Extremely Low Temperature Systems

Understanding the nuances of low-temperature engineering is key to safe and efficient operations

Figure 1. This temperature scale from 212°F to absolute zero shows the boiling points of nitrogen, oxygen, and helium

Extremely low temperatures — below –100°F (–73.3°C) — have many widely diverse commercial uses, from de-flashing of molded rubber parts on the plant floor to pristine storage of biological materials in the medical and pharmaceutical industries. Understanding the special characteristics of low-temperature fluids and systems, and properly managing the associated risks, are important to the safe design and handling of low-temperature processes.

Temperatures this cold (Figure 1) rarely occur naturally, but are commonly found in modern industry and healthcare. Two technologies are typically used to generate extremely low temperatures: consumable refrigerant systems, which use products such as liquid nitrogen or dry ice, and mechanical refrigeration systems. While both mechanical refrigeration and consumable refrigerant systems share common risks of operating at extremely low temperatures, they have different costs and operational characteristics. Selecting the right technology to generate low temperatures depends on a number of specific process parameters.

Whether the refrigeration is from a mechanical or consumable source, extremely low temperatures can provide innovative solutions to problems such as how to pulverize an intractable material or recover a difficult-to-condense volatile compound. Yet extremely low temperatures present technologists with a number of unique risks that must be safely managed.

Being aware of safety and process necessities, such as suitable materials of construction, appropriate venting, and proper personal protective equipment, will enable engineers to continue to harness the power of extreme cold to improve their operations in a safe and efficient manner.

COMMON CRYOGENIC APPLICATIONS

Extremely cold temperatures are routinely encountered in many products and processes, including the following:

• Magnetic resonance imaging (MRI) scanners, most of which use liquid helium to cool superconducting magnets to temperatures below –441.7°F (–263.2°C)
• Superconducting magnets, refrigerated with liquid helium, are the heart of the Large Hadron Collider — the machine used to discover the Higgs Boson — at the European Organization for Nuclear Research (CERN) facility in Meyrin, Switzerland
• Production, distribution and storage of industrial gases, such as oxygen, nitrogen, argon, hydrogen, and helium; for example, many plants store nitrogen as a cryogenic liquid at –320°F (–196°C)
• Natural gas liquefaction, which reduces the cost of transporting natural gas by liquefying it at temperatures below –250°F (–157°C)
• Polymerization of isobutylene, to make products such as butyl rubber, at temperatures from –130°F to –148°F (–90°C to –100°C)
• Testing aerospace components and assemblies, which must operate at very low temperatures
• The pharmaceutical and fine chemical industries, which use low temperatures in the synthesis of many intermediates, for freeze drying, and for storing biological samples and drugs
• Low-temperature metallurgy, such as quenching steel to reduce the presence of residual austenite
• Cryogenic grinding (Figure 2)

There are many additional opportunities within the industrial processing space to take greater advantage of the benefits of low temperatures and cryogenic liquids.

Figure 2. Cryogenic grinding allows even difficult materials like silicones to be reduced to a uniformly fine particle size

TYPES OF REFRIGERATION SYSTEMS

The low temperatures used in the processes outlined above are produced using either a consumable refrigerant, such as liquid nitrogen or liquid helium, or a mechanical refrigeration system.

A consumable refrigerant is produced at a central plant and delivered to the process site, where it is melted, evaporated, or sublimed to produce refrigeration. A bag of water ice in a food cooler is a simple example of a consumable refrigerant at a relatively warm temperature.

A mechanical refrigeration system uses mechanical energy to transfer the thermal energy from a refrigerant (or the machine’s working fluid) at a low temperature to a higher-temperature heat sink, such as the ambient air or cooling water.

Mechanical refrigeration systems are commonly found in home air conditioning units, refrigerators, and freezers, although these systems operate at warmer temperatures than those we are concerned with here.

Both mechanical refrigeration and consumable refrigerant systems have some common risks associated with low-temperature operations that must be properly managed. These risks include the effects of exposing people to low temperatures, the effect of low temperatures on the ductility of many materials, thermal contraction, and managing the inventory of refrigerants in closed systems. Because of their unique characteristics, the risks associated with low temperatures and the proper handling and use of cryogenic liquids may not be as well understood as other hazard classes.

SAFETY CONSIDERATIONS

Extremely low temperatures require special attention and handling to avoid risks to plant and personnel safety.

