Physical properties of covering materials for naturally

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J. Trop. Agric. and Fd. Sc. 28(1)(2000): 55–69

R. Kamaruddin, B. J. Bailey and M. P. Douglass

Physical properties of covering materials for naturally ventilated tropical greenhouses (Ciri-ciri fizikal bahan atap dan dinding untuk pengalihudaraan semula jadi pada rumah tanaman tropika) R. Kamaruddin*, B. J. Bailey** and M. P. Douglass*** Key words: transparent roofing, insect screen, direct and diffuse light transmission, coefficient of discharge, airflow characteristics

Abstrak Bahan binaan bagi atap dan dinding rumah tanaman di kawasan tropika biasanya menggunakan bahan filem plastik lut sinar dan jaring kalis serangga polietilena. Sifat-sifat bahan binaan tersebut perlulah dahulu diketahui sebelum kerja-kerja merekabentuk dan pengiraan kadar pengalihudaraan semula jadi dibuat. Artikel ini membincangkan bagaimana sifat-sifat fizikal bahan atap dan dinding ditentukan. Kadar lut cahaya filem plastik dan jaring kalis serangga didapati hampir sama iaitu melebihi 75%. Ini bermakna, tiada kesan yang ketara terhadap penerimaan cahaya oleh tanaman di dalam rumah tanaman, jika kedua-dua bahan binaan tersebut digunakan untuk mengatapi dan mendindingi rumah tersebut. Pekali kadar alir bagi jaring kalis serangga N50, N32 dan N24 masing-masingnya ialah 0.411, 0.520 dan 0.547. Jaring kalis serangga yang terkecil memberi pekali kadar alir yang terendah. Perbezaan tekanan udara menjadi bertambah dengan pengurangan saiz lubang jaring apabila angin melaluinya. Di samping itu, perhubungan linear telah terbentuk antara susutan tekanan dengan halaju angin kuasa dua. Perhubungannya juga menunjukkan susutan tekanan melintasi jaring telah bertambah dengan pertambahan kelajuan angin mengikut hukum Forcheimer’s atau kuadratik. Abstract In the tropics, covering materials such as transparent polyethylene film and insect screens are commonly used for greenhouse roofing and sidewall respectively. The importance of covering material properties is exhibited in the greenhouse design and also in quantifying natural ventilation rate. This paper discusses how measurements of physical properties of roofing and sidewall materials are made. Light transmission of polyethylene film and insect screens was found to be almost similar, that is more than 75%. This will give similar light transmission to be received by the crop. Coefficient of discharges of the screens N50, N32 and N24 was found to be 0.411, 0.520 and 0.547 respectively. The smallest screen had the lowest value coefficient of discharge. When air flowed through the screen, the pressure drop increased linearly with the square of approach airspeed. *Strategic, Environment and Natural Resources Research Centre, MARDI Headquarters, P. O. Box 12301, 50774 Kuala Lumpur, Malaysia **Silsoe Research Institute, Wrest Park, Silsoe, Bedford, MK45 4 HS, United Kingdom ***Cranfield University, Silsoe, Bedford, MK45 4DT, United Kingdom Authors' full names: Rezuwan Kamaruddin, Bernard Bailey and Paul Dauglass ©Malaysian Agricultural Research and Development Institute, MARDI 2000

55

Covering materials for tropical greenhouses

It also increased with increasing screen hole. The relationship also showed that the pressure drop across the screen had increased with increase in apparent airspeed according to Forchheimer’s flow regime or a quadratic law.

Introduction The main constraints of conventional high value crop production in the tropical open fields are uniformly hot, humid, high rainfall and extreme solar radiation throughout the year. In addition, weeds, insect and disease damages, and intensive labour requirements are in the attentive state. Selected temperate vegetables, flowers and fruits can be grown in the hot and humid lowlands. However, the yields are generally low, poor quality and crops are sometimes totally destroyed by insects and diseases, if the conventional open farming methods are applied. Furthermore, the intensive spraying of pesticides results in high residual levels in many vegetables and fruits, which have to be rejected after harvest as they are unfit for human consumption (Hawa and Rezuwan 1990; Hawa et al. 1992; Illias et al. 1992, 1994). Heavy rainfall throughout the year will cause crop damage, difficulty of working in wet conditions, susceptibility to diseases, fertilizer loss by surface runoff and there are adverse effects of continuous cropping. Besides that, hot and humid weather seems not to be conducive to crop growth and production. These conditions are more pronounced in enclosed greenhouse structures. Thus requirements of ventilation are increased. To solve those problems, Malaysian Agricultural Research and Development Institute (MARDI) has developed tropical version greenhouses. They are rainshelter (RS), insect-proof (IP) and insect-proof rainshelter (IPRS) structures (Yeoh 1992; Rezuwan 1995). RS consists of simple structural frame, open sidewall and transparent roofing; IP is totally covered with insect screen; IPRS has transparent roofing and sidewall entirely covered with insect screen. RS is suitable for crops 56

