Water in compressed air can damage production machinery, resulting in downtime and spoiled product.
Dew point and its importance
As already mentioned, wet air adds to plant operating expenses through the cost of:
• repair parts
•product damage, and
The economic advantages of reducing or eliminating these detriments of moisture build a strong case for installing a dryer. Once the decision to install a dryer has been reached, two questions arise: how dry must the air be, and what type of dryer should be used?
The most important criterion in choosing an air dryer is the pressure dew point that it must produce. The required dew point of an air system determines how dry the air must be and to a great extent, which type of dryer to use. Dew point varies with pressure. For example: the dew point conversion chart, Figure 6, shows that air at atmospheric pressure with a dew point of 12° F has a pressure dew point of 35° F at 100 psig. Dryer manufacturers may specify the dew point that a particular model can attain at atmospheric pressure or at a typical system pressure, such as 100 psig. If performance is specified at atmospheric pressure, use a chart like Figure 6 to find what the minimum dew point will be at the system's operating pressure.
The required dew point varies with each application. If preventing condensation in compressed air lines is the main concern, then the lowest ambient temperature to which air lines will be exposed will be the controlling factor. However, for some applications, dew point requirements will be more stringent, possibly as low as 100° F at line pressure. An example might be the air used for spraying a powdery substance. Even the slightest trace of moisture in such air could condense and cause particles to stick together.
If all the compressed air will be used inside a building where temperature is maintained at a stable level, then the required dew point can be fixed within a few degrees. But if some or all of the compressed air is subjected to outdoor temperature variations, the required dew point can change from day to day, or even hour to hour.
Do not be too aggressive by estimating an unjustifiable margin for error. Stating a dew point much lower than that actually required wastes money. A rule-of-thumb margin for error is about 20° F maximum.
Extremely low dew points may be required at only a few isolated locations. If this is the case, consider using individual small heatless regenerative dryers at locations requiring pressure dewpoints below 35° F. A less-expensive dryer to dry the air to less-stringent requirements can then be installed for the rest of the air system.
Evaluating flow capacity
An air dryer not only must dry compressed air to the required dew point, but also must be able to handle the required air flow without causing excessive pressure drop. Flow capacity of a dryer depends on:
•inlet air temperature
•ambient air or cooling water temperature, and
•required dew point.
When any of the above conditions changes, flow capacity of the dryer also changes. Dryer manufacturers can supply performance curves that show the relationship of their dryer's flow capacity to these four factors. Evaluating characteristics of the different types of dryers will help indicate which is best for a particular application. This is where cost finally can be considered. Purchase price of the dryer is only one factor to evaluate when choosing an air dryer. A deliquescent chemical dryer, for example, has a relatively low initial cost, but its chemical must be replaced periodically, adding to the operating cost. This cost is offset somewhat because the deliquescent chemical dryer requires no external power source.
Other dryer types may cost more initially, but have lower operating costs because they can run for long periods with little or no maintenance required. It should be clear, then, that cost analysis should be conducted based on manufacturers' specifications as they relate to an individual application's physical and economical requirements.
Installation and maintenance
Location can affect how well an air dryer performs. The site for an air-cooled dryer should be well ventilated, so heat can be carried away, and readily accessible to aid maintenance. The maximum ambient temperature for a refrigerated dryer is about 100° to 120° F. Higher temperatures prevent the dryer from exchanging heat with its surroundings and keep it from operating properly. Dryers with water-cooled condensers can tolerate higher ambient temperature because they transfer heat to the cooling water instead of to the surrounding environment. Refrigerant dryers, whether air- or water-cooled, should not be exposed to ambient temperature below 32° F unless optional low-ambient-temperature controls are installed.
If a deliquescent dryer is used in a central compressed-air system, bypass piping should be installed around the dryer to maintain air supply whenever the dryer is taken off line to add desiccant. There should also be no set of operating conditions that permit system pressure to drop low enough to allow high, turbulent air flow through the dryer that might carry chemicals into system air lines. It is important to shut off the water in water-cooled aftercoolers when the air system is shut down. A leak in the aftercooler could flood the deliquescent dryer and fill downstream piping with desiccant, making all pneumatic components inoperable.
Refrigeration and deliquescent dryers should be drained regularly, depending on the volume of liquid accumulated. Most refrigeration dryers have automatic drains, at least as an option.
It should be noted that dryers remove water vapor, while filters remove liquid water. A good drying system always has a filter with an automatic drain installed upstream from the air dryer. Air dryers of all types are not stand-alone components. The cost of adequate prefilters, both particulate and oil coalescing, is a wise investment to protect the more expensive dryers. Postfilters are necessary for several reasons. For refrigerated dryers, a coalescing filter can catch any oil from a refrigerant leak. For deliquescent dryers, a particulate filter downstream will catch any carryover of the corrosive desiccant. For regenerative dryers, a 0.5-m postfilter is necessary to catch desiccant dust, which is common to all adsorptive desiccants.