The use of heated models to describe the thermal environment in shelters for farm animals
MarkusPyykkönen
Pyykkönen,M. 1992.Theuse ofheated modelstodescribethethermal environment in shelters for farm animals. Agric. Sci.Finl. 1: 539-545. (Dept. Agric. Eng. and HouseholdTech., Viikki F, SF-00014 UniversityofHelsinki,Finland.)
Thedrybulb air temperature is still the mostcommonlyusedparametertocharacterize the thermalenvironment, even thoughit disregardsthe effect of airvelocityand the thermal properties of the flooring materialonthe heat loss from the animal.
Measurements inthelaboratoryconfirmed thatanuninsulated heated model withan overall thermal resistance of0.11
m 2 KW' 1is sensitiveenoughtodifferentiate between changes in conduction,convection and radiation conditions.
Measurements onfarms showed that the heat loss simulatedbymechanical models givesa more diversifieddescriptionof the thermal environment than thedry bulb air temperature.Although the uninsulated mechanical model is notastandardizeddevice, it isauseful method formeasuringthe thermal environment especially under sheltered winter conditions.
Keywords: heatdissipating model,heatloss,sensible heatloss,drybulb air temperature
Introduction
Mathematical animal heat loss modelsare used in converting the heat loss data obtained infeeding experiments into design values for ventilation engi- neers. In theseconversions, gross simplifications are introduced, i.e. the heat loss is onlyafunction of airtemperature,live weight and production level
(STRÖM and Zhang 1989). This approach is a major reason why the dry bulb air temperature measured at a representative location is still the parametermost commonly usedtocharacterize the thermal environment (HAHNet al. 1983) in practice.
Anotherreason may be the abundance of simple and reliable methods for measuring the airtemper- ature. Anyway, the dry bulb air temperature dis-
regards the effect of air velocity and the flooring materialon the heat loss from the animals. It also disregards the radiative heat loss.
The quantification of the thermal environment would be moreaccurateif the dry bulb air temper- ature were replaced by an effective temperature combining the cooling effect ofconvection, con- duction and radiation with the cooling capacity of evaporation.
The aim of this projectwastodevelopamethod to simulate the sensible heat loss from the animals in order to obtain a good characterization of the thermal environmentatthe level of the individual animalon farms and tostudy whether the charac- terization given by the heat dissipating model is different from that given by the dry bulb airtemper-
ature. The method was tobe sensitive enough to measure the small changes in the environment in order to evaluate the function of the ventilation systemfrom the thermal point of view. The model was tobe suitable for continuousmeasurement in pensevenin the presence of animals.
Review ofthe literature
The cooling effect of the environment has been measured withakatathermometer(MOTHES 1971, Trippe 1984, Kunz 1985)which mesures the time needed foraheated bulbtocool from 35°Cto30°C.
The coolingeffect,measured by the katathermome- ter, does not measurethe total heat loss from the animal. It gives anestimate of the maximum heat loss to the surrounding air (Trippe 1984),but it disregards the heat loss by conduction tothefloor, which is important for juvenile animals spending mostof their time lying.
The heat loss from animals has been simulated by heat dissipating models. These models generally include the following assumptions(HahnandBÖE
1985):
I.
ignoring the evaporative component of energy exchange.2. the use of uniform thermal insulation over the total surface of the model.
3. alevel of thermal insulation correspondingtothe vasoconstricted stateof the animal.
4. nopostural adjustments.
In the simplest heat dissipating models the heat flow from thewamibody wascalculated from the cooling time foracertaintemperatureinterval(Ny- GAARD1966).This introduced the problem of chan- ging temperature difference during the measure- ment.The problem was usually solved by measur- ing the cooling time for a narrow temperature inter- val, and by calculating thetemperature difference from the averagetemperature difference during the
measurement.
Insulated full-scalecow models have been used in estimating the feed energy requirement of beef
cattle and suckler cows under unsheltered winter conditions(Webster 1971, Burnett and Bruce 1987). The modelswere basedon aconstantinter- naltemperature and on anoverall thermal insula- tion, corresponding to that ofareal animal. They ignored the evaporative component of energy exchange. A cylindrical 0.4-scale model gave sim- ilar thermal responses inachanging climateasthe full-scale model(Jones 1982).
