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 [ passed by a point within the crop, there was a short-lived increase in the temperature recorded with exposed thermocouples. It is possible that these short-lived temperature increases have a positive effect and prevent freezing of the plant tissue; however, more research is needed to verify if this is true. Recently, some researchers have suggested that the mobile heater might be beneficial because it dries the plant surfaces. Since water typically freezes on the outside of plant tissue and then propagates inside the tissue to cause freezing in intercellular spaces, there may be some validity to this theory. However, more research is clearly needed to validate effectiveness of the machine. WIND MACHINES Conventional wind machines Wind machines (or fans) that blow air almost horizontally were introduced as a method for frost protection in California during the 1920s. However, they were not widely accepted until the 1940s and 1950s. Now they are commonly used in many parts of the world. Wind machines are used on a wide variety of crops including grapevines, deciduous trees and citrus. California citrus orchards are nearly all protected by wind machines. Wind machines generally consist of a steel tower with a large rotating fan near the top. There is usually a two- or four-blade fan with a diameter typically varying from 3 to 6 m. The typical height for fans is about 10–11 m above ground level. However, lower heights are used for lower canopies. To our knowledge, the fan height is set to avoid hitting the trees and there is no aerodynamic reason for the height selection. The most effective wind machines have propeller speeds of about 590 to 600 rpm. Fans rotate around the tower with one revolution every four to five minutes. Most wind machine fans blow at a slight downward angle (e.g. about 7 °) in the tower direction, which improves their effectiveness. When the fan operates, it draws air from aloft and blows at a slightly downward angle towards the tower and the ground. Power to operate the fan usually comes from an engine mounted at the base of the tower; however, some of the older machines have engines that rotate with the fan at the top of the tower. Matching the rotation of fans around their towers so that all fans are blowing in the same direction is believed to improve mixing effectiveness. Before investing in wind machines, be sure to investigate the local climate and local expenses. For example, if there is little or no inversion, then wind machines are not recommended. In California, wind machines are widely used in citrus orchards, which are mainly protected during December through January, but not in deciduous orchards, because inversions tend to be strong during winter months 156
ACTIVE PROTECTION METHODS when citrus need protection, but not in the Central Valley in the spring when deciduous trees need protection. There are reports that wind machines work better after deciduous trees leaf out in the spring. Consequently, fans are not often used in almond orchards that commonly need protection before leaf-out. Conducting a temperature survey to measure temperature inversions during the frost protection period before purchasing wind machines is advisable. If there is little or no inversion, then select a different protection method. Locate machines in places where the wind drift is enhanced by the fans. In some instances, installing machines where they can push cold air out of low spots can be beneficial. Wind machines generally have lower labour requirements and operational costs than other methods. This is especially true for electric wind machines. However, when electric wind machines are installed, the grower is required to pay the power company “standby” charges, which cover the cost of line installation and maintenance. The standby charges are paid whether the wind machines are used or not. In fact, because of increases in the cost of power, electric wind machines have become marginally cost-effective for citrus protection in some regions of California (Venner and Blank, 1995). Internal combustion wind machines are more cost-effective because they do not have the standby charges. However, they require more labour. The capital cost to install wind machines is similar to sprinkler systems, but operational costs are higher. Generally, except for noise, wind machines are environmental friendly. Wind machine noise is a big problem for growers with crops near cities and towns. This should also be considered when selecting a frost protection method. Theory of operation Wind machines provide protection by increasing the downward sensible heat flux density and by breaking up microscale boundary layers over the plant surfaces. Fans do not produce heat, but redistribute sensible heat that is already present in the air. The fans mix warm air aloft with colder air near the surface (Figure 7.7). They also help by removing the coldest air close to the leaves and replace it with slightly warmer ambient air. The amount of protection afforded depends mainly on the unprotected inversion strength. The inversion strength is calculated as the difference between the 10 m and 1.5 m temperatures in an unprotected orchard. Within the region influenced by a wind machine, the average air temperature measured at 1.5 m increases by about 1/3 of the inversion strength. Near the wind machine tower, the protection achieved is better (Figure 7.8). The actual benefit depends on inversion characteristics, which cannot be generalized. However, stronger inversions clearly give better protection. 157
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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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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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APPENDIX 2 CONSTANTS Specific heat
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- FAO ENVIRONMENT AND NATURAL RESOU
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