Trying to evaluate energy savings due to implementation of a VFD on a scrap suction system. Need som 1

[immsk]

I am trying to calculate the current running costs of a a system very simillar to what can be seen here:

http://upload.wikimedia.org/wikipedia/en/6/67/Dust_collector.jpg

my system utilizes a 60 hp centrifugal fan that is not modulated in anyway. However, I believe that when the dampers at a certain work station (at the pickup hood if you are reffering to the attached image) are closed off the total amount of air being moved by the fan is reduced and hence the total energy expended by the motor is also reduced. My questions are as follows:

1) Am I correct in assuming that the total amount of energy being expended by the system is reduced when the dampers are closed?
2) How do i calculate this reduction in energy?

Any input would be greatly appreciated.

 

[willard3]

The fan laws will give you help with this.

http://www.engineeringtoolbox.com/fan-affinity-laws-d_196.html

 

[urgross]

I don't think you'll see energy reduction, but to the contrary, completely wasted energy. You may want to consider change in duct static pressure along with flow. If not considering modulating the fan, and not already turning the fan off when not in use, putting end switches on the dampers to shut off when not in use (all dampers, no simultaneity). At a later date, you might consider a VFD with present frequency points coinciding with number of open end switches. Minimize flow to required capture velocities for all hoods, keep the required static as low as possible, and you will drop HP requirements.

 

[ozmosis]

Immsk
The application you indicate on your link (baghouse filter system) does actually lend itself to the use of a VFD for both energy savings and improvement of product quality. We have just finished a process for a large aggregate company who have included a VFD on the exhaust fan linked into the delta pressure sensor to maintain a constant pressure as the filters get loaded and then shaken and moved. If the load/pressure is constantly varying, there is scope to fit a VFD as long as you have the correct feedback
First, to answer your original specific questions:
1) If the dampers are closed, will the energy reduce? Slightly but not by much because the motor is still running at full speed. Closing the dampers will reduce load on the motor slightly and this slight reduction in load will have a slight reduction in energy. You should be able to measure this now.
2) calculate the energy reduction? I assume you mean, with a VFD. First, you need to work out what it is you are measuring and over what period of time. If the dampers are manually adjusted, you need to work out what is the reason the damper is being moved and by how much. An operator manually adjusting a damper must have some reason and this is what you need to find out and measure. Log this measurement and then use this data into the many (free) software packages available from drive suppliers and also government agencies.
If the dampers are simply open or closed for application reasons at each point, this is fairly straightforward but not as easy to simulate without knowing the actual airflow or pressure in the system.
As already pointed out, controlling the speed of a centrifugal fan could realise energy reduction by a cube proportional to speed of the fan. However, be realistic.
A key point is how long do your fans run for. The longer the better for a good ROI.
However, when looking into savings, always take into consideration the real costs.
A VFD will need an automated signal somehow to control the feedback of what it is you are measuring. Leaving something manual will inevitably mean it does not get adjusted and therefore not fully maximise any savings. Sensors will cost money.
Then you need to look at the installation of the VFD. Take into consideration where it has to be mounted, ingress of dust/moisture/fingers etc and build this into the cost.
Determine issues such as RFI interference and harmonic issues.
Factor in the efficiency of the VFD. You will get losses (2-3%).
A badly planned installation of a VFD will cost you money, not save. Ensure you work with a reputable company if going down this route.

 

[Compositepro]

Actually, closing a damper will have a large effect on energy usage. Power used by a blower is directly proportional to flow rate. However, slowing the blower speed will have a exponential effect on suction pressure so you will not be able to slow it by much and still have adequate suction. This seems to be a classic case of where a VFD is not a good application.

A blower works just like a centrifugal pump. Very little energy is required to spin a fluid. Some energy will be lost as heat due to turbulence. This is all that happens when there is no flow through the blower. When you let this spinning fluid out at elevated velocity or pressure, the fluid entering the blower must be accelerated to the speed of the blower wheel. This flow is what take the vast majority of the energy. In addition, an electric motor running at full speed with no mechanical load, uses little energy. And slowing it down will save very little.

