Cryogenic Systems How to Dose Liquid Nitrogen

1 Cryogenic SystemsMost production facilities aresearching for ways to reducecosts. A small savings on thecost of a single container for afood or beverage packer quickly adds up totremendous savings, based on the numberof containers processed. Utilizing lighterweight containers or reducing utility costsare good savings methods. But lighter weight containers for noncar-bonated products can collapse whenstacked unless special handling require-ments are satisfied. When dosed into a con-tainer, Liquid Nitrogen will provide someinternal pressure, which allows the contain-ers to be stacked several pallets Nitrogen is one utility used in thefood and beverage industry to expel oxygenfrom products and increase shelf consumption can be reduced by asmuch as 80% using a Liquid Nitrogen dosingsystem instead of gaseous Nitrogen air is composed of 78% nitrogenby volume, Nitrogen is abundant.

2 Liquidnitrogen has a boiling point of -320 F (-196 C) at atmospheric pressure. Handlingliquid Nitrogen when pressurizing or inert-ing food and beverage containers on a pro-duction line poses challenges. To use liquidnitrogen injection, a production facilitymust have a storage vessel, Liquid nitrogenpiping and an injection device capable ofmetering small amounts of Liquid nitrogenaccurately and consistently. To store, trans-fer or inject Liquid Nitrogen , insulatedequipment is a necessity because liquidnitrogen will boil away rapidly whenexposed to room Systems and PipingStorage vessels generally come in twoforms: large bulk outside storage tanks andsmall portable storage vessels calleddewars. Both vessels use a double-wall con-struction creating a vacuum between thewalls. This vacuum jacket is an effectiveinsulation that allows the container s outsidesurface to be at nearly room temperatureyet hold the Liquid Nitrogen inside for longperiods of time.

3 While no insulation systemis perfect some heat leak is inevitablewith any storage container most storagevessel manufacturers report a loss rate rang-ing from to 2% per day. A piping systemfrom the tank to the use point is requiredfor large bulk tanks, but dewars need onlya small amount of pipe because they typi-cally are placed near the use point. Liquid Nitrogen piping must have someinsulation to effectively transfer the liquidnitrogen with minimal vaporization or are two types of Liquid Nitrogen pip-ing: vacuum jacketed and nonvacuum jack-eted. Vacuum-jacketed lines are more effi-cient than unjacketed lines and operatefrost-free. A vacuum jacket is an annulus positionedaround the inner Liquid Nitrogen pipe. TheHow to DoseLiquid NitrogenEffectivelyBefore adding a Liquid Nitrogen system to a food or beveragepackaging production line, many factors must be examined.

4 By John W. Ross, Vacuum Barrier bottles can be pressurized at speeds of 1,000 bottles/min. evacuated annulus reduces the conductionand convection heat losses to extremely lowlevels, creating efficient inner Liquid nitro-gen pipe insulation. Dynamically pumpedand static vacuum are types of vacuum-jack-eted lines. Static vacuum lines typically areevacuated by the manufacturer and sealedoff for vacuum integrity. This vacuum even-tually will degrade, resulting in increasedheat losses and decreased performanceover time. Dynamically pumped lines utilizea vacuum pump on the vacuum vacuum pump must run continuously,which adds slightly to operating , vacuum integrity will improvewith time and last for years. Jacketed lines can be either rigid or flex-ible pipes. Rigid pipes must be accuratelydimensioned for proper installation into afacility, whereas flexible lines are easier toinstall and allow more versatile routing.

5 Bycontrast, unjacketed lines typically are foaminsulated and have a heat loss as much as20 times greater than that of a jacketed also have a larger outside diameterthan jacketed lines. Foam-insulated linestypically lose their insulating qualities asthe foam degrades over time. Evaluating Production GoalsFood and beverage plant personnel shouldconsider a number of goals and criteriawhen selecting a Liquid Nitrogen system ,including: Consistent container pressures or consis-tent oxygen reduction. Plant personnel safety. Reliable system operation. Reasonable acquisition and operating costs. Each producer has different priorities foreach goal, but safety usually is high on thelist. It is important to remember that liquidnitrogen becomes a gas at room tempera-ture and expands to 700 times its volume asa Liquid . Adequate piping system and injec-tion equipment protection, including safetyrelief valves, must be used to prevent over-pressurization or equipment rupture.

