ARTICLE IN PRESS. Ventilation Flow in Pig Houses measured and calculated by Carbon Dioxide, Moisture and Heat Balance Equations

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1 Biosystems Engineering (25) 92 (4), doi:1.116/j.biosystemseng SE Structures and Environment ARTICLE IN PRESS Ventilation Flow in Pig Houses measured and calculated by Carbon Dioxide, Moisture and Heat Balance Equations V. Blanes; S. Pedersen Research Centre Bygholm, Danish Institute of Agricultural Sciences, Schüttesvej 17, DK-87, Horsens, Denmark; s: (Received 29 April 25; accepted in revised form 7 September 25; published online 21 October 25) Ventilation flow in commercial livestock buildings can be estimated by means of relatively simple indirect methods based on carbon dioxide (CO 2 ), moisture or heat balances. However, ventilation flow on an hourly basis forthese balances needs adjustment fordiurnal variation in animal CO 2 and heat production. This work examines the agreement between ventilation flow measured in a pig house over a period of 41 days and that estimated from the three balances, on a 24 h and on an hourly basis. The study shows that, all three methods can give reasonably good estimations of the ventilation flow on an hourly basis. On average, the calculated ventilation flow was 8% lower than measured ventilation flow for the CO 2 balance, and 9% lowerforthe moisture and heat balances. A good agreement between measured and calculated ventilation flow was obtained on a 24-h basis (coefficient of determination R 2 between 92 and 97) and on an hourly basis (R 2 between 83 and 92). The study indicates that the agreement between measured and estimated ventilation flow on an hourly basis can be improved by taking into account the diurnal variation in CO 2, moisture and heat production. r 25 Silsoe Research Institute. All rights reserved Published by ElsevierLtd 1. Introduction Ventilation flow in livestock buildings is related with two important aspects of the animal production. Firstly, it determines indoor climate and air quality inside the house, and so it affects the comfort of the animals. Secondly, ventilation rate is also connected with environmental issues, as it has a great influence on gas emission rates from animal houses. As a consequence, ventilation flow in an animal house is a fundamental variable in the calculation of climatisation systems and gas emission rates, and so it is important to determine it in a reliable way. Direct measurement and indirect methods can both be used to estimate ventilation flow in a mechanically ventilated livestock building. However, direct measurement has several important drawbacks. Accurate registration requires the use of measuring fans installed in series with ventilation fans. Unfortunately, the large numberand the location of ventilation fans in real livestock buildings make theiruse very difficult in practice. The estimation of ventilation flow by indirect methods based on balance equations avoids these drawbacks by calculating the ventilation flow from some specific parameters of the indoor and the outdoor air, whose measurement is not so complicated and time-consuming as the direct measure of ventilation rate. Three different balances are possible (CO 2 balance, moisture balance and heat balance) depending on the parameter being measured (CO 2 concentration, content of moisture or temperature, respectively). However, these balance methods require an accurate prior determination of the production of CO 2, latent heat and sensible heat from the animals, which also take part in the equations. Therefore, it is a question of interest to assess the reliability of the balance methods to estimate the ventilation flow in livestock buildings, by comparing the estimated ventilation flows, from the three /$ r 25 Silsoe Research Institute. All rights reserved Published by ElsevierLtd

2 484 V. BLANES; S. PEDERSEN Notation A relative animal activity A CO 2 coefficient forthe diurnal adjustment of the CO 2 production due to animal activity A moist coefficient forthe diurnal adjustment of moisture production due to animal activity A heat coefficient forthe diurnal adjustment of the heat production due to animal activity a, b, c constants forthe activity models and d C prod production of CO 2 on a 24-h basis, m 3 h 1 C ind concentration of CO 2 in the indoorair, p.p.m. C out concentration of CO 2 in the outdoorair, p.p.m. E intake daily feed energy intake, W H ind content of moisture in the indoor air, kg[h 2 O] m 3 [air] H out content of moisture in the outdoor air, kg[h 2 O] m 3 [air] h time of day (24 hourclock), h h min time of day with minimum activity, h h sp specific heat of air, J kg 1 K 1 h vap heat of vaporization of water, J kg 1 k s correction factor for sensible heat at house level L corrected