Planning Foundations in Aeration Tanks

Technical and design features and the changing air demands in aeration tanks are considered to be a challenge for the ventilation technology of all wastewater treatment plants. The reduction of operating costs and demand for energy efficiency is particularly important. However, the most vital criterion when planning wastewater treatment plants is still the achievement of the highest level of availability and reliability. The key principle of engineering “each chain is only as strong as its weakest link” still applies. That is the reason this article provides some basic information on the machine design, pipework routing and room design.

Calculation of Operating Data

Standard State (Physical)

The standard state of a gas refers to the pressure of pNabs = 1,01325 bar at a temperature of TN = 273 K - 0 °C, for example, volume flow in standard state: QN [mN3 /h] density in standard state: ρN [kg/m3] relation between Celsius temperature t[°C] T=TN+ t [K] and the thermodynamic temperature T[K].

Suction State

The compressors/blowers are constructed in the intake state, that is, at the actual pressure in the inlet socket, p1abs[bar] at the average or maximum temperature of the gas in the inlet socket t1 [°C] or T1 =TN+T1[K]. For the suction of atmospheric air, it is essential to aim for the average absolute intake pressure p1abs = 1.0 bar, intake temperature t1 = 20 °C or T1 = 293 K.

Impact of Installation Height

As the height of the installation increases, the atmospheric pressure p1abs decreases:

mNN P1abs[bar] mNN P1abs[bar] mNN P1abs[bar] mNN P1abs[bar]
0 1.031            
100 1.001 1100 0.888 2100 0.785 3100 0.692
200 0.989 1200 0.877 2200 0.776 3200 0.684
300 0.977 1300 0.867 2300 0.765 3300 0.675
400 0.967 1400 0.856 2400 0.756 3400 0.667
500 0.955 1500 0.845 2500 0.747 3500 0.657
600 0.944 1600 0.835 2600 0.737 3600 0.649
700 0.932 1700 0.824 2700 0.728 3700 0.641
800 0.921 1800 0.815 2800 0.719 3800 0.632
900 0.909 1900 0.805 2900 0.709 3900 0.624
1000 0.899 2000 0.795 3000 0.701 4000 0.616

 

Calculation of Volume Flow Q from Prescribed Mass Flow ṁ

1. For the standard state: with mass flow ṁ in [kg/h] Density ρ in [kg/m3]

2. For the intake state:

Density ρn of gases in standard state and specific heat capacity cp

  Air Natural gas Town gas Landfill gas Nitrogen Hydrogen
ρn [kg/m3] 1.293 0.96 0.61 1.21 1.25 0.09
Cp [kJ/kgK] 1.005 1.926 2.227 1.314 1.038 14.051

Calculation of Intake Volume Flow Q1 from Prescribed Standard Volume Flow QN

1. Dry gas

2. Humid gas

The conversion according to equation (1) for transport and compression of atmospheric air (relative humidity on average 60% and installation height up to 500 mNN) is adequately accurate!

Calculation of Density “p1” in intake state

1. Dry gas

2. Humid gas

PNabs abs. = Pressure in standard state in [bar]
P1abs abs. = Pressure in suction state in [bar]
T1 = Temperature in suction state in [K]
TN = Temperature in standard state: TN= 273 K
QN = Standard volume flow in [mN3 /h], dry
      = Humidity, cp = RF 1 00 [ I ]
RF = Relative humidity in [%]
PS = Partial pressure of steam in [bar], see Table: Partial pressure of the steam, saturation state
ρN = Density in standard state in [kg/m3 ]
ρs = Density of steam in [kg/m3]

Density “p1“ in Intake State of Atmospheric Air p1abs = 1.0 bar

. . . . . . . . . .
Intake temperature [°C] -20 -10 0 5 10 15 20 30 40
Density ρ1 in [kg/m3] 1,377 1,325 1,276 1,253 1,231 1,210 1,19 1,15 1,113

 

