Why is the compression ratio calculated using absolute pressure?
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This post was last edited by A'Dai A Mu on 2012-1-9 at 22:41. The compression ratio ε = (exit gauge pressure P1 + 0.1) / (inlet gauge pressure P0 + 0.1). Why?
This is a math problem, not an engineering problem. The ratio theorem from mathematics is applied; for example, (2+3)/(2+4) is not equal to 3/4. Here, 2 above and below the semicolon represents atmospheric pressure, 3 represents your inlet pressure, and 4 represents your outlet pressure. Therefore, the compression ratio should also use the ratio of absolute pressures.
To use an analogy, how is the compression ratio calculated when the import pressure is zero? Considering this, absolute pressure should also be used.
The two people upstairs should not mislead others; take a look at the basic principles of compressors: the first and second laws of thermodynamics. 1) The first law of thermodynamics states that in any process (such as a compression process), energy cannot be created nor destroyed; it can only change from one form to another. 2) The second law of thermodynamics can be expressed in several ways: a) Heat cannot spontaneously transfer from a cold object to a hot object ； b) Heat can only flow from a colder object to a hotter object when work is applied. c) In a real process, the useful energy of an isolated system always decreases. d) Heat or energy (or water) always flows from high to low. 3) Definition of entropy: Energy exists at different levels, and it can be utilized only when it moves from a higher level to a lower one. Entropy is used to represent the unavailability of energy. The system releases heat, and entropy increases ； The system absorbs heat, and entropy decreases ； The system neither absorbs nor releases heat, so its entropy remains constant. Compression process 1) Isothermal process PV=C: It is necessary to remove the heat generated during compression, which is impossible. 2) In the adiabatic process PVk=C, no heat is absorbed nor released; it is a reversible process. It is also impossible because there is always heat absorption or release during the process. 3) Isentropic process: S = constant 4) Multi-variable process: PV^n = constant. This applies to real processes; note that n ≠ k. 3.2.3 Ideal gas law: PV = MRT (for ideal gases); PV = ZMRT (for real gases). Z (the compressibility factor) indicates the degree to which a real gas deviates from an ideal gas.
What was said upstairs is right – principle requirements! I work on the design of centrifugal compressors; let’s discuss more issues in the future!
From the perspective of the principle of compressor compression ratio, or in engineering terms, the compression ratio is determined based on the absolute pressure conditions.
I read the following in a handbook for water treatment engineers: Liquid ring compressors usually use single-stage compression (pressure ratio