Compressed Air Dryers

The natural water vapor content of air is concentrated and is carried through the compression process in a compressor as a vapor in high temperatures. A proper means of dehydration must be incorporated in the conditioning of compressed air where demands of high quality compressed air are entailed. A dryer can be used to receive wet compressed air from the compressor, remove moisture from the compressed air, and then deliver dry compressed air to the associated system. Remember, dryers are pressure vessels and hence must comply with the relevant pressure vessel standards of the concerned region or country.

The most common methods of compressed air drying are the: (1) absorption process, (2) adsorption process, (3) membrane drying, and (4) refrigeration method. The following sections present the details.

Pressure Dew Point

An important term used in the realm of dryers is the pressure dew point. It is the lowest air temperature reached during a drying process at the specified operating pressure.

Absorption Dryers

Figure 1 shows an absorption (deliquescent) dryer. An absorption dryer consists of a shell enclosed typically with many different types of media. These media are arranged as liquid drying section, semi-solid drying section, and solid deliquescent section. The deliquescent materials used are sodium, calcium chloride, etc. It may be noted that each drying section offers different levels of drying capacity.

Figure 1 | Absorption dryer

Pressure dew point: Absorption dryers can lower dew points to a limited extent and can be as low as -5°C (23°F) depending on operating conditions.

Application of Absorption Dryers: Absorption dryers are commonly used in applications where more complex drying solutions are not required. As absorption dryers require no electricity, they are extensively used in construction, sawmills, mining, petrochemical, and mobile equipment.

Advantages of Absorption Dryers: The advantages of absorption dryers are that they are easy to install, their operation is simple, they require no electrical connection, they can be installed outdoors in hazardous or corrosive environments, they have no moving parts, and they have the lowest initial cost as compared to other types of dryers.

Disadvantages of Absorption Dryers: The disadvantages of absorption dryers are that they have limited dew point suppression capability and their deliquescent bed must be refilled periodically.


Book on ‘Compressed Air Dryers’ by Joji Parambath

The book describes the working of absorption dryers, adsorption dryers, refrigerant dryers, and membrane dryers, in detail. Heatless, heated, and heated blower types of adsorption dryers are also presented. The features, advantages, disadvantages, and specifications of these types of dryers are also given in the book. Further, the book presents the maintenance, troubleshooting, and safety aspects of dryers. The topics are presented in a logical sequence and simple to understand language.


Adsorption Dryers

Adsorption is the physical process of collecting moisture on the porous surfaces of certain granular desiccant materials such as silicon dioxide (Silica gel), activated alumina, copper sulphate etc. An adsorption dryer must use a sufficient quantity of adsorbent materials as the drying agent. Adsorption dryers can be configured in two versions: (1) Single tower and (2) Twin tower.

Single Tower Adsorption Dryer: Figure 2 shows the constructional features of a typical single tower adsorption dryer. It consists of a tower with a drying agent, like silica gel, and an inlet port at the bottom and an outlet port at the top of the tower. When compressed air is passed through the drying agent upwards, the moisture present in the air is adsorbed by the drying agent. Dry compressed air is then delivered out of the adsorption dryer through the outlet connected to the top of the tower.

Figure 2 | Adsorption dryer, single tower

A given quantity of adsorbent materials has a fixed adsorption capacity and the adsorbent materials get saturated upon reaching their adsorption capacity. When saturated, the adsorbent material can be regenerated by removing the moisture. The moisture can be driven off by the application of dry air or hot air through the desiccant materials over time.

The drawback of a single tower dryer is that the associated system should be stopped during the regeneration phase of the single tower adsorption dryer. Therefore, in practice, two desiccant-filled parallel towers along with a valving system are used for the non-stop working of the system. The valving system selects one chamber for drying and the other chamber for regeneration. This arrangement is shown in Figure 3. The change-over from one chamber to the other chamber can be realised automatically using a time-based switching system or dew-point-dependent switching system.