latent heat production at house level, W m live mass, kg S corrected sensible heat production at house level, W S trans transfer of sensible heat through the building, W S rad heat gain due to the thermal radiation transmitted through the ceiling, W S slurry heat gain owing to the difference of temperatures between the slurry and the indoor air, W T airtemperature, 1C T ind indoorairtemperature, 1C T out outdoorairtemperature, 1C V CO2 ventilation flow calculated from CO 2 balance, m 3 h 1 V moist ventilation flow calculated from the moisture balance, m 3 h 1 V heat ventilation flow calculated from the heat balance, m 3 h 1 V measured measured ventilation flow, m 3 h 1 F tot total heat production for fattening pigs at 2 1C, W F tot total heat production at temperatures different to 2 1C, W F sen sensible heat production at house level, W F lat latent heat production at house level, W r airdensity, kg m 3 balances, with the real ventilation flow, in order to validate the method and reinforce the credibility of the results. In previous works, some authors have dealt with different aspects of this matter. In that sense, Pedersen et al. (1998) investigated the agreement between the ventilation flow calculated from the CO 2, moisture and heat balances in houses forcattle, pigs and laying hens, but a comparison with the real ventilation flow was not carried out. Pedersen and Gaardbo Thomsen (2) used the moisture and heat balance equations to estimate the heat production in a broiler house. Schauberger et al. (2) applied the three balances to predict the indoor climate in a finishing pig unit. Li et al. (25) compared the ventilation flow measured and calculated from CO 2 balance in a laying hen house. In resume, as far as is known, there are very few works that compare the measured ventilation flow in a livestock building, and the ventilation flow calculated from the balance equations. The production of CO 2, moisture and heat from the animals, depend on different factors (live mass, energy intake, etc.), and it has been demonstrated that it is also related with the animal activity which varies diurnally (Van Ouwerkerk & Pedersen, 1994; Pedersen & Rom, 1998; Ni et al., 1999a; Jeppsson, 22; Chwalibog et al., 24; Sousa & Pedersen, 24). Nonetheless, most of the values and equations forthe calculation of CO 2, moisture and heat production from the animals, that can be found in literature have been obtained from long-term experiments (more than 24 h), and so they assume intrinsically a diurnal average animal activity. As a result, calculating ventilation flow from balance equations, on less than a 24-h basis, from literature data, without any adjustment foranimal activity, can make the balance method inaccurate. The main objective of this work is to examine the agreement between ventilation flow measured in a pig house overa period of 41 days and ventilation flow calculated from CO 2, moisture and heat balances, on a 24-h basis and on an hourly basis. The agreement on an hourly basis is improved by taking into account the diurnal variation of animal activity.

3 VENTILATION FLOW IN PIG HOUSES Heat production and animal activity 2.1. Total, sensible and latent heat production In a recent report (CIGR, 22), an equation forthe calculation of total heat loss F tot in W forfattening pigs at 2 1C is presented [Eqn (1)] where m is the live mass in kg and E intake is the feed energy intake in W: F tot ¼ 59m 75 þð1 ð47 þ 3mÞÞðE intake 59m 75 Þ In that equation, the total heat loss in W is calculated as the sum of heat loss formaintenance (59 m 75 ) and heat loss for growth (second part of the equation). For a live mass above 16 kg the factor(47+3 m) should be fixed at 79. The total heat production F tot in W at temperatures different from 2 1C can be calculated from Eqn (2) (CIGR, 22): (1) F tot ¼ F tot þ 12F tot ð2 TÞ (2) where T is the air temperature in 1C. Pigs lose heat as sensible heat, due to the temperature gradient between their body temperature and the surrounding air, and as latent heat, by evaporation from respiratory tracts. However, the partition of the total heat into sensible and latent heat at house level is different to the sensible and latent heat at animal level. Sensible and latent heat at house level depends on the housing conditions, as part of the sensible heat produced by the animals can be used for the evaporation of wet surfaces inside the building. The following equations [Eqns (3) and (4)] were proposed in the same report (CIGR, 22) forthe calculation of sensible F sen and latent F lat heat production in W at house level forfattening pigs on partly slatted floor (at normal housing conditions of Northern Europe): F sen ¼ 62F tot T 6 (3) F lat ¼ F tot F sen (4) The heat production expressed in heat production units (originally defined