requiring protection from extreme solar radiation and rainfall damages. IP structure protects crops from insects and heavy rain droplet damages. However, IPRS is suitable for crops that require total protection against extreme solar radiation, high rainfall, insect and diseases, and wind damage. Horticultural transparent polyethylene, acrylic, polycarbonate and glass sheets have been widely used for the roofing materials. These materials emit short wave radiation (400 to 700 µm wavelengths) that is required by crop for photosynthesis and has an opaque property that can filter long wave radiation (between 5.0 and 60 mm wavelengths) which is harmful to the crop. In addition, they keep rain from entering, which keep the cultivation area dry for easy working and less disease borne, and fertilizer will not be washed away by surface runoff. Studies on mechanical properties of roofing materials have been done by Briassoulis et al. (1997). They pin point the current inadequacies of the material specifications given by the manufacturers. They found that mechanical properties vary considerably between the materials considered. Also in many cases, significant variations are found between the reported values describing a specific property of the same material. In addition, they suggested that further research work are needed to determine the method of testing required for characteristics of both mechanical (e.g. strength) and the physical (e.g. light transmission) requirements for the roofing materials. Insect screens have been widely used for covering the greenhouse sidewall. The main functions of insect screens are to exclude the target insects from entering, provide an opening for natural ventilation and also as a wind barrier. Insect screens

R. Kamaruddin, B. J. Bailey and M. P. Douglass

have been reported to be an efficient method for reducing the number of pesticide treatments (Berlinger et al. 1991; Baker and Shearin 1994; Ross and Gill 1994). Several commercial screens are available for covering the greenhouse. To decide which screen is suitable, it is important to know the maximum size of opening that can be tolerated in the screening to exclude the targeted insects. For example, greenhouse whitefly and western flower thrips require screens with hole sizes of 288 µm and 192 µm, respectively, for their exclusion (Ross and Gill 1994). If a grower applies a fabric with such a small hole size, but underestimates the necessary area of ventilator opening, the screen may cause a large static pressure drop, inadequate air exchange, excessive energy consumption by the fans, excessive wear and tear on the fan motors and high greenhouse temperatures (Baker and Shearin 1994). Natural ventilation is necessary to limit temperature rise, remove excessive humidity transpired by crop and maintain CO2 level from depletion inside the structures. It is quiter and cheaper compared to the forced ventilation. In order to quantify the ventilation, physical properties of covering materials are required. Material properties such as thickness, direct and diffuse light transmissions are important parameters for roofing material. However, hole area, thickness, direct and diffuse light transmission, coefficient of discharge, and airflow characteristics across the screen are also important parameters for the side wall insect screens. The covering material properties are needed in the greenhouse structural design and to quantify the natural ventilation rate induced by stack, wind and combination of both effects. However, these properties are always not available by the manufacturers and in the standard code of practices. Therefore, measurements of the covering material properties are sensible (Rezuwan 1999).

The objectives of this study are to provide some of the important physical properties of transparent polyethylene film and polyethylene insect screen, which are useful in designing and quantifying the naturally ventilated tropical greenhouse structures. In addition, the testing equipment and methods are also presented. Theoretical consideration Light transmission High light transmission through greenhouse covers is one of the important parameters required for crop photosynthesis. Studies of light transmittance through the covering materials have been made in the laboratory and also on greenhouses by many researchers. Russell (1985) studied on twinwalled materials. He introduced a general expression for calculating the average of light transmitted at any incident angle. Thermal screens used in glasshouses at night can reduce the heat loss by between 35% and 60%. Bailey (1981) and Nijskens et al. (1985) had studied radiation transfer through thermal screens in greenhouses. They described the methods used to measure transmittance, absorptance and transmittance of the screens, and presented results for potential thermal screen materials. However, specific light transmission for polyethylene and insect screen has yet to be made. Global solar radiation comprises beam radiation received directly from the sun and diffuse radiation from the hemispherical sky vault. The radiation received at the surface of the earth varies with latitude, the times of the year and day. Not all the radiation incident on a greenhouse is transmitted to its interior, some is reflected and absorbed by the cover and structure. The ratio of the solar irradiances on horizontal surfaces inside and outside the greenhouse is known as the light transmission of the greenhouse. This depends on the detailed design of the greenhouse and on the covering material, but it is also influenced by latitude, the orientation of the greenhouse and it varies

