Centrifugal Compressor

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Compressor is used to take a definite quantity of fluid which is usually a gas or air and deliver it at a required pressure. In other words, the job of a compressor is to increase the pressure of the incoming fluid. Choice of centrifugal compressors is determined by their characteristics curves based on the pressure required and the amount of input of mechanical work that is power input. From the result that we obtain, the highest efficiency of compressor was running with the speed 10 000 rpm. The efficiency at speed 10000 rpm higher than others and it only produced lower flow rate than other. The lowest was at speed 10000 rpm curve that in unstable condition. For all the speeds measurement, the inlet temperature values were almost similar. The most important are the outlet temperature values. While doing the experiment, the butterfly valve is very difficult to handle. The computer that we used also have problem because the software that been used sometimes detect faster sometimes slower so the value that we need is not accurate. For the conclusion, the highest was at speed 10000 rpm but it produce lower flow rate than other speeds. 2.0OBJECTIVE;To study the characteristics curves of a centrifugal compressor.


Compressor is a part of a system that used conservation of energy to change the energy from one to another. It is used in many mechanical systems such as power plant, refrigerator and jet engines to increase the pressure of the fluid. Several types of compressor are used such as axial compressor and centrifugal compressor. A compressor is called axial compressor when the air is turned perpendicular to the axis of rotation from left to right, whereas it is called centrifugal compressor as the flow through the compressor is turned perpendicular to the axis of rotation spin around the shaft. In general, the compressor consists of two main parts; blades and shaft. The fluid can be air or gas flows through the moving and fixed blades. The work input to the shaft is transferred by the moving blades to the air. A centrifugal compressor is made up of an impeller with a series of curved radial vanes. Air is drawn in near the hub, called the impeller eye and is spin round at high speed by the vanes on the impeller as the impeller rotates at high rotational speed. The static pressure of the air increases from the eye to the tip of the impeller. Centrifugal compressors or blowers are used for a wide range in engineering and there is no basic difference in the design for any of the different applications. [1]

Figure 1; a centrifugal compressor [2]Centrifugal compressors are widely being used for many applications such as in pipeline transport of natural gas to move the gas from the production site to the consumer, in oil refineries, natural gas processing plants, petrochemical and chemical plants, in air separation plants to manufacture purified end product gases, in refrigeration and air conditioner equipment refrigerant cycles and also in industry and manufacturing to supply compressed air for all types of pneumatic tools. [3]4.0THEORY

The performance of the compressor is characterized by the pressure ratio across the compressor (CPR), the rotational speed of the shaft necessary to produce the pressure increase and an efficiency factor that indicates how much additional work is required relative to an ideal compressor. [1]

P2, T2 (exit)

Q out


P1, T1 (enter)

Figure 2: schematic diagram of typical compressor

The increase of the pressure is measured by CPR. This is the ratio of the air total pressure, pt exiting the compressor to the air pressure flowing in the compressor. This CPR number must be always greater than 1.0. [1]

CPR = Pt2 / Pt1 or P exit / P enterIn order to produce the increase in pressure, the compressor must perform work on the flow. The shaft turns the blades at a high rate of speed. Several stages are usually employed to produce a high CPR, with each stage producing a small pressure increase. In the centrifugal compressor, additional pressure increase is obtained from turning the flow radically, radiating from or converging to a common center. Since no external heat is being added from the compressor during the pressure increase, the process is isentropic. The total temperature ratio across the compressor is related to the pressure ratio by the isentropic flow equations. [1]

Total temperature ratio=Tt2=Pt2-1Tt1

Pt1 Where, is the ratio of specific heats.

Work must be done to turn the shaft on which the compressor is mounted. From the conservation of energy, the compressor work per mass of airflow CW is equal to the change in specific enthalpy, ht of the flow from the entrance to the exit of the compressor.


The term specific means per mass of airflow. The enthalpy at the entrance and exit is then can be related to the total temperature, Tt by the equation;


Where Cpi is the specific heat at each particular point.

Performing rearrangement, the equation of compressor work per mass of airflow can be written;

CW=CpTt1 (CPR(-1)/ -1)

CThis equation relates the work required to turn the compressor to the compressor pressure ratio, the incoming total temperature, some properties of gas and an efficiency factor, C. The efficiency factor is included to account for the actual performance of the compressor as opposed to the ideal isentropic performance of the compressor. In an ideal performance, th value of the efficiency would be 1.0. However, in reality, the value is always less than 1.0. So, additional work is needed to overcome the inefficiency of the compressor to produce a preferred CPR. The work is provided by the power turbine which is connected to the compressor by the central shaft. It is worth to note that the CPR is related to the total temperature ratio across the compressor. Since the CPR is always greater than 1.0 and the value of the ratio of specific heats is about 1.4 for air, the total temperature ratio is also greater than 1.0. It means air heats up as it passes through the compressor. The efficiency of a compressor can also be improved by carrying out the compression in several stages. This is called multistage compression. [1]

Volumetric flow rate, Q= d2 2(100) p (3600)4


Diameter, d=0.044m

=1.21 kg/m3Both at 20C and 1013 mbar and the pressure drop p at the nozzle in mbar.

=Phyd 100%

PelWhere Phyd can be calculated from the total pressure head and the flow rate.

Phyd=100(dp1 + dp2) Q3600



Two stage compressor

Transparent intake

Shaped inlet for good flow

A protective plate

Transparent outlet

Butterfly valve

Pressure measuring point

Electric motor in the housing

A speed adjuster

An optical sensor



1. The windows and the analysis software is being start.2. The actual measure values is being display by choosing the command system diagram on the menu 3. The measure data has been record in an ASCII file. 4. Every time the save measurement button is click is will be save into the previously into the ASCII file. The data that have been recorded consists of

(a) Time

(b) Volumetric flow rate

(c) Speed n of the compressor

(d) Electrical power

(e) Efficiency

(f) Differential pressure 1st stage dp1(g) Differential pressure 2nd stage dp2(h) Compressor total pressure

(i) Inlet temperature Tin (j) Outlet temperature Tout5. The compressor characteristic curves is being recorded(a) The interface module is being switch on

(b) The power meter at the switch on the rear is being switch on

(c) The butterfly valve is being close completely

(d) The speed of the compressor is being set by the speed adjuster

(e) The butterfly valve is being opened a little and the flow rate for the first measurement point is being set

(f) Whenever the flow rate is being dropped the speed need to be adjust(g) The measurement is being recorded when it is in steady state

(h) The process is being repeat until the butterfly valve is fully open

6. The pressure-flow rate characteristic curve was recorded for 4 different speeds which are 10000 rpm, 11000 rpm, 12000 rpm and 13000 rpm.

6.0RESULTS Table 1: run 1 at speed 10000rpmFlow in m3/hTin in degCTout in degCdp1 in mbardp2 in mbarPower in WEfficiency in %dptotal mbarPhyd WPel W











Table 2: run 2 at speed 11000rpmFlow in m3/hTin in degCTout in degCdp1 in mbardp2 in mbarPower in WEfficiency in %dptotal mbarPhyd WPel W