The quasi stationary changes in the conditions of state of the gas when flowing through a compressor stage can be represented in a T,s- diagram.Such a compressor stage can be either part of  a multistage  or a single stage compressor.

Gas enters the cylinder from the suction chamber in the condition S. During the flow into the cylinder throttling and heating up of the gas occurs.

The non stationary change of state in the working chamber is limited by points 1 and 2 of condition of state  at the beginning and at the end of compression. For the quasi stationary evaluation of  the stage the mean pressures during the intake and discharge are relevant.

The outflow of gas out of the working chamber is combined with a pressure loss and a cooling of the gas down to the conditions of the receiver  D on the delivery side.

With the gas flowing through the interstage system its temperature will be reduced  to the suction temperature T s+ of the next stage. The pressure is reduced due to friction losses to ps+. ( The index “ + “ symbolises the next stage ).

For the function of the stage the pressure ratios below are characteristic. (All pressure ratios correlate to a line in the T,s – diagram.)

According to the first law of thermodynamics the specific work put into a stage equals the heat removed from the working chamber and the interstage system plus the  mostly relatively small change in  enthalpy of that stage.

 

Usually the gas contains a certain amount of water vapour, in the extreme case it can be saturated with water vapour. For the absolute humidity ( mass ratio of water vapour to dry gas ) there exists a limit , which is proportional   to the ratio of vapour generation pressure  and the pressure of the gas.

The temperature and thereby the pressure at which vapour is generated  at the inlet of subsequent stages only differ slightly .Therefore , with rising pressure of the gas from stage to stage its capacity for holding water vapour is reduced and partly condensation occurs. The condensate in the form of droplets has to be removed, as accumulation of water could lead to damage in the next compressor stage ( “water hammer”, corrosion ).

The oil content of the gas behind a lubricated compressor stage  lies in the range of 10 mg oil / kg gas. The diameter of the droplets  is  0,1 to 10 & # 956;m ( most frequent occurrence at approx. 1 &#956,m ).  These droplets result from being carried away  from the oil film on the walls of the intercooler system  by the gas flow. Also the oil should be separated behind each stage.

( Designs and application areas of separators )

 

The flow of gas though the interstage systems is only stationary under ideal conditions. In  the real compressor the flow can not be stationary.

The periodic load cycle in the working chambers result in changing flow velocities  and causes therefore periodic quasi stationary pressure changes in the interstage system. These pressure changes are the smaller, the smaller  the flow difference at inlet and outlet  and the larger the volume of the  interstage  system.

Due to the compressibility of the gas the interstage system represents a system capable of oscillations, which is incited by load cycles of the adjacent  working chambers . The thereby  resulting pressure and gas velocity changes  are called gas pulsations.

When designing the interstage system one tries to avoid resonances. With compressor installations with varying working conditions this is, however, not generally possible. Therefore sufficiently large  volumes  on the suction and delivery  side  are the best pre-condition to limit gas pulsations.

The main aim of pulsation studies is to calculate the  pressure pulsation amplitudes  and their effect on the given compressor installation. They should also show ways how to reduce these pulsations [ Eijk and others]