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Fan location. Assuming an enclosed building, one to three fans can be located at ceiling level midpoint on the protected side (opposite the prevailing wind) of the building. A greater number of fans may be distributed along the protected side. The high level on the wall is desirable for summer heat removal and has little effect on the efficiency of moisture removal in cold weather. Efficiency in this case means the amount of moisture removed per unit of heat used or lost. If outlet ducts are required, they should be insulated to an R of 0.5 to prevent condensation.
In addition to ventilation rate, it is necessary to consider the distribution of incoming air throughout the building. This is particularly important in both livestock production buildings and product stores.
When considering fresh-air distribution, two distinct temperature situations are involved. In areas with winter frost, outside air is cooler than that inside the buildings and fresh air must be delivered away from the stock so as to avoid cold draughts. In summer, however, the animals may be subject to heat stress and may suffer considerably unless cooling air currents are directed so as to remove excess heat from their vicinity. A good air-distribution system also ensures that the animals receive an adequate supply of oxygen and that noxious gases are removed.
Ventilation is accomplished in an exhaust-type mechanical system by reducing the pressure within the building below outside pressure, causing fresh air to enter wherever openings exist. The principal factors affecting the air-flow pattern in a building are the speed and direction of the incoming fresh air. The size, location and configuration of the air inlets are, therefore, most important in designing the distribution system.
The flow of air streams through openings has been closely investigated and the results can be summarized into the following statements:
It can be deduced from these findings that in winter, openings should be small enough to provide sufficiently high velocities to avoid cold air falling directly onto the stock, to provide good air mixing, and to maintain the required air-flow pattern at the low winter ventilation rate.
Velocities of around 3.5 to 5m/s usually satisfy these requirements. However, at these velocities it is important to consider the effect of internal partitions, structural members and other obstructions to flow, and it also becomes important for the building to be relatively airtight.
When air flows through an opening of any shape, the cross-section area of the issuing jet is reduced to 60 to 80% of the total free area of the opening. 70% is a reasonable design value. This phenomenon, the venacontracta effect, increases the velocity of air emerging from the opening. The total area of air inlet must be proportional to total fan capacity. A common rule of thumb sizes air inlets at 0.4m² of area for each m³/s of fan capacity.
The pressure drop across the inlet affects fan performance and therefore should be no higher than necessary. A draught gauge may be used to check the pressure difference across the inlet (between the inside and outside of the building at the inlet). A pressure difference of 10 to 20Pa indicates a velocity of 4 to 6m/ s. Inlet openings, regardless of type, must be adjustable so that the correct air velocity can be maintained throughout the year.
Compared with inlets, the fan outlets have a minor role to play in the distribution of fresh air in a livestock building. The effect of an outlet is to cause a general slow drift of air towards the outlet position. This drift is easily overcome by convection, animal movement or pattern of air movement established by the inlets. Only near the fan (within approximately 1 m) can a positive air movement be detected. This applies to outlets in both exhaust and pressurized systems of ventilation. However, it is recommended that no inlet be placed closer than 3m from a fan.
Figure 7.9 Air inlets - winter adjustment.
Table 7.5 Ventilation Inlet Data (Vena Contracta = 0.7)
|Static Pressure mm H2O||Velocity m/s||Inlet Area m² per m³/s|
Wind has a major effect on ventilation systems since it causes pressure gradients around buildings and directly impinges on components of the system. This pressure will cause problems of uneven air entry, with more entering on the windward side than the leeward side of the building.
Wind blowing against a fan reduces output and hoods do little to alleviate the problem. Wind blowing across a ridge chimney-type outlet may cause overventilation. Wind effects can be reduced by the following steps:
In situations where air must be distributed and wall or ceiling inlets are not feasible, polythene tubes punched with holes along their length work well. Usually two rows of holes are spaced at 600 to 750mm intervals along the tube. The total hole area should equal approximately 1.5 times the tube cross-section area. Ducts should be sized to provide 4 to 6m/ s velocity. They may be used either to distribute air in a pressure system or as an inlet for an exhaust system. Sizing is the same in either case.
Simple on-off thermostats have given dependable and satisfactory control of many ventilation systems. If the building is small and served by one fan, then a two-speed motor with a thermostat provided with two set temperatures will work well. When several fans are required, one or more may be operated continuously to provide the necessary minimum ventilation rate. Others may be controlled by a thermostat set at the minimum design temperature. These will cycle on and off in cold weather. The balance of the fans may be controlled with a thermostat set at the maximum design temperature. These will only operate in warm weather when excessive heat must be removed.
