capacity control

8/2/2019 Capacity Control

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Capacity Control of

Centrifugal Compressor

By: Raghda Ahmed



It is used for the following reasons:

1. Process flow control.2. fuel/power efficiency.

3. Pressure regulation.



Capacity can be changed in several ways:

1. Speed control.

2. Suction throttling.

3. Variable inlet guide vane.



1. Speed control

• It is the most widely used.

• Gas turbines and steam turbines were among the best

options that offered energy-efficient throughput

variation.

• Both driver types allow speed control over a fairly

wide operating range.

• Also electric variable speed motors offers a

continuation to the speed control practice.

• Variable frequency drive motors have joined turbines

as a viable option.



1. Speed control



1. Speed control • Variable speed electric motor drivers are produced in

sizes up to, and even exceeding, 100 MW.

• They offer similar flexibility and part-load efficiency

as gas and steam turbines.

• The trend is toward more variable speed drive motor

applications because of the simplicity and relative

cleanliness of these installations.

• Also, variable speed electric motors tend to requireless maintenance than other variable speed drives.



2. Suction throttling

• It represents another capacity turn- down option.

• This is mainly achieved by reducing the inlet

pressure, which in turn reduces weight flow and,

therefore, reduces power demand.

• As with the other options, precautions must be taken

to keep the compressor away from surge.



Flow rate Q

Pressure rise

P

Surge line

Choke limit

Closing the valve

ΔP

Q part load



3. Variable Inlet Guide Vane (IGV)

• The movable inlet guide vane adds pre-whirl to thegas stream entering the impeller, which in turn,

reduces the axial component of the absolute velocity,

which controls the capacity to the impeller.

m· = ρ * A * C 1• By modifying the inlet whirl component, the capacity

is reduced with little loss in efficient.



3. Variable Inlet Guide Vane •

It is more efficient than the previous method, whichcan be adjusted over a wider range

• The largest problem with the use of movable inletguide vanes is the danger of the vanes. Obviously, the

vanes are not suitable for dirty or fouling gas service.• It would be most effective on single stage

compressors.

• It can be used on multi-stage compressors, but it can

only be installed in front of the first impeller. If thecompressor has a complex arrangement, the conceptis not practical.



Advantage & Disadvantage

Speed control

• Wide control range.

• Expensive.

Suction throttling• Reasonable control range.

• low cost.

Inlet guide vane control

• Little loss in efficiency.• Suitable only for single stage or few stage compressors.

• Not suitable for dirty or fouling service.



Reference

• Compressor and modern process application

• Compressors selection and sizing , 3rd Edition

• Compressor handbook

capacity control

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