Implementing “time-of-day” energy load shed project reduces costs

Valuable lesson learned at huge aero-structures fabrication plant about energy conservation projects from the maintenance department.

10/12/2011


A few years back, as an energy manager in a huge aero-structures fabrication plant in the Midwest, I learned a valuable lesson about energy conservation projects from the guys in the maintenance department.

As a major subcontractor to the Dept. of Defense (DOD), our facility was producing structural assemblies for military aircraft. The site operations contained 50 buildings with over 8,000 employees working on a 24/7 schedule to meet the stringent production schedules demanded by DOD program managers and the prime contractors.

Basic manufacturing processes included machining, forming, heat treatment, subassembly components, fabrication, painting, final assembly, and full operational testing of each completed structure for the various military aircraft models and two commercial contract passenger planes as well. Total energy costs at the site averaged over $450,000 per month.

Part of the DOD contractor certification process requires having a formal energy conservation program in effect and using energy management systems (EMS). The energy program also required quarterly progress reviews with DOD officials.

The plant had an early generation IBM EMS that was hardwired to most of the major equipment on-site. We were able to monitor and control operation of many utility systems and some larger process systems to optimize energy usage. Energy awareness and conservation measures were fully expected. Progress was being made.

Huge structural components (after machining and forming) required chemical cleaning, acid etching, and rinsing prior to military specification painting and drying. Those processes were significantly energy intensive. Analysis of those various process energy systems had been given a priority, and we were analyzing the equipment with most potential.

Energy project opportunity identified

The ventilation systems for those specific chemical cleaning processes included numerous 100 hp supply and exhaust fans to maintain process conditions and control fumes abatement. Energy costs were being delineated, especially the heating load to maintain ambient plant conditions during winter months. Those fans had been designed to operate continuously, with manual control only.

Most of those processes generally operated on a two-shift/six-day schedule. The fans ran from 6 a.m. on Mondays to midnight on Saturday.

We decided to implement ”time-of-day” energy-saving control measures on eight of those 100 hp fans (four supply and four exhaust) during the third shift. Electrical power savings were significant ($350 per day). I was quite pleased with the idea and our programming capability (which could be controlled from my office remote terminal or from the IBM mainframe in the maintenance electrical shop. Over $9,000 per month of cost savings was immediately apparent on the No. 2 electric meter supplying that area of the plant.

Prior review of our plan with process engineering and the safety department had been conducted and subsequent air quality testing confirmed that indoor air quality was not being adversely affected. The ventilation system appeared to have been very conservatively designed.

In addition to the direct power savings, we had estimated building heating and cooling energy savings of approximately $4,000 per month during the 8-month heating and cooling seasons.

The next quarterly energy review meeting with the site leader and the DOD officials received compliments and praise.

Subsequent issue addressed

About three months later, I was in my office on a Monday morning working on another project. In walked the mechanical maintenance supervisor and an HVAC technician. They dropped a large set of worn V-belts in the middle of the office floor. There were six D-size belts each about 10 ft long. The supervisor then asked me how much money we were saving by turning the chemical process ventilation fans “off and on” every day.

I quickly indicated about $9,000 per month. The guys were a little surprised at this amount, but the supervisor then proceeded to explain that belt failures, ventilation system complaints, and maintenance labor had gotten out of control over the past few weeks. He further explained that a new set of matched belts cost about $900 plus 10 man-hours of mechanical time to replace the worn belts and perform a necessary realignment.

We had neglected to consider that the 1,800 rpm induction motors on these large fans caused significant motor sheave slip and belt wear during every start. The maintenance team had changed four sets of belts in the past month. That was work that maintenance certainly did not need.

After offering some coffee and removing a little bit of “egg” from my face, I agreed to set up a brainstorming session with a couple of the plant engineering and maintenance guys for later in the week. The rather obvious solution was to add “soft-starters” on the motors, which took about 2 months to get approved and installed. (They cost about $1,800 for each motor.)

The new starters worked just fine. Everyone was happy, and I had gained some valuable experience. Moreover, the motors were happier too, not being instantly started daily under heavy load. I also committed myself to conducting a more thorough “risk” analysis on future unique projects.

Summary

Variable speed drives are a fairly standard option on many larger HVAC and process ventilation fan systems. Since there are now several excellent VSD suppliers, the costs have become very reasonable and VSD reliability is excellent.

If your facility has motor driven fans, pumps, rotary air compressors, machine drives, and so on (40 hp and larger) that stop and start frequently, a drive motor soft-starting feature can significantly reduce your ongoing maintenance work in addition to generating good energy savings.

One caution: If your process conditions are critical (i.e., precise pressure, flow, and/or temperature control, etc.), be sure to specify high-quality VSDs. More economical VSD units can produce motor speed variability (regulation control overshoot) during every process setpoint change. I recall installing VSDs on FD fan motors for three 75,000 PPH oil-fired steam boilers back in the late 1990s.

Every time the steam load changed, the boilers produced that gray haze out the stack. We had to go back and install a better quality VSD unit. Review this issue thoroughly with the VSD suppliers.

Wamsley is a mechanical engineer with more than 40 years of plant operational experience. He is president of JoGar Energy and Utility Services, Inc. located in Alpharetta, Ga. JoGar Energy Services offers on-site energy reviews, boiler efficiency testing services, technical assessments of specific utility systems, and training seminars. Wamsley can be reached at 678-977-1508.



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