Agriculture Reference
In-Depth Information
Soil surface
20°C
20°C
22°C
22°C
50 cm
25°C
25°C
30° C
30°C
Heating pipe
35°C
80 cm
Fig. 7.11.
Thermal profiles of a soil heated with pipes.
7.4.4
Heat production
The boilers
Boilers are composed (Bordes, 1992) of: (i) a
combustion chamber with a furnace (if
solid fuels are used) or a burner (if the fuels
are liquid, or powder or gases); (ii) a heat
exchanger, where the combustion gases at
high temperature transfer their heat to the
fluid of the heating circuit (which can be
water) through the conducting walls that
separate the fluid and the air; (iii) a chim-
ney to evacuate the smoke efficiently, pas-
sively with a good draught or actively with
a fan; and (iv) safety devices and automatic
equipment needed for their operation.
It is normal to recover the water vapour
from the smoke to decrease the thermal
losses (in the form of water vapour latent
heat) and improve the performance of the
boiler. The combustion must be done in the
presence of an excess of air to achieve com-
plete combustion and to optimize the per-
formance of the boiler.
Energy sources
When choosing the type of energy for heat-
ing it is necessary to know the main techni-
cal characteristics of the fuel, what is its
state (solid, liquid or gas) at the usual condi-
tions of temperature and pressure (an essen-
tial aspect for their use), its heating power
and its content of impurities, mainly sulfur
(Bordes, 1992).
Coal is cheap, but it is very polluting. It
usually contains sulfur and its gases are
corrosive. Coal boilers are more complex
and expensive.
Wood and vegetable waste are quite
voluminous. Their transport and storage
is very expensive. They are also quite
polluting.
Diesel is expensive. Fuel oil is cheaper
and of lower performance. Natural gas is a
fuel obtained from the purification of meth-
ane deposits. The disadvantage of natural
gas is its high price, but its advantages are:
(i) it contains no sulfur; (ii) it produces very
little pollution; (iii) it is easy to use; and (iv)
CO
2
can be recovered from its combustion
gases.
The liquefied petroleum gases (LPGs)
butane and propane are used. They have the
same advantages as natural gas.
Gas boilers may use natural gas or
LPG. Nowadays, the recovery of CO
2
from the combustion gases for carbon
enrichment in these boilers is becoming
widespread.
Heat distribution
When these systems were initially con-
ceived circulation of hot water was sponta-
neous by 'thermosyphon'. The weight/
pressure differences, due to water tempera-
ture differences, generated a circulation of
water. Therefore, this type of heating was
usually called 'thermosyphon' (Bordes,
1992). Nowadays these systems are pro-
vided with a circulation pump, to improve
their efficiency and to distribute the heat
without restrictions.
