INVESTMENT-GRADE COMPRESSED AIR SYSTEM AUDIT, ANALYSIS, AND UPGRADE

IN A PULP & PAPER MILL

Paresh S. Parekh, P.E. Principal Unicade Inc. Bellevue, Washington

ABSTRACT

An investment-grade compressed air audit, analysis, design, and upgrade in an industrial facility requires a thorough understudying of original design basis, current problem definition, goal settings, load assessment, base case model development, and an ability to conduct unlimited "what if" analysis to determine the cost-effective efficiency improvement measures. Most successful projects are justified using energy savings and non-energy benefits. A life-cycle cost analysis will help the customer focus on cost of owning, operating, and maintaining the compressed air motor system over it’s useful life. A software application that easily integrates and calculates interactive impact of process and mechanical variables on electric power use would be of immense help in conducting such "systems analysis" of motor systems. This article presents a real-life successful project, with an investment of about $0.75 million dollars, documented energy savings of 2,935,000 kWh/year, and other non-energy benefits that eventually justified the project.

INTRODUCTION

In 1996 Abitibi Consolidated Sales Corporation, West Tacoma Division, located in Washington State, had production capability of 540 tons per day of recycle newsprint manufactured from TMP and recycled paper. Over the years, as mill capacity increased, various sizes and types of air compressor units were added at different locations to meet the increased air demands. As usual, the compressed air utility was expanded on as-needed basis, but without optimizing electrical energy consumption of various compressor units that were partly loaded, under utilized and poorly designed as an integrated utility system.

Four years ago, the pulp and paper mill authorized a complete compressed air system evaluation with a goal to improve air quality, reduce energy use and provide a reliable, cost-effective air utility to various mill end-users. On the basis of a systematic energy audit and process needs evaluation, six (6) old air compressors with a total connected 875 horse power electric motors were replaced with two tandem screw compressors each with a 300 horse power electric motor. The systems analysis was extensive, exploring process changes as well as mechanical changes. The process audit and energy

analysis identified ten (10) energy conservation projects, procured an energy savings grant covering 40 % of the project capital cost from Tacoma Power, and identified several non-energy benefits to the industrial client. This article will present our approach and the software application that accurately modeled base-case and conducted several "what if" case studies resulting in successful energy conservation measures (ECMs) and project implementation. We will briefly review the systems approach in optimizing the compressed air systems.

SYSTEMS APPROACH

A process (and energy) engineer takes a broader holistic systems approach to improve a compressed air system performance. It is essentially broken down into design, operational, and maintenance issues.

1. Design issues such as pipe size, equipment layout, control valve selection, air receiver sizing, location of air inlet suction (air temperature), and efficiency of electric motor drive, type of compressor selection. It also evaluates an impact of air receiver volume on compressor operating time and impact of air dryer on the air quality. Trade off between high purchase price of refrigerated air dryer versus energy savings should also be evaluated.

2. Operational issues such as set point of compressed air outlet pressure, multi-compressor staging or sequencing order, part load operation, unloading controls, how and where the compressed air being used, operating time (less is better), and finally compressed air requirement (load) reduction that includes elimination of inappropriate uses.

3. Maintenance issues such as plugging the air leaks, wasteful use of compressed air for low priority uses such as dusting, blowing, sparging etc., lub oil change, and compressed air heat recovery are thoroughly investigated.

An understanding of systems analysis along with a holistic approach help us focus on other variables that would improve the compressed-air system efficiency. The compressed air "system" can be broken down into mechanical system and process system.

The mechanical systems may consist of various end-users of compressed air, delivery or distribution system (piping, valves, fittings, hose, lubricators, pressure regulators, controls, etc.), compressor (centrifugal or positive displacement), its associated supply side equipment (aftercoolers, receiver, separators, dryers, and filters), and a driver (electric motor).

The process system will consist of air pressure at end-users, quantity and quality of air requirement, physical properties (molecular weight, polytropic exponent), ambient air temperature and air pressure, moisture in the suction air, etc. The design of mechanical and process systems as well as end-user load profile and hours of operation impact the energy consumption. It is very important to understand the technical equations used in the calculations of flow rate at standard temperature and pressure (STP), actual volumetric flow rate, free air flow, inlet air density, outlet air density, compressor capacity, adiabatic or polytropic head, discharge temperature, break horsepower, and compression ratio. A systems analysis integrates mechanical as well as process systems. This will allow the analyst to understand the impact of change in one variable or multiple variables on energy consumption. Let us start with determination of load.

