Audit of the Month
Boeing Canada has replaced their onsite membrane style Nitrogen generator with a new more modern system with increased capacity and higher efficiency. As a result, the company is now using minimal amounts of expensive liquid Nitrogen, and has reduced the energy cost per unit of gas produced by 83%.
Inert Environment Required
Boeing uses Nitrogen to provide an inert atmosphere for pressurizing the large autoclaves at the company’s Winnipeg production facility. These autoclaves operate like huge pressure cookers, heating and curing the various composite aircraft parts fabricated by production personnel. The parts made in Winnipeg are used on such high technology products as the new Boeing 787 Dreamliner. These parts are baked in the autoclaves under a combination of heat, vacuum and pressure to ensure the layers of composite material successfully bond together to provide the necessary strength to form strong, safe and lightweight aircraft fuselage parts.
|This massive autoclave requires 69,000 cubic feet of Nitrogen at a flow rate of 3,450 cfm to fill to 100 psi in 20 minutes.|
Old System Inefficient
Many years ago Boeing had installed an on-site Nitrogen generator to provide lower cost gas than could be provided by trucked-in liquid Nitrogen service. This system used a 250 HP two-stage high-pressure screw air compressor to feed a membrane-style Nitrogen generator. The generator separated the high pressure air into two streams; one of waste gases vented to atmosphere, the other a flow of high purity Nitrogen that was directed to the plant Nitrogen system. Boeing’s plant Nitrogen demand is normally very low, but has huge peaks when the autoclaves are pressurized. This required the compressor to operate at very high pressures to keep as much Nitrogen in system storage tanks as possible. Due to this high-pressure requirement, the compressor operated using the inlet modulation control mode greatly reducing its efficiency when low Nitrogen demands occurred. The compressor and Nitrogen generator was also installed outdoors and consequently had to be run 24 hours a day and 7 days a week during cold Canadian winters to keep from freezing up.
Over the years the original system had aged and was experiencing maintenance problems and reduced capacity. This increased the quantity of expensive liquid Nitrogen that had to be used to supplement the old Nitrogen system. The cost of liquid Nitrogen is substantial - up to 38 times the cost of producing the gas on site. As a result, this rising liquid Nitrogen consumption had the production manager’s attention.
Huge Peak Flows
The Boeing autoclaves are more than just simple curing ovens, these mammoth pressure vessels are big enough to accommodate very large aircraft parts. The largest unit is 50 feet long with a diameter of 16 feet, big enough to swallow a large transport truck. The volume of the largest of these vessels is just over 10,000 cubic feet. To maintain adequate production output levels, the operating procedures require these vessels be filled to pressures as high as 100 psi in as little as 20 minutes. This equates to peak Nitrogen flows of 3,450 cubic feet per minute, much less that the existing Nitrogen generator capacity.
Because of this very high peak flow, Boeing had installed a large 30,000 gallon receiver to stabilize pressures and reduce the required size of the Nitrogen generator. The plan was to charge this receiver to high pressure using the main screw compressor as a booster. This strategy required that all of the gases, even the waste stream vented to atmosphere, be boosted to high enough pressure to provide an adequate reserve. This pushed the specially designed screw compressor to its design limits and forced it into modulation mode, where it produced less and less air as the storage receiver filled up. Due to the characteristics of membrane style generators, this lower air flow increased the purity of the Nitrogen produced, but also significantly increased the losses of the system, with the membranes venting more and more gas per unit output. In fact, much of the time the unit operated with no Nitrogen output, with all the produced gas being vented directly to atmosphere.
Old System Pressure Limited
This system was at a real disadvantage in terms of Nitrogen storage. The high pressure screw compressor was limited to an output of about 175 psi. Since the required pressure for the autoclaves was 100 psi, this left only 75 psi of differential in the large storage tank that could be used to supplement peak flows. This equates to about 20,700 cubic feet of reserve, far short of the 69,000 cubic feet that was needed for an autoclave fill to the maximum pressure. Since the old Nitrogen generator could only produce about 150 cubic feet per minute of Nitrogen, the shortfall had to be covered by expensive liquid Nitrogen reserves.
To solve their Nitrogen production shortfall, and provide increased capacity for future production expansion, Boeing approached Air Liquide, a leading supplier of industrial and medical gases, for a new solution to its Nitrogen needs. Boeing challenged the company’s engineering team to not only design a system with increased output capacity, but to do it at reduced energy costs.
The team successfully met the capacity requirements and greatly exceeded Boeing’s energy cost reduction expectations. A new system was installed that could produce enough gas to supply Boeing’s peak Nitrogen demand for the foreseeable future and provide an innovative staged production process that can be shed in steps to take the Nitrogen production “off peak”- saving significant utility peak demand charges.
|Variable Speed Compressor efficiently matches the Nitrogen generator demand./td>|
VSD Matches the Flow
The new Nitrogen system uses two 100 HP rotary screw air compressors on the front end that run at a lower pressure than the original system. One compressor uses variable speed drive technology, which allows the complete system to efficiently match the air demand of the membrane-type Nitrogen generators. The other compressor, a fixed speed unit, runs at full load at its best efficiency point and quickly unloads and turns off when demand drops off or a signal to shed load is received. The combined system gives the front end of the system the capability of automatically adjusting the output capacity to meet all levels of Nitrogen generator demand, even as the Nitrogen membrane bundles age.
