Throughout the building products manufacturing industry, there is a persistent effort to reduce thermal energy consumption in order to remain cost competitive and to meet growing requirements for reduced carbon emissions. In areas where the cost of thermal energy is relatively low, it is difficult to justify capital expenditures for energy saving initiatives. This article describes an effective approach to identifying opportunities for thermal energy savings and for implementing energy-saving measures with minimal risk. Case studies demonstrate energy-saving projects that have resulted in substantial energy savings with minimal capital investment.
Most wallboard manufacturers place importance on minimising energy consumption. Energy represents a significant part of manufacturing costs and there are growing disincentives for carbon emissions. This article is directed towards engineering and management professionals that are responsible for identifying opportunities for saving energy and for managing the implementation of energy saving initiatives.
In many parts of the world, energy costs are low. In the United States, for example, natural gas prices are at the lowest point in more than a decade (See Figure 1).
In combination with the common requirement for short-term returns on investment for capital projects, the low cost of energy makes it difficult to identify qualified energy-saving initiatives and to realise them successfully. This article proposes a sequential approach to evaluating whole-system thermal performance to determine the root-causes of thermal losses and a method of minimising risk by staging investments in preliminary thermal analysis, efficiency benchmarking, feasibility studies, testing to determine operational limits and comprehensive implementation.
Although these methods are effective for any scale of production rate and equipment complexity, the case study uses a relatively uncomplicated example for demonstration purposes. A two-zone wallboard dryer is used as the case-study prototype. The scope of the paper is limited to operating procedures, control strategies and minor equipment modifications that do not require major capital investment.
Thermal analysis: Define mass components
An effective model for evaluating thermal energy consumption requires definition of the categories of materials that undergo temperature or phase changes as they move through the process. For most analysis, it is sufficient to generalise components of total mass rates into general categories. Using a wallboard dryer as an example, gypsum, paper and various additives can be generally categorised as 'solids,' as long as the specific heat of the composite material is understood. Usually, similar generalisations are adequate for non-air and non-water gaseous components, such as the products of combustion, where the composite of non-water gaseous components may be categorised as 'air.'
A two-zone board dryer without any form of heat recovery is used as a prototypical example throughout the paper (See Figure 2).
For components of mass that are changing temperature, the required energy input is the product of the mass-rate of the material, the positive temperature change of the material and the material's specific heat value. For the component of mass that undergoes a phase change (evaporating water), the required energy input is the product of the mass-rate of the material and the latent heat of vaporisation. Figure 3 shows the relative energy consumption for each component of mass.
Efficiency benchmarking: Measure mass component energy
It is helpful to understand how the equipment's operational energy efficiency compares to that of similar equipment configurations throughout the industry. Although each factory has different performance limiting factors, benchmarking in this way can provide an early indicator of the scale of opportunities for energy reduction.
Using the components of mass method is valuable for comparing against efficiencies derived from fuel-flow instrumentation. For example, very low measured energy consumption that is near or below the latent heat of vaporisation of water or very high measured energy consumption that would require the application of heat beyond the capacities of the thermal delivery systems or the transport of heated air beyond the capacity of the exhaust systems, should be called into question.
Continuing with the example of the wallboard dryer, energy consumption can be accurately estimated by measuring the mass rates and temperature changes of each component. With reasonable accuracy, the mass rates of solids and evaporated water can be estimated based on board weight samples before and after the dryer. With the addition of data showing the temperature that the panels enter and leave the process, the energy associated with heating solids and liquid water can be estimated.
Estimating the energy associated with heating air is a task of varying difficulty that depends on the complexity of the exhaust system and the number of points at which gaseous components exit the system. For example, determining the mass rate of air through a single-point exhaust system is quite simple. It can be estimated with a reasonable degree of accuracy by knowing only the evaporation rate of the dryer and the humidity ratio of the exhaust.
When there are multiple points of exhaust, such as with the example used in this case, the ratio of mass rates through each stack must also be known to accurately estimate the air rate. For dryers that are equipped with multiple points of exhaust or with systems that exchange exhaust between zones, manual flow measurements of the various stacks and ducts are usually required.
Benchmarking should yield a 'report card' showing how energy consumption for various mass components, and how energy efficiency as a whole, compares to efficiencies derived by other methods, and against industry standards (See Figure 4).
When energy consumption for mass components is compared against industry benchmarks, opportunities for focused process adjustments become clearer. This method is most easily applied when a broad data set is known for similar processes.
Feasibility study: Hypothesise mass component changes
Isolated relationships: Once the energy consumption related to each mass component has been estimated, it is possible to hypothesise the costs and benefits of reducing the degree of temperature increase or reducing the mass rate through the process.
Costs include the monetary costs associated with equipment and technology implementation, in addition to less tangible costs related to equipment maintenance, ease of operation and risk to product quality. Figure 5 describes some ways in which energy consumption may be reduced for isolated mass components by either reducing the ΔT or reducing the mass rate.
