Compressed Air, What About It?

Compressed air is primarily used for two completely different applications on a ship: starting diesel engines and supplying service air.

For starting an engine, a large quantity of high pressure air is needed on a sporadic basis. For the service air, a small quantity of continuously flowing, low-pressure air is required.

Producing compressed air is expensive. Several government agencies that focus on energy efficiency regularly raise awareness of this reality with industrial companies.

 

What About Ships?

When conducting our energy audits, we often find compressed air systems in poor shape. It seems that, as long as there is pressure in the tanks, they are not given much attention, if any. It is often when the compressors operate continuously and the air pressure is barely maintained that they are given attention. This critical stage often occurs after months of deteriorating and neglected operating conditions.

Producing compressed air is relatively expensive, and the higher the pressure the more expensive it is. One cubic metre of compressed air produced at 20 bars cost more than twice as much as the same amount of compressed air produced at 6 bars.

It quickly becomes obvious that the best way to reduce energy consumption is to use a compressor adapted to a ship’s needs. Taking the compressed air destined to start the engine and using it to produce service air is the best way to double the compressor’s energy consumption. Yet, this is what is being done on many ships. Moreover, this does not take into account the added maintenance costs that this practice engenders.

The main reason for this is that the economic criteria retained during the design/construction of a ship are not the same as those found when it’s in operation.

For example, installing a screw compressor that modulates according to pressure demand is less expensive to purchase than a piston compressor of the same capacity. However, the screw compressor can consume 30% of its power capacity even when it’s not pumping, which has the effect to raising the cost of each cubic metre of air produced.

A screw compressor can be useful to provide a large quantity of air intermittently (‘air on deck’, for example), but on a small system with modulating demand, it is definitely not the right device for the task.

The least expensive energy is the one that is not consumed. A 5 mm leak in a compressor set at 6 bars requires approximately 8 kW of energy. Because of this leak, 15 tons of fuel will be consumed over a period of one year.

During the course of our energy audits, it is not rare that we are able to reduce the amount of energy required to produce compressed air on a ship by 30% to 50%.

With our calculations, we are able to demonstrate that the actions taken to improve the performance of these systems often pay for themselves in less than two years.

All ships are different but, as a rule,  a ship should not consume more than 2,000 cubic metres of compressed air per day. And the majority of this air should be service air produced at a maximum of 6 bars.

 

 

Here is an example of the worst and the best case scenarios:

An energy audit will allow you to update your compressed air system and much more.