- Email: [email protected]
A method of forming dew on plants under controlled conditions
328
A Method of Forming
Dew on Plants Under Controlled Conditions D. PESCOD*
1.
Introduction
For some time CSIRO biologists have been studying the effect of dew on the growth of plants in hot arid areas. One important problem to be investigated is water movement between plant, ambient air and soil, in particular, moisture absorption through the leaves. In the laboratory, fine water sprays have been used to give a wetting effect similar to dew, but it was not known whether the results obtained would be the same as moisture condensed on the leaves directly from the air. To answer this problem, a cabinet has been developed and has demonstrated the feasibility of producing dew on plants in the laboratory under controlled conditions in a manner similar to the natural formation in the field. 2.
Theoretical principles
The essential requirements for the formation of dew are:(a) High relative humidity of the ambient air. (b) A large absorber of radiant heat. (c) Limited air movement. It is not difficult to provide a high relative humidity in an enclosed space. For example, a large pan of water with its temperature controlled at a point close to the temperature of the air would be satisfactory. However, when a radiant cooling panel or set of panels is introduced’ this leads to complications, because the panels would be maintained at a temperature well below that of the air to provide adequate radiant cooling, and consequently they would cool and dehumidify the air. To overcome this problem, panels have been developed in the Division of Mechanical ??Division of Mechanical Engineering. CSIRO, Highett, Victoria, Australia
Engineering, with vapour barriers which are substantially transparent to radiant heat.233 The vapour barriers consist of clear polyethylene sheets and are maintained at a temperature above the dew point of the air to prevent condensation on the surfaces. If condensation were to form on the polyethylene sheet, the panel would emit long-wave radiation’instead of being transparent to it and continued condensation on the panel rather than on the plants would be encouraged. This type of cooling panel thus makes dew formation in a cabinet a practical proposition. The plant loses heat by radiation through the polyethylene window to the cold panel, thus dropping the leaf temperature below dew point. The process is therefore the same as would occur in nature. The third requirement, that of limited air movement, makes accurate control of the air temperature difficult, but it is not an insuperable problem. 3.
Description and performance Fig. 1, left shows a section through the dew cabinet. The plants are contained in a sealed and thermally insulated space, 3 ft 6 in wide, 2 ft 3 in deep and 3 ft high. The pots are supported by narrow wooden strips on steel bars over a water bath (A) and care must be taken to ensure free air movement between the plants and the water. The temperature of the water is controlled by a sensitive single acting thermostat (B) in conjunction with a 500 W heater (C) and cooling coil (D). The water in turn controls the air temperature above it and at the same time maintains a relative humidity at the plants up to 95 ‘;$ A horizontal copper panel (E) is fitted above the plant space and is controlled at temperatures down to - 20 “C by a small refrigeration unit (F) in the base of the cabinet. The effective size
I).
I’FS(‘OI)
E E Q
‘\
‘\\
r
-K
/
_\
I
I
I
-M
_J
I R-
,A
-D
B -*
c -” A-F
’
L-h \ AD
__ m II
I
I
p
F
G
--
Fig. I.
