To save energy, first find where it’s used

One of the most important factors to saving energy in the industrial environment is to obtain the knowledge that equipment is running, and how well the shafts and rollers are moving.

12/12/2014


One of the most important factors to saving energy in the industrial environment is to obtain the knowledge that equipment is running, and how well the shafts and rollers are moving. Even when the plant runs 24 hours every day, not every piece of equipment runs continuously. Some things are working constantly, like air handling fans, while other equipment runs periodically, on a regular schedule or only as needed.

Knowing the length of service is an absolute requirement, and knowing the time of day of equipment operation will give you the information needed to help keep your plant electrical energy use lower, especially during the peak demand periods, when the utility charges a premium for the power supplied.

Measuring electrical power consumption is the easiest energy source to monitor when adding this capability to an existing system. Measuring natural gas or other petrochemical fuel use accurately would require metering devices that are integral to the piping systems, often very exotic and elaborate, while measuring electrical power can be added externally, at relatively low capital investment levels without the need for cutting and threading. Disconnecting the incoming power and adding suitable current sensing devices and connecting to the mains power for a voltage monitoring sensor is simple and completed quickly.

Deciding whether the energy use could be measured closely enough by monitoring only the current used, or more precisely by monitoring power consumption (current and voltage measured concurrently, taking power factor into account), will be easier after taking some preliminary measurements. As an example, one process that could benefit from monitored electrical energy use is a plastic injection molding machine in operation during all three shifts.

Fig. 1 Typical plastic injection molding machineThere are several separate parts which use significant amounts of electrical power: the screw auger drive motor (sometimes electric but often hydraulic), the heaters helping to melt the plastic pellets inside the barrel, and the conveyor taking the finished pieces away from the equipment. Since different plastics melt at higher or lower temperatures, the heating of the barrel and the nozzle are controlled and measured with thermocouples or other sensing components, and the power consumption is often varied using solid state switching to turn the heating elements on and off several times each second or each cycle. With this type of process, the power consumed varies according to the material being molded; the size of the finished product and amount of extra heating required to produce quality parts. It would be very difficult to estimate the power use over a period of time without doing actual measurement of the power consumption continuously. (See Fig. 1.)

Some plants might decide to measure only the electrical heating of the barrel, along with the fans or vacuum used to load pellets into the hopper and keep them clean and dry. Others might choose to monitor the consumption of each molding machine, and still others might decide that one power measurement of all the molding machines in one plant will give the information needed to make intelligent decisions about maintenance, operating schedules, or possibly adding equipment like more complex heating controls, timers to allow operation only during set periods, or variable speed drives on the fans and vacuum.
The key is that to initiate any energy saving program, one must first determine where the energy is used.

Managing motors

Motors use approximately 60% of the electrical energy consumed in industry in North America, while lighting uses about the same portion of the total in commercial operations. There have been great advances in the development and implementation of lighting sources which use less energy than in the past, specifically LED and other solid state variations.

The advances in lower consumption motors have been minimal, although using controls such as pulse width modulated drives to control the speed helps significantly. Before variable speed drives gained acceptance by improving reliability and reducing some less than desirable characteristics of an inverter like bearing deterioration, motor overheating, harmonic current, vibration, and excessive noise, the output of a fan or pump was controlled by partially closing dampers or valves. The drive motor would still be running at base speed, delivering lower quantities of air or liquid through the physically reduced duct or piping.

The best results for saving energy and high reliability use a drive matched with a motor designed for variable speed rather than just adding a drive to an existing motor. Replacing existing motors with the latest energy-efficient models will result in energy savings, but the total cost should be considered and calculated against the time of operation.

The difference in the purchase price of the two motors is minimal, but the labor to substitute one for the other is substantial. Energy savings will be best when a motor runs continuously rather than periodically. With a power cost of $0.10 per kWh, the cost to run a motor continuously amounts to $2.00 per day per hp.

Consider a 100 hp motor: The annual cost of operation would be $70,000.00 ($2.00 per day times 350 days times 100 hp). By spending approximately 30% more for a premium efficiency motor, one with 2.4% better efficiency, the annual cost will be $1800.00 lower. The larger the motor, the lower the efficiency gained when stated as a percentage. A standard 5 hp motor may be 84.0% efficient (at 75% of load), while the premium motor is 88.2%. A standard 50 hp motor may be 91.6% efficient, while the premium is 93.9% efficient.

However, the standard 5 hp would use approximately 26,644 kWh annually, and 25,374 kWh in the case of the premium design, saving 4.76% overall. The standard 50 hp uses 244,211 kWh while the premium model uses 238,027 kWh, or 2.53%.

Using the same high efficiency motor in a new installation or to replace a failed motor is sensible, but double-check the torque and speed requirements of the driven load before specifying to be sure the newer design will work (and produce the expected savings) as needed.

Finding what’s “normal”

Even a fully operating plant running every day can find energy saving opportunities by measuring the current or power use of any operation using electricity. Once the “normal” consumption level has been established, energy usage will increase as the rotating parts of the machine begin to wear. This increased consumption should be a flag to check bearings and lubrication at regular intervals.
As an added benefit to the operation, if the current decreases from “normal” (actually reducing the amount of energy used), other major problems can be mitigated immediately. Open pump discharge or blocked intake (or other dead headed pump conditions) can result in many hours of downtime.

Taking a thorough motor inventory may also help find areas of energy overconsumption. Comparing the normal running current and/or wattage consumption with the motor nameplate data often shows where a motor is oversized. At some point a 25 hp motor might have been replaced with a 30 hp motor due to availability or time constraints, and the old 25 might have been driving a load requiring only 17 hp. The end result: A load requiring 12.682 kW is driven with a motor with a capacity of 22.38 kW.

While the system works and there is never any overload or excess heat, the pairing is going to use more power, and the utility may well penalize the user for poor power factor, one result of using an oversized drive motor. The best designs have the continuously running drive motor loaded between 75% and 80% of the nameplate rating. With cycling loads, it is sensible to use a motor with capacity to handle the worst-case loading conditions.

Checking the supplied voltage, either with a handheld meter or continuously with a power monitoring device, any unbalance larger than 1% (average voltage minus maximum deviation from average divided by average times 100) should be cause for concern.

Calculating savings

Assume that a 460 Vac 100-hp motor was fully loaded and operated for 8,000 hours per year (hrs/yr), with an unbalanced voltage of 2.5%. This would cause the motor efficiency to drop from 94.4% (with imbalance of 1%) to 93.0%. With energy priced at
$0.08/ kWh, the annual energy and cost savings after corrective actions are taken are:

Annual Energy Use:
100 hp x 0.746 kW/hp x 8,000 hrs/yr x (100/93 – 100/94.4) = 9,517 kWh
Possible Annual Cost Savings:
9,517 kWh x $0.08/kWh = $760

Again, without the knowledge that the condition exists, no action would be taken. One cause which is often missed is that one phase is leaking current to earth. Adding a ground fault detector or ground fault transducer can help solve this problem.

Measure, compare, and uncover places in your process that are energy vampires, and the result will show with decreased energy use and improved reliability. 

Will Delsman is an applications engineer for San Jose, Calif.-based NK Technologies. The company’s website is www.nktechnologies.com.



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