Figure 3 | Twin-tower adsorption dryer

Types of Adsorption Dryers

There are many types of adsorption dryers available in the market. The types and their operation are described in the following sections. When saturated, the drying agent in an adsorption dryer can be renewed by blowing warm or cold air through the material, which then takes up the moisture. Accordingly, adsorption dryers can be of the following three types: (1) Heatless type, (2) Heated type, and (3) Heated blower type.

Regenerative dryers typically can deliver pressure dew points of -40°C [-40°F] at 7 bar [100 psi]. However, it is to be noted that adsorption dryers can also deliver pressure dew points down to -70°C [-94°F].

Heatless Type Adsorption Dryer

In a heatless regenerative desiccant dryer, no heater is used. A heatless twin tower dryer diverts a portion of the dried compressed air to the off-line tower. This dry purge air then flows through the saturated desiccant and regenerates it. The purge air, now moisture-laden, is harmlessly exhausted to the atmosphere through a silencer.

Heated Type Adsorption Dryer

In the heated desiccant type dryer dried purge air, diverted from the compressed air system, is first passed through a high-efficiency external heater before entering the off-line tower to regenerate the desiccant. Since the heated compressed air can hold more moisture than unheated compressed air, only about 5% of the dried compressed air is needed for regeneration.

Heated Blower Type Adsorption Dryer

A heated blower type dryer employs a high-performance centrifugal blower to direct ambient air through a heater and then through the off-line tower. The stream of heated air then regenerates the desiccant. The heated blower technology requires the highest initial capital investment. However, with no or little diversion of compressed air from the system for regeneration, it offers significantly lower operating costs than other types of regenerative desiccant dryer technologies.

Refrigerant Dryers

In a refrigerant dryer, compressed air is cooled to a temperature as low as possible to condense the moisture present in the air. The schematic of a refrigeration air dryer is shown in Figure 4. It consists of a heat exchanger and refrigerating unit. The heat exchanger is an air-to-air pre-cooler. The refrigerating unit is an air-to-refrigerant cooling unit. Warm and humid compressed air is first passed through the heat exchanger. The air gets precooled to a near ambient temperature condition of the heat exchanger. The moisture present in the air gets condensed corresponding to the temperature in the heat exchanger and water is precipitated.

Figure 4 | Low-temperature dryer

The partly prepared air is passed through the refrigerating unit (air-to-refrigerant section) to reduce the temperature of the compressed air to as low as 1.7°C (35°F). The moisture is condensed again corresponding to the temperature in the refrigerating unit. The condensed water can be collected in the water traps provided at appropriate points.

Types of Refrigerant dryers: There are two types of refrigerant dryers. They are: (1) Non-cycling type and (2) Cycling type. In a non-cycling refrigerant dryer, the refrigerant is made to circulate continuously through the system. In a cycling type refrigerant dryer, a heat sink like an Aluminum block and thermostat are provided. The refrigerant compressor runs and cools the incoming compressed air and the heat sink. The excess cooling capacity of the refrigerator under partial load conditions is used to cool the heat sink until the switch-off point is reached. The compressor is then switched off by the associated thermostat.

Pressure Dew Point of Refrigerant Dryers: Refrigeration dryers can produce dew points in a range from 1.7°C to 10°C (35°F to 50°F) at system operating pressure. A lower dew point is not feasible in this type of dryer as the condensate would freeze at 0°C (32°F) or lower temperature.

Advantages of Refrigerant Dryers: The advantages of refrigerant type air dryers include compact dimensions, easy to install, operate, and maintain, low capital cost, maintenance costs, and operating cost, constant dew point, no need for chemical or desiccants, and easy condensate separation.

Disadvantages of Refrigerant Dryers: The disadvantages of refrigerant type air dryers include limited dew point capability andthe problem of refrigerant leakage.

