Core Operational Factors of Compressors: Temperature, Humidity, Cleanliness, and Key Structural Concepts

This article delves into the key factors affecting compressor performance and reliability: temperature, humidity, air cleanliness, clearance volume and its impacts, as well as methods to increase discharge capacity and the critical reasons for limiting discharge temperature. Understanding these elements is fundamental to ensuring the efficient and safe operation of compressors.

1. What is the relationship between temperature level and the compressor?

The degree of hotness or coldness of an object is called temperature. According to the law of conservation of energy, work and heat are interchangeable. The rise in temperature at various parts of a compressor is converted from mechanical friction work and compression work. For example, improper assembly or inadequate lubrication of bearing shells will increase friction work, which is dissipated as heat, causing the bearing shell temperature to rise and potentially even leading to bearing failure. Therefore, the quality and condition of the machine can be judged by monitoring the temperature levels at different points.

High and low ambient temperatures and oil temperatures affect the compressor in the following ways:

    1.  Excessively high intake gas temperature reduces the discharge capacity.

    2.  Excessively high gas temperature during compression increases power consumption and reduces productivity.

    3.  Excessively high cylinder temperature causes coking of lubricating oil in the valves and piston rings, losing its lubricating effect. This poses an explosion risk if exposed to sparks. It also leads to poor operation of components like piston rings, valves, and packing, increasing wear and compromising sealing.

    4.  Excessively high bearing temperature can burn out bearings or shells, potentially halting operation.

    5.  Overheating of other components reduces mechanical strength and can cause deformation.

    6.  Excessively high lubricating oil temperature reduces oil viscosity and oil pressure, impairing lubrication effectiveness.

    7.  Excessively high cooling water temperature reduces cooling efficiency.

    8.  Excessively high temperature in electric motors or internal combustion engines carries the risk of burnout.

However, temperature should not be too low either. If cooling water temperature falls below 0°C, it can freeze, disrupting circulation and potentially damaging the machine. Excessively low lubricating oil temperature increases viscosity excessively, hindering lubrication. Excessively low temperature also makes internal combustion engines difficult to start, etc.

Therefore, an important skill for compressor operators is to judge whether the machine is operating normally by observing temperature changes and to strictly control temperatures at all points within specified limits to ensure stable equipment operation.

2. What is the relationship between humidity level and the compressor?

Air humidity changes with its state. When air is compressed, its temperature rises and its relative humidity decreases. When compressed air expands, its temperature falls and its relative humidity increases, usually resulting in moisture condensation.

If the air contains excessive moisture, it adversely affects the compressor as follows:

    1.  Increases flow resistance: Moisture in the air narrows compressed air passages, increasing flow resistance.

    2.  Affects volumetric efficiency: Reduces the volumetric efficiency of the gas.

    3.  Causes hydraulic shock and equipment damage: Impedes the compression process, subjecting compression equipment and pneumatic tools to hydraulic shock. Accumulation of significant water in coolers and cylinders can cause machine damage incidents.

    4.  Corrodes equipment: Moisture in air is highly corrosive, causing compression equipment and pneumatic tools to rust more easily, shortening service life.

    5.  Impairs lubrication: Moisture in the gas mixes with lubricating oil during compression, reducing lubrication effectiveness and accelerating component wear. In packing using recirculating lubrication, it not only causes poor sealing but also degrades the lubricating oil.

    6.  Reduces weight-based production capacity: The weight per unit volume (one cubic meter) of moist air (i.e., gas molecule density) is less than that of dry air of the same volume. Furthermore, as compressed air passes through coolers, air receivers, and pipelines, most water vapor condenses and is removed, reducing the production capacity calculated by weight.

    7.  Causes pipeline freezing: If moisture is present in the air supply system and ambient temperature falls below 0°C, water will freeze on the inner walls of air pipes, similarly reducing pipe diameter. Worse, it can sometimes cause complete freezing of individual pipe sections, blocking work in specific areas.

Therefore, the quality of compressed air depends not only on its pressure but also closely on its humidity.

3. What is the relationship between cleanliness and the compressor?

Due to wind, air always contains varying degrees of dust and other impurities. Excessive sand and impurity content in the air is quite harmful to the compressor:

    1.  Accelerates wear: Sand particles are quite hard and will wear down cylinders, piston rings, piston rods, packing, and other components, shortening the machine's service life.

    2.  Forms carbon deposits and explosion risk: Dust entering the cylinder mixes with lubricating oil and forms coke deposits in valves and piston rings. This impedes mechanical lubrication and can cause cylinder scoring or bearing seizure. Moreover, under high compressor temperatures with abundant sand, it may create an explosion hazard.