Two heatedmodels, one insulated, of the size and overall thermal resistance ofa40 kglamb,and another uninsulated black copper spherewere used by Hahn and Böe(1985) in estimating the energy demand of lambs in different environments. The modelswere placedataheight of 0.5 m above the floor. The results of the two models were highly correlated(r=0.96).The good correlation between the heating requirements of insulated and uninsu- lated models indicates that uninsulated models could provideanacceptablemeasureof the thermal environment (Hahn and Böe 1985).The sensible heat flow from heat dissipating models is essential- ly the same asthat from real animals, as summa- rized by the equation presented by Esmay and Dixon(1986):
Qs= A*C*(Ta-Te)
where:
Q
s= sensible heat loss,W; A=surfacearea ofanimal; Ta= surface temperature ofanimal, K;Te= average temperature of surroundings,K; c = overall sensible heat transfer coefficient.
Construction of the heated model
Totest whether uninsulated heated models provide anacceptable measureof the thermalenvironment, aheated model withanoverall thermalresistance of
2 1
0.1 lm" KW was constructed. The surfacearea of the modelwaschosensothat thecontactarea of the modelto thefloor, 23.3% of the totalsurface, was about the same asthat ofalying animal, which is about20%(Gommers etal. 1970).
Agric.Sd.Finl. 1(1992)
With a given resistance and a constant voltage, the energy requirement ofan electrically heated modelcan be calculated by measuring the time for the powertobe switchedon,and by multiplying it by the electrical power. The heat loss from the
2 .
model(Wm")isthen computed by division by the total measuring time and the surface area of the model.
The heated modelwasmade ofastandard alumi-
nium box used in electronics(125 *80*57.5 mm) which wasfilled with ethylene glycol (50%). The amount of fluidwas0.3 kg. Themean temperature of the fluid during a heating cycle was 32.5°C, ranging from 31.5°C to 33.5°C. The model was heated witha heatingelement, placed about2mm from the bottom surface of the vessel. The heating wascontrolled through a thermistor,which started and stoppeda timer when switching the heatingon lig. I.Working principleof the heated model.
A Voltage supplyandregistering unit A 1Transformer
A 2Rectifierand voltage stabilizer
A 3 Voltageregulatorand stabilizer
A 4Relay switch for the timerA 5
A 3Timer B Regulating unit
B IRegulation of the controlvoltage B2 Comparator
B 3Relay for theheating voltage,B2controllingtherelay.
C Measuring device
C 1Heatingelement (approx.5ohm) C2 Sensorlube with two thermistors
Table I. Coefficients of the regression lines between the temperature difference and the heat loss for the different flooringmaterials with the heated model.
Flooring Coefficientof Intercept Coefficient
material determination a b
Concrete 0.99 -31.2 14.3***
Plastic 0.99 -21.7 11.2***
Steel 0.98 -19.1 15,4***
Straw 0.99 -17.2 B.6***
Wood 0.99 -26.1 11.2***
***p<o.l %
and off. The working principle of the model is shown in Figure
1.
Thetemperature difference,definedasthe differ- encebetween the ambient temperature atthe floor level and the mean temperature measured in the sensor tube during a heating cycle of the model, explained 98- 99% of the variation of the heat loss from the model in the laboratory which is in agree- ment with the general laws of sensible heat flow (Table 1).These results indicate that the accuracy of the method is acceptable. When the temperature difference ranged from 7°C to 30°C, the linear regression line was:
Q=a+bdT
where: dT= temperature difference, °C;
Q
=heatloss from themodel,Wm'2.
The values of the intercepts and the coefficients of the regression lines for the different flooring materialsarepresented in Table
1.
The fact that the regression coefficients calculated for the floorma- terialsaredifferent indicates that the model differ- entiates between conduction conditions which may occurin practice.The effect of air velocity on the heat loss from the modelwas measured ina wind tunnel. To be abletomaintainaconstanttemperatureof themo- vingair, anairtemperatureof 15°Cwasused which corresponds to the temperature generally used in calf shelters. The regression between the air velo-
-2 2
city ranging from 0ms to Ims and the heat loss
from the model was:
Q=282.7+6.9 *(v'
1
)- 78.5 *ln(v'');
(r2=0.99)where: v=air velocity, ms'1;
Q
=heat loss from the model,Wm2.The temperature of the upper surface of the model decreased with increasing air velocity, which explained the non-linearity of the regression between the air velocity and the heat loss in agree- mentwith the general theory of convection.
