Considering that most cryogenic systems use temperature differences approaching 400oF (222oC), it is easy for heat to sneak into the system. Any heat that transfers to the cryogenic fluid reduces the system's efficiency and increases the cost of operation for that process. These higher costs can present themselves in higher liquid/gas consumption, slower processing speeds or lower yields. These heat losses affect the gas producer that operates the air separation/liquefaction plant as well as the firm consuming the liquid/gas as part of their manufacturing or testing process. The entire cryogenic value chain needs to defend itself from heat leak.
The Big PictureWhen evaluating a cryogenic system, companies tend to focus on the large portions of the system. For example, it is easy to see how investing in a high-quality bulk storage tank can significantly reduce the overall heat leakage of the system. Today's tanks with composite super-insulation provide considerable thermal efficiencies compared to the older perlite insulation technology (figure 1). Depending on the size and age of the tank, a perlite-insulated tank might have twice as much heat leak as the same size composite super-insulated model. Further, perlite tanks have a tendency for the insulation material to settle within the annular space, leaving voids in various areas of the tank that become local areas of increased heat transfer. A composite super-insulated tank does not have these insulation-settling issues.
It also is a good idea to have the vacuum level checked on any bulk tank that is older than five years. The performance of vacuum-insulated systems is highly dependent on the vacuum level. If the vacuum level is compromised, the heat leak rate will be greatly increased.
The next large portion of many cryogenic systems is the piping system that carries the fluid from the storage tank to the point of application. The three main insulation techniques used for cryogenic piping systems are foam insulation, dynamic vacuum and static vacuum. Foam-insulated pipe is by far the least efficient of the three and becomes even less efficient over time. Dynamic vacuum insulated pipe uses a vacuum pump installed with the pipe system to continuously pump the annular space to maintain a vacuum. Dynamic vacuum systems are significantly more efficient than foam-insulated systems; however, they have significant operating cost and reliability issues. Static vacuum insulated systems have lower operating costs because they do not include the vacuum pump and its operating costs (figure 2).
Don't Overlook the DetailsMost plants are aware of the fundamental issues in cryogenic systems as they relate to the bulk tank and piping system design and insulation technologies. It is easy to consider the importance of the insulation system on a 6,000 gal tank, or 150' of pipe. Unfortunately, many facilities tend to ignore details like the connection of the pipe system to the storage tank, or the connection of the pipe system to the end use equipment. Shouldn't the 80/20 rule apply to cryogenic systems like it does to most other applications? After all, as long as the overall tank and pipe system are well insulated, how much harm can be done in just a couple feet of uninsulated pipe? The answers are simple -- the 80/20 rule does not apply, and plenty of harm can be done in just a couple feet of uninsulated pipe.
Due to the extreme temperature differences in a cryogenic system (liquid nitrogen is often near -300oF [-184oC]), a large amount of heat will transfer quickly at any location that is not extremely well insulated. These locations are often easy to identify by the presence of ice, frost or condensation (sweating). Typically the connections (tank to pipe system, old pipe system to new pipe system, pipe system to end use equipment, etc.) are the most common locations neglected. This oversight often occurs because one firm might have ownership of the bulk tank while another firm has ownership of the pipe system, and neither firm wants to take ownership of the interface or connection. As a result, the connection is made in a short period of time with readily available materials that are considered “good enough,” rather than being carefully selected and integrally designed for optimal efficiency.
Any heat that enters the system at the tank-to-pipe-system connection is carried through to the application at the end of the pipe system. The heat influx at the beginning of the system can cause local boiling and two-phase liquid/gas flow. Two-phase flow will create significantly higher pressure drops through the pipe system, irregular liquid delivery, warmer liquid, increased wear on soft goods such as valve seats, and other system complications.
Vacuum-insulated withdrawal lines have been available on bulk storage tanks (1,500 to 15,000 gal) for many years; however, end users are not likely to have this feature on their tanks unless they make a specific request to their gas supplier. As the concept of microbulk systems (1,000 to 3,000 liter capacity) becomes increasingly popular with smaller companies, these tanks now are also offered with vacuum-insulated withdrawals.
A Complete System PerspectiveUnless someone takes accountability for the complete cryogenic system design and performance, multiple items can easily be overlooked or neglected. Due to the extreme temperatures in cryogenics, a slight oversight in the design or installation can create a major inefficiency in the system's operation. One of the worst places, and unfortunately the most common, to have such an oversight is the connection of the vacuum-insulated pipe system to the liquid supply tank. If this connection is not all vacuum insulated, the heat transferred into the system at this location can be greater than the heat leak from the rest of the pipe system.
With today's high energy costs, any new cryogenic system should be evaluated and designed from a complete system perspective to obtain the most efficient system for the long-term lowest cost of operation. Any existing cryogenic systems should be reviewed for opportunities to improve system efficiency and lower operating costs. Particular attention should be given to the interfaces of various portions of the system. In the course of reviewing the system, leave no ice ball unturned. PCE