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(a)
Fresh
air
Fan
Cooled
water
Hot water
Hot air
(b)
Chimney
Hot air
Combustion
chamber
Hot air
Heat exchanger
Burner
Fuel
Combustion
air
Fresh
air
Fresh
air
Fan
(c)
Injector
Combustion head
Flames
Mix of
gas and air
Fresh
air
Hot
air
Fan
Air
Pressurized fuel
Fig. 7.6. Air heating systems: fan coil (a); hot air generator using indirect combustion (b) and direct
combustion (c).
with fins and a fan improves the exchanges.
Electric fan coils are expensive due to their
power consumption.
Hot air generators burn fuel, normally a
gas, which heats the air that is forced to
circulate inside the greenhouse. They can
use direct or indirect combustion. In those
using direct combustion, the products of
the combustion are sent to the greenhouse,
so the fuel must be clean, with less than
0.03% sulfur content (Urban, 1997a). As
they may generate other gases such as ethyl-
ene (Hanan, 1998), direct systems are nor-
mally used only as a defence against
exceptionally low temperatures (of short
duration) and as a support.
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