A-
--T$q
---
w
G
Section through cabinet (top left); Plan of refrigeration system (top right) ; Detail of refrigeration system (bottom)
330
A METHOD
OF
FORMING
DEW
ON
of the cold panel is 3 ft 3 in x 2 ft 1 in; it is fixed at a distance 3 ft 4 in above the supports for the plants. Two sheets of clear polyethylene film (G) 0.002 in thick below the panel form the vapour barrier and convective cooling of the lower sheet is prevented by an electric heater (H) between the two sheets. This consists of a grid of fine resistance wire at I in spacing. Side panels(J) of bright commercial aluminium sheet reflect radiant heat and give an effect of horizontal extension of the absorbing surface. The main lights (K) consist of four 300 W tungsten floodlights and are intended to simulate the effect of sunlight drying the dew. The total radiant energy on the plants and the spectral composition are comparable with direct sunlight, though the energy level is higher in the near infra-red. When these lights are on. the refrigeration is run without the electric demisting and the door of the cabinet (L) left ajar to prevent excessive temperature rise in the cabinet. An auxiliary fluorescent light (M) of 8 W is provided for inspection during the formation of dew and this may be left on. An inspection window (N) with an insulated cover is provided in the door. The controls (0) are located below the door. Fig. I, right shows the refrigeration system. A i hp commercial air-cooled condensing unit (F)
PLANTS
UNDER
(‘0NTROI.I.t.I)
(‘ONI)I
IlO4h
is used. and is run continuously. It‘ a reduced rate of radiant cooling is required. a by-pass valve consisting of a commercial capacity regulator (P), accessible from the front control panel (0). is adjusted. The main evaporator the radiant cooling panel (E)---is provided with a liquid recirculation system to ensure uniform panel temperature, using a simple design ot injector (Q). The end of a restrictor (R) (Fig. /. hotfom) or expansion tube5 (A in o.d.. 20 S.W.G.) projects into a specially shaped copper fitting (S) with & in holes, made to be inserted in an ordinary refrigeration A in copper tee (T). The fluid emerging from the end of the expansion tube at high velocity mixes with surplus liquid to be recirculated (U) and gives enough pressure recovery in the mixing tube (V) to balance the pressure drop through the evaporator. Some liquid refrigerant is carried with the gas in the return line, and this is evaporated in a small coil (D) immersed in the water bath (A). This can cool the water considerably below ambient if desired. A compressor (W), tilterdrier (X) and liquid receiver (Y) complete the circuit. Fig. 2, I&, is a front view of the complete cabinet. The overall dimensions are 6 ft 5 in high x 4 ft I in i( 2 ft 10 in. If necessary. the door may be removed to allow the cabinet to be
moved through an ordinary 2 ft 8 in wide doorway. Castors are provided for ease of movement. The cabinet may be operated from a 240 V single-phase power point; the total capacity is approximately 2000 W. The electric demister requires a 40 V, 100 W supply. A variable voltage supply up to 50 V. 150 W would enable the oxact rate of heating for a particular condition to be selected. Fig. 2, rig/If, shows the interior of the cabinet with tomato plants under test. As the plants lose heat by radiation to the refrigerated panel and their temperature falls below the dew point of the air. condensation takes place on the exposed surfaces. The air is cooled in the process and convection currents are established between the foliage and the water bath. With no heat transfer through the walls of the cabinet, the temperature of the air remains close to that of the water and the relative humidity may be varied up to 95j!,i by variation of the open spaces between the plants and the water. Since the heat flux from air to plants is very small, heat transfer through the walls to the air can change its temperature enough to disturb seriously the desired air movement. The thermal insulation provided on the walls allows the cabinet to operate with a temperature difference up to 10 “C below that. outside, but at this condition the formation of dew becomes slow and less regular. It is preferable to limit the difference to 5 C. The formation of dew also depends upon the relative humidity of the air, the type and position of the leaf surface and the temperature of the cold panel. Under favourable conditions, dew has been observed within a few minutes of starting the refrigeration ; under other conditions, several hours may pass before the surfaces are fully saturated. Ice has been formed on plants experimentally with the temperature of the water bath at 1 “C. The ice did not have the crystalline appearance of frost. Evidently it was condensed on the leaves as water droplets and was subsequently frozen.
4.
Discussion
Preliminary work at the CSIRO Division of Land Research and Regional Survey has confirmed that the cabinet appears to be suitable for
all the biological requirements for which it had been designed. It is believed that normal crystalline frost could be formed by placing the cabinet in an environment of about 0 “C and using ice in the water bath, but no experiment to confirm this has yet been conducted. One of the problems associated with radiant cooling in high humidities is that the temperature of the polyethylene sheets can fall below the dew point unless special provision is made to prevent it. Demisting of the polyethylene sheets by circulation of warm air from the condenser was attempted. but this was found to be inadequate, as the air was cooled rapidly while passing between the layers and condensation could occur over a large area. By using very fine wire as an electric heater, spread over the whole area of the panel. uniform heating was possible with negligible radiation or blockage. The best position for the heater was found to be between the two polyethylene sheets. Wire trays for supporting the plant pots were found to be unsuitable, as they were cooled by radiant heat exchange and caused excessive dehumidification of the air as it passed through. Narrow wooden strips, as shown in Fig. 2. were found to be satisfactory. Extra thermal insulation had to be provided on the walls and door to reduce the influence of room temperature on the performance of the cabinet. Forced air circulation was also tried with the object of improving the moisture transfer from the bath to the air and to promote air movement between the bath and the leaves of the plants. Dew would form readily provided the air velocity was very low. Higher velocities increased the heat transfer coefficient between air and leaves and could prevent the surface temperature of the leaves reaching the dew point. However. the dew formed at low air velocities was rather less even than with natural convection. and there was a greater tendency to condensation on the lower polyethylene sheet. Water vapour diffuses through polyethylene very slowly and eventually pools of water may form on the polyethylene sheets when the unit has been shut down. As water is partly opaque to thermal radiation, provision is made for
332
A
METHOD
OF
FORMtNG
DEW
ON
PLANTS
UNDER
CONTROLLED
C‘ONDII‘IONS
removal of the polyethylene for cleaning. It is not known yet how often this will be necessary, but it is expected to be at intervals of several months of operation, provided the lower sheet is properly sealed. Occasional draining and cleaning of the water bath is necessary as no form of screen is permissible between the pots and the water. The sealed unit refrigeration system will operate for long periods without maintenance. A possible future development would be to make the cabinet independent of outside conditions by accurately controlling the temperature of the walls or providing an outer jacket of air at a controlled temperature. These complications were not considered to be justified at this stage.
