Cooling with Liquid Nitrogen
Done properly, a cryogenic cooling system provides high quality, low cost cooling.
Essentially, there is a choice between mechanical and cryogenic cooling. The basic difference between mechanical cooling and cryogenic cooling is that mechanical cooling systems use a closed-loop system that recovers and reuses the coolant. Traditional cryogenic cooling systems use an open system that expels the spent vapor into the atmosphere.
Mechanical Cooling Systems
With a mechanically cooled system, the equipment for recompression and cooling of the spent coolant vapor — in order to convert it back to a reusable liquid — is part of the system. The system includes at least one compressor, two heat exchanger coils, a fan and several process controls. These systems are somewhat complicated and relatively expensive. Also, mechanical cooling systems are limited by the capacity of the compressors, which are, in turn, limited by available power, space and tolerance for noise and heat.
Mechanical refrigeration can be advantageous over expendable gas for cooling applications where expendable liquid nitrogen or liquid carbon dioxide is not readily available for long periods of cooling. Also, mechanical refrigeration can be advantageous where the time required to ramp down to temperature is not critical. Finally, mechanical refrigeration may be the only choice for clean environments and for areas where the spent CO2 or N2 exhaust gas could pose a risk to workers.
The amount of heat that can be removed by a mechanical cooling system is related to the difference between the current temperature of the chamber and the evaporator. The performance of the chamber will degrade substantially as the limits of the system are approached. Therefore, unless one has a great deal of time for a ramp down, using a single-stage refrigeration system to achieve -31°F (-35°C) in a midsize chamber will be possible but tedious.
Cryogenic Cooling Systems
Cryogenic cooling systems leave the task of compressing and liquefying the coolant to the coolant supplier. One simply buys the coolant, uses it and discards it into the atmosphere. Generally, from the user’s perspective, the system is simpler, faster and often better performing.
Cryogenic cooling can deliver extremely low temperatures should they be necessary. Liquid nitrogen can reach near -320°F (-195°C). By contrast, single-stage mechanical refrigeration has temperature limits around -65°F (-53°C).
It is worth noting, however, that cryogenic coolants pose certain health risks that are easily tolerated but important to consider. Neither liquid carbon dioxide (LCO2) nor liquid nitrogen (LN2) is poisonous to humans, but both have the potential to displace breathable oxygen in a closed room. While excessive quantities of carbon dioxide (CO2) will usually — but not always — manifest with signs of headache or dizziness, nitrogen (N2) makes up 78 percent of the Earth’s atmosphere, and excessive concentrations that displace oxygen may accumulate without warning. Any device that uses expendable CO2 or N2 must be vented to the outside atmosphere or be used in a well-ventilated area with an oxygen monitor.
These deficiencies aside, cryogenic cooling has advantages over mechanical cooling. When plant layouts change, it typically is less costly to relocate the end of a flexible vacuum-insulated hose than to relocate a mechanical freezer. Cryogenic cooling is the better choice for applications that require fast downward temperature transitions or very low temperatures (below the limits of mechanical refrigeration). If low temperatures are not held for long periods, cryogenic cooling can be cost effective.
Cryogenic cooling is simple to use, and cryogenic cooling chambers are less expensive to buy. However, users are cautioned to consider the costs of not only the cryogenic coolant used but also the cost of managing and changing the coolant tanks in facilities that do not have built-in cryogenic liquid coolant delivery systems. Long-period cooling can become expensive when done cryogenically.
How to Design a Cryogenic Cooling System for Industrial Processes
When designing cryogenic cooling systems, some common pitfalls befall many designers. But, if the designer is aware of these, steps can be taken to mitigate the negative results.
The most common cooling system simply delivers liquid nitrogen from a bulk storage tank to the point requiring the cooling activity. There, the liquid nitrogen or solid CO2 (dry ice) impinges on the work to be cooled. It evaporates or sublimes upon arrival by absorbing heat from its surroundings and from the workpiece, thereby lowering the workpiece temperature. In this scenario, the process uses the latent heat of vaporization of the liquid nitrogen or liquid carbon dioxide, which is the most efficient cooling process of a cryogen.
Other common systems cool by delivering low temperature vapor to surround the workpiece and rely on sensible heat to transfer heat energy from the workpiece and lower its temperature. This process is less efficient than using the latent heat of vaporization.
