Compressed Air – Utilities in a production facility
Compressed air is one of the primary utilities serving a production facility and has multiple applications, including the operation of pneumatic motors (e.g., hammers, screwdrivers, mixers), the functioning of pneumatic machinery (machine tools, pumps), specific production activities (drying, painting), and in the process industry, it is used as Instrument Air (valves and instruments).
Proper management, sizing, and installation of the compressed air system are important for three reasons:
- Reduction of Operating Costs
- Prevention of Damage to Machinery and Equipment
- Avoid Production Downtime
Table of Contents
How to size your compressor
The two fundamental data points for correctly sizing a compressed air system are as follows: total air flow rate (usually measured in Nm³/h or scfm) and operating pressure (usually in bar, atm, or psi).
Difference Between Nm³/h and m³/h
When estimating the total air flow rate for a compressed air system, it is important to remember the difference between Nm³/h and m³/h.
Nm³/h (“normal cubic meters per hour”) does not represent the actual air flow rate but is a standardized flow rate under so-called Normal Conditions (0°C, 1 bar). A similar concept applies to the imperial unit scfm (standard cubic feet per minute). Typically, manufacturers will provide the estimated air consumption data in Nm³/h or scfm. If the data is provided in m³/h or other actual flow rate units, it must be converted to standard units. To convert m³/h to Nm³/h, knowing the actual pressure and temperature of the application, you can use the following formula:
Flow rate in Nm³/h = Actual flow rate in m³/h * (Pressure in bar) * (273.15 / (273.15 + Temperature in °C))
For example, for a machine requiring an air flow rate of 1 l/s at 6 bar and room temperature:
Flow rate in Nm³/h = 3.6 m³/h * (6 bar) * (273.15 / (273.15 + 25)) = 19.78 Nm³/h
Actual flow rate in m³/h = 1 l/s * 3600 / 1000 = 3.6 m³/h
How to Estimate the Total Air Flow Rate for the Compressor
To estimate the total air flow rate for sizing the compressor, you need to create a list of all the equipment that requires compressed air and determine the required flow rate for each. Typically, manufacturers will provide this data in the technical specifications. If the data is unavailable, you will need to make an estimate. For example, for pneumatic valves, a safe estimate is typically 1 scfm (1.61 Nm³/h) per valve.
To estimate the total compressed air flow rate, you need to sum the flow rates required by all the equipment. To avoid oversizing the system, consider the following:
- Continuous-use equipment: For these, you can simply add the manufacturer’s data.
- Intermittent-use equipment: For these, you need to estimate the degree of contemporaneity, as not all equipment will require compressed air at the same time. Examples include spray guns, screwdrivers, pumps, and valves. The degree of contemporaneity depends on the production process, but for standard applications, a 20-30% contemporaneity factor is typically considered.
- Safety factor: Finally, add a safety factor to account for potential peaks, future expansions, or network losses. Typically, a 25-50% safety factor is added to the calculated value.
In conclusion, the total compressed air flow rate can be calculated as follows:
Total flow rate (in Nm³/h or scfm) = (Sum of all continuous-use equipment + Sum of all intermittent-use equipment * Degree of contemporaneity) * (1 + Safety factor)
For example, for a new production facility, we estimated:
- The flow rate required for continuous-use equipment is 200 Nm³/h.
- The flow rate required for all intermittent-use equipment is 4000 Nm³/h.
- A contemporaneity factor of 20% is considered.
- A safety factor of 25% is added.
The total required flow rate will be:
Total flow rate = (200 + 4000 * 0.20) * (1 + 0.25) = 1250 Nm³/h
How to Determine the Operating Pressure of the Compressed Air System
In general, you need to determine the maximum pressure required for the operation of the equipment. Typically, a value of 7 bar is sufficient for most standard applications.
This pressure is required at the end-use point, but you must account for pressure losses along the network, which are influenced by the system design. Pipes and accessories must be sized to minimize pressure losses. A 2-3% pressure loss is typically considered a good balance between investment costs (pipe diameter) and operating costs.
How to Determine the Compressor Size in kW of Electric Power
Compressor manufacturers can recommend the appropriate compressor size based on air consumption data, required pressure, and application type.
