final capacity control - copy

8/6/2019 Final Capacity Control - Copy

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Capacity: full rated volume of flow of

compressed gas

Capacity is the full rated air volume deliveredat compressor inlet conditions

Compressor-capacity control refers to the

system that matches the compressed-air

output to the system-air demand


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Reduce power and energy consumption

Decrease the starting load

Minimizing electrical problems

Improving lubrication.


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1- By pass control

2- Start and stop control3- Three step capacity control

4- Clearance control

5- Five step capacity control

6- Reverse flow control


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At a preset pressure limit , a pressure sensing element pilot device

causes air to be admitted to the suction valve unloaders

The unloader mechanism, attached directly to the suction valve,

consists of valve fingers operating in the unloader cylinder directly

on the valve strips. The spring-loaded fingers are forced to theunloading position either by the movement of the diaphragm stem

through apiston and power spring in the unloader cylinder or

directly by the stem.

This mechanism, when actuated , holds the suction valve strips in

the open position allowing the air or gas to pass freely in and out of

the suction valves without compression in the cylinder, thus

reducing the delivered capacity of the cylinder and unit.


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y Start-stop capacity control is commonlyused with small, electric motor-driven

compressor

y Start-stop control uses a pressure-sensitive

switch to start and stop the compressor tomaintain a preselected pressure range

y pressure switch will be wired in the pilot

circuit of a magnetic starter as well as in the

solenoid valve electrical circuit

Thus, an increase in pressure will break thestarter circuit, stopping the compressor

motor and, at the same time, causing the

compressor to unload


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This control, which uses suction valve unloading,regulates capacity in steps of100%, 50% and 0% offull capacity and is activated by some sort ofgovernor. The actual unloading of the compressor

can take place electrically and pneumatically, orpneumatically only


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clearance pocket, when open, increases the

cylinder clearance volume, which causes the

compressor capacity to decrease . An indicator

card taken from a cylinder end with a 50%

capacity reduction

see next Figure shows this effect more clearly.


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The volumetric efficiency for reciprocating compressor is

given by

vol = actual volume of drawn air per cycle/stroke volume.

!

n

s

d

vol

P

PC

1

11L where;

C is the clearance ratio Vc/VsPd is the delivery pressure

Ps is the suction pressure


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When more control is required , it is possible to

use suction valve unloaders and a clearance

pocket in each cylinde r to regulate the capacity

in steps of100%, 75%, 50%, 25%, and 0% of full

compresso r capacity.

The pressure fluctuation of the system is less

with five-step control than with a three-step

control.


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Figure shows the control sequence of a

compressor with five-step capacity control


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approximately 1% in power is saved when using a

five-step control instead of a three-step control. This

amount represents a considerable savings whenevaluated over a year of operation


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An unloading device is fitted to each suctionvalve. At partial loads , this device doesnot allow

the suction valve to close

when the piston is in its bottom dead-centerposition, but delays this closing in a controlled

way. Thus, a certain amount of gas, which issteplessly adjustable, is allowed to returntowards the suction manifold before the

compression starts.


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As the piston accelerates , it pushes gas in a

"reverse flow" back out the suction valve faster and

faster, which increases the drag forces on the valve

plate . These forces will eventually overcome the

unloader forces and the compressor valve will close

. At that point, compression will start with thereduced capacity output

final capacity control - copy

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