Case Studies

a. Case Study: Replacement of Eddy-Current Drive with a VFD

Company Background

The company is a 100,000 square foot plant that produces approximately 1 million linear feet of stainless steel tubing per month for its customers in the automotive, food, pharmaceutical and petrochemical industries.

Project Overview

The production process consists of drawing stainless steel tubing through dies to reduce their diameter and/or wall thickness. This drawing process is carried out on a drawbench.

Each tube typically goes through several breaking draws which rapidly form the tube close to its final dimensions. Next, the tube undergoes a few final “finishing” draws to achieve the exact tube size. The drawbench used for breaking draws operates 24/5, performing approximately 1,200 draws per day.

The breaking drawbench was powered by a 150 HP motor running at 1,800 rpm. This motor was coupled to a speed reducer through an eddy current clutch which is known to be an inefficient although reliable technology.

Approach

The implementation team used data collected by the plant engineering department to analyze the existing system. This included observing the operation of other equipment similar to the breaking drawbench and noting where similar operating parameters could be applied to the breaking drawbench.

Due to the wide variety of tube diameters, wall thickness, material used and orders received each week, a single representative product does not exist. To obtain data representative of actual operation, the project team randomly selected orders and then performed a detailed analysis of the intermediate steps to which the tubing undergoes. The team compared the power requirement that plant engineering measured in the base case study against the measured system power requirements.

Project Implementation

A VFD using vector drive options was selected as it can continuously monitor the current, voltage and angular position of the AC induction motors. Prior to the development of vector drive controls, only DC motors, which are less reliable and required more maintenance than their AC counterparts, could be used in applications requiring accurate torque and speed control. Today, vector drive options are a regular feature of VFDs.

In order to accomplish the project goals, the team replaced the magnetic starter and eddy current clutch with a VFD vector drive and line reactor. A line reactor was included as a system specification to avoid and harmonic issues on the distribution system. The plant engineer also wanted to increase the torque output to the drawbench, improve overall drive efficiency, and reduce energy consumption, so the standard efficiency 150 HP, 1,800 rpm motor was replaced with a high efficiency 200 HP, 1,200 rpm motor. The lower speed motor was chosen as it produces greater torque than the higher speed motor.

Results

As a result of the changes implemented by the team, the breaking drawbench now requires less energy to draw a tube, even though the motor size was increased from 150 HP to . For a typical draw, the eddy current coupling system required 190 HP to draw a tube, while the more efficient VFD drive requires less than 90 HP. The projected total annual operating time was also reduced by 623 hours as a result of the modifications, since the greater horsepower available enabled many of the tubes to be reduced to the desired size with a smaller number of draws.

The modifications reduced the breaking drawing bench’s total annual energy consumption from 439,065 kWh to 290,218 kWh and reduced the total annual electricity costs by 34 percent from the base case cost of $20,812.

An estimated 2,762 hours of labor per year will be saved as the result of these changes. Personnel estimate that one draw was eliminated from 1/2 of the orders processed. Time is not only saved through the reduced number of draws required to “break” a tube, but is also saved other operations required by the drawing process, such as degreasing, cut-off, swaging, and annealing. Assuming a labour rate of $8.50 per hour, the labour reduction amounts to labour cost savings of $23,473 per year. The reduced number of draws necessary also saved an estimated $41,322 of stainless steel, as fewer draws equate to fewer swaged ends cut off (waste) after each draw. Including other direct savings of $5,415, the total cost savings is $77,266. When measured against the project’s $34,000 cost, the simple payback came in at about 6 months.

Energy and Cost Savings
Project Implementation Costs $34,000
Annual Energy Cost Savings $7,056
Annual non-energy cost savings (labour & scrap)    $70,210
Simple Payback (years) 1/2
Demand Savings (kW) 189
Annual Energy Savings (kWh) 148,847

b. Case Study: Replacement of Damper Controls with VFDs in an HVAC System

Company Background

A small textile processing plant processing raw fiber wanted to improve the HVAC system performance in its plant. The company worked with a VFD specialist to retrofit 15 of the system’s fan motors with variable frequency drives (VFDs). As the plant operated 24/7, savings on the HVAC system could be cost efficient and provide better space conditioning and air quality for its workers.

Project Overview

The ventilation system uses nine supply fans and nine return fans to control and maintain proper ambient conditions, cool process machinery and provide proper air quality to its workers. Initially, a mixture of return air and fresh air is cleaned, cooled and humidity adjusted by four air washers. This air is then supplied to the facility by the nine supply fans and distributed to the plant through ceiling mounted ducts and diffusers. Nine return fans pull air through the processing area into a network of ducts. Any suspended particulate is filtered out at the inlet of each return fan.

