1.Desiccant dryer basics and pressure drop overview

(1) Functions and classification of desiccant dryers

In modern industrial production, compressed air has become an important power source and process gas. However, untreated compressed air contains a large amount of moisture, oil and particulate matter. These pollutants will not only corrode equipment and affect product quality, but may even cause production accidents. As a key equipment in the compressed air post-processing system, the core function of the desiccant dryer is to remove moisture from the compressed air to meet the dew point temperature requirements of the production process, thereby ensuring the normal operation of downstream equipment and product quality.

According to different drying principles, desiccant dryers can be mainly divided into the following categories:

Refrigerated desiccant dryer: uses refrigeration technology to cool the compressed air to a preset dew point temperature, so that the water vapor in the air condenses into liquid water and then discharges it. Its advantages are simple structure and relatively low operating cost, and it is suitable for occasions where the dew point requirement is not high.

Desiccant desiccant dryer: uses solid adsorbents with strong moisture absorption capacity to absorb moisture from compressed air. Desiccant dryers usually use a dual-tower alternating working mode, with one tower performing adsorption drying and the other performing regeneration. Desiccant dryers can achieve a lower dew point and are suitable for industries such as precision instruments, medicine, and food that have high requirements for the dew point of compressed air.

Membrane dryers: use membrane materials with selective permeability to allow water vapor in the compressed air to permeate to the other side of the membrane and be discharged to achieve drying. Membrane dryers have a compact structure and do not require power or regeneration gas. They are suitable for small, mobile or noise-sensitive occasions, but their drying effect and processing capacity are relatively limited.

In industrial applications, desiccant dryers are widely used because they can achieve a lower dew point and a wide range of processing capacity. This article will focus on desiccant dryers.

(2) What is pressure drop?

Pressure drop is the pressure loss caused by overcoming friction resistance and local resistance when a fluid flows through a pipeline, equipment, or filter medium. In a dryer system, pressure drop refers to the pressure difference between the compressed air inlet and outlet of the desiccant dryer.

Ideally, the gas flows through the system without pressure loss. However, in actual systems, due to the friction of the inner wall of the pipe, the viscosity of the fluid, the resistance of the equipment, and the local resistance caused by various pipe fittings, the gas will inevitably suffer pressure loss. Pressure drop is an important indicator to measure the efficiency of the fluid passing through the system or equipment.

In the dryer system, pressure drop mainly occurs in the following links:

Inlet pipe and valve: the pipe and valve before the compressed air enters the desiccant dryer.

Inlet filter: a filter that removes solid particles and oil in the compressed air.

Adsorption tower: compressed air flows through the adsorbent bed.

Outlet filter: a filter that removes adsorbent dust.

Outlet pipe and valve: the pipe and valve after the dried compressed air flows out of the desiccant dryer.

The size of the pressure drop is directly related to the energy consumption of the system, the operating efficiency of the equipment, and the gas pressure of the downstream equipment. A well-designed and well-operated dryer system should have the lowest possible pressure drop. The industry usually sets a reasonable pressure drop standard to guide equipment selection, design, and operation and maintenance. For desiccant dryers, the required pressure drop range is usually between 0.02 MPa and 0.05 MPa, but the specific standards vary depending on the equipment type, specifications and manufacturer.

Understanding the concept of pressure drop and its importance is the basis for in-depth understanding of the operating status of the desiccant dryers and troubleshooting.

2.Detailed explanation of factors affecting the pressure drop of the desiccant dryers

The pressure drop of the desiccant dryers is not a constant value. It is affected by a combination of factors. These factors include the characteristics of the equipment itself, as well as the operating conditions and pipeline design of the system. In-depth analysis of these influencing factors will help us find effective ways to control and reduce the pressure drop.

(1) Equipment factors

The design and composition of the equipment itself are the basis for affecting the pressure drop.

① Adsorbent type and quality

The adsorbent is the core of the desiccant dryer, and its physical properties directly affect the flow resistance of the gas.

Adsorbent particle size: Smaller adsorbent particles have a larger specific surface area, which is conducive to improving adsorption efficiency, but it also means that the gaps between particles are smaller, and the gas encounters greater resistance when flowing through, resulting in an increase in pressure drop. Conversely, adsorbent particles with larger particle sizes have larger gaps and lower pressure drop, but the adsorption efficiency may be slightly reduced. Equipment manufacturers usually choose the appropriate adsorbent particle size range based on the design and expected performance of the adsorption tower.

Adsorbent shape and surface characteristics: Adsorbents of different shapes and their surface roughness will affect the smoothness of gas flow. Smooth, regular particles usually produce less resistance than rough, irregular particles.

Adsorbent bulk density and porosity: Bulk density refers to the mass of adsorbent per unit volume, and porosity refers to the volume ratio of pores between adsorbent particles or inside particles. High bulk density and low porosity mean fewer channels for gas flow, greater resistance, and higher pressure drop.

