Danfoss SC compressors were designed to allow more frequent cycling in a smaller, more efficient package.
Like most other commercial refrigeration equipment, ice machines are being scrutinized for potential energy efficiency improvements. In November 2000, the Federal Energy Management Program (FEMP) included ice machines in its recommended purchases program. The federal government is the nation's largest consumer of appliances of all kinds. FEMP establishes specifications for purchases made by government logistic agencies.

The rest of the country is now following that lead.

In April 2005, the California Energy Commission (CEC) established new Appliance Efficiency Regulations. For California, these new regulations established requirements that mean 85 percent of the ice machines sold there today will be unacceptable as of January 2008.

How unacceptable? On average, considering all current designs, the CEC regulations mandate a 32 percent reduction in consumed energy per 100 pounds of ice produced.

Now, federal legislation coming out of committee would establish national standards following the lines established in California. If passed, these standards will affect every ice machine sold in the United States by January 2010. In anticipation of these changes, manufacturers are taking a hard look at ways to increase efficiencies. Some are ahead of the curve; one such company is Manitowoc, whose new S-Line machines currently meet California's 2008 standards.

The S-Series machine applies air pressure to the back of the ice, gently removing cubes from the evaporator.

Changing The Process

In traditional cube ice machines, an evaporator is constructed by brazing tubing to one side of a plate. A grid of cube-shaped compartments is mounted on the other side of the plate.

In service, water is run into the compartments. The evaporator removes heat. A sensor determines when cubes have been formed, and triggers control circuitry to actuate a solenoid valve that bypasses the condenser; cooling is thus interrupted. Hot gas passes through the evaporator. The ice cube surfaces in contact with it melt slightly, and the ice is harvested as the cubes fall into a receiver.

In that traditional model, three areas present opportunities for reducing energy usage:

1. Evaporator technology.

2. Insulation.

3. The refrigeration system.

Manitowoc dealt with all three.

The key to the S-Line is a reduction in freeze-harvest cycle time.
The key to the S-Line is a reduction in freeze-harvest cycle time by up to 50 percent, the company explained. A smaller machine using less energy is said to produce the same amount of ice.

The crucial factor in evaporator design is to maximize heat transfer while eliminating corrosion. The S-Line's evaporator uses nickel-plated copper; the copper allows greater heat conductivity, and thus faster freezing. The nickel-plating satisfies sanitary requirements.

Cubes are harvested more quickly from an S-Line evaporator because of some changes from more traditional approaches, the company continued. In older machines, cubes are freed by gravity and must overcome the tension created by the film of water created by the action of the hot gas.

The S-Series machine applies air pressure to the back of the ice, gently removing the ice from the evaporator. Minimizing the time the cubes are on the evaporator during the harvest cycle also increases the available time for ice making, in turn improving the efficiency of the machine. In addition the ice melt is minimized, increasing the size of the cubes.

Heavily insulating the food zone, the area around the evaporator, prevents warming of the area by ambient conditions.

Manitowoc chose Danfoss as supplier of compressors, expansion valves, and solenoid valves for the refrigeration system itself. The two companies worked together to specify and develop products that could meet efficiency requirements and withstand the rigors of more-frequent cycling.

Compressor First

The first step was redesigning a compressor that would function efficiently with more frequent and shorter cycling. Danfoss said its Type SC compressors were designed to allow more frequent cycling in a smaller, more-efficient package.

Design enhancements include improved bearings that withstand high hot gas temperatures and frequent thermal cycling. Also, the valve plate is made of a higher-temperature alloy, with machining precision to closer tolerances for the ice machine application.

Low and accurately controlled evaporator superheat, along with high-cycling capability, is critical to the ice machine application; Danfoss developed a version of its stainless steel TUC expansion valve with a special charge, custom formulated for ice machine applications.

The valve's Precision Port design incorporates a precision-machined pushrod and bushing that was designed to allow for a more frictionless seal without a packing gland, the company said. The free-floating pushrod is self-aligning. Laser welding of the power element helps preserve the structural uniformity of the stainless steel diaphragm for more consistent operation over an extended service life.

A precision-machined cone and the special ice machine charge provide repeatable refrigerant metering, the company added.

Accurate control of the freeze-harvest cycle, plus sharp separation between its freezing and harvesting phases, also are essential in ice machine applications. A critical component there is the solenoid valve that functions as a hot gas bypass valve.

If a traditional valve allows a small amount of leakage, that situation has to be overcome by added compressor work to remove the heat. Manitowoc selected the Danfoss EVU valve, designed to have virtually no leakage and a lower coil wattage.

Service Guidelines

To keep these machines operating at peak performance, the employees using them need to clean them carefully and monitor their performance, calling for service when signs of inefficiency appear.

Several changes have been made to machine design to make cleaning easier, including an auto-clean option, isolation of the food zone from the back of the machine, front-accessible components with rounded edges, and adding broad-spectrum antimicrobials to molded products to reduce slime.

The combination of energy enhancements means that a given ice machine production rate, say 500 pounds per day, can be maintained with a smaller, more efficient compressor using less energy and meeting stringent new requirements.

Publication date: 07/04/2005