as the heat production of a dairy cow), is shown in Fig. 1, where 1 heat production unit (hpu) is equal to 1 W of total heat produced by the animals at 2 1C. A further adjustment can still be necessary if the housing conditions are slightly different from the previously mentioned. A correction factor k s forthe sensible heat production S in W at house level, was introduced in Pedersen et al. (1998), forthat adjustment [Eqn (5)]. The adjusted latent heat L in W can then be Heat production per hpu, W Total heat 8 Latent heat 6 4 Sensible heat Ambient temperature, C Fig. 1. Distribution of total heat as sensible and latent heat for fattening pigs on partly or fully slatted floor (CIGR, 22); hpu, heat production unit equivalent to 1 W at 2 1C obtained by Eqn (6): 2.2. Carbon dioxide production S ¼ k s F sen (5) L ¼ F tot S (6) The CO 2 production in a pig house is important because it can be used forcalculation of the ventilation flow. The CO 2 production is the addition of that emitted by the animals in the respiration, and that released by the slurry. Van Ouwerkerk and Pedersen (1994) estimated that the total CO 2 production in animal houses was between 17 and 2 m 3 h 1 hpu 1, and in average of 185 m 3 h 1 hpu 1. In that work, it was considered that 4% of the total CO 2 production came from the slurry. That value of 185 m 3 h 1 hpu 1 was 135% higher than the value of 163 m 3 h 1 hpu 1 that was recommended previously in CIGR (1984). A value of 185 m 3 h 1 hpu 1 forthe total CO 2 production was used also in Pedersen et al. (1998), who found that it provided a good agreement between ventilation flow calculated from the CO 2 balance and the heat and moisture balances, and in Sousa and Pedersen (24), where those authors stated that using a CO 2 production of 185 m 3 h 1 hpu 1 gave ventilation rates of the same level as measured by the measuring fans. However, Ni et al. (1999a) found that the CO 2 production of a pig of 15 kg of body weight was equal to 8129 g h 1 (that is approximately 173 m 3 h 1 hpu 1 ), and the CO 2 production from the slurry, in a partly

4 486 V. BLANES; S. PEDERSEN slatted pig house, was on average about 34% of the CO 2 emitted by the animals (Ni et al., 1999b) Animal activity models Animal activity shows daily cycles, according to the different animal actions (feeding, moving, resting, etc.) during every 24-h period. There are several alternatives to quantify the animal activity. One possibility is to assess it by sensing systems with passive infrared detectors (Pedersen & Pedersen, 1995). To obtain the diurnal pattern of the animal activity, the absolute measurement can be expressed in relation to daily average, to get the relative animal activity A. However, if it is not possible to measure the animal activity, the relative animal activity can be approximated directly, by one of the following diurnal variation models (CIGR, 22): the single sinusoidal model or one maximum model [Eqn (7)] and the double sinusoidal model or two maxima model [Eqn (8)]: A ¼ 1 a sin 2p 24 ðh þ 6 h minþ (7) A ¼ 1 a sin 2p 24 ðh þ 6 h minþ b sin 2p c ðh dþ (8) where: h is the time of day (24 hourclock), h; h min is the time of day with minimum activity, h; and, a, b, c and d, are constants. As the production of CO 2, moisture and heat is positively related with the animal activity, the hourly adjustment coefficients for production at every specific hour A CO 2 ; A moist and A heat (forthe CO 2, moisture and heat production, respectively) follow a similar pattern as the relative animal activity (measured or estimated). 3. Materials and methods 3.1. Experimental room The investigation was carried out in an experimental building forfattening pigs (Fig. 2), at Research Centre Bygholm (Danish Institute of Agricultural Sciences). The house is provided with four pens with partially slatted floor. The room is equipped with a neutral pressure mechanical ventilation system that consisted of one inlet and one outlet unit with fan. Incoming air passes through the inlet pipe, and enters the room through a circular horizontal inlet slot, placed in the centre of the room at 22 m above the floor m 2.35 m 4.6 m Computer room The building is insulated and the different materials that compose the walls, the ceiling and the floorwere taken into account in the estimation of the coefficients of heat transmission or U-values. The duration of the experiment was 41 days, starting in November month, therefore winter conditions can be considered. Two products (maize silage and straw) were used as rooting materials for the pigs, during two different periods. In that respect, it is known that when maize silage is used in the pen as a rooting material, the pigs eat part of it every day. In the calculation of the total heat production [Eqn (1)], an increase in the energy