Energy efficiency in industrial compressed air systems

Liran Dayssi, EL-AV Compressors Ltd. March 14, 2021

Recent years have seen a significant increase in the energy costs of industrial plants, namely water, gas and electricity. This has caused manufacturers to put an emphasis on energy efficiency, while reducing wasteful energy consumption in the plant and making production processes more efficient and cost-effective. Central to this is the cost of electricity, which constitutes a significant part and at times the main part of the plant’s energy costs. The various electricity costs consist of all kinds of consumers such as heating, air-conditioning, lighting, work tools, and more. However, one of the largest consumers of electricity in most manufacturing plants is the compressed air system, which could be 50% and even more of the plant’s electricity costs.

A large portion of manufacturing plants tend to ignore the compressed air system, a mistake that costs the plant a great deal of money. Outdated compressed air systems or systems that are not properly maintained lead to high, wasteful electricity consumption, something that can be significantly remedied by making several changes. In most cases, there are solutions that can significantly reduce electricity costs, possibly even 70-80% of electricity costs for compressed air production.

Savings in electricity costs for production of compressed air will directly affect the plant’s production costs, and thus improve its profitability.

In the following are several ways in which we can improve the efficiency of the compressed air system:

Energy survey for compressed air system

The survey is a tool that can be used in order to get a status report of the compressed air system and its efficiency. Performing an energy survey by a professional company with sophisticated equipment, including a detailed report when completed, will give the person in charge [of the compressed air system] an accurate picture of the plant air requirement throughout the workday, as well as its efficiency. In addition, the report will give a picture of the quality, efficiency, and general level of all system components (dryer, water drains, etc.). The report will give the reader the tools with which to improve the efficiency and operational condition of the entire plant air system.

It has been proven that performing a professional, in-depth energy survey of the compressed air system will lead to significant reduction (in the tens of percent) of the system’s energy and operations costs.

Example from energy survey report:

The air compressor is the central component of electricity costs in the production of compressed air. There are several technologies of compressors for the production of compressed air, such as piston compressors, vane compressor, and more. At present, the most common and effective of them is rotary-screw compressors; however, even with this technology there are different levels, quality degrees, transmission technologies, and more.  The oldest and least efficient technology is that of start/stop (on/off), which does not take the plant air demand into account at all. The next in line is the variable speed (inverter) compressor, which changes its output according to plant air consumption, and thus significantly reduces electricity consumption. The most efficient technology today is that of a variable speed compressor with a permanent magnet motor, which is significantly more efficient than the standard motor, from both output and flexibility of speed variation aspects.

In the compressor itself, the highest cost throughout its length of life is the cost of electricity, as can be seen in the following illustration:

As can be seen in the diagram, about 85% of the compressor’s costs throughout its life is that of electricity; the remaining 15% are its actual purchase and current maintenance costs. An average of about 35% of electricity costs can be saved by upgrading the compressor to a variable speed (inverter) compressor. For example: A 100 hp (75kW) compressor working about 7,000 a year, with hourly electricity cost of NIS 45-50.

Its annual cost of electricity will be about NIS **350,000 and up, considering that it is not variable speed, of high quality and in good condition. If we upgrade this compressor to an identical compressor with variable speed, the annual savings will be about NIS 122,500. In essence, throughout the life of the compressor, the cost of electricity will be about NIS 3.5 million, and the savings about NIS 1.25 million.

At present, in addition to the inverter there is also an electric motor technology called permanent magnet, that provides 15% higher efficiency compared to a variable speed compressor without a permanent magnet motor, and improves the compressor’s air production capacity and electricity consumption.

By upgrading to a compressor with permanent magnet technology, an average of about ***50% can be saved compared 35% for a standard inverter type compressor.  In addition to the technological upgrade, it is recommended to adjust the working pressure to actual plant requirements. Working at pressure higher than that required, leads to higher air production costs, sometimes by tens of percent.

^{* The measured length of life is 10 years.
** In order to calculate the annual cost: motor power is calculated 75 kW x 1.2 SF - 90 Kw; the cost of electricity according to the Israel Electric Company is NIS 0.5309 and 7,000 annual work hours: 90X0.5309X7000=334,467.
*** The calculated savings is vis-à-vis a compressor without variable speed (inverter).}

In addition to the air compressor, there are additional components in the air system, the upgrade or replacement of which would reduce electricity consumption, such as:

• Old and inefficient air dryers, mainly adsorption dryers that lose a large amount of air in each work cycle. In the new models with advanced control systems and valves, savings of about 50% can be achieved in energy consumption, compared to the old models. Upgrading old air dryers operating with inefficient refrigeration compressors, and obsolete liquid drain systems could improve energy consumption by another 15% to 20%.
• Timer-operated water drains that lose a great deal of air while draining liquids can be replaced with smart drains operated by liquid level, thus saving about 70% electricity costs.
• The air reservoir, when matched to plant needs and the air system together, will have a significant impact on the efficiency of the air system, as well as the plant air piping.
• The plant air piping is an important component in the energy efficiency of the compressed air system. Proper design of the pipe routing, piping diameter and correct routine maintenance, including detection and repair of leaks, can improve a compressed air system’s efficiency by tens of percent.

Repair of air leaks in plant piping and production machines will lead to savings of tens of percent. Many factories fail to attribute importance to this issue; nevertheless, air leaks are equal to wasted energy, estimated in hundreds of millions of shekels a year in Israeli industry.

Table for calculation of air leaks and their cost

This table presents the amount of air that leaks each year, according to hole diameter and working pressure in the air pipe. In order to demonstrate the cost of a leak (or leaks) in money, we shall convert the amount of air into kWh (kilowatt-hour), the energy we would have invested in producing the air. For example, at 1/4” diameter and 7 bar pressure, the amount of air leaking is 2,945 liters of air per minute. In order to produce this amount, wi will invest from 18 to 22 kWh (depending on the type and condition of the air compressor).
An average of about 20 kWh, cost of 1 kWh for industry (from Israel Electric Company data) is NIS 0.5309 per kWh. A factory working about 7,000 hours a year with a compressed air system will waste about NIS 74,000 a year on such an air leak.

Summary

To conclude, we can determine that compressed air systems may be very large energy consumers. As you’ve read in the article, there are many different ways of improving the efficiency of the compressed air system in your factory. It is highly recommended to consult with a professional company, with broad knowledge in this area. It is recommended to invite a professional to perform a survey of the compressed air system, and receive recommendations on how to act in order to improve its energy efficiency, and achieve savings in electricity costs to the factory.