Natural refrigerants such as ammonia, carbon dioxide and hydrocarbons are providing an efficient and environmentally friendly process cooling alternative.

Two rotary screw compressor units provide cooling energy at Bayer's Leverkusen, Germany, refrigeration plant, for which York designed the process cooling plant.

The European Environment Council adopted the EU F-Gas Directive in October 2004, and the Kyoto Protocol went into effect in February 2005. Both agreements aim to reduce the emission of greenhouse gases, which include the fluorinated hydrocarbons (FCs) and partially halogenated fluorinated hydrocarbons (HFCs) used in refrigeration and air conditioning.

Lately, natural refrigerants have increasingly been used as environmentally friendly alternatives in process cooling. Ammonia makes up the largest proportion -- more than 40 percent -- followed by carbon dioxide and hydrocarbons. Ammonia's important role is hardly surprising. “Against the backdrop of carbon dioxide emission trading, for many users, energy efficiency is the most important criterion in selecting the appropriate refrigerant,” explains Georges Hoeterickx, an executive board member of eurammon, the European initiative for natural refrigerants. “Because ammonia has proven efficient and effective, many companies choose to use this traditional natural refrigerant.”

Two factory-built, skid-mounted air-cooled ammonia systems supplied by Star Refrigeration provide cooling energy to Lundbeck Pharmaceuticals' production process.

A Remote Cooling System for Economic Refrigeration

The Bayer chemicals group, for instance, commissioned York's refrigeration specialists to build an ammonia-based refrigeration plant. The company chose ammonia because it is an efficient, economical and environmentally friendly refrigerant. The job was part of Bayer's extension of the remote cooling system at its primary plant in Leverkusen, Germany, one of the largest production plants in the world. Here, in roughly 600 buildings scattered on more than one square mile, Bayer manufactures pharmaceuticals, dyes, rubber, polyurethanes and organic and inorganic products.

The heat generated by all production-related processes is dissipated through a remote refrigeration plant with several refrigeration centers. Three different temperature levels are available for this process: 23, -4 and -49oF (-5, -20 and -45oC). Decentralized cascade chillers, also hooked up to the main supply, provide temperatures of -49oF and below. Bayer uses the remote refrigeration plant, which comprises several large units, to economically generate cooling energy and ensure a steady supply.

At the York refrigeration plant, two rotary screw compressor units provide cooling energy. The refrigerant, ammonia, is liquefied in condensers that are supplied with cooling water. The refrigerant is then fed into the liquid line through a subcooler and part-time jetting pumps as needed to supply the process loads. The overheated ammonia gas returned by the network is cooled in a suction separator and is then returned to the suction side of the compressor. The plant's refrigeration output exceeds 5 MW.

Table 1. While many conventional refrigerants deplete the ozone layer and contribute to global warming, natural refrigerants such as ammonia, carbon dioxide and hydrocarbons are an environmentally friendly alternative. Click image for larger view.

An Air-Cooled Ammonia-Based Plant

Lundbeck Pharmaceuticals, a Danish drug manufacturer, needed to expand its production capacity in Seal Sands, England, to meet increased demand. The production process requires adhering to precisely specified temperature gradients, with temperatures ranging from -121 to 500oF (-85 to 260oC). Lundbeck required a cooling solution that was efficient, reliable and environmentally friendly -- a challenge that Star Refrigeration, based in Glasgow, Scotland, was happy to take on.

The plant builders produced two factory-built, skid-mounted air-cooled ammonia systems that each supply one loop with cooling energy. Factory packaging significantly reduced onsite assembly and facilitated the production startup. The low-temperature cycle at 41oF (5oC) cools 400 m3/hr of the coolant Therminol D12 from Solutia with a refrigeration output of 1.4 MW. The ultra-low temperature cycle chills 50 m3/hr of Therminol D12 to -13oF (-25oC) with an output of 220 kW. Both cycles use soundproof rotary screw compressors and high-performance fully welded plate and shell evaporators, which assist in the design of a system with a small refrigerant charge. The refrigeration systems have tanks for storing the secondary refrigerant, which helps to bridge downtimes and facilitate maintenance work. Star Refrigeration equipped the plant with modern controls, including ammonia detectors. Thanks to the modular design, the operator can easily and inexpensively expand the installation at any time.

An ammonia-based refrigeration plant built by Dresen + Bremen cools equipment and air conditions the premises for a German sweets manufacturer.

One-Step Process Cooling and Air-Conditioning

On its factory premises in German Halle/Westfalen, a leading Germany sweets manufacturer erected a new production building that required a system for cooling processes, as well as air-conditioning the premises.

Following a detailed economic feasibility study that weighed the pros and cons from an installation engineering point of view, the operator chose an ammonia-based refrigeration plant built by the German company Dresen + Bremen. “The plant uses a minimum of energy and is reliable thanks to its redundant structure. The additional investment compared to Freon chillers pays for itself in four years. What's more, the operator benefits from the positive image of ammonia as a natural refrigerant,” explains Dresen + Bremen's Norbert Hackmann.

Besides ensuring a controlled dissipation of heat during the production of chocolate, bonbons and gummy candies, the refrigeration system keeps the machines cool. At the heart of the centralized system are four RPM-regulated rotary screw compressors. Two liquid cycles at temperatures between 41 and 52oF (5 and 11oC) supply cooling energy to the consumer loads. Cold water circulates in the process cooling cycle; propylene glycol is used in the air-conditioning. Plate heat exchangers handle the energy transfer, based on the principle of gravity-feed evaporation. The evaporating temperature is 37oF (3oC) in each case, while the condensation temperature is 91oF (33oC). The waste heat from the rotary screw compressors is used to heat the process water through a self-contained glycol cycle.

The refrigeration plant is housed in a steel machine room on the roof of the production hall. Once the two-step system extension is complete, the refrigeration output will be nearly 2 MW in the production area, and nearly 3 MW for the air-conditioning.

A Sustainable Future

Technological evolution and innovations have helped to establish natural refrigerants as an economical, safe solution for applications in numerous areas. In addition to the medium-range cooling temperatures mentioned in this article, eurammon sees significant potential for natural refrigerants in manufacturing cooling energy in the -22 to -148oF (-30 to -100oC) range, particularly in the chemicals and pharmaceuticals industries.

“Natural refrigerants including hydrocarbons and carbon dioxide are high-performance and environmentally friendly alternatives for such applications,” concludes eurammon's Hoeterickx. PCE