Membrane Conversion Loss
The membrane Nitrogen generators are set to maintain a purity of 98% or better and to produce about 11,600 cubic feet per hour (190 cfm) of Nitrogen. This output illustrates the typical conversion ratio of the membrane separation process at high purity levels, with the full load capacity of the front end air compressors totaling 765 cfm. Rather than trying to push this full air output capacity to the 330 psi needed for optimum high pressure Nitrogen storage, the input air is produced at only 165 psi and the smaller flow of Nitrogen at the output of the generator is fed into two small booster compressors that pump up the existing 30,000 gallon receiver tank to full pressure.
Boosters Add Storage Capacity
Boosting the Nitrogen to pressures higher than it is used slightly increases the energy required for the smaller Nitrogen flow produced by the membrane filters, but this is much less costly than producing the much larger amount of higher pressure air feeding the front end of the system. Storing the Nitrogen at 330 psi in a 30,000 gallon storage receiver provides over 64,000 cubic feet of reserve capacity - enough Nitrogen to fill the largest autoclave in 20 minutes.
|Booster Compressors charge a large 30,000 storage tank to 330 psi.|
The plant Nitrogen demand has very high peak flows during autoclave filling but low Nitrogen demands during normal periods. This gives the system time to catch up after filling operations, slowly recharging the storage tank over a number of hours. During this filling operation the generator is kept at 100% output capacity, its most efficient point. When the storage is full the control system completely turns off the generator, avoiding inefficient partial load operation.
With current production levels, the generator needs to run only 30% of the time. This gives the system the flexibility of selectively operating at only optimum times, allowing a further cost reduction opportunity through the elimination of the Nitrogen generators’ peak demand from the total facility electrical demand.
Boeing’s electricity bill includes both energy (kWh) and demand (kVa) components. The demand charge billed by the utility each month is calculated as the highest 15 minute peak recorded by the utility meter in the billing month. In Boeing’s case the facility peak demand occurs at a low frequency, less than 4% of the time. Because of this and because the Nitrogen system was designed with large storage, has numerous periods of inactivity, and controllable compressors, the production can be turned off during peak plant electrical demand, thereby saving significant demand charges. Boeing’s control system uses a Manitoba Hydro supplied, specially programmed power meter that constantly monitors the total facility peak and automatically turns off one or two stages of compressor power, as required, to limit the demand contributed by the Nitrogen system. This system has a bypass feature that allows full Nitrogen production should storage be depleted and full production required.
|Table shows the savings for the new system and compares the charges with or without demand.|
A major byproduct of the air compression process is heat. The new system is designed to recover the heat of compression to supplement building heat in the winter and further reduce costs.
The projected energy consumption for a system equivalent to Boeing’s original Nitrogen generator, but with higher capacity, was an estimated 2,150,000 kWh per year with a peak each month of 225 kW. Projected annual electrical costs would have been \$76,400 including peak demand charges. The electrical operating cost of the new system, without the demand charges, has been verified at only \$12,730 per year for a savings of 84%. Further liquid Nitrogen savings are estimated at \$30,000 per year.
|This operator’s screen allows Boeing to monitor the efficiency of the Manitoba Hydro designed control system.|
Reliability and Efficiency
“Our new Air Liquide nitrogen system is much more reliable and cost effective than either the gas generation or liquid vaporization we previously used.”, says Gerry Glor, Boeing Winnipeg’s Autoclave Subject Matter Expert. “And our partnership with our local utility really helped out.”
This project is another example of how an innovative design team can come up with a superb solution to a production problem, in an energy efficient way. Boeing and Air Liquide’s design team are commended for their innovative design and the resulting low per-unit Nitrogen cost. A great example of being Power Smart!
How a Nitrogen Membrane Works
Selectively permeable Nitrogen membranes separate gases by the principle of selective permeation across the Nitrogen membrane’s wall. For Nitrogen polymeric membranes, the rate of permeation of each gas is determined by its solubility in the Nitrogen membrane material, and the rate of diffusion through the molecular free volume in the Nitrogen membrane wall. Gases that exhibit high solubility in the Nitrogen membranes, and gases that are small in molecular size, permeate faster than larger, less soluble gases.
Fast gases permeate through the Nitrogen membrane wall more readily than "slow" gases, thus separating the original gas mixture into two streams. The purity of the desired streams can be adjusted by changing the operating conditions.
The ability of Nitrogen membranes to separate two gases is determined by their selectivity, the ratio of permeability of the two gases. The higher the selectivity, the more efficient the separation and less energy is needed to run the system.
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