Owners and operators of chemical production facilities place a far greater emphasis on the reliability and energy efficiency of process areas than they do on the energy efficiency of the HVAC systems that condition their office spaces. When they engage a consulting engineer for an energy assessment, they are often focused on cost-effective production, and understandably so. Yet owners may achieve significant savings on energy costs if they include offices and other occupied spaces in the assessment.
The SSOE Group was hired to complete an energy assessment for a chemical company and clearly demonstrated the benefits of a comprehensive assessment. The 1,600 acre facility has an electrical power feed of 50MVA and consumes over 1,435,000 MMBTU of natural gas per year. Based on current best industry practices, the owners wanted to know what energy savings could be realized through improvements in the energy flow from the plant’s utility systems to its chemical processes. SSOE was asked to identify energy savings opportunities with a short-term simple payback. The assessment also took into account various risk management scenarios of escalating energy prices to identify additional money-saving opportunities.
Although this plant’s energy consumption was largely attributable to chemical processes, SSOE proposed expanding the assessment beyond the process to include HVAC services in five administrative, engineering and laboratory buildings. The additional research did not significantly increase the total cost of the study, and so the owner agreed.
Here is a look at the key findings and energy savings opportunities.
Reduce Standby Process Boilers
Steam production trend charts showed that all five boilers were typically in year-round service except for scheduled maintenance. Boilers operate most efficiently at high firing rates, but the plant did not require any of the boilers to operate above 50% of their capacity. Discovering that on any day excess boiler capacity exceeded demand for production steam by a factor of two, SSOE concluded that the plant could shut down two boilers, meet production needs and still maintain one boiler on hot standby.
Engineers discovered this opportunity while reviewing the recorded steam production and natural gas consumption data. The plant meters the gas each boiler consumes and the gross steam production from their two steam generation buildings. The total steam generation capacity was graphed along with actual steam produced for one year. The difference in actual generation to total capacity was three boilers worth of steam. From each boiler’s gas usage, engineers discovered long periods of time when the boilers were idling. Assuming only one spare idling boiler, avoidance of the annual gas consumption of the other two idling boilers was calculated at $500,000 in savings.
Optimize HVAC Performance
SSOE recommended eight changes to optimize HVAC performance based on the findings from their inspection of these systems and industry best practices:
- Replace existing pneumatic controls in 10 buildings with a direct digital control (DDC) system. From the inspection of existing equipment and interviews of the maintenance personal, it was found that the old pneumatic equipment was problematic, requiring many service calls every year for adjustments and repairs. New DDC controls would not only improve occupant comfort but also enable implementation of various energy conservation strategies, such as temperature setbacks, vacation mode, optimum warm up, demand control ventilation, VAV reset and others.
- Reset space cooling setpoints from 68 degrees to 72 degrees in three buildings. This no cost recommendation was based on the fact that there was no building automation system (BAS) or supervisory panels to control all the separate HVAC units and zones that served these buildings. Lacking automated controls, best practices are to set the thermostats at 72 degrees when occupied in the cooling season. Each degree increase in setpoint can save as much as 3% on cooling costs per day.
- Replace existing chillers in one building with high-efficiency chillers. During their inspection of the equipment, engineers noted that the nameplate indicated it was more than 20 years old, approaching its useful life expectancy. Current chiller technology with multiple compressors are 5-15% more efficient than old equipment, especially in partial load operation, which is where this equipment will operate during the majority of the cooling season.
- Replace the existing packaged roof top unit (RTU) in one building with a high-efficiency packaged rooftop unit with economizer control. Similarly, engineers’ inspection of this equipment found that most were over 20 years old. Similar to chillers, new unitary RTU air conditioning technologies are more efficient than old equipment due to multiple compressors to better match partial load operation. Economizer systems allow for nearly free cooling in temperate weather as large volumes of cool outside air replace the stale warm air in the building.
- Convert air handling systems in one building from constant volume to variable air volume (VAV) control. This building was found to have a constant volume AHU that served VAV boxes for each zone. Fan energy is wasted by recirculating the excess air volume that was not used. If the air handler were converted to a VAV operation, the fan speed would be slowed down to just match the air volume needed for cooling. This would result in significant fan energy savings; a 2% reduction in fan speed equals an 8% reduction in fan energy in accordance with the fan law cube rule.
