The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of the American Society of Agricultural Engineers (ASAE), and its printing and distribution does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by ASAE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is from an ASAE meeting paper. EXAMPLE: Author's Last Name, Initials. 2002. Title of Presentation. ASAE.CIGR Meeting Paper No. 02xxxx. St. Joseph, Mich.: ASAE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASAE at hq@asae.org or 616.429.0300 (2950 Niles Road, St. Joseph, MI 49085.9659 USA).

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��������

First Name Middle Name Surname Role Email

Raphael

Moshe

Ido

Linker

Tarnopolsky

Seginer

Research Ass.

Research Ass.

Professor

linkerr@tx.technion.ac.il

���� ������

Organization Address Country

Technion.Israel Institute of Technology

Agricultural Engineering Department, Technion

Haifa 32000

Israel

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Pub ID Pub Date

024040 2002 ASAE.CIGR Annual Meeting Paper

The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of the American Society of Agricultural Engineers (ASAE), and its printing and distribution does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by ASAE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is from an ASAE meeting paper. EXAMPLE: Author's Last Name, Initials. 2002. Title of Presentation. ASAE Meeting Paper No. 02xxxx. St. Joseph, Mich.: ASAE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASAE at hq@asae.org or 616.429.0300 (2950 Niles Road, St. Joseph, MI 49085.9659 USA).

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Agricultural Engineering Department, Technion, Haifa 32000 Israel.

Email: linkerr@tx.technion.ac.il

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Agricultural Engineering Department, Technion, Haifa 32000 Israel.

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Agricultural Engineering Department, Technion, Haifa 32000 Israel.

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4������� natural ventilation, fan ventilation, 50.mesh screen

2

� ��������� �

In recent years growers have been forced to fit greenhouse openings with insect.proof screens. These screens increase the resistance to flow, resulting in a reduced ventilation rate and increased temperature. Screens of different hole sizes are used for the exclusion of different pests. Based on Bethke and Paine (1991), Bethke et al. (1994), and Both and Albright (1990), the limiting sizes of the individual openings are, approximately, as follows:

Green peach aphid Myzus periscae 0.9 mm2 Bethke et al.

Serpentine leaf miner Liriomyza trifolii 0.4 mm2 Bethke et al.

Melon aphid Aphis gossypii 0.2 mm2 Bethke et al.

Siverleaf whitefly Bemisia argentifolii 0.2 mm2 Bethke et al.

Bemisia tabaci 0.2 mm2 Bethke & Paine

Western flower thrip Frankliniella occidentalis 0.03 mm2 Both & Albright

While insect exclusion depends mainly on the size of the individual openings (Bethke et al., 1994), the resistance to flow is mainly a function of the porosity (open to total area) of the screen (Dierickx, 1998) and of the Reynolds number. A common approach to modeling the

effect of porosity is to make the resistance proportional to 22 /)1( αα− , were α is the porosity

(Pinker and Herbert, 1967; Brundrett, 1993). At low Reynolds numbers (fine screen), when flow is laminar, the resistance to flow is inversely proportional to Re. At very high Reynolds numbers the resistance approaches a constant.

When operated under field conditions, the screens accumulate dust, and the resistance to air flow increases over time. Both and Albright (1990) reported that dust accumulation on a 100 mesh screen, over a period of three days in a laboratory test setup, resulted in doubling of the pressure drop across the screen. On the other hand, Roberts et al. (1995), working in a fan ventilated greenhouse, noted that $����� ��������������� �%��������&��������������� ����� �' ����������������������������������� ���������(.

It turns out that the reduction in ventilation rate as a result of the screens is quite different for fan.ventilated and naturally ventilated greenhouses. Regarding fan.ventilated greenhouses, Roberts et al. (1995) suggested that such greenhouses $������������������������ ������ ���� ���� ������)���� * ���������� �������������������������� ������(. For natural ventilation, on the other hand, Sase and Christianson (1990) estimated that a greenhouse outfitted with a dense screen $� ���������������+,������ ������������������������������������������ ������ ��(. Montero et al. (1997) showed that anti.thrips and anti.aphid screens reduce the discharge coefficient by 50% and 35% respectively. Muסoz et al. (1999) found that placing an anti.aphid screen over the vents of a naturally ventilated multi.span tunnel reduced the ventilation rate by about 70%. Teitel et al. (1999) showed that the temperature.rise (difference between greenhouse and outdoors) when the vents are open and covered with a 50 mesh insect screen, is 45% of the rise when the vents are closed. Another, less dense screen, resulted in only 20% rise, meaning that the temperature.rise due to the insect screen was larger by at least a factor of two compared to unscreened vents (>45%/20%).

The objective of this study was to determine the loss coefficient of the insect screen used in our experimental greenhouse, to quantify the effect of dust accumulation, and to compare these findings with results from the literature.

