Views: 16736 Author: Site Editor Publish Time: 2025-03-01 Origin: Site
1. What is a Compressor?
A compressor is a machine that compresses gas to increase its pressure or transport gas. It is also called an air compressor or blower.
Various types of compressors are classified as power machinery, capable of reducing gas volume, increasing pressure, and imparting a certain amount of kinetic energy, which can be used as mechanical power or for other purposes. Depending on the gas being compressed, they are referred to as air compressors, oxygen compressors, ammonia compressors, gas compressors, and so on.
2. What Are Compressors Used For?
With the rapid development of the national economy, compressors are widely used in industry. Due to their extensive applications, compressors are known as “general-purpose machinery.” Based on the different purposes of compressed gas, their applications can be classified into the following categories:
1. Compressed Air as Power:
Used to drive various pneumatic machinery and tools. Exhaust pressure ranges from 7–8 kgf/cm² for pneumatic tools, about 6 kgf/cm² for control instruments and automation devices, 2–4 kgf/cm² for vehicle braking and door/window operation, 4 kgf/cm² for mixing in pharmaceutical and brewing industries, 1–2 kgf/cm² for weft yarn blowing in air-jet looms, 25–60 kgf/cm² for starting medium and large diesel engines, 150 kgf/cm² for oil well fracturing, about 50 kgf/cm² for “secondary recovery” in oil extraction, about 800 kgf/cm² for high-pressure blasting in coal mining, and various pressure levels for defense applications such as submarine diving, torpedo launching, and shipwreck salvage.
2. Compressed Gas for Refrigeration and Gas Separation:
Gas is compressed, cooled, and expanded to achieve liquefaction for artificial refrigeration (e.g., freezing, refrigeration, and air conditioning), as seen in ammonia or Freon compressors. Compression pressures typically range from 8–12 kgf/cm². Such compressors are often called “refrigeration machines” or “ice machines.” Additionally, if the liquefied gas is a mixture, its components can be separated in a separation device to obtain high-purity gases. For example, liquefied air can be separated to obtain pure oxygen, pure nitrogen, and rare gases such as xenon, krypton, argon, and helium.
3. Compressed Gas for Synthesis and Polymerization:
In the chemical industry, compressing gas to high pressure often facilitates synthesis and polymerization reactions. Examples include ammonia synthesis from nitrogen and hydrogen, methanol synthesis from hydrogen and carbon dioxide, and urea synthesis from carbon dioxide and ammonia. In the chemical industry, the pressure for producing high-pressure polyethylene can reach 1500–3200 kgf/cm².
4. Compressed Gas for Hydrogenation and Refining of Oils:
In the petroleum industry, hydrogen is artificially heated, pressurized, and reacted with oil to crack heavy hydrocarbon fractions into lighter ones, such as in heavy oil hydrocracking and lubricant hydrofinishing.
5. Gas Transportation:
Compressors used for pipeline gas transportation vary in pressure depending on pipeline length. For long-distance gas transmission, pressures can reach 30 kgf/cm². Chlorine cylinder filling requires 10–15 kgf/cm², while carbon dioxide cylinder filling requires 50–60 kgf/cm².
3. How Are Compressors Classified?
Compressors are classified based on their structural forms as follows:
According to their operating principles, they can be divided into:
Reciprocating (piston) compressors
Rotary compressors (including turbo, liquid-ring, and centrifugal types)
Axial flow compressors
Jet compressors
Screw compressors
Among these, the reciprocating (piston) compressor is the most widely used.
4. How Are Piston Compressors Classified?
Piston compressors can be classified in various ways, often with different names. Common classification methods include:
(1) Based on Cylinder Position (Cylinder Centerline):
Horizontal compressors: Cylinders are arranged horizontally.
Vertical compressors: Cylinders are arranged vertically.
Angular compressors: Cylinders are arranged at angles such as L-type, V-type, W-type, and star-type.
(2) Based on the Number of Compression Stages:
Single-stage compressors: Gas is compressed once in the cylinder.
Two-stage compressors: Gas is compressed twice in the cylinder.
Multi-stage compressors: Gas is compressed multiple times in the cylinder.
(3) Based on Cylinder Arrangement:
In-line compressors: Multiple cylinders are arranged sequentially on the same shaft (also called single-row compressors).
Parallel compressors: Multiple cylinders are arranged in parallel on several shafts (also called double-row or multi-row compressors).
Compound compressors: Combines in-line and parallel arrangements for multi-stage compression.
Horizontally opposed (balanced) compressors: Cylinders are arranged horizontally on both sides of the crankshaft with 180° opposed crankpins, forming an H-type configuration to balance inertia forces (common in large compressors).
(4) Based on Piston Compression Action:
Single-acting compressors: Gas is compressed only on one side of the piston (also called single-motion compressors).