Membrane Dryers

A membrane dryer consists of a compressed air inlet, fibre membranes, purge air nozzle, purge air outlet, compressed air outlet, as shown in Figure 5. The membrane module is a specially designed module with finer and densely packed hollow fibre membranes. It is arranged in multiple layers and housed in the inner chamber of the module.

Figure 5 | Membrane dryer

Operation of a Membrane Dryer: Moisture-laden compressed air from a compressor flows into the module housing and flows downward through the central structure of membrane fibers. Only the water vapor molecules in the compressed air can permeate through the membrane pores while the air continues to flow and gets discharged as a dry medium in a manner as explained below.

Pressure Dew Point of Membrane Dryers: The pressure dew point of this type of dryer is typically 4°C [40°F], but lower dew points to -40°C [-40°F] can be achieved at the expense of additional purge air loss.

Advantages of Membrane Dryers: Membrane dryers have the advantage of low installation and operating costs. They have no moving parts and are suitable for installation in outdoor locations as well as in hazardous areas.

Disadvantages of Membrane Dryers: However, they are high-cost devices and their use is limited to systems with small to medium volume flow rates. Another disadvantage is the requirement of a large amount of purge air to achieve required pressure dew points. Further, the membrane may be contaminated by oil.

Applications of Membrane Dryers: Membrane dryers find applications in mobile vehicles with limited space and in garage workshops, petrol stations etc. They also find direct application in CNC machines for drying relatively small volumes of air.

General Features of Dryers

Four main factors, namely, the flow rate capacity, pressure dew point, operating pressure, and inlet temperature must be considered while selecting a dryer for a compressed air system. An electronic controller can be incorporated into a dryer to execute all valve switching functions and monitor the operations of the dryer. An LCD can be included in a dryer for viewing critical dryer parameters, such as tower status, process value status, load-factored purge savings, alarm, filter and drain service remainder for filters, and switching failure alarm.

Specifications of Dryers

As we are aware, there are many different types of compressed air dryers employing varied technologies for their drying processes. Therefore, the parameters of one type of dryer may not be the same for other types of dryers. However, the common specification parameters of dryers are as follows: rated flow rate capacity, maximum working pressure, pressure drop, pressure dew point, maximum inlet air temperature, electrical supply, power consumption, noise level etc.

Dryers Standards

The standard ISO 7183:2007: ‘Compressed-air dryers – Specifications and testing’ specifies the performance data that are necessary to describe dryers and applicable test methods to be used for different types of compressed air dryers.

Selection of Dryers

The selection of dryers depends on the variables, such as, system demand, air quality requirements, etc., that are unique to a compressed air system. The requirements for the degree of drying vary depending on the application, but should always be achieved with the least possible consumption of energy.

Installation and Maintenance Aspects of Dryers

The key maintenance points may be kept at the operator level for faster maintenance and increasing operator safety. Dryers are pressure vessels and must conform to the rules concerning pressure vessels in one’s region. All safety precautions must be taken during their installation, operation, and maintenance.

Tips for General Maintenance

Generally speaking, dryers are maintenance-free. However, certain maintenance activities are to be carried out during the service life of a dryer. Some of the general maintenance activities for a dryer are listed below:

  1. Switch off the dryer while carrying out any maintenance activity
  2. Isolate all pressure sources and vent the internal pressure of the dryer before dismantling
  3. Use proper technique to tightly charge the drying chambers with adsorbent desiccant material
  4. Check for correct operation after maintenance

Desiccant Filling

The desiccant bed must be filled fully with desiccant materials utilizing all of the available space in the charging bed.

Disposal of Condensate Substances Generated in Dryers

The condensate released by a dryer may contain mineral oil aerosols, particles of dust and dirt, cooling and lubricating oil, rust, wear debris, pieces of sealing material, and weld from the pipeline. Therefore, the condensate must be disposed of properly and responsibly. Remember, incorrect disposal of the condensate tends to be detrimental to the environment.

Joji Parambath

Author


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