Investing in the future
The use of natural refrigerants on board ships has proven to be an efficient practice that yields low operating costs, in addition to reducing the impact of refrigeration on the environment.
In its environmental report, the United Nations concluded that the only way to limit the effects of climate change globally is to radically reduce greenhouse gas emissions. The European Union (EU) has assumed a leading role in this regard, and in 2006 approved a mandate for the reduction of some fluorinated greenhouse gases, which came into force this July.
The "F-Gas Directive" includes regulations that specify that air conditioning and refrigeration systems must be provided with regular maintenance by qualified personnel, in addition to being tested to ensure that there are no leaks; this leads to decreasing greenhouse gas emissions.
"Refrigeration systems using natural refrigerants will become increasingly important," says Monika Witt, CEO of Eurammon, the European Initiative for Natural Refrigerants. "If you use ammonium, carbon dioxide or hydrocarbons, which are not only climate-neutral natural refrigerants, but are also very efficient, then you will have low operating costs, which is a safe investment in the future."
Shipping companies rely on natural refrigerants
For example, Axima Refrigeration France, supplied a cooling system that uses propane, a natural refrigerant, for a ship operated by Philips Petroleum that processes crude oil (Floating Production, Storage and Discharge -FPSO-).
These FPSO-specialized vessels collect oil and gas from oil drilling, process and store them until the raw materials can be transferred to a tank car or arranged in a pipeline. The FPSO ship operating in the Timor Sea between Australia and East Timor has tanks for temporary storage of propane and butane.
Given the high ambient temperature of approximately 30º C, it is necessary to constantly refrigerate the crude oil, (propane at -39º C and butane at -7º C); for which four chillers with a total cooling capacity of 5 MW are used. Also, to increase the efficiency of the plant, contractors connected the generators in cascade, which allowed the propane refrigerant to cool in two stages, and the condensation heat was then dissipated into the surrounding seawater. The main component of the cooling system is an oil-lubricated screw compressor with a volume flow of 8,000 m³/h.
The fourth cooler is a backup that makes it possible for the ship's crew to stay cool and continue to process the oil, even when some of the cooling systems break down. All screw compressors are equal in decreasing the number of spare parts needed, which is an important factor given the limited space on board a ship. The cooling system was successfully put into operation aboard the FPSO ship in 2003.
To protect the equipment against corrosive saline water, Axima Refrigeration built all the pipes and inrush drums in stainless steel, while the heat exchangers, condensers and oil coolers are made of titanium. It should be noted that for the operation of coastal technical plants very strict standards are applied.
For its part, Atlantic Pelagic Seafood (international shipping company of the United States) commissioned Grenco B.V. Marine to adapt in the "American Freedom", a refrigeration ship for fish, a cooling system that was large and environmentally friendly.
Put into operation towards the end of 2006 and with a processing capacity of approximately 400 tons of sea fish per day, the "American Freedom" is one of the largest reef ships in the world. It operates as a mother ship in Atlantic regions, that is, it serves as a base for fishing boats that bring their herring and mackerel catch, and pumps it directly into a hold while at sea.
The contractor planned and built a cascade cooling system using ammonia and carbon dioxide. The high-pressure side uses four screw compressors with ammonia with a total output of 4,500 kW. This circuit is loaded with 1,500 kg of ammonia; the evaporation temperature is -6º C and the condensation temperature is 36º C. The low-pressure side employs four screw compressors with carbon dioxide, supplying 2,000 kW of cooling capacity; the evaporation temperature on this side is -50º C and the condensation temperature is -2º C.
The fish is subjected to shock freezing in 36 vertical plate freezers and then stored in cold holds that are refrigerated with air coolers that use carbon dioxide. Heated carbon dioxide is used to thaw coolers, eliminating the need for a conventional defrosting unit, and achieving additional energy savings.
The low-pressure circuit contains a total of 10,000 kg of carbon dioxide, while the ammonia circuit is part of the waterfall and also offers two chillers that are used to cool seawater (this combination is a total novelty in cooling technology for ships). All parts of the cooling system are automatically monitored and can be controlled from a central computer.
Technology with a bright future
"The planners, contractors, component manufacturers and operators who have joined forces and crossed competitive boundaries to form the European Initiative for Natural Refrigerants, Eurammon, are constantly working on developing environmentally benevolent solutions for refrigeration technology," says Monika Witt of Eurammon.
"Natural refrigerants are already used in commercial air conditioning, in the beverage and food industry, in sports and in different recreational parks, as well as in the chemical and pharmaceutical industry and in the automotive trade. The goal is to make more adjustments and improve technical processes and strengthen energy efficiency, to contribute to climate protection."
Natural refrigerants
Ammonia (NH3): Ammonia has been successfully used as a refrigerant in industrial refrigeration plants for over 130 years. It is a colorless gas that liquefies under pressure and has a pungent smell. Ammonia has no ozone degradation potential (PDO= 0) nor does it have direct global warming potential (GWP= 0).
Thanks to its high energy efficiency, its contribution to the indirect global warming potential is also low. Ammonia is flammable and toxic to the skin and mucous membranes. However, its ignition energy is 50 times higher than that of natural gas, and ammonia does not burn without a support flame.
Due to ammonia's high affinity for atmospheric moisture, it has been rated as "hardly flammable." Ammonia is toxic, but it has one characteristic: its strong odor, which gives it a warning capacity below the possible concentrations of 3 mg/m³ of ammonia in air. This means that ammonia is evident at levels well below those that endanger health. In addition, ammonia is lighter than air and therefore rises rapidly.
Carbon Dioxide (CO2): Carbon dioxide has a long history in the field of refrigeration that extends into the mid-nineteenth century. It is a colorless gas that liquefies under pressure, with a slightly acidic smell and taste. Carbon dioxide has no ozone degradation potential (PDO= 0) and has negligible direct global warming potential (GWP= 1) when used as a refrigerant in closed cycles.
It is non-flammable, chemically inert and heavier than air. Carbon dioxide is narcotic and harmful to human health at moderately high concentrations. Because carbon dioxide has a lower critical temperature than other refrigerants, recent research has focused particularly on optimizing system design, and increasingly effective cooling plants are being developed to close this gap. Carbon dioxide is available in abundance and there is no need to recycle or dispose of it.
Hydrocarbons: Refrigeration plants using hydrocarbons such as propane (C3H8) or butane (C4H10) have been in operation around the world for many years. Hydrocarbons are colorless gases and have almost no odor, liquefy under pressure and have no ozone degradation potential (PDO = 0) or significant direct global warming potential (GWP = 3).
Thanks to their outstanding thermodynamic characteristics, hydrocarbons particularly produce energy-efficient refrigerants. They are heavier than air and have an anesthetic and suffocating effect at high concentrations.
Hydrocarbons are flammable and capable of forming explosive compounds with air; however, with current safety standards, refrigerant losses are almost zero. Hydrocarbons are available at low cost worldwide and thanks to their ideal refrigerant characteristics, they are commonly used in small plants with low refrigerant loads.
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