Why does the LoW8 cylinder show a better Vapour Capacity compared to Glass Fibre Composites?
The vapour capacity of a LPG gas cylinder is an important performance indicator. For certain high LPG usage applications, e.g. a laundry dryer or big grills, a minimum supply of gas is required. The vapour capacity is generally expressed in Joules or mass per unit of time delivered by the LPG cylinder.
The LPG is stored in liquefied form (The L stands for Liquefied) to a maximum of 80% where the remaining space in the cylinder is filled by LPG in a gaseous state. Due to subtracting gaseous phase (by all kind of appliances like burners, stoves, BBQ) the equilibrium inside the cylinder needs to be balanced by evaporation of the liquid state. This process requires heat, which is extracted from the surroundings. This explains condensation or even ice on the outer surface of the cylinder exactly at the location of the liquid state of the LPG when high capacity appliances are connected and in use.
LPG is a mixture of propane and butane, with a boiling point of -42°C and 0°C respectively. Using a cylinder with propane, butane or a mixture at the temperature below the boiling point is impossible because there is simply no gas pressure available.
Aspects influencing vapour capacity
The vapour capacity of a gas cylinder depends on the following aspects:
- The temperature of the surroundings. A higher temperature will result in a higher vapour capacity.
- The fill grade of the cylinder. The vapour capacity decreases with decreasing fill grade of the cylinder. Under normal circumstances it is impossible to use all the (liquid) gas in a cylinder.
- The heat conductivity of the cylinder wall. Heat transfer is required to create the gas state of LPG by evaporation of the liquid state. Insulating the cylinder or using poorly thermal conductive materials decreases the vapour capacity. This is the main reason why in general conventional composite cylinders perform less with respect to steel or aluminium cylinders. Composite cylinder walls are made of less conductive materials such as thermoplastics, thermosets or glass fibres. The use of protective casings to prevent impact damage further reduces the vapour capacity.
- Radiation and convection of heat. For instance an air flow on the cylinder (forced convection) will increase the vapour capacity.
Design Aspects LoW8 in relation to Vapour Capacity
The vapour capacity is included in the design requirements for the loW8 cylinder. The loW8 foot-ring for instance has grooves to prevent build-up of cold air below the cylinder.
The LoW8 cylinders does not require any protective casing (like conventional glass fiber composite cylinders) around the cylinder body; this is one of the reasons the LoW8 shows a higher Vapour Capacity.
Vapour Capacity Tests
LoW8 has evaluated the vapour capacity of various cylinders including the LoW8 cylinder. Comparable cylinder volumes are selected and environmental conditions are kept equal. Although there are different test set ups we selected two types of test requires by our customers.
- The Laundry Dryer Test. This test has been performed in Malaysia. The Dryer is an appliance which uses the most LPG per time unit. Therefor the Vapour Capacity might be critical. The requirements to the LPG Cylinder: Sufficient Vapour Capacity to feed the burner and at full usage be able to supply for 30 cycles (which is the same like a steel cylinder). A typical Cycle is about 30 minutes where the Vapour Capacity required varies between 0,7 and 1,5 kg/hour.
Test Conditions: LPG (60% propane 40% butane), Temperature Surroundings (28 Celsius), Minimum Temperature Bottom Cylinder: 18C, remaining LPG after completing test: 1,1 kg
Results: The LoW8 cylinder showed a Vapour Capacity equalling the steel cylinder and even the number of cycles (31) outperformed the steel cylinder!
- Big Burner Test. This test has been executed in The Netherlands with a so called Big Burner (see picture). With a regulator the maximum pressure has been set at 5 bar resulting in an output according to the graph (see below).
Test Conditions: Temperature surrounding, 19C. LPG (60% propane 40% butane).
Test requirement: Minimum Vapour Capacity of 400 gram/h after 5½ hours constant burning.
Results: The LoW8 cylinder shows a more or less constant Vapour Capacity after 5½ hours of 695 grams/hour. Temperature at the Bottom of the Cylinder: – 20.2 C.