Controlling compressed air moisture

Print page

Email page

By Timothy J. Fox, P.E.

PlantServices.com

Keywords: compressed air, energy consumption and compressed air system performance

Before specifying a refrigerated air dryer, it's important to understand the relationship between energy consumption and compressed air system performance.

How refrigerated dryers work

Humid air enters the system at the air compressor intake. With refrigerated dryer technology, this moisture is removed by condensing the water vapor into a liquid by cooling the air stream to the desired pressure dew point temperature in one, or more, heat exchangers. Well-designed refrigerated dryers achieve outlet pressure dew points of 38F (3C). As long as the compressed air piping downstream of the dryer is not below that temperature, liquid water will not be present in the system.

Dryers that do not perform to their specifications deliver pressure dew points higher than expected.  In some cases, this performance discrepancy — defined by the amount of water not removed by the dryer — can be dramatic. Figure 1 presents the detrimental effects that a non-performing dryer can have on a compressed air system. As shown, the difference between a 38F (3C) and a 60F (16C) pressure dew point for a 1,000 standard cubic foot per minute system can be more than 70 gallons per week. 

This additional moisture is simply passed downstream into the piping.

Refrigerated dryer designs

The most common refrigerated dryer design is shown in Figure 2. It uses an air-to-air heat exchanger (pre-cooler / re-heater), an air-to-refrigerant heat exchanger (evaporator), a moisture separator and a refrigeration unit.

The pre-cooler

The pre-cooler section partially cools warm, saturated incoming air using the colder air exiting the moisture separator. This reduces the cooling required in the air-to-refrigerant heat exchanger. As a result, the size of the refrigeration unit and electrical consumption are reduced.

The evaporator

The evaporator chills the pre-cooled air exiting the air-to-air heat exchanger to the lowest temperature in the dryer.  At this point, the maximum amount of water vapor has been condensed into a liquid. If the separator provides proper and efficient liquid removal, this temperature should closely match that of the outlet pressure dew point. 

The lower the outlet pressure dew point temperature, the greater the amount of moisture removed and discharged from the compressed air stream. However, to avoid condensate from freezing in the dryer, the pressure dew point temperature can’t drop below 32F (0C). 

Another limitation is that air can only be cooled to some temperature above that of the refrigerant. The more heat exchange surface that is available, the closer the evaporator outlet temperature will be to the refrigerant temperature. This temperature difference between the air exiting and the refrigerant entering the evaporator is called the evaporator approach temperature. The lower it is, the closer the exiting air temperature is to the refrigerant temperature.

The separator

After the air exits the evaporator, it enters the separator where the condensed liquid water is removed from the air stream. An efficient separator is critical for moisture removal because liquid carryover is vaporized in the re-heater portion of the air-to-air heat exchange and recondenses in the air piping. This often underestimated step is responsible for many elevated pressure dew points in dryers capable of achieving low evaporator approach temperatures.

The re-heater

After separation, the cool, dry air enters the secondary side of the air-to-air heat exchanger where it is reheated to a temperature less than the warmer incoming air. The temperature difference between the air exiting and entering the dryer is called the dryer approach temperature.  The lower the dryer approach temperature, the closer the exiting air temperature is to the inlet air temperature. The dryer approach temperature influences the demand placed on the evaporator and refrigeration system.

Performance standards

To compare one dryer design and manufacturer to another, the industry has adopted rating standards for refrigerated compressed air dryers. In the United States, the Compressed Air and Gas Institute (CAGI) uses its standard, ADF-100. It defines the dryer rating inlet condition as 100F (37.8C) inlet temperature, saturated with water vapor, 100 psig (6.9 barg) inlet pressure and 100F (37.8C) ambient temperature. The pressure drop across the unit must be less than 5 psi (0.34 bar). The manufacturer then assigns an inlet compressed airflow rate (expressed in standard cubic feet per minute or scfm) and an outlet pressure dew point temperature for each model. 

In the European Community, the standard is ISO-7183.  It sets the dryer inlet condition as 35C (95F) inlet temperature, saturated with water vapor, 7 barg (101.5 psig) inlet pressure and 25C (77F) ambient temperature. Again, the manufacturer must state the inlet flow rate (in normal cubic meters per hour or Nm3/hr) and the outlet pressure dew point.

Figure 2. Typical refrigerated dryer

When comparing equipment, it’s imperative that the purchaser confirm that all equipment is being rated to the same standard, and that it produces equivalent pressure dew points at the stated compressed air flow rates.

The business of heat loads

When evaluating the energy required to remove moisture from the air stream, one must first assess the amount of heat that must be removed from the air.

The incoming compressed gas has two relevant components: compressed air (primarily, nitrogen and oxygen) and water vapor. Both must be cooled simultaneously. As the mixture is cooled, the water vapor condenses.  Because air remains as a gas, it’s cooled, but there’s no phase change. Heat removed from the water vapor is known as the latent heat. Heat removed from the air is termed sensible heat. The sum of the two determines the total cooling required to reduce the compressed air from its incoming temperature to the stated outlet pressure dew point temperature.