Controlling compressed air moisture

July 18, 2006
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.

QTotal  = QSensible + QLatent

Table 1 compares latent heat, sensible heat and total heat loads for different inlet temperatures, inlet pressures and outlet pressure dew points. Sensible heat is a function of inlet and outlet dew point temperatures. Higher inlet temperature increases the sensible heat load; higher outlet dew point temperatures reduces it. The latent heat is a function of inlet temperature, inlet pressure and outlet dew point.  Higher inlet temperatures, lower inlet pressures and lower outlet dew points work to increase latent heat. The values in Table 1 substantiate the argument that, for comparability, all equipment must be rated at equivalent inlet conditions and outlet pressure dew points. 

Table 1. Heat loads in refrigerated dryer

For example, a dryer designed for the CAGI ADF-100 inlet criteria and an outlet pressure dew point of 38F is expected to handle a total heat load of 8,963 BTU/hr for every 100 scfm of inlet flow. A different dryer, rated for the same flow rate, but delivering an outlet pressure dew point of 50F, will only handle 7,469 BTU/hr for every 100 scfm, a reduction of 17%.

Theoretical limits

 A more efficient air-to-air heat exchanger results in a smaller refrigeration system. But how efficient can an air-to-air heat exchanger be?  For example, is it possible to design an exchanger so large that it requires virtually no refrigeration? The answer is, of course, no. 

In a heat exchanger of infinite size and length, the air exiting the separator can only be reheated to the air temperature of the incoming air (a 0F dryer approach temperature). Because there is no liquid water present after the separator, heat added to exiting air only increases the air temperature and with no change of phase (i.e., sensible heat). Sensible heat added to reheat the air stream from the separator temperature to the inlet temperature is equal to the sensible heat removed from the air stream as it is cooled from the inlet temperature down to the outlet pressure dew point temperature. 

As shown in Table 1, subtracting the sensible heat load from the total heat load results in a difference equivalent to the latent heat load. Therefore, the latent heat load is the theoretical minimum heat load that must be removed by the refrigeration system in a compressed air dryer while still providing the stated level of moisture removal. Without the refrigeration capacity to remove the latent heat, the dryer simply can’t perform as advertised. Table 2 presents the theoretical minimum heat load distribution for a +38F (+3C) refrigerated air dryer at different inlet temperatures and pressures.

Practical expectations

While an air-to-air heat exchanger producing a dryer approach temperature of 0F (0C) sounds interesting as a discussion point (i.e., outlet temperature equals inlet temperature), it is not realistic from a design and manufacturing point. Even extremely large heat exchangers are not practical. Limitations quickly become evident in the areas of packaging and airside pressure drop control. Most notably, the manufacturing costs make extremely large heat exchangers impractical. 

What is the dryer approach temperature of a practical air-to-air heat exchanger? An examination of current designs suggests its 10F to 20F (5C to 11C). 

Table 2. Heat load comparisons for a +38 degree F pressure dew point

One can calculate heat loads on air-to-air and air-to-refrigerant heat exchangers as a function of appropriate temperature and outlet dew point. As approach temperature increases, a higher percentage of the total heat load can be shifted to the air-to-refrigerant heat exchanger. These results are shown in Table 2.

Energy consumption

Once evaporator heat demand has been established, the size of the refrigeration unit can be determined and  power consumption of the dryer calculated. Power is consumed by the refrigeration compressor, the refrigerant condenser cooling fan motors (assuming air-cooled units) and the electrical control system.

For this analysis, assume that the units are air-cooled and the electrical power consumed by the control system (indicator lights, electronic control boards, electronic condensate drain valves, etc.) is negligible. 

Refrigeration compressor

The simplest way to quantify the electrical power that produces the required refrigeration is to use the compressor’s energy efficiency rating (EER). This value is the quotient of refrigeration produced (expressed in BTUs per hour or Watts) divided by the energy consumed by the refrigeration compressor (expressed in Watts). For a given compressor, these figures vary as compressor suction and discharge conditions change. Therefore, it’s important that compressors be analyzed at equivalent inlet and outlet conditions. 

