Centrifugal and Reciprocating Compressors
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18-02-2011, 09:33 AM

.ppt   Centrifugal Compressors.ppt (Size: 388 KB / Downloads: 189)
Centrifugal and Reciprocating Compressors.
Centrifugal Compressors

Immense popularity due to large high capacity process plants and NG distribution systems requiring compressors of extreme reliability to handle large gas volumes.
Very long service life between overhauls; No surprise outages. Reasons – NO rubbing or reciprocating parts; few high clearance rotating parts only is the essence of the success story.
Thanks to high RPM > 5000 RPM, suitable for high-speed steam turbines powered by process waste heat steam generators. Reliability builder and cost saver
Gas turbines and electric motors via gear enhancers.
CC construction Details
Casing contains all internals – Rotors, Diffusers, SBW, BP, SRC ES, & EBB
Horizontally or Vertically Split
HS for < 40 bars; too tough bolting
VSC for > 20 bars for Hazardous gas
VS – all internals assembled into a bundle and shoved in a high pressure forged barrel.
Gasketed and Bolted Suc Head
O-Ring sealed and Split Ring retained Discharge Head
Wheels, Diffusers, Shaft Sleeves
Wheels: One or more Wheels on Rotor Shaft
Sleeves also spacers– 250 BHN – keep them apart.
6-8 Wheels per rotor for practical shaft lengtrh, bearing capability, deflection etc.
6-8 vanes originate at the eye and terminate at the wheel tip.
The gas enters the inlet eye, jets out at high velocities at the wheel tip due to the kinetic energy added to the gas by the spinning wheel.
Stationary vanes alias Diffusers
Positioned concentric to the rotor wheels; increasing cross-section area flow path, velocity diminution & increased gas discharge pressure (Bernoulli’s laws of conservation of energy in a flowing stream of fluids.)
Machines < 1000-mm WC Head ‘Fans’. ≤ 15,000 mm WC boosters;>15000≤ compressors.
Wheel equations
Q a n; P a N2 for a given dia whl & g;
P a D2 for a given RPM & g;
P a g for a given wheel dia and RPM;
Q-compressor throughput; N-rpm; P- discharge pressure or more accurately DP across the compressor; g - Density of the gas.
Gas densities << e.g. air of 1.293 Kg/Nm3, water of 1000 Kg/m3.
CR <1.1 for a practical sized wheel and RPM. E.g. for 30 to 250 bars 22 wheels rotor of 10,000 - 15000 RPM.
>>long! Impractical! >>shaft deflections, >>bearings, etc.
So only 8-10 W in a rotor, and more casings.
In our example 2 or 3 casings;
Cooling in Intercoolers;
Gas temp < limit temp of compresor parts especially elastomers. < power > eff.
Wheel tip discharge enters the diffuser.Increasing flow path area
< vel > Pr; enters eye of next wheel; > Pr;
several wheels raise the gas pressure to desired levels.
Seals between wheels alias interstage seals minimizes leak back to prev impellor eye.
Benefits: < gas inlet temp to wheel; < disch temp; > throughput; >eff; < power
Laby seals as in fig is the only possible seal between wheels.
Tongued split halves slip into casing grooves with rotor in position; Tongues secure the half to case.
Gas expands in v-grooves and dissipates DP; Thus leak is << despite liberal rotor clearance for rub free runs.
Laby of » 80 BHN on spacers of » 250 ideal.
Corrosive gases may require Laby of SS.
Strip type laby runs rub damage free in case of failed bearing / surge etc.
Rotor Axial Thrust, Seal Ref Pr
Rotor Axial Thrust: DPw*Aw on each wheel towards Suction.
å DPw*Aw of all wheels is net axial thrust.
BP keyed to the shaft rotates - at shaft RPM - in its Laby sealed chamber.
BP leaks discharge end seal chamber ds BPC.
A 2” pipeline – >> AF - connects to SSC.
So DSC Pr » SSC Pr.
BP Benefits:» RAT cancellation. < thrust bearing, < DSC Pr; Same seal for both. < inventory and initial costs.
Shaft End Seals: SES
SES << or stops SE leaks. (Both = pr)
LS at SE leaks > 0.5% capacity. (Liberal cl)
LS are ok for low cost, nonflammable, environment friendly and no hazard gases only e.G. Air, CO2.
Inert gas injection in LS middle; Moderately obnoxious gases.
Case study-1 LS mom importance; CS-2 ill effects from liquid carry over with gases.

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