LOAD ESTIMATE

Most of the industrial processes are continuous in nature, operating 7 days a week, 24 hours a day, with a planned yearly shutdown of a week or two. In many cases, not all of the processes are down at the same time and the rotating shut down requires that the compressed air utility be available around-the-clock. Whether designing a new system or re-rating an existing operation, it is prudent to make a reasonable estimate of end-user load profile, resulting in total computation of compressed air flow rate in cubic foot per minute (CFM) or pounds per hour (lbs./hr) that a compressor system must deliver. The load profile will also give a projection of peak load, part load and no load time periods. A graphical

presentation of total CFM versus 24-hour time period will allow development of lead/leg compressor selection and deployment strategy.

The existing compressed air system at the pulp mill comprised of eight compressors. Table 1 lists their location in the mill, type and capacity in ACFM. All eight compressors were metered for a month. The total design capacity of six compressors was 4,137 ACFM at 100 psig, the compressed air load was estimated to be 3,675 ACFM and the upgrade plan was based on the average compressed air use of 3,300 ACFM. Table 2 documents the compressed air load in the mill.

BASELINE METERING

An establishment of base line operation is utmost important in energy savings and conservation analysis. The base line energy consumption should be correlated to one or more dependent variables such as plant production and other process variables or major events when compressed air is predominantly required (for example, sheet breaks on paper machines). A regression analysis that correlates energy use to multiple plant or process variables is useful in determination of impact production variability has on the compressed air system energy use.

The simplest way to measure the electricity usage is to connect power metering equipment to each compressor motor in the motor control center and log amperage, voltage, power factor, and kW over a period of time. The data can be averaged over 15 minutes or one-hour intervals and stored in the logger for further analysis. Figure 1 plots compressor load in kW over one month time period for a 50 horsepower Worthington reciprocating air compressor. Similar pre-retrofit energy consumption readings were taken at the other seven air compressors to establish baseline energy use.

Table 1: Existing Air Compressors

During the same time period, plant production data and other dependent variable data were also collected and correlated to energy consumption. Other process variables such as discharge air pressure, temperature, and flow rate (if available) were also noted at each compressor for creating existing compressed air operating system computer model. It should be noted that part-load compressor air capacity can be estimated from the electrical load data using vendor furnished part-load compressor curves.

BASE CASE MODEL

The base case (pre-retrofit) computer model development process was broken down into three separate areas: Data Collection (including personnel interviews), Model Development, and Testing / Verification. More than one interview with plant operators, process engineers, maintenance personnel, and management were required. This process of data gathering took several days due to complexity of the plant and compressed air system. A thorough understanding of plant’s current operation, original design, and future requirement will help the energy engineer evaluate all facets of their operation and propose energy conservation measures that not only save energy but provide a more reliable system. A system design and engineering experience is extremely valuable in this phase.

An ideal goal will be to model and simulate original design operation and current operation under various case study scenarios. Once the current operation of various compressors is modeled off-line on a computer program accurately, the energy engineer can then create various "what-if" case studies with proposed energy conservation measures. Unicade’s Fluid Flow Systems Analysis software’s Compressor module was used in modeling the base case as well as subsequent energy savings case studies. Figure 2 shows the baseline case calculation for Worthington HBB (one of eight) compressor. All eight compressors were modeled in C-MAXÔ .

Figure 1. One of the eight compressors’ baseline metering

Figure 2. Base Case Calculation for Worthington W HBB Compressor

ENERGY CONSERVATION MEASURES

The next phase of the project was to evaluate various ECMs and then make specific recommendations based on the life cycle economic justifications. The operations and maintenance (O&M) improvements were also evaluated. The following is the summary of analyzed ECMs.

1. Replace existing two instrument air compressors and four mill air compressors with two Sullair TS32-300 water-cooled tandem screw compressors. A third compressor will be installed as a stand-by spare.
2. Specify premium efficiency motors for all new compressors. An energy reduction of over 3% can be achieved over vendor’s standard motors.
3. Install Sullair MEC controller to sequence three compressors that efficiently matches compressed air supply to the mill air demand. The vendor predicts 10 to 25% power savings over systems without this automatic control.
4. Increase compressed air pipe distribution line sizes to reduce the pressure drop.
5. Relocate filtered air intake to outdoors. The cold air will lower the energy consumption.
6. Implement heat of compression energy recovery in cooling water that will save 150 psig steam.
7. Elimination of fan power used or reduction in package horsepower in case of air cooled compressor (i.e.: 400 hp Ingersoll-Rand Rotary Screw air cooled exchanger).
8. Install one Zeks #4800 HSDM water cooled, cycling, refrigerated dryer system.
9. Add a dew point demand control on the desiccant dryer to conserve energy. Utilize existing desiccant dryer in winter months.
10. Consider installation of local receiver tanks near PM#2 and PM#3. Size receiver tanks generously.
11. Implement the following compressed air use reduction and/or replacement projects:

• Use a low horsepower blower and a chiller for calendar stack cooling on the paper machine no. 3.
• Improve or "tune" distribution and air discharge off paper machine #3 hood that will eliminate the pockets of dead air. Currently compressed air is introduced to induce turbulence inside the hood, which has poor ventilation.
• Implement an aggressive air piping inspection program to reduce leaks.

