What to expect from an effective compressed air audit

Compressed air has moved to a higher level in the field of energy saving, as the production of compressed air is up to eight times more expensive than electricity, a quarter leakage of wasted energy costs 9,000 euros. This increased awareness has also brought with it compressed air audits. With audits available from many sources, it is important to understand what plant operations, engineers and maintenance managers should expect from a full audit – or more appropriately – a full review of the air system.

Start from scratch

Compressed air system costs are rated with a medium to high four value or a low five value. However, payback is often measured in months – not years. These investigations enable the development of longer-term downstream options that continue to reduce operating costs. Some devices have seen reductions of up to 80 percent.
Although many companies view energy costs as a variable cost, in reality, the savings in energy production costs from a well-managed and well-controlled compressed air system is a reduction in fixed costs. The reason is that when electricity consumption decreases, production remains the same or even increases.
An energy saving of 100,000 euros will have the same effect on net profit as sales of 2,000,000 euros at a net margin of 5 percent. Expect an effective plant air system survey or system audit to reduce air emissions and energy costs from 25 to 50% almost immediately. Below are some frequently asked questions and questions about what causes energy costs in compressed air systems found during the survey:

Is it a leak?

Excessive pressure?

Lack of air saving devices such as venturi nozzles, air curtains and the like?

Producing high pressure air for low pressure use only?

Bad choice of primary air supply?

Bad choice or use of compressor controls?

Bad selection or use of filters, dryers, connectors and regulators?

Bad placement of the air receiver?

Wrong pipe?

Bad choice of compressed air to get the job done vs an alternative power source?

How can you compare relative costs if you don't know the cost of compressed air?

Or, all of the above?

The truth is, it's always "all of the above" and more.

The survey must provide an overview of compressed air system quality at a cost commensurate with system size, complexity and potential renewal. The survey should develop both short- and long-term plans for establishing basic control and management of the air system.
Focus on what is needed to bring the interrelated parts of the system together and allow the user to understand the "basics" of these parts and their relationship. Create a general guide that you can use to continue to improve the efficiency of the system. Identify specific programs and actions to be implemented with estimated costs and returns. Develop a full sustainability program that would include baseline plans for expansion if needed.
This minimal effort is not expected to yield the ultimate return associated with a full and detailed system analysis and training. An initial audit should lead to additional control programs or even to fully controlled and fully controlled compressed air systems. A well-thought-out, simple audit has its place. This can result in significant savings, a success that should lead the user to pursue deeper programs in the future for efficiency and return. Audits that do not include this should be considered unacceptable.

Evaluation of the system from the supply side

Evaluate the suitability of existing air compressors in terms of use and general apparent performance and condition (without disassembly or mechanical work). The report should address the assessment of efficiency, the suitability of insertion controls and the capacity to translate lower air demand into lower energy costs. If necessary, obtain facts about modification options, including: system reduction capabilities; installation and support systems such as cooling water and ventilation; and general advice on alternative types of equipment and controls that may be more suitable or more effective.
Expect an evaluation of the compressed air conditioning equipment, specifically:

- In the case of an aftercooler, you want to know the efficiency it has when delivering 38°C of the highest ambient temperature. If not, then determine the possibility of using auxiliary coolers at the dryer inlet. You will also want the installation geometry manual for the dryer.

- Ensure dryers and filters are suitable for the application with respect to size, efficiency, pressure drop and controls. Learn about possible modifications that would improve performance and efficiency. Are automatic drains applied correctly, equally well dimensioned? Is the sizing and installation correct to meet the highest ambient temperature?

- Supply-side piping between the compressor and system reservoir should be evaluated for suitability, efficiency and pressure drop. Finally, the location of the air receiver should be consistent with proper control procedures, storage of dry air, and its ability to operate with adequate storage to compensate for the delay in response to capacity control.

- It is essential that the operational control band for the compressor capacity controllers includes the primary dry air reservoir for the production area to ensure that the control will operate in relation to actual production and not in relation to spurious signals from internal piping and other pressure spikes.

An effective review or audit evaluates all generated compressed air, treatment and distribution of air to the production area. They may not recommend new equipment, but should identify any currently available operating cost advantages and/or generic alternatives for future consideration.
All recommendations should refer to the impact on productivity and quality. Are the unloading controls capable of translating lower airflow demand into lower electrical usage? This is the most critical issue and capabilities depend on things like compressor type, control type, equipment condition, piping and storage capacity.

Proper audits reveal significant savings

Once the supply side is re-evaluated and adjusted as required to achieve controlled and monitored operation for sustainability, performance should remain optimal unless improper changes are made.
For example, a Central European plant had twenty-two lubrication-cooled rotary screw compressors running simultaneously (Figure 1). A previous audit had them all at full load and recommended three more units. The factory staff also felt that all units were at or near full load and that additional units would be needed.
A second professional system analysis revealed that several units were actually at no load and all other units at part load (Figure 2). Duct reconfiguration allowed the device to deliver the same airflow at the same pressure with only 11 units. This resulted in energy savings of 800,000 euros per year with a simple payback of 6 months.

Diagram 1: Current system

Scheme 2: Proposed and modified system

Additional savings on maintenance were also significant.

