Design and Operation of a Prototype Mechanical Ventilation System for Livestock Transport Vehicles

J. agric. Engng Res. (2001) 79 (4), 429}439 doi:10.1006/jaer.2001.0713, available online at http://www.idealibrary.com on AP*Animal Production Technol...
Author: Leo White
14 downloads 0 Views 314KB Size
J. agric. Engng Res. (2001) 79 (4), 429}439 doi:10.1006/jaer.2001.0713, available online at http://www.idealibrary.com on AP*Animal Production Technology

Design and Operation of a Prototype Mechanical Ventilation System for Livestock Transport Vehicles P. J. Kettlewell; R. P. Hoxey; C. J. Hampson; N. R. Green; B. M. Veale; M. A. Mitchell Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK; e-mail of corresponding author: [email protected] Forge Yard, Checkendon, Reading RG8 0SP, UK Roslin Institute, Roslin, Midlothian EH25 9PS, Scotland; e-mail:[email protected] (Received 15 April 2000; accepted in revised form 21 February 2001; published online 8 May 2001)

A prototype, mechanically ventilated, livestock transport vehicle (for pigs, sheep or cattle) is described. The design complies with current legislation and meets the &higher standard' ventilation requirement for vehicles which are to be used to transport animals for over 8 h. Extraction fans are located at regions of low external pressure on the moving vehicle to optimize performance in transit and provide a controlled variable throughput of air. The system provides air movement over all the animals and is independent of vehicle movement. The design of this prototype system has enabled detailed measurements of heat and moisture production of the animals. Preliminary assessment of the system has been e!ected during two commercial journeys moving pigs from farms to an abattoir. The variability in heat loss from the animals ranged between 1)4 and 1)9 W kg\ liveweight but in both cases the split between sensible (45%) and latent (65%) heat loss was comparable. These initial data, when augmented with further studies over a wider range of ambient conditions, can be used as the basis for guidelines for the development of improved forced ventilation systems. Such systems will be an essential component in vehicles which are being designed to improve animal welfare in transit.  2001 Silsoe Research Institute

1. Introduction The welfare of animals during transport is a matter of great concern to producers, hauliers, welfare organisations, legislators and the general public. There are published data to suggest that the transport of farm animals is a stressful procedure (Bradshaw et al., 1996; Hails, 1978; Hall & Bradshaw, 1998; Warriss, 1998) which may compromise welfare, reduce meat quality and may, in extreme cases, result in an increase in mortality. It has been documented (McGlone et al., 1993; Jarvis et al., 1996; Hall & Bradshaw, 1998; Knowles, 1998) that although many stressors a!ect animals in transit, it is the thermal micro-environment within the transport container which poses the greatest threat to the animals' welfare and well being. Other stressors may compromise welfare but the thermal e!ects can, under extremes, result in mortality in transit. During transportation, the large numbers of animals carried lose large amounts of heat and water inside the 0021-8634/01/080429#11 $35.00/0

vehicle. In warm conditions, they lose even more water by panting or sweating. The net e!ect is the creation of a hot, humid transport micro-environment in close proximity to the animals (Kettlewell et al., 2000). The ideal thermal micro-environment should allow optimal sensible and latent heat exchange and thus require minimal adjustment of heat loss. It would also require minimal adjustment of heat production and therefore pose minimal threat to homeothermy. There are published equations (Commission Internationale du Genie Rural, 1984) which allow the theoretical heat production of animals to be estimated. Using these equations, typical theoretical values of total heat production can be calculated. For example, cattle of 500 kg produce 560 W per animal; pigs of 100 kg, 160 W per animal; sheep of 30 kg, 78 W per animal. On a typical livestock transporter, with a deck length of 13)6 m and stocked at recommended stocking densities, the total amount of heat produced per deck is: cattle, 13 400 W; pigs, 11 500 W; sheep, 8000 W.

