Although most liquid nitrogen-based equipment will specify pressure and flow requirements at which the user should set their source tank, there are still several common processes that do not specify optimal source pressure. Processes such as filling pressurized cylinders, charging vapor shippers or hooking to automated freezers are among those where users often lose efficiency due to incorrect source pressure.
Liquid nitrogen is at its coldest state at atmospheric pressure, when it is approximately -320°F (-196°C). Higher pressure LN2 is warmer, and less saturated, as LN2 at 100 psi is already warmer than -280°F (138°C). When liquid nitrogen is dispensed from a tank at 100 psi and brought into an already chilled dewar, as much as 30 percent of the nitrogen has flashed off in pressure reduction alone. By contrast, that loss drops to approximately 7 percent when filled from a 22 psi tank.
Medium Pressure Applications
Filling a pressurized cylinder is a process for which users often have a hard time nailing down the correct parameters. Most industries find that a fill pressure of approximately 30 psi higher than the set tank pressure provides an optimal range of fill time versus pressure losses. Trying to fill a 22 psi dewar at 22 psi may result in very long fill times. Likewise, trying to fill much higher than 30 psi over the setpoint will result in filling the cylinder with a low saturation of liquid nitrogen, and your cylinder will appear much less filled when the liquid settles.
Many cylinder fillers will manually open valves to fill cylinders, leaving the vent side wide open. While this is acceptable, and some automated systems work on this very principle, others find that putting a back-pressure regulator on the vent side of the cylinder will reduce losses due to pressure even further. They will set the back-pressure regulator to the set pressure of the tank (often 22 psi for low pressure applications).
Low Pressure Applications
Filling an open-mouth dewar is one of the simplest tasks to perform with liquid nitrogen. It should be done with as low pressure as possible to reduce loss due to pressure reduction but still fill the flask in a reasonable amount of time.
Although automated freezers are also non-pressurized dewars, freezers are typically much larger and, therefore, would require a minimum of 18 to 22 psi to fill at a rate deemed acceptable to facility managers. Filling at pressures lower than this will increase the fill time dramatically, which can also be inefficient due to heat input. The longer the fill process takes, the longer the liquid nitrogen flows through the uninsulated components of the fill process, allowing for more ambient heat input into the liquid nitrogen fill. Most automated freezers are sold with 50 or 100 psi safety relief devices, but the manuals will state that optimal fill pressure is 18 to 22 psi.
Freezers can be filled at higher pressures under certain circumstances. For example, if the facility has several floors of liquid nitrogen piping on a bulk system, the bulk tank may be set at 45 psi to attain at least 22 psi on the top floors while leaving the bottom floors with the 45 psi seen at the bulk tank. This can be acceptable; however, the freezer user should check with the freezer manufacturer to find the pressure rating on the solenoids that are being used. The pressure rating should be at a minimum higher than the safety relief device installed, but it is still a good idea to check.
Filling a freezer at a higher pressure may finish faster; however, the liquid introduced is at a slightly lower density, and users may or may not see a slight drop in the liquid level when the fill stops and the liquid nitrogen settles at atmospheric pressure. This is not necessarily a problem as long as the user is aware of these effects because sometimes these higher pressure fills are necessary when the situation arises.
Adjusting Liquid Nitrogen Pressure
Many users have liquid nitrogen bulk tanks that are at a high pressure because they are supplying high pressure nitrogen gas, not liquid, to their building. Trying to pull liquid nitrogen off of these tanks becomes trickier because one cannot put a simple regulator on the liquid use side to bring down pressure. A vacuum-jacketed piece of equipment, sometimes called an adjustable pressure phase separator (APPS), is required to correctly bring a high pressure liquid to a low pressure liquid. While this equipment is very effective and reliable, there will still be nitrogen losses when lowering pressure. The most efficient way to supply low pressure liquid to a building is to have separate tanks for liquid and gas use.
However, because not all facilities can have separate tanks, the user needs to decide if a pressure changing system is right for the application. While the adjustable-pressure phase separator is effective at extracting low pressure liquid from a high pressure tank, a system can be installed to build high pressure gas from a low pressure liquid tank. Every facility needs to consider their specific factors, including liquid usage, gas usage, pressure requirements and cost of different volumes of liquid, to correctly determine what the best method for delivery is for them.
All scenarios have different parameters that need to be considered when designing a liquid nitrogen supply system. There are multiple ways to go about setting up a delivery system; therefore, the best way to decide how to proceed is to first contact your local cryogenic pipe and equipment shop to set up a consultation.