Why the right blower flow rate for aeration is important
Reading time: 3.5 minutes
As the operator of a wastewater plant, you may already have realized that not all blowers are alike. Therefore, one of the biggest decisions you have to make is which one is the optimal solution for your requirements. Comparing different solutions can be tricky, especially if each blower manufacturer uses its own way to report flow rate and air pressure data.
The right flow & pressure for your aeration process
Before you do anything else, you want to make sure you define the optimal flow rate and air pressure for your aeration process. Getting those requirements right, will ensure you have enough aeration capacity during peak demand hours and a smooth and efficient process during normal operating hours.
When you get the blower air flow rate calculation wrong and undersize your equipment, then your plant will not be able to reach the design treatment capacity. That means you can treat less water or, even worse, you end up with an effluent that does not meet the required standards. Oversizing the blower, on the other hand, will result in a waste of energy because it will either start-stop frequently or run in a blow-off state.
Replacing or adding to existing equipment
If you are replacing an existing unit, checking the old unit’s data plate is most often not enough. The original capacity calculation (at the time of the plant’s construction) will tell you something about the theoretical air flow rate. In the meanwhile, the capacity demand may have increased.
So, for an existing plant, it often no longer reflects reality. It is crucial to figure out whether you want a new model with the same motor power and flow or if you need a different size. The same applies if you need to add new equipment: getting the same size to match the existing units is usually an inefficient way to increase capacity.
An exact flow rate calculation can be done by an EPC or via one of your internal process engineers. Alternatively, some blower manufacturers can also offer you a compressed air audit that can help you get a close estimate of the flow rate needed.
Additional factors that influence your comparison
After you’ve defined the flow capacity for your aeration applications, you can start comparing solutions. This is where it may get even more tricky. Different blower solutions providers will use different ways to report flow and pressure. Since the air flow rate is such an important factor to consider when choosing the right solution, it is important you compare apples to apples.
Different ways to measure and report flow
For example, you have to determine whether you’re dealing with an inlet flow or delivered flow. Furthermore, there are many different units to specify the flow requirement, which can make comparisons complicated.
Some indicate a volume flow (the volume of fluid flowing per unit of time) and others a mass flow (the mass of fluid passing per unit of time). The former is measured in units like cubic feet per minute (cfm), cubic meter per minute (m3/min), or cubic meter per hour (m3/hr), and the latter in Nm³/hr or kg/hr.
Some of them depend on inlet conditions and others on pre-defined reference conditions. For example, is your standard reference condition Nm³/hr at 0°C or 20°C? This can make a significant difference (of about 8%) in terms of flow and power.
Then there are the inlet filter losses that have to be considered. Some plug & play blowers come with integrated filters while the filters of others have to be installed on-site. This, as well as the selection of the filter, can affect the pressure drop that has to be taken into account for the blower air flow rate calculation.
Another factor is flow tolerance. That means the range of the flow that the blower is expected to deliver. Usually, it is 4%. So for example, with a flow of 1000 m3/h, the range of the actual flow can be between 960-1040 m3/h. However, if your flow is very critical, then that tolerance should be 0%, which means the blower will deliver a flow of at least 100% of what it is designed to provide.
Comparing apples to apples
As you can see, there are many different ways to report flow data. The most important thing to do here is to make sure you’re comparing apples to apples.
However, when it comes to replacing your blower, looking for a one-on-one replacement will make you miss out on a lot of potential savings. Make sure to (re)define the capacity you actually need before you decide on a specific type of blower.
Checklist for comparing blower performance
- What flow rate and air pressure do I need for my process?
- How is the flow rate reported?
- Inlet or delivered flow?
- In which unit is it reported?
- Inlet conditions or pre-defined reference conditions?
- Are there any pressure drops that have to be taken into account?
- What is the flow tolerance for my process?
