Iacdrive|Automation Solution

Rotary Tube Furnace Efficiency

There are many factors that govern the performance of rotary tube furnaces. A direct fired rotary unit has a potential for much higher thermal efficiency due to the direct contact of the hot gases with the material in process. Cement kilns are the most common large scale unit operation with direct fired units. Any articles you find on this will be helpful. Thermal efficiency can be estimated by dividing the inlet temperature minus the outlet temperature by the inlet temperature minus the ambient temperature in absolute scales either Rankine or Kelvin. Then there is the issue of co-current versus counter-current firing and heat recovery from the hot material and the exit gas for which standard designs are available. Indirect fired rotary kilns have heat transfer limitations due to the thickness and alloys needed for high temperature calcination >500 C. There is no simple way to measure the equivalent of the inlet and outlet temperatures on a direct fired unit. There are simply exit gas temperatures from each zone and an approximate shell temperature on the hot side of the shell which is lower than the zone exit gas temp. These are useful for control purposes and consistent operation. The higher the temperature the material requires to achieve conversion the higher the shell side fired temperature has to be to provide the delta T necessary to drive heat through the shell into the material zone.

Some materials further limit heat transfer by adhering to the inside of the shell and acting as an insulator! This requires trial and error application of “knockers” at the ends of the shell or sometimes internally secured chains that bang around and knock the adhering material loose. This is a potential nightmare as the learning curve to install chains so that the securing lugs and the chains themselves will stay attached for acceptably long service before failing and ending up in the take off conveying equipment with usual breakage and downtime is an uncertain one. From Perry’s one can find thermal efficiencies for indirect fired rotary’s given as less than 35%. The bed fill can be 10-30% depending on the heat demand of the material and the heat transfer limitations. You will want to have real time gas usage metering on the burners so that you know the theoretical energy input. From that you can subtract the theoretical heat needed to complete your reaction and compare that to the input to see how efficiently you have used the energy input.

The few large high temperature direct fired rotary kilns I have seen had view ports for measuring the local wall temperatures by optical pyrometer. It can be a challenge to get a protected thermocouple sheath down into the moving bed for an actual bed temperature and even just to hang it in the gas streams at the outlet or inlet area. See if you can contact cement kiln suppliers for some configurations of temperature sensing elements for your application. Bed fill effects on heat transfer are related to several parameters. Above ~500 C gas and refractory liner temperatures, the main heat transfer mode will be radiative as far as the surface of the bed material. Within the bed it will be conduction and some convection at the surface. A thin bed will reach max. temperature in shorter time, but this reduces through put for a given gas temperature. If you increase bed fill to increase production you will have to increase the firing temperature and the outlet temperature will probably increase lowering your thermal efficiency. This becomes a trade off between production rate and energy efficiency. Countercurrent firing usually maintains the highest driving force for heat transfer along the bed and gives the highest temperature of the bed just before exit of the bed material.

Perry’s may have a useful section on direct fired rotary kilns and lime or cement manufacturing references may help you as well. Please make sure lead emissions to air are properly captured

Calculate Capacitors Power

In general, to calculate the necessary Power of Capacitors, we can use the following formula:

Qc = P ( tgφ1 – tgφ2 )

where :
– Qc : the Power of Capacitors.
– P : the total Power of Loads that are running during normal working.
– tgφ1 : the tangent of φ1 ( the angel between current & voltage before compensation )
– tgφ2 : the tangent of φ2 ( the angel between current & voltage after compensation )

In all cases, we should take into consideration the following points :
1- It will be better to oversize the calculated Qc by ” 10 to 15% “.

2- Be careful when compensate the PF of a Motor to avoid the Over-excitation case, but we can verify it by using the following formula : Qc (motor) = 2 x P (1 – Cos φ ), where :
– P : the Motor’s Power.
– Cos φ : the PF of the motor before compensation.

3- After calculation of Qc, the choosing of Capacitors type will be done according to the Harmonic Distortion percentage. Noting that in some case where the Harmonic Distortion percentage is high, we should use ” Detuned Reactors ” with Capacitors, and when this percentage is too high, we can’t install the Capacitors before minimizing or eliminating the harmonics that their percentages are too high.

Resistance to ground

Resistance to ground is greatly influenced by the ambient conditions and the state of the motor when tested.

Factors Affecting Insulation Measurement:
First, it is important to understand that we are measuring a motor circuit. We are connecting our test instrument at a point where we can measure the majority of the de-energized circuit. As such, we do not necessarily know where an insulation anomaly is located when identified. We also have the motor circuit potentially exposed to differing environments. Ambient temperature and humidity can have a significant effect on any insulation measurements. When a motor circuit’s insulation is tested is also a major variable. Testing a motor circuit immediately after shut down will most likely yield good results. This is because the motor is warm and dry. Testing a motor after it has been shut down for a while may indicate insulation problems, but if the motor is allowed to reach ambient temperature, the insulation integrity may appear normal. This is because while cooling, particularly in somewhat humid conditions, moisture (condensation) will accumulate within the motor and lessen ground resistance. Is this a problem? Yes, particularly if starting from a partially cooled state. Most motor failures occur during starting. This is when the insulation is exposed to the most stresses. If your motors are only down for a few hours at a time, then this is when insulation testing should be conducted.