Many other types of delivery systems are used, depending upon the application. These include secondary cooling circuits and immersion freezers. LCO2 is most common in food cooling while LN2 is used in food cooling and in industrial applications such as environmental chambers, machining, metal treating and molecular beam epitaxy applications.
In many applications, it is important to control the workpiece or environment within close temperature tolerances. This can become a challenge for any cooling system where there are different items or products to be cooled or maintained at a specific temperature.
The best rapid temperature control is achieved by the delivery of high quality liquid cryogen, meaning a cryogenic liquid that is consistently very cold and dense. The nemesis of a cryogenic liquid is its tendency to absorb heat as it travels from the storage point through a pipe to the delivery point. It is important that a high quality, vacuum-insulated pipe is used to prevent significant heat absorption and the development of significant two-phase flowing liquid. Two-phase liquid can result in what is known as slug flow, which is liquid interspersed with slugs of warm vapor. When slug flow occurs, the mass flow rate is diminished, and piping vibration appears along with noise.
Foam-insulated pipe is attractive because of its low initial material cost, but it is disastrous when used to transfer a cryogen that must be consistently cold and dense. Foam-insulated pipe typically has a much higher heat leak than the vacuum-insulated pipe, resulting in far greater two-phase flow. This two-phase cryogen flow results in the inability to control temperature consistently and to achieve low temperatures.
Reasons to Add a Phase Separator Near the Cryogenic Delivery Point
To improve the ability of the cryogenic cooling process to deliver consistent temperature, the design engineer should consider the use of a phase separator near the cryogen delivery point. The phase separator removes vapor that was introduced as the cryogen traveled from the bulk storage tank toward to the point of use. The vapor is caused by a small amount of heat leak and pressure drop due to pipeline friction and sometimes elevation change.
Even the best piping cannot totally eliminate two-phase flow. By placing a phase separator near the delivery point, the vapor can be removed from the cryogen and vented away to the atmosphere, leaving high quality liquid to be delivered only a short distance to the use point.
Another point to be considered regarding the phase separator surfaces when the cooling process requires high volume flow to cool large masses with high heat capacity. To comprehend how this works requires a basic understanding of the typical phase separator.
The phase separator is simply a well-insulated vessel with liquid level and pressure control. Liquid level is maintained by sensing the liquid level in the vessel. It notifies an inlet valve when the level needs replenishing. Pressure inside the vessel usually is maintained by a pressure-regulator relief valve that may be set to the needs of the application. Many times, the phase separator is positioned above the delivery point of the cryogen and only head pressure is used to cause the liquid delivery. Internal pressure is maintained at atmosphere. This results in the coldest, densest liquid possible. To maintain the most consistent liquid temperature requires maintaining the pressure of the liquid as constant as possible. Remember, a cryogenic liquid changes temperature as its pressure changes.
Phase separators, by nature, experience a pressure rise when the inlet valve opens to replenish the liquid lost by delivery to the workpiece or cooling chamber. The inrush of liquid immediately begins vaporizing in order to release energy from the cryogen that was stored in a bulk tank at higher pressure. The vapor is released through a vent mechanism, but nonetheless, pressure temporarily rises during the refilling. This temporary rise in pressure — or pressure spike — results in a temporary pressure and temperature change of the cooling liquid at the delivery point. The higher the required flow rate, the greater and more frequent the pressure spikes occur due to more frequent filling of the vessel. Phase separators with high flow capacity require large inlet and vent valves with inevitably greater hysteresis than small components.
Some phase separators are designed to mitigate the phenomena described. Some companies offer phase separators with PLC keypad-adjustable, liquid-level high and low limits and pressure control. In cases where the application does not demand close tolerance temperature control, the liquid level limits can be set substantially apart, resulting in infrequent filling and exhausting of vapor. In cases where close tolerance pressure and temperature control are needed, the liquid level limits can be set close, resulting in more frequent filling but far less overall variation in output pressure and temperature.
Careful selection of components to fit the application and working with the component manufacturers to engineer the best possible cooling system ensures that a cryogenic cooling system will provide close tolerance, rapid response and ultra-low temperature cooling. Selecting the right components can prevent surprises at startup and result in long-term, low-maintenance system life. PC