For a quick sizing reference for most applications, you can refer to the following table:
| Consumption (Nm³/h) | Pressure in bar | Power in kW |
| 8 | 7 | 2 |
| 32 | 7 | 4 |
| 65 | 7 | 7 |
| 100 | 7 | 11 |
| 130 | 7 | 13 |
| 220 | 7 | 22 |
| 320 | 7 | 33 |
| 410 | 7 | 42 |
| 540 | 7 | 56 |
| 880 | 7 | 91 |
| 1000 | 7 | 103 |
| 1200 | 7 | 124 |
| 1500 | 7 | 155 |
| 2000 | 7 | 206 |
Energy Costs of a Compressed Air System
Regarding operating costs, it is important to consider that compressed air represents a significant portion of total energy costs. Roughly, every 1 kW of energy produced requires 8 kW of electrical energy.
Additionally, considering the lifecycle of a compressed air system (about 10-15 years), the total costs can be broken down as follows:
- 70-75%: Energy costs
- 15-20%: Compressor, accessories, piping, and installation costs
- 10%: Maintenance costs
The two fundamental principles for cost reduction are:
- Minimize leaks: A single small leak at 7 bar can cost up to €1000 per year. Older facilities may have up to 20% of compressed air production costs due to system leaks.
- Reduce system pressure: Every 140 mbar reduction can save 1% of energy costs. Therefore, it is crucial to size and install the system correctly to minimize pressure losses. Another important question to ask is: What pressure do we actually need?
Other useful considerations for reducing energy costs:
- Use variable speed compressors with inverters.
- Select the best compression technology based on system characteristics (reciprocating, scroll, screw compressors, etc.).
- Recover heat for other production processes or simply for heating.
How to Size the Piping for a Compressed Air System
When sizing the piping for a compressed air system, the main goal is to keep pressure losses low (<2-3%). Pressure losses are influenced by:
- System type (loop or single-branch)
- Pipe length (distributed pressure losses) and system details (number of bends, elbows, valves, restrictions, couplings, etc.)
- Pipe material and surface roughness
- Pipe diameter
System Type
Loop systems are preferred over single-branch systems because they reduce pressure losses, pressure fluctuations, and facilitate maintenance at individual points.
Pipe Length
The total pipe length depends on the application layout. It is always advisable to minimize the number of bends or other elements that can add pressure losses to the circuit. For very long straight pipes, thermal expansion must be considered, as it can create overpressures and lead to pipe failure. It is recommended to insert a U-bend every 50 meters to act as an elastic joint that absorbs thermal expansion.
The simplest method to account for pressure losses due to accessories is to convert them into equivalent meters of linear pipe. For example, a 90° bend can be converted into a certain number of equivalent linear meters.
For a quick estimate, refer to the following table:
| Equivalent Linear Meters | |||||||
| Pipe Diameter | 25 mm | 40 mm | 50 mm | 80 mm | 100 mm | 125 mm | 150 mm |
| Bend where R = d | 0.3 | 0.5 | 0.6 | 1.0 | 1.5 | 2.0 | 2.5 |
| Bend where R = 2d | 0.15 | 0.25 | 0.3 | 0.5 | 0.8 | 1.0 | 1.5 |
| 90° Elbow | 1.5 | 2.5 | 3.5 | 5 | 7 | 10 | 15 |
| T-Joint | 2 | 3 | 4 | 7 | 10 | 15 | 20 |
| Check Valve | 8 | 10 | 15 | 25 | 30 | 50 | 60 |
| Diaphragm Valve | 1.2 | 2.0 | 3.0 | 4.5 | 6 | 8 | 10 |
| Gate Valve | 0.3 | 0.5 | 0.7 | 1.0 | 1.5 | 2.0 | 2.5 |
For example, four 90° bends for a 50 mm pipe are equivalent to 3.5 * 4 = 14 meters of linear pipe.
Therefore, the total pipe length will be equal to the linear pipe length plus the equivalent linear meters for all points that introduce additional pressure losses.
Pipe Material
The material depends on technical applications, but typically the following materials are used:
- Galvanized Steel: Low cost and suitable for most cases. Susceptible to corrosion.
- Stainless Steel: Expensive but corrosion-resistant.
- PVC: Economical but less durable.
- Aluminum: Expensive but can achieve low roughness levels, reducing pressure losses.
Pipe Diameter
The pipe diameter must be properly sized to reduce pressure losses. The simplest method is to refer to sizing tables. For example, for a 7 bar circuit, you can refer to the following table, which sizes the diameter to keep pressure losses below 4% (0.30 bar). Choose the diameter based on the total pipe length (including equivalent lengths for pressure drop points) and the total required flow rate.
For example, for a circuit of about 500 meters and a required flow rate of 150 m³/h, a 40 mm diameter would be appropriate.