Division of Mechanical Engineering, CSIRO, for their advice during the design of the cabinet, and D. Sheppard and A. Gilbert of the Division of Mechanical Engineering, for carrying out most of the development and testing.
Acknowledgements The author wishes to thank Dr R. 0. Slatyer of the Division of Land Research and Regional Survey, CSIRO, and R. N. Morse, Chief of the
4 (6) 122 4 Trickett, E. S.; Goulden, J. D. S. The radiation trans-
REFERENCES
Raber, B. F. ; Hutchinson, F. W. Panel hearing and cooling analysis. John Wiley &Sons Inc., N.Y., 1947 ’ Morse, R. N.; Kaletzky, E. A new approach to radiant cooling .for human corqfort. J. lnstn Engrs Amt., 1960,33, 181 3 Morse, R. N. Radiant cooling. Archit. Sci. Rev., 1963, 6 (2) 50-53 5 Kowalczewski, J. J. Performance of refrigeration
’
systems with fixed restriction operating under variable evaporator and condenser conditions. J. Refrig., 1961,
mission and heat conserving properties of glass and some plastic films. J. agric. Engng Res., 1958, 3 (4)
281
A Method of Forming
Dew on Plants Under Controlled Conditions D. PESCOD*
1.
Introduction
For some time CSIRO biologists have been studying the effect of dew on the growth of plants in hot arid areas. One important problem to be investigated is water movement between plant, ambient air and soil, in particular, moisture absorption through the leaves. In the laboratory, fine water sprays have been used to give a wetting effect similar to dew, but it was not known whether the results obtained would be the same as moisture condensed on the leaves directly from the air. To answer this problem, a cabinet has been developed and has demonstrated the feasibility of producing dew on plants in the laboratory under controlled conditions in a manner similar to the natural formation in the field. 2.
Theoretical principles
The essential requirements for the formation of dew are:(a) High relative humidity of the ambient air. (b) A large absorber of radiant heat. (c) Limited air movement. It is not difficult to provide a high relative humidity in an enclosed space. For example, a large pan of water with its temperature controlled at a point close to the temperature of the air would be satisfactory. However, when a radiant cooling panel or set of panels is introduced’ this leads to complications, because the panels would be maintained at a temperature well below that of the air to provide adequate radiant cooling, and consequently they would cool and dehumidify the air. To overcome this problem, panels have been developed in the Division of Mechanical ??Division of Mechanical Engineering. CSIRO, Highett, Victoria, Australia
Engineering, with vapour barriers which are substantially transparent to radiant heat.233 The vapour barriers consist of clear polyethylene sheets and are maintained at a temperature above the dew point of the air to prevent condensation on the surfaces. If condensation were to form on the polyethylene sheet, the panel would emit long-wave radiation’instead of being transparent to it and continued condensation on the panel rather than on the plants would be encouraged. This type of cooling panel thus makes dew formation in a cabinet a practical proposition. The plant loses heat by radiation through the polyethylene window to the cold panel, thus dropping the leaf temperature below dew point. The process is therefore the same as would occur in nature. The third requirement, that of limited air movement, makes accurate control of the air temperature difficult, but it is not an insuperable problem. 3.