intake, has been considered owing to that ingestion. The slurry was removed twice during the 41 days: in the middle of the experiment and at the end Animals The experiments were carried out with 4 pigs in the first 25 days and then reduced to 28 pigs, to make more space due to increased live mass. The initial average body weight was 85 kg increasing to 1225 kg after41 days. The pigs were fed ad libitum, with feed whose composition was 68% of grain, 16% of soya bean cake, 1% wheat bran and 6% of others. The feed intake was measured daily Registrations Adjacent building Experimental room 11 m Fig. 2. Experimental room, with partially slatted floor Indoor and outdoor climate Climate measurements comprise temperature, humidity and CO 2 concentration of the indoor and the outdoor air, and solar radiation outside of the building.

5 VENTILATION FLOW IN PIG HOUSES 487 Forthe balance calculations on an hourly basis, the difference between indoor and outdoor temperature perhourwas calculated as an average of 12 data, and in the case of humidity and CO 2 differences, as averages of six data. Solar radiation was measured at hourly intervals Activity The animal activity was measured by three passive infrared sensors (Pedersen & Pedersen, 1995) located inside of the room Ventilation flow Ventilation flow in the outlet pipe was measured V measured in m 3 h 1 by a measuring fan (FANCOM) that records 12 measurements per hour. Ventilation flow on an hourly basis was calculated as average of those measurements Carbon dioxide, moisture and heat balance equations The CO 2, moisture and heat balance equations are based on the conservation of mass and energy in the building, understeady-state conditions. The CO 2 balance can be expressed by the following equation [Eqn (9)]: C prod A CO V CO2 ¼ 2 ðc ind C out Þ1 6 (9) where: V CO2 is the ventilation flow in m 3 h 1 from CO 2 balance; C prod is the production of CO 2 in m 3 h 1 on a 24-h basis; and C ind and C out are the CO 2 concentrations in the indoorand outdoorairin p.p.m. In this work, a value of 185 m 3 h 1 hpu 1 forthe total CO 2 production in the farm has been used for the CO 2 balance. The suitability of this value is discussed below. The ventilation flow can be calculated also from the moisture balance [Eqn (1)]: V moist ¼ 36A moist L (1) h vap ðh ind H out Þ where: V moist is the ventilation flow from moisture balance in m 3 h 1 ; H ind and H out are the contents of moisture in the indoor and outdoor air in kg[h 2 O] m 3 [air] and h vap is the heat of vaporisation of waterin J kg 1, that can be obtained, according to the indoortemperature T ind in 1C from the expression: ð T ind Þ1 3 : Finally, the balance equation forthe sensible heat is described by Eqn (11): V heat ¼ 36ðA heats þ S trans þ S rad þ S slurry Þ (11) h sp rðt ind T out Þ where: V heat is the ventilation flow in m 3 h 1 from the sensible heat balance; S trans is the transfer of sensible heat in W through the building (negative when it is heat loss); S rad is the heat gain in W due to the thermal radiation transmitted through the ceiling; S slurry istheheatgaininw owing to the difference of temperatures between the slurry (when it is excreted) and the indoor air; T out is the temperature of the outdoor air in 1C; h sp is the specific heat of airin J kg 1 K 1 ;andr is the airdensity in kg m 3. The solar radiation affecting the roof varies over the daytime, and a certain space of time is required to sense its effect in the indoor air. The outdoor temperature also varies diurnally, although this variation is slower. In this work, a time delay for the heat gain due to the thermal radiation, and for the heat loss of sensible heat through the building, has been considered. On the other hand, it is known that only a specific fraction of the energy from the total thermal radiation is transmitted through the ceiling. That proportion has also been included in the calculations. Regarding the heat gain from the slurry, an initial slurry temperature of 39 1C (equal to the body temperature of the pigs) was used. When these balances [Eqns (9), (1) and (11)] are performed on a 24-h basis, A CO 2 ; A moist and A heat are equal to 1. Finally, note that, theoretically, all three calculated ventilation flow, should give the same value, and equal to the measured ventilation flow, as it is indicated in the following expression [Eqn (12)]: 4. Results V CO2 ¼ V moist ¼ V heat ¼ V measured (12) 4.1. Live mass, energy intake and total heat production Table 1 shows the results on live mass, energy intake and total heat production from the animals, over the experiment period. Table 1 Live mass,energy intake and heat production (average for every week) Week Live mass, kg Energy intake Heat production, W MJ day 1 W Maintenance Total m 75, where m is the live mass in kg.

