Agriculture Reference
In-Depth Information
areas it is recommended that the system is
used in association with thermal screens
and with a complementary heating system
(hot air) which has a fast response.
The heat transfer from the heating pipes
will depend on the type of pipe used (mat-
erial and characteristics, such as diameter,
with or without fins, etc.) and the temperature
difference between the pipe and the green-
house air. Table 7.2 summarizes some data
in this respect.
Sometimes, low-temperature water heat-
ing systems are used to warm up the roots,
besides heating the aerial part of the crop, in
which case they heat by convection, radia-
tion and conduction. The pipes may be made
of PE or corrugated polypropylene. The
water temperature is regulated by mixing hot
water (from the boiler) with cold water
(return) by means of three-way valves.
Low-temperature water heating systems
are able to maintain higher values of soil or
substrate temperature than hot-air heating
systems (due to their location near the soil)
if the same set temperatures of the green-
house air are used (Lorenzo
et al
., 1997a).
This may be attributed to the higher thermal
inertia of water heating systems that involves
slightly higher energy consumption.
The response time of heating is of the
order of 40-60 min in an efficient hot water
system, whereas it decreases to 10 min in a
hot air system (Day and Bailey, 1999).
By IR radiation
In these systems, tubes heated to high tem-
peratures (250-375°C) emit IR radiation
which heats the plants (Van de Braak, 1995).
The source of energy is usually propane or
natural gas, for economic reasons, because
electricity is very expensive. This system
has a low thermal inertia and is not homo-
geneous, because the leaves that receive the
radiation are much warmer than those in
the shadows not exposed to this radiation,
thus water condensates on these leaves
(Hanan, 1998). Nowadays they are seldom
used in greenhouses.
7.4.3
Soil or substrate heating
The heating of the roots is achieved by
means of hoses or pipes buried in the soil,
packed into concrete slabs or in contact
with the crop substrate (Figs 7.8 and 7.10).
Heat is transferred by conduction.
They require a conductive soil, which
requires it to be wet. Insulation may be imple-
mented at a certain depth to limit the losses.
The water temperature must not exceed
45-50°C, to avoid drying of the roots and of
the soil (Urban, 1997a), because dry soil is a
bad heat conductor (Fig. 7.11). Polypropylene
pipes of 20 mm diameter are commonly used,
with water velocities of 0.6-0.9 m s
−1
, provid-
ing up to 50 W m
−2
, with pipes separated by
30-40 cm and avoiding tube lengths longer
than 120 m (ASAE, 2002).
Heating pads are used in nurseries. They
are composed of pads with a circulating
water or electricity heating system. Careful
irrigation must be performed, because the
pads desiccate quickly, so it is necessary to
install sprinklers or thin fogging.
In soilless growing systems it is fre-
quent to heat the roots simply by locating
the pipes of a conventional heating system
near the substrate. Sometimes, a pipe is
placed inside or below the substrate (with
insulation below) (Fig. 7.10). Growing tables
and crop benches may also be heated by
radiant pipes or integrated heating elements
(Fig. 7.8). These are used in nurseries and
for cultivation of ornamental crops.
Table 7.2.
Heat transfer, in watts per linear metre
of heating pipe, for several temperature differences
between the pipe and the surrounding greenhouse
air (adapted from Van de Braak, 1995).
Steel pipe
diameter
(mm)
Plastic pipe
diameter
(mm)
Temperature
difference (°C)
51
33.2
26.4
25
10
15
10
8
6
20
34
23
18
14
30
55
38
31
24
40
77
53
44
35
50
101
71
58
46
60
128
90
73
-
70
156
108
90
-
80
185
129
107
-
