Dimensioning Compressor Installations
25 April, 2022
A number of decisions must be made when dimensioning compressed air installation for it to suit different needs, provide maximum operating economy and be prepared for future expansion. Learn more.
Appropriate compressed air distribution
Inadequate compressed air distribution systems will lead to high energy bills, low productivity, and poor air tool performance. There are three demands which must be met to avoid inefficiency.
- Low pressure drop between the compressor and point of consumption
- Minimal leakage from the distribution piping
- Proper condensate separation, if no compressed air dryer is installed
In this article, we'll explain how to meet such factors for optimal performance.
How to maintain a low pressure drop between the compressor and point of consumption
The three demands mentioned above primarily apply to the main pipes for current and planned compressed air consumption. If you need to install a larger pipe at a later date, the cost is relatively low compared to rebuilding the entire distribution system. Routing, design, and dimensioning are important for the efficiency, reliability and cost of compressed air production.
Sometimes, compensation for large pressure drops is attempted by increasing a compressor’s working pressure from 7 bar(e) to 8 bar(e) (for example). This approach offers inferior efficiency, and can cause the point of consumption to rise above the allowed level. Instead, it's recommended to evaluate the fittings.
Measuring your pipe network
Fixed compressed air distribution networks should be dimensioned, so pressure drops in the pipes don’t exceed 0.1 bar. This measurement is in relation to a compressor's most remote point of consumption. When calculating pressure, connected flexible hoses, couplings, and other fittings must be considered. The largest drop frequently occurs at these connections.
The longest permitted length in the pipe network for a specific pressure drop is calculated using the following equation.
l = overall pipe length (m)
∆p = permitted pressure drop (bar)
p = absolute inlet pressure (bar(a))
qc = compressor Free Air Delivery, FAD (l/s)
d = internal pipe diameter (mm).
Creating an optimal system
The best solution involves designing a closed loop ring pipe system. From this starting point, branch pipes can run to various consumption points. This approach provides uniform compressed air supply, as air is led to the consumption point from two directions.
To maintain ideal pressure, all air compressor installations should use this system. The only exception is if there's a great distance between the machine and point of consumption, where a separate main pipe is added.
The importance of an air receiver
One or more air receivers are included in each compressor installation. Their size is related to the compressor capacity, regulation system, and the consumer's air requirement pattern. The air receiver creates a buffer storage area for compressed air, balances pulsations, and cools and collects condensation.
Achieve the right volume
Consequently, the air receiver must be fitted with a condensate drainage device. The following equation applies when dimensioning the receiver's volume. Note that this calculation only applies for compressors with offloading/loading regulation.
V = air receiver volume (l)
qC = compressor FAD (l/s)
p1 = compressor inlet pressure (bar(a))
T1 = compressor maximum inlet temperature (K)
T0 = compressor air temperature in receiver (K)
(pU -pL) = set pressure difference between Load and Unload
fmax = maximum loading frequency (1 cycle every 30 seconds applies to Atlas Copco compressors).
For variable speed drivel (VSD) compressors, the required air receiver volume is substantially reduced. When using the above formula, qc should be considered as the FAD at minimum speed. It's also worth noting that it is not advised to dimension the compressor/pipe network for high air demand in short periods of time.
Compensating for high air demand
In the scenario above, a separate air receiver should be dimensioned for maximum output and placed near the consumer point. In more extreme cases, a smaller, high pressure compressor is used with a larger receiver. This set up meets short-term, high volume air requirements at long intervals.
Calculating mean consumption
Keeping in mind your overall usage, the following equation is used to meet mean consumption.
V = air receiver volume (l)
q = air flow during emptying phase (l/s)
t = length of the emptying phase (s)
p1 = normal working pressure in the network (bar)
p2 = minimum pressure for the consumer's function (bar)
L = filling phase air requirement (1/work cycle).
This formula does not take into consideration how a compressor can still supply air during the emptying phase. Learn more about air receivers and how to size them.
Designing & dimensioning the compressed air network
When designing and dimensioning a compressed air network, it's good to start with an equipment list detailing all consumption points and their locations. It's ideal to group these points into logical units and use the same distribution pipe for air supply from air compressor plant risers.
A large compressed air network is typically divided into four main parts.
- Risers
- Distribution pipes
- Service pipes
- Compressed air fittings
The risers transport compressed air from the compressor plant to the consumption area. Distribution pipes split air across the distribution area. Service pipes route air from distribution pipes to workplaces/consumption points.
A proper pipe system
Distribution of compressed air generates pressure losses caused by friction in the pipes. With this in mind, the pressure generated directly by the compressor is usually not fully ready for utilization. In addition, throttling effects and changes in the direction of flow occur in valves and pipe bends. Losses, which are converted to heat, result in pressure drops.
Therefore, required pipe lengths for different parts of the network (risers, distribution and service pipes) need to be determined. A scale drawing of the probable network plan is a suitable basis for this calculation. The length of the pipe is corrected through the addition of equivalent pipe lengths for valves, pipe bends, unions etc. as illustrated below.
As an alternative to the above formula, a nomogram (shown below) can be used to find the most appropriate pipe diameter. The flow rate, pressure, allowed pressure drop and the pipe length must be known in order to make this calculation. Standard pipe of the closest, largest diameter is then selected for the installation.
Equivalent pipe lengths for all parts of the installation are calculated with a list of fittings and pipe components. In addition, flow resistance is expressed by correlating pipe length. The network's selected dimensions are then recalculated to ensure pressure drop won't be significant. Individual sections (service pipe, distribution pipe and risers) should be calculated separately for large installations.
Flow measurement in a compressor installation
Strategically placed air flow meters facilitate internal debiting and economic allocation of compressed air utilization within the company. Compressed air is a production medium that is part of the production cost for individual departments within the company. From this viewpoint, all parties concerned could benefit from attempts at reducing consumption within the different departments.
Flow meters available on the market today provide everything from numerical values for manual reading to measurement data. This information is fed directly to a computer or debiting module. Flow meters are generally mounted close to shut-off valves. Ring measurement requires particular attention, as the meter needs to be able to measure both forward and backward flow.
Understanding proper compressed air distribution
We hope this article helps you evaluate your setup for optimal performance with minimal pressure drops and leakage. Using the equations mentioned is a good starting point. If you're still uncertain of the best approach, feel free to get in touch. Our team is happy to help.
Learn more about the process of installing a compressor system below.
Compressor Placement and Optimal Working Conditions
![Technician in Compressor Room, Optimization]()
Compressor Placement and Optimal Working Conditions
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