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ENERGY EFFICIENCY THROUGH THE INTEGRATION OF COMPRESSED
AIR SUPPLY CONTROL WITH AIR NETWORK DEMAND CONTROL
Mr L Padachi*, Dr GD Bolt**, Dr M Kleingeld**, Mr JH Marais**, Prof EH Mathews**
*Eskom Demand Side Management and North West University, ** Contracted to North West University and
consultants to HVAC International and TEMM International
ABSTRACT schedule, following a traditional trend of drilling, blasting
and cleaning as seen in Figure 2.
In the mining sector compressed air is used for drilling,
loading boxes, refuse bays, cooling and agitation.
Compressed air systems in large industrial applications
generally operate inefficiently and are costly energy
components. Intricate air reticulation networks supply
air to working levels with very little control over air
demand.
The implementation of an Energy Management System Figure 2: A typical mine operation schedule
for the optimisation of compressed air networks has
shown that a large scope for energy efficiency projects This has the effect that the air demand will vary
exists. throughout a 24-hour profile, having peak compressed air
usage periods and low compressed air usage periods.
An in-depth investigation into the compressed air During the low demand period, less compressed air is
networks of a gold mine operation reveals an average needed and the standard procedure should be to operate
energy efficiency potential of 2.2MW. This will save the compressors according to the demand.
mine in excess of 13,700MWh per annum resulting in
electricity cost savings of more than R3.4 million. The Current trends show that this is not the case on many
benefits of such a system go beyond that of only the mines and this opens up a window of opportunity for
mine. Eskom will also benefit from fewer supply power saving projects. By implementing an efficient
constraints and lower carbon emissions. control strategy of a mine’s compressed air system, large
power savings can be realised. This will not only benefit
Eskom during peak periods, but throughout the day, as
1. INTRODUCTION energy efficiency can also be utilised in such a project.
The industrial and mining sector contributes
approximately 60% towards the maximum demand during 2. DEMAND FOR COMPRESSED AIR
national peak periods [1]. Compressors are responsible for
approximately 9% of the industrial demand as shown in The supply of compressed air is an expensive and energy
Figure 1 and merits further investigations into energy intensive process. Most mines have intricate underground
efficiency innovations. air reticulation networks through which compressed air is
distributed to various sections of the mine. Many of these
mines are not aware of how much or where exactly the
compressed air is used underground.
The installation of underground flow and pressure
monitoring equipment, coupled with pressure and flow
control valves, make it possible to monitor, optimise and
automatically control underground compressed air
networks. Figure 3 shows an example of a typical control
valve setup with the required auxiliaries.
The ability to control the mass flow of air to a mining
level makes it possible to optimally control the air
Figure 1: Maximum demand in the industrial sector
pressure on that level. Each underground level in a mine
has a unique pressure requirement.
Various investigations have shown that compressors are
not always being operated efficiently. On most mines
centralised blasting is being implemented. This means
that the mine operates according to a fixed mining
When building the simulation model for the mine air
network, each level has to be simulated in precise detail.
For each level mathematical models must be built which
will accurately represent the specific air usage. The
simulation model for the level is verified to ensure that it
reacts in exactly the same way as the actual operations on
that mining level.
Again, it is important to remember that all the energy
users must be integrated to arrive at optimum DSM and
Figure 3: Control valve with bypass valve energy cost. All the simulated components are combined
into one integrated model, which represents the integrated
Through the optimised control of the pressure operation of the complete mine. This will provide an
requirement of each level, air wastage is minimised. accurate simulation corresponding to the actual conditions
When the demand for compressed air drops, a reduction encountered in the mine’s air network as depicted in
in the supply of compressed air will result. This in turn Figure 5.
manifests itself as an electrical power saving (the 1 # West 2 # East
compressors “cut-back” on absorbed power via guide
vane control).