Adsorbent quality: During long-term use, adsorbents may deteriorate, agglomerate, or pulverize due to the adsorption of oil, moisture, or other impurities. These changes will lead to structural changes in the adsorbent bed, obstruction of the gas channel, and a significant increase in pressure drop. Powdered adsorbents will also produce a large amount of dust, which will clog subsequent filters and further increase pressure drop. Therefore, regular inspection of the state of the adsorbent and timely replacement of inferior adsorbents are one of the key measures to control pressure drop.

② Adsorption tower structure design

The design of the adsorption tower directly affects the flow path and speed of the gas in the tower.

The inner diameter and height of the adsorption tower: The cross-sectional area of ​​the adsorption tower determines the linear velocity of the gas flowing through the adsorbent bed. At the same flow rate, the smaller the tower diameter, the higher the linear velocity, the greater the fluid resistance, and the higher the pressure drop. The height of the tower determines the distance that the gas flows through the adsorbent bed. The longer the distance, the greater the accumulated friction resistance and the higher the pressure drop. Therefore, it is the basis for reducing the pressure drop to reasonably determine the size of the adsorption tower so that it can adapt to the design flow rate.

The filling method and uniformity of the adsorbent bed: The adsorbent should be evenly filled in the adsorption tower to avoid “channel effect” or “short circuit”. The channel effect refers to the fact that the gas flows rapidly mainly through certain areas of the adsorbent bed, while the adsorbent in other areas is not fully utilized. This will lead to a decrease in the drying effect, and due to the uneven gas flow rate, excessive local pressure drop may occur in some areas. Layered filling, vibration compaction and other methods can help improve the uniformity of the adsorbent bed.

Gas distributor and gas collector design: A gas distributor is usually provided at the inlet of the adsorption tower to evenly distribute the compressed air to the entire adsorbent bed cross-section; a gas collector is provided at the outlet to collect the dried air. The design of the distributor and gas collector directly affects the uniformity of the gas flow in the tower. If the design is unreasonable, it may generate greater resistance in local areas, resulting in an increase in the overall pressure drop and affecting the drying effect.

Support structure in the tower: structures such as grids and sieve plates that support the adsorbent bed will also generate resistance to the airflow. The porosity, strength and corrosion resistance of these supporting structures need to be fully considered in the design to reduce the impact on the pressure drop.

③ Performance of the filter device

Desiccant dryers are usually equipped with filters at both the inlet and outlet. The inlet filter is used to remove solid particles and oil in the compressed air and protect the adsorbent; the outlet filter is used to remove dust generated by the adsorbent to prevent it from entering the downstream system.

Filter element material and pore size: The material and pore size of the filter element determine its filtration accuracy and resistance to airflow. The higher the filtration accuracy, the better the filtration effect, but the greater the resistance generated and the higher the pressure drop.

Filter flow capacity: The designed flow capacity of the filter should be greater than or equal to the maximum flow of the system, otherwise it will generate too much resistance when high-speed airflow passes through.

Filter element contamination degree: With the increase of usage time, the filter element will gradually accumulate pollutants, resulting in a decrease in the effective channel of the filter medium, an increase in gas flow resistance, and an increase in pressure drop. Therefore, regular inspection and replacement of filter elements is an important measure to maintain low pressure drop. Severely clogged filters are one of the common reasons for excessive pressure drop in the desiccant dryer.

(2) System operating conditions

During the operation of the desiccant dryers, changes in its working state will also directly affect the pressure drop.

Flow rate and flow rate: Flow rate is one of the most important factors affecting pressure drop. According to the principles of fluid dynamics, the pressure loss of a fluid when it passes through a pipeline or equipment is roughly proportional to the square of the flow rate.

Inlet pressure: The inlet pressure of the desiccant dryer will also affect the density and flow rate of the gas.

Inlet pressure is lower than the design pressure: At the same flow rate, a decrease in inlet pressure means a decrease in gas density and an increase in volume flow, which leads to an increase in gas flow rate, thereby increasing fluid resistance and causing a relatively higher pressure drop. At the same time, too low an inlet pressure will affect the adsorption capacity and regeneration effect of the adsorbent, indirectly leading to an increase in pressure drop.

Inlet pressure is higher than the design pressure: An increase in inlet pressure will increase gas density, reduce volume flow, and reduce linear velocity, which may reduce pressure drop. However, excessive inlet pressure may also affect equipment strength and safety.

Adsorbent regeneration condition: The regeneration process of the desiccant dryer is a key link in removing moisture from the adsorbent and restoring its adsorption capacity. The quality of the regeneration effect directly affects the adsorption performance and bed structure of the adsorbent.

Incomplete regeneration: If the regeneration temperature is insufficient, the regeneration time is insufficient, or the regeneration gas volume is insufficient, the moisture in the adsorbent will not be completely desorbed. Adsorbents with high water content not only have reduced adsorption capacity, but also may change their pore structure, resulting in increased gas flow resistance and increased pressure drop. In addition, adsorbents that are not completely regenerated will quickly reach saturation in the next adsorption cycle, requiring frequent switching, which may also cause pressure drop fluctuations.

