The first computers such as the ENIAC, built in the second world war, were huge. ENIAC filled a 20-ft by 40-ft room and weighed 30 tons. It had 18,000 vacuum tubes, and required programmers to path cables and flip any of 3000 switches to run each and every program. The vacuum tubes alone sucked 174,000 watts of power! Its processing speed of 100,000 cycles/second was about 0.005% of the speed of the average smartphone today.
Like computers, speed drives have become smaller, more affordable, and have more abilities as each year goes by. Their cost can now be quickly recovered even on fractional horsepower motors. Given that the price of energy isn’t likely to drop anytime soon, here are a few guidelines for the use of speed drives in aquaculture.
WHAT ARE SPEED DRIVES?
A speed drive is sometimes referred to as any of the following: adjustable speed drive, variable frequency drive (VFD), frequency converter, inverter, or simply a drive. I prefer the term VFD.
A VFD takes in the electrical power supplied by your utility at a frequency of 50Hz or 60Hz and converts it to electrical power at a programmed frequency desired. The typical VFD is capable of operating an output of 0-400Hz.
How does this help? Motor speed is determined by the number of poles in a motor and the frequency of the power supplied to it. Controlling the frequency means controlling the motor speed.
If this motor is connected to a centrifugal pump, a fan, a chiller or other equipment, the benefits can be numerous and valuable.
A pump without a VFD operates along a ‘pump curve’ supplied by the pump manufacturer. At any given flow rate, the pump will have an associated pressure it generates. But, the system that the pump is in also has a curve-the ‘system curve’. This curve describes what pressure will be required to generate any specific flow rate through the system.
Controlling a pump by opening, restricting, and shutting valve(s), will move the operational point of the pump along the pump curve. Controlling a pump by changing its speed will move the duty along the system curve.
At Scotian Halibut we have employed VFDs on pumps with variable flow requirements since 2000. And on our heat pump since 2012. The National Research Council in Nova Scotia and Fisheries and Oceans Canada in New Brunswick both have sites using VFDs to control heat pumps for many years.
UNDERSTANDING AFFINITY LAWS
If you are not familiar with pump affinity laws, that’s ok. But to really understand the benefits of a VFD to a pumping system we have to go there. The affinity laws state relationships between speed, pressure, flow, and more for a centrifugal pump or fan.
Of primary interest is the relationship between speed and power. The affinity laws state that:
New Flow= Old Flow * (New Speed/Old Speed)
New Head= Old Head * (New Speed/Old Speed)²
New Energy Draw= Old Energy Draw * (New Speed/Old Speed) ³
The first is easy math: Half the speed results in half the flow. The second is not much harder to follow: Half the speed delivers one quarter the pressure. But the third is where the magic happens. Half the speed only uses one eighth of the energy! This is the secret primarily driving VFD application.
If you have a variable flow requirement, the VFD can save you significant percentages of energy. Typical systems that operate seasonally for example might assume only 1% operation at full capacity. Part loads may be something like 42% of time at 75% load, 45% of time at 50% load, and 12% of the energy compared to running the system at full flow. However, using a VFD would save you 75% of the energy compared to full flow.
VFDs have other advantages too. They can be used to create three phase power from a single phase supply. Or to use a motor rated for a different voltage than what you have available. Chiller compressor too big for your evaporator? A VFD can help remedy this.
Newer versions are being produced now capable of being programmed like a computer, and capable of monitoring multiple facets of a system and relaying that information to almost any monitoring or alarm system around the world.
There is another affinity law that states: New Pump Wear=Old Pump Wear (New Speed/Old Speed)²/³ As a result, the wear on a pump at half speed is one third the wear at full speed!
Before you go out and install VFDs on all pumps and fans, remember that there may be applications where they may not be helpful. For example, in a system where the flow is constant, more energy can be saved by sizing the pump to operate as its designed flow rate. Select a pump whose “best efficiency point” matches the constant flow and pressure you need.
To conclude, VFDs have a place in aquaculture where variable loads are experienced on pumps, fans, chillers, and heat pumps.