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Compressed Air Explained This section explains the basics behind compressed air.

 

Why Do We Need It?

 

Compressed Air is often described as the fourth utility, although not as ubiquitous as electricity, petrol and gas, it plays a fundamental part in the modern world. The main difference is that users generate their own air and so have a choice in the way that air is generated.

The importance of compressed air is often over looked, but in reality it plays a vital part in most modern manufacturing processes and modern civilization. Although we may not realize it most products we use today could simply not be made without compressed air. Compressed air accounts for about 10% of the global energy used in industry today.

With so many applications in different environments being dependant on compressed air, the compressors not only have to compress the air to a specific pressure, at a certain flow, it has to deliver air of the right quality. To most people, a compressor is all that is required to compress air, but to obtain the right quality of the compressed air, more equipment is often needed. Filters and dryers are often needed to remove oil and water before it reaches the application. CompAir has a range of completely oil-less compressors where air comes into contact with the process it serves and so the quality is critical, for example in where a compressor may be used in a food packaging role.

 

The three different types of compressors

 

Piston Compressor: The piston compressor is one of the earliest compressor designs, but it remains the most versatile and is still a very efficient compressor. The piston compressor moves a piston forward in a cylinder via a connecting rod and crankshaft. If only one side of the piston is used for compression, it is described as single acting. If both sides of the piston, top and underside are employed, it is double acting.

The versatility of the piston compressors knows virtually no limits. It compresses both air and gases with very little alterations. The piston compressor is the only design capable of compressing air and gas to high pressures, such as breathing air applications.

The configuration of a piston compressor can be a single cylinder for low pressure/low volume to a multi-stage configuration cable of compressing to very high pressure. In these compressors, air is compressed in stages, increasing the pressure before entering into the next stage to compress the air into even higher pressure.

 

Rotary Screw: The screw compressor is a displacement compressor with pistons in a screw format; this is the predominant compressor type in use today. The screw compression element main parts comprise male and female rotors that move towards each other while the volume between them and the housing decreases. The pressure ratio of a screw is dependent on the length and profile of the screw and of the form of the discharge port.

The screw element is not equipped with any valves and there are no mechanical forces to create any imbalance. It can therefore work at high shaft speed and combine a large flow rate with small exterior dimensions.

 

Rotary Vane: Based on traditional, tried and tested technology, the vane compressor is directly driven at very low speed (1450rpm), offering unrivalled reliability. The rotor, the only continually moving part, has a number of slots machined along its length into which fit sliding vanes that ride on a film of oil.

The rotor rotates within a cylindrical stator. During rotation, centrifugal force extends the vanes from their slots, forming individual compression cells. Rotation decreases the cell volume, increasing the air pressure.

The heat generated by compression is controlled by pressurized oil injection.

The high pressure air is discharged through the outlet port with the remaining traces of oil removed by the final oil separator.

 

Once air has been compressed, why refine it?

 

Air is a colorless, odorless, tasteless mixture of many gases, primarily nitrogen and oxygen. Air is naturally contaminated with solid particles, such as dust, sand, soot and salt crystals. This contamination varies with differing environments and altitude.

Water vapor is another natural ingredient which can be found in variable amounts in the air. The amount of water vapor and contamination of the air plays a vital role in the compression process and in the quality of the air delivered by the compressor.

The damaging and corrosive properties of water are well known. Untreated air at atmospheric pressure contains large amounts of water and other contaminants such as oil droplets and dirt particles.

When the air is compressed the concentration of moisture and other contaminants increases. If allowed to remain in the system this corrosive mixture has a detrimental effect on pneumatic equipment, causing unnecessary production downtime, product spoilage and reduced equipment life.

Compressed air filters remove the oil and dirt content while compressed air dryers remove water vapor before air reaches the point of use.

 

Different ways of treating/refining compressed air

 

Aftercoolers: Aftercoolers are a good first step in removing moisture and air contaminates. They lower the temperature to safe, usable levels, thus reducing the air's ability to hold water vapor, removing 70%. However the air is still saturated. A further drop in temperature will cause additional condensation to occur in downstream airlines.

Refrigerant drying Refrigerant drying cools the compressed air, whereby a large amount of the water condenses and can be separated. The compressed air is then heated so that condensation does not form on the outside of the pipe work system.

The compressed air cooling takes place, via a closed coolant system, where a refrigerant cooling agent is employed. CompAir uses environmentally friendly gasses for this. By cooling the compressed incoming air with the cooled outgoing air in the heat exchanger, the energy of the refrigerant dryer is reduced.

 

Desiccant Drying: Desiccant drying works on the principle of absorption of water vapor through a bed of desiccant material, in a pair of chambers. Two types are used, heatless regenerative and heat regenerative. The heatless type uses a percentage of the dried air (purge), for re-generation of the desiccant material, while the heat regenerative type uses an electric heat disk, which reduces the amount of purge air needed for regeneration.

 

Filters and Water Separators: WS Water Separators have been designed for the efficient removal of bulk liquid contamination from compressed air. Filters are used both prior to, and after compression. Filtering the intake air reduces the intake wear on the compressor by removing larger particles. During the compression air can be contaminated with oil (in oil lubricated machines) which is removed by filters. The filter efficiency is dependant on compressor type, age, design and condition. Filters are often setup in multistage. Fiber filters can only trap oil as droplets, while more efficient active carbon can trap oil as a vapor. We can help you select the right filters for your needs.

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