    3.  Clogs systems, reduces air output: Sand and dust entering the machine easily clog valves, coolers, air pipelines, and pneumatic tools, causing leaks in the compression equipment and thereby reducing discharge capacity.

    4.  Increases energy consumption: Dust increases compressor wear, impairs lubrication, affects gas cooling, raises the final temperature of the compressed gas, and leads to a sharp increase in power consumption.

Therefore, before air or other gases enter the compressor, they must be filtered through equipment equipped with air filters to prevent dust and impurities from entering the cylinders, avoid rapid wear of relatively sliding parts, and also prevent oxidation of the lubricating oil.

4. What is the clearance volume in a compressor?

Due to requirements related to compressor structure, manufacturing, assembly, and operation, certain spaces or gaps must be left in the cylinder. This space or gap is called the clearance volume (also known as harmful volume or dead space).

Clearance volume exists in the following parts of a compressor:

    1.  The gap between the piston face and the cylinder head when the piston reaches the end of its exhaust stroke.

    2.  The annular gap between the cylinder liner surface and the piston outer diameter (from the face to the first piston ring).

    3.  The volume formed by the passage from the valve to the cylinder volume, and the volume inherent to the valve itself (such as valve seat passages, spring chambers, etc., with passage volume accounting for the largest proportion).

Some clearance volume in compressors is structurally necessary, while some is unavoidable. For example, the gap between the piston and cylinder head is primarily based on the following considerations:

    1.  Provision for thermal expansion: During periodic motion, the piston heats up and expands radially and axially due to friction and heat generated from compressing gas. Clearance volume is needed to prevent piston-cylinder head collision or piston seizing in the cylinder.

    2.  Prevention of water hammer: When compressing gas containing water droplets, moisture may accumulate. Clearance volume can prevent "water hammer" phenomena due to the incompressibility of water.

    3.  Accommodating manufacturing and assembly tolerances: There are always tolerances in component manufacturing precision and assembly. Moving parts may also loosen during operation, increasing joint clearances and overall component dimensions.

As for the clearance volume formed by valve passages, it is mainly unavoidable due to valve arrangement.

During compressor operation, the presence of clearance volume reduces the actual gas volume drawn in by the intake valve, correspondingly lowering the discharge capacity. Therefore, when designing cylinders, the impact of clearance volume on discharge capacity must be considered. In compressor design, while considering factors like productivity, manufacturing, assembly, and safe operation, the clearance volume should be minimized as much as possible. However, sometimes to adjust piston force, the clearance volume is correspondingly increased, which is a common practice in designing opposed-piston compressors.

5. How to increase the discharge capacity of a compressor?

Increasing the discharge capacity (air delivery) of a compressor, i.e., increasing its output coefficient, is typically achieved by the following methods:

    1.  Correctly selecting the size of the clearance volume.

    2.  Maintaining the tightness of piston rings.

    3.  Maintaining the tightness of valves and stuffing boxes.

    4.  Maintaining the sensitivity of intake and exhaust valves.

    5.  Reducing resistance during gas intake.

    6.  Drawing in drier and cooler gas.

    7.  Maintaining the tightness of discharge pipelines, valves, air receivers, and coolers.

    8.  Appropriately increasing the compressor speed.

    9.  Employing advanced cooling systems.

    10. Cleaning cylinders and other components when necessary.

6. Why is the discharge temperature strictly limited in compressors?

For compressors with lubrication, excessively high discharge temperature causes a series of serious problems:

    Reduces lubricating oil viscosity and degrades its performance.

    Causes rapid evaporation of light fractions in the lubricating oil and leads to "carbon deposit" formation.

Practice has proven that when discharge temperature exceeds 200°C, carbon deposits become quite severe. Carbon deposits lead to the following consequences:

    1.  Block passages in exhaust valve seats, spring seats (valve guards), and exhaust pipes, increasing passage resistance.

    2.  Cause piston rings to seize in piston ring grooves, losing their sealing function.

    3.  Under electrostatic action, carbon deposits may cause explosion incidents.

Therefore, discharge temperature must be strictly limited. It is generally specified that for power compressors, the discharge temperature must not exceed 160°C for water-cooled types and must not exceed 180°C for air-cooled types.

Conclusion

Temperature, humidity, cleanliness, and clearance volume are core parameters determining compressor efficiency, reliability, and lifespan. Precise control of intake and discharge temperatures, effective management of air humidity and cleanliness, and rational design with an awareness of clearance volume impact are key to optimizing compressor performance (such as increasing discharge capacity) and ensuring its long-term safe and stable operation. Operators and maintenance personnel must deeply understand the interaction of these factors and prioritize them in daily monitoring and maintenance.