6 488 V. BLANES; S. PEDERSEN 4.2. Animal activity measured and estimated from diurnal variation models Measured animal activity has been expressed in terms of relative activity (in relation to average activity for every day) and then averaged over 41 days. Relative animal activity from measured data (Fig. 3) indicates that a low, and relatively constant, animal activity occurs from 2: to 3:. The minimum animal activity during a 24-h period is about 25% lower than the daily average. On the other hand, relative animal activity shows two maxima during the day, probably affected by the times of the day when the feeders are replenished. These two maxima are relatively close to each other. This can be associated with the shortness of daylight in the winterperiod. Relative animal activity (averaged over 41 days) has been approached by two different activity models: the single sinusoidal model and the double sinusoidal model (Fig. 3). The correlation coefficients between the measurements and the estimations are shown in Table 2, where two cases are considered for each model. In the first case, we have used in the equations, the standard coefficients proposed in CIGR (22). In the second case, we have found the optimum coefficients: those that minimise the standard deviations of the differences between measured and estimated animal activity. The great improvement (coefficient of determination R 2 from 55 to 95) in the double sinusoidal model when the coefficients are optimised is mainly related to the change in the coefficient c which determines the distance between the two daily maxima foranimal activity. The inaccuracy when using the CIGR coefficients is probably connected to the fact that, in that case, the coefficients were estimated according to average annual conditions. However, this work has been carried out underwinterand natural lighting conditions, and so the time between the two daily maxima has to be reduced from 11 to about 6 h. On the otherhand, as measured animal activity shows two main maxima during the day, the double sinusoidal model, when using the appropriate coefficients, fits betterthan the single sinusoidal model. In conclusion, it can be stated that in general terms, the diurnal variation in animal activity can be explained both by the single sinusoidal model ðr 2 ¼ 88Þ and by the double sinusoidal model ðr 2 ¼ 95Þ: Relative activity Time, hours after midnight Fig. 3. Diurnal variation in relative animal activity:, measured;, single sinusoidal model;, double sinusoidal model; (activity models from optimised coefficients) Table 2 Values for the constants a, b, c and d,and the time of the day with minimum activity h min in h in the activity models; and the coefficients of determination R 2 for the correlation between measured and estimated animal activity Single sinusoidal model Double sinusoidal model a (h min ), h R 2 a b c d (h min ), h R 2 Standard Optimised

7 VENTILATION FLOW IN PIG HOUSES Comparison between measured and calculated ventilation flow The relation between calculated and measured ventilation flow, is shown in Table 3. The pigs get straw as rooting material in the first part of the experiment and maize silage in the last part but no offset on calculated ventilation flow is observed, why all 41 days measurements are considered in the analyses as a whole. Table 3 presents the results from two different approaches in the calculation of the ventilation flow, namely: no adjustment of the partition between sensible and latent heat ðk s ¼ 1Þ; and adjustment of the partition by applying a correction factor k s of 93. Table 3 shows that, when no correction is applied (k s ¼ 1), ventilation flows from moisture balance and from heat balance are in disagreement. However, in theory, moisture and heat balances should provide the same ventilation flow. That disagreement can be attributed to an inaccuracy in the estimation of the partition of the total heat into sensible and latent heat, as it is known that that division is highly influenced by the housing conditions. In this work, the same ventilation flow from the heat and the moisture balance is achieved when