429

 2001 Silsoe Research Institute

430

P. J. K ET TL E W EL L E¹ A¸ .

The only published data based on "eld measurements rather than predictive models for animals under transport conditions, is that for broiler chickens (Kettlewell et al., 2000). In recent years there has been a major e!ort to formalize improvements in the transport of livestock. Directives and Council Regulations (EEC Council Directive, 1991) have been developed within the European Union framework and these were implemented within the UK in the Welfare of Animals (Transport) Order 1997 (Stationery O$ce, 1997), which has been revised more recently under the Welfare of Animals (Transport) (Amendment) Order 1999. All vehicles used to transport animals must comply with the &basic standards' speci"ed within the Order but animals must not be transported for more than 8 h in these vehicles. After this 8 h period, the animals must have 24 h rest before the journey can be continued. Vehicles which are to be used to transport animals for periods in excess of 8 h must comply with the additional requirements for &higher standard' vehicles set out in the Order. One of these requirements is the need to ensure adequate ventilation, which may be adjusted depending on the prevailing ambient temperature. This requirement stems from the need to maintain a stable, acceptable thermal micro-environment around the animals throughout the whole transport period. An Animal Welfare Sub-Committee of the Scienti"c Committee on Animal Health and Welfare of the European Commission DG XXIV has developed proposals for the standards for active ventilation on livestock vehicles moving animals for more than 8 h. The SubCommittee has yet to make "nal recommendations and although various suggestions for minimum ventilation rates have been recommended (equivalent to 0)1 m h\ kg\ liveweight), the interim report (EEC, 1999) has not given consideration to the implementation of these air#ow rates, nor its dependence upon the prevailing climatic conditions. It does, however specify temperature limits for the di!erent species and how these should be modi"ed for conditions of high humidities. Air movement through vehicles may be used to remove heat and moisture, and provide direct convective cooling. Appropriate ventilation will have two e!ects, "rstly it will provide a better temperature gradient between the animal and its immediate surroundings for heat exchange, and secondly by removing moisture from the immediate environment around them, it will create an improved water vapour density gradient favouring enhanced evaporative loss, thus allowing the animals to better tolerate higher dry bulb temperatures. Research into livestock vehicles has generally focused on naturally ventilated vehicles to study the e!ects of transport on the animals per se. One study has sought to

determine which environmental parameters are necessary to ensure the comfort of animals in transit (Randall, 1993). The conclusions included quantitative criteria for a number of environmental parameters (temperature, relative humidity, carbon dioxide concentration, space requirements) to aid shippers when planning journeys. Thermally induced ventilation has also been considered (Randall & Patel, 1994) and a mathematical model produced to predict the thermal gradients which might be achieved on a stationary vehicle. Passive (or natural) ventilation however is very variable and dependent primarily on vehicle movement and wind speed and direction. There is little control available over such ventilation regimes other than opening and closing ventilation apertures which requires the vehicle to stop for the driver to make adjustments which he perceives to be appropriate. There is scope for inadequate ventilation in the summer or excessive ventilation in the winter. Some research has addressed the use of mechanical ventilation on livestock vehicles (Van Putten & Lambooy, 1982; Lambooy, 1988) but this has been investigating the e!ects on animals of a given ventilation system rather than de"ning the requirements for such a system. Mechanical ventilation was expected to limit the extreme values of temperature and humidity but this was not seen. The description of the ventilation system suggests that it may not have been optimally designed to ensure a de"ned air stream over all the animals, with the consequent shortcomings. Studies on the air movement around vehicles have usually concentrated on the determination of methods for the reduction of aerodynamic drag (Hurst et al., 1983; Najlepszy, 1988). Wind tunnel modelling has been employed to try to improve the internal environment particularly with regard to reducing the ingress of aerial pollutants such as dust (Town & Lapworth, 1990). More recently, aerodynamics and ventilation characteristics of poultry transport vehicles with full-scale studies and scale model investigations in wind tunnels have been investigated (Hoxey et al., 1996; Baker et al., 1996; Dalley et al., 1996). It is the extension of these latter studies which has led to the development of the prototype transport vehicle described in this study. This paper presents current information on the mechanisms behind the passive ventilation which occurs on transport vehicles and explains how this is determined by the external pressure "elds around the moving vehicle. The external pressure "eld provides the &driving force' for internal air#ows so the ventilation is greatly reduced on a stationary vehicle. A prototype mechanical ventilation system has been designed for &red meat' transporter vehicles (cattle, sheep and pigs) which ensures an air#ow across all the animals throughout the whole transit

