6.3.1 Air Compressor Definition and Principle

“Compressed air”

An air compressor is described as a pump which takes air from the atmosphere and, with an input of energy (using an electric motor, diesel or gasoline engine, etc.), compresses it in one or more stages to a smaller volume with higher pressure and temperature.

By one of several methods, an air compressor forces more and more air into a storage tank, increasing the pressure. When the pressure increases, the temperature also increases, as per the basic laws of compression.

6.3.2 Reason for Cooling the Air During and After Compression

When the air is compressed, its temperature increases. This increase in air temperature causes several problems:

1. Lubrication and Fouling: High temperatures can cause problems with the lubrication of the cylinders and lead to fouling as the oil breaks down on the discharge valve surfaces.

2. Density Loss: There is a reduction in air mass flow (throughput) because air at a high temperature is less dense (less useful air is stored).

In order to overcome these problems, cooling is necessary during and after compression. Additionally, air contains moisture; cooling helps to remove this moisture from the compressed air, preventing damage to downstream equipment.

Compression may be done in multi-stages with intercooling to increase energy efficiency.

6.3.3 Storage of Compressed Air

Compressed air is stored in steel reservoirs until required for purposes.

The compressed air has to be stored in order to ensure that a supply is readily available at all times. The air storage tanks or reservoirs are pressure vessels and must conform to all rules relating to the construction of such vessels. The steel from which air storage tanks are made is good quality low carbon steel, similar to that used for boilers.

6.3.4 Purpose of Compressed Air

Compressed air serves several critical functions on a vessel:

• Starting of the marine I.C. engines present onboard the vessel.

• It also supplies control air to the machines and systems. (This air can be used for various uses such as control purposes, and this is known as control air.)

• Service air is supplied from the air bottle for general service purposes.

• If the quick closing valves are air operated, safety air is supplied through an air bottle. (Quick Closing Valves can be controlled by mechanical, hydraulic, or pneumatic transmission. They are suitable for marine services, providing a quick shut-off in emergency situations from outside the engine room, mainly used in pipeline systems with combustible liquids.)

  

6.3.5 Air as an Ideal Gas and Basic Theory

Air can be treated as ideal gas (The perfect gas) and relationship is PV/T=C

Basic Theory of Compressing Air:

The air we breathe has two major constituents: Nitrogen and Oxygen. The approximate composition of atmospheric air is:

• By Volume: 78% nitrogen, 21% oxygen, and 1% other gases.

• By Weight: 76% nitrogen, 23% oxygen, and 1% other gases.

A perfect gas is a theoretical gas that differs from real gases in a way that makes certain calculations easier to handle. Even though air is not a “perfect” gas, the presence of nitrogen and oxygen in major proportion makes it obey very closely to a “perfect” gas.

A “perfect” gas obeys some laws:

Boyle’s Law (PV =C)

For a fixed mass of gas at constant temperature, the volume (V) is inversely proportional to the pressure (P).

Charles’s Law (V/T =C)

For a fixed mass of gas at constant pressure, the volume (V) is directly proportional to the Kelvin temperature (T).

Combination Law

The above two laws can be combined to form a combination law which can be represented as:

PV/T = C (Constant)

6.3.6 Compression Process Relationship: PV^n = Constant

What Happens to Air When it is Compressed?

Keeping the combination law in mind (PV/T =C), when the air is compressed, the pressure and temperature of the air increase as the volume of the space containing air reduces.

• Boyle’s Law (P is inversely proportional to V): As the volume of the space containing air reduces, the pressure increases.

• Charles’s Law (When V is constant, P is directly proportional to T): As the pressure of the air is increased due to compressing, the temperature also increases.

Thus, as a result of compressing air, the pressure and temperature increase as the volume decreases.

Compression Cycles and Types

Air can be compressed in three ways:

1. Isothermal compression.

2. Adiabatic compression.

3. Polytrophic compression.

4. 

6.3.6.1 Isothermal Compression

Isothermal compression is compressing air with no change in temperature, or at a constant temperature (T =Constant). To achieve this, the total amount of heat produced during compression has to be removed at the same rate.

To achieve this same rate of heat exchange, the design of the compressor must be such that it moves slowly, allowing more time to extract the heat as it is generated. Thus, during isothermal compression, the work input is the least and involves no temperature change.

The isothermal compression process can be symbolically represented as:

PV^n =C  (Where n = 1)

6.3.6.2 Adiabatic or Isentropic Compression

Adiabatic compression means that no heat is given to or taken from the cylinder walls of the compressor Also. all work done in compressing the air is stored in the compressed air itself. Thus, this process takes maximum energy input, as no heat loss takes place through the cylinder walls.

Theoretically, the compression process can be represented as:

PV^n =C  (Where n larger than Y)

6.3.6.3 Actual or Polytrophic Compression

The actual compression process is termed as polytrophic compression. From the pressure-volume curves, it is evident that the work done in the adiabatic process is more than the isothermal process. Since isothermal compression requires the least work input, it is the most desired process in an air compressor.

However, in practice, true isothermal compression is not possible to achieve. The polytrophic process is the only practical process, falling between the ideal isothermal and the theoretical adiabatic.

Theoretically, the compression process can be represented as:

PV^n =C  Where n = Y