For a refrigerated dryer, use the suction and discharge values at its rated condition (either CAGI ADF-100 or ISO-7183). For most dryers using the CAGI ADF-100 rating point, the approximate values are 35F (1.7C) saturated suction temperature and a 130F (54C) saturated discharge temperature. For ISO-7183 conditions, the saturated suction temperature is also 35F (1.7C), but the saturated discharge temperature drops to 105F (41C) because of the lower ambient temperature. Most refrigeration compressors at  CAGI operating points generate EERs in the range of 8.0 to 10.0 BTU/Watt  (2.3 to 2.9 Watts/Watt).

Table 3 shows the expected compressor power consumption for both theoretical (infinite air-to-air) and the practical cases (dryer approach temperature of 15F) discussed in Table 2.  Table 3 assumes a dryer inlet condition of 100F, saturated with water vapor, 100 psig and a 38F outlet pressure dew point. An average refrigeration compressor EER value of 9.0 BTU/Watt (2.6 Watts/Watt) is used. In most cases, dryer designers will not use compressors with the exact refrigeration capacity necessary for the evaporator heat load.  Compressors are only available in incremental sizes, and the specifier will likely be forced to use one with more capacity than required. In these cases, the air-to-air heat exchanger may be reduced in size to optimize design and reduce manufacturing costs and pressure drop.

Table 3. Power consumption in a refrigerated dryer

Cooling fan motors

Evaluating power consumed by the cooling fan motor is more difficult than that of the  refrigeration compressor. Fan motors are smaller and less efficient. Variations affecting air-cooled condenser designs include fan blade pitch, condenser face area, static pressure drop and desired air velocities. These design variables can result in fan motors of drastically different sizes and energy requirements. Fortunately, the power consumed by fan motors is much less than that of refrigeration compressors. A study of current manufacturers’ data provides the necessary information to develop an industry average. A good approximation of  fan power required for air-cooled refrigeration units is expressed by the equation:

P = (1.5 x 10-5) Q + 0.3,
Where, P = fan power, in kW
Q = refrigeration capacity, in BTU/hr
The results are shown in Table 3.

Total Power

The total power consumed by the refrigerated dryer is simply the sum of the compressor power and the cooling fan power:
PowerTotal  = PowerCompressor + PowerFan

The results

Table 3 displays the total theoretical minimum power consumption for air-cooled refrigerated compressed air dryers at different compressed air flow rates. It also shows the expected practical values. This information can be used when evaluating air-cooled equipment. When evaluating water-cooled designs, subtract the power consumed by the fan motors. For example, in a 1,000 scfm refrigerated dryer, power consumption less than 3.16 kW is simply not possible; however, a value closer to the expected number of 5.20 kW is acceptable.

In another example, Manufacturer A promises a 2,000 scfm refrigerated air dryer will deliver a 38F (3C) outlet pressure dew point at the CAGI ADF-100 rating conditions. This manufacturer also claims the unit will consume 7.0 kW of electrical power. Referring to Table 3, this value is only slightly above the theoretical minimum power consumption of 6.02 kW. It is highly unlikely that the performance claims of this particular dryer are going to be met. Either the unit will consume more power than listed, while providing the promised moisture removal, or it will not remove the stated moisture amount. In the worst scenario, neither the power consumption nor the moisture removal performance will be met.

Making an educated decision

Evaluating brands of refrigerated compressed air dryers can become a difficult and confusing task. The extensive array of performance criteria (flow rate, inlet temperature, inlet pressure, outlet pressure dew point, pressure drop, etcetera) permits manufacturers to publish data at conditions that benefits them. An educated buyer can wade through these issues by first confirming that all equipment under consideration is rated at the same industry standard, the same inlet flow rate and the same outlet pressure dew point. 

When evaluating energy consumption, any unit claiming figures below the theoretical minimum values in Table 3 is suspect for actual energy consumption or outlet pressure dew point performance.  In these cases, buyers beware. Energy consumption values should be centered on the practical expected values listed Table 3. By confirming that a reasonable amount of energy is being consumed, the user has additional assurance that the expected quality of the compressed air system will be realized.

Timothy J. Fox is an engineer with SPX Dehydration and Process Filtration. He can be reached at 724-873-8443 or at [email protected].

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