3

5���������������������, ������� �5����

The two major contributors to pressure drop across a greenhouse are the inlet(s) and outlet(s)

oi ppp += . [1]

If an opening is screened, the pressure drop across it may be increased considerably.

The pressure drop across a screen, sp , is a function of the approach velocity, u. The two

simplest (and equivalent) forms of this function are:

K The Forchheimer equation (Miguel et al., 1997; Muסoz et al., 1999; Dierickx 1998)

buaup 2s += , [2]

K The use of a variable loss coefficient (Kosmos et al., 1993; Idelchik, 1996; Montero et al., 1997. Teitel and Shklyar, 1998)

2

21

s ukp ρ= , [3]

where ρ is the fluid density, and the pressure loss coefficient, k, is a function of the Reynolds number, Re

η+ζ

=Re

k . [4]

In Equation [4]

ν

=ud

Re , [5]

where d is the thread (wire) diameter and ν is the kinematic viscosity of the fluid. Substituting from Equations [5] and [4] into Equation [3], the result is seen to be equivalent to Equation [2], with

ηa21 ρ=

d

ζνb

21 ρ= . [6]�

Using the formulation of Equation [3], the loss across the greenhouse (Equation [1]) becomes

2

o21

o

2

i21

i ukukp ρ+ρ= . [7]

Defining a velocity ratio (outlet/inlet)

i

o

u

u≡β , [8]

the pressure drop becomes

( )o

2

i

2

i21 kβkup +ρ= . [9]

When p is known, the resulting inlet approach.velocity is

)kβ(k

2pu

o

2

i

i +ρ= . [10]

4

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Typical wind pressure coefficients of a greenhouse are: windward +0.7, leeward –0.5, totaling 1.2 (Albright, 1990). Hence, at a wind speed of 2 m/s, the pressure drop across the house is about 3 Pa (Equation [3]). Assuming that two opposite walls serve as screened inlet and outlet, that a typical clean insect screen loss coefficient is 10, and that it increases to 50 when the screen is dirty, the inlet velocity becomes

no screens ki = 2 ko = 2 β = uo/ui = 1 p = 3 Pa → ui = 1.12 m/s

clean screens ki = 10 ko = 10 β = uo/ui = 1 p = 3 Pa → ui = 0.50 m/s

dusty inlet ki = 50 ko = 10 β = uo/ui = 1 p = 3 Pa → ui = 0.29 m/s

both dusty ki = 50 ko = 50 β = uo/ui = 1 p = 3 Pa → ui = 0.22 m/s

This suggests that a screen may reduce the natural ventilation flow by a factor of two, and dust – by another factor of two.

����������������

The screened inlet area is assumed to be an order of magnitude larger than the unscreened outlet (fan) area. A very crude fan characteristic (linear) is

β−=

−=

m

i

m

m

o

mu

u1p

u

u1pp . [11]

From Equations [10] and [11]

0p2u

up2u)kk( m

m

im

2

io

2

i =−β+β+ρ . [12]

Typical parameter values are pm = 50 Pa and um = 10 m/s. Hence

no screen ki = 2 ko = 1 β = uo/ui = 10 → ui = 0.89 m/s

clean screen ki = 10 ko = 1 β = uo/ui = 10 → ui = 0.83 m/s

dusty screen ki = 50 ko = 1 β = uo/ui = 10 → ui = 0.67 m/s

The reduction of ventilation rate in this case is less than 10% when the screen is clean and about 25% when the screen is dusty. These are overestimates, perhaps by a factor of two, resulting from the unrealistic linear fan characteristic (Equation [11]).

The results of these order.of.magnitude estimates are in agreement with the impression, based on the literature review, that screening in general and dust accumulation in particular, are not a significant problem in fan ventilated greenhouses, but are of major concern in naturally ventilated systems.

"����*���� ��$����� ���

The screen under consideration is a nominal 50 mesh insect screen, made of woven nylon threads, 0.25 mm in diameter. The wire spacing is 26 per inch by 52 per inch, resulting in

rectangular openings of approximately 0.18 mm2 area and porosity, α, of about 0.36. Such a screen should be able to exclude aphids and whiteflies but not thrips.

The pressure loss coefficient of the screen was measured in a small laboratory test rig and in a 50 m2, fan.ventilated greenhouse in Haifa, Israel. The laboratory screen sample was a 300 cm2

5

circle, while the greenhouse air.inlet over which the velocity was integrated, measured 0.80

(vertical) × 5.40 (horizontal) m2. The approach velocity in the laboratory could be varied from 0.2 to 1.0 m/s, equivalent to a wire.based Reynolds number of 3 to 17. The approach velocity in the greenhouse was 0.6.0.7 m/s. In both setups, the mean approach velocity was calculated from several velocity profiles measured parallel to the screens. The mean velocity and the measured pressure drop across the screen were used to calculate the loss coefficient via Equation [3]. The fit of the laboratory results to Equation [4] produced

5.5Re

6.87k s += , [13]

which can be rearranged as the product of a function of porosity and a function of the Reynolds number (Pinker and Herbert, 1967).

