How does the pressure dew point of the refrigerated air dryer affect the operation of the air compressor?

In modern industrial production, compressed air is widely used in various fields as an important power source and process gas. However, untreated compressed air often contains a large amount of impurities such as moisture, oil and solid particles. These impurities will not only corrode equipment and block pipelines, but also affect product quality and even cause production accidents. Therefore, it is very important to purify compressed air. Among many purification equipment, refrigerated air dryer have become a key component of compressed air treatment systems with their efficient and stable dehumidification performance. One of the core performance indicators of refrigerated air dryer is pressure dew point. This article will explore in depth the definition of refrigerated air dryer pressure dew point, its working principle, and how pressure dew point directly or indirectly affects the normal operation of air compressors, and put forward suggestions for optimizing pressure dew point and jointly managing refrigerated air dryer and air compressors, in order to provide readers with a comprehensive understanding and guidance.

1.What is a refrigerated air dryerand its working principle

A refrigerated air dryer, as the name implies, is a device that removes moisture from compressed air by lowering its temperature. Its basic working principle is to use a refrigeration system to cool compressed air to below its pressure dew point, condense water vapor in the air into liquid water, and then separate and discharge the liquid water through a gas-water separator.

The main components of the refrigerated air dryer include:

Heat exchanger: used to exchange heat between high-temperature and high-humidity compressed air and low-temperature and dry compressed air, precool the incoming compressed air, and reheat the dried air.

Evaporator: the core component of the refrigerated air dryer, where the refrigerant evaporates and absorbs heat, cools the compressed air to the dew point temperature, and condenses the water vapor.

Condenser: The refrigerant condenses and releases heat in it, dissipating the heat to the environment.

Compressor: the power source that drives the refrigerant cycle.

Expansion valve/capillary tube: throttle and reduce the pressure of the refrigerant to make it evaporate in the evaporator.

Gas-water separator: separates and discharges the liquid water formed by condensation.

Automatic drain valve: automatically discharges the separated water at a fixed time or according to the liquid level.

The working process of the refrigerated air dryer is as follows: the high-temperature and high-humidity compressed air first enters the precooler (heat exchanger), and exchanges heat with the low-temperature dry air cooled by the evaporator, and the temperature drops. Then, the pre-cooled compressed air enters the evaporator, where it is further cooled by the low-temperature refrigerant of the refrigeration system, and the temperature drops below the dew point. At this time, the water vapor in the air condenses into liquid water. The condensed water is separated in the air-water separator and discharged through the automatic drain valve. The dried low-temperature air passes through the pre-cooler (heat exchanger) again, exchanges heat with the incoming compressed air, and the temperature rises, and then leaves the dryer and enters the subsequent pipe network or equipment. During this process, the refrigeration system continues to operate, continuously circulates the refrigerant, maintains the low temperature of the evaporator, and thus realizes continuous dehumidification of the compressed air.

Depending on the refrigeration method, the dryer can be divided into air-cooled and water-cooled. The air-cooled dryer dissipates heat into the air through a fan, which is suitable for small dryers and occasions where the ambient temperature is not high. The water-cooled dryer takes away the heat through cooling water, which is suitable for large dryers and occasions with high requirements for ambient temperature.

In addition to the basic refrigeration cycle, modern refrigerated air dryer are usually equipped with intelligent control systems, pressure sensors, temperature sensors, etc., which can monitor the operating status in real time, perform fault diagnosis and alarm, and automatically adjust the operating parameters according to the actual working conditions to ensure the best dehumidification effect and energy efficiency.

2.Definition of pressure dew point

Before discussing the impact of pressure dew point on air compressors, we first need to clarify the definition of pressure dew point.

Dew point refers to the temperature at which water vapor in the air reaches saturation and begins to condense into liquid water under a certain pressure. Simply put, the lower the dew point temperature, the less water content in the air, and the drier the air.

Pressure dew point refers to the temperature at which water vapor in the air reaches saturation and begins to condense into liquid water under a specific working pressure. Unlike the atmospheric dew point, the pressure dew point is related to the actual working pressure of the compressed air. When the pressure of the compressed air increases, the amount of water vapor it can hold decreases, so its dew point temperature also increases. Conversely, when the pressure decreases, the dew point temperature decreases.

