Welcome to the first installment of Compressor University. The mission of this column is to educate readers in a practical manner about industrial compressor technology with an emphasis on efficiency, reliability and safety.
Pumps Versus Compressors
Some people say that compressors are simply pumps that pump gas. This may be true to some extent, but significant differences exist between them. First, let's explore the similarities. They both have positive displacement and dynamic designs capable of satisfying a broad range of flow and head requirements. Reciprocating pumps and compressors are both used for low-flow, high-head applications; centrifugal pumps and compressors are used for high flow, low-head applications and screw pumps and compressors are used for mid-flow, mid-head applications. As with pumps, multistaging can be used with any of these compressor designs whenever higher heads are required. Both pumps and compressors use seals, bearings, lubrications systems and some type of driver. This is where the similarities end.
The T Word (Thermodynamics)
The key difference between pumps and compressors is related to the thermodynamic nature of gases. Because liquids are incompressible, their behavior can be explained with the relatively simple Bernoulli's equation, in which liquid density is assumed constant throughout the flowstream.
Due to the compressibility of gas, compressor performance is much more complex than moving liquids. The head realized in dynamic compressors is a function of the gas properties, i.e. density, molecular weight, specific heat ratio, etc., at each impeller's inlet. Another significant difference is that energy is stored in gases by increasing pressure and physically compressing the gas molecules. Furthermore, as gas mixtures are compressed, liquid fractions can fall out, depending on the compression ratio and the actual composition of the gas.
An interesting byproduct of compression is that gases usually increase in heat when compressed and cool when expanded. (The heat produced when gas is compressed is called the heat of compression.) The effect is commonly observed in spray cans (like deodorant or paint), but is also used in refrigerators, air conditioners and the liquefaction of gases. At ordinary temperatures and pressures, all real gases except hydrogen and helium heat up when compressed. British physicists James Joule and William Thomson investigated this phenomenon.
To reduce the temperature of the gas as it moves to the next stage of compression and improve efficiency, interstage cooling is often required. This requires a heat exchanger along with a cooling medium of either water or air. A possible byproduct of interstage cooling is liquid. Remember: Liquids can kill a compressor! For this reason, most compressors require knockout drums in the suction piping and between stages. Excessive liquid ingestion in centrifugal compressors can lead to impeller erosion, overloading of the driver, and even bearing failure. In reciprocating compressors, liquid carryover will invariably lead to immediate and certain internal damage due to tight internal clearances.
Compressors tend to be more critical than pumps because they are typically unspared since they are more expensive than pumps. Their high cost is related to their complexity and higher horsepower ratings. In process settings, unspared compressors are usually one of the most critical elements in the process. They can cause machinery engineers to lose sleep.
Since compressors are typically unspared, their reliability is paramount. Process compressors are required to operate continuously for up to 5 years between repairs. To accomplish this goal, compressors require highly engineered ancillary components, such as lubrication subsystems, seals and bearings. Additional protection in the form of surge control for centrifugal compressors and vibration sensors are usually integrated into high speed machine installations.
Compressors can be hazardous to your health. Compressed gas is a form of potential energy that must always be respected. Combining this risk with the fire risk related to flammable gases results in a potentially lethal energy combination. For this reason, the design, selection and assembly of compressors require great skill, knowledge and experience.
Bottom line, compressors are not simply pumps that pump gas.
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Pumps & Systems, July 2008