3.3. M ASS BALANCE MODELS – NATURAL TRACER GASES
3.3.2. Moisture balance
The humidity balance is a tool normally used to obtain a minimum ventilation rate, which guarantees acceptable humidity and CO2 levels within the barn. To maintain the animal heat balance and the body temperature, latent heat will increase with increasing temperature to substitute the decrease in sensible heat. In the humidity balance method it is assumed that only ventilation can remove moisture produced by the animals.
To calculate the humidity ratio (Albright 1990), i.e. mass of water vapour evaporated into dry air (kg/kg) for the livestock house and outdoor air, several parameters must first be calculated. By knowing the dry-bulb temperature, the water vapour saturation partial pressure (pws) was determined, as well as the actual partial pressure of water vapour (pw) and then the humidity ratio (kg water vapour/kg dry air) within the stall and outside air were calculated.
Psychometric equations
Temperature range from 0 to 200°C, suited to the livestock house conditions.
)
Coefficients to calculate water vapour saturation partial pressure (pws) are:
A1 = -5.8002206 E + 03
A2 = 1.3914993 E + 00 water vapour (pw) in the measured air can be calculated.
ws
w p
p /
=
φ (16)
With these values it is now possible to calculate the humidity ratio (kg water vapour/kg dry air) with the stall and outside air.
)
The total heat production Φtot from the broilers is derived below
(
t)
Φtot= total heat from all animals (W) Φs = sensible heat from all animals (W)The latent heat production is described in terms of energy, so to get the amount of water
Just like the heat balance the study is done under steady state conditions so the sources of water should be equal to the losses. The source of water is in this case only the animals, with sources from faeces, urine and feed included, and the loss the ventilation air.
wi, wo = indoor and outdoor humidity ratios (kg water/kg dry air)
Mass Ventilation Rate
In this case it is assumed that the mass flow rates are assumed to be the same at the inlets and outlets (Albright 1990), calculated by the following psychometric equations:
(
−)
( / −1)Finally the air flow rate (m3/s-1) is obtained by adding the water content of the air in ambient conditions and applying the density, with the following expression:
ρ ) (mair wi
Q= + (23)
Q = air flow rate rate (m3/s-1)
mair = mass air flow rate (kg dry air/s-1) wi = indoor humidity ratio (kg water/kg dry air)
Once again, this model follows the assumption that the animal latent heat production is constant on a diurnal basis, in spite of the fact that the heat production varies diurnally.
The diurnal variation will for instance be influenced by the feeding strategy, lighting and hygro-heat parameters. Both livestock houses tested in this study used ad lib feeding systems. The Louisiana stall combined the use of natural and artificial lighting, whereas the mechanically ventilated stall used artificial lighting. Animal activity can also be used with the heat and moisture balance models to try and improve the AER calculations.
The minimum moisture difference between inside and outside conditions to obtain reliable results is 0,5x10-3 kg water/m3 dry air (Pedersen et al. 1998), therefore if the moisture difference were below this level, then the value was corrected and the minimum difference value (0,5x10-3 kg water/m3 dry air) was assigned for the calculation.
3.3.3. Carbon dioxide balance
The CO2 mass balance requires measuring actual carbon dioxide concentrations in combination with evaluating a model of carbon dioxide production. In the model, carbon dioxide production in livestock buildings is compared to the carbon dioxide release that escapes via the ventilation, therefore, the carbon dioxide terms have to be considered separately (Van Ouwerkerk & Pedersen, 1994). Research since the last 20-30 years has indicated that the average CO2 production from cattle, pigs and poultry over a 24h period and corresponding to a medium feeding level amounts to 0,185m3h-1.
To improve the accuracy of the calculation, i.e. incorporate diurnal variation into the model, it is recommended to include animal activity.
Ventilation flow per hpu (m3/h) = 6 passive infrared detectors (PIDs) and a data logger. The activity signal is dependent on the temperature and velocity of a heated body, the distance to the sensor and for two simultaneously moving bodies. Two PID units were mounted 2 m above the ground in the centre of the stall at an angle of 45°, and activity data was accumulated every 5 minutes over 24 hour periods. The measurements were transformed and integrated into the calculations to account for the diurnal variation which is not accounted for with the balance models alone, as discussed in the CO2 theoretical section.
The accuracy of the CO2 technique is affected adversely by artificial CO2 sources, e.g.
manure and depends on the estimate of metabolic CO2 production, which varies according to body weight, health status, etc (Seedorf et al. 1998). In the literature (Van Ouwerkerk and Pedersen 1994) it is stated that under farm house conditions with a medium to high feed intake CO2 production from manure is 4% of the total production, this was corrected for in all CO2 mass balance calculations. Furthermore, if direct-fired gas or oil-fuelled heaters are used at any time, particularly when the chicks are young, then additionally amounts of carbon dioxide will be produced, and must be corrected for. Numerous authors have stated that a minimum difference of 200ppm between inside and outside conditions is necessary for reliable results (Pedersen et al. 1998), in Experiment 4.1 because of the high air exchange rates under summer conditions a minimum difference of 150ppm was used.
Overall, it is often stated in the literature that the typical accuracy of the mass balance models in estimating ventilation rates falls with in the acceptable range of ± 20%, this was checked, on three occasions under different conditions. The first round of measurements involved testing the three mass balance models in a full sized naturally ventilated livestock house under summer conditions (Experiment 1).