Factors that influence the pressure, volume or resistance of the system directly impact the fan power requirements. Therefore, air density, changes to damper positions, system pressure and air filter pressure drops, supply and return air system interaction and parallel fan operation all affect how much power the fans require and must be monitored to ensure the efficient functioning of the system. Variable inlet guide vanes and outlet dampers initially controlled the system’s air flow, but were highly inefficient. Setting these devices was imprecise and resetting them could only be done manually.

Approach

To determine how to improve ventilation system performance, the company and VFD specialist collected base case system data over two weeks to measure the performance of the existing system. Motor power was electronically logged, damper positions were manually recorded based on visual inspection of the damper linkages and power was measured on each fan at ten minute intervals. These data were analyzed and the team then developed a load duty cycle to calculate energy demand, operating hours (peak, and off-peak periods) and annual energy consumption during this period for both the return fans and supply fans. This data was later compared to the new system data, collected from after installation.

Project Implementation

After determining that the ventilation system’s fans were significantly oversized, the team retrofitted 15 of the 18 fans with VFDs. The remaining fans always ran at full flow and did not need VFDs. A power and energy measurement was connected to each of the VFDs to gather load data for the new system. The system logged each fan’s speed and power consumption in 15 minute intervals and savings analysis reports were generated. With the VFDs installed, damper control was no longer necessary so the fan control dampers were left fully opened.

Results

Installation of the VFDs reduced the ventilation system’s total electricity demand from approximately 322 kW to 133 kW, a 59 percent reduction. The total annual energy consumption for the fans similarly fell 59 percent from approximately 2,700,000 kWh to 1,100,000 kWh. The energy efficiency gains were possible because the VFDs enabled plant personnel to fully open the fan control dampers and reduced fan speed. This resulted in a large drop in power consumption and allowed the system to operate more efficiently. These electricity savings translated to annual energy cost savings of about $101,000. The cost of the project was $130,000 and included:

  • Cost of the feasibility study
  • Capital cost
  • Installation cost
  • Engineering
  • M&V activities
Energy and Cost Savings
Project Implementation Costs $130,000
Annual Energy Cost Savings $101,000
Simple Payback (years) 1.3
Demand Savings (kW) 189
Annual Energy Savings (kWh)     1,579,400

c. Case Study: Replacement of Vacuum Pump Controls with a VFD in a Dairy Facility

Company Background

A dairy farm uses a vacuum system to operate automatic milking equipment that milks cows and sends the milk to a holding tank. Prior to undertaking the project, the dairy vacuum system was utilizing a vacuum pump with a 30 HP motor that controlled vacuum levels by bleeding in air from the atmosphere, which is standard practice in the industry.

Project Overview

Modern dairy milking systems used in dairy farms utilize a vacuum for automatically milking the animals. In typical dairy vacuum milking systems, several milking units are attached to the animals with teat cups and a vacuum is introduced into the milking lines by a vacuum pump which draws milk from the teats into a storage tank.

The vacuum pump is driven by a constant speed AC motor. To maintain the stable vacuum needed for milking cows, air must be removed from the system at the same rate at which it leaks into the system. Air typically enters the system through pulsators, leaks and units becoming detached from animals.

Conventional vacuum control is accomplished by running a constant speed vacuum pump sized to the largest possible airflow and allowing air to bleed into the system through a pressure regulating system.

With some technical assistance from their utility, the farm examined their vacuum pumping system to determine whether it was operating efficiently. The evaluation determined that the system’s motor was oversized for the required vacuum and the farm could lower its energy costs with a smaller, more efficient system.

Project Implementation

Using the recommendations provided, the farm decided to implement a system which reduced the motor size and operated with a VFD. The motor was replaced with a new, energy efficient 20 HP motor. In addition, the farm installed a VFD on the new motor to adjust the pumping system speed based on the system load. The dairy could have installed a larger VFD on the existing 30 HP motor, but that would have increased the capital cost and missed the opportunity for a more efficient motor.

Project Results

The implementation of the new vacuum pump system resulted in energy savings and more efficient production for the farm. While the original system drew 16 kW, the new system never uses more than 4.5 kW, even during peak needs. The farm was able to decrease the horsepower required by the vacuum system by 30% of the system’s total capacity without any decrease in vacuum pressure or system capacity. In addition, the VFD was able to change the pump’s speed to more precisely match the process vacuum requirements.

The project’s implementation has allowed the farm to achieve annual energy savings of $5,520 and 55,000 kWh, representing over 70% of the electricity used by that system. With a total cost of $8,200 the simple payback was only 1.5 years. The project also reduced maintenance costs and will lead to increased equipment life.

Energy and Cost Savings
Project Implementation Costs $8,200
Annual Energy Cost Savings $5,520
Simple Payback (years) 1.5
Demand Savings (kW) 11.5
Annual Energy Savings (kWh)     55,000

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