Frost Protection - UTL Repository

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] F R O S T P R O T E C T I O N : F U N D A M E N T A L S , P R A C T I C E A N D E C O N O M I C S [ consumes about 1.0 litre h -1 , then dividing the fuel requirement by the consumption rate gives a range between 133 and 265 liquid-fuel heaters per hectare (H H ). For more efficient protection, it is best to keep the fuel consumption per heater low and use more heaters. The energy output for commonly used liquid and solid fuels is provided in Table 7.2. Note that the energy output is in MJ l -1 for liquid-fuel, MJ per cubic metre for gas and MJ per kilogram for solid fuels. If the fuel consumption rate (F C ) and energy requirement (E R ), including additional energy required for inefficiency, are known, then the number of heaters per hectare can be determined. Use Equation 7.1 to determine the number of liquid-fuel heaters per hectare from the energy requirement (E R ) in W m -2 , the fuel energy output (E O ) in MJ l -1 and the fuel consumption rate (F C ) in litre h -1 per heater: HH = [ E ( × 10 ) ] 6 R E O ( 10 7 3.6 × ) F C heaters ha -1 Eq. 7.1 The 3.6 × 10 7 coefficient converts E R in W m -2 to J h -1 ha -1 . For solid fuels, use Equation 7.1 to determine the number of heaters per hectare (H H ) from the energy requirement (E R ) in W m -2 , the fuel energy output (E O ) in MJ kg -1 and the fuel consumption rate (F C ) in kg h -1 per heater. An Excel application program “HeatReq.xls” for calculating both liquid-fuel and solid-fuel heater requirements is included on the computer application disk. Heater placement and management Heater distribution should be relatively uniform with more heaters in the borders, especially upwind and in low spots (Figure 7.5). If the crop is located on a slope, then more heaters should be placed on the upslope edge where cold air is draining into the crop. Under freezing conditions, when the wind speed exceeds 2.2 m s -1 (7.9 km h -1 ), considerable heat loss occurs due to horizontal advection and higher concentrations of heaters are needed on the upwind border. Low spots, which are colder, should also have higher concentrations of heaters. Heaters on the borders should be lit first and then light more heaters as the need increases (e.g. if the wind speed increases or the temperature drops). Heaters are expensive to operate, so they are commonly used in combination with wind machines or as border heat in combination with sprinklers. 150
ACTIVE PROTECTION METHODS TABLE 7.2 Energy output for a variety of common fuels FUEL OUTPUT PER UNIT OUTPUT RELATIVE TO 1 LITRE OIL SYSTEM OUTPUT Liquid fuel Oil (2.8 litre h -1 H -1 x 100 H ha -1 ) Kerosene (2.8 litre h -1 H -1 x 100 H ha -1 ) Propane (2.8 litre h -1 H -1 x 150 H ha -1 ) Gas Fuel Natural gas (1.0 m 3 h -1 H -1 x 265 H ha -1 ) Solid fuel Wood Coal (0.5 kg h -1 H -1 x 360 H ha -1 ) Coke bricks (0.5 kg h -1 H -1 x 365 H ha -1 ) MJ l -1 37.9 37.3 25.9 MJ m 3 40.1 MJ kg -1 20.9 30.2 29.1 litre 1.00 1.02 1.46 m 3 0.95 kg 1.81 1.25 1.30 MJ h -1 ha -1 10 612 10 444 10 878 MJ h -1 ha -1 10 627 MJ h -1 ha -1 See note (1) 5 436 5 311 NOTE: (1) Output depends on wood type, water content of fuel, and size and number of fires. Energy outputs are expressed in MJ per litre, MJ m 3 or MJ kg -1 for liquid, gas and solid fuels. FIGURE 7.5 Sample arrangement of heaters (small dots in the figure), with higher concentrations along the upwind edge and in low spots (after Ballard and Proebsting, 1978). LOW SPOT WIND 151
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- F A O E N A N D V I MO N I T O R
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FOREWORD Agrometeorology deals with
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ACKNOWLEDGEMENTS We want to thank D
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LIST OF PRINCIPAL SYMBOLS Roman Alp
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T n °C Observed minimum temperatur
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CHAPTER 1 INTRODUCTION OVERVIEW Whe
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INTRODUCTION Freeze and frost defin
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INTRODUCTION FIGURE 1.2 Mean air an
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INTRODUCTION FIGURE 1.4 Air (T a )
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INTRODUCTION TABLE 1.2 Categories a
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INTRODUCTION example, because bloom
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INTRODUCTION south in Florida in re
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INTRODUCTION assistance might be in
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CHAPTER 2 RECOMMENDED METHODS OF FR
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RECOMMENDED METHODS OF FROST PROTEC
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] F R O S T P R O T E C T I O N : F
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CHAPTER 4 FROST DAMAGE: PHYSIOLOGY
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FROST DAMAGE: PHYSIOLOGY AND CRITIC
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CHAPTER 5 FROST FORECASTING AND MON
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FROST FORECASTING AND MONITORING di
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FROST FORECASTING AND MONITORING A
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FROST FORECASTING AND MONITORING Fo
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APPENDIX 2 CONSTANTS Specific heat
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- FAO ENVIRONMENT AND NATURAL RESOU
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