Partial Pressure of Steam, Saturation State

t [°C] Ps [bar] ρs [kg/m3] t [°C] Ps [bar] ρs [kg/m3] t [°C] Ps [bar] ρs [kg/m3]
-20 0,001029 0,000881 13 0,014965 0,01134 42 0,08198 0,05652
-18 0,001247 0,001059 14 0,015973 0,01206 44 0,09100 0,06236
-16 0,001504 0,001267 15 0,017039 0,01282 46 0,10086 0,06869
-14 0,001809 0,001513 16 0,018168 0,01363 48 0,11162 0,07557
-12 0,002169 0,001800 17 0,019362 0,01447 50 0,12335 0,08302
-10 0,002594 0,002136 18 0,02062 0,01536 52 0,13613 0,09108
-8 0,003094 0,002529 19 0,02196 0,01630 54 0,15002 0,09979
-6 0,003681 0,002986 20 0,02337 0,01729 56 0,16511 0,1092
-4 0,004368 0,003517 21 0,02485 0,01833 58 0,18147 0,1193
-2 0,005172 0,004133 22 0,02642 0,01942 60 0,1992 0,1302
0 0,006108 0,004847 23 0,02808 0,02057 62 0,2184 0,1420
1 0,006566 0,005192 24 0,02982 0,02177 64 0,2391 0,1546
2 0,007055 0,005588 25 0,03166 0,02304 66 0,2615 0,1681
3 0,007575 0,005946 26 0,03360 0,02437 68 0,2856 0,1826
4 0,008129 0,006358 27 0,03564 0,02576 70 0,3116 0,1982
5 0,008781 0,006795 28 0,03778 0,02723 72 0,3396 0,2148
6 0,009345 0,007258 29 0,04004 0,02876 74 0,3696 0,2326
7 0,010012 0,007748 30 0,04241 0,03037 76 0,4019 0,2515
8 0,010720 0,008267 32 0,04753 0,03382 78 0,4365 0,2718
9 0,011472 0,008816 34 0,05318 0,03759 80 0,4736 0,2933
10 0,012270 0,009396 36 0,05940 0,04172 90 0,7011 0,4235
11 0,013116 0,01001 38 0,06624 0,04624 100 1,0133 0,5977
12 0,014014 0,01066 40 0,07375 0,05116      

 

Explanations of Individual Standards for Performance Measurement and Calculation of the Standard Volume Flow

Performance Measurement and Calculation of the Standard Volume Flow

The Power Requirement and Its Characteristics

Generally, different performances can be defined based on the losses which need to be taken into account for the specification. The more complex a system, a packaged unit or a plant, the more complex the performance specification or performance comparison.

The following presents the performance levels relating to compressor or blower packages.

Starting from the lowest power requirement, which is to say, the mechanical shaft performance at the stage, followed by the wholly driven packaged unit, the power requirement decreases or increases.

  • Shaft performance at the stage (unit capacity) (without periphery)
    Describes the mechanical performance which is directly taken at the drive shaft of the stage
  • Coupling power (with periphery)
    Also takes into account, in addition to mechanical performance, the intake and discharge-side losses of the stage’s periphery
  • Required drive power
    Also includes performance losses because of slippage of the belt drive upon power transmission
  • Terminal power – motor
    Efficiency losses by the motor and mechanical auxiliaries of the packaged unit are also considered
  • Terminal power - packaged unit
    Electrical auxiliaries, which have a separate electrical connection, are also considered
  • Terminal power - frequency converter
    The performance losses brought about by a frequency converter and, consequently, the whole drive system, are considered

Explanations for ISO 1217

This international standard, which is applied by Aerzener Maschinenfabrik GmbH, insists on methods for a performance measurement, with reference to the power and the volume flow requirements of positive displacement machines. This international standard points out that the test and operating conditions must be taken as a basis for a complete performance measurement.

The test runs of compressors with a fixed speed, which are developed in batches or in series and supplied according to particular performance values, are described in annexes B, C and D. The relevance of the respective annex relies on the type and design of the compressor.

The test runs of compressors with a variable speed, which are developed in batches or in series and supplied based on specific performance values, are described in annex E. In general, the volume flow has been defined as per 3.4.1, i.e. as “volume flow measured at the discharge nozzle and calculated back to the conditions of the suction side”. Annex C applies for packaged units with a fixed speed for compression of nitrogen or air. According to chart C2, the maximum permissible deviations are defined as follows:

Volume flow range (m3/min) Usable intake volume flow (%) Spec. power consumption (%)
< 0,5 ± 7 ± 8
0,5...1,5 ± 6 ± 7
1,5...15 ± 5 ± 6
> 15 ± 4 ± 5

 

The tolerance values of the a./m. chart include all measuring and manufacturing tolerances. According to annex C.2.4, electrically driven compressors will have to be measured as completely mounted packaged units (as specified by the customer) and examined by their terminal power.

