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Chapter 2 - Control and Operation of Reciprocating Gas Compressors

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Emily Wright

on 3 May 2013

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Transcript of Chapter 2 - Control and Operation of Reciprocating Gas Compressors

Compression Principles Chapter 2 V= constant
P/T = constant
Amonton's Law Adiabatic Compression Adiabatic compression: No heat added or removed during the compression process. Follows the formula: This is never achieved because there is always heat added or removed Adiabatic Process Q=0
No heat transfer between gas and surroundings Example of Adiabatic Process Gas held in compression on left
Weight removed and gas expands slowly
No friction, perfect lubrication
Piston and cylinder constructed from a perfect insulator
No heat transfer between gas and its surroundings Polytropic Compression Isothermal compression and adiabatic compression are never obtained and actual compression follows the polytropic compression formula: n is not the same as k (ratio of specific heats) and should not be used as such Polytropic Process The real world! Isentropic Process Constant entropy (s=contact)
Reversible adiabatic or constant entropy Compression Cycles Double Acting Double Acting Compressor Head End Performance Crank End Performance Adiabatic Compression (Temperature Calculation) To estimate the discharge temp of a compressor the following formula may be used (good for recips, not centrifs) 1= suction, 2= discharge
NB: API 618 limits discharge temp to 150C or 135C if a non-lubricated cylinder is used Typical Cp/Cv Data Discharge Temperature Calculations Question:
A compressor is compressing gas at 40C from 200kPa to 650kPa in a single stage.
Calculate the compressor discharge temperature for the following gases:
Natural gas (Mw=15)
Natural gas (Mw=25)
Ethylene Volumetric Efficiency The volumetric efficiency (Ev) of a reciprocating compressor is the ratio of the volume of gas delivered by the compressor (Vs) to the piston displacement (Vpd) The volumetric efficiency is always less than one or 100% Clearance The clearance volume (Vc) is the volume remaining in the cylinder when the piston has reached the end of its discharge stroke It is usually expressed as a percentage as in the range 15-25% but can be higher or lower Volumetric Efficiency (2) A more accurate equation for volumetric efficiency Ev (%) is: Where
Zs= suction gas compressibility
Zd= discharge gas compressibility
L is Rc/100 for compressor speeds up to 500rpm and Rc/50 for compressor speeds over 500rpm Volumetric Efficiency Changes Multi - Stage Limitations in the use of single compression stages:
Discharge temperature
Pressure differential
Effect of clearance
Power saving
Separate cylinder for each stage
Intercooling of gas between the stages Multi-Stage PV Diagram Multi-Stage Compression Ratio For minimum power with perfect intercooling, the compression ratio for each stage (rs) can be determined from the overall compression ratio (Rt) s is the number of stages Multi-Stage Compression Ratio For example: Two, three and four stage machines follow the equations Constant Volume Process Compression Principles Constant Volume Process Isothermal Process Adiabatic Process Isentropic Process Constant Pressure Process Polytropic Process Compression Cycles Constant Pressure Process P=constant
V/T= C
Charles' Law Isothermal Process Boyle's Law:
At constant temp the volume of an ideal gas varies inversely with the pressure Compressor Action PV Diagram Discharge (2-3) Expansion (3-4) Suction (4-1) Single Acting Compressor Stroke Single Acting Compressor Stroke PV Diagram Head End Head End Compression (1-2) Compression (1-2) Discharge (2-3) Expansion (3-4) Suction (4-1)
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