Description and performance Fig. 1, left shows a section through the dew cabinet. The plants are contained in a sealed and thermally insulated space, 3 ft 6 in wide, 2 ft 3 in deep and 3 ft high. The pots are supported by narrow wooden strips on steel bars over a water bath (A) and care must be taken to ensure free air movement between the plants and the water. The temperature of the water is controlled by a sensitive single acting thermostat (B) in conjunction with a 500 W heater (C) and cooling coil (D). The water in turn controls the air temperature above it and at the same time maintains a relative humidity at the plants up to 95 ‘;$ A horizontal copper panel (E) is fitted above the plant space and is controlled at temperatures down to - 20 “C by a small refrigeration unit (F) in the base of the cabinet. The effective size
I).
I’FS(‘OI)
E E Q
‘\
‘\\
r
-K
/
_\
I
I
I
-M
_J
I R-
,A
-D
B -*
c -” A-F
’
L-h \ AD
__ m II
I
I
p
F
G
--
Fig. I.
A-
--T$q
---
w
G
Section through cabinet (top left); Plan of refrigeration system (top right) ; Detail of refrigeration system (bottom)
330
A METHOD
OF
FORMING
DEW
ON
of the cold panel is 3 ft 3 in x 2 ft 1 in; it is fixed at a distance 3 ft 4 in above the supports for the plants. Two sheets of clear polyethylene film (G) 0.002 in thick below the panel form the vapour barrier and convective cooling of the lower sheet is prevented by an electric heater (H) between the two sheets. This consists of a grid of fine resistance wire at I in spacing. Side panels(J) of bright commercial aluminium sheet reflect radiant heat and give an effect of horizontal extension of the absorbing surface. The main lights (K) consist of four 300 W tungsten floodlights and are intended to simulate the effect of sunlight drying the dew. The total radiant energy on the plants and the spectral composition are comparable with direct sunlight, though the energy level is higher in the near infra-red. When these lights are on. the refrigeration is run without the electric demisting and the door of the cabinet (L) left ajar to prevent excessive temperature rise in the cabinet. An auxiliary fluorescent light (M) of 8 W is provided for inspection during the formation of dew and this may be left on. An inspection window (N) with an insulated cover is provided in the door. The controls (0) are located below the door. Fig. I, right shows the refrigeration system. A i hp commercial air-cooled condensing unit (F)
PLANTS
UNDER
(‘0NTROI.I.t.I)
(‘ONI)I
IlO4h
is used. and is run continuously. It‘ a reduced rate of radiant cooling is required. a by-pass valve consisting of a commercial capacity regulator (P), accessible from the front control panel (0). is adjusted. The main evaporator the radiant cooling panel (E)---is provided with a liquid recirculation system to ensure uniform panel temperature, using a simple design ot injector (Q). The end of a restrictor (R) (Fig. /. hotfom) or expansion tube5 (A in o.d.. 20 S.W.G.) projects into a specially shaped copper fitting (S) with & in holes, made to be inserted in an ordinary refrigeration A in copper tee (T). The fluid emerging from the end of the expansion tube at high velocity mixes with surplus liquid to be recirculated (U) and gives enough pressure recovery in the mixing tube (V) to balance the pressure drop through the evaporator. Some liquid refrigerant is carried with the gas in the return line, and this is evaporated in a small coil (D) immersed in the water bath (A). This can cool the water considerably below ambient if desired. A compressor (W), tilterdrier (X) and liquid receiver (Y) complete the circuit. Fig. 2, I&, is a front view of the complete cabinet. The overall dimensions are 6 ft 5 in high x 4 ft I in i( 2 ft 10 in. If necessary. the door may be removed to allow the cabinet to be
moved through an ordinary 2 ft 8 in wide doorway. Castors are provided for ease of movement. The cabinet may be operated from a 240 V single-phase power point; the total capacity is approximately 2000 W. The electric demister requires a 40 V, 100 W supply. A variable voltage supply up to 50 V. 150 W would enable the oxact rate of heating for a particular condition to be selected. Fig. 2, rig/If, shows the interior of the cabinet with tomato plants under test. As the plants lose heat by radiation to the refrigerated panel and their temperature falls below the dew point of the air. condensation takes place on the exposed surfaces. The air is cooled in the process and convection currents are established between the foliage and the water bath. With no heat transfer through the walls of the cabinet, the temperature of the air remains close to that of the water and the relative humidity may be varied up to 95j!,i by variation of the open spaces between the plants and the water. Since the heat flux from air to plants is very small, heat transfer through the walls to the air can change its temperature enough to disturb seriously the desired air movement. The thermal insulation provided on the walls allows the cabinet to operate with a temperature difference up to 10 “C below that. outside, but at this condition the formation of dew becomes slow and less regular. It is preferable to limit the difference to 5 C. The formation of dew also depends upon the relative humidity of the air, the type and position of the leaf surface and the temperature of the cold panel. Under favourable conditions, dew has been observed within a few minutes of starting the refrigeration ; under other conditions, several hours may pass before the surfaces are fully saturated. Ice has been formed on plants experimentally with the temperature of the water bath at 1 “C. The ice did not have the crystalline appearance of frost. Evidently it was condensed on the leaves as water droplets and was subsequently frozen.