+=

+α

α−= 82.0

Re

0.1372.682.0

Re

0.13)1(k

2

2

s [14]

Figure 1 shows the individual laboratory and greenhouse results, as well as Equation [13]. In addition, the laboratory results of Teitel and Shklyar (1998) and Teitel et al. (1999) for a screen with the same specifications are also shown. The agreement is fair, indicating that a representative value of the loss coefficient for a clean 50.mesh screen is about 12, for a typical inlet velocity of 0.65m/s (Re approximately 10, as in the greenhouse experiment). In other words: a pressure loss of about 12 times the dynamic pressure, or about 3 Pa for an approach velocity of 0.65 m/s. This is in fair agreement with other results for woven insect screens of around 50 mesh (Kosmos et al., 1993; Sase and Christianson, 1990; Miguel et al., 1997: Econet SF). In contrast, the anti.thrips screen tested by Roberts et al. (1995) had a loss coefficient of about 40, and screens with punctured holes had loss coefficients of 90 and 180 (Sase and Christianson, 1990), making them much less attractive from the pressure loss point of view.

The greenhouse measurement of Figure 1 was taken shortly after the screen had been thoroughly scrubbed clean in February 2000. Following the measurement, the greenhouse was operated continuously at the maximum ventilation rate (approach velocity of 0.65 m/s) for four months, until the end of June. Several additional measurements were taken over the first ten days and one final measurement was taken at the end of the period. The results (Figure 2) show an initial sharp increase of the loss coefficient, reaching a level of 75 in the first 10 days. From then on, until the final measurement, the increase was more moderate, perhaps asymptotic, reaching finally a level of ks=200. At this point a very considerable amount of dust had accumulated, and the pressure loss across the screen reached about 35 Pa. Figure 2 shows, however, that the approach velocity at the inlet screen has declined only slightly, by about 20%. This supports the previous impression that fan ventilation is hardly affected even by a dramatic increase in the resistance to flow of the inlet screen.

6

0

10

20

30

0 10 20 30 40

Re

Pre

ssu

re lo

ss c

oe

ffic

ien

t, k

s

ks(lab)

ks(grh)

ks(Eq. [13])

ks(Teitel, 1998)

ks(Teitel, 1999)

Figure 1: Pressure loss coefficient, ks, for a 50)mesh insect screen. Laboratory (lab)

and greenhouse (grh) data, the best)fit curve for the laboratory data (Equation [13]),

as well as results of Teitel and Shklyar (1998) and Teitel et al. (1999) are shown.

0

10

20

30

40

1/2 3/3 2/4 2/5 1/6 1/7

Date, day/month

Scre

en p

ressure

diffe

rence

p,P

a

0

60

120

180

240

Re

sis

tance

co

eff

icie

nt, k

s, a

nd

ap

pro

ach v

elo

city,

u,

cm

/s

p

u

ks

Figure 2: Influence of dust accumulation on screen loss coefficient, ks, pressure drop,

p, and approach velocity, u.

�������������� �6���������, ������� � ������

Having established that screening and dust accumulation are not likely to affect significantly the environment of well designed fan.ventilated greenhouses, naturally.ventilated greenhouses are

7

now considered. To simplify the analysis, a greenhouse without roof vents is considered, where the inlet and outlet openings (in opposite walls) are of equal size.

A simple energy balance of a greenhouse at times of maximum ventilation is (ASAE, 2000)

)TcQ(T)Sε(1 io −ρ=− , [15]

where S is the solar load inside the greenhouse, Ti is the inlet air temperature, To is the outlet air

temperature, ρ is air density, c is specific heat of air at constant pressure, Q is the ventilation

rate per unit ground area and ε is the so.called evaporation coefficient, the fraction of radiation load that is dissipated as latent heat. The relation between the inlet velocity ui and Q depends on the dimensions of the greenhouse:

w

uhQ ii= , [16]

where hi is the height of the inlet opening, assumed to span the whole length of the inlet wall, and w is the width of the greenhouse, from inlet to outlet wall (again, no roof windows). Substituting from Equation [16] into Equation [15], yields

)T(Tuhw

c)Sε(1 ioii −

ρ=− , [17]

Substituting for ui from Equation [10], with β=1, the result is

oioi

i

io kk}p,S,w,{Fp

S)kk(

h

w

2c

1TT +ε≡+

ρ

ε−=− , [18]

where the first factor of the middle expression is a function of the ‘wetness’ of the greenhouse, via

ε; the second is a function of greenhouse dimensions; the third is a function of the inlet and outlet properties, particularly the screens; and the last factor represents the relevant weather variables: solar radiation and wind (via p). The temperature.rise for the assumed configuration is seen to be proportional to the square root of the sum of the pressure loss coefficients. It doubles when the pressure coefficients quadruple, which in view of Figure 2, is not unlikely to happen in practice. Doubling a design temperature.rise of 3.4K, may have significant undesirable consequences.