When conducting insulation testing, the most important consideration is consistency. Always test at the same location, use the same test voltage, perform the test for the same amount of time, and use the same test instrument. It is also important to note the motor temperature, ambient temperature, and relative humidity. It is also helpful to compare like motors and the motors that are operating within the same environment.

Insulation testing is somewhat ambiguous. Although there are reference standards, they cannot be rigidly followed because they do not factor in all of the potential variables that may be encountered. Temperature is the biggest variable. Temperature of the motor and the ambient temperature are of primary concern. One method to help negate the influence of temperature is performance of a “Timed Resistance Test.” This testing is comprised of “Dielectric Absorption,” “Polarization Index,” and “Step Voltage” testing. Dielectric Absorption is a 1 minute test. The resultant values at 30 seconds and 1 minute are logged and the ratio of the 30 second value divided into the 1 minute value, is a relative indicator of insulation integrity. A polarization index is a 10 minute test with the resultant ratio derived from the 1 minute value divided into the 10 minute value.

So, if ground resistance is low after prolonged shutdown and it is at ambient conditions, then you probably have an insulation issue. Conditioning of the insulation may be required. A motor shop can perform a “Clean, Dip and Bake.” process which will prolong the motor longevity. If the motor is several years old you may want to HiPot the insulation but if you are not using one of the newer units that automatically shut down upon a jump in current, you may cause insulation failure and that would necessitate a rewind.

Generator reactive power

After the generator connected to grid, the generator will be more stable than before connected to grid, because in this situation the frequency and voltage are fixed and controlled by the grid, not the independent generators. How much active and reactive power you can contribute to the grid depends on the grid requirement, such as when the grid shorts of active power, the frequency of the grid will drop, and then the grid will ask you or other generators to contribute more active power, and if short of reactive power, voltage will drop, then you could be asked to contribute more reactive power, and vice versa, which depend on the balance of power which is generated from generators and consumed by the users.

From generator side, the less reactive power, the better, as this power increase the VA and then the current to increase the losses on the transmission line which will be carried by the plant. But from grid side, as not too many equipment can generate the reactive power, the more contribution of the reactive power, the better.

At the full load operation of generator, the maximum contribution of reactive power should depend on the PF of the generator at full load (manufacturer provided for each generator). If your PF is too low and it could affect your active power transfers to the grid and will be punished by the grid. At the not full load situation of the generator, the PF could not be decided by the generator, if the grid does not need too much active power from you, but needs more reactive power and asks you to contribute more, PF could be more than 1 at the moment, but never over the Max reactive power calculated from full load.

Solar power

On a purely theoretical level and ignoring interrelated economics and energy usage, it makes sense to charge EVs during the day – though never in non-distributed environments, IMO.

In reality, and the reality for likely the rest of my life, it makes more economic and particulate emissions sense to distribute solar power during the day to decrease, and ultimately decommission, fossil fuel sources used for peak demand supply that occurs during the day.

Thus, using solar output distributed to offset the dirtiest, most expensive and most distribution grid loading power enhances and optimizes the value and worth of that solar generated power – both economically and ecologically. Attempting, therefore, to do all of ones’ EV charging off peak is the optimal solution until the mix of energy sources changes dramatically – likely a 20 plus year process even in the most environmentally friendly “energy generation mix” regions of the world. Even if one charges during “peak”, it is better to simply charge from the grid as the distributed energy is allowed to go to areas of peak demand. Again, for at least my lifetime, I don’t project a more optimal use of that generation even assuming the archaic state of most “grids” persist.

Right now, even for a 1 story commercial building, solar cannot supply the energy needs used in the office, much less a manufacturing facility. In fact, it can normally only supply 1/3 or less for the most energy and resource intensive commercial environment in a UV intense region (and that is quite an optimistic calculation, more likely 1/5th). Once you get to two or more stories on the building, one is not even close. On a modest tower with a tower parking garage, the footprint is likely to small to even generate the needs on a theoretical basis. Distributing the energy to location of greatest needs will allow us to dial down and decommission peak sources, which again are the dirtiest and most wasteful.

At some point, we will hit a new equilibrium where the energy generation mix is much cleaner, solar generation specifically is much more efficient, and peak power generation is handled more efficiently and ecologically cleaner. I still believe, however, that distributed power is better than “off grid” type of scenarios as it allows the energy to go where it is being demanded at the moment, decreasing the need for redundant sourcing. And, even in the cleanest energy generation mix, redundancy means building more of something and is by definition more energy wasteful and ecologically wasteful than a scenario where the redundancy buffer that is required is lesser.

Much of this type of debate reminds me of the consumer sort recycle versus the destination sort recycle debate. Even with the advances in trash collection and recycling processes, 20 years later we are suboptimizing the recycling process. Much of the reason for that is the “style” statement, making people feel like they are contributing by sorting themselves. It may make some people “feel” better by imagining “independent” off grid or semi off grid solutions. In reality, however, we live in an interconnected world where “sharing” or distributing solutions to leverage scale and minimize redundancies is far more advantageous, economic, and a faster route to a solution to both particulate emissions issues and energy independence for groups of people.

