DESIGNING AND OPERATION OF THE EVAPORATIVE SYSTEM FOR AIR COOLING IN STABLES

Pavel Neuberger, Vladimír Šleger, Martin Polák
Department of Mechanics and Engineering, Czech University of Life Science, Prague, Czech Republic

ABSTRACT

Contribution describes procedures during designing of evaporative cooling system for inner air cooling in stables under different climatic condition. There are described an effect of different device types and recommendation for its operation. Two types of equipment are used in agriculture for evaporative cooling: pressure nozzles and evaporative panels. The sufficient 80 % adiabatic efficiency can be expected in the equipment with spray nozzles at the 5 MPa water pressure before the nozzle and with the properly installed standard wetted surface 100 mm thick, over which air flows at a rate of 1 m·sec^-1. When deciding the installation of the cooling equipment, it is important to take into account local climatic conditions. The future evolution of climate is estimated by means of mathematical models. Should the limit be determined for the efficient use of the evaporative cooling for example in the stable for growing and finishing pigs at an annual temperature maximum above 33°C, it is suitable to install the equipment in the current period in places with the average summer temperature above 16.5°C. However, around 2050, this would already be in places where the average temperature is now above 15.5°C in the summer months. Thanks to knowledge of the future values, it is possible to find out what is called a space analogue, i.e. a place where a similar situation occurs at the present time and will be there in the future. In the current and future period, it is sufficient to design equipment that is able to evaporate about 5-6 grams water into each cubic meter of the ventilation air. On the basis of this value and the given volume of the ventilation air in the stable, it is possible to determine the necessary number of nozzles or, as the case may be, the volume of the evaporative panel. Simultaneously with the evaporative cooling, other important means should be used for increasing the comfort of animals, such as shading the buildings, increasing the airflow rate around animals and directing the flow of ventilation air directly to the zone of animals.

Key words: stables, cooling and ventilation, evaporative systems, climate modeling.

INTRODUCTION

The use of suitably designed evaporative cooling is one of the methods how to reduce the consequences of heat stress on animals kept in stables during the summer months. The amount of water just of magnitude of several grams evaporated into one cubic meter of air cools the air by several Kelvin due to a high evaporation heat (2500 kJ·kg^-1). In order to achieve the most efficient evaporation, it is necessary to maximally increase the water surface area. Two types of equipment are used in agriculture for this purpose. Pressure nozzles (Fig. 1), which spray water directly into ventilation air, and evaporative panels (Fig. 2), i.e. walls with a large surface wetted with water running down, through which air is sucked into the stable.

The advantage of the pressure nozzles is a lower water consumption, high efficiency, accurate dosing and sanitary operation. The disadvantage is a high water pressure, clogged nozzles and sometimes even undesirable humidification of the interior space. The advantage of evaporative panels is a lower energy consumption with the simultaneous air purification. The disadvantage is unsanitary operation, non-uniform thermal field and if brand materials are used for filling, also a higher price.

The adiabatic efficiency of the pressure nozzles η_ad  (-), defined as the ratio of the achieved cooling to the maximally possible cooling, depends on water pressure p (Pa) before the nozzle according to the equation η_ad = 0.124 + 1.35·10^-7·p [1]. Theoretically, 100% efficiency can thus be achieved at a water pressure of 6.5 MPa. Air cannot be cooled more by evaporation because its 100% water vapour saturation has been achieved. The adiabatic efficiency of the evaporative panels is in the range from 60% to 90%. It depends primarily on the filling material used, thickness of the wetted layer and the flow rate of treated air [7]. The dependence of efficiency η_ad (-) on the flow rate v (m·s^-1) is described by the equation η_ad = 86.62 – 20.787·v + 2.755·v² (corrugated cellulose) or η_ad = 76.055 + 2.909ˇv – 17.414·v² (wood wool) [12]. The optimum PAD thickness is 50-100 mm.

When deciding the installation of the cooling equipment, it is important to take into account local climatic conditions. These conditions can be assessed by means of long term detailed meteorological data. The measured and observed values of basic meteorological factors during 1961-2000 on the territory of the Czech Republic are available in the literature, e.g. Květoň [6] or directly in climatic stations. Apart from humidity, the maximum and average air temperature in a given region is primarily important for designing the evaporative cooling. For example, in stables where poultry is kept, Shane [9] recommends using evaporative cooling if a temperature of 35°C occurs in a given place every year. Growing and finishing pigs belong to animals even more sensitive to increased temperature and in this case, it is therefore possible to assume a limit for the efficient use of the evaporative cooling by 1-2 K lower. For the assessment of losses due to the thermal stress of animals in the event of daily temperature fluctuations, average temperature plays a key role [4,8].