Double-acting compressors: Gas is compressed on both sides of the piston (also called double-motion or multi-motion compressors).
Multi-cylinder single-acting compressors: Multiple cylinders compress gas on one side of the piston.
Multi-cylinder double-acting compressors: Multiple cylinders compress gas on both sides of the piston.
(5) Based on Final Discharge Pressure:
Low-pressure compressors: Final discharge pressure is 3–10 gauge pressure.
Medium-pressure compressors: Final discharge pressure is 10–100 gauge pressure.
High-pressure compressors: Final discharge pressure is 100–1000 gauge pressure.
Ultra-high-pressure compressors: Final discharge pressure exceeds 1000 gauge pressure.
(6) Based on Discharge Volume (Capacity):
Micro compressors: Discharge volume is less than 1 m³/min.
Small compressors: Discharge volume is 1–10 m³/min.
Medium compressors: Discharge volume is 10–100 m³/min.
Large compressors: Discharge volume exceeds 100 m³/min.
(7) Based on Rotational Speed:
Low-speed compressors: Speed is below 200 rpm.
Medium-speed compressors: Speed is 200–450 rpm.
High-speed compressors: Speed is 450–1000 rpm.
(8) Based on Drive Type:
Electric compressors: Driven by electric motors.
Steam-driven compressors: Driven by steam engines.
Internal combustion engine-driven compressors.
Turbine-driven compressors.
(9) Based on Cooling Method:
Water-cooled compressors: Use circulating water to remove heat generated during compression.
Air-cooled compressors: Use air and fins to dissipate heat generated during compression.
(10) Based on Transmission Method Between Power Source and Compressor:
Direct-coupled compressors with rigid couplings.
Direct-coupled compressors with flexible couplings.
Gear-reduction-driven compressors.
Belt-driven compressors (flat or V-belts).
Free-piston compressors (without crankshaft-connecting rod mechanisms).
Integral compressors: Power engine cylinders and compressor block are cast as one unit with a shared crankshaft.
Additionally, compressors can be classified as fixed or mobile, and with or without crossheads.
5. What Are Booster Compressors and Recycle Compressors?
In general chemical processes, high-pressure compressors are sometimes needed to further compress gases with initial pressures several times atmospheric pressure, thereby increasing their pressure even more. These are called booster compressors.
Recycle compressors are also a type of booster compressor, often called recycle pumps. Their function is to increase the pressure of gases with inlet pressures of 50–1000 gauge pressure by an additional 10–50 gauge pressure to overcome system resistance and compensate for pressure drops in the circulation system. Their characteristics include operating at high pressures with small compression ratios and low discharge temperatures, often without cooling water jackets.
6. What is Atmospheric Pressure?
The layer of air surrounding the Earth is called the atmosphere. Due to gravity, air envelops the entire planet. The weight of air exerts pressure on objects, referred to as atmospheric pressure. Air consists of extremely small gas molecules with volume and weight. At a pressure of one atmosphere and a temperature of 273 K, the number of molecules in 1 cm³ of any gas is 2.683 × 10¹⁹.
We do not feel atmospheric pressure because our bodies are surrounded by air, balancing internal and external pressures. Atmospheric pressure exerts approximately 1 kgf per square centimeter, which we define as one standard atmosphere.
7. What is Gauge Pressure?
The gas pressure indicated on a typical pressure gauge does not represent the true gas pressure but rather the pressure exceeding atmospheric pressure. In other words, atmospheric pressure is not included. Gauge pressure is measured with atmospheric pressure as the zero reference point. It is also called indicated pressure or gauge pressure, abbreviated as “gauge pressure.”
Gauge Pressure = Absolute Pressure – Atmospheric Pressure
8. What is Absolute Pressure?
Absolute pressure is gauge pressure plus atmospheric pressure. It is measured with a perfect vacuum as the zero reference point.
Absolute Pressure = Gauge Pressure + Atmospheric Pressure
Absolute Pressure = Atmospheric Pressure – Vacuum Pressure
In calculations, absolute pressure is denoted as P.
9. What is Vacuum?
When the gas pressure inside a container is lower than atmospheric pressure, a vacuum is created, also called negative pressure. A space completely devoid of matter (100% vacuum) is referred to as an absolute vacuum, which is difficult to achieve in practice.
Typically, 760 mm Hg (at 0°C) is used as the standard reference. If the indicated pressure of a container is lower than atmospheric pressure, the reading is called the vacuum degree. The vacuum degree indicates the pressure difference by which the container’s gas pressure is lower than atmospheric pressure, also called vacuum pressure or low pressure.
The lower the atmospheric pressure inside the container, the higher the vacuum degree. Conversely, the higher the atmospheric pressure inside the container (not exceeding 1 atmosphere), the lower the vacuum degree. If the gas pressure inside the container equals atmospheric pressure, the vacuum degree is zero, indicating no vacuum.
