Cooling with Cryogenics
While cryogenics may appear to cost more than other cooling methods, the speed of chill, the very cold endpoints that can be attained, the smaller footprint in the plant, the lower capital investment, and the ease of use can make it the most cost effective. The most common cryogenics are liquid nitrogen (LIN) and liquid carbon dioxide (LCO2), and one or the other or both are commonly available throughout most of the United States and Canada. While technically LCO2 is not a cryogen, it is the most common liquefied gas alternative to LIN.
Typically 99.999 percent pure, LIN is produced in an air separation plant that has liquefaction capabilities. Such a facility also will produce liquid oxygen and limited quantities of liquid argon. The major cost components for LIN supply are electrical energy for separation and liquefaction, and trucking the liquid to the customer. The air is free.
By contrast, LCO2 is produced either from a vent or well out of a carbon dioxide gas pocket deep within the earth, or as a byproduct of a chemical operation. Raw carbon dioxide can contain impurities, and while LCO2 is available in many grades, 98 percent purity is common. Furthermore, because LCO2 commonly is used for seasonal cooling in the food industry, it may be in short supply during the summer, which generally results in customers paying a surcharge. The LIN supply is more constant in many areas.
It is common to store and handle LIN at pressures of 20 to 50 psi for cooling operations. At these pressures, LIN is approximately -300oF (-184oC) and a typical use temperature is about -180oF (-118oC). As the working pressure increases, LIN delivers fewer BTUs to use for cooling. Furthermore, in using LIN, a design team should confirm that the very great temperature differential (∆T) between the cryogen and the product will not result in thermal shock, cracking or crazing of the product. Many polymer plastics cannot withstand freezing temperatures below -40oF (-40oC). They are subject to shatter if exposed to lower process setpoints.
Handling CareLCO2, however, normally is stored at pressures of 250 psi or higher and is at approximately 0oF (-18oC) in the storage vessel. Typical use-temperature for LCO2 is approximately -90oF (-67oC). This largely is due to a need to avoid the triple point of LCO2 when the liquid turns to a solid, which will freeze the distribution lines and even the storage vessel itself.
LCO2 requires more care in material selection and fabrication for distribution piping than LIN, but each demands due regard for both pressure and working temperature. Insulated Type K copper, a common pipe selection for handling either cryogen, is available with preinsulated joints. Vacuum-jacketed stainless steel pipe is available at higher cost but offers increased R values. Typically, the heat gain of pipe-runs more than 150' demand vacuum-jacketed piping. Process control will always be optimal if vacuum-jacketed grades are used.
When cryogen is used, safety concerns arise, the foremost of which is human health and safety. There are literally thousands of installations around the world safely using cryogens for process or product cooling. However, consider these facts. Humans commonly breathe in 78 percent nitrogen and 20.9 percent oxygen. Use of pure LIN for cooling can exhaust concentrated clouds of cold nitrogen vapor. Normally, nitrogen is slightly lighter than air and will rapidly rise and return to equilibrium in the atmosphere. However, using LIN in confined or poorly exhausted spaces may dilute the air's oxygen to levels approximately 18.5 percent, below which personnel begin to feel dizzy and pass out. Cold nitrogen vapor tends to sink, so passing out puts a person into a zone of even lower oxygen, resulting in anoxia or oxygen deprivation. Fortunately, well-designed exhaust systems do not allow nitrogen exhaust into the working space, and oxygen safety monitors can be used to alert operators when oxygen drops to a certain level.
The situation with LCO2 is more critical. OSHA's exposure limit is just 0.5 percent carbon dioxide in the breathing atmosphere. Because the human bloodstream preferentially absorbs carbon dioxide over oxygen, a worker quickly will be overcome. LCO2 installations must have well-designed exhaust systems and appropriate monitors with warning alarms and lights. Furthermore, carbon dioxide gas is naturally heavier than air and, when cold, flows downhill like water, collecting in low spots.
In addition, if a cryogenic liquid is allowed to sit in a pipe long enough, even an insulated one, it eventually will absorb enough heat from the surrounding environment to change phase to a gas. LIN can expand almost 700 times when it sublimes to a gas, and LCO2 can expand about 550 fold. Either situation will cause pipe pressures to skyrocket to dangerous levels. Every pipe-run design should include a properly sized safety relief valve in the distribution line between each set of points at which liquid could be isolated.
