Choosing a compressed air dryer
Ambient air entering an air compressor always contains water vapor. At 75 F and 75% relative humidity, a 25-hp compressor will produce 20 gal. of water per day. This water vapor must be condensed and removed from the air system. Condensate is a contaminant that adversely affects end use applications.
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Ambient air entering an air compressor always contains water vapor. At 75 F and 75% relative humidity, a 25-hp compressor will produce 20 gal. of water per day. This water vapor must be condensed and removed from the air system. Condensate is a contaminant that adversely affects end use applications. Failure to remove condensate can result in corrosion, air leaks, pressure drops, and scale formation in the distribution system.
Compressing air concentrates water vapor content. Compressing air also heats it and keeps water vaporized. When compressed air enters the distribution system and cools, the vapor condenses. The compressed air is further cooled when it expands from an elevated pressure to atmospheric pressure. Liquid water, or condensate, is unacceptable for many process applications and can contaminate an entire compressed air system.
When air is cooled to the point that relative humidity reaches 100% and then is cooled further so some of the moisture begins to liquefy, the air is said to have been cooled to its dew point. At 100% saturation, a 20-deg-F drop in temperature condenses 50% of the water vapor present in the compressed air.
It is important to have dryer performance quoted in pressure dew point. There is a considerable difference between atmospheric dew point and pressure dew point. A —30-F dew point at atmospheric pressure is only +14 F at 100 psig (Fig. 1).
Types of dryers
Refrigerated
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Noncycling
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Cycling (thermal mass or cylinder unloaders)
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Desiccant
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Heatless
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Internally heated
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Externally heated
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Blower purged (heated)
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Heat of compression
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Deliquescent
Membrane
Refrigerated dryers
The principle used in a refrigerated air dryer is essentially the same as that used in a refrigerator or air conditioner. Refrigerated dryers provide pressure dew points of approximately 35 F to 40 F.
Refrigerated dryer advantages:
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Low initial capital cost
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Relatively low operating cost
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Low maintenance cost
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Not damaged by oil in the air stream.
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Disadvantages:
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Limited low dew point capability.
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Noncycling , direct-expansion refrigerated dryers use a capillary tube system or a thermostatic expansion valve in conjunction with a hot gas bypass valve to compensate for load fluctuations and to prevent freezing under no-load or low-load conditions.
Noncycling advantages:
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Low and precise dew point
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Quick response to air flow changes
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Compressor runs continuously.
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Disadvantages:
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No energy savings at partial and zero flow.
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Cycling refrigerated dryers use a thermal mass surrounding the air passage in the evaporator. The mass may be liquid or metal. Under reduced load conditions, the thermal mass tends to stay cold, allowing the refrigeration compressor to shut down. As the thermal mass warms up the compressor starts up. Some larger dryers use cylinder unloaders to reduce energy consumption at part load.
Cycling advantages:
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Energy savings at partial and zero air flow.
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Disadvantages:
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Dew point swings
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Reduced compressor life due to frequent starts and stops
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Increased size and weight from heat sink mass
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Increased capital cost.
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Desiccant dryers
Regenerative desiccant dryers use a desiccant, which adsorbs water vapor in the air stream. Moisture is driven off in the regeneration process by applying dry, expanded purge air, heat (internal or external), or a combination of both.
These dryers usually have twin towers. One tower dries the air from the compressor while the desiccant in the other tower is being regenerated. Purge air requirements range from 10% to 18% of the total airflow in pressure swing (heatless) dryers. Dew points of approximately —40 F to —100 F can be obtained.
Advantages:
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Very low dew points can be achieved without potential freeze-up
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Heatless type can be pneumatically operated remotely and in hazardous locations.
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Disadvantages:
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Relatively high initial capital cost
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Periodic replacement of desiccant bed
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Oil in air stream can coat desiccant, rendering it useless
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Purge air is usually required
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High cost of operation.
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Heat-of-compression dryers are regenerative dryers that use the heat generated during compression to regenerate the desiccant. They can be considered heat-reactivated. In the case of lubricant-free compressors, optimum energy savings are possible.
There are two designs. A single-vessel or rotating-drum type can provide a dew point, which, depending on cooling water temperature, can vary between —40 F and 4 F. A twin-tower type, with some purge air, can provide a constant dew point of —40 F.
Advantages:
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Low electrical installation cost
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Low power cost
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Minimum floor space
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No loss of purge air
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Can reach low dew points.
