Here is a brief summary of each of those papers.
CO2 AS BRINEUsing CO2 and ammonia has typically been done with cascade systems. A paper from a team of engineers at Mycom, however, described a system it said was “not an NH3/CO2 cascade system, but one that uses CO2 as a brine or secondary fluid.
“Most refrigeration systems require -40°F or higher evaporating temperature, where a cascade system is less energy efficient and initial cost is high,” they wrote. “By utilizing a CO2 brine system, these most common temperature requirements can be achieved at equal or lower initial costs and reasonable energy efficiency.”
To achieve this, the researchers said they needed:
• “Development of synthetic oil soluble in ammonia which enabled use of a dry-type evaporator, thereby drastically reducing the refrigerant charge.”
• “Use of a semihermetic motor, welded heat exchangers, and fabrication of the unit package in the factory to minimize refrigerant leakage.”
• “Use of an evaporator condenser to lower condensing temperatures, thus improving the COP [coefficient of performance] of the system and at the same time, using the condensing unit as a scrubber system in case of emergencies.”
PIPEWORK FRACTUREA paper from Bent Wiencke of Nestlé focused on the importance of system design and appropriate monitoring to prevent operating conditions and circumstances that can lead to a large ammonia release.
His case study examined an ammonia release caused by hydraulic shock that ultimately led to a deflagration (combustion that is transmitted through a gas). He warned of designs that could allow the possibility of hydraulic shock, a vapor-propelled liquid slug, in the liquid transfer vessel. He urged use of ATM A-106 or ASTM A-333 piping material for the coil manifolds to minimize risks.
He also noted the need for safeguards, such as high-level alarms of the liquid level controller connected properly to the liquid transfer vessel; proper setting of the soft hot gas feature; and a pressure transducer or pressure switch installed in the valve group and interlocked with the large gas solenoid valve.
ROTARY SCREW COMPRESSORSA paper from John Cosner of Johnson Controls/Frick carried the title, “Energy Efficiency and Enhanced Performance by Applying Variable-Speed Drives to Rotary Screw Compressors.”
According to Cosner, “With rotary screw compressors being the largest consumers of electricity in an industrial refrigeration process, better part-load efficiency of these compressors has become a growing concern within our industry.” He then suggested, “One solution is the use of a variable-speed drive (VSD) in lieu of a constant-speed drive.”
He said such an approach “has become a more common occurrence from even five years ago. This trend will continue to grow in order to operate refrigeration processes as efficiently as possible while performing to system requirements as well as possible.
“In almost all industrial refrigeration processes, there is an opportunity to improve the process, either by improved part-load efficiency, or enhanced control and performance from the application of a variable-speed drive to a rotary compressor.”
ROTARIES AND RECIPSUsing variable-frequency drives (VFDs) on both screw and reciprocating compressors was discussed in a paper from Gary Schrift and Greg Klidonas of FES Systems. The emphasis was on recips.
“Depending on the application and load profile, the most efficient solution may be a reciprocating compressor with conventional cylinder unloading or speed control.
“An alternative to an all-screw installation with a VFD may be a hybrid system,” they wrote. “A hybrid solution may be more energy efficient. However, the savings must be sufficient to offset the increase in maintenance costs sometimes associated with reciprocating compressors.
“The addition of a VFD on a reciprocating compressor in a hybrid system may further increase the energy savings. However, as with screw compressors, the savings must offset the VFD equipment installation premiums.
“In any case, the decision to apply a VFD must carefully consider all of the factors that can influence the resulting energy consumption and payback. Trying to force a VFD as an energy-saving solution when the application does not warrant it may actually increase your energy usage, cause unforeseen problems, or result in a long payback period.”
AMMONIA-HALOCARBON COMPARISONAlex Gooseff of ALTA Refrigeration and Jamie Horton of ElectroMotion Refrigeration presented a paper comparing ammonia and halocarbons in terms of sustainability. “When contemplating the question, ‘Should our facility utilize an ammonia or halocarbon refrigeration system,’ an owner should perform a detailed financial analysis of the two systems. The first cost difference of the two systems may be easily returned via the savings in operating costs and the long-term benefits can be significant.”
They offered “rules of thumb that apply for a distribution facility application.”
• Less than 50,000 square feet of refrigerated space: Halocarbon split-circuit systems are normally accepted.
• At 50,000 to 200,000 square feet of refrigerated space: Both halocarbon split-circuit systems and a central ammonia system are common. The owner’s priorities must be considered. A lifecycle cost analysis should be performed.
• Over 200,000 square feet of refrigerated space: Central ammonia refrigeration systems are most common.
AIR-COOLED AMMONIA CONDENSERSHeinz Jackmann of Guntner Germany and Ian Runsey of Guntner U.S. looked at air-cooled ammonia condensers as an alternative to evaporative condensers.
“The technology of ammonia plants has changed and therefore the possibilities for using air-cooled condensers have increased. In many regions, the cost of electricity, water, and wastewater have increased. In some regions, water is scarce or there are restrictions on water use.
“What has changed?”
They pointed to compressors (“it is technically possible to use screw compressors in ammonia plants for maximum condensing temperatures up to 120° or 130°”), controls (“modern control technology optimizes the operation of a plant”), cost of resources (“for the design of an ammonia refrigeration plant in countries with high costs for fresh water and wastewater, it is important to calculate the operating costs and compare with the alternative of air-cooled condensers”), and environmental awareness (“in most regions, water chemically treated cannot be discharged into wastewater treatment systems with unclarified water”).
REFRIGERANT RELEASESDouglas Reindl and Todd Jekel of the University of Wisconsin/Industrial Refrigeration Consortium reviewed what they called “techniques suitable for use in estimating the quantity of refrigerant lost as a result of a leak.” The research involved vapor-only, liquid-only, and flashing liquid leak scenarios.
“In the event of a refrigerant leak from a system, the quantity of refrigerant loss must be estimated as accurately as possible,” they wrote.
“It is essential that operations staff gather the following information: time incident began, location of refrigerant release, state of the refrigerant upstream of the leak site, geometry of the leak site, pressure of the refrigerant upstream of leak location, temperature of the refrigerant upstream of the leak location, behavior of the leak, room concentration, and leak duration.”