SCREW COMPRESSOR – MAIN COMPONENTS 1 Rotor Set

Figure 4,2 – 2 Dry Screw Compressor Element

Instrument air (“dry”) screw compressors require 2 screw stages in series to achieve the required pressure. The High Pressure (HP) stage or element will be physically smaller, to suit the increased density. Otherwise the 2 elements are very similar.

A dry screw element comprises a matched pair of helical screw rotors running in mesh. The screw thread profile is special, as it forms a series of air seals between air pockets of increasing pressure. There are several proprietary designs, typical of which is the 4 / 6 design. In this design a 4 lobed “male” rotor with bulbous lobes meshes with a 6 lobed “female” rotor with narrow tapered lobes. The “male” rotor acts as a set of continuous rotary pistons, and the female rotor as matching cylinders. The rotors should not actually touch each other or the casing, rotor timing is by means of timing gears. The “male” rotor is driven by the input shaft and does virtually all the compression work, thus the timing gears carry very little load.

Each rotor runs in two sets of rolling element bearings, one at each end. The bearings are set up and pre-loaded very precisely to maintain shaft position. The timing gears sit outboard of the Non-Drive End bearings. Bearings and timing gears are oil lubricated. Sophisticated labyrinth seals separate the oil from the oil-free air, and minimise the air loss.

As the pair of screws rotate, air is drawn in from a shaped port in one end of the casing, compressed by the meshing screws, and discharged at the other end of the screw set. Since the screws have a fixed throughput volume and a fixed geometry, the capacity and pressure ratio is, in effect, designed in. Some degree of capacity control can be achieved by a combination of suction throttling, internal recycle valve (slide valve) and, more recently, inverter- driven variable speed drive.

Dry screw compressors generate significant temperatures due to the heat of compression, and air leakage friction losses. Hence they must be built with mechanical clearances which permit a substantial back-flow of air. This reduces the capacity of the compressor compared to its displacement, and reduces efficiency.

Figure 4,2 – 3 Twin Screw Compression Principle

OIl-flooded screw compressors can have the same basic geometry, but allow oil into the rotor mesh, which now acts as a timing gear. The timing gear is not required, and the seals become much simpler. The exhaust air is saturated with oil (which must be separated later). A radically different compressor design, which achieves exactly the same results, uses a single oil-flooded screw rotor, fitted with slotted “gate” rotors which mesh with the screw and act as seals. The “gate” rotors run at right angles to the main rotor and are made of special non-metallic materials. Oil-flooded compressors run at lower temperatures than dry compressors (the oil acts as a heat sink during compression), the oil also acts to seal up leaks through clearances. Hence these machines are more efficient. They can also produce a higher pressure ratio per stage and can often deliver the required duty with one stage, making them significantly cheaper than dry machines.

4.2.6.2 Element Casing

The element casing consists of one or more matched and machined cast iron housings, enclosing the rotors, housing the bearings and seals, and forming the oil chambers for the bearings. Each casing is a matched and doweled set of parts. On package air machines, the casing is flange mounted to the drive gearbox. The casing has cast-in oil ways and a cast cooling jacket.

4.2.6.3 Gearbox

Package air compressors have an integral speed-increasing gearbox, in the form of a small Bullgear machine. The Bullgear shaft is driven by the electric motor, and the pinion is mounted directly on the end of the driven rotor. Dry compressors normally have two stages thus two driven pinions. The set-up of the gearbox is determined by the element build quality and the spigots & fits of the casings.

Oil-flooded compressors, which may have only one stage, still require the gearbox to drive the compressor element at the appropriate speed.

The gearbox is mounted to the baseframe and forms the structure of the compressor. It may contain or enclose the Intercooler.

4.2.6.4 Intercooler & Aftercooler

Packaged dry screw air compressors normally include an Intercooler and Aftercooler. The Intercooler is integral to the machine as it cools the LP stage discharge air for inlet to the HP stage. If the intercooler were not present or not working, HP discharge temperatures would be excessive and the rotors would rub. The intercooler is normally a proprietary shell and tube design, integrated into the compressor construction. Condensation produced by the intercooler must be drained off by an automatic device, to prevent damage to the HP stage.

The aftercooler is not integral to the operation of the compressor, it may be dispensed with if high temperature (about 150 C) air is required. The main purpose of the aftercooler is to cool the air sufficiently to knock out most of the moisture, prior to entry to an air drier. The aftercooler is typically of shell and tube design, mounted local to the compressor package or inside the housing. The aftercooler and automatic moisture drain must work, or the drier will be overloaded.

Oil-flooded compressors work on a different cooling principle, most of the heat of compression goes into the oil. Hence the large cooling load is the oil cooler. A small aftercooler may be fitted, to reduce outlet air temperatures and permit moisture removal.