PLC PLC
P-98
VBar VBar
P-103 P-123 P-104 P-129
When the valve is partially closed the pressure A
I-49
B
E-81
A
I-50
B
E-82
VBar Vbar
downstream of the valve will be reduced. The upstream A
I-48
B
A
I-51
B
P-99 P-122
network pressure will increase. This increase in upstream
VBar VBar
P-106 P-81 P-105
A A
P-124
B B
I-47 I-52
pressure will signal the control system of the compressors A
B
VBar E-83
P-116 A
B
VBar
P-100
E-84
to adjust the guide vane angles and reduce the amount of I-46
A
B
VBar
P-111
V-30
P-125
I-53
A
B
VBar
V-32
P-114
P-118
P-115
air drawn into the compressor, causing the compressor to I-45
A
B
VBar
I-54
A
B
VBar
P-101
P-115 LEGEND
absorb less electrical energy. Figure 4 shows the guide I-44
A
B
VBar P-112
I-55
A
B
VBar P-113 P-119
VBar
Verabar (P,F,T)
V-31 V-33
vane controller and the inlet guide vanes on a compressor. I-43
A
VBar
P-117
I-56
A
VBar
P-102 P-119
PLC Surface PLC
B B
I-42
VBar
I-57
VBar
Pressure sustainng
P-121 P-120 P-110
A P-108 A valve
Optimised control of the network pressure build-up will I-41
A
B
VBar
P-107
I-58
A
B
VBar
P-109
A
V-29
B
I/O connection at
PLC
ensure that the inlet guide vanes of the compressors are I-40
B
E-85
E-87
P-127
I-59
B
E-86
E-88
P-128
set at the optimum angles. In some cases compressors will
even be stopped due to the optimised control of the entire Figure 5: Simplified layout of underground air
network pressure. network control equipment.
The fully integrated dynamic system and control model
for the mine is extensively verified with detailed
measured data. The necessary update of the integrated
model is repeated until verification proves to the client
that a successful computer model of the compressed air
network has been achieved.
This verification process ensures that the simulation
model correctly represents the integrated air network of
the mine. After the simulation model is complete and the
confidence of the client is obtained, the model is
integrated with the SCADA control system of the mine.
Figure 4: Compressor guide vanes.
Other daily varying influences on the final electricity
3 ENERGY MANAGEMENT FOR THE costs, which must inter alia be included in the system, are:
OPTIMISATION OF AIR NETWORKS
• Varying production schedules;
To reduce the energy consumption of the compressor • Maintenance schedules;
system at the mine the operation schedule must be • Demand control levels;
changed. However, the mines will only consider a change • Pressure demands; and
to their operational procedures if there is a very high level • Flow demands.
of confidence that these changes will not affect the safety
and production of the mine. A detailed integrated,
dynamic control simulation procedure, running
successfully for the full mine operation is required.
The user supplies the simulation model with certain leading into the levels that do not have a controllable
constraints including minimum pressure settings, valve will be installed. All pipe sections leading into the
minimum flow settings, production schedules, equipment levels have a diameter of 300mm. The underground air
requirements, upper/lower limits for services, e.g. the pipes of the various levels are not interconnected.
drilling equipment on/off limits, any other constraints and
the maintenance schedule for the following day. This
information is then integrated into the system before
energy management can be optimised for maximum load
reduction and minimum energy cost accounting.
A dynamic optimisation procedure is then integrated with
all the parameters to obtain the optimum schedule for all
the components. This will ensure minimum energy cost
and maximum DSM, taking into account all the safety,
health, operational, maintenance and other constraints.
4 GOLD MINE CASE STUDY
The site under investigation consists of ten shafts, with
seven compressor houses that supply compressed air to
ten shafts (# symbol represents a shaft) and five plants.
These compressor house and plant locations can be seen
in Figure 6. The intricate compressed air system consists
of surface compressors, pipe work, valves, drills, Figure 7: Simplified underground layout for 11#.
agitators, loading boxes, loaders, shotcrete and other
pneumatic equipment. The diameter of the main air The proposed valves will be installed on the production
compressor line into the shaft is 500mm. Air is also used levels and the levels using the most flow. The verabars
at the surface fridge plant. are installed on all the levels, including the levels that do
not have controllable valves. This will ensure that the
levels without controllable valves can still be monitored
for leaks and faults. If there is a leak on a level, the flow
on this level will rise above its normal threshold. The
proposed infrastructure can be seen in Figure 8.
