In general, vending machine compressors have considerable excess capacity during most of their run time. A high-capacity compressor is required only during pull-down, which is 12 percent of the cycle. This is the time it takes to fully chill the products after they are loaded into the machine in order to bring the beverages to sales temperature as quickly as possible.
There are opportunities for saving energy during the 88 percent of the cycle when the system doesn't need to work so hard - when it is only called upon to maintain the target temperature.
Danfoss performed tests at its lab facilities in Monterrey, Mexico, using a cola vending machine to compare the type of compressor just described with the Danfoss NF Series hermetic compressor, which the company said is quieter and more energy efficient.
The unit had been using a 1/2-hp compressor from another manufacturer. It was replaced with a Danfoss NF11FX2, a 1/3-plus-hp compressor, part of the company's high-efficiency compressor family. Danfoss designed the compressors using technologies learned from the household compressor market. That includes more efficient motors, optimization of the suction side of the compressor, improvements in suction and discharge valves, and improvements in the refrigerant side routing through the compressor.
Testing was performed in accordance with ANSI/ASHRAE Standard 32.1-1997, which is a method of testing for rating bottled and canned beverages vending machines.
The machine was loaded with 768 soft drink cans. Measurements of power consumption, pull-down time, and average can temperature were taken.
Using the NF compressor, energy consumption was reduced by 25 percent.
The Valve AspectTraditionally, no expansion valve is used in vending machine systems. Rather, refrigerant is metered through a capillary tube. But in the second phase of the testing, further efficiencies were achieved by applying a thermostatic expansion valve (TEV). Such a valve maximized evaporator efficiency, helping to reduce the pull-down time.
The second test compared a system with a TEV to one with a capillary tube under constant ambient temperature and relative humidity to analyze performance and energy consumption during initial pull-down time and during stabilized operation.
Test data showed the expansion valve system was, on average, 12 percent more efficient than one with a cap tube.
Most of the savings come in the pull-down cycle. When applied to conditions other than the design ambient, the savings with the TEV increased dramatically - up to 20 percent in outdoor conditions over a machine with a capillary tube optimized for indoor conditions.
System ApproachThe final phase of testing combined a TEV with a smaller, high efficiency compressor.
The NF7.3FX2 is a 1/4-hp compressor, also part of the Danfoss high-efficiency compressor family. Because of the expansion valve, Danfoss was able to use the entire evaporator capacity, allowing the company to reduce the size of the compressor while maintaining the pull-down and steady state cooling targets.
That combination yielded a 36-percent improvement in efficiency while still meeting the objectives of the cola company.
Combining system strategies and evaluating the complete system rather than individual components attained such improvements.
"Danfoss is a manufacturer of both controls and compressors, which enables us to have a system perspective that results in unique solutions to help our customers achieve their objectives," said John Galyen, vice president of OEM sales and marketing for Danfoss Air-Conditioning and Refrigeration.
The Refrigerant IssueAlong with the objective of reducing energy consumption, the cola manufacturer also announced an initiative to reduce the environmental impact of its cooling equipment over time, based on the Kyoto Protocol.
The soft drink company said in a press release, that it will "no longer purchase new cold-drink equipment using HFCs wherever cost-efficient alternatives are commercially available."
Said Galyen, "This is another challenge Danfoss has embraced. With our extensive research into alternative refrigerants, we are making the solution to this challenge a reality."
Publication date: 04/04/2005