Heating/Ventilation Control Systems

Transcription

1 Heatig/Vetilatio Cotrol Systems ANALYSIS OF STABILITY OF LIVESTOCK BUILDING HEATING/VENTILATION CONTROL SYSTEMS Y. Zhag, E.M. Barber, H.C. Wood Executive Summary Mathematical models ca be used to predict the outcomes of scearios where there are may variables. The advatage of beig able to model the outcome is that fewer experimets eed to be coducted. For example, if the model ca accurately predict the outcome of a chage i oe or more variables the it is oly ecessary to coduct a few tests to demostrate the effectiveess of the model. This eables researchers to focus limited resources toward aswerig specific questios that either caot be aswered by the model or are predicted by the model to give a favourable respose. A mathematical model is used i this study of vetilatio cotrol systems. A importat issue related to vetilatio cotrol systems is stability. Stability refers to the ability of the cotrol system to provide a eviromet with miimal temperature/humidity fluctuatios i a attempt to maitai predetermied set poits. I the future, as humidistats become available for use i livestock buildigs, temperature-humidity-cotrol (THC) strategies will be used. Simulatio results reported i this paper show that temperature cotrol (TC) is absolutely stable ad temperature-humiditycotrol (THC) is coditioally stable. Two THC systems, humidistat cotrolled heater ad thermostat cotrolled secod stage fa (THC1), ad thermostat cotrolled heater ad humidistat cotrolled secod stage fa (THC2), were ivestigated. A upper temperature limit to deactivate the heater for THC1 should be devised, otherwise istability may occur whe the weather is both warm ad wet. A lower temperature limit to deactivate the secod stage fa for THC2 should be devised, otherwise istability may occur whe the weather is cold. A THC system likely gave a wider temperature fluctuatio tha temperature based cotrol systems. INTRODUCTION The cotrol strategy for a livestock buildig airspace ca be defied with the use of vetilatio graphs (Christiaso ad Fehr, 1983; Zhag et al, 1989). However, the practical problem of how to achieve cotiuously the optimal eviromet has remaied usolved. Covetioal cotrol strategies for a livestock buildig airspace ofte fail to meet their objectives. As a result of iadequate kowledge of the miimum vetilatio requiremets of livestock buildigs, heatig/vetilatio systems ofte are improperly desiged. For example, a simple cotrol strategy (such as temperature cotrol aloe) caot meet all objectives of the cotrol system such as moisture balace vetilatio i witer. There is also a lack of kowledge about the resposes of the thermal eviromet to differet cotrol systems. A importat problem i cotrol systems cocers stability. There are very few studies o the stability of livestock buildig thermal eviromet cotrol systems. The reaso for the lack of cosideratio of stability may be that stability for covetioally used simple temperature cotrol systems has ot bee a problem. "Possible ucotrollability" was metioed by Nakaishi et al (1973) for decoupled temperature ad humidity cotrol systems, but the ucotrollability was either demostrated or aalyzed because of the difficulty of the oliear ature of the system. Cosiderig the overall tred to use of more idustrial vetilatio techologies i livestock bars ad with the use of alterative moitorig ad cotrol equipmet, the aalysis of stability will be beeficial for both desig ad trouble shootig purposes. The objective of this study was to test the followig hypothesis: "For o/off cotrol systems, cotrollig the heatig/vetilatio equipmet usig either temperature cotrol (TC) strategy or temperature humidity cotrol (THC) strategy will always result i a stable cotrol system". GENERAL CONTROL SYSTEM STRUCTURE FOR LIVESTOCK HOUSING The cotrol termiologies for Aimal Eviromet Cotrol System (AECS) have bee systemized by Cole (1980). AECS terms used i this paper are cosistet with Cole's defiitios. First, a AECS is specified, the each system elemet is represeted as a trasfer fuctio. The resultig trasfer fuctios are the utilized to formulate a block diagram ad to perform 38

