Step By Step Guide to Sizing an Air Pump for Aquaponics
- December 19, 2018
- Posted by: Ahmed
- Category: Uncategorized
In this article we will walk you through the process of sizing an aeration for your aquaponics system so that you can easily identify the equipment you will need without having to deal with confusing manufacturers specs. For an introduction to aeration system and learn more about the what you need to consider before designing the aeration read blog Important Parameters for Sizing An Air Pump
Steps used to Size the Air Pump
Step 1: Determine the rate of oxygen diffusion of the diffuser or air stones used in your system. In this blog we will limit our discussion to 3″ air diffusers, you can easily adapt the example to other sizes using the formula.
Step 2: Calculate the amount of oxygen needed for the water in your system in relation to the daily feed rate.
Step 3: Determine the number of diffusers or air stones required to achieve the oxygen diffusion rate in step 2.
Step 4: Finally, calculate the minimum amount of CFM produce by an air pump that will supply the rate of oxygen diffusion to each air stone at a certain depth.
The formula we will use to calculate the volume of air needed in your system is as follows:
CFM diffuser (ft3/min) = 0.3 cfm (Using our 3″ medium pore SWEETWATER® Air Diffusers from Pentair).
Lbs of air/ft3 = 0.075 (The weight of a cubic foot of air)
Lbs of Oxygen per lb/air = 0.23 (The weight of the Oxygen in a pound of air)
SOTE = Standard Oxygen Transfer Efficiency rate (SOTE) or rate at which oxygen will transfer to water under “standard” conditions per 1 foot of depth. For a 3″ medium pore diffuser it is .01 lbs per foot of depth.
Depth = 1′ (assuming a standard grow bed of 1 foot depth).
FTE = Field Transfer Efficiency (FTE) – The actual tested transfer efficiency based upon the existing oxygen content of the water (the higher it is, the less will transfer), temperature and salinity. We assume the input water is at 20-25 C (68-78 F) and 4.0-5.0 mg/L DO which gives an FTE of approximately 0.5[i]
Time = 60 (minutes per hour to get to O2 per Hour)
Now, let’s calculate the amount of oxygen needed for the water in your system in relation to the daily feed rate. The recommended oxygen to feed ratio from aquaculture literature is given as 1.0 kg of oxygen per 1.0 kg of feed fed[ii].
We will use this ratio to calculate the oxygen requirement of our system. Note: we have seen in practical application especially in smaller systems that half the recommended ratio which would be 0.5 kg oxygen per kg of feed can be adequate.
Let’s assume we are growing tilapia in a 650 gallon tank at a final stocking density of 5.5 gallons/lb or 0.2 lb/gallon. The fish consume 2% of their body at the current stage which is about 4.8 lbs (2160 g) of feed per day.
Using the 1:1 ratio we need 4.8 lbs (2160 g) of Oxygen every 24 hours or 0.2 lbs of O2 per hour (4.8 lbs/24 hr) which is the oxygen requirement for the complete system per day (or using the 0.5:1 ratio we need 0.1 lbs per hour).
Next, we divide the oxygen requirement of the entire system by the oxygen produced by each air stone to determine the number of air stones needed.
Since each air stone uses 0.3 cfm, we need an air pump or blower that produces a minimum of 38.6 cfm at 1 ft depth.
Other Oxygen Loads Should be Accounted For
This size of an air pump will insure that there is more than enough oxygenation occurring in your system even if stocking densities increase. However, in practical application a much smaller blower can used (using the 0.5:1 ratio discussed above, that would give you a 19.3 cfm air blower) but this is a decision to be made by the designer of the system knowing the trade-offs involved.
Since this is an aquaponics application, plants in the system may also require oxygen. Additionally, in a RAS system, there may be other components that require oxygen. You must take other significant loads into consideration when sizing your system; however for the purposes of this article we have taken only the fish as the one and only load, assuming 1lbs of Oxygen per 1lb of feed fed.
So please use this calculation as a guideline with discretion.
If you need more help with sizing an aeration system for your own aquaponics system or if you’re looking for a tool that can help you with almost any aspect of the design then check out the 5in1 Design Calculator which comes with 5 separate calculators that give you all the design parameters you need so you can quickly design or test your system without having to do worry about any complicated equations.
[i] Pentair Aquatic Master Catalog 39th Ed. Tech Talk 35
[ii] Recirculating Aquaculture – M.B.Timmons & J.M. Ebeling – 2007 Edition, P. 87