Multiple compressor installations

In the past, our approach to selling compressor installations was centered around the notion of combining Fixed Speed and VSD (Variable Speed Drive) compressors. However, with our current wealth of knowledge and compelling evidence, we are now in a position to question this belief. There are at least seven compelling reasons to consider multiple VSDs as a superior choice.

An installation with multiple VSDs has the lowest optimal Total SER, or the minimum Total SER that can be achieved.

The graph above illustrates the Total SER for three different compressor scenarios: 1xVSD+2FS (red), 2xVSD+1xFS (blue), and 3xVSD (green).

All scenarios start with the same curve because a VSD machine always initiates the operation. As the Free Air Delivery (FAD) increases, the typical VSD curve emerges. During this phase, the VSD compressor adjusts its speed to match the required air demand, thereby optimizing energy consumption.

When the second compressor comes into play, the behavior differs based on the specific scenario. In the case of 2xVSD+1FS, the second VSD compressor starts operating, whereas for the 1xVSD+2FS scenario, the FS compressor starts. The difference in SER arises because the FS compressor delivers a constant flow, pushing the VSD compressor to a lower speed that is not optimal. This results in an increase of the Total SER.

On the other hand, in the 2xVSD scenario, either one of the VSD compressors operates as FS but at an optimal speed, enabling the second VSD compressor to work at a higher speed, closer to the optimal range. Alternatively, both VSD compressors can regulate their speeds, resulting in a lower SER.

The pattern is repeated when the third compressor goes into action. The specific behavior depends on the scenario and the combination of VSD and FS compressors.

It is important to note that this study is based on optimal conditions and environments. To mitigate the impact of deviations from the optimal conditions arising from other equipment in the airnet and downstream, a central controller is required. Such a controller ensures coordination and synchronization among the compressors, maintaining efficiency and minimizing energy wastage.

Total SER

2. Stable flow and no over-pressurization

When comparing multiple VSD compressor installations to setups with FS compressors, one important consideration is the ability to maintain a stable flow and prevent over-pressurization within the airnet. Multiple VSD compressors offer distinct advantages in achieving these goals.

In multiple VSD compressor installations, the regulation of the VSD compressors is configured based on a pressure setpoint. This allows for a narrower pressure band of approximately +/- 0.1 bar (1.45 psi), ensuring a more stable compressed air flow. In contrast, FS compressors typically operate within a pressure range with a typical difference of 0.3-0.5 bar (4.35-7.25 psi) between the minimum and maximum pressures. The narrower pressure band of VSD compressors minimizes fluctuations in the compressed air supply, resulting in a more consistent and stable flow throughout the system.

Multiple VSD compressor setups are designed to operate within the defined pressure setpoint, minimizing the risk of over-pressurization of the airnet, which could lead to a higher air leakage losses.

3. Smaller VSD compressor

In installations that combine FS and VSD compressors, it is necessary to choose a larger VSD compressor to compensate for any potential regulation gaps. This means that the VSD compressor should have a higher capacity to ensure a continuous and stable air supply, even during peak demand. The presence of a regulation gap can lead to an insufficient air supply and adversely affect overall performance.

However, when a system exclusively uses VSD compressors, there is no need for a larger VSD unit to compensate for a regulation gap. In such cases, all VSD compressors can be sized equally, resulting in a more cost-effective solution. By selecting identical VSD compressors, the investment costs, energy bills, and maintenance requirements can be significantly reduced. Additionally, having uniformly sized VSD compressors simplifies the system design and improves overall system efficiency.

By choosing equally sized VSD compressors, customers can enjoy lower investment costs, reduced energy bills, and simplified maintenance. This optimized approach ensures efficient operation and longevity of the compressors while maintaining a consistent air supply throughout the system.

4. Reduced long-term costs

In an installation with a combination of FS and VSD compressors, the VSD compressor is set to start first and run continuously. This arrangement aims to reduce energy consumption and ensures a smoother air delivery. However, the continuous operation of the VSD compressor in this setup may cause it to wear out quicker than the FS compressor. Consequently, it would require more frequent maintenance and potentially necessitate new investments sooner.

A multiple VSD setup allows the running hours to be balanced without increasing the energy consumption. This extends the lifetime of the equipment.

5. Lower operational costs

When comparing the operational costs of compressor installations with different types of compressors to those with multiple VSD compressors, several key factors should be considered.

A. Inventory and stock costs

A compressor room that utilizes various types of compressors often requires a larger inventory of spare parts to accommodate the different equipment models. This increases the internal operational costs associated with stock management and inventory control. In contrast, multiple VSD compressor setups offer a standardized approach, minimizing the need for a wide variety of unique parts. This standardization reduces inventory complexity and associated costs.

B. Staff training and expertise

Maintaining a compressor room with different compressors requires additional staff training to ensure proper handling, maintenance, and troubleshooting of each specific compressor type. This training can be time-consuming and costly. However, in installations with multiple VSD compressors, the uniformity of the equipment simplifies the training process. Staff members can gain expertise in a single compressor model, reducing the training time and costs.

C. Maintenance activities and costs

A compressor room with different compressor types usually requires more frequent and specialized maintenance due to the varied equipment specifications and operational characteristics. The need for different maintenance routines, spare parts, and troubleshooting procedures contributes to increased operational costs. Conversely, multiple VSD compressor installations benefit from standardization, which enables streamlined maintenance activities. The use of identical compressor models simplifies maintenance procedures, reduces the need for unique spare parts, and ultimately lowers service costs.

In summary, compressor setups with different compressor types tend to incur higher internal operational costs related to inventory management, staff training, and maintenance activities. In contrast, multiple VSD compressors offer advantages in terms of standardization, reducing the number of unique parts, simplifying the training process, and streamlining maintenance activities. By minimizing complexity and optimizing efficiency, multiple VSD compressor installations contribute to lower operational costs over the long term.

6. Lower noise level

When comparing the noise levels of multiple VSD compressor installations to FS and VSD compressor combinations, several factors come into play.

A. Less unload noise

In multiple VSD compressor setups, less unload noise contributes to lower overall noise levels. Unload noise typically occurs in FS compressors when the compressor switches from load to unload mode. VSD compressors, on the other hand, can adjust their speed to match the required air demand, minimizing the need for unload and reducing noise levels.

B. Reduced gear noise

In combined FS and VSD compressor installations, FS compressors often incorporate gears to maintain a fixed rotational speed. The operation of these gears can produce additional decibels, adding to the overall noise level in the compressor room. Multiple VSD compressor setups, however, do not generate this noise since VSD compressors operate without gears.

In summary, multiple VSD compressor installations offer lower noise levels compared to FS and VSD compressor combinations. Lower blow-off noise and reduced gear noise contribute to a quieter operating environment. By minimizing these noise sources, multiple VSD compressor setups provide a more comfortable workplace and reduce the impact of noise-related issues.

7. Higher power factor

Reactive power is an important consideration when comparing installations with VSD compressors to those with a combination of FS and VSD compressors. Higher reactive power results in a lower power factor.

In FS and VSD combinations, the FS compressor typically has a lower reactive power factor. When the FS compressor operates with a lower power factor, it can lead to reactive power losses, resulting in increased energy costs and possible penalties, depending on the contractual agreements with the energy supplier.