The future evolution of climate is estimated by means of mathematical models. The future amount of the CO₂ emissions cannot be determined accurately; neither can the climate sensitivity be determined to the increased concentration of greenhouse gases. For this reason, several different groups of emission scenarios SRES (Special Report on Emission Scenarios) have been set up, where evolution alternatives are included leading to the compensation of differences between rich and poor countries and the evolution is oriented to a very heterogeneous world. In the world centres for climate modelling, several global climatic models have been created, by means of which the climate changes and their impacts have been estimated. The outputs from these models are freely available on the following address: http://ipcc-ddc.cru.uea.ac.uk. The HadCM2 (Hadley Centre for Climate Prediction and Research) model from Great Britain meets best the conditions of the Czech Republic.

In 2000, several alternatives scenarios were created for climate changes on the territory of the Czech Republic by using a procedure recommended by the Intergovernmental Panel on Climate Change. Period 1961-1990 was chosen as a reference period for the assessment of changes. The World Meteorological Organization considers this period as normal and it is denoted as the current state. The calculated temperature changes that are expected to occur in the Czech Republic until 2050, are indicated in Table 1 for two extreme emission scenarios: A relatively optimistic scenario when considering low sensitivity and a pessimistic one for a high climate sensitivity [5].

Although there are uncertainties in regional estimations of the future warming and considerable differences between individual models, their conclusions agree that the increase of the summer temperature above the continents of mild latitudes will exceed the increase of the global annual average air temperature.

While the expected changes should not affect the thermal balance of ruminants considerably, the situation is opposite for pigs and poultry [13]. The probability of the occurrence and increased frequency of temperature extremes significantly increases even if the average temperature values increase mildly [2]. These facts show on the growing importance of the evaporative air cooling also in stables in mild geographical latitudes.

The primary goal of this contribution is to assess the effect of climatic conditions on the design of equipment for evaporative cooling in a given region.

MATERIAL AND METHODS

The current climatic conditions can be determined on the basis of meteorological elements in selected climatic stations. In the calculations, data on the average daily temperature were used that were determined from the values measured at a certain time (at 7:00 a.m., 2:00 p.m. and 9:00 p.m.) and the maximum daily air temperature that was determined according to the extreme thermometer in the summer months (June, July, August) from 1961 until 1990. Stations were chosen with respect to their uniform distribution and continuity of measurements, if possible at the same place. The accessibility and form of the provided data is also important. For making it possible to generalize the results obtained in a specific place to other regions with analogous climate, the average temperature in the summer months for the entire period 1961-1990 was chosen as the basic parameter characterizing the climatic conditions of a given place. The data from the following 27 stations were available for the calculation: Brno-Tuřany (average summer temperature 17.9°C), Olomouc-Slavonín (17.9°C), Zatec (17.9°C), Kuchařovice (17.7°C), Semčice (17.7°C), Hradec Králové-Nový Hradec Králové (17.5°C), Holesov (17.4ºC), Doksany (17.4°C), Mosnov (17.1°C), Klatovy (16.8°C), Tábor (16.6°C), Valasské Meziříčí (16.5°;C), Třeboň (16.5°C), Kralovice (16.4°C), Ondřejov (16.2°C), Velké Meziříčí (16.2°C), Havlíčkův Brod (16.1°C), Husinec (16.0°C), Město Albrechtice Záry (15.9°C), Ústí nad Orlicí (15.9°C), Kostelní Myslová (15.8°C), Cheb (15.8°C), Liberec (15.6°C), Přimda (14.5ºC), Červená (14.4°C), Svratouch (14.4°C), Churáňov (12.°C).

For the comparison of the results for the current and future conditions, it was necessary to create the corresponding series of the expected future average and maximum daily temperature during the equally long period. The temperature increments according to Table 1 were added to the values obtained from individual stations for the optimist and pessimistic evolution alternative.

From the everyday average and maximum temperature, the number of days is determined in the summer months, when the determined value of the average or maximum temperature is exceeded. Furthermore, it is possible to determine the temperature maximum that occurs every year in a given locality.

When designing the evaporative equipment, the maximum amount of water ρ_p (kg·m^-3) that the equipment can evaporate into the ventilation air must be considered as an important factor. This quantity can be calculated from the relation ρ_p = (x₂ - x₁)·ρ_sv. Specific humidity x₁ (kg·kg^-1), corresponding to the maximum air temperature t₁ (°C) in the given place during a period of thirty years, can be calculated from the relation [3]:

x₁ = 5.8·(1 + tgh(0.05·(t₁ – 10)))·10^-3.