These results confirm the conclusion that an uninsulated heated model with an overall thermal
2 1
resistance of less than 0.11 m KW candifferenti- ate between conduction and convection conditions which may occurin practice.
Measurementsonfarms
The first farmtestsonthe modelweremadeincow shelters, simultaneously in acalf pen witha woo- den slatted flooronSuitia experimental farm and in anuninsulated shelter with deep litteratMuurla, an experimentrunby the College ofVeterinary Medi- cine (between 2 Feb and 22 Mar, 1990).
Additionaltestsweremade in three piggeries, in farrowing house A between 27 Mar and 6 Apr, 1990 and in farrowing houses B and C between 14 Feb and 15 Apr, 1991. In the piggeries the model wasplaced on concrete floor in the farrowing pen in the lying areaof the piglets, with 20 - 30 mm of straw under the model. In farrowing house C there werehotwaterheating tubes under the littered lying area. In theweaner house of piggery C the model wasplaced on a slatted floor made of galvanized welded mesh. Therewere animals in the pen during themeasurementsin theweanerhouseandatMuur-
la.
On thefarms,the dry bulb air temperature, i.e.
the ambient temperature, was measured with Hg thermometersatan accuracy of O.l°C. The values of the thermometers wereread daily at 8.30a.m.
and4.30 p.m. The function of the thermostat of the Agric. Sei.Finl. 1 (1992)
Table 2.Thermal environment asweeklymeans inapen at Table 3.Heat loss from the model in farrowinghouses.
Suitiaandinanuninsulated shelter at Muurla.
Parameter Farrowinghouse
Period Heatloss, Wm 2 Ambient temperature, °C ABC
Suitia Muurla Suitia Muurla
Mean ambient temperature, °C 19 19 19
2-9Feb 211.8 199.1 13.5 3.3 Measuring period, days 11 17 5
9-16Feb 211.0 230.8 13.5 1.5 Observations 22 34 10
16-23Feb 212.7 205.4 13.5 1.1 Mean heat loss from the 153“ 104c 110b
15-22Mar 188.8 175.6 13.5 4.3 model, Wm“2
Mean 206.1 202.7 13.5 2.6 S.D.,Wm“2 9.6 8.1 3.4
S.D. 11.5 22.7 0.0 1.3 T~
lvalue 0 27ns significantdifferences are shown by different letters,
p>o.ool
nsnotsignificant
ventilationsystem wastested with athermohygro- grafatanaccuracy of I°C.
Resultsfrom thecowshelters
The heat loss was measured in one week periods overfive weeks. The lowest outside airtemperature was -11.0°C, and the weekly means varied from -l.l°Cto +2.B°C.
The average ambient temperature was 13.5°Cat Suitia, ranging from 12°C to 15°C. The weekly means of the ambient temperature varied from
+I.l°C to+4.3°C in the uninsulated shelter at Muurla. The heat losswas206.1+11.5 Wm'2 in the
penand202.7±22.7 Wm ~in the shelter(Table 2).
The difference in the thermal environment be- tweenthe pen atSuitia and the uninsulated shelter with deep litteratMuurla isobvious,if the environ- mentis described by the ambienttemperature.If the thermal environment is described by the heat loss from the heated model, there was no difference between the environments. The heat loss in the uninsulated shelterwas highly influenced by the heat produced by fermentation in the deep litter.
Results fromthe piggeries
Therewere significant differences in the heat loss measured by the model when placed in different
Table4.Heat loss from the modelin farrowinghouse C and inweanerhouse of thesamefarm before and afterreducing the ventilation rate.
Farrowin Weaner house g house
Parameter
before after
Observations 10 8 9
Mean ambient temperature, °C 19 24 24 Mean heatloss, WirT2 110“ 133° 108b
S.D.,WirT2 3.4 22.4 9.4
“,b significant differences are shownby different letters, p>o.ol
farrowing houses even though the measuredaver- age ambient temperatures were the same, 19°C (Table 3).
