect and clean your filters. Replace your filters regularly and when damaged.
- Drain your air compressor regularly: Just above the drainage area is where all removed debris — including particles, oil and excess moisture — are stored when separated by your tank’s filter. The drain needs to be emptied to avoid rust, clogs, back-up and to improve performance. Remember to do this after every use to keep moisture out of your air compressor.
- Schedule preventive maintenance checkups: Preventive maintenance is the best way to keep your air compressor running effectively for as long as possible. Preventive maintenance can allow a technician to catch issues before they grow into more significant, costly problems later on. It also minimizes the chance you will have to cease operations in the event of a compressor break down or repair.
- Replace your air compressor when necessary: Quality air compressors are designed to last a long time, but that doesn’t mean they won’t eventually need to be replaced. If you are having trouble with excessive moisture, noticed additional concerns that repairing has not fixed or if a service technician has recommended it, consider a machine replacement.