4.2.6.5 Seals and Bearings

In a dry screw compressor, the seals are sophisticated labyrinth seals, located between each of the 4 rotor bearings and the compression element. The primary purpose of the seal is to retain lubricating oil in the bearings, and to prevent it from entering the compression air. The secondary purpose is to minimise air leakage to atmosphere. A small amount of air, with traces of oil, is vented from the breather, via a mist filter. Oil is returned to the lubrication system. The bearings are basically high precision ball bearings, set in groups to achieve rotor support and control pre-load. They are not standard stock bearings, and the set-up requires special tools. Provided that the oil quality is maintained, the bearings fail predictably according to fatigue, this permits elements to be changed out on the basis of time or vibration monitoring. There is always the risk of random bearing failures, good vibration alarms may be able to detect the failure before rotor damage occurs, although the element must be changed.

In oil-flooded machines, the seals are much simpler, their purpose is to control the air flow from the compressor element into the bearing area. There is no direct leakage path to atmosphere. 4.2.6.6 Control Devices

Screw compressors are nominally fixed-ratio and fixed-volume machines. This is set in manufacture and cannot be changed in the field. If the discharge pressure does not match the compressor rating, power is wasted and the compressor also becomes very noisy. Control measures adopted include suction throttling, internal slide valve (internal compression ratio adjustment) and variable speed control. Each manufacturer offers standard and optional control devices, to suit the application. The overall control system is managed by a proprietary PLC controller, running proprietary software. Most vendors are prepared to offer tailored systems but these often then mean non-standard PLC's which give problems to field service engineers. The suction throttle may be a modulating device, to help match the required load, or simply an on/ off device which works as part of a load/ unload control. The matching part of the load/ unload control is a vent valve that opens the compressor discharge to atmosphere, minimising the motor drive load while permitting instant loading.

Internal slide valves are more usually fitted to oil-flooded compressors, as a more sophisticated load control system. They work by reducing the working length of the screw, thus reducing the internal compression ratio and the absorbed power. They are particularly effective on variable pressure ratio systems e.g. refrigeration compressor duties.

A recent design option, made practical by the availability of robust and cost-effective inverter drives, is variable speed operation. This is mechanically simple, saves power and reduces noise and machine wear. Below a minimum load the compressor must still unload and stop.

4.2.6.7 Silencers

Screw compressors produce very high internal noise levels because of the opening and closing of rotor pockets at the lobe pass frequency. This gives a narrow tone band of the order of 800 Hz. These high frequency tones are very identifiable and annoying, equally they are easily silenced by small inlet and outlet silencers. Noise breakout inside the package enclosure is easily dealt with by foam lined panels, provided that all gaps are closed, and doors are not left open.

4.2.6.8 Water and Oil Separators

Water separators are simple devices located downstream of each inter- and after-cooler unit. They comprise a simple baffle or demister device, a collecting chamber and a drainage device. Traditionally, the drainage device was a float trap as used in steam service, but these are notorious for blocking or jamming. More modern systems tend to use a timed solenoid operated drain, with a liquid sensor to check for correct operation. Oil-free compressors can discharge clean water condensate to open or domestic drains, or to deck. Oil-flooded compressor condensate tends to be an oily emulsion, which must be run to oily water drains for separation. Oil-flooded compressors have an integral oil separator, this vessel is physically larger than the compressor itself, in order to accommodate the oil separator filter unit. The base of the separator normally acts as the lubricating oil sump. The separator comprises a mechanical separator followed by a fine fibre coalescer filter element. This is designed as a wick to draw oil down to a collector rim which returns the liquid oil to the sump. The air/ oil mixture can generate high static voltages, if not earthed these can produce dangerous sparks. The known risk area is the coalescer filter, this element contains earthing wires or straps which must be in good order. The risk is not present beyond the filter element as there is not enough oil present to either carry the charge or to fuel an explosion. Even so, downstream pipework should be metal and fully

4.2.6.9 Dryers

Air dryers are frequently installed in conjunction with instrument air compressors. The working principle is normally pressure swing absorption onto silica gel beads, in a pair of pressure vessels. Regeneration is by hot air, or cold dry air. Dryers should be able to achieve a dew point of – 40 to – 60 C (atmospheric), these values are required to prevent liquid water or ice from appearing at solenoid valves, especially on exposed equipment during severe winter conditions. Dryers should always be fitted with a dew-point monitor to confirm that they are actually working properly.

Refrigerated dryers can only work down to a dew point of about + 3 C (pressure) which is not dry enough for instrument air service. It is generally satisfactory for service air.

Dryers can only work effectively if the inlet air has been cooled and has had all liquid water removed. Otherwise this water load will tend to overload the dryer. If a dryer is being used with an oil-flooded compressor, effective coalescer filters are required upstream of the dryer, or oil will coat and destroy the drying media.

4.2.6.10 Control & Management Systems

Control of the compressor will be part of the vendor's proprietary package controls. Typically, the control system will monitor discharge pressure, at the very least the compressor will switch on and off, and load and unload, according to demand. More sophisticated control will attempt to load follow by modulating the compressor capability.

The control system will also monitor oil pressure, temperature, etc., carry out start-up and controlled stopping of the compressor. Many modern systems include action logs and alarm logs, although these often can only be read by the service engineer, not the operator.

Because the control system is proprietary, it will often be very limited in its potential for tailoring or compliance with customer specifications. That leaves the choice between accepting a "standard" system, and insisting on a "special" system with attendant costs and risks.

4.2.7 INTEGRATION ASPECTS