The difference in the heat loss between farrow- ing house A and farrowing house B, about 50 Wm , correspondsto anincrease of air velocity of 0.2ms2atanairtemperatureof 19°C.
The thermal environment of farrowing house C was compared tothat of the weaner house of the samefarm. Despite the higher ambienttemperature the heat loss from the model was higher in the weanerhouse(Table 4).After reducing the airve- locityatthe floor level by decreasing the ventilation
rate in theweaner house,the difference in the heat loss disappeared.
Conclusions
Themeasurementsonthe farms showed that the dry bulb airtemperaturedoesnotalways givean accur- ate picture of the thermal environment. In condi- tions where animals should be sheltered free from draft, the heat loss simulated by mechanical models gives a more diversified description than the air temperature. These results completely agree with the conclusion of Hahn andBÖE(1985) that unin- sulated models can provide an acceptable method of measuring the thermal environment.
The model isnot able toregulate the heatloss, which in combination with the rather low heatre-
2 i
sistance of0.11 m KW means that the model is more sensitivetochanges in the environment than a live animal. The sensitivity of the model isnot a disadvantagehowever, asthe function of the venti- lationsystem is measured. Even small changes in the environmentcan be detected with the model.
Altough the model doesnotmeasure the heat loss from theanimal, it gives anestimate of the maxi- mumsensible heat loss. In thisrespectit resembles the katathermometer used byTrippe(1984).
Themeasurements onfarms showed that the he- ated model was durable enough to be used on
farms, even on the floor in pens where there are animals. Yet, the model cannotbe used in un-
sheltered winter conditions, where the heat loss from the model exceeds 450Wm .A heat loss of 465Wm meansthat the heating element is switch- edon all the time.
Although the uninsulated model is not a stand- ardized device for measuring the thermal environ- ment,it isauseful method especially under shelter- ed winter conditions asindicated by the measure- mentsonthe farms. The modelcanbe usedtoadjust the ventilation andtoestimate the effect ofachange in the thermalenvironment, except achange in the relative humidity,onthe heat loss from the animals.
In studying the correlation between the thermal environment and the animal health the model pro- vides abetter description of the thermal environ- ment than that given by the dry bulb air temper- ature.
As the thermal properties of the model are known, it is possibletocalculate the corresponding effective temperaturewhere the heat loss is only a function of thetemperature. The effectivetemper- aturecan then be used in the mathematical animal heat loss models to estimate the heat loss from animals in the situation measured.
References
Burnett,G.A.&Bruce, J.M. 1978.Thermal simulation of sucklercow.FarmBldg. Progress 54: 11-13.
Esmay.L.M.&Dixon,J.E. 1986.Environmental control for agricultural buildings. 287p.Westport,Connecticut.
Hahn,G.L.&Böe, K. 1985.Evaluating thermal demand in cold sheephousing. ASAE-paperno.MCR85-150. 11p.
—,Nygaard.A.&Simensen.E. 1983.Towardsestablishing rational criteria for selection and design of livestock environments. ASAE-paper83-4517.
Gommers. F. 1., Cristison,G.I. & Curtis, S.E. 1970.
Estimatinganimal floor contactareas.J.Anim.Sci. 40:
552-555.
Jones,C. G. 1982.Modellingthe integrated climatic energy demand on animals. Interim report to theAgricultural Research Council (U.K.). 12p.
Kunz,P. 1985.KälberhaltunginHiitten. FAT-Berichte 269.
12p.
Mothes.E. 1971.Stallklima. 196p. Berlin.
Nilsson. C. 1988.Floors inanimal houses. Institutionen för
lantbruketsbyggnadsteknik. Rapport 61.
Ström.J. S. &Zhang.G. 1989.Thermal control inanimal buildings. In: Dodd, V. A. & Grace, P. M. (eds.).
Agricultural engineering 2.Rotterdam,p. 1265-1278.