applying a correction factor k s of 93. That means that 7% of the sensible heat produced by the animals [calculated according to CIGR (22) equations], is used to evaporate water from wet surfaces (drinking water, slurry, etc.) inside the animal house. Table 3 Relation between calculated and measured ventilation flow from different correction factors k s of the sensible heat Balance Ratio of calculated/measured airflow k s ¼ 1 k s ¼ 93 CO Moisture Heat Ventilation flow calculated from the CO 2 balance is, on average, 8% lower than averaged measured ventilation flow (Table 3). This result indicates that the value of 185 m 3 h 1 hpu 1 forthe total CO 2 production should be reconsidered and slightly adjusted to improve the estimation of the ventilation flow from the CO 2 balance. In that sense, a perfect agreement between estimated and measured ventilation flow would involve considering a total CO 2 production in the house, equal to 21 m 3 h 1 hpu 1. However, in any case, that increase in the total CO 2 production seems not to be related with an underestimation of the slurry emissions, as it has not been observed any development in the differences between measured and calculated ventilation flow, as the days passed and the slurry was being accumulated in the house (maximum depth of 6 m). In Table 4, the standard deviation s in m 3 h 1, and correlation coefficients R 2 between measured and calculated ventilation flow are shown. Four cases are considered depending on the adjustment criteria by animal activity being applied. In the first case, estimated ventilation flow was calculated from animal emissions not adjusted by activity. In the following three cases, those emissions were adjusted by measured activity, activity from the double sinusoidal model and activity from the single sinusoidal model, respectively. In the calculations that provide the results shown in Table 4, the relation between measured animal activity A and the coefficients forthe adjustment of CO 2, moisture and heat production on an hourly basis (A CO 2 ; A moist and A heat ) were optimised, just like the coefficients of the single and the double sinusoidal models (a, b, c, d and h min ), which define the diurnal variation of those productions. The correction factor k s of 93 was also applied, according to the results presented in Table 3. The results indicate that the agreement between measured and calculated ventilation flow is generally high (Table 4), but the agreement can be improved by including in the calculations the diurnal variation on the production of carbon dioxide, moisture and heat by the animals. When adjusting the equations by measured Table 4 Standard deviation s in m 3 h 1,and coefficient of determination R 2 for the correlation between measured and calculated ventilation flow on hourly basis Balance Not adjusted by activity Adjusted by measured activity Adjusted by activity from double sinusoidal model Adjusted by activity from single sinusoidal model s R 2 s R 2 s R 2 s R 2 CO Moisture Heat

8 49 V. BLANES; S. PEDERSEN CO 2 balance, m 3 h 1 (a) y =. 99x 5 R 2 = moisture balance, m 3 h 1 (b) y =.98x 5 R 2 = heat balance, m 3 h 1 (c) y =.92x 5 R 2 = Fig. 4. Measured and calculated ventilation flow on hourly basis from: (a) carbon dioxide, (b) moisture and (c) heat balance adjusted by activity from single sinusoidal model; R 2, coefficient of determination activity, between 83% and 9% of the total variation of the ventilation flow can be explained by balance equations; and between 84% and 92% when using the single orthe double sinusoidal model. A comparison between measured ventilation flow and ventilation flow calculated from the CO 2, moisture and heat balances, adjusted by activity from the single sinusoidal model, is shown in Fig. 4, by way of example. Regarding the ability of the different balances to provide accurate ventilation flow, Table 4 shows that CO 2 and heat balances provide better results than moisture balance. The correlation coefficient from

