Considerations for Using Liquid Nitrogen
Installing Liquid Nitrogen Chambers For The First Time
Here are some facts and tips you may find useful if you are installing Liquid Nitrogen chambers in the workplace for the first time.
Facts.
- Liquid Nitrogen (LN2) is liquid because it is cold and not because it is under pressure.
- It has a boiling point of -196°C and has an expansion ratio from liquid to gas of 1 : 682.
- As a gas it is odourless.
- As a gas/vapour it is heavier than air when cold but lighter than air at room temperature.
- In major installations LN2 is stored externally to the building in insulated vessels at low pressure (commonly at 2 Bar) and piped to the chamber(s) throughSIVL (Super Insulated Vacuum Line).
Safety - Contact your LN2 supplier for Safety Advice
As with all refrigerant gases you need to observe certain precautions. Unlike some compressor refrigerants however, LN2 is not toxic, not carcinogenic, can be released freely into the atmosphere and is not restricted in any way by the Montreal Protocol.
Hazards :
As a refrigerated liquefied gas, contact with the product may cause cold burns or frostbite. In high concentrations it may cause asphyxiation due to Oxygen depletion.
Consider these easily overlooked points when doing your risk assessment :
Chamber exhausts should be connected to the outside atmosphere and should not be linked to each other.This is because Nitrogen gas from one chamber may displace the Oxygen in another chamber that may have personnel working in it.
A storage vessel containing 20,000 Litres of LN2 can deposit 13,600m3 of Nitrogen into the workplace. In worst-case conditions this is enough to create an Oxygen depletion risk in an area of 100m x 100m.
Clean Room "make up air" is 80% Nitrogen therefore the Oxygen depletion risk may be higher than in an open factory.
Does the design of your clean room air distribution system give potential for depletion in areas that do not have nitrogen facilities installed?
Costs
LN2 chambers are less expensive to buy than compressor versions.
LN2 chambers are less expensive to maintain than compressor versions.
LN2 chamber spares are less expensive to buy and less bulky to store than spares for compressor versions.
LN2 is a more expensive source of energy than electricity.
LN2 chambers provide rapid heat transfer capability resulting from high convection coefficients, which are not achievable by compressor versions. This capability enables dwells to be minimised, thus saving the cost of heating and cooling the chamber lining material.
LN2 chambers can be more expensive to install due to the storage vessel, exhaust, Oxygen depletion monitoring equipment and SIVL.
In use or not, LN2 systems accrue some costs through facility charges, SIVL maintenance contracts and losses.
LN2 costs vary with the volume you use and can be high, particularly if you are a low volume user in a remote area and delivery transport incurs ferry charges etc. In this case you may benefit from a larger storage capability than the same volume user who is close to a supply depot.
Environment
LN2 is recyclable and to that extent is "GREEN".
LN2 chambers have a smaller footprint and therefore take up less space.
LN2 chambers do not need cooling towers and therefore eliminate the risk of Legionnaires Disease.
LN2 requires a lot of energy to produce but is essentially a by-product of the production of Oxygen. Nevertheless you should use it economically by effective screen process management and by maintaining your equipment and supply pipework in good order.
LN2 chambers are quiet in operation.
LN2 deliveries can be very noisy. Consider this when planning your vessel location close to residential dwellings. If this is a problem restrict delivery times and review vessel size accordingly.
Compressor cooled chambers usually have remote condensers or water towers which are noisy most of the time the chambers are running.
Supply Facts
SIVL is a fixed installation and is expensive to re-route.
SIVL to chamber connections frost up and produce puddles of water.
SIVL that has lost vacuum will collect condensation and ice.
A filter is essential at each chamber outlet.
The high pressure Nitrogen supply you may have for inert gas processes is not suitable for chambers.
LN2 deliveries can be very noisy. Consider this when planning your vessel location close to residential dwellings. If this is a problem restrict delivery times and review vessel size accordingly.
Compressor cooled chambers usually have remote condensers or water towers which are noisy most of the time the chambers are running.
Generating your own LN2 is not viable except under extraordinary production circumstances. I.e. Currently in the UK you would have to commit to a 10 to 15 year facility rental and be confident that you could maintain usage at 3,000,000 litres/year minimum.
When new SIVL has been installed or an existing SIVL altered there will be a build up of oxide particles within the SIVL pipe work. These need to be flushed out before connection to the chamber, otherwise chamber cooling solenoid valves will intermittently stick open over a period of several months. Flushing needs to be done by a "competent person" with the chamber disconnected and the SIVL delivering liquid (not gas) Nitrogen for at least 5 minutes under controlled conditions. The SIVL should be allowed to warm up and the process repeated.