57

Covering materials for tropical greenhouses

through the day and over the year (Bot 1983). Radiation emitted by the sun is commonly used to indicate energy transfer by electromagnetic waves. Two wave length regions are important for the greenhouse climate; first the shortwave, solar irradiation with wavelengths of about 400–700 nm and the second that of the longwave, thermal radiation with wavelengths about 5.0–60 µm. The shortwave irradiation is called photosynthetically active radiation (PAR) which is important for photosynthesis for greenhouse production. However, the long wavelength radiation is not required by plants and has to be shielded by an opaque covering materials. When the radiation strikes a body, it can be absorbed, reflected or transmitted through the body. The fractions of the radiation absorbed, reflected and transmitted are as follows;

α+ρ+τ

(1)

where α is the absorptivity, ρ is the reflectivity, and τ is the transmissivity. A real body absorbs less radiation than a black body at the same temperature and it also emits less radiation. A black body is a perfect emitter and absorber of thermal radiation. Kirchoff’s law states that the emissivity (ε) of a real surface is equal to the absorbtivity for radiation of the same wavelength. The values of emissivity, transmissivity and reflectivity depend on the material and the wavelength of radiation. For example, the transmissivity of glass is close to 0.9 for photosynthetically active radiation, but is almost zero for infrared radiation. The emissivity of a material depends on its temperature and surface finish. Coefficient of discharge Sase and Christianson (1990) determined the coefficient of discharge for several selected screen materials for use in greenhouse applications. They found that for all the materials tested the pressure drop varied linearly with the square of the approach 58

velocity. Kosmos et al. (1993) determined the force and static pressure resulting from airflow through screens. They found the wind pressure on screens and static pressure drops across screens increased exponentially with increasing approach air velocity. The coefficient of drag was unstable at lower approach air velocities but tended to stabilize to a constant value for the higher velocities through each screen. Miguel et al. (1997, 1998) used porous media flow analysis to characterize airflow through thermal, shading and insect screens. Instead of using the discharge coefficient for characterizing the screens, they used porosity and inertial factors, and concluded that the use of a discharge coefficient is appropriate only when Re > 150. Experiments on a small-scale greenhouse model were carried out by Montero et al. (1997), who measured the discharge coefficients of screens installed on continuous roof openings. Three types of screens with different hole sizes were tested. They found that the discharge coefficients decreased as the hole size decreased and that windows equipped with anti-thrips screens had a discharge coefficient approximately half of those from the same windows without screens. When air moves through a screen, the screen creates a resistance to the flow and a static pressure drop develops across the screen. These phenomena make the actual airflow reduce and can be expressed in the ventilation equation as a coefficient of discharge. The coefficient of discharge is the product of the coefficients of velocity and contraction: both of which are 1.0 or less in value. It is also a function of inlet type and design. It can be expressed in the form of Euler-like relation, 1 2

∆P = – Fo p V 2

(2)

and Cd =

1

F

(3)

o

where ∆P is the static pressure drop across the screen (Pa), Fo is the friction factor

R. Kamaruddin, B. J. Bailey and M. P. Douglass

(dimensionless), ρ is the air density (kg/m3), V is average air velocity through screen (m/s) and Cd is coefficient of discharge (dimensionless). Airflow characteristics Screens have been reported to be an efficient method for reducing the entry of pests into greenhouses and also providing an opening for natural ventilation system. The physical properties of screens are important because they affect the pressure drop across the screen, the air exchange rate, energy consumption by fans and temperature in the green house. As air flows through the screen a static pressure drop occurs across it. Brundrett (1993) and Teitel and Shklyar (1998) have studied the pressure drop across the insectproof screens. They found that the pressure drop was a function of the upstream velocity, the air density and a pressure loss coefficient. The latter is a function of the screen porosity and the Reynolds number. Several studies were carried out on the flow through the different screens for agricultural activities. Kosmos et al. (1993) tested screens used in agriculture to determine the force on them and the static pressure drop across them. They found that the wind pressure on a screen and the pressure drop across it increased exponentially with increasing apparent air velocity. In addition, the fabric density and configuration greatly affected the pressure drop. Miguel et al. (1998) derived the motion equation for one-dimensional mass transfer through a permeable material that can be expressed as;

( ) ( )( ) ( )

( ) ( )( )

ρ µ ρ µ p µ Y µ 2u – –– + ––2 u –– = –– - –– u-ρ ––1 |u|u+u – ––2 ε x ε  ε x x  / 2