For annex E (compressor with frequency converter) the same tolerances and rules apply as for compressors with a fixed speed.

Explanation of PTC 10 - 1997

This standard applies to South and North America. It explains the procedure for determining the thermodynamic power of centrifugal and axial compressors and fans (blowers) under certain conditions.

ASME PTC 10 acceptable deviations for test parameters with comparable conditions:

Condition Acceptable deviation in %
Inlet pressure 5
Inlet temperature 8
Specific gravitation 2
Power 4
Input density of gas 8

Explanations of ISO 5389

This standard defines the test conditions for compressor units that comprise of a centrifugal compressor and are driven by an electric motor. It applies to drive power of 75 kW to 1865 kW.

The maximum permissible deviations are defined as follows:

Volume flow (according to ISO 5167 or 9300) in % Specific power requirement in % Power consumption without load in %
± 4 ± 5 ± 10

 

AERZEN, with more than 150 years of experience as a manufacturer of displacement machines, has played a major role in developing the ISO 1217 standard. At present, the American standard PTC 13 is a work in process.

AERZEN, as a German company, also carries out its performance tests for displacement machines according to ISO 1217, as standard.

The majority of wastewater treatment plants are presently equipped with frequency converters. When it comes to upgrades, the company offers its customers low-investment costing solutions. Here, it is essential for AERZEN as a machine supplier, to comply with the demands posed by different brands, efficiency levels and performance ranges.

For instance, a power rating using the tolerances of ISO1217, annex E insists that the operator of the wastewater treatment plant receive a new frequency converter and a new compressor package. However, most plants have already been provided with a superior-quality frequency converter. Thus, it is important for the customers to have this flexibility of decision making. That is why the company strives to provide its customers with bespoke, project-based solutions.

This may include:

  • Packaged units with internal frequency converter
  • Individual blower and compressor stages
  • Packaged units without motor
  • Packaged units with motor
  • Packaged units without frequency converter
  • Packaged units with external frequency converter (for an installation in a separate room)

The combination of products provided in the scope of supply defines which tolerance or standard is used. By way of example: for turbo compressor packages, the ISO 5389 standard is employed in Europe with a different set of tolerances. As a company with a global presence and a product portfolio encompassing a wide range of compressor technologies that particularly cater to wastewater treatment plant technology, AERZEN will have to take into account several standards with a range of varied tolerances.

AERZEN makes best use of the standard tolerance in all cases, and for a good reason. Using the standard tolerance, both the company and its customers can rest assured that every packaged unit will offer maximum energy efficiency and, most significantly, safe operation, even in combination with external brands. This is what the company has learned from 150 years as a compressor manufacturer that has emerged in parallel with the wastewater treatment industry.

Applied Standards in Wastewater Technology Regarding Standard Volume Flow

In the field of wastewater technology, there are several applicable standards for the calculation of a standard volume flow. The following is an indication of the three most important standards:

  1. DIN ISO 1343: volume flow in standard state where T1 =273 K, p1=1,013 bar, rF=0%
  2. ISO 2533: volume flow in standard state where T1=288 K, p1=1,013 bar, rF=0%
  3. ISO 1217: volume flow in standard state where T1=293 K, p1=1,000 bar, rF=0%

Example:

Medium   Air  
Intake volume flow
t1=20°C, p1=1,013 bar, rF=0%
Q1 m3/h 1500
Volume flow in standard state where
T1=273 K, p1=1,013 bar, rF=0%
QN Nm3/h 1397
Volume flow in standard state where
T1=288 K, p1=1,013 bar, rF=0%
QN Nm3/h 1474
Volume flow in standard state where
T1=293 K, p1=1,000 bar, rF=0%
QN Nm3/h 1519

 

t1= Inlet temperature

After the consultation, the company will calculate the best power rating in accordance with the customer’s preferred tolerance or standard.

Aerzen Machines Ltd

This information has been sourced, reviewed and adapted from materials provided by Aerzen Machines Ltd.

For more information on this source, please visit Aerzen Machines Ltd.

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