Can You Use a 9kg Gas Bottle Instead of a 45kg?
Often asked: “Can I use a small gas cylinder or bottle instead of a bigger one?”
Another common question is on the use of a 9kg gas bottle instead of a 45kg.
Yet other queries relates to using a 2kg, or other small gas bottle, instead of a 9kg on a standard size BBQ.
Let’s look at the potential issues of downsizing the gas bottle…
The Answer is “It Depends…”
There are a number of factors that can affect the answer:
What is the gas load of the appliance or appliances you are trying to power?
How full is the gas bottle?
What is the ambient temperature?
Is the gas in the bottle propane, butane or a mixture of the two?
Is the installation safe for a 9kg?
All these affect the rate of vaporisation or safety.
Vaporisation is how fast the liquid LPG can turn into gas, inside the bottle.
The LPG (propane and/or butane) turns from liquid to gas when it boils.
This is called vaporisation.
To boil, the liquid LPG draws heat from the steel walls of the gas bottle which, in turn, get heat from the ambient air.
LPG liquid boils and turns back into gas vapour when you release some of the pressure in the gas bottle by turning on your gas appliance.
As with water, the more heat applied, the more rapidly it boils, vaporising at a faster rate.
Vaporisation Must Match Consumption
The amount of gas that the appliance or appliances are drawing from the gas bottle must be matched by the rate of vaporisation.
Gas fitters use vaporisation tables for proper cylinder selection.
These match the required vaporisation rates to the corresponding vessel size.
Vaporisation tables show the maximum continuous vaporisation rates, in MJ/hr or BTU/hr.
This is calculated at different ambient temperatures for each available vessel size.
Gas Bottle Size Matters
The size of the gas bottle makes a difference.
Switching to a smaller vessel results in a lower rate of vaporisation.
Heat is absorbed through the vessel shell and into the liquid.
This area of liquid to steel contact is known as the “wetted area”.
The larger the tank or the fuller the tank, the larger the wetted area.
This means more gas that can be vaporised at a given temperature.
Lower Fill Equals Less Vaporisation
Keeping in mind the concept of "wetted area", the maximum rate of vaporisation drops as the fill level of the bottle drops.
If there is less LPG in the vessel, there is less contact area between the liquid LPG and the steel.
This reduces the available heat for vaporisation.
For example, if the bottle is half empty, the vaporisation rate will be roughly halved.
If the consumption rate of the attached appliances is low, this may make no difference at all.
However, if the consumption rate is high, the vaporisation rate may not keep pace with the consumption.
It may work fine when full but then have problems as the level decreases.
Depending on the appliance, this starvation may cause the appliance to function poorly or not at all.
Temperature Also Affects Vaporisation
If the ambient temperature drops, it decreases the amount of heat that the steel can absorb.
This, in turn, means that the liquid LPG has less heat transferred to it by the steel.
Less heat results in a lower rate of vaporisation.
If vaporisation and consumption were already closely matched, this could be problematic.
A decrease in heat means that the vaporisation rate may not keep pace with the consumption.
Propane vs Butane
Propane and butane have different boiling points — the temperature at which it goes from liquid to gas (vapour).
Propane boils at -42°C whereas butane boils at -0.4°C.
This means you have a huge problem if you try to use pure butane when the temperature drops below freezing.
No Boiling = No Vapourisation = No Gas
So, with butane only, you could find yourself with no gas for your heater and cooking appliances when it gets cold.
In some areas, LPG suppliers provide a mixture of propane and butane to address this problem.
This can work well when there are temperatures both below and above freezing.
However, the mixture in the cylinder can become butane rich if there is too much cold weather.
This is because only the propane is vaporising.
Needless to say, propane is the preferred choice for cold weather climates.
This is not an issue in either Australia or the US, as both countries have propane only supplied.
Important LPG Safety Warning Regarding Using 9kg in Lieu of 45kg Gas Bottles
We do not recommend connecting a 9kg gas bottlein place of a 45kg gas bottle, if the installation was designed for a 45kg gas bottle. There are a number of safely issues that can affect both your personal safety and the proper operation of your gas appliance.
9kg LPG Gas Bottle Safety Issues
The following potential safety hazards will exist of you try to use a 9kg gas bottle in an LPG installation designed for a 45kg gas bottle:
The 9kg gas bottle could end up hanging from its supply hose.
The 9kg gas bottle has no place to sit upright and level
The 9kg gas bottle is not secured and may fall.
The 9kg gas bottle was not leak-tested after connecting.
The 9kg gas bottle may not be grounded, creating an ignition hazard. Plastic milk crates are a good example of a non-grounded support.
It must not be in an unstable position where it could fall and start leaking or be damaged
9kg Instead of 45kg?
In addition to the safety issues, a 9kg gas bottle only vaporises at a rate of about 19MJ/hr when 80% full at an ambient temperature of 20°C.
In winter, at 0°C, the rate would drop to around 12MJ/hr.
Then, as the contents are used and you get down to 40% remaining, the vaporisation output would be about 6MJ/hr.
As the level gets even lower, the output will continue to drop.
Now compare this to the consumption of common gas appliances:
♦ 26L continuous flow hot water = 199MJ/hr
♦ Most common size indoor gas heater = 25MJ/hr
♦ Typical cooktop burners = 6 to 13MJ/hr (each)
So, with the exception of the smallest cooktop burners, in the scenario above you are going to be starving the appliance for gas.
In most cases, they will either function very poorly or simply stop working.
It is important to stay safe and have the right size of gas vessel to provide the required rate of vaporisation,
This is especially true if you have more than one appliance or if you have appliances with a high MJ/hr or BTU/hr rating.
It is also critical in colder weather or if you are supplied with LPG that contains butane.
The use of undersized vessels can cause significant problems.
Using a larger gas bottle – cylinder – helps ensure the proper operation of your appliances.
The best advice is to go with the original recommendation of your gas fitter.
Comments, questions or feedback?
The information in this article is derived from various sources and is believed to be correct at the time of publication. However, the information may not be error free and may not be applicable in all circumstances.