VFD help

[coolhand 0]

I've been lurking around here for a couple of weeks and I think I might find an answer or some direction. I need to find a formula or program on the web to help me with proving energy savings with using a VFD on a blower. Project scope: I'm in the process of specing out controls and VFD's for a project at work. We have 40 3hp blowers that are controled with dampers to adjust output. Most of these fans only need to be run half speed for the desired results and we have a few VFD's in place to do this. What I am propossing is 40 Power Flex 70 drives connected with a ControlNet and controlled with a Control Logix 5000 and a Versa View HMI with RS View 32. The drives will each have a presure transducer for feed back for PID control. They will all be set with different "recipies" for different production needs and will eventualy be self tuning for building dynamics -temp/humidity. I'm up to 80 grand so far, but this "back bone" of the Logix 5000 and the Versa View will let us communicate and control the other processors on this line as well as data tracking, etc. One of the extra benifits to this system is a reduction in energy consumption from the blowers since I can usually run them 1/2 speed. The specifics of the fan and "fan curve" is 8" 40 tabbed wheel. 3hp, @ 3450 = 1250 cfm producess 3" hg. At 1.5hp (half speed), 1750 = 500 cfm producess 1" hg. Theses presure readings are consistant with what I've measured at the fans. I would anticpate a 50% or more reduction in kilowatt usage but would like to be able to prove it to myself before I show it to the managers who will sign the check for the project. These fans run 24/7 and from what I can figure cost over $90,000 a year to run (we have ten lines like this one!). If we can cut that energy consumption in half that's a pretty quick project payback along with a nice ata boy from the bosses. I could make it a much simpler project but the PowerFlex drives have remote I/O along with the analog input and output. They are also "tag based" and will make for a "cut and paste" project....Plus it will be alot of fun. Thanks for the help and input. Any suggestions are welcome. Luke

[Guest HalBishop]

Just beacuse you reduce the fan speed by 50% the actual power used to run the fans will not decrease by 50%. If the dampers are closed to the 50% level you will see very little power savings. The power consumed by a fan is a direct product of how much air you are moving. It is a measure of work being performed. When you close the dampers on a fan it is doing less work thus less energy is being consumed. The other thing you will have to consider is the fact that the fan output is not linear.

[coolhand 0]

Well Hall, I've read quite a bit of info over the last few weeks talked to a lot of folks and even tried another Forum and learned alot on the way. One of the things I started to read and hear about was the affinity law. The basic affinity law states that in a variable torque load profile (like a fan or centrifugal pump), the power consumed varies by the cube of the speed. So at 1/2 speed, the kW consumed will be 1/2 x 1/2 x 1/2, or 1/8 of the power consumed at full speed. The reason is that with that type of load, the amount of work done is related to amount of load being coupled to the mover (fan blades or pump impeller vanes), and as the speed goes down, less and less of the load (air or fluid) is being picked up and moved. How much less is what the affinity law states: that it is not linear. Less work output = less energy input, and the VFD has a variable torque algorithm that tweaks the v/hz pattern to optimize the energy input and maximize the savings. We have several blowers without dampers but not integrated like we will do. One of those blowers is a PowerFlex 40 and I have the "Drives Explore" software to read the drives power consumption on the fly. Here is what I found out the other day when I hooked up: @ 60 hz the drives output power is 1.88 kw @ 50 hz the drives output power is 1.11 kw @ 40 hz the drives output power is .55 kw @ 30 hz the drives output power is .24 kw @ 25 hz the drives output power is .14 kw With the affinity laws to back this data there will be a significant reduction in kw consumption because it is not linear at all but cubed every time there is a reduction in power output to the fan. If the load were not variable torque such as the fan or pump the reduction would not be there. It has been a lot of interesting reading and too much time with the calculator. We pay .04 per kwh and for one blower running @ 60Hz we would pay $658.00 per year or $26350.00 for a line of forty blowers. When these go to VFD control we will average between 25-35 hz. If I figure 30hz as the average that line will only cost around $3300.00 per year! Thats better than $23,000! I would encourage a google search and some dull talks with the fan engineers (theier too excited about fan curves etc.) to anyone interested. As far as this project goes I need to cut a bit of fat out of it and I think it might be a go. Luke

[ECSI 17]

To save some capital costs you could group some fan motors and put them on one drive. ie: It's usually cheaper to put three 3 HP motors on one 10 HP drive than to use three separate 3 HP drives. Of course you would need separate OLR protection on each motor but you should still save a bundle. There would also be less installation space required, power feed wiring and control logic. just a suggestion Regards

[coolhand 0]

ECSI, the real reason for the project is better individual fan control durring our manufacturing process and being able to return to those stored setpoints when a change-over occurs. Furtunatly each of the fans have a disconnect mounted to it and the VFD's meet NEMA 1 and will be mounted remote with the fan. It's a very clean and dry factory so this is a plus. We realized awhile ago that aside from controlling our process for better efficency there could also be an energy savings going on with the better quality. The bean counters would like this and probablly be willing to invest in this project. The PF 70 is alot more expensive than the PF 40 but will make for a cleaner install and simpler program with the ControlNet. I'll track the annual kwh consumption for proof. If quality, production time, and energy savings all happen I will probablly be retrofitting 7 more lines over the next few years. It could save our plant over $175,000 every year. I'm comming up on third plate for my price estimate and need to start cutting a little fat. If I can get it down a bit more the boss sould let me go with a nice selection of hardware and the project will be a go. It should be a fun one. Luke

[Guest HalBishop]

The energy consumed by the fan is not just a product of the speed. The amount of air that the fan is capable of moving is a product of the speed. The amount of energy consumed is directly related to the amount work being performed. If the dampers are closed to 50% then the fan is moving less CFM and consuming less energy. You can check the energy usage on a fan with dampers closed to 50% and then compare that to the energy usage of the fan running on a VFD and moving the same amount of air. This is the only way you can calculate your energy savings.