2 stability aalysis. Figure 1 illustrates the mai features of a AECS. The cotrol system is comprised of a cotroller, cotrol forces, ad a plat. The referece iputs for a livestock buildig airspace are setpoits of temperature (Tref), relative humidity (RHref) ad cotamiat level (Cref). The cotroller cosists of all sesig elemets, such as thermostats, ad the effector mechaisms such as electric relays to tur heaters o ad off. Almost all cotrollers curretly available i the Caadia market place are temperature based cotrollers, i.e., the sesig elemets are temperature sesors. There are few humidity based cotrollers used i livestock buildigs because the sesors are geerally ureliable i dusty ad corrosive airspaces. The cotrol forces are oe of the mai cosideratios i selectig cotrol strategies. The thermal resposes of the airspace ad the supplemetal eergy cosumptio are completely defied by the cotrol forces oce the parameters ad the disturbaces of the buildig are specified. The cotrol forces for the livestock buildig system ca be iterpreted as the supplemetal heat ad the vetilatio rate. Alterative cotrol strategies ca be cosidered to be just differet levels ad combiatios of cotrol forces. The plat of the cotrol system i this case is the livestock buildig airspace. There are several sources of disturbace iputs to the plat: weather coditios, which cosist of wid velocity, temperature, relative humidity ad solar radiatio, heat trasfer with the groud, heat ad moisture productio of aimals withi the airspace, ad the heat ad moisture loads due to maagemet activities. Parameters defiig the plat iclude buildig heat ad mass trasfer characteristics, buildig orietatio, stockig desity, ad the heatig ad vetilatig requiremets. The cotrolled variables, i.e., the outputs of cocer, are the temperature, relative humidity ad cotamiat levels of the airspace. I tur, those cotrolled variables are the moitored parameters which are fed back to the cotroller. The feedback variables are the compared with the referece iputs to decide if a ew cotrol force should be activated. Oly temperature ad humidity are aalyzed i this paper. Temperature Cotrol The temperature cotrol block diagram of a livestock housig system icludes a temperature sesor, cotroller, vetilatio ad supplemetal heat equipmet. The relatioship betwee these compoets is described i figure 1. The stability of a closed-loop system ca be determied by may methods. The Roth s stability criterio was used ad was foud to be appropriate for this study. Temperature ad Humidity Cotrol (THC): I the case of combied temperature ad humidity cotrol, the aalysis of stability becomes more complicated ad it ca ot be performed usig the method used for the TC system. THC systems are ot curretly a commo logic used i livestock buildigs, however, the developmet of reliable humidistats suitable for bar eviromets should make these more available i the ear future. With THC, the cotrol forces, heater ad fa are activated separately i a attempt to maitai both the temperature ad relative humidity at desired levels. There are two potetial cotrol patters for a THC system: 1.THC1: Thermostat activated fa for temperature cotrol, ad humidistat activated heater for relative humidity cotrol. 2.THC2: Thermostat activated heater for temperature cotrol, ad humidistat activated fa for relative humidity cotrol. The trasfer fuctios of relative humidity cotrol could be derived usig the equatios describig the psychrometric properties of air. However, because relative humidity ad temperature are ot idepedet ad the relatioship is highly oliear, the trasfer fuctio method is ot valid for oliear system aalysis. Graphical aalyses by meas of a psychrometric chart ad simulatio have bee performed. For THC1, thermostat cotrolled vetilatio is absolutely stable as aalyzed by Roth s stability criterio. The humidistat activated heater will always decrease the relative humidity but will icrease the temperature. This part of the cotrol may cause wide temperature fluctuatio ad a udesirable high 39

