How To Scale A System Larger Or Smaller
If you want to build a system that is smaller or larger than ones in this manual, simply multiply the amounts and system numbers up or down proportionately, remembering that the Styrofoam sheets for the rafts come in 4-foot by 8-foot sizes. The only exception is the water flow rate, which doesn’t vary until the trough series gets really long (as we’ll show you in a moment).
How did we discover this? Well, we used the information UVI gave us in the 2007 short course about scaling systems. They showed us how to scale the pump up or down in size depending on how many square feet of trough area you had in a system. They taught us that bigger systems needed bigger pumps because they had more trough area, right? Well, we scaled a system down to 64 square feet, using the following: UVI has a 2,400 square foot system with an 85-gallon per minute pump. We wanted to build a 64 square foot system, which is 0.02667 times smaller than their system, so we multiplied their 85 gpm pump by 0.02667 and came up with a 2 gpm pump, bought and installed a 2 gpm pump in a system with a 4-foot wide, 10-inch deep, 16-foot long trough (SAME depth and width as all our other systems).
It was only after we’d operated this system for a while, and had seen how incredibly prolific it was, that we realized what we’d done: without even thinking about it, we had created our FIRST experiment in water flow rate in aquaponics systems. Here’s why: the speed the water flows past the plant roots in a trough is determined by how fast the water flows into the trough, NOT by how long the trough happens to be. We had systems with 20 gallons per minute going into the trough, and a system with 2 gallons per minute going into the trough, and they were both growing the same, even though one had ten times the flow rate over the plant roots that the other had! We had a successful experiment, and new information to redesign our aquaponics systems with to save energy!
What does this mean? Although our flow rate experiments have indicated that 5 gpm is a safe minimum, there’s a nice little 2 gpm pump that we recommend MicroSystem 64 and 128 builders use, and has worked fine in those systems. We’re being a little more conservative with our Family System and suggesting a pump with a 5-gpm flow rate. We’re being even more conservative with our 1,024 square foot Commercial Systems and suggesting a pump with a 10-gpm-flow rate.
up to THIS SIZE (below).
IMPORTANT! These systems all have a single set of 4-foot wide series-plumbed troughs, so the flow rate through the trough is 2 gpm for the MicroSystem, 5 gpm for the Family System, and 10 gpm for the 1,024 square foot Commercial System. We’re pretty certain the little 2-gpm pump would work for all these systems, if you didn’t let it get clogged, and it didn’t have to lift the water more than two feet of head.
Here are the specifications for the MicroSystem 64 (one of our two sizes of Backyard Systems)
64 square foot Off-Grid LD Aquaponics System Proportions
|Pounds of fish in rearing tank||20|
|Rearing tank, depth||20″ of water min.|
|Rearing Tank gallons||150-300|
|Troughs, area in square feet||64|
|Water Flow Rate GPM||2|
|Troughs, depth (inches)||8-10”|
|Air Pump size||0.5 cfm @ 40″ H2O|
|Total system water||550-700|
REALLY IMPORTANT! When you change system proportions or design your own custom system, you will need to find fish tanks, air pumps and blowers (or air pump) that fit the rescaled system’s requirements, based on these approximate ratios (hint: begin by defining your desired square footage of raft area):
- 0.3 pounds of fish per square foot of raft (yes, 3/10 of a pound), and
- 4 gallons of fish tank water per pound of fish in the tank.
- 1.5 cfm of air @ 40” H2O per 100 pounds of fish in the system.
- 1.5 cfm of air @ 10” H2O per 100 lineal feet of trough 4 feet wide.
- 5-gpm minimum water flow rate into each trough circuit.
Let’s do a sample system design from these numbers. Say we want a system that has 8,000 square feet of raft area. The first number we find is the amount of fish required to power the system: 8,000 X 0.3 is 2,400 pounds. We need a fish tank that has 4 gallons of water for each pound of fish, or 4 X 2,400, which is a 9,600-gallon tank. A 20-foot circular tank is a little small at 8,500 gallons, so we’ll use a 24-foot diameter circular tank that holds about 11,800 gallons. We need 1.5 cfm of air @ 40” H2O for each 100 pounds of fish, so we need 24 X 1.5, or a blower that provides 36 cfm of air @ 40” H2O. We divide our 8,000 square feet of trough by a width of 4 to find out we have 2,000 lineal feet of trough, and we need 1.5 cfm of air @ 10” H2O for each 100 feet of this, or 1.5 times 20, which is 30 cfm @ 10” H2O.
Based on numbers from a student (Zac Hosler), we know for sure that a 900-foot-long continuous trough circuit works just fine without nutrients getting depleted during their circuit through the trough. So, to be conservative, we’ll split the 2,000 lineal feet of trough into four circuits, each with 500 lineal feet of trough. Each circuit will need our 5-gpm minimum, times four, so we need a pump with a minimum 20-gpm flow rate at whatever head the system operates at.
What’s head, you ask? Head is the vertical distance (height) that the pump has to pump the water, from the intake of the pump up to the place the water comes out at the other end of the pipe. Every pump is rated for flow rate at a certain head. If you ask your pump to lift the water higher by increasing the vertical distance the pump has to pump to, the flow rate will decrease because you’re asking the pump to do more work. If you use a pump that has a flow rate of 20 gpm at 3 feet of head, then put your fish tank 10 feet above the pump, you WON’T GET 20 gpm. You’ll probably get more like 7-8 gpm. IMPORTANT! Make sure your pump is rated for the flow rate you want at the height (head) you need to lift the water to.
If you have any confusion or difficulty with custom system design and equipment specification, email us, we’ll help you with sizing of equipment and plumbing and layout, including designing custom aquaponics systems with complete CAD construction drawings. We will charge for this, but it will cost far, far less than purchasing the wrong equipment or putting in the wrong size piping.