V EN TI LA T I O N F O R L IV ES TO C K VE HI C L ES

period, whether the vehicle is moving or stationary. This system has been developed in the light of current knowledge and allows controlled ventilation to be provided at an appropriate level consistent with the needs of maintaining a stable, acceptable thermal micro-environment. The performance of the system is still being assessed but, by way of example, its performance is described during the transportation of two separate groups of pigs. The data collected during these commercial journeys provide further information for the re"nement of the system and, with further evaluation, will provide a sound basis for design guidelines for mechanical ventilation systems for livestock transport vehicles.

2. Vehicle ventilation There are two ventilation regimes in which the air movement through a vehicle should be considered, passive (or natural) and active (or mechanical) ventilation. Passive ventilation is the situation which exists on most standard transport vehicles. Air exchange within the container is driven by virtue of thermal buoyancy, movement of the vehicle itself and by the prevailing wind. With active ventilation, powered fans are "tted to the vehicle to provide air movement at all times.

2.1. Passive ventilation The external pressure "eld around a moving vehicle has been reported (Hoxey et al., 1996) and modelled (Baker et al., 1996; Dalley et al., 1996) and it is this pressure "eld which determines the internal air#ow patterns. A typical livestock transporter has a solid headboard and standard side grille vents: as the vehicle travels down the road, air passing over the front edge of the container separates from the vehicle creating a region of low pressure (suction). The air#ow re-attaches along the length of the vehicle and by the rear grilles, although there is still a region of suction, the magnitude of that suction is much less than at the front grilles. The net e!ect of this pressure "eld is that air tends to enter at the rear grilles, move forward within the container over the animals and leave through the front grilles. It should also be noted that any holes drilled through the front headboard will allow air to enter the container but this air stream will tend to be drawn out through the front grilles. It will not travel through the length of the vehicle and may therefore reduce ventilation e$ciency. When the vehicle is stationary, the external pressure "eld associated with vehicle movement disappears and internal air#ows are driven primarily by the prevailing wind. As there is little control over the air#ow through

431

the container, on windy days the prevailing wind direction will dominate the air#ow pattern. On occasions when there are strong cross winds, the resultant air#ow will be across the vehicle. Parking vehicles at right angles to the wind direction can be used to allow air#ow amongst the animals during hot weather.

2.2. Active ventilation Using fans to provide air movement within the container can ensure that adequate ventilation is provided for all the animals throughout the whole transit period (including stationary periods). The nature of the internal air#ow will be determined by the location of air inlets and outlets and the di!erential pressure between them. However, in employing active ventilation it must be recognized that extracting air from the container is preferable to trying to blow air into the container. This removes the possibility of air jets that in close proximity to animals can be detrimental to their welfare. The resultant air#ow must pass over all the animals to a!ord the opportunity of heat exchange between the animal and the air stream. An optimal design for an active ventilation fan system will have extraction fans mounted at regions of low pressure to enhance their performance when the vehicle is moving, and have air inlets and outlets at locations which will ensure that the ventilating air passes over all the animals within the container. Such a system design lends itself to automated control. There are some livestock transport containers which already have fans "tted but most, if not all, su!er from some fundamental limitations. (i) Such vehicles have been constructed to meet the perceived legal requirements rather than being designed to a!ord appropriate ventilation for the needs of the livestock. (ii) Typically, banks of fans have been "tted along the length of one side wall onto existing retaining grilles. The fans blow into the vehicle so heat generated by the fans will be transferred to the air moving across the animals. (iii) The throughput of fans is uncontrolled and all fans operate together. Consequently the ventilation may be excessive in all but very hot weather. (iv) There is no provision to control where air enters and leaves the vehicle. If the requirement is to provide positive ventilation by forcing air into the vehicle then this may be better accomplished using ducted air. This will avoid having excessive air movement over the animals but it will also