 

[ozmosis]

I should have added that maintaining a minimum velocity is crucial but the use of VFDs in dust extraction or whatever is being extracted, is a pretty well defined use of these devices but needs to be taken as part of a complete system view.
An interesting paper is attached:

http://www.eceee.org/conference_proceedings/ACEEE_industry/2005/Panel_1/p1_11/paper

A VFD will save energy if applied correctly. It will be more than simply controlling the dampers but then, I don't know the scope of control the original poster requires. They have the benefit of being there.

 

[EnergyProfessional]

Ozmosis: interesting study, and I agree you could save quite a bit of energy, especially since nowadays VFDs are pretty cheap.
I couldn't decipher if they had the pressure sensors in the dirt-stream, ("a control system consisting of motorized blast gates at equipment intake ducts, pressure sensors, and a VFD.") but if it was n the dirt-stream I would be concerned the pressure senor clogging up.

 

[urgross]

Not sure why you'd want to use a dP after TAB (as part of controls) as everything monitored would be related to velocity.

Each hood will have a capture velocity related to a flow, as well as duct transport velocity requirement. With damper end switches, you would know which hood is open, and corresponding total flow. At TAB, verify flow for capture velocity, and corresponding VFD Hz.

For theoretical energy savings, measure the static pressure under current conditions. Then, measure static for each combination of damper openings. You may want to estimate simultaneity of use based on current observations for hoods in use. Horsepower savings would be calculated by (CFM * Pstatic)/(6356*Motor Efficiency * Fan Efficiency).

The lower you can keep flow to minimum capture velocity or transport velocity, and more you can reduce the static pressure, the greater the HP savings.

 

[EnergyProfessional]

urgross: you assume each damper is on/off while in reality the use may just throttle the flow. And each branch may have different design flow rates. so just saying 50% of dampers open means running VFD at 50% won't work.

 

[urgross]

No, I don't assume that, my writing is just bad.

Thinking in terms of fume hoods or BSC's may be helpful. For fume hoods, if each hood is not mapped to a contact switch for that hood, you could not tell if you needed to flow for a 3', 4' or 5' hood. For a limited number of hoods, and no simultaneity assumed, the same would be done with sash contacts.

The volume damper on the hood exhaust may be used as a throttling damper, not the sash. Throttling position would be set at capture distance and capture velocity. Separate damper would be needed for control and operate only on/off.

 

[hoodsman]

You should not be adjusting dampers in a dust control system after the system is balanced generally speaking. Any damper adjustment will affect the system balance and the carrying velocities in the main duct.
A VFD is a good thing to have on a system that includes a baghouse or dust collector. The VFD should be controlled by the static pressure at the inlet to the dust collector.

 

[ozmosis]

I work for a VFD manufacturer but not one of those people who says 'put a drive on any motor and it will save money'. In my original response, I was simply trying to articulate the fact that somewhere in the system, you need to measure the pressure or flow and this will determine the position of a damper or the frequency in the VFD. Once you know what it is you are measuring, you can make a decision based on that. Vfd's these days are intelligent but they still need something to tell them what to do.
In the original post from immsk, it was not clear what is going to decide how dampers open or close. Often, it is an operator 'knowing' what is going on in the system and doing it manually. Not always the best solution.
I recall a coating plant in the UK once where we fitted a VFD to the extraction filter system. We utilised the existing magnahelix (I think that was the type) delta pressure system to maintain a constant pressure. However, the operator had for years only looked at the ammeter as a gauge on how to adjust the damper. Fitting the VFD meant substantial energy savings and hence the amps dropped. He was all at sea as this was his only guide to what setting to make and we'd automated it. The company was thrilled at the savings but the operator was not, he was moved to roles 'outside of his cosy office' after it was clear, his only role was to look at a pencil line on an ammeter...