This Midwestern prepared food company now spends $269,463 annually on energy to operate their compressed air system. This figure will increase as electric rates are raised from their current average of 6.2 cents per kWh. The set of projects recommended below will reduce these energy costs by $112,902 or 41%. In addition, these projects will enhance productivity and quality and reduce equipment maintenance costs. Estimated costs for completing the projects total $146,102, which represents a simple payback of 15.6 months.
This plant produces prepared foods, which are frozen and then shipped for distribution. The production is vertical as meals are cooked and prepared on site. The plant runs production six days a week with limited or no production on Sunday. Some clean up and maintenance is implemented on Sunday. Due to space constraints in this article, we will focus on the demand-side system opportunities that were found to realize energy savings.
Demand-Side System Projects
Please note that all the demand-side system optimizations, listed in this article, only became realized energy savings because we completely modified the capacity control strategy on the air compressors. Compressors were manually turned on and off by maintenance personnel and without change, the actions below would have yielded $0 dollars in savings. It’s important that customers understand that demand-side actions, without the accompanying investments in control strategies, may not yield any results.
Demand-side projects described in this article include a leak management program, reconfiguring cabinet coolers, and replacing open blows.
Compressed Air Leak Survey
A survey of compressed air leaks was conducted at the plant and 41 leaks were identified, quantified, located, and listed. Potential savings totaled 195 cfm for the 41 leaks that were identified.
Ultrasonic leak locators were used to identify and quantify the compressed air leaks. These tools included a VXP AccuTrak manufactured by Superior Signal, and a UE Systems Ultraprobe 2000. Estimation of leak size was achieved by noting the intensity of the signal by the operator, type of leak, and observation. The estimates are made on a conservative basis and probably understate the magnitude of the volume of leaks. The results of the leak survey are summarized and tabulated on the following page.
Shutting off the air supply to these leaks when the area is idle would save significant energy use. Reducing the overall system pressure would also reduce the impact of the leaks, when air to the air cannot be shut off. Repairing the leaks can save additional energy. The savings estimates associated with a leak management program are based on the unloading controls of the compressors being able to effectively translate less air flow into lower cost.
With a few minor exceptions, most of the leaks cold not have been found without the use of an ultrasonic leak detector and a trained operator. Leak locating during production time with the proper equipment is very effective and often shows leaks that are not there when idle.
However, a regular program of inspecting the systems in “off hours” with “air powered up” is also a good idea. In a system such as this one, some 90 to 95% of the total leaks will be in the use of the machinery, not in the distribution system.
The area surveyed in the leak study included a great deal of high background noise from steam leaks that shield many of the smaller leaks. In continuing the leak management program, plant staff should perform leak detection during non-production hours in order to eliminate some of the high ultrasonic background noise.
Cabinet cooling is often required to obtain reasonable life and performance of the electronic equipment in control cabinets. Blowing straight compressed air into the cabinet for cooling is generally not an efficient or cost-effective use of compressed air. Vortex coolers deliver chilled air with no moving parts, and therefore, may use less compressed air when properly applied. They should always be:
• Regulated to the lower effective pressure
• Equipped with a flow generator set to the lowest effective flow
• Equipped with automatic temperature controlled shutoffs
Refrigeration units should be carefully selected and equipped with automatic regulation control.
Alternatives to using compressed air for cabinet cooling include “heat tubes” or “heat pipes,” whose cores are made from highly conductive alloy materials for fluid cooling. In operation, the heat flows from the outside by means of a fluid phase change. As heat is applied to the core (inside the cabinet), the fluid inside the pipe absorbs heat, is vaporized, and then travels to the condenser end (outside the cabinet), where it is cooled back to a liquid. The liquid returns to the evaporator inside the cabinet, etc. An exterior fan usually draws heat away from the core, which cools the internal cabinet without exposing the interior of the cabinet to ambient air.
Heat tubes are most energy efficient when applied to cool a “sealed cabinet.” The only mechanical parts are the fans — heat pipes are passive and require no power water-cooled – they cannot cool below ambient temperature unless water-cooled.
Another very effective cooler that can cool below ambient when the heat load is within its capability is a Thermoelectric Air Conditioner which uses the “Pelletier effect” to convert low electric flow into refrigeration.
The following table lists some observed cabinet coolers that require installation modification. In this case, we are suggesting automatic temperature control shut offs.
Replace Blowoff Air with Amplifier Nozzles
Regardless of application, there are several guidelines that should always be applied to compressed air being used for open blow off:
Use high pressure only as a last resort
All blow off air should be regulated
All blow off air should be regulated to the lowest effective pressure—higher pressure means higher flow, which may not be needed
Use Venturi air amplifier nozzles whenever and wherever possible—this will usually reduce blow off air at least 50%, freeing up more air flow for other applications
All blow-off air should be shut off (automatically) when not needed for production.
Plants with many 1/8 and 1/4 inch lines running as blow off on units will use approximately 10 and 25 cfm each, respectively, at 60 psig.
One savings approach is to use an air amplifier, which requires less compressed air. Air amplifiers use “Venturi” action to pull in significant amounts of ambient air and mixing it directly into the air stream, which amplifies the amount of air available at the point of use. Air amplifiers have amplification ratios up to 25:1. Using 10 cfm of compressed air can supply up to 250 cfm of blow off air to the process and generate a savings of a 15 cfm compressed air per 1/4-inch blow off. Savings may be available using 1/8-inch lines, but the cost effectiveness will not be as great.
Another method for blow off to be investigated is the use of “blower generated” low pressure air. This air is much less costly to produce on a $/scfm basis. It is the volume of air (scfm) that creates the mass or weight of the air that performs the blow off. The pressure influences the “thrust” out to the end of the nozzles where it quickly dissipates. Often a “higher volume” or weight of air at lower thrust (pressure) improves productivity and quality of the blow off over the higher pressure version.
Most plants can benefit from an ongoing demand-side monitoring and leak management program. Generally speaking, the most effective programs are those that involve the production supervisors and operators working in concert with the maintenance personnel. Accordingly, it is suggested that all programs consist of the following:
* Short Term – Set up a continuing leak inspection by Maintenance Personnel so that for a while, each primary sector of the plant is inspected once each quarter to identify and repair leaks. A record should be kept of all findings, corrective measures, and overall results. The PROJECT COST SECTION below includes current price quotes for ultrasonic leak locator equipment.
* Long Term – Consider setting up programs to motivate the operators and supervisors to identify and repair leaks and to continue monitoring blowoffs, cabinet coolers, and other potentially inappropriate uses of compressed air. One method that has worked well with many operations is to monitor the air flow to each department and make each department responsible for identifying its air usage as a measurable part of the operating expense for that area. This usually works best when combined with an effective in-house training, awareness, and incentive program.
By Hank Van Ormer, Air Power USA