Freezing of tissue. Extremely low temperature can rapidly freeze tissue, a characteristic that enables high-quality, long-term storage of cell lines and biological samples. For example, cryogenic tissue banks enable long-term storage of organs for research, while pharmaceutical companies also use cryopreservation to store cells. However, contact between a worker’s bare skin and a low-temperature vapor, liquid, or solid can quickly freeze the skin tissue, resulting in a cryogenic burn.

Contact is most likely to occur when objects are being moved into or out of a low-temperature zone, such as placing samples into a liquid nitrogen storage bath; during maintenance activities; or when low-temperature fluids are being transferred. To perform these tasks safely, workers should wear long sleeves, long pants, thermally insulating gloves, and face and eye protection. Eyes are the most sensitive area and can easily be harmed by cold vapor, so a full face shield over safety glasses is advisable. Pants should not have cuffs, and gloves should be loose so they can be quickly removed if necessary.

When an object is placed in a cryogenic liquid, or a cryogenic liquid is poured into a warm container, boiling and rapid vaporization will occur. These tasks should be done slowly, and tongs should be used to handle objects being dipped.

Even when low-temperature liquids are not directly handled, it is still important to identify any uninsulated pipes or vessels that contain them. If unprotected skin comes into contact with these surfaces, the skin may stick to them.

Embrittlement. Many materials will embrittle at cold temperatures. This is a powerful benefit of low temperatures that facilitates the size reduction of materials that would otherwise be too soft, oily, or volatile to grind. For example, rubber and other soft polymers can yield fine particles only when milled at low temperatures. Also, many spices contain volatile components that are essential to their quality. If spices are not milled at low temperatures, qualities such as aroma and taste will diminish due to the heat generated during grinding.

Many materials commonly used for ambient temperature systems, such as carbon steel or galvanized steel, lose their ductility as their temperature is lowered. This can result in a catastrophic failure of equipment and piping made from these materials if subjected to excessive stresses under low-temperature conditions. Piping and pressure vessel design and fabrication codes, such as ASME B31.3 (Process Piping) and B31.5 (Refrigeration Piping and Heat Transfer Components), address this hazard by specifying minimum temperatures for materials of construction, plus materials testing and design restrictions for selecting and using materials at low temperatures. Materials that remain ductile at low temperatures include austenitic stainless steel (including types 304, 316 and 321), copper, red brass, and many copper alloys and aluminum. These are the preferred low-temperature materials of construction.

In addition to the embrittlement of materials of construction, thermal contraction must also be considered when designing an extremely low-temperature system. Most materials of construction will shrink as their temperatures decrease. For example, a stainless steel or copper pipe that is 100 ft. (30 m) long will contract linearly by about 3.5 in. (90 mm) as it cools down from 70°F to –320°F (20°C to –195°C). This thermal contraction is independent of the diameter of the pipe. The stresses generated by thermal contraction are large and will severely damage an improperly designed pipeline or piece of equipment.

Vapor expansion. Liquid nitrogen, the most common cryogen, expands to over 700 times its liquid volume when warmed to 68°F (20°C). This expansion property is used commercially to purge, inert, and pressurize containers housing foods, drugs, and chemicals that are sensitive to air or moisture by dropping a small amount of liquid nitrogen into the container during packaging.

Low-temperature systems may need to be designed to accommodate the pressures that can be generated whenever a liquid refrigerant is trapped in a closed volume. For example, liquid nitrogen or liquid trifluoromethane can become trapped in a pipeline between two closed valves. As the cold liquid warms up to ambient temperature, the increase in vapor pressure can spring flanged joints and burst pipes.

In liquid nitrogen piping systems, this expansion is usually managed by installing a pressure relief valve, typically known as a thermal relief valve, in every piping segment that can potentially trap liquid. All thermal relief valves in a liquid nitrogen system should discharge to a safe location, ideally outdoors.

In mechanical systems, the higher unit cost and other properties of the refrigerants used, such as trifluoromethane, means that thermal relief valves are not a feasible solution. Instead, these systems typically incorporate expansion vessels to accommodate expansion without loss of refrigerant.

Condensing or solidifying surrounding materials. The ability of cryogenic temperatures to liquefy substances with low boiling points is useful in many process operations. For instance, it is used to help plants operate more sustainably by condensing volatiles that cannot be separated from exhaust air streams at ordinary refrigeration temperatures. Many times, the condensed material can be reused and recycled instead of being incinerated in a thermal oxidizer, and hence wasted.

If cryogenic liquids contained in vessels or piping are colder than the oxygen dewpoint of the surrounding air, an oxygen-enriched liquefied-air condensate will form on uninsulated surfaces. This can drip onto surrounding equipment and personnel, causing cryogenic burns. As the condensate warms and re-evaporates, the resulting local raised oxygen levels can also create a serious fire hazard.