9 VENTILATION FLOW IN PIG HOUSES 491 moisture balance when ignoring the diurnal variation of activity, is relatively low ðr 2 ¼ 66Þ: Nonetheless, ventilation rate from moisture balance also shows a high correlation when considering the diurnal variation ðr 2 ¼ 84Þ: The standard deviations of the differences between measured and calculated flow (Table 4), are about 11 12% of the average measured ventilation flow (1285 m 3 h 1 ) when considering the CO 2 and heat balance, and about a 15% in the case of the moisture balance. 3 CO 2 balance, m 3 h 1 (a) y =.915x R 2 = moisture balance, m 3 h 1 (b) heat balance, m 3 h 1 (c) y =.921x 5 R 2 = y =.95x 5 R 2 = Fig. 5. Measured and calculated ventilation flow on 24-h basis from: (a) carbon dioxide, (b) moisture and (c) heat balance; R 2, coefficient of determination

10 492 V. BLANES; S. PEDERSEN Table 5 Effect of the sun radiation on the standard deviation s in m 3 h 1,and coefficient of determination R 2 between measured and ventilation flow calculated from heat balance (not adjusted and adjusted from the single sinusoidal model) Factors considered in the heat balance (besides the heat emission from the animals) Not adjusted Adjusted from single sinusoidal model s R 2 s R 2 Heat transmission Heat transmission+solar radiation Heat transmission+solar radiation+time delay in the effect of solarradiation Table 6 Sensitivity analysis (% change in calculated ventilation flow) Parameter Basic value (average of 41 days) Increment Change in airflow, % CO 2 balance Moisture balance Heat balance Temperature indoor 176 1C 11C Temperature outdoor 42 1C 1 1C 95 RH indoor71% 1% 35 RH outdoor97% 1% 37 CO 2 conc. indoor19 p.p.m. 5 p.p.m. 36 CO 2 conc. outdoor39 p.p.m. 5 p.p.m. 39 U-value y 43 W/m 2 K 1% 1 Solarradiation z 1% 1 Live mass 136 kg 1% RH, relative humidity. y U-value, coefficient of heat transmission. z The maximum value of solar radiation for the whole period was 198 W m 2, and the transmission of the energy from the solar radiation has been set as 1% of the total radiation measured every hour. When calculating the ventilation flow from the heat balance [Eqn (11)], we have taken into consideration some different factors (the thermal radiation that is transmitted to the indoor air, a time delay in the solar radiation and in the heat loss, and the heat gain from the slurry) that take part of the heat balance in the real farm, but whose importance is unknown. Including these factors makes the calculation more complicated and therefore it is interesting to analyse if they involve a relevant improvement in the estimation of the ventilation flow. Table 5 shows the correlation coefficients that have been obtained forthe heat balance (not adjusted and adjusted from the single sinusoidal model) as the calculation was refined. Considering a time delay in the heat transmission did not mean any change in the standard deviation and the coefficient of determination under winter conditions. Finally, Fig. 5 shows linear regressions and coefficients of determination between measured and calculated ventilation flows (from the three balances), on a 24-h basis Sensitivity analysis The measurement of each parameter that takes part in the balance equations inevitably entails a measurement error, which involves a lack of precision in the estimated ventilation flow. Table 6 shows the sensitivity of the calculated ventilation flow to inaccuracies in the measurement of temperature, relative humidity, CO 2 concentration, coefficient of heat transmission, solar radiation and the live mass of the animals. As seen in Table 6, an error of 1% in estimated ventilation flow corresponds approximately to an error of 1 1C in the measurement of temperature, 3% for the relative humidity or 15 p.p.m. for the CO 2 concentration. 5. Conclusions In this work, a comparison between measured ventilation flow in a pig house during 41 days, and ventilation flow calculated from CO 2, moisture and heat

