Components

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

The most important type for low and middle pressure range are the tube bundle heat exchanger, see Figure 4.10.

It consists of a cylindrical outer shell (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 air-cooled 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 helical coil tube cooler, where the gas is led through a helical coil 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, see Table 4.5.

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 below relies on the assumption that the heat flow depends mainly 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 helical coil 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 if the gas outlet temperature gets closer to the coolant inlet temperature, see Figure 4.12.

Table 4.6 shows the special features with heat exchangers in piston compressor installations.

The separation of droplets can be done either via the inertia or via 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, see Figure 4.13.

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 slows down due to 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 flows into the filter centrally and crosses the filter in a radial direction, see Figure 4.14.

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 liquefaction) 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 with droplets flows 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, see Figure 4.15.

In adsorption dryers, the gas with droplets flows 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.