Trippe,M. 1984. Priifung des KriteriumsAbkiihlungsgrösse auf seineEignung zurweitergehenden Charakterisierung des Mikroklimas inTierställen. Institut furTierhygiene der Tierärtzlichen Hochschule Hannover. 106p.
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30: 684-690.
Manuscriptreceived April1992 MarkusPyykkönen
Department of Agricultural Engineering and Household Technology
Viikki F
SF-00014UniversityofHelsinki,Finland Agric. Sei.Fint. 1(1992)
SELOSTUS
Termisen ympäristön mittaaminen eläinsuojissa
MarkusPyykkönen Helsingin yliopisto
Ilman keskimääräistä lämpötilaa käytetään yleisesti kuvaa- maan eläinsuojien termistä ympäristöä. Ilman lämpötilaei sisällä tietoa ilmanliikenopeudeneikä lattiamateriaalin vaiku- tuksesta eläimen lämmönluovutukseen.
Eläinmallin luovuttamaa lämpömäärää voi periaatteessa käyttää eläimen termisenympäristön kuvaamiseen,sillä mal- linvapaa lämmönluovutusonsamojen fysiikanlakien alainen kuin eläimen vapaa lämmönluovutus. Jotta termisenympäris- tönjatkuvamittaaminen olisimahdollista,rakennettiin läm- mitettävä mekaaninenmalli,jonka lämpövastusoli0,11 m’
KW1.
Lämmitettävänä mallinakäytettiin etyleeniglykolilla (50
%)täytettyäalumiiniastiaa (125 * 80 *57,5 mm). Nesteen määräoli0,3kg. Lämmitysvastuksen sisältäväkuparikotelo oli 2mm alumiiniastianpohjan yläpuolella. Lämmitystäoh- jaavatermistönkytki käyttöjännitteen aikalaskuriin, joka oli ainakytkettynä päälle lämmitysvastuksenlämmittäessä.
Koska tehoP=
U 2
*Rl
, niinmallin luovuttamalämpö- määräon lämmitysajan jatehon tulo,kun vastusja jännite ovatvakioidut. Mallin luovuttama vapaa lämpö saadaanjaka- malla lämpömäärämittausajallajamallin pinta-alalla.Lämmitettävää mallia kokeiltiin käytännön olosuhteissa Suitiankoetilallavasikkakarsinassa, jossaolipuinenrakolat- tia, ja eristämättömässä olkipohjaisessa makuusuojassa Muurlassa. Lämmönluovutus mitattiin viikonjaksoissaviiden
viikon aikana.
Olosuhteissa oli selvä ero, kun niitä kuvattiin ympäristön lämpötilalla,sillä karsinassalämpötilaoli 13,5°Cjamakuu- suojassa 2,6 °C. Mallin lämmönluovutuksen perusteella ei olosuhteissa ollut eroa, sillä mallin lämmönluovutus karsinas- saoli206,1±11,5Wm
2
jamakuusuojassa 202,7+22,7 Wm2.Mallin avulla mitattiinporsituskarsinan termistäympäris- töäkolmessa sikalassa. Vaikka ilman lämpötilanasetusarvo oli kaikissa sikaloissa 19°C,niinmallin lämmönluovutus oli tilastollisesti merkitsevästi erilainen eri sikaloissa. Ero suu- rimman ja pienimmän lämmönluovutuksen välillä, noin 50Wm2,vastaao,2ms
1
eroailmannopeudessa.Sikalassa Cmallin lämmönluovutus varhaisvieroitusosas- tossa (133 Wm'2)oli suurempikuin porsituskarsinassa(110
Wm"),vaikka ilmanlämpötilaoli27°Cvarhaisvieroitusosas-
tossa.Kun ilmanvaihdonsäätöä muutettiin varhaisvieroitus- osastossa, mallin lämmönluovutus oliyhtäsuuri (108 Wnr) kuinporsituskarsinassa.
Mittaukset osoittivat, että ilman lämpötila ei aina anna tarkkaa kuvaa termisestä ympäristöstä. Lämpöä luovuttavat mallit antavatmonipuolisemman kuvan termisestäympäris- töstäkuin ympäristön lämpötila jamallien avulla voidaan mitata eläinten oleskelualueellatapahtuvattermisenympäris- tönmuutokset.
