Type of gas in the cylinders
LPG (liquefied petroleum gas) contained in the cylinders are supplied either as Butane or Propane. – Commercial Butane, which contains approximately 80-90% of Butane whilst commercial Propane contains approximately 90% propane.
Large quantities of flammable vapour can be produced from relatively small amounts of liquid LPG stored in cylinders and gas tanks. This makes LPG an ideal portable fuel, cylinders must always remain upright to ensure only vapour exits the cylinder valve before it enters the regulators.
At atmospheric pressure BUTANE boils at -2°Cand PROPANE boils at -45°C
Contained in cylinders or bulk tank, LPG remains a liquid under pressure and this pressure depends on the type of LPG and the ambient or cylinder temperature surrounding the cylinder . The chart shows the vapour pressure within a gas cylinder at various ambient temperatures
Vaporisation in the cylinder
In a cylinder LPG is liquid at the bottom and vapour under pressure at the top. When there is an off-take of gas, the gas volume is regenerated by boiling off the liquid part. This vaporisation cools down the liquid. The heat required to continue the LPG to boil is the surrounding air or ambient temperature in contact with the cylinder.
Propane delivers high pressure at colder temperaturers and is vaporised faster so is used and stored outside. Butane is used mainly for indoor or summer use.
During off-take, the temperature decreases, then the pressure decreases. When only a small amount of liquid remains in the cylinder, the pressure is lower than when the cylinder was full. Likewise the larger the cylinder the greater the surface area and so more vapour can be produced this is the vapourisation rate.
Typical vapourisation rates on cylinders
The maximum flow rate depends on:
– the type of gas
– the level in the cylinder
– the ambient temperature
– the using time
– the dimension and material of the cylinder
– the number of cylinders
Cylinders can only supply a certain rate of vapour and must be sized so as to meet the heat input of the appliances. Often an overlooked part of a LPG installation the correct sizing and quantity of cylinders will ensure that the cylinder regulator delivers the correct delivery pressure or automatic changeover using the full contents of a cylinder before eventually selecting the reserve cylinder contents.
At Clesse and Novacomet the capacity we declare is normally “worst case scenario” to ensure the regulator or ACO operate in both very cold or hot climatic conditions and low cylinder contents. This means in most cases our regulators are “understated”
Off-take rates for cylinders are typically as indicated below and based on a continuous off take rate, some installations will require more than one cylinder and so Automatic Changeovers are use.
Recommended maximum offtake rates for LPG cylinders
When supplying Cooker 12kW, CH boiler 28kW and fire 14kW total load is 54kW – Therefore 2x47kg cylinder combined will need to be used. An ACO will need to used with 4 cylinders (2 per side) Compact 800 -5kg/h supplying 69kW at 37mb outlet pressure would be ideal.
Note: 1 kg/h Gas Flow rate = 13.9kW = 47,500BTU/h