Principle of Operation
A centrifugal compressor accelerates the velocity of the gases (increases kinetic energy) which is then converted into pressure as the gas flow leaves the volute and enters the discharge pipe.
Centrifugal compressors consist of stationary casing,
Containing rotating impeller (imparts a high velocity of air),
Fixed diverging passage (The air is decelerated with rise in static pressure).
Impeller may be single or double-sided
Basic Components
Suction Vane Tips = Part of the impeller vane that comes into contact with gas first.
Discharge Vane Tips = Part of the impeller vane that comes into contact with gas last
2. Types of Centrifugal Compressors
Single- Stage: Compress the gas once
Use for high gas flow rates, low discharge pressures
Multi- Stage: Take the discharge of one stage and pass it to the suction of another stage
Use for high gas flow rates, high discharge pressures
Usually operate at speeds > 3,000 rpm.
Air is sucked into the impeller eye and whirled at high speed by the vanes of the impeller disc.
The static pressure increases from eye to tip.
Remainder of static pressure rise occurs in diffusers.
Normally half of pressure rise occurs in the impeller and 50% in diffuser.
Some stagnation pressure loss occurs.
Why centrifugal compressor?
Centrifugal compressors are extremely popular because most are close to being oil-free. Although oil is used in the compressor which can create an aerosol, the special sealing systems used in most centrifugals reduce the level of oil contaminants to very low levels. Centrifugals are also popular because of the very large capacities (> 100,000 CFM) that are possible with a single compressor, combined with fairly high pressures (100 to 350 Kg/cm2). Centrifugals are economically attractive when flows are high.
If other compressors are available which will meet the same pressure/flow requirements, centrifugals are preferred when the process requirement allows for a fixed pressure ratio and requires oil-free gas.
Centrifugal units often have higher installed costs than reciprocating and screw compressors for the same pressure/flow range, and they are inflexible to changes in pressure ratios.
Stall
Defined as the (aerodynamic stall) or the break-away of the flow from the suction side of the blades. A compressor stall occurs when the pressure of air entering the engine drops below the pressure in the compressor or air within the compressor drops momentarily as a result of stalling air (disruption in air pressure). When this happens the compressed air expands and travels toward the area of less pressure. This happens quite fast and can be explosive.
Stall is a local disruption of air flow within the compressor which continues to provide compressed air but with reduced effectiveness
Stall happen due to too much suction feed but less discharge and will prevent by implemented inlet guide vanes,
Surge
Surge is associated with a sudden drop in delivery pressure and with violent aerodynamic pulsation which is transmitted throughout the whole machine.
Surge happen due to less suction feed at operating speed and will prevent by installing recycle line or anti surge line.
For a given discharge pressure a compressor has a certain minimum flow rate. Below this flow rate the compressor becomes unstable. a decrease in flow below the minimum flow can cause a series of momentary reversal of flow through the compressor. This situation is called surge.
Surging results in violent fluctuations in discharge pressure. When an electric motor is used as driver surging can cause extreme variation in motor current. Symptoms of surging are low gas flow, excessive vibration and banging sound inside compressor
Compressor Controls
Compressor controls typically match compressed air output to compressed air demand by maintaining discharge air pressure within a specified range. There are five primary control strategies for maintaining the pressure within the desired range.
Variable-Speed Control
Rotary-screw air compressors can be equipped with variable frequency drives to vary the speed of the screws and the corresponding compressed air output. As in other fluid flow applications, the variation of speed to vary output is extremely energy efficient.
On/Off Control
In on/off control, the compressor turns on and begins to add compressed air to the system when the system pressure falls to the lower activation pressure. The compressor continues to run and add compressed air to the system until the system pressure reaches the upper activation pressure when the compressor shuts off. Reciprocating compressors typically employ on/off control. On/off control is the most efficient type of part-load control, since the compressor draws no power when it is not producing compressed air.
Modulation Control
In modulation control, the position of the inlet air valve is modulated from full open to full close in response to compressor output pressure. Inlet modulation is a relatively inefficient method of controlling compressed air output.
Blow-off Control
In centrifugal compressors, the quantity of air flow through the compressor can only be controlled by modulating the inlet air valve over a relatively small range. When flow is reduced below this range, the flow becomes unstable in a “surge” condition. To avoid surge, centrifugal compressors may discharge compressed air to the atmosphere to control compressed air output to the system. Blow-off control is the least efficient method of controlling compressed air output, since input power remains constant as the supply compressed air to the system decreases.
Load/Unload Control
In load/unload control, the compressor “loads” and begins to add compressed air to the system when the system pressure falls to the lower activation pressure. The compressor continues to run and add compressed air to the system until the system pressure reaches the upper activation pressure. It then “unloads” and does not add compressed air to the system until the system pressure drops to the lower activation pressure. When unloaded, rotary screw compressors typically partially close the air inlet valve and bleed the remaining compressed air in the sump to atmosphere.
Power draw when fully unloaded varies from about 60% of full load power to about 30% of full-load power, depending on compressor design and on the length of time the compressor runs unloaded. To fully unload, the load/unload cycle time must be long enough to allow the compressed air in the sump to bleed to atmosphere when the compressor unloads. Thus, load/unload control works best when coupled with adequate compressed air storage, which lengthens load/unload cycles while modulating pressure variation to end uses.