Dimensioning Compressor Installations
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.
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Compressed Air Distribution
Inadequate compressed air distribution systems will lead to high energy bills, low productivity and poor air tool performance. Three demands are placed on a compressed air distribution system: A low pressure drop between the compressor and point of consumption, a minimum of leakage from the distribution piping and efficient condensate separation if a compressed air dryer is not installed.
How to keep the pressure drop low between the compressor and point of consumption?
These three demands primarily apply to the main pipes, and to the planned compressed air consumption for current needs as well as for the future. The cost of installing larger pipe dimensions as well as fittings than those initially required is low compared to the cost of rebuilding the distribution system at a later date. The airline network's routing, design and dimensioning are important for the efficiency, reliability and cost of compressed air production. Sometimes a large pressure drop in the pipeline is compensated by increasing the working pressure of the compressor from 7 bar(e) to 8 bar(e), for example. This offers inferior compressed air economy. Moreover, when compressed air consumption is reduced, so is the pressure drop and the pressure at the point of consumption consequently rises above the allowed level.
These three demands primarily apply to the main pipes, and to the planned compressed air consumption for current needs as well as for the future. The cost of installing larger pipe dimensions as well as fittings than those initially required is low compared to the cost of rebuilding the distribution system at a later date. The airline network's routing, design and dimensioning are important for the efficiency, reliability and cost of compressed air production. Sometimes a large pressure drop in the pipeline is compensated by increasing the working pressure of the compressor from 7 bar(e) to 8 bar(e), for example. This offers inferior compressed air economy. Moreover, when compressed air consumption is reduced, so is the pressure drop and the pressure at the point of consumption consequently rises above the allowed level.
Fixed compressed air distribution networks should be dimensioned so that the pressure drop in the pipes does not exceed 0.1 bar between the compressor and the most remote point of consumption. The pressure drop in connecting flexible hoses, hose couplings and other fittings must be added to this. It is particularly important to properly dimension these components, as the largest pressure drop frequently occurs at such connections.
The longest permitted length in the pipe network for a specific pressure drop can be calculated using the following equation:
l = overall pipe length (m)
∆p = permitted pressure drop in the network (bar)
p = absolute inlet pressure (bar(a))
qc = compressor Free Air Delivery, FAD (l/s)
d = internal pipe diameter (mm)
The best solution involves designing a pipe system as a closed loop ring line around the area in which air consumption will take place. Branch pipes are then run from the loop to the various consumer points. This provides uniform compressed air supply, despite heavy intermittent usage, as the air is led to the actual point of consumption from two directions. This system should be used for all installations, except if some points of large air consumption are located at a great distance from the compressor installation. A separate main pipe is then routed to these points.
What is an air receiver?
One or more air receivers are included in each compressor installation. Their size is a function of the compressor capacity, regulation system and the consumer's air requirement pattern. The air receiver forms a buffer storage area for the compressed air, balances pulsations from the compressor, cools the air and collects condensation. Consequently, the air receiver must be fitted with a condensate drainage device. The following relation applies when dimensioning the receiver's volume. Note that this relation 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)
One or more air receivers are included in each compressor installation. Their size is a function of the compressor capacity, regulation system and the consumer's air requirement pattern. The air receiver forms a buffer storage area for the compressed air, balances pulsations from the compressor, cools the air and collects condensation. Consequently, the air receiver must be fitted with a condensate drainage device. The following relation applies when dimensioning the receiver's volume. Note that this relation only applies for compressors with offloading/loading regulation.
For compressors with Variable Speed Control (VSD) the required air receiver volume is substantially reduced. When using the above formula, qc should be considered as the FAD at minimum speed. When the demand for compressed requires large quantities over short periods of time, it is not economically viable to dimension the compressor or pipe network exclusively for this extreme air consumption pattern. A separate air receiver should be placed near the consumer point and dimensioned according to the maximum air output. In more extreme cases, a smaller, high pressure compressor is used together with a large receiver to meet short-term, high volume air requirements at long intervals. Here, the compressor is dimensioned to satisfy 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)
The formula does not take into consideration the fact that the compressor can supply air during the emptying phase. A common application is starting large ship engines, where the receiver's filling pressure is 30 bar.
Learn more about air receivers and how to size them.
Designing the compressed air-network
The starting point when designing and dimensioning a compressed air network is an equipment list that details all compressed air consumers, and a diagram indicating their individual locations. The consumers are grouped in logical units and are supplied by the same distribution pipe. The distribution pipe is, in turn, supplied by risers from the compressor plant. A larger compressed air network can be divided into four main parts:
- Risers
- Distribution pipes
- Service pipes
- Compressed air fittings
The risers transport the compressed air from the compressor plant to the consumption area.
Distribution pipes split the air across the distribution area. Service pipes route the air from the distribution pipes to the workplaces.
Dimensioning the compressed air network
The pressure obtained immediately after the compressor can generally never be fully utilized because the distribution of compressed air generates some pressure losses, primarily as friction losses in the pipes. 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.
The required pipe lengths for the different parts of the network (risers, distribution and service pipes) are determined. A scale drawing of the probable network plan is a suitable basis for this. The length of the pipe is corrected through the addition of equivalent pipe lengths for valves, pipe bends, unions etc. as illustrated below.
The required pipe lengths for the different parts of the network (risers, distribution and service pipes) are determined. A scale drawing of the probable network plan is a suitable basis for this. 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, when calculating the pipe diameter, 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.
The equivalent pipe lengths for all the parts of the installation are calculated using a list of fittings and pipe components, as well as the flow resistance expressed in equivalent pipe length. These "extra" pipe lengths are added to the initial straight pipe length. The network's selected dimensions are then recalculated to ensure that the pressure drop will not be too significant. The 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 should be a 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 fed directly to a computer or debiting module. These 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.
Learn more about the process of installing a compressor system below.
Related articles
Installing a Compressor
Installing a compressor system is easier than it used to be. There are still a few things to keep in mind though, most importantly where to place the compressor and how to organise the room around the compressor. Learn more here.
What is an Air Receiver?
An air receiver, sometimes referred to as a compressed air tank, is an integral part of any compressed air system. Learn more about them here.