11 VENTILATION FLOW IN PIG HOUSES 493 balances has been carried out. Results indicated that, in general terms, ventilation flow calculated from the three balances were in good agreement, although they were slightly lowerthan measured ventilation flow. (1) Ventilation flow estimated from CO 2 balance (from 185 m 3 h 1 hpu 1 of CO 2 ) on an hourly basis was, on average, 8% lower than the averaged measured ventilation flow. To bring a perfect agreement between estimated and measured ventilation flow would involve considering a total CO 2 production in the house, equal to 21 m 3 h 1 hpu 1. (2) Ventilation flow estimated from both, moisture and heat balances, on an hourly basis, were on average 9% lowerthan the averaged measured ventilation, by converting 7% of sensible heat into latent heat. (3) The values forthe coefficient of determination R 2 for the correlation between measured and estimated ventilation flow, on an hourly basis, without adjustment foractivity, were 79 forthe CO 2 balance, 66 forthe moisture balance and 85 for the heat balance. (4) The agreement on an hourly basis can be improved by including in the calculations the diurnal variation in the CO 2, moisture and heat production (e.g. values of R 2 of 92, 84, 87, forthe CO 2, moisture and heat balance, respectively, for a single sinusoidal variation in animal activity). (5) The diurnal variation in animal activity can be fairly explained by both, a single sinusoidal model (R 2 ¼ 88) and by a double sinusoidal model (R 2 ¼ 95). (6) The values forthe coefficient of determination R 2 for the correlation between measured and calculated ventilation flow on a 24-h basis were: 97, forthe CO 2 and the moisture balance, and 92 forthe heat balance. References Chwalibog A; Tauson A H; Thorbek G (24). Diurnal rhythm in heat production and oxidation of carbohydrate and fat in pigs during feeding, starvation and re-feeding. Journal of Animal Physiology and Animal Nutrition, 88(7 8), CIGR (1984). Climatization of animal houses. Commission Internationale du Ge nie Rural. Report of Working Group. Scottaspress Publishers Limited, Aberdeen CIGR (22). Climatization of animal houses. Heat and moisture production at animal and house levels. Commission Internationale du Ge nie Rural. 4th Report of Working Group. Research Centre Bygholm, Horsens (Denmark) Jeppsson K H (22). Diurnal variation in ammonia, carbon dioxide and watervapouremission from an uninsulated, deep litterbuilding forgrowing/finishing pigs. Biosystems Engineering, 81(2), Li H; Xin H; Liang Y; Gates R S; Wheeler E F; Heber A J (25). Comparison of direct and indirect ventilation rate determinations in layer barns using manure belts. Transactions of the ASAE, 48(1), Ni J Q; Hendriks J; Coenegrachts J; Vinckier C (1999a). Production of carbon dioxide in a fattening pig house under field conditions. I. Exhalation by pigs. Atmospheric Environment, 33, Ni J Q; Vinckier C; Hendriks J; Coenegrachts J (1999b). Production of carbon dioxide in a fattening pig house under field conditions. II. Release from the manure. Atmospheric Environment, 33, Pedersen S; Gaardbo Thomsen M (2). Heat and moisture production of broilers kept on straw bedding. Journal of Agricultural Engineering Research, 75, Pedersen S; Pedersen C B (1995). Animal activity measured by infrared detectors. Journal of Agricultural Engineering Research, 61, Pedersen S; Rom H B (1998). Diurnal variation in heat production from pigs in relation to animal activity. EurAgEng, Paper No 98-B-25, AgEng Oslo98 Pedersen S; Takai H; Johnsen J O; Metz J H M; Groot Koerkamp P W G; Uenk G H; Phillips V R; Holden M R; Sneath R W; Short J L; White R P; Hartung J; Seedorf J; Schro der M; Linkert K H H; Wathes C M (1998). A comparison of three balance methods for calculating ventilation rates in livestock buildings. Journal of Agricultural Engineering Research, 7, Sousa P; Pedersen S (24). Ammonia emission from fattening pig houses in relation to animal activity and carbon - dioxide production. CIGR Ejournal, VI, manuscript BC 4 3 Schauberger G; Piringer M; Petz E (2). Steady-state balance model to calculate the indoorclimate of livestock buildings, demonstrated for finishing pigs. International Journal of Biometeorology, 43, Van Ouwerkerk E N J; Pedersen S (1994). Application of the carbon dioxide mass balance method to evaluate ventilation rates in livestock buildings. Proceedings of XII CIGR World Congress on Agricultural Engineering, Milan, Italy, vol. 1, pp