In air compression systems and compressed air applications, we are more concerned about the pressure dew point because it directly reflects the dryness of the compressed air at the actual working pressure. A lower pressure dew point means that the water vapor content in the air is very low at the actual working pressure and it is not easy to condense into liquid water. This is what we pursue because liquid water is the root cause of many problems.

Pressure dew point is usually expressed in degrees Celsius (℃) or degrees Fahrenheit (℉). For different application scenarios, the requirements for pressure dew point are also different. For example, in general industrial applications, the pressure dew point requirement may be +3℃ to +7℃. In some occasions with extremely high requirements for dryness, such as precision instruments, electronics industry, pharmaceutical industry, etc., a lower pressure dew point may be required, such as -40℃ or lower, which usually needs to be achieved through an adsorption dryer.

It should be noted that the refrigerated air dryer can usually only reduce the pressure dew point to close to the ambient temperature or refrigeration temperature, generally between +3℃ and +7℃. To obtain a lower pressure dew point, an adsorption dryer is required. The adsorption dryer achieves deep drying by adsorbing water vapor in the air with an adsorbent, and can achieve a pressure dew point of -40℃ or even lower.

Therefore, understanding the concept of pressure dew point and its relationship with pressure is crucial to evaluating the performance of the refrigerated air dryer and selecting appropriate compressed air treatment equipment.

3.The impact of pressure dew point on air compressors

Although the refrigerated air dryer is located downstream of the air compressor and post-processes the compressed air, the pressure dew point of the refrigerated air dryer has a direct or indirect impact on the operation of the air compressor. This impact is mainly reflected in the following aspects:

Impact on the air quality at the inlet of the air compressor: Although the refrigerated air dryer processes compressed air, the downstream pressure dew point is too high, which means that the possibility of water condensation increases. This water may flow along the pipeline network and even flow back to the air inlet of the air compressor, affecting the performance of the air inlet filter and the air inlet valve, resulting in filter element blockage, valve rust and other problems. Although this situation is relatively rare, it may occur under some special working conditions.

Impact on the lubrication system of air compressors: The lubricating oil of air compressors, especially oil-lubricated screw compressors, will come into contact with compressed air during operation. If the pressure dew point of the refrigerated air dryer is too high, it means that there is more water vapor in the compressed air. These water vapors may mix with the lubricating oil to form emulsions under high temperature and high pressure. Emulsification of lubricating oil will significantly reduce its lubrication and cooling performance, resulting in increased wear of moving parts, shortened bearing life, and even high-temperature shutdown and other faults. In addition, emulsified oil may also block the oil circuit and filter, further affecting the normal operation of the compressor.

Corrosion effect on the internal parts of the air compressor: The condensed water in the compressed air is corrosive, especially when the air contains acidic or alkaline gases, the condensed water will form a corrosive solution. These solutions will corrode the metal parts inside the air compressor, such as rotors, bearings, valves, pipes, etc. Long-term corrosion will lead to component damage, performance degradation, and even the need to replace expensive parts, increasing maintenance costs.

Impact on the energy consumption of air compressors: The pressure dew point of the refrigerated air dryer is too high, which means that its dehumidification effect is not good and more energy is required to operate to achieve the required dryness. Although this may seem to be an energy consumption problem of the dryer itself, the operating status of the dryer is closely related to the air compressor. If the dryer fails to meet the compressed air dryness requirements of the downstream process due to pressure dew point problems, it may cause the entire compressed air system to start and stop more frequently or operate at higher pressures, which indirectly increases the energy consumption of the air compressor. In addition, corrosion and wear caused by moisture will reduce the efficiency of the air compressor and increase energy consumption in the long run.

Impact on the maintenance cycle of the air compressor: Higher pressure dew points will cause increased corrosion and wear of the internal components of the air compressor, thereby shortening its maintenance cycle. More frequent maintenance not only increases downtime, but also increases maintenance costs. For example, wearing parts such as lubricants, filters and valves need to be replaced more frequently.

Impact on the performance of the air compressor: Moisture in the compressed air will reduce its density and volumetric efficiency, thereby affecting the actual output volume of the air compressor. Although this effect is relatively small, it may still have an impact in applications with high air volume requirements.