Figure 6: Simplified compressed air layout
There are 14 underground levels at 11 shaft. Five of these
are non production levels as seen in Figure 7. The
remaining 9 levels are fully productive. The non
production levels, 1200 and 77 house the pump stations.
Drills, loaders and boxes are being used on the production
levels. The required pressure for the drills is 500kPa.
Similar pressures are required for all other shafts. It is
proposed that controllable valves with verabars (meaures Figure 8: Proposes infrastructure upgrade.
volumetric flow) and triloops (instrument that converts
heat, pressure and flow readings to a protocol that a PLC All the input parameters can be used to simulate and
can read) be installed on production levels. On the non- determine the savings potential. The baseline and optimised
production levels one orifice flow meter and one pressure load profile is shown in Figure 9.
transmitter on each of the underground pipe sections
Baseline vs. Optimised load reduction various control valves, the line pressure changes are
50,000
monitored and optimised. Pressure build-up occurs in the
45,000
system. The compressors are controlled to run at a lower
40,000 loading by optimal positioning of the inlet guide vanes.
35,000
30,000
At the investigated gold mining operations, a potential of
2.2MW saving is possible, with a CAPEX of R11.6
Power kW
25,000
million. The payback period without any energy efficiency
20,000
funding mechanism will be 6 years and could be reduced to
15,000
2 years with the inclusion of DSM and CDM funding.
10,000
Optimised profile
Baseline
5,000
0
7. REFERENCES
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour (Wee k day)
Figure 9: Optimised load profile [1]. Eskom, “Energy Efficiency and Demand Side
Management and in South Africa”, Eskom DSM,
In order to realise full automated control certain hardware Mega Watt Park, Maxwill Drive, Sunninghill,
will be required which includes: Santon, +27 11 800 2776
• Controllable valves on the 9 production levels. [2]. Shames, I.H., “Mechanics of Fluids 4th Edition”,
McGraw Hill Companies, Inc., 1221 Avenue of
• Verabars with triloops with the valves.
the Americas, New York, NY 10020, 2003.
• Orifice flow meters and pressure transmitters.
• I/O cards, close to the valves, as well as
standalone sensors.
7. AUTHOR(S)
• Pressure sustaining compressors at the
refrigeration plants. Principal Author: Lawrence Padachi holds a Bachelor
• One main surface PLC. of Technology Degree in Electrical
• The fibre backbone from the main surface PLC Engineering Heavy Current. He is
the I/O cards and the PLC to the SCADA. currently working for Eskom and has
• A SCADA to connect to Energy Management 11 years experience in the
System. Distribution, Transmission and
• Energy Management System to be set up in the Enterprises division of Eskom.
control room. Lawrence is currently the Industrial
Sector Manager for Demand Side
5. COST BENEFIT ANALYSIS Management.
The total cost of implementing a Real-time Energy
Management System for the optimisation of the gold mines Co-author: Dr Gerhard Bolt is contracted to North
compressed air network is estimated at R11.6 million. This West University and consultant to HVAC International
will enable an energy efficiency savings of 2.2MW over a and TEMM International.
24hr period during weekdays. In total, the mine will save
13,765MWh per annum with a cost benefit of R3.4 million. Co-author: Dr Marius Kleingeld is contracted to North
West University and consultant to HVAC International
The recent slowdown in the global economy may make it and TEMM International.
difficult for mine management to approve these
compressed air projects due to the high initial capital Co-author: Mr Johan Marais is contracted to North
expenditure. The existing payback period is in excess of 3 West University and consultant to HVAC International
years compared to a required approval policy in the order and TEMM International.
of 1 to 2 years payback for the mines.
Co-author: Prof Edward Mathews is contracted to
50% energy efficiency funding from Eskom DSM will North West University and consultant to HVAC
make the payback more feasible and ensure the roll out of International and TEMM International.
many more such energy efficiency projects.
Presenter:
6. CONCLUSION The paper is presented by Lawrence Padachi.
The efficient use of compressors in the mining industry
relies on the accurate control of the compressed air demand
through an Energy Management System. When simulating,
monitoring and controlling the pressure set points of the