Filled-type or bimetallic-type thermostats placed 2 metres high near the centre of the building work well as controllers. Electronic controllers, using multiple thermistors to sense temperatures in several locations, combined with variable-speed motors and automaticallyadjusting inlets are available. Although they undoubtedly do a more precise job of controlling the building environment, their additional cost is difficult to justify. Humidistats have not proven very satisfactory as controllers for mechanized ventilation systems.
Ventilation Design Example
Although calculating the heat and moisture balance for a building in cold (below 0°C) weather is not a typical problem for Tropical Climates, a sample will show how the psychrometric chart is used and also the possible difficulties encountered in cold climates.
Assume a farm has sixty 600 kg cows housed in a 10m by 40m by 3m barn with 20m² of windows and 12m² of doors. R values are: window 0.17, door 1.0, ceiling 2.6 and wall 2.1. The temperature and relative humidity are -10° C and 90% outside and + 12° C and 75% inside. The total heat and latent moisture production from the animals is found in Table 10.2 and is 1130W and 0.485 kg/hr per cow. From Figure 7.2, the 1500m psychrometric chart, - 10° C and 90% equals -6kJ / kg enthalphy and 0.0016 kg/ kg specific humidity. Also +12°C and 75% equals 31kJ enthalphy and 0.0078 kg/ kg specific humidity. From the chart, the humid volume at 12°C and 75% equals 0.98m³/ kg, the value at which the fans are exhausting air. 1 kJ = 1/3.6W.
|Heat production 60 x 1130||= 67,800W|
|Respired moisture production 60 x 0.485||= 29.1 kg/hr|
|Heat loss from:|
|Ceiling 400 x 1/2.6 x 22°||= 3,385W|
|Wall (300-32) x 1/2.1 x 22°||= 2,808W|
|Windows 20 x 1/0.17 x 22°||= 2,588W|
|Doors 12 x 1/1.0 x 22°||= 264W|
|Total heat loss||9,045W|
|Heat available for ventilation 67,800 - 9,045||58,766W|
|Minimum air flow to remove moisture 29.1/ (0.0078-0.00 16)||4694 kg / hr|
|Fan capacity at minimum flow 4694 x 0.980/3600||1.28m³/s|
|Heat removed by air flow 4694 x (31.5-(-6)/3.6)||48896W|
As the heat available for ventilation is greater than that actually removed by the minimum ventilation rate, the inside temperature will tend to rise or the relative humidity will fall, but a cycling of additional fan capacity will maintain the desired temperature. It should be pointed out that although the values for moisture production in Table 10.2 include normal evaporation from feed, manure and urine, the evaporation may well run higher or lower, depending primarily on how much wet floor surface is exposed from which evaporation can take place. Greater evaporation would reduce the moisture to be removed with the manure.
If the heat removed by the ventilation is greater than that which is available for ventilation, a fall of inside temperature will result unless the insulation of the building is improved and/ or supplemental heating is installed. It should be noted that a lowered minimum ventilation rate aimed at maintaining the temperature may cause the inside air to become saturated and result in condensation on cold surfaces such as windows.
Calculations using outside summer temperatures, e.g. 21°C, would show the need for additional fan capacity to remove heat and thus maintain an acceptable temperature difference between inside and outside, e.g., 4°C.
Maximum ventilation rate is the product of sensible heat production divided by temperature difference (inside outside) and isobar specific heat capacity.
The sensible heat production is, according to Table 10.2, 465W per animal at 25°C (inside temperature) and the maximum ventilation rate is thus:
60 x 465 (4 x 0.35) = 19.950m³/hr or 5.54m³/s.
Between the cold and warm weather rates, thermostats cause a cycling of fan operation that will maintain temperatures within the desired range.
During high temperature periods, ventilation alone may be insufficient for maintaining satisfactory temperatures in animal buildings. The following cooling system can be effectively used in totally enclosed buildings. Other cooling techniques such as spray cooling are covered in later sections.
The evaporative cooler operates on the simple principle of a fan drawing hot air from outside through a wet pad into the building. The hot air is cooled by evaporating water which changes sensible heat in the air to latent heat in the vaporized moisture thus causing a temperature drop.
Air temperature reduction in buildings of as much as 11°C can be achieved during hot periods with low humidity. In humid weather, the cooling effect is considerably reduced, but the system may be suitable for the greater part of the hot season in many areas.