(4)

with u = εuj where u is the superficial fluid velocity (m/ s), uj is the velocity through the material in direction y (m/s), ρ is the density (kg/m3), p

is the pressure (Pa), µ is the dynamic viscosity (Pa s), Y is the inertial factor and x is the direction of flow, e is the porosity and K is the permeability of the screen (m2). Lebon and Cloot (1986) stated that the permeability is related to the reciprocal of the collision frequency of diffusing particles and the kinematics fluid viscosity. Then according to Bear and Bachment (1990), for a Reynolds number (Re) smaller than 150 the flow is incompressible. Therefore, equation (4). can be expressed in terms of quadratic law or the Forchheimer equation as; µ Y p –– u + ρ ––1 |u|u = –––  x / 2

()

(5)

where µ is the superficial fluid velocity (m/s), u is the velocity through the material in direction y (m/s), /µ/ is the absolute velocity through the material (m/s), ρ is the density (kg/m3), p is the pressure (Pa), µ is the dynamic viscosity (Pa s) and Y is the inertial factor and x is the direction of flow. Materials and methods Physical dimensions Samples of transparent polyethylene film and polyethylene insect screens were prepared according to the requirements of the physical properties to be measured. Three screens N50, N32 and N24 were used in these studies. N50 means there are 50 holes per inch. Fordingbridge Limited and Tildenet Limited, United Kingdom supplied polyethylene film and insect screens respectively. The test programme is shown in Table 1. A fluorescence microscope with CCD camera was used to measure the individual screen hole size. The image from the camera was sent to a computer that had image analysis software program to process the data. This measurement system gave the desired accuracy and the results could be obtained from the computer print out. Plate 1 shows the apparatus for measuring the hole size at Silsoe College, Cranfield 59

Covering materials for tropical greenhouses

Table 1. Experimental programme for measuring the physical properties of polyethylene film and screens Material

Thickness (m)

Hole area (m2)

Coefficient of discharge (dimensionless)

Direct light transmission (%)

Diffuse light transmission (%)

Polyethylene sheet Insect screens Screen N50 Screen N32 Screen N24











✓ ✓ ✓

✓ ✓ ✓













Note: ✓ means testing is carried out otherwise is ✕

Plate 1. Apparatus for screen hole measurement

University. The specifications of the image analysis instruments are as follows; 1. Optomax V image analysis computer: 256 grey levels, 704 x 560 pixels, image editing 2. Leitz fluorescence microscope: 1.6–100x objectives, 3 narrow band filters 3. Optical bench/macro-stand 4. Newvicon, videcon and CCD cameras 5. Custom Software The thickness of both film and screens were measured using a micrometer gauge.

60

Diffuse light transmission The diffuse light transmissions of the samples were measured using a diffuse light transmission goniometer. This is shown in Plate 2. The specifications of the diffuse light transmission are as follows; Hemisphere

Light source

: 2.0 m diameter semi hemispherical dome with an eastman white reflectance coating on the internal surface. : 120 units of 8 W fluorescent lamps.

R. Kamaruddin, B. J. Bailey and M. P. Douglass

Plate 2. Diffuse light transmission goniometer

Spectroradiometer : Bentham Instruments Ltd. Monochromator – M300 Detector – photomultiplier, Hamatsu R446 185–87 ηm Diffraction grating – 1200 line/mm, 0.1 µm Light guide – Macam UV, 100 µm fibre, 500 mm long Integrating sphere : 600 mm diameter, 2 port copper sphere with an eastman white reflectance coating on the internal surface Sphere aperture : 200 diameter aperture on top of the sphere Accuracy : ± 1% The sample was placed on top of the sphere aperture and then the measurement was taken. A second measurement was taken when the top aperture was left open. The transmittance was calculated as the ratio of the first measurement to the second.

Direct light transmission Direct light transmission of the samples was measured using a direct light transmission goniometer. This machine was developed by Silsoe Research Institute as shown in Plate 3. The specifications of the machine are as follows; Light source

Collimator

Mirror

: 600 w tungstenhalogen overhead projector lamp : Spherical mirror and Fresnel lens to produce a horizontal light beam : Two-plane mirrors mounted in a ring which can rotate through 360 ° about the horizontal axis of the light beam and rotate the beam through 360 ° in a vertical plane

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Covering materials for tropical greenhouses

Plate 3. Direct light transmission goniometer

Spectroradiometer : Bentham Instruments Ltd Monochromator – M300 Detector – photomultipler, Hamatsu R446 185– 87 ηm Diffraction grating – 1200 line/mm, 0.1 µm Light guide – Macam UV, 100 µm fibre, 500 mm long Integrating sphere : 400 mm diameter, 2 port copper sphere with an eastman white reflectance coating on the internal surface Sphere aperture : 100 mm diameter aperture on top of sphere with a second identical aperture inclined at 45 ° Accuracy : ± 1% Measurements were taken after the light source reached a constant operating temperature and its light output became