Dry air density ρ_sv (kg·m^-3) is given by the equation of state:

The relation for the calculation of the dry air pressure psv (Pa) can be obtained from the basic relations of thermodynamics in form:

where 98 000 Pa, i.e. the average value in the Czech Republic [3], is substituted for the humid air pressure p_vv;
r_sv = 287 J·kg^-1·K^-1 and r_p = 461 J·kg^-1·K^-1 are the values of the specific gas constant of dry air and of vapours respectively. Temperature T_sv (K) of dry air is given by the relation T_sv = t₁ + 273.15. Under the assumption of the same enthalpy of air in state 1 and 2, the specific humidity of air x₂ (kg·kg^-1) after cooling can be calculated from the relation:

where c_psv = 1 004 J·kg^-1·K^-1 and c_pp = 1 884 J·kg^-1·K^-1 are the specific heat capacities of dry air and of vapours respectively; l₂₃ = 2 499·103 J·kg^-1 is the specific heat of the water evaporation at 0°C, η_ad (-) is the adiabatic efficiency of the cooling equipment and t_ad (°C) is the temperature of the limit adiabatic cooling at 100% relative humidity that can be determined for air state 1 given by parameters t₁ and x₁ from the Mollier diagram; instead of it, the corresponding wet bulb temperature can be substituted with sufficient accuracy.

RESULTS AND DISSCUSION

In Fig. 3, the annual temperature maximum rounded down to an integer is displayed. In the given place with the indicated average temperature in the summer months, this temperature was observed on the average at least once during each year of the thirty year period. Should the limit be determined for the efficient use of the evaporative cooling in the stable for growing and finishing pigs at an annual temperature maximum above 33°C, it is suitable to install the equipment in the current period in places with the average summer temperature above 16.5°C. However, around 2050, this would already be in places where the average temperature is now above 15.5°C in the summer months.

In Fig. 4, the calculated amount of water is indicated that must be evaporated into the ventilation air at the extremely high temperature and the chosen adiabatic efficiency of 80%. This value does not depend too much on the average summer temperature. It is obvious that in the current and future period, it is sufficient to design equipment that is able to evaporate about 5-6 grams water into each cubic meter of the ventilation air. On the basis of this value and the given volume of the ventilation air in the stable, it is possible to determine the necessary number of nozzles or, as the case may be, the volume of the evaporative panel. The 80% adiabatic efficiency can be expected in the equipment with spray nozzles at the 5 MPa water pressure before the nozzle and with the properly installed standard wetted surface 100 mm thick, over which air flows at a rate of 1 m·sec^-1 [1,7,12].

The air temperature above 21°C is already outside the region of the optimum conditions for many animal kinds and categories [10]. If the average value of temperature is decisive during its daily fluctuation [4,8], Fig. 5 can be used in the assessment how frequently the heat stress occurs in stables in the summer period in a given place and what the losses are in the animal production. Thanks to knowledge of the future values, it is possible to find out what is called a space analogue, i.e. a place where a similar situation occurs at the present time and will be there in the future.

In Fig. 6, the number of days is indicated in the summer season when the equipment would have been in operation under the assumption that it was put into operation whenever temperature exceeded 24°C.

CONCLUSIONS

The paper describes how the effect of local climatic conditions should be taken into account when designing efficient cooling of air in stables. The number of places where it will be suitable to install evaporative cooling will increase with the increasing temperature in the summer months. Independently of the local conditions, the planned evaporative equipment should be able to evaporate about 5-6 grams water into each cubic meter of ventilation air in the current and future period. In the time when cooling is used although air does not reach the extreme conditions, it should be possible to regulate the amount of evaporated water, for example, by using a variable number of pipe branches with nozzles, independently controlled units, mobile equipment, etc.

Sometimes it is possible to encounter an evaporative cooling unit that is unsuitably designed and its effect is by several Kelvin worse than expected. The most frequent cause is a small amount of water that the equipment can evaporate. It is oblivious of the anti-drip valve in the nozzles (Fig. 7) which prevents water from dripping at a reduced water pressure. The efficiency of the evaporative panels exposed to sunshine (see Fig. 2) is decreased to mere 15% [11].

Simultaneously with the evaporative cooling, other important means should be used for increasing the comfort of animals, such as shading the buildings, increasing the airflow rate around animals and directing the flow of ventilation air directly to the zone of animals.

REFERENCES

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Accepted for print: 1.04.2008

Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.