In the case of hydrogen and helium, the surrounding air can even be solidified. This frozen air can block the discharge ports of pressure relief valves, preventing them from operating correctly.

LOW-TEMPERATURE SYSTEMS

There are two technologies for generating low temperatures in industrial processes: mechanical refrigeration systems and consumable refrigerants. Whatever the method of final delivery in the plant, however, producing extremely low temperatures requires sophisticated mechanical refrigeration technology.

Cascade refrigeration systems are very useful for producing low temperatures. A cascade refrigeration system combines two or more compressors and typically two or more different refrigerants in a series of connected refrigeration cycles operating at different temperatures (Figure 3). The low-temperature cycle removes heat from the low-temperature process and transfers it to the high temperature cycle via an intermediate condenser or heat exchanger. The high-temperature cycle removes the heat from the intermediate condenser and transfers it to the high-temperature condenser, where it is rejected to cooling water or ambient air.

“Autocascade” systems are also very effective for producing low temperatures. An autocascade system has a single compressor, a refrigerant mixture comprising two or more components with progressively decreasing atmospheric boiling points, and a series of intermediate heat exchange steps [ 1,2 ]. Most natural gas is liquefied using mixed-refrigerant autocascade-type refrigeration cycles. Closed- and open-air-cycle refrigeration systems have also been used to produce low temperatures [ 3,4 ].

A consumable refrigerant is produced at a central plant and delivered by truck to the process site for use. Low temperatures are generated by evaporating the consumable refrigerant and releasing its vapors to the atmosphere. At the point of use, consumable refrigerant systems are typically simpler than mechanical refrigeration systems, and easier to install and operate. This simplicity means that consumable refrigerant systems can easily be designed to be very reliable.

Liquid nitrogen and liquid carbon dioxide are the most common consumable refrigerants for extremely low-temperature processes. Liquid helium can be used as a consumable refrigerant for certain high-value applications, but care must be taken due to the high unit cost of helium.

Liquid carbon dioxide, with a triple point of –109.3°F (–78.5°C), can be used at the upper end of the extreme temperature range. Solid carbon dioxide, also known as dry ice, can be used to make laboratory and pilot-production-scale cold baths by mixing it with a low-freezing-point solvent such as acetone or limonene. Such bath temperatures are limited to about –109°F (–78°C).

LIQUID NITROGEN REFRIGERATION

Liquid nitrogen, with a normal boiling point of –320.5°F (–195.8°C), is the most widely used consumable refrigerant for generating temperatures below –110°F (–79°C). Figure 4 illustrates a typical liquid nitrogen refrigeration system. There are typically five major pieces of equipment:

Liquid nitrogen storage tank. This holds the inventory of liquid nitrogen for use in the process. A cryogenic liquid storage tank is a sophisticated tank-within-a-tank. The inner tank holding the liquid nitrogen is suspended from a carbon steel outer tank by a carefully engineered support system to minimize the heat transfer by conduction. The space between the inner and the outer tanks is held at high vacuum to minimize heat transfer by convection. The space between the tanks can also be filled with powder insulation, or the inner tank wrapped with super insulation blankets, to reduce radiant heat transfer.

An insulated pipeline. This transfers liquid nitrogen from the storage tank to the process. Liquid nitrogen pipelines are typically insulated to conserve the refrigeration value of the liquid nitrogen and to prevent personnel contact with the very cold surfaces of the pipe. Poly(isocyanurate) cellular plastic foam is the simplest insulation system for liquid nitrogen pipelines. Vacuum jacketed (VJ) piping is more efficient than foam insulation, but is also more expensive to purchase and install. VJ piping is a coaxial piping system: the inner pipe carries the liquid nitrogen, while the outer pipe supports the inner pipe and contains a vacuum that minimizes convective heat transfer. The inner pipe is wrapped with layers of super-insulation, a material designed to minimize radiation heat transfer.

Figure 5. This ultrafine grinding mill uses liquid nitrogen for refrigeration

When a consumable refrigerant like nitrogen is used for a super-cold application such as cryogenic grinding or de-flashing, it is important that the nitrogen be primarily in the liquid state at the point of use. The higher the quality of the two-phase liquid-gas mixture (in other words the greater the ratio of liquid to gas), the more heat it can remove from the process material. While insulated piping is crucial to keeping the nitrogen as a liquid, other factors are also important. When a cryogenic operation starts up, for instance, the need to cool the piping and equipment from ambient to operating temperature will temporarily reduce the fraction of liquid nitrogen. In addition, flash losses are created by pressure drops along the pipe run, especially at valves, sharp bends, and the tops of vertical legs. To that end the pipe run needs to be as short as possible, with smooth bends and minimal changes in direction or elevation.