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Disadvantages:
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Used only with oil-free compressors
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Used only with compressors that have a continuously high discharge temperature
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Inconsistent dew point at varying load conditions
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Affected by changing ambient and inlet compressed air temperatures
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Booster heater required for low heat conditions
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High pressure drop on single vessel type.
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Deliquescent
The single-tower deliquescent dryer has no moving parts and requires no power supply. This simplicity leads to low installation costs. This design typically provides a dew point suppression of approximately 20 F and some claim to achieve 50 F suppression. The desiccant is consumed only when moist air is passing through the dryer.
Advantages:
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Low initial capital and installation cost
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Low pressure drop
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No moving parts
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Requires no electrical power
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Can be installed outdoors if the automatic drain is protected from freezing
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Can be used in hazardous, dirty, or corrosive environments.
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Disadvantages:
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Limited dew point suppression
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Periodic replenishment and replacement of desiccant bed
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Regular periodic maintenance
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Desiccant material can get into downstream piping and do damage unless properly filtered
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Some desiccant materials melt or fuse above 80 F
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Oil in air stream can coat desiccant, rendering it useless
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Relatively high maintenance costs.
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Membrane
Specially designed membranes allow water vapor to pass through the membrane pores faster than air, which acts as purge air. This reduces the amount of water vapor in the air stream at the outlet of the dryer and suppresses the dew point to approximately 40 F. Dew points as low as —40 F can be reached at the expense of additional purge air.
Advantages:
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Low installation cost
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Low maintenance cost
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Can be installed outdoors
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Can be used in hazardous locations
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No moving parts
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No electrical power required.
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Disadvantages:
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Limited capacity
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High purge air loss and energy cost
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Oil or other contaminants can foul membrane
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Selection
Selecting an air dryer should be done by a professional acquainted with air drying techniques. Knowledge or information about the applications and the degree of dryness required is essential in making this decision.
Specifying a dew point much lower than required can add to initial and operating costs and is generally not good engineering practice. Specifying a dew point that is marginal can be even more costly since it could shut down a process or damage product.
For plant and instrument air, the primary goals are to eliminate condensation and freezing. Such considerations determine what dew point protection a compressed air system will require.
The following information is required to properly select an air dryer:
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Maximum air flow in standard cubic feet per minute (scfm)
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Inlet air pressure
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Inlet air temperature
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Ambient air temperature (and water temperature if condenser is water-cooled)
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Desired pressure dew point
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Utilities required.
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Installation
Dryers should always be installed with filters before and after. The placement and type of filter depends on the type of dryer (Fig. 3). Generally, coalescing filters are placed before all types of dryers. Some manufacturers recommend a particulate filter before a refrigerated dryer. Downstream filters are usually coalescing. Regenerative dryers may require a one-micron filter downstream.
Bypass valves should be installed around dryers and filters to permit servicing or removal without shutting down the air system.
Pressure differential indicators should be installed on each filter to indicate when replacement is required. Do not bypass any components in air breathing systems. Some instrument air systems require dual air treatment centers to allow servicing without downtime.
Dryer ratings
Standard conditions for the capacity rating of refrigerated compressed air dryers are contained in CAGI Standard ADF 100, Refrigerated Compressed Air Dryers-Methods for Testing and Rating. These conditions are commonly called the three 100s. A dryer inlet pressure of 100 psig, an inlet temperature of 100 F, and an ambient temperature of 100 F. If the plant compressed air system has different operating conditions, dryer rating will be affected and must be discussed with the supplier to ensure performance.
Similar conditions apply to dual-tower regenerative dryers (ADF 200), single-tower regenerative dryers (ADF 300), and membrane dryers (ADF 700). No standards presently exist for deliquescent dryers.
Plant Engineering magazine extends its appreciation to Bill Scales of Scales Air Compressor Corp. for his assistance in the preparation of this article.
More Info:
Here are two major resources for information on compressed air drying. Compressed Air and Gas Institute (CAGI) has a brochure titled “Compressed Air and Gas Drying.” Call them at 216-241-7333. The Compressed Air Challenge puts out a 266-p manual called “Best Practices for Compressed Air Systems.” To order, call David McCulloch at 850-651-4540.
Recommended pressure dew points, F
Air motors (high efficiency) — 40 to 38 Air motors (low efficiency) — 20 Breathing air (later humidified) — 40 to 38 Chemical processing — 40 Control air — 40 General indoor service — 40 to 50 General outdoor service — 100 to 38 Instrumentation — 40 Paint spraying — 40 to 38
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