62

stable. The mirror assembly was rotated so that the light beam was vertical and centred over the horizontal aperture at the top of the integrating sphere. The test sample was placed over the top aperture and the inclined aperture was left opened but shielded from stray light. The test sample was placed over the inclined aperture and shielded from stray light and the top aperture was left open. A second measurement was then taken. The transmittance was calculated as the ratio of the first measurement to the second. With this method of measurement the reflectivity of the sphere remains constant and the transmission value was not influenced by the reflectivity of the sample. Coefficient of discharge The static pressure difference and air velocity across the insect screens were measured using a fan test rig according to BS 848. The rig consisted of a fan to supply air at different air speeds, diffusing screens for straightening the air flow, a louvered shutter to regulate the air flow direction and

R. Kamaruddin, B. J. Bailey and M. P. Douglass

Plate 4. Fan test facility for measuring coefficient of discharge

the test material frame mounted at the outlet. In order to measure the related parameters, the instrumentation such as Type FC014 manometers by Furness Controls Ltd., platinum resistance thermometer by H. Tinsley & Co. Ltd., DOL 14 hygrometer by Skov Bros., barometer series 8190 by KDG Instruments Ltd. and the computer logging system was incorporated with the test rig. The fan test rig, used in these studies was developed by Silsoe Research Institute is shown in Plate 4. In the experiment, the screens were mounted in the frame at the outlet of the rig and the approach air speed was controlled by the fan. The approach air speed through the screen is defined as the airflow rate through the screen divided by the total area occupied by the screen. Measurement was taken by regulating the fan for different approaching air speeds. Once the desired air speed across the screen sample was made, parameters such as approaching airspeed, air pressure, air temperature and relative humidity at the test inlet and outlet were measured by the sensors. Then, the data was recorded by a data logger and could be produced graphically on the computer screen.

Results and discussion Area of screen hole The mean hole areas for screens N50, N32 and N24 are shown in Table 2. The area was measured between the edges of the threads that separate the hole. Screen N50 had the smallest hole area and screen N24 had the biggest hole area. If the openings are considered in relation to the actual area of the screens, the ratios are 53%, 66% and 68% for screens N50, N32 and N24 respectively. Thickness of materials The thicknesses of transparent polyethylene sheet and polyethylene insect screens are shown in Table 3. The means of the sheet thickness are 0.178 mm, 0.372 mm, 0.348 mm and 0.351 mm for polyethylene, screen N50, N32 and N50 respectively. Diffuse light transmission The diffuse light transmission of the transparent polyethylene sheet and the polyethylene insect screens are shown in Table 3. The means of the diffuse light transmission of the polyethylene film and screens N50, N32 and N24 are 82.0%, 75.0%, 78.9% and 83.2% respectively. 63

Covering materials for tropical greenhouses

Table 2. Area of single hole in the different sizes of insect screen (mm2) Screen

n

–x

max

min

S.E.

n-1

Screen N50 Screen N32 Screen N24

117 113 149

0.133 0.401 0.732

0.160 0.580 0.850

0.100 0.270 0.600

0.001 0.005 0.003

0.011 0.058 0.035

Table 3. Thickness and diffuse light transmission of transparent polyethylene film and polyethylene insect screens (%) Material Thickness (mm) Polyethylene Insect screens Screen N50 Screen N32 Screen N24

–x

n

max

S.E.

n-1

100

0.178

0.198

0.155

0.0010

0.008

50 50 50

0.372 0.348 0.351

0.385 0.363 0.355

0.358 0.340 0.348

0.0010 0.0010 0.0002

0.006 0.004 0.002

Diffuse light transmission (%) Polyethylene film 100 82.0 Insect screens Screen N50 50 75.0 Screen N32 50 78.9 Screen N24 50 83.2

86.1

71.3

0.20

2.01

76.1 79.7 84.0

73.7 77.6 82.1

0.07 0.06 0.07

0.51 0.46 0.49

Screen N50 gives the lowest light transmission of the three screens. This is because screen N50 had the smallest hole area to transmit light. The diffuse light transmission of the polyethylene film lies between screen N32 and screen N24. Therefore, the polyethylene film and screens are suitable for cladding the crop protection structure. Direct light transmission The results of direct light transmission at different incidence angles are presented in Table 4. The angle of incidence was relatively measured perpendicular to the sample sheet surface. An angle of 0° means light perpendicular to the sheet surface. The effect of incidence angle on the direct light transmission is presented in Figure 1. The figure shows that screen N50 gave the lowest light transmission compared to screen N32 and screen N24. This is due to the screen N50 having the smallest hole area to transmit light. However, the polyethylene film transmission fell within the range of the three screens. Therefore, the