Regeneration system failure: Failure of the regeneration heater, blockage or leakage of the regeneration gas valve, blockage of the regeneration gas pipeline, etc. will affect the regeneration effect, resulting in incomplete regeneration of the adsorbent and increased pressure drop.

(3) Pipeline design and equipment layout

The pipeline design and equipment layout connecting the dryer and various parts of the compressed air system have a significant impact on the pressure drop of the overall system.

Pipe diameter selection: Too small a pipe diameter is a common cause of excessive system pressure drop. The selection of a suitable pipe diameter should take into account flow rate, flow rate, allowable pressure loss and economy. The gas flow rate in the pipeline should not be too high to avoid excessive friction resistance.

Pipeline elbows, valves and joints: Elbows, valves and various joints in the pipeline will generate local resistance and increase pressure drop. The number of elbows should be minimized, valve types with low flow resistance should be selected, and the pipeline layout should be optimized to make it as straight and short as possible.

Equipment layout and height difference: The installation position of the desiccant dryer and the height difference with the compressor, gas storage tank, and terminal gas point will also have a slight impact on the pressure drop, especially in long-distance transmission and systems with large height differences. Reasonable planning of equipment layout, shortening of pipeline distance, and reduction of elbows will help reduce the overall pressure drop.

Pipeline inner wall condition: The inner wall of the long-term used pipeline may accumulate rust, scale or oil, increase the roughness of the inner wall, thereby increasing friction resistance and increasing pressure drop. Regular inspection and cleaning of the pipeline is necessary.

(4) Maintenance

The daily maintenance status of equipment and systems is directly related to their operating performance and pressure drop level.

Filter maintenance: Regularly check and replace the filter elements of the inlet and outlet filters to prevent blockage and increase in pressure drop.

Adsorbent inspection and replacement: Regularly check the condition of the adsorbent, such as color, hardness, and whether it is agglomerated or powdered. Replace inferior adsorbents in a timely manner according to the usage time, operating conditions and inspection results.

Valve and pipeline inspection: Check whether all valves in the dryer system are working properly and whether there are leaks or blockages. Check whether there are leaks at the pipe connections.

Instrument calibration: Regularly calibrate pressure gauges, thermometers and other instruments to ensure the accuracy of monitoring data and detect abnormal pressure drops in a timely manner.

Equipment cleaning: Clean the outside and inside of the equipment to prevent dust and oil accumulation from affecting heat dissipation and component life.

Without effective maintenance, equipment performance will gradually decline, pressure drop will gradually increase, and may even cause equipment failure.

3.Hazards and manifestations caused by excessive pressure drop

Excessive pressure drop in the dryer is not only a signal of abnormal system operation, but also brings a series of hazards.

(1) Increased energy consumption

This is the most direct and important hazard of excessive pressure drop. In order to overcome higher pressure drop, the compressor needs to output greater pressure, which means that the compressor needs to consume more electricity. It is estimated that every 0.01 MPa increase in pressure drop in the compressed air system may lead to an increase in energy consumption of about 1%. Continuous high pressure drop will significantly increase the company’s operating costs.

(2) Shortened equipment life

Increased compressor load: In order to maintain the required gas pressure downstream, the compressor needs to continue to operate at a high load or even overload state, which will accelerate the wear of the compressor and shorten its service life.

Wear of internal components of the dryer: When high-speed airflow passes through narrow channels or obstructed areas, it will generate higher shear and impact forces, accelerating the pulverization of the adsorbent and the wear of the equipment.

Frequent valve movement and wear: In order to maintain the drying effect, if the adsorbent is not completely regenerated and frequent switching occurs, the number of valve movements will increase, accelerating its wear and leakage, further affecting the pressure drop and drying effect.

(3) Insufficient system compressed air supply

When the pressure drop of the dryer is too high, even if the compressor can operate normally, the compressed air pressure reaching the end point will drop significantly, which may be lower than the minimum working pressure required by the equipment. This will cause the pneumatic equipment to fail to work properly, the production process to be interrupted or the efficiency to decrease. In situations where there are strict requirements on the gas pressure, excessive pressure drop may cause the entire production line to shut down.

(4) Increased maintenance costs

Excessive pressure drop is often accompanied by an increase in equipment operating abnormalities and failures.

Frequent troubleshooting and repairs: More time and effort is required for fault diagnosis and repair.

Increased frequency of parts replacement: Filters, adsorbents, valves and other parts need to be replaced more frequently due to increased wear.

Unexpected production stoppage losses: Unexpected production stoppages caused by equipment failures can bring huge economic losses, including production interruptions, delivery delays, customer complaints, etc.

Summary

The pressure drop of the dryer is a key indicator that affects the energy efficiency, equipment life and operational stability of the compressed air system. Excessive pressure drop not only leads to energy waste and increased costs, but may also affect downstream production processes.