Energy Efficiency Auditing through the UW-Milwaukee Industrial Assessment Center

W. Barlas, C. Zahn, and Y. YuanDepartment of Mechanical Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI, USA 53211

Mission

(1) Anysley, R. (2005). Saving heating costs in warehouses.ASHRAE journal, 47(12), 46-51. Retrieved from https://www.ashrae.org/.../docLib/.../200512265816_886.pdf

(2) Linear regression analysis of energy consumption data. (2013). Retrieved from http://www.degreedays.net/regression-analysis

(3) Muller, M., Simik, M., Mak, J., & Mitrovik, B. (2001). Compressed air leaks. In Modern industrial assessments; A training manual.

References & Bibliography

  The University of Wisconsin – Milwaukee Industrial Assessment Center (IAC) is one of 24 centers across the nation funded by the U.S. Department of Energy. In partnership with Wisconsin Focus on Energy, the IAC provides no-cost technical assessments to small and medium-sized manufacturers. Teams of engineering students, lead by faculty, conduct on-site assessments to observe, measure and improve how facilities utilize energy and other valuable resources.

Thermal Destratification (1), (2)

Introduction

1. Train the next generation of engineers capable of identifying, researching and implementing energy efficiency projects in manufacturing industries. 

2. Provide quality services to manufacturers that improve energy efficiency, reduce resource consumption, and decrease operating costs while helping companies thrive in times of rising energy costs.Examples of Energy Efficiency and Cost Savings Recommendations

Compressed Air Leak Detection (3)Thermal stratification within a building is based on the buoyancy of air. Because heat rises, there is a large temperature gradient between the floor and ceiling of a building with high ceilings. This phenomenon comes at a high cost because of the load put on the heating system to warm air close to the floor, while the ceiling is above the thermostatically controlled temperature.

Energy Usage Before ProjectUg = Ug = Natural gas usage per monthm = slope of best fit line HDD = heating degree days per month Bn = Monthly natural gas usage of heating Ec = Ec = Current annual energy usage of heating

Proposed Energy UsageWhen a building is destratified the temperature at the ceiling is reduced based on the following model from and the following equations (3)

qad =qad = Reduced heating load from destratificationTibd = Initial temperature before destratificationTiad = Calculated final temperature after thorough mixingTiad =Gq =Qp = Caloric value of natural gas

= Efficiency of gas heater (80%)Ep =Ep = Proposed Energy UsageAES =ACS = AES × $/thermACS = Annual cost savings

Energy Usage Before ProjectSignificant energy savings can be obtained by repairing the air leaks in the compressed air system. Even small leaks can significantly increase the energy use by the compressor. If the diameter of a given leak and the pressure at that point are known, the volume of air lost over a period of time can be found. The energy needed to replace this wasted air, as well as the cost of the waste can then be calculated.

Vf =

Vf = Volumetric flow rate of free air, cubic feet per minute

NL = Number of air leaks, no unitsT = Temperature of the air at the compressor inletPleak = Line pressure at leak in question, psia

Pin = Inlet (atmospheric) pressure

C1 = Isentropic sonic volumetric flow constant, 28.37 ft/sec-°R0.5

C2 = Conversion constant, 60 sec/min

Cd = Coefficient of discharge for square edged orifice, 0.8 (no units)

D = Leak diameter, inches (estimated from observations)Cb = Conversion constant, 144 in2/ft2

Tline = Average line temperature

Vf1 =

Once the volumetric flow rate of lost air is known, the power necessary to replace this air can be calculated. The power loss is calculated as follows for each leak, then summed together.

L =L = Power loss due to air leak, hp (using Vf)

k = Specific heat ratio of air, 1.4, no unitsN = Number of stages, 1C7 = Conversion constant, 3.03 x 10-5 hp-min/ft-lb

Pop = Compressor operating pressure = 100psig = 114.7psiaEa = Air compressor isentropic (adiabatic) efficiency, no units, 0.82 for rotary screw

compressorsEm = Compressor motor efficiency (85% or 0.85)

Proposed Energy UsageThe annual energy savings (AES) after the leaks have been fixed is determined as follows:

AES = LH = Annual time during which leaks occurs, 5,227.5 h/yrC8 = Conversion factor, 0.002545 C9 = Conversion factor,

ACS =

RE = Average electricity rate

Other Common RecommendationsRecommendation #1: Use Outside Air for Compressor

The work done by an air compressor is proportional to the temperature of the intake air. Lower intake air temperature results in lower energy consumption. The outside air is always cooler than the air inside of the compressor room for the facility. Use of outside air for the air compressor can save electricity by reducing the amount needed to produce the same amount of compressed air.

Recommendation #2: Install Radiant Heaters in Shipping and Receiving Area

The shipping and receiving area of the plant is currently heated by natural gas heaters. These heaters heat the air in the environment, which in turn heats the people and objects that are present. Radiant heaters do not heat the air in the environment, and provide a perceived heated effect perfect for heating environments like loading docks. They heat the people without wasting the warm air that would usually escape into the outside environment.

Recommendation #3 Install Cooling Tower

A cooling tower will reduce the load on the chiller units being used to cool the water required for Process B by reducing the temperature of the water in the loop. This will be done by adding the cooling tower to the closed loop system that the chiller is currently using.

Recommendation #4: Use Energy Efficient Cogged V-Belts

By replacing the existing electric motor drive standard V-belts with more energy efficient cogged V-belts the amount of friction between the belt and the drive system is increased. This increases the total efficiency of the system, and results in the motor using less energy to produce more mechanical work.

 Recommendation #5: Purchase Cardboard Baler

Compact corrugated cardboard using a baler. These compacted bales can be sold instead of paying someone to come remove loose cardboard.

  Recommendation #6: Use synthetic lubricants for motors

Synthetic lubricants have several benefits. They not only extend equipment life, but also increase plant productivity by allowing machinery to operate at the highest efficiency for a longer period of time. They also have high thermal, oxidation, and contamination resistance. Use of synthetic lubricants can reduce energy consumption and improve productivity.

Recommendation #7: Adjust Boiler Air-Fuel Ratio

The combustion efficiency of a boiler system can be increased by optimizing the mixture of natural gas to air that enters the chamber within the boiler. This will increase the overall efficiency of the boiler system, and therefore reduce natural gas usage.

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f(x) = 43.2529383376379 x + 1738.7890608727R² = 0.966923513677317

Heating Degree Days vs. Natural Gas Usage (Therms)

Heating Degree Days

Th

erm

s

Figure 2: Linear regression model to calculate heating load (2)