Answer: Dry enough so that no moisture will condense in your air lines. Whatever type of air dryer you ultimately select, the objective is the same – to prevent moisture from condensing in your compressed air lines where it can foul your air-operated equipment and processes.
It's a question of dew point. How does any compressed air dryer prevent moisture from condensing? By lowering the pressure dew point of your compressed air to a point that is lower than the lowest temperature to which your air lines and equipment will be exposed. Notice that we said "pressure" dew point. In compressed air, pressure dew point is the temperature at which moisture starts to condense at a given pressure. It's a lot higher than the dew point at atmospheric pressure, so it can fool you.
|Atmospheric Dew Point||-40ºF||-20ºF||0ºF||+20ºF|
|Pressure Dew Point @ 100 psig||-3ºF||+21ºF||+47ºF||+75ºF|
Raw air from your compressor is hot, dirty, and loaded with moisture. For compressors without integral coolers, the air temperature at the compressor discharge manifold ranges from 180°F to 350°F. Compressors with integral coolers typically reduce the air temperature to within 20°F of the ambient temperature. In either case, the air has substantial amounts of moisture. A properly sized after-cooler and separator with proper draining removes a large amount of this moisture and reduces the temperature to a safe, usable level. Even if your compressor has an integral cooler, it is wise to further cool your compressed air before attempting to dry it. Van Air after-coolers, both air-cooled and water-cooled models, reduce the temperature of your compressed air to within 5°F to 15°F of the cooling medium. This gets rid of up to 75% of the entrained moisture vapor, guaranteeing maximum performance and efficiency of drying equipment.
Determine the dew point you need if planning to operate sensitive pneumatic equipment or if compressed air is used as part of a process. Check with the original manufacturer of the pneumatic equipment and/or a process engineer to determine the dew point specification.
If your aim is to eliminate corrosion in air system piping, consider a -20°F pressure dew point or lower. If you simply need reliable, liquid free plant air, find the LOWEST ambient temperature in which your compressed air lines are located. If, for example, the lowest temperature in your facility is a room air conditioned to a minimum of 60°F, the pressure dew point in your air system must be lower than this temperature.
Compressor: Check the manufacturer’s specifications to determine the exact output in SCFM. The capacity of a compressor in SCFM at 100 PSIG can be estimated using a ratio of 4-5 SCFM discharged per compressor horsepower. Some useful information to gather before contacting Van Air Systems or Van Air Systems distributors:
After-cooler: As noted above, an after-cooler reduces the compressed air temperature, causing a substantial amount of entrained moisture to condense, which can be removed through a separator and drain assembly. If you currently have an installed after-cooler, it's important to know the air temperature at the discharge. If installing a new after-cooler, some important questions to answer are:
Air Receiver: After the compressed air is discharged from your compressor and/or after-cooler, it normally enters an air receiver. These tanks help eliminate pulsations in the discharge line and also act as storage capacity for intervals when demand exceeds compressor capacity. Important information to know is:
Temperature in usage areas: To determine the pressure dew point needed to protect your entire air system from condensation, a thorough survey should be made to locate the lowest ambient temperature to which the air lines and equipment will be exposed. The ambient temperature affects the temperature of the compressed air. Remember the pressure dew point must be lower than the lowest temperature your compressed air lines are likely to contact. If it's not, moisture will condense in your air lines.
First, find the location of all distribution piping before deciding the dew point required. There are some particular things to check:
Generally this is the least expensive type of compressed air dryer, both in terms of capital and operating cost. These dryers are charged with deliquescent tablets that absorb moisture and gradually dissolve (deliquesce). Freedom Dryers feature a compensating dew point. That is, the outlet pressure dew point fluctuates to compensate for the inlet air temperature. Using Dry-O-Lite desiccant, the exit dew point is 20°F lower than the inlet air temperature to the dryer. This 20°F differential is referred to as the "dew point depression." Dew point depressions of up to 65°F can be attained using more aggressive desiccant, such as SP and 10BF
Freedom Dryers are remarkably adaptable. These dryers are best suited indoors or outdoors when the air usage is close to the compressor, portability is desired, and air is used rather than stored. Outdoor installation provides a dew point lower than the ambient temperature.
These type of compressed air dryers go by various names: regen dryers, twin tower dryers, desiccant dryers, pressure swing adsorption (PSA) dryers, temperature swing (TSA) dryers. In any case, the two towers operate alternately. Wet compressed air flows through the desiccant bed of one tower, while the desiccant in the other tower is being regenerated or stripped of accumulated moisture. Regeneration of the desiccant bed is accomplished in various ways: by a purge of dry air drawn from the outlet of the on-line tower; by externally heated purge air; by internal heaters; by atmospheric air blown through an external heater and forced through the off-line tower; or with steam. The desiccant bed consists of a solid adsorbent material, such as activated alumina, silica gel, or molecular sieve.
Of the three compressed air dryer types, regenerative desiccant dryers supply the lowest dew point, usually -40ºF and, if required, as low as -100ºF pressure dew point. Regenerative desiccant dryers are often specified to protect instrumentation and control systems, laboratory equipment or moisture-sensitive process materials. These dryers can also protect against air line freeze-ups in outdoor lines in the most severe conditions.