- Cryogenic fluid flow rate.
- Fluid type.
- Usage pattern or cycle time of the cooling application.
- Required liquid quality.
- Distance from the liquid storage point to the use point.
Cryogenic applications can be segmented into four general categories: high flow continuous operation, low flow continuous operation, high flow intermittent operation and low flow intermittent operation. To determine each category's most efficient insulating system, the designer must calculate the system's total cooling losses. This total includes the initial cooldown losses to chill the process pipe to cryogenic temperature plus the steady-state heat leak into the piping system.
Cooldown Loss ComparisonCooldown losses result from cooling the pipe and some insulation from ambient temperature to -300°F (-184°C) or colder. Each time the pipe is allowed to warm up, cooldown losses will occur during the next cycle when the cryogenic fluid again is introduced into the pipe. In systems that operate continuously (24 hr per day, five days per week), the pipe stays cold and, therefore, cooldown losses are negligible. However, in systems that operate intermittently (8 hr per day, five days per week), the piping system will warm up during off hours and then must be cooled down when the process is restarted. These cooldown losses may contribute significantly to the system's total coolant loss.
Urethane foam insulation on copper tubing (Type K) and vacuum-jacketed pipe (Schedule 5 stainless steel) are two principal cryogenic pipe insulating systems. Usually, urethane foam is 3" thick and surrounded by a PVC outer jacket. In a 1" process line, this type of system uses approximately 0.83 lb of liquid nitrogen per linear ft to cool the pipeline from ambient to -300°F (-184°C). A similar size vacuum-insulated pipeline requires 0.43 lb of liquid nitrogen per linear ft. A typical cryogenic piping installation is about 200' of piping. A 1" foam-insulated copper line requires 166 lb of liquid nitrogen to cool the line to cryogenic temperature. The cost is about $6.64 per cooldown, based upon a liquid nitrogen cost of $.04 per lb. In comparison, a vacuum-insulated pipeline requires only 86 lb of liquid nitrogen and costs $3.44 per cooldown.
A continuous use process might shutdown once per week, resulting in 52 cooldown cycles per year. By contrast, an intermittent use process likely will shutdown at least once per day, resulting in more than 250 cooldown cycles per year. The resultant costs for cooldown losses are $345 per year for the foam-insulated pipe operating on a continuous basis and $1,660 per year for the same design operating intermittently. The cooldown losses for vacuum-insulated pipe are $179 per year for continuous operation and $860 per year for the intermittent application.
Another factor contributing to cryogenic coolant loss is the steady-state heat leak through the insulating system into the cryogenic fluid. The heat leak rate through a 1" copper line with 3" of foam insulation is 20 BTU/ft-hr; the heat leak at a joint is approximately the same as in a straight run of pipe.
For a vacuum-insulated pipe, the heat leak is 0.45 BTU/ft-hr for straight sections and 9.1 BTU/hr per joint. In prefabricated static vacuum-insulated pipe, the joints typically are made using vacuum-insulated bayonets. These joints have a higher heat leak than straight sections of vacuum-insulated pipe. A typical 200' long pipe run might have 10 joints. The steady-state heat leak for 200' of foam-insulated pipe is 4,000 BTU/hr and costs $1.87 per hour in liquid nitrogen loss; the same vacuum-insulated pipe has a heat leak of 181 BTU/hr and costs $0.08 per hour in coolant loss.
The total heat leak or coolant loss is a combination of the cooldown loss plus the steady-state heat leak. The foam-insulated line costs $12,000 per year when operated continuously and $5,500 per year when operated intermittently. Vacuum-insulated pipe reduces the operating cost due to heat leak to $700 per year for a continuous operation and $1,000 per year when run intermittently.
When operating a cryogenic system and looking at the heat leak costs, the operating cost advantages of vacuum-insulated piping are obvious. However, a less obvious conclusion is that it can be less expensive to keep the vacuum-insulated piping system continuously cooled down and filled with cryogen ($700 per year) than to operate on an intermittent basis and allow the pipe to warm up between uses ($1,000 per year).
Total Cost ComparisonFinally, to complete the foam insulation vs. vacuum insulation cost comparison, you must factor in the initial cost to install the pipe. Vacuum-insulated pipe typically costs two times the installed cost of foam-insulated copper. If you use representative numbers of $50 per ft for foam and $100 per ft for vacuum-insulated pipe, then the installed costs of your 200' pipe run are $10,000 and $20,000 respectively. The total cost for the pipe is equal to the initial installed cost of the pipe plus the annual operating cost due to heat leak. Table 1 shows the total system cost during the first five years of operation.
The operating cost due to heat leak for foam systems becomes even higher than indicated in table 1 as the foam begins to degrade. In almost all cases, vacuum-insulated piping systems pay for themselves in two to three years. In many installations - particularly those systems that operate continuously - the payback is six to 12 months.
The right piping system choice ultimately depends upon the user's financial position. If adequate capital funds are available to afford installing vacuum-insulated pipe, then it is usually the best choice. If capital funding is short, then the best choice is the less expensive foam-insulated pipe. The worst choice, of course, is uninsulated pipe, which carries the highest operating cost due to heat leak and which can cause frost burns if operating personnel contact the exposed pipe. By comparing cooldown losses, steady-state heat leak and initial installation costs with your available capital, you will be able to choose the system best fitted to your situation.