Drying the baled forage improves the quality of the hay. The process proves to be very effective with alfalfa in particular, because this forage is more permeable to air than stable grass and provides the great advantage of preserving almost the entire leaf.
Lorenzo Benvenuti
This type of process involves the use of specific plant that can be made in situ or in sheet metal and, in this case, operate resting on a paved area. In both cases it is useful to protect them with canopies of a certain width in order to be able to manage loading and unloading operations with peace of mind, even in the event of rain. The management of the plant is in fact different from that for loose fodder because in this case the cell is also the physical place where storage takes place, while in the case of baled fodder at the end of drying the product is removed from the plant and stored in the barns. Therefore, at the end of the process, the bales are removed from the bedmaking it possible to start a new drying cycle.
The bed of these plants has a rectangular floor and consists of a ventilation chamber with ventilation holes on which the bales are placed. On one of the short sides is a fan, which is always of the centrifugal type, and a heat generator.
The Ventilation Chamber
In in-situ plants, the ventilation chamber is made of reinforced concrete surmounted by a platform consisting of prefabricated square or rectangular plates, each of which has a hole in a central position on which the bale is placed. The platform thus formed is passable by a tractor. In sheet metal plants, loading is carried out by keeping the telescopic loader or the tractor equipped with a front loader outside the plant; for this reason, the bales are often only arranged in two rows.
The free space inside the ventilation chamber must be as large as possible. As an indication, it should not be less than 40-50 cm to ensure that the ventilated air mass can expand, equalise and then flow through the forage avoiding further pressure losses. As an indication, a good ratio between the system’s air flow rate, expressed in m3/s, and the volume of the ventilation chamber, expressed in m3, should be less than 0.6 Hz, with optimal values of less than 0.5 Hz. However, there is much variability in this respect. Plants with opposing air flow, i.e. when air reaches the bale from both below and above, have two ventilation chambers in communication with each other. The cross-section of these connections, and of all air ducts in general, must be such as not to cause an increase in the pressure drops that determine the system’s operating pressure. The connecting duct between the fan and the ventilation chamber must also have a cross-section that increases in size as it moves away from the fan outlet.
A Few Technical Data
However, the compression of the round bale, determined by the baling, offers greater resistance to the movement of air compared to the loose forage. This leads to an increase in the operating pressure, particularly at the start of the ventilation process, where total pressure values between 500 and 1,200 Pa (approximately 50 and 120 mm of water column) can be detected. Under these conditions, preferential exit paths can be established in the contact area between the round bale and the plate, through which the air returns to the environment without passing through the forage. To avoid this loss, which negatively affects the energy balance of the process, some measures are taken to improve the seal between the base of the round bale and the support plate. A balanced ratio between the diameter of the plate hole and that of the round bale, generally between 0.6 and 0.7, combined with the creation of special reliefs along the edge of the plate hole, makes it possible to reduce the extent of this phenomenon. It should be remembered that a greater ratio tends to increase air leakage, while a reduction, in absolute terms, of the hole diameter causes an increase in operating pressure.
As an indication, the centrifugal fan of a low-temperature system is generally sized to guarantee an air flow rate of between 0.6 and 0.7 m3/s per ventilation hole at an operating pressure of 500 Pa with bales of more than 2 m3 in volume. The electrical power installed for ventilation is 0.6-1.2 kW per ventilation hole and about three times higher for endothermic Diesel engines.
Dwell times on the system vary depending on the temperature and humidity conditions of the ventilation air, the moisture content of the forage and the uniformity with which the air is distributed within the round bale. They can be reduced considerably by heating the ventilation air; even at 6-8°C above room temperature, the process duration tends to be halved.
Systems that only use heat generators when necessary are characterised by a longer permanence of the fodder on the system, but also by lower energy consumption. To improve the performance of the system while keeping consumption low, it is advantageous to construct air-to-air solar panels. When the connection channels between the panel and the fan are well dimensioned, the solar panels contribute very effectively to the ventilation process.