6 Asafety relief valve must be placed betweenany two shutoff valves in the system . Onbulk tank-fed Systems , the lowest ratedrelief device typically is placed outdoors. Ifa safety relief valve does relieve, it is saferif it happens outdoors rather than insidewhere someone could get food and beverage producers aremore interested in the dosing device than inthe storage and piping system . Nitrogendosing equipment is the main componentof a Liquid Nitrogen system . For efficientoperation, Liquid Nitrogen injection equip-ment should be frost-free, safe and able toreliably meet the production facility s goalsfor pressure or inerting is important on a productionline where losses are calculated in minutesof downtime. A Liquid Nitrogen dosingdevice will need some startup time from aroom temperature condition because allinternal surfaces must be cooled down toliquid Nitrogen temperatures.

7 As with anyliquid Nitrogen equipment, operating pro-cedures must be adhered to because thedanger of contaminating the equipmentwith moisture does exist. Moisture is thebiggest enemy of the cold surfaces of liquidnitrogen equipment. It takes only a smallamount to freeze up the equipment inter-nally. A quality unit will work continuously(either idle or dosing) without being over-come with moisture or frost adjustments such as nozzlechanges for different container sizes andmaintenance must be able to be completedwithout moisture contamination or longCryogenic SystemsBoth portable dewars and bulk tank-fed Systems can be used with Nitrogen dosing of their size, portable dewars (left) must be refilled regularly, but bulk tank Systems (right) do high Nitrogen losses associated with uninsulated pipe are evident by frost (left). Vacuum-jacketed piping insulates the pipe carrying the Liquid Nitrogen , so frost does not Each production facility hasdifferent specifications for Liquid nitrogendelivery.

8 Some applications require that theliquid Nitrogen be delivered aseptically. Insuch a case, the dosing unit also must becapable of being pressurizing or inerting resultsare important to the entire operation. Awater bottle with too little pressure couldcollapse when stacked or not label proper-ly. A bottle with too much pressure possiblycould burst when stored in the trunk of a cardue to temperature effects. Inerted productscould oxidize or spoil if the Liquid nitrogendose was too small; too large a dose on aninerted product could cause an overpressur-ized container to jam the production injection can be accomplished bydosing individual containers or with a steadystream of Liquid Nitrogen . Either method canyield consistent results. Liquid Nitrogen will boil away rapidlyonce it is introduced into a container. There-fore, it is important to control the liquidnitrogen efficiently before dosing.

9 A typical18 fl oz (600 ml) polyethylene terephthalate(PET) bottle with a 1 fl oz (30 ml) headspaceand pressure specification of 17 psig willneed approximately oz ( g) ofliquid Nitrogen . The dose of Liquid nitrogenwill boil away and expand to fl oz( ml) of room temperature Nitrogen gasafter the container is sealed. Add fl ozof gas to a sealed volume of 1 fl oz, and youend up with 17 challenge for the Liquid nitrogendosing equipment manufacturer is to con-trol the boiling Liquid and deliver oz consistently at speeds from 40bottles/min to more than 1,000 dosing equipment can control the liq-uid Nitrogen up to the dosing point, but itcannot control the Liquid Nitrogen s behav-ior once it has been dosed into the con-tainer. The Liquid Nitrogen will boil awayrapidly as the container travels to the cap-per or seamer, so travel time should beminimized for accurate results.

10 The transferfrom dosing to capping also should besmooth to prevent the boiling Liquid frombouncing out of the container. Another aspect to consider is consistentcontainer fill levels. If container headspacevaries because the fill levels are wildly dif-ferent, the final bottle pressures also willbe wildly different. For example, supposethe bottle previously mentioned had an 18fl oz fill with a 1 fl oz headspace, and thenext bottle on the production line had a fillof fl oz (610 ml) with a fl oz (20ml) headspace. Both bottles receive oz charge of Liquid Nitrogen . Theliquid Nitrogen dosing is consistent; how-ever, in accordance with basic gas laws,the final bottle pressure on the 18 fl oz fillis 17 psig and the bottle with a fl ozfill has psig final pressure. Many fac-tors determine final bottle pressure accura-cy in addition to the dosing equipmentaccuracy.