3 temperature. As show i Figure 2a, poit A represets the thermal properties of room air ad poit B those of the icomig air. Whe the weather is both warm ad wet (poit B i Figure 2a), the vetilatio is maximum ad the room air relative humidity may still exceed the setpoit (75% i this case). The heater will be activated ad the room temperature will be icreased to Ta', which is udesirably high. Therefore, a upper limit temperature to deactivate the heater should be devised to prevet the istability. For THC2, as show i Figure 2b, poit C represets the thermal properties of the room air whe the vetilatio rate is icreased from the stable state (for example, after a secod stage fa is activated i attempt to lower the relative humidity back toward the set poit). If the heat ad moisture productio of aimals ca be assumed to be costat ad idepedet of Ta ad RHa over a small rage, poit C will always be o the coectig lie betwee A ad B whe the vetilatio rate chages. Because more cold fresh air is brought i, poit C moves closer to poit B, ad a higher relative humidity or eve codesatio may result. Thus, icreasig the vetilatio rate i respose to a humidistat uit will decrease the absolute humidity but may actually icrease the relative humidity. The higher relative humidity measured by the sesor is agai set to the cotroller to icrease the vetilatio rate ad as a result the cotrol system becomes a positive feedback system which is ustable. The other half of the THC2 system is temperature cotrol which is always stable. I practice, the positive feedback for relative humidity may ot be able to cotiue. As the vetilatio rate icreases, the temperature decreases ad whe the room temperature drops below the lower temperature set poit, the heater will be activated to brig the temperature up ad hece the relative humidity will go dow. This will cause wide fluctuatios of temperature ad relative humidity. The extet of the fluctuatios deped o the time respose of the cotrollers (Zhag, 1989). The ustable coditios may oly occur whe the icomig air temperature is much lower tha the airspace temperature, or if the heatig/vetilatio equipmet is improperly sized. SIMULATION RESULTS Because the previous aalyses showed that the feedback i the relative humidity cotrol loop for the THC2 system will be positive at low outside temperature ad the system may become ustable, the stability of the THC2 system was tested usig a simulatio model of the livestock buildig airspace (Zhag, et al, 1992). A farrowig room cotaiig eight sows ad litters was assumed. The vetilatio rates were 118 cfm (200 m3/h) ad 152 cfm (258 m3/h) for the first ad secod stage fas, respectively. The capacity of the heater was 9.4 BTU/h (2.75 kw). The time costats of the temperature ad humidity cotrollers (1) were cosidered to be 75 s ad 430 s i two simulatio tests. The buildig ad the heatig/vetilatio equipmet used i the simulatios were described by Zhag (1989). Durig the simulatio, two sets of 24 h weather data of Caadia Prairie area, which rage from -40 F (-40 C) to 50 F (10 C), were used. As the simulatio was advaced, airspace temperature (Ta), airspace relative humidity (Rha) ad volumetric vetilatio rate (q) were calculated i each 2 s itervals. The the mea, maximum ad miimum values of Ta ad RHa, ad q were sorted ad stored ito each outside temperature. The simulatio showed Ta was out of cotrol whe outside temperature was below -14 F (-25 C), uder which the relative humidity reached 100% ad codesatio occurred. The relative humidity varied over a large rage durig the etire simulatio period ad this variatio idicated poor cotrol. The other pheomeo observed was that the heater capacity was isufficiet whe the outside temperature was below -4 F (-20 C). The overlappig operatio of the heater ad the secod stage fa for the THC2 strategy was severe ad apparetly this overlap made the temperature ad relative humidity fluctuate widely ad resulted i more supplemetal eergy cosumptio tha ay other cotrol systems tested uder the same weather coditios (Zhag, 1989). The situatio was worse whe the cotroller had a large time costat. The slow respose of the cotroller (430 s vs. 75 s) legtheed the overshoot period ad icreased the overshoot magitude. 40

4 From the above aalysis, the THC2 system was ustable regardless of the cotroller time costat. The failure occurred whe both the heater ad the fa(2) were fully operated whe the differece betwee the airspace temperature ad the outside temperature was large (greater tha 67 F [37 C] i this case) because of the isufficiecy of the heater. The THC2 system could be stabilized by usig a lower temperature limit to deactivate the secod stage fa or by icreasig the capacity of the heatig equipmet. However, over-sizig the heatig/vetilatig equipmet is udesirable i terms of iitial costs, thermal respose ad supplemetal eergy cosumptio (Zhag, 1989). I the test of Levesque et al. (1983), a THC2 system was used but o stability problem was reported because the outside temperature was ever as low i the Otario regio, ad because the airspace temperature of the calf bar was low. CONCLUSIONS 1.Temperature cotrol (TC) system is absolutely stable; 2.Temperature humidity cotrol (THC) is coditioally stable. Specifically, a upper temperature limit to deactivate the heater for THC1 (humidistat cotrolled heater ad thermostat cotrolled secod stage fa) should be devised, otherwise istability may occur whe the weather is both warm ad wet; ad a lower temperature limit to deactivate the secod stage fa for THC2 (thermostat cotrolled heater ad humidistat cotrolled secod stage fa) should be devised, otherwise istability may occur whe the weather is cold ad the heatig system is udersized. 3.THC systems likely gave a wider temperature fluctuatio tha temperature based cotrol systems. Figure 1. Geeral cotrol system for livestock buildig airspace (Modified from Cole, 1980) Referece Iputs T ref RH ref C ref Exteral Eergy Cotroller Cotrol Forces Vetilatio, Supplemetal Heat Disturbace Iputs: Weather Coditios; Aimal Heat ad Moisture Productio; Maagemet. Airspace (Plat) Ta RH a Ca Parameters: Buildig Heat Trasfer, Orietatio, etc. Moitorig Parameters 41

5 Figure 2.Thermal properties of air i a vetilated airspace: (a) for THC1 i warm ad wet weather; (b) for THC2 whe cold air is mixed with warm air. Ethalpy icrease due to heat productio withi the airspace 100% B A 75% C Moisture cotet Dry bulb temperature T i Ta T a (a) 100% 75% Codesatio regio C A Moisture cotet B Ti Dry bulb temperature (b) Ta 42