And finally, nitrogen is chemically inert while carbon dioxide is reactive, which is demonstrated by its affinity to replace oxygen in the human bloodstream. Carbon dioxide gas will combine with water to form a weak carbonic acid. It also is bacteriostatic and will retard the growth of bacteria and mold. LCO2 has been used in food chilling and freezing to reduce microbial contamination on oysters by approximately 1 log.
Cooling CapacityAnother factor to consider in deciding on cryogenic cooling is in the form of a question: How much cooling capacity does either cryogen actually have? There is a theoretical answer and a practical reality.
Figure 1 shows that LCO2 flashed to snow has 113 BTU/lb as latent heat and 29.8 BTU/lb as sensible heat, for a total of 142.8 BTU/lb. LIN at 15 to 30 psi delivers 81.5 BTU/lb as latent heat and 88.9 BTU/lb as sensible heat, for a total of 170.8 BTU/lb.
While the LIN theoretically has the greatest potential per pound, note that more than half of the potential BTUs from LIN is from sensible heat in the cold gas. Recovering these BTUs requires blowing the cold vapor over the product at high exchange rates and may require additional feet of conveyor.
As a practical matter, either cryogen generally delivers within 10 percent of the same BTUs for common industrial operations, with the actual efficiency of the cryogen depending largely on the method of use. Common application methods are to dip or immerse the product in the liquid cryogen; spray the liquid cryogen over the product in a high gas flow blast tunnel or spiral system; inject the cryogen into either a liquid or finely ground product as it is being mixed or blended; inject the liquid cryogen into a pneumatically conveyed product stream; spray LCO2 onto the product as "snow"; or a combination of these methods.
Immersion systems that do not have a post-cool tunnel to recover sensible heat will achieve efficiencies of only 60 to 80 BTU/lb of cryogen. Either a tunnel system or a combination of immersion with a post-cool tunnel will achieve efficiencies of approximately 100 BTU/lb. If the post-cool is sufficiently long, it is possible to get as much as an additional 15 BTU/lb. LCO2 snow systems also will achieve 100 BTU/lb. For bottom injection, it is possible to get 115 BTU/lb of LCO2.
Why Go Cryogenic?Cryogenic process control systems often offer a small footprint compared to other cooling methods. Use of an immersion system for a moist food item can result in freezing to an equilibrated temperature of -10oF (-23oC) in as little as 25 sec in a footprint of 11 by 6'. The immersion unit is typically a conveyor, some fans, a control system and some insulation. These types of units can run for years with minimal maintenance. This same moist-food application could require more than two hours in a mechanical system of considerably greater mechanical complexity and more than four times the total footprint under roof.
When a mechanical system already exists in a plant, it probably will prove to be the most economical choice for controlling process temperatures. However, where mechanical systems do not exist, a cryogenic system may be the most attractive.
Cryogenic cooling is worth consideration for any process temperature control application. For those applications that offer economic or practical incentive to use cryogens, cryogenic cooling methods may allow an end-user to install a process line with shorter lead time to startup at much lower capital investment and achieve control of process temperatures in seconds or minutes rather than hours. PCE
Sidebar:Much process cooling equipment in the field today has been running for 10 to 15 years. Often, prospective buyers opt for refurbished used equipment, seeing little advantage to new construction over older units. However, there now are greater regulatory and financial impetuses for improvements in process control than have been seen for some years, and they affect both equipment makers and end-users.
Multiple Forces Pressuring Equipment Updates
The increased awareness of food-borne pathogens such as Listeria, E. coli, H0157 and mad cow disease, and the resultant liability associated with any such outbreak, are tightening process control in the food industry. Increases in metal and energy costs in the global market also are pushing control enhancements in many process industries. The impact of ever-increasing security concerns also is likely to create opportunities for improvements in process control.
After many years of incremental but minor improvements in cryogenic equipment, some substantial developments currently are being introduced. One example is an immersion freezer design that uses a tightly woven polyester belt rather than a steel mesh chain as the conveyor. With the polyester belt, small particles are not lost, nor are soft products marked by chain impressions. In addition, the LIN bath in the polyester belt unit requires a smaller volume of LIN, which can improve efficiency and minimize LIN loss.