General strategy: By rearranging the Figure 5 according to categories of process adjustments and then evaluating categories for cost and complexity versus return, a general strategy becomes clear (See Figure 6).
The bottom three categories shown in Figure 6 pass feasibility tests because they meet the objective of having relatively low costs and at least moderate return.
Application: Specific process adjustments
The procedure leading up to and including the determination of a generalised strategy for energy reduction is a linear decision making process that requires only a moderate level of thermodynamic proficiency and the employment of good measurement practices.
Each application has a different set of challenges related to machine configuration and capacity as well as factors affecting product quality and merchantability within a specific market. The following case studies are selected because they demonstrate the value of leveraging strong expertise in equipment capabilities and process limitations to translate a general strategy into specific initiatives.
Case study: Prototype equipment
The prototype equipment is a two-zone dryer equipped with direct exhaust in each zone (1A and 2A). The exhaust fan in Zone 2 may selectively extract exhaust directly from the zone (damper 2B) or through a dry-end seal section (damper 2C). Please see Figure 7.
Case study: Increase efficiency
This case study describes a real initiative that illustrates how a combination of minor capital improvements and operational changes can be applied to successfully improve the efficiency of a process with an acceptable return on investment. The capacities and configuration of the machinery are altered to disguise the owner and location of the equipment, however the methods of optimisation and magnitude of recovery have not been changed and are representative of the actual recovery.
Circulation volume: Reducing exhaust temperature
It is common for energy-saving initiatives to be prioritised according to the observable performance of mechanical systems such as leaks, air-handling system performance and burner excess air. Without discounting the value of diligent maintenance, individual component performance must always be considered in the broad scope of system and process performance. The case study offers a good example of how the optimisation of a single isolated component can be detrimental to the overall performance of the entire system.
Air is circulated through a dryer zone, where it cools as heat is transferred from the airstream into the board. A portion of the cooled airstream is then exhausted from the process, with the remainder reheated and recirculated. The heat carrying capacity of the circulation air is a function of the circulation air velocity. For constant rates of heat transfer, high-velocity air streams experience less temperature drop than do low velocity airstreams. For the same evaporation rate, a high velocity airstream will require a lower delivery temperature and will result in a higher return temperature than would a lower velocity airstream (See Figure 8).
As the exhaust is extracted from the return side of the zone, which is the coolest part of the circulation stream, a reduction in circulation volume results in a reduction in exhaust temperature.
The energy consumed by the primary mass components in the exhaust (air and water vapour) is a function of the temperature gain (ΔT) experienced by those components as they pass through the process. By reducing the exhaust temperature, the ΔT is reduced and by extension the energy required to heat those components is reduced. One may generalise that a reduction in circulation volume corresponds to an increase in energy efficiency.
It is worth noting that mechanical deficiencies resulting in reduced circulation system efficiency will usually result in increased thermal efficiency. This inverse relationship is frequently misunderstood and it is common for good-intentioned efforts to increase thermal efficiency by increasing circulation fan efficiency to have the opposite result. In the case-study dryer example, a third party energy consultant had made recommendations to increase circulation fan efficiency for the purpose of improving energy efficiency that resulted in an actual reduction in thermal efficiency. By reversing those previous recommendations, the plant was able to reduce dryer energy consumption by 1.4%.
This approach to improving energy efficiency must be employed with discretion, since the reduction in return temperature necessitates an increase in delivery temperature for constant evaporation rates. Among many risks that must be understood are the possibility of exceeding the vapour permeability of the paper in the upstream zones, increasing the magnitude of delivery temperature stratification and the possibility of a reduction in equipment longevity.
Dry-end seal utilisation: Reducing air induction
The case-study dryer is equipped with a dry-end seal section. Through an arrangement of dampers, exhaust may be selectively routed through the dry-end seal damper (2C) or directly from the return end of the zone (2B). Please see Figure 9.
The dryer was operating with the dry-end seal damper completely closed and the return damper completely open. As can be seen in the figure above, this configuration resulted in the induction of ambient air into the process through the opening at the end of the dryer.
After entering the process, the air becomes a component of mass that must be heated to the exhaust temperature of the zone. By increasing the rate of exhaust extraction through the dry-end seal it would be possible to slow or reverse the flow between the dry-end seal and the dryer body, thereby reducing or eliminating air induction at this location. Please see Figure 10.
Although there is less ambient air entering the process air stream, there is a greater volume of air passing through the exhaust fan. A preliminary step in the implementation of this strategy was to determine the fan speed and power requirements to handle the additional volume at operational temperatures, in addition to confirming that adequate power was available for starting the fan in cold conditions. In this case, the fan motor was upsized and the drive ratio was increased. This change resulted in an energy reduction of 2.4%.
Process-level flow: Reducing exhaust temperature
Prior to optimisation, the case-study dryer was operating with a pressure differential between zones which resulted in process-level gas exchange from Zone 2 to Zone 1. The volume of air transferred between zones was expelled from the process at the relatively higher exhaust temperature of Zone 1 instead of the relatively lower exhaust temperatures of Zone 2. Figure 11 shows the pressure profiles of the dryer before optimisation.
