"To avoid very high pressures, cascade designs were specified, in which CO2 is only used in the low stage," he noted. Typically ammonia is used on the high stage. To move between the two stages, a cascade heat exchanger (CHE) is used. The CHE serves as a condenser for the CO2 side and as an evaporator for the ammonia side.

"In general, the size of the ammonia charge in the cascade system is typically only 10 to 20 percent of the charge in a conventional system."

According to Christensen more than 100 ammonia/CO2 cascade systems have been installed worldwide, some have been in operation for more than five years. "This proves that the system concept is reliable and safe," he said. "All known users are satisfied with the operation and performance."

He said the approach has been tried so far on only a limited basis in the United States. "But increasing electricity prices, stricter regulations for using ammonia, and the impending termination date for use of R-22 should rapidly increase the number of installations in the U.S. over the next decade."

The defrost issue with R-744 was reviewed by Andy Pearson of Star Refrigeration, Glasgow, Scotland. "Perhaps the greatest diversity within the system is in the type of defrost used, reflecting the greater degree of technical innovation required to achieve a satisfactory result in coil defrosting," he said. "There are significant differences in the installation costs of the different systems. They also result in different operating costs."

He added, "The most common and suitable systems for defrosting air coolers are the hot gas and high-pressure liquid systems. These two direct methods are now almost universal.

"For plate freezers, the high pressure liquid is advantageous because the compressors required for that application would be significant compared to the swept volume of the whole system. Other than that, there are very few criteria when choosing between the two systems.

"Hot gas defrost systems are most efficient if waste heat can be used to provide the hot gas. Running as a heat pump can also be very attractive, particularly if the cooling effect can be used beneficially elsewhere in the system. The decision of the type of defrost to be used will not be determined by efficiency considerations, but will be more significantly influenced by convenience and reliability."

THE FREEZE FACTOR

Klaas Visser of KAV Consulting of Kangaroo Flat, Australia, discussed the actual costs of product freezing.

"Freezing is an attractive method that preserves more desirable attributes such as nutrition, taste, texture, and shelf life. However, freezing is a costly exercise with expenses comprising floor space, energy, and maintenance."

In optimizing design procedures for air blast freezing systems, Visser said: "Selecting equipment in the engine room based on the coefficient of performance (COP) alone is not the correct procedure. To evaluate the most energy-efficient freezing system, it is necessary to evaluate as energy inputs to arrive at the lowest possible energy consumption."

AN APPLE A DAY

The popularity of apples seems to be in direct proportion to the challenge of keeping them in storage but still tasty for as long as 11 months - the time between the prime picking month each fall. A report from Kem Russell of Doubl-Kold of Yakima, Wash., looked at the challenges regarding storage requirements, refrigeration, and control of the atmosphere.

In particular the report looked at several facilities in California.

"Refrigeration requirements vary greatly from pull down to holding," he said. "Heat loads to consider include product sensible heat, product respiration, envelope transmission, and fan motors. To achieve the required fruit quality over long storage periods, fruit temperature should usually be pulled down quickly. Quick pull down can be accomplished best by using a tight stacking pattern, an evaporator of sufficient capacity for the desired pull down rate, an optimum evaporator temperature, and adequate air circulation."

A REALLY OLD PLANT

So what do you do when your government wants you to do a risk assessment on an ammonia refrigeration plant at a facility that is more than 100 years old?

That was the challenge faced by Anders Lindborg and his colleagues at Ammonia Partnership AB of Viken, Sweden, when they were called in to help in such an assessment at a brewery in the Copenhagen, Denmark, area.

The local environmental protection agency wanted to know what would happen should 6 tons of ammonia be accidentally released. The first ammonia compressor at the plant was started in 1879. "Since then, the system has grown," Lindborg said, "with parts being added or modified over the years. Many parts of the refrigeration system in operation today are about 55 to 60 years old, but some parts remain original."

He said that a quantitative risk analysis was undertaken involving such factors as equipment, personnel, training, education, and protective equipment. The result did indicate that attention had to be given to the pump down system and some stress corrosion cracking. But with that being addressed, "The system does not represent a significant threat to the people in Copenhagen."

THE PEOPLE FACTOR

Successful system operations are more than equipment care and maintenance. It also involves "fine tuning the people machine," said Vern Sanderson of Wagner-Meinert Inc., Fort Wayne, Ind.

"Not everyone gives 100 percent every day," he said. "Fine-tuning the people machine is meant to help increase our productivity and the productivity of those around us. Among aspects he suggested giving attention to were using the right terms and communicating clearly, promoting interaction between employees, understanding what motivates an employee, and avoiding a sledge hammer approach to directing employees.

He even suggested employers be aware of employees who try to "avoid work through continuous and unneeded training."

ANDY AMMONIA

At the end of each conference, IIAR gives the Andy Ammonia Award to the papers presented that receive the highest recognition by attendees. The award was first presented at the 1997 IIAR conference held in New Orleans. Of the papers presented at the Reno conference, the CO2 defrost topic of Andy Pearson and the apple cooling topic of Kem Russell were given the honor.

Publication date: 05/08/2006