64

min

capabilities of the polyethylene film and screens to transmit solar radiation into the crop protection structure are similar. In addition, the effect of rough screen surface to scatter the light transmission at different incidence angles was insignificant if compared to the film surface. The figure also shows that light transmission is highest for light incident perpendicular to the sheet surface. The transmission gradually reduces when the angle of incidence increases. Theoretically, light transmission of transparent materials follows the cosine curve law where light transmission decreases when the angle of incidence increases. The figure also shows all screens and polyethylene curves do not follow the cosine curve. Light transmission curves of materials are lower than the theoretical curve at angles 0°–25° and will be higher when the angle is more than 25°. In general, light transmission is sufficient for crop production when the angle of incidence is less than 30°. However, the light transmission for angles more than 80 ° are difficult to be measured.

R. Kamaruddin, B. J. Bailey and M. P. Douglass

Table 4. Direct light transmission of polyethylene film and polyethylene insect screens (%) –x

max

min

S.E.

n-1

100 100 100 100 100 100 100 100 100

91.3 90.7 90.6 89.8 88.6 85.2 79.8 67.2 52.9

92.0 96.0 91.1 90.3 89.2 86.3 81.7 69.3 56.6

90.9 90.0 89.9 89.1 87.8 83.2 77.3 64.9 50.3

0.02 0.06 0.02 0.02 0.03 0.06 0.09 0.10 0.16

0.20 0.59 0.20 0.24 0.30 0.57 0.90 0.96 1.58

Insect screens Screen N50 Angle 0° Angle 10° Angle 20° Angle 30° Angle 40° Angle 50° Angle 60° Angle 70° Angle 80°

50 50 50 50 50 50 50 50 50

91.0 89.9 89.2 87.6 84.4 79.4 73.0 66.7 64.4

91.6 90.6 90.3 88.5 85.3 80.9 74.6 70.0 69.6

90.0 88.9 88.1 86.1 83.1 78.0 71.4 64.1 59.5

0.06 0.05 0.05 0.07 0.08 0.10 0.10 0.19 0.42

0.42 0.37 0.37 0.48 0.57 0.68 0.73 1.37 2.93

Screen N32 Angle 0° Angle 10° Angle 20° Angle 30° Angle 40° Angle 50° Angle 60° Angle 70° Angle 80°

50 50 50 50 50 50 50 50 50

93.3 91.9 90.9 89.9 88.0 84.9 79.6 70.7 66.9

94.1 92.5 91.5 90.5 89.2 86.0 81.2 71.9 69.8

92.3 91.2 90.0 88.9 87.0 83.4 77.4 69.3 65.4

0.06 0.05 0.05 0.05 0.08 0.09 0.13 0.09 0.15

0.44 0.32 0.37 0.38 0.54 0.60 0.89 0.60 1.08

Screen N24 Angle 0° Angle 10° Angle 20° Angle 30° Angle 40° Angle 50° Angle 60 ° Angle 70° Angle 80°

50 50 50 50 50 50 50 50 50

96.0 95.0 93.7 92.9 91.4 88.6 84.4 75.2 67.2

96.7 95.7 94.6 94.1 92.2 90.0 85.7 77.2 71.5

95.2 94.4 91.4 92.3 90.4 87.6 83.5 73.6 63.9

0.05 0.05 0.08 0.06 0.06 0.07 0.08 0.11 0.24

0.36 0.36 0.57 0.40 0.43 0.47 0.56 0.77 1.73

Material

n

Polyethylene sheet Angle Angle Angle Angle Angle Angle Angle Angle Angle

0° 10° 20° 30° 40° 50° 60° 70° 80°

Coefficient of discharge The coefficients of discharge of the insect screens are presented in Table 5. The means of the coefficients of discharge for screen N50, screen N32 and screen N24 are 0.411, 0.520 and 0.547 respectively. Screen N50

gives the lowest coefficient of discharge compared to other screens. This is due to the screen N50 having the smallest holes that created the highest pressure drop and the lowest airspeed through the screen.

