Frost Protection - UTL Repository

]
F R O S T P R O T E C T I O N : F U N D A M E N T A L S , P R A C T I C E A N D E C O N O M I C S
[
Liquid-fuel heaters
Liquid-fuel heaters were developed for frost protection during the early 1900s.
Use of the method decreased as oil prices and concerns about air pollution
increased. Although not widely used, the use of liquid-fuel heaters for frost
protection is still a viable method in cases where laws do not prohibit it and
the cost of fuel is not too high. Liquid-fuel heaters require considerable labour
for placement, fuelling and cleaning, in addition to the capital costs for the
heaters and the fuel. Typically, there are about 75 to 100 oil stack heaters or
150 to 175 propane-fuel heaters per hectare, and a well designed and operated
heater system will produce about 1.23 MW ha -1 (i.e. 123 W m -2 ) of power.
The approximate consumption rate is 2.8 litre h -1 per heater for oil- and
kerosene-fuelled heaters and about 1 m 3 h -1 for propane-fuel heaters. More
than half of the energy output from the heaters is lost as radiation to the sky
and convective heat losses on a typical radiation frost night, so the heater
output is high relative to the heat gained by the crop. Note that these
recommendations are for protection of large deciduous orchards that are
surrounded by other orchards that are being protected. Isolated smaller
orchards may require more heaters.
When lighting heaters, every second or third heater in a row should be lit first.
Then go back and light the remaining heaters. This helps to reduce convective
losses of heat through the top of the inversion layer. Oil-fuel heaters should be
cleaned after every 20 to 30 hours of operation, and the heaters should be closed
to prevent entry of rainwater that could cause leakage of oil onto the ground.
The stack can be blown off or the fire extinguished if too much steam is
produced. Remove oil from the heaters at the end of the season. Free-flame-type
heaters will accumulate carbon and lower the fuel efficiency level. Catalytic
sprays can be used to reduce carbon accumulation. They should be refilled
before they run out of fuel and cleaned with a stick or simply hit to free the soot
accumulation that reduces efficiency.
Various types of fuels burners are available for frost protection. A list of
fuels and heaters approved for use in Florida (USA) are given in Tables 7.3 and
7.4. Because they can be improvised with cans of paint, oil, etc., the free flame
type (i.e. without a chimney) is cheaper and easier to transport and fill. They
are smaller, so the density of heaters can be greater, giving better mixing and
less chance for the chimney effect. This sometimes results in improved
protection. However, they are less fuel-efficient because there is more
volatilization and they pollute more. In some locations, they are not approved
for use in frost protection.
152