4.Consequences of too high pressure dew point

A high pressure dew point means that the moisture content in the compressed air exceeds the standard and cannot be effectively removed by the dryer. This not only affects the normal operation of downstream gas-using equipment, but also brings a series of serious consequences to the entire compressed air system, including the air compressor itself. These consequences are mainly reflected in the following aspects:

Failure and damage of downstream gas-using equipment: This is the most direct and common consequence of too high pressure dew point. Many pneumatic components, instruments, and equipment have strict requirements on the dryness of compressed air. If there is a lot of moisture in the compressed air, it will cause:

Failure of pneumatic valves and cylinders: Moisture will corrode the metal parts inside the valve, causing jamming, leakage or inflexible movement. In low temperature environments, moisture will even freeze and completely block the valve or cylinder.

Instrument failure: Precision instruments have high requirements for the purity and dryness of compressed air. Moisture can cause corrosion and blockage of internal components of the instrument, affecting measurement accuracy and stability.

Spraying equipment problems: In spraying applications, moisture in compressed air can cause defects such as bubbles, orange peel, and flow marks on the paint surface, seriously affecting the appearance quality of the product.

Dryer failure: Some process flows require additional drying equipment. If the pressure dew point of the front-stage dryer is too high, it will increase the burden on the subsequent drying equipment and even cause it to fail.

Pipeline corrosion and blockage: Condensed moisture will corrode the inner wall of the pipeline, causing rust. Rust residue and moisture mixed together may form a paste, block the pipeline, and affect the smoothness of air flow.

5.How to optimize the pressure dew point to ensure the normal operation of the air compressor

Optimizing the pressure dew point of the dryer and keeping it within a reasonable range is the key to ensuring the normal operation of the air compressor and the entire compressed air system. The following are some strategies and measures to optimize the pressure dew point:

5.1 Correct selection of dryer:

Select the appropriate processing capacity according to the actual gas consumption and pressure: The processing capacity of the dryer should be slightly larger than the maximum gas output of the air compressor. If the dryer processing capacity is insufficient, the compressed air will stay in it for too short a time, the dehumidification will be insufficient, and the pressure dew point will increase.

Select the appropriate type of dryer according to the ambient temperature and gas requirements: In areas with high ambient temperatures, water-cooled dryers may need to be selected to ensure good cooling effects. For occasions with extremely high requirements for dryness, it is necessary to choose an adsorption dryer instead of relying solely on a refrigerated air dryer.

Consider the inlet temperature and pressure: The performance of the refrigerated air dryer is affected by the inlet temperature and pressure. When selecting, the maximum temperature and actual working pressure of the air compressor outlet should be considered, and a refrigerated air dryer that can cope with these working conditions should be selected.

5.2Ensure the normal operation and maintenance of the refrigerated air dryer:

Regularly check and maintain the refrigeration system: including checking whether the refrigerant is sufficient, whether the refrigeration compressor is operating normally, whether the condenser and evaporator are clean, etc. Poor refrigeration effect will lead to an increase in dew point.

Clean the heat exchanger regularly: Dust or dirt on the surface of the heat exchanger will affect the heat exchange efficiency, resulting in poor pre-cooling and reheating effects, affecting the dew point.

Regularly check and maintain the gas-water separator and automatic drain valve: Poor performance of the gas-water separator or failure of the automatic drain valve will cause the condensed water to be unable to be discharged in time, accumulate inside the refrigerated air dryer, and affect the dehumidification effect. The automatic drain valve should be checked regularly to see if it is draining normally, and cleaned or replaced if necessary.

Replace filters regularly: refrigerated air dryer are usually equipped with pre-filters to remove solid particles and oil from compressed air. Blockage of the filter will increase the pressure drop and affect the handling capacity of the refrigerated air dryer. The filter element should be replaced regularly according to the manufacturer’s recommendations.

5.3Optimize the working conditions of the air compressor:

Reduce the outlet temperature of the air compressor: Higher intake air temperature will increase the load of the refrigerated air dryer and affect the dehumidification effect. Ensure the normal operation of the air compressor cooling system and reduce the outlet temperature to help the refrigerated air dryer work more efficiently.

Ensure that the air compressor works within the rated pressure range: When the air compressor operates at the rated pressure, it can ensure that its gas output is stable, which is conducive to the stable operation of the refrigerated air dryer.