Commercial evaporative coolers are available in sizes varying in capacity from 1 to 95m³/s. Since they are complete with built-in fans, it is essential that suitable units with correct ducting, diffuser and register sizes be selected to allow balanced air distribution in the building. Ample exhaust vents should be provided around the perimeter of the building to allow the free outlet of air. A thermostat is advisable to control the units. Where humidity control is required, a humidistat can be added to the control circuit. Some designs incorporate a heat exchanger. In these, the air which has been cooled while passing the wet pads is used to cool other air which actually enters the building. While this results in less humid air being used for ventilation, the extra step causes a loss in efficiency.
An alternative to the packaged evaporative cooler can be assembled with a pad and fan system. Pads made of 50mm thick compressed "wood wool" or other suitable material are installed, usually in the long wall of the building, and exhaust fans are positioned in the opposite wall. Incoming air is cooled as it passes through the wet pads and then, after passing through the building, is exhausted by fans, Figure 7.10. For effective operation, the air velocity through the pad area should be limited to about 0.8m/s. This is accomplished with 1 to 1.5m² of pad area per/m³ and second of air flow. The cooled air leaves the pad at a relative humidity of 85 to 90%, but is quickly moderated by the ambient air.
Water is spread evenly over the pads from a manifold supplied from a sump with a float-controlled water level. Recirculation of water through the pads should be at the rate of approximately 1 60ml/ s for each m³/ s air flow. The actual water consumption, which is the evaporation ot water into the passing air, varies with the changing conditions of temperature and humidity. However, as a guide, it is approximately 20% of the water recirculation rate.
Evaporative coolers, which rely on wind pressure to force air through the wet pads, are less effective since the air flow is likely to be either too low or too high most of the time. While naturally ventilated evaporative coolers will require larger pad areas, the fact that no fan or power to drive a fan is required recommends these designs for small scale applications in rural areas. They can usually be constructed with local materials and be operated and maintained by the farmer at low cost.
The value of evaporative cooling systems depends on the application and on the typical wet-bulb temperatures of the region. In areas of high humidity they work well for greenhouses and potato stores, but are not satisfactory for poultry and other animals that depend on respiration for body cooling at high temperatures. Evaporative cooling is much more practical in dry regions where the air can be cooled appreciably while the humidity is still low enough to have little effect on animal comfort.
Figure 7.10 Evaporative cooler.
Meeting the temperature requirements for storing some products may not be possible with ventilation alone or with evaporative coolers. If the product has sufficient value to justify mechanical refrigeration, then nearly ideal conditions may be provided.
Principles of Refrigeration
Most fluids can occur as either a liquid or a vapour depending on pressure and temperature. The higher the pressure and the lower the temperature, the more likely that the liquid phase will occur. Whenever there is a change of phase there will be a concurrent latent heat exchange. That is, when a liquid changes to a vapour, heat is absorbed; when a vapour changes to a liquid, heat is given off. There are several materials that happen to change state at pressures and temperatures which make them useful in mechanical refrigeration systems.
A refrigeration system is comprised of four main parts:
The components are connected together in a complete circuit in the order listed. In addition, there may be a receiver (small tank) between the condenser and the expansion valve. See Figure 7.11.
Figure 7. 11 Refrigeration system.
When the system is charged with a refrigerant, operating the compressor reduces the pressure in the evaporator and causes the refrigerant to boil, evaporate and absorb heat. This causes a drop in temperature. At the same time the compressor is pumping the evaporated vapour into the condenser at high pressure. This causes the refrigerant to condense back to a liquid while giving up heat. The temperature in the condenser will rise. The receiver serves as a reservoir for liquid refrigerant. Obviously the evaporator is installed in the room to be refrigerated and the condenser is located where ambient air can readily absorb the heat produced. The expansion valve is the temperature control mechanism for the system. If it is adjusted to further restrict the refrigerant flow, both the pressure and boiling temperature in the evaporator will drop and within the limit of the system's capacity, the room temperature may be maintained at a lower level.
The pressure on the condenser side is determined largely by ambient conditions. If the air temperature is relatively low, the condenser discharges its heat easily at normal pressures.
However, in very hot weather or if the airflow through the condenser becomes restricted by dust or other debris, the temperature and pressure may rise to levels dangerous to the system, unless a high-pressure safety switch has been installed.
