Interstage systems

For the interstage and final cooling in piston, compressors have to fulfill specific requirements. [12]

The most important type for low and middle pressure range are the tube bundle coolers.

It consists of a cylindrical outer tube (1), closed on its end by covers (8) and the bundle of tubes (3) , which in turn is  usually closed on its ends by two flat plates (2). For bundles of tubes fastened on both ends special constructions are necessary to avoid buckling reduce bending ( for example a design to provide compensation for the elongation of the tubes, an axially movable cover (floating cover) or U-tubes with only one cover plate). Normally the coolant is led through the tubes and the gas, diverted by baffles, flows in the opposite and cross direction of the coolant flow. [13 – 15]

In the low pressure range, also aircooled tube coolers are being used. The tubes are close to each other and are fitted with cooling fins.

In the high pressure range double tube coolers, with the gas flowing through the inner tube, are the preferential design. A simple design is the snake tube cooler, where the gas is led through a  snake tube (sometimes also in parallel for several stages) through the hardly moving cooling liquid. It is mainly used for small volume flows and high pressures.

The calculation of the cooler starts with the following correlation between the main parameters :

For the heat flux the compression work done in a stage represents the upper limit.

For approximations values from practical experience for the coefficient of thermal conductivity can be used. The diagram as shown is based on the assumption, that the heat flow is mainly dependent on the heat transfer on the gas side. This heat transfer can be calculated for all types of coolers (1, 2 tube bundle -, 3 tube, 4 double tube, 5 snake tube coolers) approximately as a function of the product of gas velocity c_G and pressure p_G.

The mean logarithmic temperature difference between gas and coolant at equal inlet temperatures will be the lower, the closer the gas outlet temperature is to the coolant inlet temperature.

The separation of droplets can be done either via the inertia or the diffusion principle.

The cyclone works according to the inertia principle. By injecting the gas stream tangentially into the separator housing a spin of the stream is created. The gas exits through the central tube at the bottom of the cooler.

The radial moment of inertia is considerably higher for the droplets due to their higher density. This creates a relative motion of the droplets towards the outside, which is slowed down by friction. The size of the droplets, for which a balance exists on the radius of the plunger tube between friction and inertia, is called the limit droplet diameter. Droplets with larger diameter will be separated on the outside diameter of the cyclone, from where they flow downwards as a liquid film towards  the drainage at the bottom of the cyclone (drainage).

A separation according to the diffusion principle is carried out in fibre filters. The gas stream enters centrally into the filter and crosses the filter in a radial direction. Due to the molecular motion (diffusion) a higher proportion of droplets will be separated than would correspond to the ratio of area taken up by the fibres compared to the total area of the filter. The result of the diffusion will be the more effective the longer the droplets remain inside the fibres package.

The limit diameter of droplets for fibre filters is considerably smaller than the one for inertia separators. Therefore fibre filters are mainly used for fine separation, cyclones for crude separation.

The separation efficiency compares the mass flow of liquid which has been separated and the amount of liquid at the inlet. The separation efficiency of cyclones and other inertia separators (for example wire mesh filters or mesh deflectors) rises with the gas flow degressively, the overall pressure loss however rises progressively. Separation efficiency and total pressure losses  of a fibre filter improve with lower volume flows. The selection and design of separators is an optimisation problem between separation efficiency, allowable pressure drop and costs.

If there is a requirement for very clean gas (e.g. special air installations, installations for gas separation and liquification) highly effective gas cleaning systems with refrigeration dryers will be installed. Most of the components of water or oil carried as vapour will be taken out of the gas.

In refrigeration dryers the gas is led through a bath of brine (4) , which is cooled down by a refrigerator. The amount of liquid the gas can carry in  form of vapour is thereby reduced and part of it will be separated as droplets. These are then taken out of the gas in the cyclone (6), which is an integral part of the design.

In adsorption dryers the gas is led through columns filled with porous material, on the large surface of which water molecules will deposit. Once the absorbers are saturated, they will be regenerated in a drying cycle by a partial flow of the dried gas.