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Covering materials for tropical greenhouses

100

Direct light transmission (%)

90 80 70

Screen N 50 Screen N 32 Screen N 24 Polyethylene Cosine curve

60 50 40 30 20 10 0

0 10 20 Angle of incidence (°)

30

40

50

60

70

80

90

Figure 1. Relationship between direct light transmission and angle of incidence for polyethylene film and insect screens Table 5. Coefficient of discharge of polyethylene insect screens (dimensionless) Screen

n

–x

max

min

S.E.

n-1

Insect screen Screen N50 Screen N32 Screen N24

52 65 50

0.411 0.520 0.547

0.412 0.521 0.548

0.411 0.519 0.547

0.00004 0.00008 0.00003

0.0003 0.0006 0.0002

The linear relationships between the pressure drop and the square of the airspeed across screens N50, N32 and N24 are presented in Figure 2. This relationship is in good agreement with the findings of Sase and Christianson (1990) and Baker et al. (1994). The figure also shows that the pressure drop increases with decreasing screen hole and with the square of the approach airspeed. The linear regression equations and coefficients of determinations derived from Figure 2 are summarized in Table 6. The relationship between pressure drop and airspeed for the three screens according to Forchheimer’s flow regime is presented in Figure 3. This relationship is in agreement with the finding of Miguel (1988), where the pressure drop across the screen increases with increasing apparent airspeed according to a quadratic law. The figure also shows that the smallest screen hole gives the highest pressure drop. This is because the 66

smallest opening has the smallest value of the discharge coefficient. The regression equations and coefficients of determination for the screen are summarized in Table 7. The results of airflow across the screens in the forms of linear relationship are useful to quantify natural ventilation rate according to equation (5), while the quadratic can be used to fit ventilation equation according to Miguel (1988). Conclusion The physical properties of covering materials varied according to the nature of materials. Diffuse and direct light transmission of transparent polyethylene sheet and insect screens are relatively close to each other and so do not have much effect on the overall light transmission of the tropical greenhouse structures. Direct light transmission decreases when the angle of incidence is closer to the sheet or screen surface. In general, the direct and diffuse

R. Kamaruddin, B. J. Bailey and M. P. Douglass

35

Screen N50

30

Pressure drop ∆P, Pa

25 Screen N32 Screen N24

20 15 10 5 0 0 2 4 Square of airspeed U (m2/s2)

6

8

10

12

Figure 2. Relationship between pressure drops and square of airspeed through the insect screens Table 6. The linear regression equations and coefficients of determination derived from Figure 2 Screen

n

Screen N50 Screen N32 Screen N24

52 65 50

Equation 3.412U2

DP = DP = 2.120U2 DP = 1.908U2

Adjusted R2

S.E.

0.971 0.983 0.979

0.028 0.006 0.006

35 Screen N50

30

Pressure drop ∆P, Pa

25 20

Screen N32 Screen N24

15 10 5 0 0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

Square of airspeed U (m/s) Figure 3. Relationship between pressure drop and airspeed through the insect screens

67

Covering materials for tropical greenhouses

Table 7. The summary of quadratic regression equations and coefficients of discharge of screens derived from Figure 3. Screen

n

Equation

R2

Screen N50 Screen N32 Screen N24

52 65 50

DP = 2.536U2 + 2.324U DP = 1.962U2 + 0.410U DP = 1.794U2 + 0.309U

0.9997 0.9997 0.9994

light transmission of both polyethylene film and screens is more than 90% and 75% respectively. However, the direct light transmission does not follow the theoretical cosine law. The coefficients of discharges for screen N50, screen N32 and screen N24 are 0.411, 0.520 and 0.547 respectively. The smallest screen gives the smallest value of the coefficient of discharge. When air flowed through the screen, it was found that there was a linear relationship between pressure drop and the square of approaching air speed. In addition, the relationship between the pressure drop and apparent airspeed is in accordance with Forchheimer’s flow regime, where the pressure drop increases with airspeed according to a quadratic law. Finally, information on physical properties of polyethylene and insect screens are useful for the greenhouse designed and quantifying natural ventilation. Acknowledgements The authors wish to express their appreciation to Mr. A. Hilton, Mr. D.J. Wilkinson and Mr. A.G.T. Lockwood for their assistance in this study. Special appreciation also goes to Cranfield University, Silsoe and Silsoe Research Institute for providing the testing facilities, and MARDI for the financial support. References Bailey, B. J. (1981). The reduction of thermal radiation in glasshouses by thermal screens. J. Agric. Eng. Res., 26: 215–4 Baker, J. R. and Shearin, E. A. (1994). An update on screening for the exclusion of insect pests. N.C. Flower Growers Bulletin 39(2): 6–11