There are a number of fluorocarbon refrigerants used for various temperature applications. The most common, refrigerant 12, is used for applications in the -15 to 10°C range. Ammonia, refrigerant R717, is also used in this temperature range. R12 is odourless, non-toxic, nonflammable and is piped with copper tubing. R717 is toxic, has a strong pungent odour, burns in certain concentrations in air, is prone to leaking and is piped with steel pipes. However, ammonia is cheaper and more efficient because it has a much higher heat of evaporation thus requiring smaller component parts throughout. Consequently ammonia systems are, because of the economies, often chosen for large stores in spite of the disadvantages, but R12 is almost universally the choice for small systems.
Fabricating a refrigeration system requires the specialized equipment and knowledge of a contractor. However, it is a distinct advantage for the customer to know how the evaporator size and corresponding operating temperature relate to the conditions required in the cold store.
A given storage room and product quantity will impose a particular load (watts) on the refrigeration system. That load can be met by operating a relatively small evaporator at a very low temperature (heat moves to its limited surface rapidly), or by operating a larger evaporator at a more moderate temperature (heat moves more slowly but to a much greater surface area). Air passing through an evaporator will, in nearly all cases, be cooled sufficiently to reach saturation (100% RH).
The psychrometric chart shows that the moisture holding capacity (specific humidity) of air at two slightly differing temperatures will be nearly the same, while air at widely differing temperatures will have quite different specific humidities.
For example, assume a store temperature of 10°C and an evaporator temperature of 8° C. The absolute humidity of saturated air at 8° C is 0.0066 kg/ kg. That will allow a relative humidity at 10°C of 89% which is desirable for a potato store.
In contrast, onions store best at 0° C and 75%RH, so a smaller evaporator operating at -5°C and 0.0025 kg/ kg at saturation would provide the desired 75%RH.
Unfortunately refrigeration contractors may not understand this relationship or they may not care and therefore present a bid for a system based on too small an evaporator which would need to be operated at too low a temperature. This would have a lower purchase cost, but fail to provide the proper conditions.
Finally it should be pointed out that in air conditioners for homes one of the objectives is to reduce humidity. Consequently small-size evaporators operated at low temperatures are quite in order.
The use of solar energy dates back to before recorded history and in fact has been and is being used by all farmers in the production of their crops. The purpose here is to note the nature of solar energy and relate that to some applications.
The energy reaching the earth from the sun is referred to as solar flux. The energy approaching the earth's atmosphere perpendicular to the surface is 1.27kW/m². Due to the earth's atmosphere only lkW/m² reaches the earth under optimum conditions and for practical purposes a value of 0.9kW/m² is often used for latitudes where the altitude (angle of the sun's rays to earth) is close to 90°.
Factors that affect the actual amount of energy available in a particular area are:
1 Latitude and season: As the earth is inclined 23.5° degrees, the angle that the sun makes with the earth is continually changing throughout the year. Between latitudes 23.5° north and 23.5° south, the sun will be perpendicular for two days each year and its noon altitude never drops lower than 43°. However, farther north or south the sun never reaches 90° and in winter the angle may be very low. (Only 16 1/2° in winter at 50° latitude north or south).
2 Weather: The frequency of cloudy days is an important factor in the amount of radiation received over a period of time. Although the belts around the earth falling between 20° and 30° both north and south receive nearly 90% of the total solar radiation, there are great regional variations from this. Consequently, in doing design work it is imperative to have solar information for a local area, including seasonal variations.
There are several types of solar collectors including:
A flat-plate collector can be as simple as a water tank painted black or it can be more complex, e.g., a collector surface painted black with one or more transparent layers that allows the sun's rays to enter while reducing the reradiation of heat, all mounted in a tight frame with insulation on the back side. Figure 7.13
Table 7.6 Mean Daily Solar Radiation on a Horizontal Surface (k Wh/m²)
|Dar es Salaam||7°||55||5.42||3.89||4.27||5.22||4.86|
* Max. daily values
Figure 7.12 Mean annual solar radiation on a horizontal surface k Wh/m².
In most cases the heat collected is removed with either air or water. Which one is used depends on the purpose of the collector. That is, to dry products, air would be used; to heat water, water would be used.
Collector plates may be made of metal with water tubes bonded to the plate. Copper has high conductivity and is easily soldered to the plate. Aluminium also has good conductivity but is difficult to bond to the plate. Manufactured aluminium plates have the water lines pressed into the surface.
