Energy saving with VSD controlled compressors

2011-06-01 - Using a variable-speed AC drive is one of the most effective ways to save energy. While other energy saving methods may shave singular percentage points off the overall consumption, a variable-speed drive frequently saves 30 percent or more of the energy in many centrifugal compressor applications

Electric motors use two-thirds of all electricity in industry, so any chance to reduce this load, even by single figures, is highly significant. Offered the chance to cut energy consumption by up to half, industry ought to jump at the opportunity. Yet, variable-speed drives (VSDs) are only installed in a minority of applications. The reasons often boil down to the fact that the people who design these applications are not responsible for the energy bill. At the same time, the people in charge of the energy bill, in most cases, are not engineers and are not aware of the benefits VSDs can offer.

How VSDs can help
The main applications for VSDs have been pump and fan applications. But as VSD technology has progressed other applications, such as compressors, are beginning to reap the benefits of VSD control.

A VSD controlled air compressor, for example, uses an AC drive to control the speed of the unit, which in turn saves energy compared to a fixed speed equivalent.

VSDs reduce the energy output of a compressor, by controlling the speed of the motor, ensuring it runs no faster than necessary. The traditional way of controlling a compressor is by running the motor at full speed and then stopping it when the air has been compressed to the correct pressure. It is then stored in a reservoir at a slightly higher pressure than is needed, to allow a hysteresis in the pressure. This “on-off” method is wasteful, because the motor keeps running at its nominal speed regardless of the requirement. Some compressors are designed with a bypass system which returns air from output to input, which is again wasteful.

The problem of energy waste is made worse by the fact that many motors are undersized. This is because they are short term rated and only pull high currents when they load up. For example, a recent a 75 kW motor on a compressor was monitored and found to have an absorbed power of 83 kW on load. This is very common in screw applications.

The 'oversizing' with compressors is related to the actual output of the compressor not the motor. Compressor manufacturers often sell compressors for worst case scenarios that are essentially too large for the applications.

These 'oversized compressors' can easily be identified by the off load running hours when compared to on load hours. Compressors with high run hours and low load hours are ideal for conversions.

VSDs work by controlling the waveform of the current and voltage supplying the motor. A VSD converts the incoming AC power to DC and then back to a quasi-sinusoidal AC power using an inverter switching circuit. The movement of the motor shaft can be adjusted with great accuracy, ensuring that the application gives the performance needed. The benefits of this technology included reducing power cost, reducing power surges (from starting AC motors), and delivering a more constant pressure.

Typically, a fifth of a factory's electricity bill is attributed to the production of compressed air. The majority of modern factories are heavily involved in cutting costs, and energy awareness is a key concern. For example, 10-12 percent of all power generated in the UK is dedicated to the production of compressed air, and a portion of this power is wasted energy.

Large electrical cost savings can be achieved by installing a VSD compressor in place of an existing rotary screw or piston machine. Because of this, many governments are pushing the industries to move towards this technology in hopes of reducing wasted energy.

So which type of compressors can benefit the most from the application of VSDs?

There are two different basic compressor designs - positive displacement and dynamic – and, as such, applying VSDs to compressor applications are bit more challenging than with pumps and fans.

Positive displacement compressors
There are two main types of positive displacement compressor – reciprocating and rotary-screw.

Reciprocating compressors use pistons driven by a crankshaft. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Discharge pressures can range from low pressure to very high pressure (>35 MPa). In certain applications, such as air compression, multi-stage double-acting compressors are said to be the most efficient compressors available, and are typically larger, noisier, and more costly than comparable rotary units.

Rotary screw compressors use two meshed rotating positive-displacement helical screws to force the gas into a smaller space. These are usually used for continuous operation in commercial and industrial applications and may be either stationary or portable. Their application can be from 2 kW to over 375 kW and from low pressure to very high pressure (>8.3 MPa).

These kind of compressors, piston and screw type, for example, have the capability to create pressure increase, independent of driven speed.

This implies that this type of compressor does not need any mechanical volume control devices, but that the required capacity can be controlled directly by operational speed.

Energy consumption of positive displacement compressors is directly proportional to the speed and this type of compressor, unlike a centrifugal compressor, does not have limitations to its speed range, especially when lubrication and cooling will not be hazardous.

Positive displacement compressors with VSD control save energy at any load condition.
Care must be taken when installing VSDs to ensure that the machine is suitable for variable speed operation, by examining minimum speed for the lubrication system and for the seals, but this does not normally create problems.