Cleaning solvent for motor windings

Usually, the dry ice approach is the best bet because it leaves no real residue from the cleaning material. If the insulation is “fluffing”, the likely problem is that the air pressure used to move the dry ice particles is too high.

A second alternative that can be used is “corn cob blasting”. The media is reusable, biodegradable particles of corn husks. Again, a relatively low pressure air stream is required. It WILL damage the insulation if the pressure is too high, just as in the dry ice case.

Most solvents will aggressively attack the insulation systems used for windings: this is specifically true for the larger machines where mica tapes are coated / filled with a resinous material (vacuum pressure impregnation). However, it is equally true for smaller machines where the primary insulation is at the strand level and is essentially a varnish or enamel coating on the wire. If you’re worried about how the solvent will affect the insulation system, get in touch with the motor supplier for their suggested approach.

If a solvent-based cleaner must be used, it should be applied sparingly – BY HAND – on the areas to be cleaned to break up the oily / greasy contaminant and then rewashed with some other (non-solvent) approach to clean away any solvent residue. This also will require a “dry out” of the equipment after the second washing. This three-stage approach tends to minimize damage done by solvent that may be left behind to “eat away” at the varnishes, enamels, and resins comprising the insulation system.

One last thing – pretty much ALL solvents are going to be designated as hazardous materials in most regions, due to health concerns. Therefore it is more a case of “pick your poison”!

Can I operate a 50Hz transformer at 60Hz power supply?

Well first let get one thing straight for transformers: the higher the line frequency, the lower the core (iron) losses! The core power loss are proportional to kf*B^2 approximately for any machine, dynamic or static. But transformers are self-excited static machines, meaning the flux density B is reverse proportional to the line frequency, therefore Pcoreloss = kB^2*f=k*(1/f)^2*f=k/f… so the higher f, the lower the losses. However, increasing the frequency also increases the magnetizing inductance – lowering the magnetizing current. For if you increase the frequency you may want to increase the voltage. But of course this is not usually practical, as line voltage of 60Hz systems is usually lower than those of 50Hz systems. So operating a 50Hz motor at 60Hz should be safe, but may result in higher voltage drop because of lower magnetizing current and because of higher leakage inductance (the series inductance).

It is true that the higher the frequency, the higher the hysteresis (and eddy current) losses will be. But is it a common misconception to assume higher power losses when frequency increases in a transformer. Simply because the hysteresis losses depends not only on frequency, but on the max magnetic flux density as well (Bmax^2). The flux density is reversely proportional to the line frequency, which eventually causes lower core losses as you raise the frequency. This holds true for low and mid frequency ranges. For higher frequencies, skin effect and eddy currents dominates, so the picture may be different. However, iron core transformers do not operate in such high frequencies. We use ferrite core instead. In a practical transformer model, the core losses are represented by a parallel resistor (Rc). The resistor’s value is linearly dependent of the line frequency (Rc=k*f), and the core losses are given by Pc=U^2/Rc… Of course this model is limited to mid-low frequencies…

Electrical drives for off-highway vehicles

I’ve seen some attempt of electrical driven prototypes in the field, but is still not an enough big sector that let you find specific literature. Excluding the large dumpers for mining, probably the only machine that is built in series is D7E from CAT.

One of largest engineering challenge that you will face on a similar application, is the cooling to the power electronic. You can consider that you will have to dissipate 3-5% of the power that your driver is processing and the max temperature of IGBT’s is not so far from the max temperature in that your vehicle can operate. A small temperature delta, mean a large heat exchanger and/or pretty high speed of air through it. (That with all the problems related to that). A possible solution is liquid cool the IGBT’s mounting them on the aluminum plate. You can’t use the engine cooling fluid because it too warm, but you may can use hydraulic oil (that should never get warmer of 55C).

If you are thinking to expand some gas from the AC, please take in account the possible condensation issues (your voltage on the DC bus can arrive around 800V when the vehicle is breaking, you do not want condensation around). Using SR motors is opening another challenge. For take max advantage of the technology, you want the motor spinning pretty fast (motor get smaller for same size of rotor and with that design, no problems retaining magnets). That means use high ratio gears. In off road vehicle are often used planetary gears because they are compact and cheap. As soon you rise the input speed, the efficiency of those kind of gears drop because you incur in hydrodynamic loss (for a series of problems that are connected to the level of oil that you need to keep in the gear housing). Probably if you are using an SR motor, you want consider to use an angular stage like first reduction after the motor.

I’m not too sure if I would use a battery like energy storage. Batteries take time for convert from electrical to chemical. Most of the braking will happen in a short time so you will end up burning most of the regenerated energy trough a braking resistor (the DC bus can’t go up to infinite about voltage). If you are driving a dozer that has a very low efficiency (most of the vehicle kinetic energy will be burnt in the tracks etc. and very little will arrive to the SR motor to be regenerate), probably the regeneration is not too important, on other vehicle is maybe more important so look to capacitors or flywheels for storage is probably more appropriate.