instrument air and meet its dew point specification before usage. This is achieved with the help of an instrument air dryer, which is explained in the subsequent section. Instrument air dryer An instrument air dryer is a piece of equipment or machin - ery used to dehumidify compressed (process) air to signif - icantly lessen or eliminate moisture in the air stream. The most common types of dryers being used are based on the adsorption principle, where the drying agent is a spheri- cal or granular form of material known as a ‘desiccant’ (see Figure 2 ). The most common drying agents are silica gel, molecular sieve, and activated alumina. The dryer selection is based on the mode of regeneration. Generally, the air dryer utilises two towers: one containing a desiccant that removes moisture from the air stream from the compressor, while the other regenerates the used des- iccant. The main distinction between types of instrument air dryers is based on the regeneration process, specifically the ‘heat-reactivated regeneration system’ and the ‘heat - less regeneration system.’ In the heat regeneration system, the adsorbent is reused after desorbing its water vapour through either of the means, such as: • Direct heating of the adsorbent bed by passing hot air, which is heated by an electrical heater. • Indirect heating of adsorbent bed by embedded steam coils in the adsorber through which steam is passed. • Heating of adsorbent bed through the hot compressed air (before air compressor aftercooler), utilising HOC. Instrument air dryer prevalent practice Energy conservation by an HOC-type compressed air dryer is a breakthrough in compressed air drying technology. The hot air from the oil-free air compressor at 120°C or higher is used directly to regenerate the desiccant bed in the com - pressed air dryer (see Figure 3 ). After regeneration, this air is cooled to 40°C in the water- cooled aftercooler and then dried in the second tower. Thus, the use of electrical heaters is avoided. The main advantages of HOC-type compressed air drying are energy conservation and heat recovery, which were wasted in the aftercooler. Conventional ‘no purge loss type’ air dryers are now being used to reactivate the desiccant. Fixed-cycle dryer The instrument air dryers are designed for a fixed cycle time, where the switching from adsorption to regeneration mode takes place after a predefined set point to reach cycle time. Dryers are mainly designed to function at maximum operating conditions, the highest flow rate, the highest temperature, and the lowest pressure. Fixed-cycle dryers are constantly switching towers and regenerating based on worst-case scenario, whereas the actual operating conditions may be quite different. With a fixed-cycle dryer, regeneration is constant and designed for the maximum incoming water load. In reality, the aver - age amount of moisture entering the dryer is less than the design. However, the fixed-cycle dryer cannot take
Compressed air dryers
Membrane
Refrigerant
Desiccant
Regenerative
Deliquescent
Pressure swing (Heatless/heated)
Heat of compression
Figure 2 Typical instrument air dryer system and related functions
advantage of a reduced water load since it continues to regenerate at a predefined cycle time. Regulating dew point – an innovative approach The dew point demand controller makes it possible to reduce the operating cost of any dryer by regulating the dew point rather than regulating the sequence through a fixed time-based control panel. The outlet air dew point will determine the operating cycle time of the dryer. If the dew point is higher than the desired level, the adsorp- tion cycle can be extended beyond eight hours until the required dew point of -40°C is achieved. The controllers can be set for the specific dew point desired in the system. Changeover would take place only at the adjusted dew point. The dew point demand controller utilises a state-of-the- art moisture analyser to accurately measure the actual dew point that the dryer is always delivering. The instrument can also be set for a precise dew point at which the dryer is desired to switch towers. Every time a fixed cycle dryer switches towers, it slightly damages the desiccant, thereby reducing its life. In addition, all the switching valves experi - ence wear and tear with every tower shift. Converting from a fixed cycle to a demand cycle would increase the effective life of desiccant and reduce the maintenance required on the dryer’s switching valves. Overall benefits include: • Saving electrical power. Potential electrical power saving in the dryer may vary from one to two years. • There is no requirement for additional hardware to be installed. Also, an analyser transmitter and soft signal to dryer PLC can predicate new requirements. • Because of the extended adsorption cycle time, the num - ber of adsorption cycles/changeover will be reduced, and the regeneration cycle also will be reduced. The extended hours of operation of the adsorption cycle can also vary from ~10 minutes to three hours. • Reduction in the number of changeover cycles will, in turn, reduce the wear and tear of the changeover valves, control valves, and other components. • Operating efficiency of the air dryer will be high. Case study The grassroots compressed air plant of a recently commis - sioned Indian refinery is considered for the case study. The
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PTQ Q1 2025
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