By adjusting the way in which the exhaust system is controlled, this pressure differential was eliminated. By extension, the energy associated with expelling the exhaust at a higher temperature was eliminated (See Figure 12). This change resulted in a reduction in energy consumption of 1.7%.
Summary of efforts to increase efficiency
This case study is a good example of how relatively significant energy savings can often be achieved with minimal capital investment. The factory's risk was minimised by employing consulting services in a staged manner that first explored potential savings with a less detailed study of the process before investing in a detailed analysis to forecast cost and recovery. Implementation took place after the cost and recovery analysis demonstrated adequate payback. By using highly-qualified consulting services, the factory achieved total energy savings of about 5.5%.
Case study: Reduce evaporation rate
The preceding case study demonstrates how a wallboard dryer can be optimised to reduce energy consumption relative to evaporation rate.
The following case study describes a procedure for reducing energy consumption by reducing the evaporation rate itself. Recognising that reductions in evaporation rate can be affected by a variety of changes to upstream processes that reduce the free water in boards entering the dryer, the scope of this article is limited to operational changes to the dryer itself. Therefore a strategy will be described for increasing the free water in boards leaving the dryer.
It is noteworthy that such a strategy reduces energy associated with temperature gain and phase transitions of evaporated water, while energy associated with heating the other components of mass do not necessarily increase. As a result, a dryer that has been energy-optimised by maximising finished moisture will consume less energy on a production basis, but will be less efficient when standardised to evaporated water. It is necessary to understand this distinction and to select the appropriate basis for evaluating initiatives to increase finished moisture.
All dryers have some degree of variability in finished moisture that is caused by uneven cross-sectional heat application. Figure 13 illustrates typical variability as indicated by a moisture-profiling measurement device.
Limits and variability
Figure 14 summarises the accepted limits in terms of the moisture content of board leaving the dryer. The definitions are as follows:
Machine variability: The finished product variability (moisture) caused by mechanical and process deficiencies that are outside of the control of human operators.
Operational variability: The finished product variability caused by inconsistent operating controls and operating procedure employed by human operators.
Limits: The allowable limits for finished board moisture are ultimately determined by modes of failure that occur when board is either too wet or too dry, but can also be determined by standard operating procedures related to efficiency targets.
In a dryer that has been optimised in this way, the top limit which defines 'too dry' is changed from a limit defined by modes of quality failure to limits defined by rigorous energy standards. Implementing such a change can be difficult if the machine variability is too great or if the operational variability cannot be managed within limits.
Usually, machine variability must be minimised through a variety of tactics to reduce the effects of variable heat application to different board positions in the dryer. Commonly called 'balancing the dryer,' such an initiative usually requires that a number of performance controls are in place. A partial list of prerequisite steps for minimising machine variability includes air mixing apparatus to minimise duct stratification, effective deck speed synchronisation controls (butting controls) to minimise variation in dwell times between decks, standardised evaporation rates for the range of products, exhaust controls that minimise zone-to-zone exhaust exchange and deck-damper adjustments.
Operational variability is reduced through standard operating procedures and effective automated systems. Most finished product variability coincides with variability in the incoming material. Therefore, systems that predict and respond to interruptions in production and variable incoming material are a crucial part of a strategy to reduce energy use by minimising water loss.
Figure 15 shows limits that have been narrowed for higher finished moisture and the requisite narrowing of machine variability and operational variability bands to accommodate such a change.
Summary of efforts to reduce evaporation rates
Successful implementation of initiatives to increase finished moisture usually requires a committed investment in process improvements and training. Results vary greatly depending on initial conditions. Energy reductions between 1% and 2% can be reasonably expected in many cases.
Summary of key recommendations
1. Vigilantly consider process risks to quality when implementing energy saving initiatives. Ultimately, this is a business of making high-quality wallboard, not minimising energy use.
2. Benchmark the process against industry standards. Rely on process experts that have experience with a broad data-set of similar machines.
3. Understand the theoretical limitations of the process and test the validity of measured and calculated efficiencies against these limitations.
4. Invest in high-quality fuel flow instrumentation and periodically verify instrumentation against mass-energy calculations.
5. Use a scientific approach to identify thermodynamic opportunities for savings rather than focusing primarily on observable mechanical deficiencies.
6. Developing focused energy-saving initiatives based on a general strategy requires deep expertise in the capabilities of the equipment and the quality limitations of the process. Seek the help of industry experts that have true experience in studying feasibility and successfully implementing recommendations.
7. Do not assume that OEM equipment was commissioned according to its intended mode of operation or that it is capable of operating according to its intended mode of operation.
8. Recognise the value of building energy expertise at the plant level. If relying on the expertise of outside consultants or equipment manufacturers, insist on an implementation plan that includes intensive process training.