These proposed changes were reevaluated after factoring in the non-energy benefits. The final list of ECM, with respective energy savings estimate, is presented in the Table 3 below.

Table 3: ECM Summary List

RULES OF THUMB

The positive displacement compressors have a fixed displacement volume; therefore inlet cubic feet per minute (also called compressor capacity) will remain the same. The changes in inlet air temperature, pressure, molecular weight, relative humidity and outlet air pressure will have an impact on energy consumption and up-time of the compressor system. Assuming an industrial plant requires a specific mass flow rate of dry air, we can develop a set of rules of thumb using the compressor equations for a full load positive displacement compressor system. The "Rules of Thumb" are as follow.

LIFE CYCLE COST ESTIMATE

Integrating these variables into a financial analysis would give a better and accurate picture of economics of such a project. The total cost can be normalized using total compressor capacity over its predicted life. In some cases, it may reveal that a compressor equipment that cost less initially to purchase may actually be significantly more expensive using normalized cost indices. Undoubtedly, experience and professional judgment will play a key role in evaluating the life cycle cost of a system improvement. Again, a powerful software tool can integrate, calculate and present various life cycle case study scenarios with ease, and help you make an optimal decision.

VERIFICATION OF ENERGY SAVINGS

Figure 3: Life Cycle Cost Analysis Summary

Figure 4: Project Cost and Financial

Figure 5: Life Cycle Maintenance Cost

Figure 6: Life Cycle Energy Recovery Credit

Figure 7: Non-Energy Benefits’ Present Value

SAVINGS VERIFICATIONS AT PULP MILL

At the paper mill, it was determined that in addition to the paper production in tons/day, number of sheet breaks at paper machine winder requires significant amount of compressed air. The daily data was correlated for one month and presented as shown below in Figure 8. The correlation with net daily production was significantly better than with number of sheet breaks at the paper winder.

The regression analysis of each dependent variable (production and number of sheet breaks) indicated poor compressed air energy consumption correlation to # of breaks. This was indicated by R²=0.0353. However, daily paper production has much stronger correlation (R2=0.9612), as shown in Figure 9.

Figure 8: Correlation between Total Daily Compressed Air Power & Production

Figure 9. Regression Analysis: Compressed Air Power & Production

We concluded that at an average production rate of 500 tons per day for 355 days a year, 24 hours a day operation, this new compressed air system has reduced energy utilization between 2,922,000 to 2,949,000 kWh per year. This project had far exceeded the project proposal energy savings estimate of 2,086,000 kWh a year. On the basis of the pre and post retrofit metering data, mill production information and subsequent regression analysis, the compressed air upgrade project had reduced energy usage by 2,935,000 kWh a year.

It should be noted that energy savings would increase if the actual pulp production rate increases above the average paper production rate of 500 tons/day. On the other hand, the savings would decrease if the production were to fall below the annual target production rate. In other words, the energy savings would change as the production in tons/day varies. However, at the lowest production rate (285 tons/day) during the post retrofit, the measured savings was 2,828,000 kWh/year which is still significantly higher than the estimated project proposal energy savings. A summary of compressed air estimated energy savings and verified energy savings is in Table 5.

Table 5: Energy Savings Summary

An estimated energy savings contribution by each energy conservation measure is presented below in Table 6.

Table 6: Savings Contribution by ECMs

CONCLUSION

An investment-grade professional compressed air system energy audit and analysis require the following.

1. An excellent understanding of end-user processes,
2. Comprehensive knowledge of compressed air system design and,
3. A thorough understandings of process and mechanical variables that impact the energy usage.

A methodical approach starts with field visits, operator interviews, data collection, computer modeling of current system, pre and post retrofit metering, ECM analysis with previously set-up computer model, cost-effective measure selection and comprehensive life cycle cost and economic analysis. After installation of energy conservation measures, the field verification of energy savings must be completed in accordance with an approved completion plan. Finally, the project concludes with an operating personnel training to maintain the new compressed air system in optimum condition assuring persistence of energy savings over a long period of time. Utilization of a software application, specifically designed to perform motor systems energy analysis and life cycle cost analysis, will enhance the accuracy of energy savings forecast as well reduce the engineering analysis time

In an industrial environment, non-energy benefits must be quantified and factored into the justification process. A well planned energy analysis and implementation will meet the success criteria of the project - that is to reduce operating cost while increasing system reliability and meeting the rate of return goal of a plant’s capital investment.

ACKNOWLEGEMENT

This article was presented at 22^nd Annual Industrial Energy Technology Conference in Houston, Texas, USA on April 5-6, 2000.

REFERENCES

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