Determining the best method for cryogenic liquid supply begins with an analysis of the volume of liquid required for a particular system. For example, a single cryogenic freezer in a small laboratory most likely should be fed from a low pressure portable liquid cylinder connected with flexible hose. As additional freezers are in-stalled and more liquid is required, however, liquid cylinders may be inadequate to handle the increased capacity.
Larger cryogenic systems often require a bulk-storage station. Due to space limitations, bulk-storage stations typically are located outside the facility and connected via cryogenic piping. The transition from liquid cylinders to bulk storage supply is driven by the economics of liquid costs, delivery, safety and convenience.
Whether it is the economic advantage of bulk-liquid supply, employee safety considerations, the labor reduction associated with not having to handle liquid cylinders, or simply the convenience of having cryogenic liquid at the turn of a handle, many companies transition from cylinder-fed systems to the bulk storage approach. With the number of these bulk-storage stations increasing, many designers of cryogenic systems are asking: "What is the best way to supply cryogenic liquid from the bulk-storage tank to the point of use?"
Cryogenic liquids have a normal boiling point far below room temperature (e.g., nitrogen is -320°F [-195.5°C]); thus, they are boiling constantly. Any heat that passes through the piping's insulation results in irreversible losses. In general, more efficient - and expensive - insulation systems have higher heat leak resistance. Selection of cryogenic piping largely is based on the cost trade-off associated with a high or low heat leak rate.
3 Piping ChoicesThe basic styles of insulated pipe used today are urethane foam-insulated, dynamic vacuum-insulated and static vacuum-insulated.
Urethane Foam-Insulated Piping. Also known as foam pipe, urethane foam-insulated pipe (figure 1) requires the least amount of initial expense. This can be attributed to the low cost materials as well as a relatively simple manufacturing process. Also, foam pipe is produced in standardized lengths, which lends itself to an automated manufacturing process, and it does not require welding or the evacuation of annular space. Standard sizes and components are available from stock with short lead times.
Despite the lower initial costs of foam pipe, liquid nitrogen operating costs are high due to the heat leak rate. Also, because of moisture intrusion into the foam, the heat leak rate will increase every year the pipe is in service. In general, foam pipe has an expected lifetime of five to seven years.
With dynamic vacuum-insulated piping, the heat leak rate is considerably lower than foam pipe. This allows for lower liquid nitrogen operating costs and a longer expected life of 10 to 20 years. Dynamic vacuum piping and components typically are kept in stock at standardized lengths. This allows for short lead times and flexibility when designing and installing the system.
Static vacuum piping has the least amount of heat leak (as much as 40 times less than foam pipe), resulting in the lowest liquid nitrogen operating costs. The expected lifetime is 10 to 20 years, and it does not require routine maintenance.
Calculating Real CostAs any cryogenic piping system is a sizeable investment, it makes sense to calculate the real cost rather than just consider the initial installed costs. To measure real costs, you must include:
- Initial Investment - cost of entire piping system and components, including installation and production down time.
- Operating Liquid Nitrogen Costs - expense of liquid nitrogen that is lost due to heat leak into the piping system.
- Maintenance Costs - costs associated with routine upkeep of the piping.
- Lifetime of System - expected lifetime for a cryogenic piping system before it needs to be repaired or replaced.
The impact of these costs should be assessed. A cost savings calculation is one way to compare the operating liquid nitrogen costs of cryogenic piping systems.
While economics are a big part of the selection criteria for cryogenic piping, other design criteria - tank location, application cycle time, liquid quality and piping size - also are important considerations in the selection process.
Tank Location. Ideally, the tank would be located right next to the point of use. Often, this is impractical due to factors such as available space, access for supply tankers, building and safety codes, and even aesthetic considerations. As the distance from the tank to the point of use increases, the initial piping cost increases, and potential for heat leak becomes even more critical.
Application Cycle Time. Cycle time, or the frequency at which liquid must be supplied at the use points, plays an important role in the efficiency of any cryogenic system. Will cryogenic liquids be used monthly, weekly, daily, hourly or continuously? When a cryogenic liquid first is introduced into a warm piping system, the liquid vaporizes and creates gas. This process of cooling the pipe until it is full of liquid can take from 10 min to 1 hr, depending on the size and length of the piping and the rate at which gas is vented away. Gas vented from the piping is an unrecoverable loss.
When the pipe is left unused for a period of time, the piping will warm up at a rate equal to the heat leak rate. The higher the piping's heat leak rate, the faster it will warm up and the more liquid will be required to recool it. Urethane foam-insulated piping will warm up in 2 to 4 hr while static vacuum-insulated pipe will remain cold 24 hr.
Liquid Quality. This is defined as the ratio of liquid to gas supplied at the use point. The higher the percentage of liquid, the higher the liquid quality. High liquid quality is important for smooth fluid flow through the piping system and proper equipment operation. Nitrogen expands 700 times in volume as it changes phase from liquid to gas. When a high percentage of gas is present in the piping, the gas will displace the liquid and reduce the piping's liquid flow capacity. Poor liquid quality also causes delays in the liquid supply because the gas must be vented prior to liquid use.
Certain types of equipment such as environmental chambers require high quality liquid on-demand for proper performance. One way to ensure a quick response is by installing a keep-full device on the piping system. A keep-full operates by removing the boil-off gas through a vent valve in the piping. With this device, the piping line remains full of liquid ready for immediate use. Keep in mind that when a keep-full is employed, heat leak calculations must be based on 24-hr/day usage, as any gas that forms will be vented.
Piping Size. This depends on your anticipated flow requirements at each use point. Generally, cryogenic piping is sized for relatively low flow velocities to reduce pressure drop. The size of the pipe should be based on current and possible future flow requirements.
Each of these design criteria plays a part in the proper selection of the cryogenic piping system. It is important to know the benefits and limitations of each type of piping. Most importantly, work with an experienced cryogenic equipment supplier that will help you analyze your requirements, size your piping and provide a cost-effective design.