Glass, fiberglass-reinforced plastic, and plastic films may be used to cover the collector. Glass passes over 90% of the solar energy; fiberglass about 80% if kept clean, and polythene film 90%. However, polythene loses a great deal of heat through reradiation. Glass has the longest life; fiberglass can be expected to last 10 years, and polythene only a year or two.
Figure 7.13 Exploded view of typical flat-plate collector (By courtesy of Cooperative Extension Service, Cornell University).
The efficiency of collectors varies greatly. The parabolic units mentioned earlier may reach 50 to 75%. Flat-plate units operate in the range of 25 to 50% depending on design and position of mounting. Some simple designs may be even less efficient. In many cases an inexpensive, simple design is the most practical to choose. Often an increase in size will offset low efficiency. It is important to remember that no matter what type of collector is used or how efficient it is, it can never collect more energy than the product of the local flux rate and the collector area. In fact, it may be said that size (area) of a collector is its most important characteristic.
Orientation of Flat-Plate Collectors
Collectors of any type are more effective if they are moved so that they are continually perpendicular to the sun's rays. However, controls to accomplish this are expensive and not practical for rural operations. Instead an effort is made to orient the collector to the best average position. To understand this requires the explanation of two angles, azimufh and altitude. Figure 7.14.
Figure 7.14 Azimuth and altitude (southern hemisphere).
The azimuth is the horizontal angle of the sun in relation to the true south meridian. In the morning it will be measured in an easterly direction and in the afternoon, in a westerly direction. The altitude is the vertical angle the sun makes with the horizontal plane at the earth's surface. At the equator the sun's altitude will be to the north from March to September and to the south from September to March. As one goes farther south the sun has a north altitude for a longer and longer time, until south of latitude 23.5°S the altitude is always to the north.
Since the sun's altitude is so high in the small latitudes, placing a collector horizontal works quite well. However, some angling of the collector will improve the average performance. The following angles from the horizontal are suggested:
Year round operation - The latitude angle
Summer operation only - Latitude minus 10°
Winter operation only - Latitude plus 10°
For example, a collector to be installed in Lusaka, latitude 15° S. for year round use should be tipped 15° to the north and faced within 10° east or west of north.
Application of Solar Energy
Increased use of solar energy depends in large part on the cost of alternate sources of energy and on the improvement in designs of equipment for the use of solar energy. Although this energy is free, the equipment to use it is not. This means that applications that can be used throughout the year and those that are simple enough to be low in cost are most likely to be practical.
Some possible applications in rural areas are:
Figure 7.1 5 Solar water heater.
From room to room:
Sound transmission through a wall occurs as a result of the structural members being set into vibration by the sound waves, which in turn cause vibrations in the air on the opposite side. Therefore the heavier the construction the less easily it is set into vibration and the better its sound-insulating value. However, the sound-insulating value of a dense barrier such as a masonary wall may be seriously diminished if the sound is transmitted along structural members which link the rooms, e.g., ceilings, floors and plumbing lines. In addition, any openings such as gaps around doors or between ceiling and walls will allow noise to bypass a sound-insulating member. Noise from a roof due to the druming of rain and cracking of metal roofing can be reduced by installing a ceiling or panelling on the underside of the rafters. The sound insulating value of a ceiling is further improved by adding a layer of insulation which tends to absorb some of the sound before it is transmitted. Heavy construction will help to attenuate the sound.
Within a room:
Rooms having many hard surfaces tend to be very noisy and speech becomes distorted. This is because the sound is reflected and rereflected several times by the surfaces, thus creating an echo effect. Sound absorbents will reduce the time taken for the sound vibrations within the room to decay. Fibre boards and other soft materials are very efficient in dampening high frequency sounds, but for low frequency sounds a thin panel covering an air space works best.
Lightning striking a building can cause substantial structural damage and a fire may be started. Buildings with thatched roofs located in prominent positions present the worst risk, while concrete and steel frame buildings offer a low risk. A lightning-protective installation has three major parts; an air termination, a down conductor and an earth termination and its function is to provide a simple and direct path for the lightning to discharge to the ground.
The air termination consists of one or several pointed copper rods fixed above the highest point on the roof. One down conductor (e.g., 25 x 3mm copper tape) can serve a building of up to 100m². The earth termination consists of a 10 to 12mm copper-plated rod driven into the ground at least 2m. If the soil tends to become very dry at any time during the year, additional ground rods driven 2.5m deep will offer greater protection.
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