Dynamic compressors
The most common dynamic compressor is the centrifugal type which uses a rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser section converts the velocity energy to pressure energy. These compressors are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants. Their application can be from 75 kW to thousands of kW. With multiple staging, they can achieve extremely high output pressures greater than 69 MPa.

The most common way to control the capacity of centrifugal compressor is to modulate inlet guide vanes. As the load decreases, the mass flow of the air needs to be reduced. Traditionally the compressor is run at the same speed, but the air flow is reduced by closing inlet vanes to match the required capacity. Centrifugal compressors follow the same affinity laws as that associated with centrifugal fans and pumps.

This implies that speed reduction also reduces the compressors capability of generating pressure increase, or lift. This means that a centrifugal compressor’s operational range through speed variation is limited.

However, remarkable savings can be achieved thanks to the cubic relationship between speed and power which states that the need for power increases with the cube of the speed. This means that a small increase in speed requires a lot more power, but also that a modest speed reduction can give significant energy savings. A compressor running at half speed consumes only one-eighth of the power compared to one running at full speed.

Applying VSDs to centrifugal compressors, while still maintaining inlet vanes, brings energy savings that can be estimated by comparing the required pressure to the pressure generating capability of compressor at a certain speed.

Benefits of VSDs
So the advantages of using VSD control are that production of compressed air precisely follows the varying demand. As a result, direct energy savings of between 5 to 35 percent can be achieved with lower energy consumption at partial loads. Furthermore, precise control allows lower discharge pressure. For example reducing system pressure from 7 to 6 bar reduces energy consumption by 7 percent.

Indirect energy savings add up to another 10 percent due to lowered net pressure, thereby reducing system pressure and reducing leakages.

VSD control of compressors also brings improved process control by way of a stable pressure independent of air consumption along with less mechanical stress.

Other benefits include a stable system pressure through a rapid reaction to pressure changes, typically within 0.1 bar; smooth starting leading to no peak current penalties, less stress on mechanical parts and unlimited start-ups; and a fixed power factor.

Proven VSD-controlled compressors
Despite VSD-controlled compressors being relatively new applications, several installations have been successfully tried and tested.

In Belgium, a 275 kW fixed speed compressor was converted to a 315 kW variable speed compressor. During testing with three different load profiles, savings between 18 and 25 percent resulted in savings of over €15,200 per year, with a payback of just three years.

Meanwhile a European biochemical company had a demand for oxygen with very constant pressure but where the oxygen volume is highly variable. The system was earlier controlled by switching on and off two compressors of different sizes. Because of problems with high power consumption, noise level and maintenance cost the larger compressor was equipped with a VSD. This resulted in energy saving of about 1,700,000 kWh/year with a resulting reduction in CO2 emissions of 850,000 kg/year.

Life cycle costs
Many users favour traditional control methods because these methods are easy to implement and straightforward to understand. Many will also justify this approach by assuming that the cost of the wasted energy is less than that of buying a VSD. But this is not true. Unlike many other energy saving methods which may only reduce energy consumption by one or two percent, the very significant savings produced by a VSD means payback is often achieved in a year or less. The application will then continue to make a very significant contribution to energy savings, year after year, for as long as it remains in use.

Energy appraisal
In order for a company to reduce energy costs, it needs to evaluate how it uses energy and air. A professional air audit is the best way to see if a VSD compressor is right for your application. These audits are available from various compressed air specialists, which can tell you how much your company can save by installing a variable-speed drive compressor.

In addition, VSD manufacturers, such as ABB, offer an energy appraisal which is a systematic examination of key applications that include the monitoring of energy consumed before and after the change to VSDs.

An energy appraisal defines where energy can be saved and quantifies how much energy can be saved with the installation of VSDs. These figures are then translated into a possible monthly saving, the amount of money that will be saved, in energy bills alone, if the equipment is installed.

It is not unusual for users to dismiss the promise of 50 percent energy saving on a 20 percent speed reduction as the exaggerated claims of a manufacturer. However the savings can be verified and the best way to start is with an energy survey. This will enable you to see the potential savings in black and white, enabling you to make the decisions that bring your company improved profitability.


Steve Ruddell
ABB Limited
Tel: 01925 741 111; Fax: 01925 741 212;
Email: energy@gb.abb.com; Web: www.abb.co.uk/energy

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