*� ������ ��1. Measured loss coefficient for a 50.mesh insect screen agrees with results in the

literature.

2. The loss coefficient of a 50.mesh screen may increase 20 fold through dust accumulation.

3. The pressure loss across insect screens has a significant effect only in naturally ventilated greenhouses.

4. In naturally ventilated greenhouses the accumulation of dust may significantly reduce the ventilation flow and hence increase the temperature.rise. Doubling of the temperature.rise is not unlikely in practice.

�������� �������

This study has been supported by Israel Ministry of Agriculture Grant 838.0453 and BARD Grant IS.2680.96.

8

�� ���

Albright, L. D. 1990. %�� ������������������� ��������������. St. Joseph, MI: ASAE.

ASAE, 2000. Heating, ventilating and cooling greenhouses. ANSI/ASAE EP406.3 MAR98. In #-#%���������, 675.682. St. Joseph, MI.

Bethke, J. A., and T. D. Paine. 1991. Screen hole size and barriers for exclusion of insect pests of glasshouse crops. .���������%��������-� ���� 26(1):169.177.

Bethke, J. A., R. A. Redak, and T. D. Paine. 1994. Screens deny specific pests entry to greenhouses. /�� ��� ��#� ������, May.June:37.40.

Both, A. J., and L. D. Albright. 1990. Thrips exclusion from greenhouses by screening. Part One. Unpublished report. Cornell Univ. ABEN Department. 12 pp.

Brundrett, E. 1993. Prediction of pressure drop for incompressible flow through screens. .��0�� ����%�� ��� ��� 115:239.242.

Dierickx, W. 1998. Flow reduction of synthetic screens obtained with both a water and airflow apparatus. .���������#� ��������%�� ��� ���1������ 71:67.73.

Idelchik, I. E. 1996. 2������3���������� ���� ������. Washington, MD: Hemisphere Publishing Corporation.

Kosmos, S. R., G. L. Riskowski, and L. L. Christianson. 1993. Force and static pressure resulting from airflow through screens. ������� �����������#-#% 36(5):1467.1472.

Miguel, A. F., N. J. van de Braak, and G. P. A. Bot. 1997. Analysis of the airflow characteristics of greenhouse screening materials. .���������#� ��������%�� ��� ���1������ 67:105.112.

Montero, J. I., P. Muסoz, and A. AntSn. 1997. Discharge coefficients of greenhouse windows with insect.proof screens. #����2�� ������� 443:71.77.

Muסoz, P, J. I. Montero, A. AntSn, and F. Giuffrida. 1999. Effect of insect.proof screens and roof openings on greenhouse ventilation. .���������#� ��������%�� ��� ���1������ 73:171.178.

Pinker, R. A., and M. V. Herbert. 1967. Pressure loss associated with compressible flow through square.mesh wire gauzes. .��!�����%������-� � 9(1):11.23.

Roberts, W. J., L. Vasvary, and S. Kania. 1995. Screening for insect control in mechanically ventilated greenhouses. ASAE Paper 95.4541, 12pp.

Sase, S., amd L. L. Christianson. 1990. Screening greenhouses – some engineering considerations. ASAE paper NABEC 90.201, 13 pp.

Teitel, M., and A. Shklyar. 1998. Pressure drop across insect.proof screens. ������� �����������#-#% 1998, 41(6):1829.1834.

Teitel, M, M. Barak, M. J. Berlinger, and S. Lebiush.Mordechai. 1999. Insect.proof screens in greenhouses: their effect on roof ventilation and insect penetration. #����2�� ������� 507:25.34.

9

6�� �������

������!����

a coefficient in Equation [2] s2Pa/m2 b coefficient in Equation [2] sPa/m c specific heat of air at constant pressure J/kgK d thread (wire) diameter m h height of inlet opening m k pressure loss coefficient – p pressure difference Pa pm maximum fan pressure at zero flow Pa Q ventilation discharge per unit ground area m3/m2s Re Reynolds number – S solar load inside greenhouse W/m2 T air temperature K u air velocity m/s um maximum air velocity, when fan operates against zero pressure m/s w width of greenhouse m

α porosity of screen (open to total area ratio) –

β outlet.to.inlet velocity ratio –

ε evaporation coefficient –

ζ coefficient in Equation [4] –

η coefficient in Equation [4] –

ρ air density kg/m3

ν air kinematic viscosity m2/s

����������

i air inlet m maximum o air outlet s screen w wind