The Generation of Compressed Air

The Generation of Compressed Air (1)

Objective

To understand the properties of gases, the fundamental gas laws and the physical effects of compression and how to control them.

Some definitions

Air pressure

Air exerts a pressure on its surroundings that gradually reduces with altitude. It also varies slightly with the weather. At sea level the Atmospheric Pressure is 1.0 bar abs. A pure vacuum has a pressure of 0 bar. There are two ways of stating the pressure of a compressed gas: Gauge pressure and Absolute pressure.

Gauge Pressure is the pressure of a system above atmospheric pressure. The units are bar gauge (bar g) and are the most commonly used units.

Absolute Pressure is the system pressure above a vacuum as described above. It is also measured in bar. Absolute pressure is always 1 bar above gauge pressure.

Temperature

The 2 common fixed points for temperature on the Celsius or Centigrade scale are the freezing point of water = 0 deg. C and the boiling point of water =100 deg. C. Absolute Zero is the lowest possible attainable temperature and this is minus 273 deg. C (-273 deg. C). Often for calculations, the temperatures are quoted as absolute temperature in degrees Kelvin (deg. K). These temperatures are obtained by simply adding 273 to the Celsius scale. For example:

                  0 deg. C = 273 deg. K

                 20 deg. C= 293 deg. K

               100 deg. C= 373 deg. K

Pressure ratio

This is the ratio of the absolute outlet pressure to the absolute inlet pressure of a compressor or a compression stage.

Volume

Air volume is usually quoted as litres or cubic metres. 1 cubic metre contains 1000 litres. The output from a compressor is usually quoted in litres per second or, for large installations, cubic metres per second. The volumes quoted are for the input volume, not the smaller compressed volume from the output side.

Gases and their properties  

There are three well-known states of matter

·       Solid

·       Liquid

·       Gas

Solids do not easily change shape and cannot be compressed. Liquids however, can easily change shape but cannot really be compressed. Gases are completely different. They are very much lighter, and are easily compressed.

If a gas is compressed, its volume gets smaller but its pressure goes up. There is a direct relationship between pressure and volume known as Boyle’s Law:

Pressure (P) x volume (V) =constant.

This works if the temperature does not change. The effect of a temperature change is that the pressure or volume will change in proportion. This is known as Charles’ Law:

Pressure (P) = constant x Temperature (T)

The units of pressure and volume can change depending on how much weight of gas is being described. The temperature (T) however is defined as the absolute temperature.

The absolute temperature scale starts at absolute zero that is –273 deg. C. This means for example that water freezes at 0 deg. C or 273 o Absolute, usually written as 273 o K (After Kelvin, who first described the scale). Water boils at 100 o C, or 373 o K. These adjustments to the temperature scale need to be made when calculating pressure or volume changes if a gas is heated. Fortunately, this is a minor factor when describing air compression. 

Energy and air compression

Air compressors are usually driven by electric motors. The cost of doing this is significant.

 Let us look at the pressure of air.

The most common unit of pressure is the bar. Air at sea level has a pressure of one bar. This drops to around 0.5 bar at an altitude of 5500 metres and only 0.25 bar at 11000 metres. This is why breathing is difficult at high altitudes and why aircraft cabins must be pressurised.

The most common units of volume are the litre and cubic metre. There are 1000 litres in a cubic metre. A big air compressor will have a capacity of about 1000 litres per second, or 1 cubic metre per second. The pressure most commonly used in breweries for general distribution is 7 bar (absolute). By taking in 1000 litres per second of air, the compressed volume drops to about 140 litres per second. This capacity requires a very powerful electric motor. In practice for this rate of production, a compressor with a motor of about 400 K.W. is needed. The graph below shows an example of how much energy is needed to produce 100 litres per second of air at varying pressures.

The motor size for an air compressor will depend on the friction inside the compressor. These energy losses are usually about 25%. They can be higher if the machine is badly maintained or running intermittently. They perform best when fully loaded.

When a gas is compressed, it heats up. This effect is noticeable when you pump up a bicycle or car tyre. Inside a fast moving air compressor, this effect is much greater. If the heat is not removed, the air temperature would rise by about 200 deg. C. The graph below shows how much the temperature of air rises as it is compressed.

(The Diesel Engine makes use of this heating effect. Instead of going up to 7 bar, a Diesel will compress the air by a factor of about 20. This causes the air to get so hot that fuel will ignite without needing a spark.)

This heating effect can be reduced by constantly cooling the air and by taking the air up to 7 bar (absolute) pressure in 2 stages. It is not normal for a single stage compression to go above 5 bar, so that 6 bar needs a second stage of compression. This method is also more energy efficient.

The equation mentioned above P x V= constant x T shows that temperature has an effect on pressure and volume. This means that compressors are more efficient at very low temperatures. Cold air is denser than hot air and occupies a smaller volume. If air is compressed in several stages and cooled down at the same time, more air can be pushed through, so the compressor is more efficient.

In summary, the energy consumed by a compressor is controlled by 3 factors:

·       The overall pressure achieved (usually 6 bar)

·       The temperature of the air entering the compressor

·       The design and efficiency of the compressor

In order to achieve this efficient use, a compressor should be operated under the following conditions:

·       Make sure the discharge pressure is only slightly above the pressure needed in the system.

·       Use the coolest air available, i.e. from outside the compressor room and at an elevated level above the roof. Compressor rooms are usually hot.

·       Avoid pressure drops going into the compressor. In practice this means wide pipework

·       Regularly clean air filters before the compressors, so that they do not block.

Control of compressors

The demand for compressed air will vary continuously in a brewery.

A pressure sensor in the system will indicate when air is needed or not. Usually there are several compressors and these can be started up or stopped as required.

It is not desirable for large motors to frequently stop and start. Small machines can do this more easily.

If a brewery has a mixture of large and small compressors then the large compressors usually run continuously. The small compressors are used to meet variations in demand.

The manufacturers of air compressors now usually supply complete control systems. This can involve the use of variable speed drives on the compressor motors. This meets small variations in demand. Only larger variations lead to a machine being stopped or started. This means that there is less wear and tear on the machines. There is also a saving in running costs.

Water in compressed air

Air can contain quite a lot of water vapour. In cold dry climates, the amount is small, maybe only 2 gms in a cubic metre.

In tropical maritime climates with a temperature of 30oC or more, this rises to about 30 gms in a cubic metre. In hot countries, a change in temperature leads to heavy rain. In contrast, at the South Pole it is so cold that it hardly ever snows because the air is so dry. The graph below gives you an idea of how much moisture is held in air as its temperature rises.

When air is compressed, this water vapour is also compressed but the temperature rises.

Once the air is cooled down, it is over-saturated with water vapour and condensation occurs.

This water has to be removed; otherwise it will gradually fill up the system. It will also cause corrosion, and may freeze and cause blockages. Some applications need very dry air. It is important to dry air for powder and grain conveying so that malt or flour is kept dry and does not stick and form lumps, or go mouldy. Machinery and controls need dry air so that corrosion does not take place.

Dew point 

When describing the amount of water held in air, we often refer to the Dew Point. The Dew Point of air is defined as the temperature at which air becomes saturated and moisture starts to condense at a given pressure. For example, air may feel dry during the day but will contain quite a lot of water. When night comes the temperature drops and dew forms. The air outside feels damp, but the total water content is still the same. It is important to install a dew point meter at the air dryer to contain the moisture in the air.