Imagine the difference in fixed maintenance costs between 11 operating rotary screw compressors and 22!

Summary on the offer page

What are the key points we are looking for and what is their potential range of savings? For supply management, these questions are:

- Different types of compressors and their base performance can vary by 5 to 10 percent at full load. With conventional motors and controllers, this can affect the energy bill by 5 to 10 percent. A generally acceptable installation arrangement includes suction and discharge pipes.

- Storefront control types and their ability to translate reduced demand into lower energy consumption can reduce savings from 10 to 70 percent at no-load for conventional motors and controllers and impact energy costs by up to 50 percent.

- What is important is the efficiency of the engine and the basic performance characteristics of the main controller. Is it possible to install an efficient variable speed drive (VSD)? If so, the potential part-load savings could be between 5 and 25 percent, but more than one VSD is rarely required. VSD drives do not have to bring significant energy reduction with oil-free rotary screws.

- Dryers, filters, and every 1 bar of lost pressure can represent 1/2% of wasted energy if the compressor is running at a higher pressure. Each automatic drain has the potential to waste between 300 and 1000 euros per year. Too much pressure drop will also affect available effective storage and performance control.

Analysis of the compressed air system on the demand side

Another case is the demand side. The identification and implementation of energy-saving projects is always very important. Continuous and rigorous monitoring and review is essential to maintain sustainability.
Current projects will ultimately outpace supply-side savings. Many of the general areas of energy conservation continue to create significant new opportunities for the future as technology (especially electronic) continues to improve. Many processes such as blowdown efficiency, air driven vacuum generators, cabinet cooling and diaphragm pump control are evolving with new energy efficient designs and controls. All processes should be reviewed every two to three years.

Demand side management / waste management

We have often classified waste management and demand-side management as separate sectors, but they are actually two parts of the same entity. You should look at waste management separately, because it goes without saying, but is often ignored.
It requires the right equipment in the hands of trained personnel to accurately evaluate the size of the leak and the measured performance of the system to determine the true euro recovery value of the electrical energy recovery for its repair and removal. For example, an air leakage of 1/4 could be equivalent to the cost equivalent of running about 286 light bulbs at 60 watts.

Leakage Implementation and Control Program

A continuous, economical program must be in place that effectively conserves air and controls leaks in the plant. Generally speaking, the most effective programs are those that involve production supervisors and operators working with maintenance personnel in a positive manner. Most leak surveys identify a leak rate of up to 20-35% of the total air flow. This has a minimal impact on the energy bill - 10 to 20%.
Consider these suggestions:

- Set up an ongoing leak inspection program by maintenance personnel so that each sector of the factory is inspected once every four to six weeks. Identify and repair leaks. Keep records of these findings and results. Leakage can be quantified and measured to estimate the flow and assign a measurable cost value to it.

- Install a leak detection and repair program that positively motivates production workers (especially operators and supervisors). Establish and monitor air conservation programs.

- Monitor air flow to each responsible sector (eg use of recording or non-recording flowmeters) and identify air usage as a measurable part of that area's operating costs. Production supervisors, who are closest to the problems, are not too often responsible for compressed air costs.

- This usually works best in combination with an effective internal training and awareness program for those involved. Leak detection and repair programs provide greater energy savings at significantly lower costs than any other procedure.

- Go through and evaluate the compressed air distribution system. Examine and identify air using tools and processes. The most important thing is to talk to the operations and maintenance staff to find out the options for waste management and demand side management.

Operate at the lowest effective pressure

This is one of the most overlooked areas of opportunity in compressed air system cost management. Identify the areas in the plant and the lowest possible pressure at which they can operate with optimal production.
Identify areas in systems experiencing unregulated air with resulting "pressure spikes". Note that the actual volumetric air flow rate is directly proportional to the inlet pressure for any fixed demand. Extra pressure means extra airflow without a corresponding increase in production. Excessive pressure from too many processes (air tools, air driven vacuum generators, etc.) can have a negative impact on production and quality.
Identify areas of wasted but controllable compressed air performance in the air distribution system – particularly areas of excessive system or end user pressure drop.

- Develop methods to measure airflow supplied to various departments and areas of use for downstream programs.

- Identify the minimum optimum flow and pressure required to perform each task. Find ways to use flow regulators, air reservoirs, intermediate regulators, etc.

- Look at unregulated air use. Maintaining a regulated, stable, minimum effective pressure uses as little air as possible, and a stable, predictable pressure increases the productivity and quality of almost any process.

- Examine blowdown procedures and consider using air-saving devices such as venturi air boosters, venturi baffles, and vortex cooling tubes instead of open tube cooling of compressed air.

- Look at areas of use or event requirements that may need their own storage space, then use flow controllers to convert short-term high flow to low average flow.

- Examine the position and efficiency of all isolation valves. Review equipment operating procedures to ensure these valves are closed, except during the production period of their area or machine. This can be done automatically and/or controlled remotely.

- Explore higher pressure areas than most systems that can be accommodated with such things as a smaller satellite unit, small hydraulic system, pneumatic booster, etc. and will allow for an overall reduction in air pressure production.