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Bear, J., and Bachment, Y. (1990). Theory and applications of transport phenomena in porous media. New York: Kluwer Academic Publishers Berlinger, M., Mordechai, J. S. and Leeper, A. (1991). Application of screens to prevent whitefly penetration into greenhouses in the Mediterranean basin. Bull. IOBC/WPRS XIV (5): 105–10 Bot, G. P. A. (1983). Greenhouse climatic: from physical process to a dynamic model. Ph. D. Thesis, Agricultural University of Wageningen, The Netherlands Briassoulis, D., Waaijenberg, D., Gratraud, B. and von Elsner, B. (1997). Mechanical properties of covering materials for greenhouses. Part 2: Quality assessment. J. Agric. Eng. Res. 67:171–217 Brundrett, E. (1993). Predication of pressure drops for incompressible flow through screens. J. Fluids Eng. 115: 239–42 Hawa, J. and Rezuwan, K. (1990). Performance and feasibility of high value vegetable cultivation under rainshelters. Special report presented at MARDI scientific Council Meeting, No. 66, 17 Dec. 1990 Serdang Malaysia. 40p Hawa, J., Embi, Y. and Rezuwan, K. (1992). Vegetable cultivation under simple rainshelters in Malaysia. Extension Bulletin No. 350: FFTC. Teipei City, Republic of China on Taiwan Illias, M. K., Rezuwan, K., Hawa, J. and Mohd. Khairol, M. A. (1992). High-value vegetable’s production under rainshelter. Paper presented to MARDI Senior Staff Conference, Hilton Kuala Lumpur Malaysia Illias, M.K., Rezuwan, K. and Tengku Ariff, T.A. (1994). Peluang pelaburan dalam pengeluaran sayur-sayuran di bawah struktur pelindung hujan. Serdang: MARDI Kosmos, S. R., Riskowski, G. L. and Christianson, L. L. (1993). Force and static pressure resulting from airflow through screens. Transactions of the American Society of Agricultural Engineers 36(5): 146–72

R. Kamaruddin, B. J. Bailey and M. P. Douglass

Lebon, G. and Cloot, A. (1986). A thermodynamically modelling of fluid flows through porous media: application to natural convection. Int. J. of Heat and Mass Transfer 29: 381–90 Miguel, A. A. F. (1998). Transport phenomena through porous screens and openings: from theory to greenhouse practice. Ph. D. Thesis. Agricultural University of Wageningen, The Netherlands Miguel, A. F., Van de Braak, N. J. and Bot, G. P. A. (1998). Analysis of the airflow characteristic of greenhouse screening Materials. J. Agri. Eng. Res. 67: 105–12 Miguel, A. F., Van de Braak, N. J., Silva, A. M. and Bot, G. P. A. (1997). Forced fluid motion through openings and pores. Building and Environment (in press) –––– (1998). Free-convection heat transfer in screened greenhouses. J. Agric. Eng. Res. 69: 133–9 Montero, J. I., Munoz, P. and Anton, A. (1997). Discharge coefficients of greenhouse windows with insect-proof screens. Acta Horticulturae 443: 71–7 Nijskens, J., Deltour, J., Coustisse, S., and Nisen, A. (1985). Radiation transfer through covering materials, solar and thermal screens of greenhouses. Agricultural and forest Meteorology 35: 229–42

Rezuwan, K. (1992), Rekabentuk, membina dan penyelengaraan struktur rumah pelindung hujan berdininding jaring kalis serangga. Nota Bahagian Kejuruteraan Pertanian, MARDI Serdang –––– (1995). Insect-proofed rainshelter structures for temperate vegetable production in the lowlands. Paper presented to Malaysian Sciences and Technology Congress, Universiti Malaya, Kuala Lumpur, Malaysia –––– (1999). A naturally ventilated crop protection structure for tropical conditions. Ph. D. Thesis. Cranfield University, United Kingdom Ross, D. S. and Gill, S. A. (1994). Insect screening for greenhouses. Information Facrs. Facts 186. College Park, Md., University of Maryland Russell, R. W. J. (1985). An analysis of the light transmittance of twin-walled materials. J. Agric. Eng. Re. 31: 31–53 Sase, S and Christianson, L. L. (1990). Screening greenhouses - Some engineering considerations. American Society of the Agricultural Engineering. Paper No. NABEC 90–201 Teitel, M. and Shklyar, A. (1998). Pressure drop across insect-proof screens. Transaction of the American Society of the Agricultural Engineers 41(6): 1829–34 Yeoh, K. C. (1992). Design and construction of rainshelters. Extension Bulletin No. 350:FFTC. Taipei City Center. Teipei City, Republic of China on Taiwan

Accepted for publication on 23 August 2000

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Physical properties of covering materials for naturally

J. Trop. Agric. and Fd. Sc. 28(1)(2000): 55–69 R. Kamaruddin, B. J. Bailey and M. P. Douglass Physical properties of covering materials for naturall...

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