4.
Discussion
Preliminary work at the CSIRO Division of Land Research and Regional Survey has confirmed that the cabinet appears to be suitable for
all the biological requirements for which it had been designed. It is believed that normal crystalline frost could be formed by placing the cabinet in an environment of about 0 “C and using ice in the water bath, but no experiment to confirm this has yet been conducted. One of the problems associated with radiant cooling in high humidities is that the temperature of the polyethylene sheets can fall below the dew point unless special provision is made to prevent it. Demisting of the polyethylene sheets by circulation of warm air from the condenser was attempted. but this was found to be inadequate, as the air was cooled rapidly while passing between the layers and condensation could occur over a large area. By using very fine wire as an electric heater, spread over the whole area of the panel. uniform heating was possible with negligible radiation or blockage. The best position for the heater was found to be between the two polyethylene sheets. Wire trays for supporting the plant pots were found to be unsuitable, as they were cooled by radiant heat exchange and caused excessive dehumidification of the air as it passed through. Narrow wooden strips, as shown in Fig. 2. were found to be satisfactory. Extra thermal insulation had to be provided on the walls and door to reduce the influence of room temperature on the performance of the cabinet. Forced air circulation was also tried with the object of improving the moisture transfer from the bath to the air and to promote air movement between the bath and the leaves of the plants. Dew would form readily provided the air velocity was very low. Higher velocities increased the heat transfer coefficient between air and leaves and could prevent the surface temperature of the leaves reaching the dew point. However. the dew formed at low air velocities was rather less even than with natural convection. and there was a greater tendency to condensation on the lower polyethylene sheet. Water vapour diffuses through polyethylene very slowly and eventually pools of water may form on the polyethylene sheets when the unit has been shut down. As water is partly opaque to thermal radiation, provision is made for
332
A
METHOD
OF
FORMtNG
DEW
ON
PLANTS
UNDER
CONTROLLED
C‘ONDII‘IONS
removal of the polyethylene for cleaning. It is not known yet how often this will be necessary, but it is expected to be at intervals of several months of operation, provided the lower sheet is properly sealed. Occasional draining and cleaning of the water bath is necessary as no form of screen is permissible between the pots and the water. The sealed unit refrigeration system will operate for long periods without maintenance. A possible future development would be to make the cabinet independent of outside conditions by accurately controlling the temperature of the walls or providing an outer jacket of air at a controlled temperature. These complications were not considered to be justified at this stage.
Division of Mechanical Engineering, CSIRO, for their advice during the design of the cabinet, and D. Sheppard and A. Gilbert of the Division of Mechanical Engineering, for carrying out most of the development and testing.
Acknowledgements The author wishes to thank Dr R. 0. Slatyer of the Division of Land Research and Regional Survey, CSIRO, and R. N. Morse, Chief of the
4 (6) 122 4 Trickett, E. S.; Goulden, J. D. S. The radiation trans-
REFERENCES
Raber, B. F. ; Hutchinson, F. W. Panel hearing and cooling analysis. John Wiley &Sons Inc., N.Y., 1947 ’ Morse, R. N.; Kaletzky, E. A new approach to radiant cooling .for human corqfort. J. lnstn Engrs Amt., 1960,33, 181 3 Morse, R. N. Radiant cooling. Archit. Sci. Rev., 1963, 6 (2) 50-53 5 Kowalczewski, J. J. Performance of refrigeration
’
systems with fixed restriction operating under variable evaporator and condenser conditions. J. Refrig., 1961,
mission and heat conserving properties of glass and some plastic films. J. agric. Engng Res., 1958, 3 (4)
281