Liquid nitrogen flow controls. The flow of liquid nitrogen to a refrigeration process is typically controlled using a three-valve manifold. The first valve is a manual valve that is used to stop the flow of liquid nitrogen to the process. This valve should be able to be locked out. The next valve is an automatic safety valve to stop the flow of liquid nitrogen in an emergency. The final valve is a flow control valve. Valves are typically made from stainless steel, brass, or bronze; carbon steel components must not be used. Low-temperature valves are similar to those used at ambient temperatures, but feature modifications such as extended stems to keep their packing glands warm.

Heat exchanger. The heat exchanger is designed to evaporate the liquid nitrogen, and possibly to heat the resulting nitrogen gas, by transferring heat from the substance being refrigerated. The heat exchanger in Figure 5 is a modified screw auger for cooling plastic pellets prior to cryogenic grinding. The heat exchanger could also be a cooling tunnel for refrigerating solids that will not convey well in a screw auger. Liquids or gases can be cooled, or vapors condensed, using liquid nitrogen in heat exchangers of more familiar types: shell-and-tube, spiral plate, spiral tube, and plate heat exchangers.

Gaseous nitrogen exhaust system. A stream of nitrogen gas will be generated as the liquid nitrogen evaporates in the heat exchanger. This nitrogen gas must be discharged to a safe location outdoors to prevent the formation of an oxygen-deficient atmosphere. Oxygen-deficient atmospheres can be very dangerous to people if their hazards are not managed safely. Wall-mounted ambient-air oxygen sensors should be included with any consumable refrigerant system.

References 5 and 6 discuss the safe handling of liquid nitrogen in more detail.

SELECTING A LOW-TEMPERATURE SYSTEM

While mechanical refrigeration and consumable refrigerant systems face common technical challenges associated with low-temperature operation, they have very different cost and operational characteristics. Selecting the right technology to generate low temperatures depends on a number of specific parameters and process goals, as discussed below:

Process temperature. Table 1 shows some potential refrigerants, with their normal boiling points and critical temperatures. This is not a complete list, and does not include multi-component refrigerants as used in autocascade cycles. Below –100°F (–73°C) the choice of refrigerants for mechanical refrigeration systems becomes restricted primarily to flammable materials, plus nitrogen, argon, and helium. Using a non-flammable consumable refrigerant, such as liquid nitrogen, can be an attractive alternative to a mechanical refrigeration system containing flammable refrigerants, especially at process temperatures below –100°F.

Helium’s properties make it an attractive refrigerant below –320°F (–195°C). Helium can be used either as a consumable refrigerant or as a working fluid in a mechanical refrigeration system.

Installed capital costs. Consumable refrigerant systems have relatively low installed capital costs. The cost breakdown between the variable and fixed (capital) costs of a consumable refrigerant system is weighted toward the variable costs; that is, the cost of the refrigerant.

The installed capital costs of mechanical refrigeration systems are typically higher than those of consumable refrigerant systems, often including a significant cost for the refrigerant charge. Mechanical refrigeration may also depend on using existing cooling towers for heat rejection. The cost breakdown for mechanical refrigeration is weighted toward the capital costs. The variable cost of a mechanical system should be lower than that of a consumable refrigerant system. However, if the mechanical refrigeration system is under-utilized, the total cost can be significantly higher than expected.

The system that provides the highest return on invested capital should be selected. In many cases, this is a consumable refrigeration system.

Process duration. Consumable refrigerant systems can be attractive for processes that only require low-temperature refrigeration for a small portion of the overall cycle, even though the process as a whole may run for extended periods. An example of this type of process would be low-temperature synthesis of pharmaceuticals, where cleaning between batches consumes a significant amount of time.

Consumable refrigerant systems may also be able to provide a much faster cool-down time than a typical mechanical refrigeration system. Mechanical refrigeration and consumable refrigerant systems can even complement each other, with the consumable refrigerant system providing fast cooling cycles and very low temperatures, while mechanical refrigeration is used for the baseload.

Low capital costs can make consumable refrigerant systems attractive for processes with uncertain market projections. Consumable refrigerants are also ideal for processes that do not reoccur frequently, such as cooling a reactor during a refinery turnaround, or using liquid nitrogen to cool down an LNG plant during startup.