Carbon Dioxide: The New, Old Refrigerant
March 1, 2017
For the past 30 or 40 years, the refrigeration system of choice on many vessels has been some type of chlorofluorocarbons (ie: Freon) or hydroflourocarbons. The most common, R-22, which can be used until 2025, is facing restrictions on charging new refrigerant and the price is getting to be sky high. New, almost drop-in replacements are available, although an oil change is necessary and some O-rings might need replacing, but with less efficiency as a drawback. R-507 and a very similar R-404A can still be used and are viable replacements in many cases. They will also, however, eventually be phased out.
Natural refrigerants with zero ozone depletion factors are definitely on their way in. The natural refrigerants, such as R-717 ammonia, R-290 propane and R-744 CO2 (carbon dioxide) are beginning to see more extensive use. Ammonia has, of course, been in use for many years; but its downside, the pungent odor and aggressiveness, makes it a doubtful choice for use on smaller fishing vessels. In areas where leaks are possible, ammonia requires enough room to allow crew to escape to safe areas in case of a leak.
R-290 – propane – is an excellent refrigerant, and very similar to R-22 in almost all ways; using the same components, compressors and oil. Propane adds the risk of explosion in case of a refrigerant escape into enclosed areas, and with a spark setting it off.
With the expanding use of LNG (Liquefied Natural Gas) for clean burning propulsion fuel, the ventilation and storage for propane refrigerant needs to be approved by the US Coast Guard and the Classification societies, so the foundation is laid for the use of propane as a refrigerant on vessels, as well.
Most of us are familiar with gas as a heating medium in our houses and we might even have a gas stove, fireplace or barbeque in our house without worrying. In a tight, properly installed system, R-290 propane will have a future in many places. It is widely used as a refrigerant in Europe.
Carbon dioxide (CO2, R-744) was a very popular refrigerant in early industrial refrigeration systems around the year 1900. It was replaced with R-717 ammonia as the compressors and components were cheaper because of the lower pressures operating with ammonia versus CO2.
Today's new technology has meant a re-invention of R-744, CO2, as a refrigerant, using modern, high pressure rated components, stainless steel piping and new, reliable semi-hermetic compressors without shaft seals. The shaft seals are vulnerable to failure with high operating pressures. These compressors have opened new possibilities for the use of this once almost forgotten refrigerant. It has some benefits:
CO2 is very dense, requiring smaller refrigeration lines and valves compared to R-717 ammonia. In one case, we replaced an 8-inch ammonia line with a 4-inch CO2 line for the same refrigeration load.
CO2 has an excellent heat transfer ability. On a flake ice machine, for example, the capacity increase by using CO2 versus ammonia, will be in the area of 15 to 20 percent at the same temperature. Even with correctly designed plate freezers, however, the effect is less even though the plate is very cold, as the heat transfer through the seafood being frozen sets the pace for the freezing.
The technology is consistently improving, making new components, compressors, freezers and chillers available at competitive costs. The cost of, for example, an RSW system with CO2 is less than a system with R-717 ammonia, but still a little bit more than a system using Freon, R-507, as refrigerant.
CO2 can be used as a "volatile brine" refrigerant, being cooled by a small charge ammonia refrigeration plant. It will be pumped through the refrigeration system and partly evaporate. The remaining liquid CO2, together with the liquid will return to the ammonia-cooled condenser. The CO2 gas will condense and be pumped around the evaporators again. Additionally, operating such a system can be power efficient because the compressor's system is operating at only part load.
Carbon dioxide is also relatively safe, however:
• CO2 must be contained in pressure vessels designed for higher pressures than for ammonia or Freon, adding to plant cost.
• CO2 must be kept cold to keep pressures within reason. The pressure at, for example, 75°F will be 900 psig. Even compressors, freezers and chillers being available at competitive costs demand expensive pressure vessels. Therefore, measures must be taken to keep part of the system colder at all times to avoid release of CO2. Systems must be specifically designed for high pressure, using Transcritical compressors, (compressors that can work with CO2 gasses over 88°F,) in order to keep the systems compact and not dependent on being kept cooler than a seawater cooled condenser can accommodate.
Recently a progressive fishing company, Global Seas, decided to replace the existing RSW system using Freon refrigerant on one of their trawlers. They came to Highland Refrigeration with a request for a quotation for replacement of their old system. Highland Refrigeration calculated the cost for the system using R-717 ammonia, R-507 Freon and a Transcritical R-744 CO2 refrigerant system and made a comparison of the prices, power consumption and space requirement of the three types of systems.
Bob Desautel, the owner and CEO of Global Seas, together with his Port Engineer Stan Frech, made the bold decision to be the launch customer for the CO2 system. The entire refrigeration package with a 140TR system is being built at Highland Refrigeration's shop in Seattle's Fishermen's Terminal to be ready for March 2017 testing and installation.
It is skid mounted with Titanium RSW chiller and condenser, (Titanium heads, tube sheets and tubes,) and with 3 VFD driven semihermetic GEA Bock CO2 compressors. Dividing the load on 3 compressors ensures redundancy and excellent part load power consumption.
The system is compact, only approximately 6 feet by 8 feet, and 6-feet tall, piped, wired and tested, ready to be lifted on board to be either in a deck mounted container or below deck.
The operation is controlled by an open source PLC, making for easy operation with minimum operation input. This system also allows for remote monitoring with an added communication module.
Lars Matthiesen established Sabroe in North America in 1979 and ran the company for 14 years before consulting for GEA (Kuhlautomat Berlin) and establishing their screw compressor presence in the USA, now GEA Refrigeration USA. In 1998 he opened his own company, Highland Refrigeration.