- Rebalance existing fume hoods in the laboratory to 100 fpm face velocity at 18” sash height for energy conservation. Engineers inspecting the lab hoods found that they did not have adjustable sashes and the air volumes were in excess of 100 fpm. The excess sash velocities translate to more air being removed from the building than necessary. Rebalancing hood exhaust to recommended levels would save energy by reducing the amount of conditioned air that replaces air exhaust from the lab.
- Install VAV controls in the large fume hood lab. No controls were present to reduce the air volumes to hoods when not in use. During this time or when the hood sash is in a closed position (cracked open just a couple of inches), the air volume is reduced to maintain the recommended opening velocity. This strategy takes advantage of the two energy saving principals previously discussed; it not only saves on conditioned air replacement costs but also saves hood fan energy.
- Minimize unused or underutilized spaces that are being air conditioned in three buildings. On the walk-through of all buildings, several were found to have only 10% to 50% occupancy. Moving occupants from the nearly empty buildings to other available spaces reduces maintenance costs and saves energy. The empty buildings are then kept just warm or cool enough to prevent damage to the structure and contents.
The projects were estimated to save a total of $ 244,000 per year on steam costs and $9,900 on electricity costs. Conversion to DDC controls yielded the greatest single savings on combined energy costs at $134,000 (of the above total)—with a simple payback in just eleven years. The average simple payback on all of these projects combined occurs in just six years—an excellent return on investment—with the replacement of chillers having the longest single payback of 29 years.
Additional Opportunities from Existing Resources
SSOE found the following additional opportunities to improve HVAC efficiency using existing resources:
- Use recoverable waste heat to meet heating requirements. Currently, condensate return from the boiler distribution system is approximately 70%, and the balance is unrecovered. Engineers determined the condensate recovery of 70% from the plant measurement of boiler make-up water as a percentage of the total measured steam generation. It was information frequently reviewed to facilitate ordering of the chemicals required to treat the make-up water and determine process efficiencies. Of the unrecovered condensate, 5% was used for direct injection into the process and the balance (25%) was lost to vents or drains. It was generally accepted that the lost 25% was just the cost of doing business and not worth trying to recover. However, the industry standard for condensate return, excluding direct injection, is 90%. Recoverable condensate could clearly be used for the heating needs of occupied buildings if piping were run to these buildings.
- Use water cooled condensers instead of direct expansion (DX) units to cool occupied spaces. Engineers’ inspection of the rooftop units (RTU’s), discussed above, were the basis for this recommendation. Office cooling has been provided by DX units, and yet the plant has an extensive and energy-efficient cooling water system. SSOE recommended that new RTU’s utilize water from the cooling towers, which would improve efficiency by 3% or greater.
SSOE’s comprehensive energy assessment identified ways to save more than $2 million a year overall, including $100,000 on the cost of steam and more than $200,000 on the cost of electricity to heat and cool administrative, engineering and laboratory spaces. At the same time, the engineers’ assessment of natural gas usage throughout the facility was the key to identifying over $500,000 in potential savings on natural gas costs from a reduction in the number of standby boilers on the process side. Engineers also found enough recoverable waste heat on the process side to provide all of the heating for occupied spaces. In addition, they discovered a way to increase cooling efficiency by 3% using existing cooling towers.
This project illustrated the importance of always looking at the big energy picture: a comprehensive energy assessment can reveal savings opportunities on both the process and the people sides of a chemical facility. And, inclusion of occupied spaces in an energy assessment does not substantially increase the cost of the study.
James L. Yerke, PE, CEM, is an Engineering Supervisor and Senior Associate of SSOE Group, a global engineering, procurement, and construction management (EPCM) firm. Yerke is responsible for identifying existing and proposed capacities, as well as engineering and schematic design of mechanical systems. He has over 30 years of experience, including energy calculations energy conservation in industrial process and energy centers. Yerke can be reached at 248-643-6222 or [email protected].