Reduce leakage of the compressed air pipeline network: Leakage will cause the air compressor to start and stop frequently or run for a long time, increase the load fluctuation of the refrigerated air dryer, and affect its dehumidification effect. Regularly check and repair pipeline leakage to help stabilize the system operation and reduce the load of the refrigerated air dryer.

Install a post-filter after the refrigerated air dryer: Install a high-efficiency post-filter after the refrigerated air dryer to further remove the tiny moisture, oil and solid particles that may remain in the compressed air, improve the quality of compressed air, and provide cleaner gas for downstream equipment.

Monitor the pressure dew point: Install a dew point meter at the outlet of the refrigerated air dryer to monitor the pressure dew point in real time. If the pressure dew point is detected to be exceeded, the operating status of the refrigerated air dryer should be checked in time, the cause should be found and handled.

6.Joint management of refrigerated air dryerand air compressors

refrigerated air dryer and air compressors are two important components of the compressed air system. There is a close connection between them and they affect each other. Therefore, it is crucial to jointly manage them instead of treating them in isolation to ensure the optimized operation of the entire system. Joint management can be started from the following aspects:

System overall design and planning: In the system design stage, the air compressor and refrigerated air dryer should be considered as a whole. According to factors such as actual gas consumption, gas quality requirements, and environmental conditions, the model and quantity of air compressors, as well as the type and processing capacity of refrigerated air dryer should be reasonably selected. Consider the matching between the two. For example, the maximum inlet temperature and pressure of the refrigerated air dryer should match the outlet conditions of the air compressor. Reserve a certain margin to cope with possible future gas consumption growth or changes in working conditions.

Linkage control and optimized operation: Modern compressed air systems are usually equipped with intelligent control systems. The air compressor and the refrigerated air dryer can be included in the same control platform to achieve linkage control and optimized operation. For example:

Adjust the start and stop and operating parameters of the compressor and the refrigerated air dryer according to the gas consumption: When the gas consumption decreases, the loading/unloading state or variable frequency speed regulation of the compressor can be adjusted according to actual needs, and the operation mode of the refrigerated air dryer can be controlled in linkage to reduce energy consumption.

Monitor the operating status of the refrigerated air dryer and feedback to the compressor control system: For example, when the refrigerated air dryer fails or the pressure dew point exceeds the standard, the information can be fed back to the compressor control system to trigger an alarm or take corresponding measures, such as reducing the compressor load or shutting down, to avoid the problem from expanding.

Optimize the coordination of compressors and refrigerated air dryer according to the network pressure and gas demand: Through real-time monitoring of the entire system, the intelligent control system can adjust the loading order, unloading pressure and other parameters of the compressor, and optimize the operation of the refrigerated air dryer to maintain a stable network pressure and meet the gas demand.

Unified maintenance and management strategy: Develop a unified maintenance plan to coordinate the maintenance of air compressors and refrigerated air dryer. For example, you can arrange downtime maintenance at the same time to reduce the number of production downtimes. Establish a unified equipment file to record the operating data, maintenance records, troubleshooting and other information of both, and provide a basis for subsequent fault diagnosis and performance evaluation.

Energy efficiency evaluation and optimization: Evaluate the energy consumption of air compressors and refrigerated air dryer as a whole. Analyze the energy consumption data of the two to find out the energy saving potential. For example, by optimizing linkage control, reducing system leakage, and improving equipment operation efficiency, reduce the energy consumption of the entire compressed air system.

Collaboration of fault diagnosis and troubleshooting: When there is a problem with the compressed air system, fault diagnosis should be carried out from the perspective of the system as a whole. For example, when it is found that the compressed air is not dry enough, the possible cause is not only the failure of the dryer, but also the high outlet temperature of the air compressor, leakage of the pipe network and other problems. Through joint diagnosis, the root cause of the problem can be found and solved more quickly and accurately.

Summary

As a key post-processing equipment in the compressed air system, the pressure dew point of the dryer directly determines the dryness of the compressed air. A lower pressure dew point means less moisture in the compressed air, which can effectively prevent the failure of downstream gas-using equipment, the decline of product quality, the increase of maintenance costs and the occurrence of safety hazards.