In cases where there are technical or economic impediments in the supply of electricity, some farms have installed diesel engines. In this case, it is interesting to use part of the heat produced by the engine, which is able to increase the temperature of the ventilated air by about one degree, which is equivalent to approximately 4 percentage points of relative humidity (of course, the reduction in the relative humidity value depends on the starting conditions).
Due to the larger volume and the not always orderly distribution of the forage inside the bale, the drying process inside the round bale may present additional difficulties compared to prismatic bales. Indeed, with round bales, drying may be less uniform because it is slower in those areas where there is less air flow. This occurs due to the presence of areas in which the forage has a higher density and, most noticeably, this phenomenon manifests itself in the outermost layer of round bales made with constant volume compression chamber round balers. The application of special covers to be placed on the upper base of the round bales, the overlapping of two bales, and the adoption of systems with opposing flows favour the diffusion of the air throughout the round bale, helping to complete the drying of the forage located in the outermost position.
However, it helps to improve the efficiency of the process by creating round bales with a more even distribution of the forage, such as those obtained with variable compression chamber round balers. However, care must be taken not to make round bales that are too dense and heavy, especially with stable grass forage or grass-rich forage. It is therefore necessary to have balers on which it is possible to modify the compression exerted on the forage: this must be reduced as the moisture content increases. The air permeability of the round bale is in fact correlated with its density, which can be modified at harvesting time by acting on the adjustment devices on the balers
Energy Consumption
Establishing energy consumption a priori is complex precisely because it is difficult to assess aspects that essentially depend on the moisture content of the forage and the process conditions. In plants operating at low temperature increases (approximately maximum 10°C), the factors that affect energy expenditure include the air flow rate, the natural drying capacity of the air and the change in the temperature of the ventilated air.
Particularly for the type intended for drying baled forage (precisely because it operates discontinuously), it is advisable to take advantage of the natural drying capacity of the air and passive heat generation systems such as the solar air collector and cogeneration, reserving the intervention of the heat generator for situations of actual need. In fact, the use of the hot air generator or any other energy-consuming device intended to modify the psychrometric parameters of the air should be operated only when the relative humidity of the air is close to the hygroscopic equilibrium point.
In fact, if we examine the energy behavior of a rational bale-drying plant, one can easily determine the significant difference that exists between the energy consumption used by the fan and that to heat the air. The former rarely exceeds values of 1 kWh (electrical) per round bale (less for prismatic bales), while the latter is generally between 4 and 8 kWh (thermal) per round bale, respectively, with a temperature difference of 5 or 10°C. However, it is possible (and necessary!) to rationalize ventilation and in particular the use of the heat generator also through automatic systems that carry out the appropriate adjustments based on the thermo-hygrometric parameters detected.
When the fan is driven by an electric motor through the simple adoption of a timer, it is possible to reduce ventilation during the wettest hours (for example at night) by adopting ventilation cycles to avoid the problems of thermal rise in the forage. However, this technique should be adopted with caution especially when the forage tends to heat up easily.
Reduction in Energy Expenditure
Reduction in energy expenditure can also be achieved by using alternative energy sources to fossil fuels. In fact, plants are always equipped with heat generators, fueled by diesel or other fossil fuels, capable of producing a heat increase that should not be higher than 8-10°C, given that the increase in operating temperature leads to a reduction in drying efficiency, which negatively affects process costs.
Fossil fuel burners can be replaced by devices designed to exploit biomass from agriculture, forestry processing, sawmills and the wood industry. However, their cost-effectiveness is strongly conditioned by the market trend or by the actual possibilities of procuring the biomass directly.
When sizing these heating systems, it should be considered that to raise one cubic meter of air by one degree, on average no more than 1.25 kJ are needed and that, therefore, biomass consumption will be indicatively between 1.3 and 2.6 kg/h/ventilation hole while those of diesel fuel consumption will be between 0.44 and 0.88 kg/hour/hole. The ratio of energy yield between woody biomass and diesel fuel is therefore 3:1.
Among the supplementary energy sources for air heating, however, the most reliable and widely used in this sector is the one based on solar radiation recovery by means of solar air collectors, which can also be realized with hired labour system once the correct technical solution is identified
