ThyssenKrupp Elevator: Reducing Energy Use in a Curing Oven

BACKGROUND

ThyssenKrupp Elevator (TKE) is a global manufacturer of elevators and other mobility systems for buildings and cities. The company’s Groupwide Energy Efficiency Program (GEEP) focuses on measures to address waste heat, the reduction of stand-by times, and the replacement of plant components. At their ISO 50001 and LEED Operations and Maintenance Gold certified plant in Middleton, Tennessee, the TKE Energy Team works to continuously identify sources of inefficiency and implement solutions to improve energy performance. The Middleton plant worked with the University of Memphis Industrial Assessment Center to perform a comprehensive energy assessment and the resulting report identified the oven used to cure powder coating on structural elevator parts as a significant source of inefficiency. A preliminary energy measurement indicated that the structural oven consumed up to 35% of the plant’s natural gas use, and almost 15 percent of total plant energy use. Improving the efficiency of the structural oven was critical to reaching plant energy reduction targets and was achieved by adding insulation and a rear enclosure to the oven and shutting down one of the exhaust stacks to reduce heat losses. These improvements resulted in a 35 percent reduction in oven gas consumption.

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The structural oven contains a 3,700 square foot oven chamber and operates up to 20 hours per day. Elevator parts are routed through four oven hot zones; two zones being single pass and the other two being traversed twice. Each zone has a dedicated burner and circulating fan to ensure proper mixing of burner heat throughout the oven. The temperature profile for each elevator part calls for approximately 30 minutes of cure time in the oven at a holding temperature of 350 to 400 Fahrenheit. The oven is vital to the curing process, so the TKE Energy Team made sure to evaluate how any energy-saving modifications would impact oven uptime and structural part finish quality. 

Modifying the oven is challenging due to its complexity and age. Because the oven is decades old, original design specifications were lacking and detailed documentation of oven design was not available. With this in mind, the TKE Energy Team moved forward with a conservative, phased approach. This process is characterized by making key energy improvements one-at-a-time, pausing to collect metrics and confirming energy reductions, and then testing the oven to ensure the thermal profile was maintained. The team identified and moved forward with three structural oven upgrades:

  • Installation of rock wool insulation;
  • Moving from two exhaust stacks to one exhaust stack under steady-state operation; and
  • Constructing a structural oven enclosure.

These upgrades yielded a 19% reduction in oven shift firing rates (from 5.2 MMBTU per hour to 4.2 MMBTU per hour). The oven modifications also reduced startup procedures from 3 hours to 60 minutes, along with production time being reduced by an hour, resulting in a 15% decrease in oven shift time (20 hours to 17 hours). These collective improvements produced a 35% reduction in fuel use and additional labor savings that were not quantified.

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SOLUTIONS

As a first step, the TKE Energy Team contacted the DOE Industrial Assessment Center (IAC) at Tennessee Tech for an assessment of the entire plant, including the structural oven. IAC’s are located throughout the country and specialize in identifying energy saving opportunities at manufacturing plants. At the Middleton plant, the IAC team examined the structural oven and recommended oven insulation as a method of reducing heat loss and improving overall oven efficiency. The TKE Energy Team confirmed this theory with an insulation calculator customized to account for insulation materials. Rock wool insulation was installed in the structural oven and the team continued to monitor performance and evaluated additional modifications to reduce heat loss.

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The TKE Energy Team consulted with their Better Plants Technical Accounts Manager (TAM) who performed a visit to the plant about the IAC findings and additional opportunities to reduce oven heat loss. Using data loggers and auditing tools from the Better Plants program diagnostic equipment loan program, the TKE Energy Team and TAM logged structural oven energy use data. The team observed and documented the 4 am oven startup, oven firing rates, oven dimensions, temperatures, and exhaust rates. This data was analyzed using the Process Heating Assessment and Survey Tool (PHAST), a software platform in the DOE MEASUR tools suite that enables industrial facilities to assess the energy efficiency of process heating applications. The plant developed a robust baseline of the oven’s energy use and fuel consumption. The analysis revealed a high proportion of exhaust heat, which prompted further evaluation of whether the full exhaust gas flow rate for two exhaust stacks was required during steady-state operation. The team concluded that two exhaust stacks were not necessary and that one exhaust stack can accommodate the structural oven exhaust while still maintaining the required ventilation rates and cure time. Thus, the second exhaust stack was disabled for steady-state operation and would only be used for initial purge prior to startup and safety measures. 

The TKE Energy Team made a final, small upgrade to the structural oven after the previous two measures were completed. The team identified heat losses in the oven rear due to an ineffective air knife and the oven being unenclosed. A contractor was brought into the plant to construct an enclosure and shut off the 5-horsepower air knife motor. These upgrades yielded some electricity savings and reduced heat loss in the oven. 

Using an approach developed by Middleton plant staff, the TKE Energy Team tested the structural oven to see if any energy-saving efforts affected production adversely. This approach involved using a parts handling system to route a part assembly with imbedded thermocouples through the oven. The team then operated the oven at production firing rates while the part assembly records the temperature profile in each oven zone. Results were then compared with previous readings to ensure that the appropriate cure times and temperature requirements were maintained after the energy-saving upgrades were completed. The team conducted this test after each energy efficiency improvement to confirm that the necessary oven parameters were met by the individual improvement.

Structural oven energy related savings were then quantified using a corporate reporting metric developed by the TKE Energy Team. The metric records project-specific natural gas savings and the associated dollars saved and is based on production rates by month and the change in oven firing rates that were recorded before any upgrades took place. It allowed the TKE Energy Team to compare the six-month period after upgrades were completed with the same period from the year before. Results indicated a 35% reduction in oven gas consumption and a six-month bill reduction of more than $15,500. This projects to yearly savings of over $31,000 in natural gas costs and when compared to project costs of $8,000, a payback period of 3 months. Based on the success of the structural oven improvements, further energy savings opportunities are being evaluated, including exhaust gas heat recovery and VFDs for the recirculation fans.

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OTHER BENEFITS

For future projects, the plant is following a new evaluation process based on the oven improvement project’s monthly corporate reporting metrics framework. This process enables the TKE Energy Team to report to management on energy reductions, utility cost savings, environmental impacts, and return on investment of any given project, and to measure its progress towards reduction goals and document actions taken as part of its ISO 50001 energy management system.

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The improvements to the structural oven are also being evaluated for replication in the plant architectural oven. Although this unit is newer than the structural oven, it has significant room for improvement. To investigate this further, the TKE Energy Team initiated a study to evaluate whether they can eliminate the use of the architectural oven during the second production shift, by consolidating both shift’s production into the first shift operations.

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Annual Energy Use

Baseline Oven Shift Firing Rates
5.2 MMBTU per hour
Actual Oven Shift Firing Rates
4.2 MMBTU per hour

Energy Savings:

19%

Annual Energy Cost

Baseline Annual Natural Gas Bill Cost
1
Actual Annual Natural Gas Bill Cost
0.81

Cost Savings:

$31,000

Sector Type

Industrial

Location

Middleton, Tennessee

Project Size

700,00 Square Feet

Financial Overview

$8,000

Parts entering structural oven
Parts entering structural oven