Isothermal and isentropic air compression process | Safeties of air compressor on ships
There are two ways of air compression: isothermal and isentropic
Isothermal air compression:
We know that the isothermal process is a constant temperature process, which is not optimal for compressing air since it will be a very slow process. In an isothermal process, the work done is less and the area of the curve is less. The area below the dotted line represents an isothermal process in the figure below.
          Isothermal compression can be a useful process in certain applications, such as in refrigeration and air conditioning systems. In these applications, compressing a gas while keeping its temperature constant can help to prevent the gas from overheating and potentially damaging the equipment.
Isentropic air compression:
Adiabatic air compression is a thermodynamic process in which air is compressed without any heat transfer between the air and its surroundings. In other words, no heat is added or removed from the air during the compression process.
 During adiabatic compression, the temperature of the air increases due to the work done on the gas by the external force compressing it. This increase in temperature can be calculated using the adiabatic compression equation, which is given by:
P2/P1 = (V1/V2)^γ
where P1 and V1 are the initial pressure and volume of the air, P2 and V2 are the final pressure and volume of the air, and γ is the ratio of specific heats of the air (which is approximately 1.4 for air).
The work done during adiabatic compression is also greater than in isothermal compression, as the air is being compressed against its own pressure due to the increase in temperature. This results in a higher compression ratio, which can be useful in some applications where a high-pressure output is required. Â
Adiabatic compression is commonly used in compressors for industrial applications such as air compressors, gas turbines, and engines. However, the increase in temperature during adiabatic compression can also be a disadvantage, as it can cause the air to reach high temperatures that may cause damage to the compressor or other components if not properly managed. As a result, adiabatic compression is often combined with other cooling methods to prevent the air from overheating.  Â
         In an isothermal process the air will be compressed by a compressor in a single stage so work done will be more to produce high-pressure compressed air. But in the multistage process we see in the curve work done is the less blue area in the pic. This green area will save the work done because the isentropic line approaches the isothermal line due to which the air temperature drops to ambient temperature and workdone is saved. For this reason, multistage is used over a single stage compressor.
Safeties of air compressor in ship:
For Lube oil low-pressure trip:
In ships, compressors use a splash lubrication system. Many times this lube oil low pressure can damage the compressor as the lube oil pressure will drop, due to which we need to shut the compressor in such case.
Discharge valve temperature:
After the discharge of air the temperature of the air should be within a certain limit, when this temperature rises above a certain limit we should take action because it is a fire hazard. For this reason here a fusible plug is fitted which will melt and release all the air if the temperature is high.
Safety valve after discharge:
A safety valve is fitted after the discharge port to release the pressure when it gets higher than a predetermined pressure.
Water jacket high temperature trip fitted:
Jacket water temperature will be high this means the cylinder side has some problem causing the heat due to which the compressor can be damaged. So it trips the compressor.
Note:
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