432

P. J. K ET TL E W EL L E¹ A¸ .

introduce greater pressure losses, hence requiring more energy, than comparable non-ducted systems. The prototype vehicle described here has addressed these constraints and has been designed accordingly. The performance of the ventilation system, with regard to the internal thermal micro-environment has been investigated during two exemplar journeys transporting slaughter weight pigs from rearing farms to a commercial abattoir.

3. Materials and methods Measurements were made on a standard 13)6 m long, tri-axle, air suspended, three deck semi-trailer (North West Trailers Limited, Houghtons Parkhouse Coachwork Limited) operated by Silsoe Research Institute. The vehicle is con"gured as a three-deck transporter but for the purpose of this research, the lower deck has been modi"ed to provide controlled active ventilation. The remaining two decks are passively ventilated. The vehicle contains a self-powered instrumentation system for data collection during transportation. The mechanically ventilated lower deck features four fans, one pair on each side of the vehicle, located at the front side of the container as shown in Fig. 1. These fans extract air from within the container and are located where regions of low external pressure develop on the moving vehicle. To operate on a vehicle, 24 V DC

Fig. 1. Silsoe livestock transporter; extraction fans are mounted at the front sides; air inlets are at the rear sides

Fig. 2. Diagram of plan view of lower deck of vehicle showing location of temperature/humidity sensors (S)

powered axial #ow fans were selected each with a nominal maximum throughput of 0)5 m s\. The two rear grilles, one on each side of the vehicle, were used as air inlets, each with an area of 0)76 m. All the other grilles along the side of the vehicle were kept closed. If the fans are operated at maximum speed then the average inlet air speed will be 1)32 m s\. The static pressure drop across the fans during operation was typically 10}20 Pa. This regime results in air being drawn in at the rear of the vehicle, moving forwards over all the animals and being extracted through the fans at the front sides of the container. An array of nine combined temperature/relative humidity sensors were mounted at the locations shown in Fig. 2. Three sensors on the mid-line of the vehicle in the front, middle and rear pens were secured to the roof structure. At the rear air inlets, two sensors were located one on each side of the vehicle on the metal grilles. These sensors were mounted within aspirated tubular housings, Fig. 3, to protect them from direct solar radiation. The remaining four sensors were mounted in close proximity to the front outlet fans, one by each fan. All of the sensors were guarded to protect them from interference by the animals. The location of these sensors enabled the temperature and humidity distribution to be assessed throughout the container. In addition to the temperature and humidity measurements, the levels of carbon dioxide were monitored at the inlet and outlet to provide information on the generation of carbon dioxide by the group of animals. Air#ow rate was determined by measuring the pressure drop across the extraction fans and relating this to calibration curves from laboratory testing of the fans under equivalent conditions. The pressure di!erence across the fans was sensed by pressure tapping points, each consisting of a #at metal plate with a 9 mm central tapping point, located at the outer face of each fan compared with the internal pressure being measured on the front headboard of the lower deck inside the container. Data collection on the middle deck was con"ned to temperature and humidity with an array of sensors at

433

V EN TI LA T I O N F O R L IV ES TO C K VE HI C L ES

Fig. 3. Aspirated housing for ambient temperature/humidity sensor, positioned inside rear air inlet

nine locations. Sensors were located, attached at roof level, on the nearside, mid-line and o!side of the vehicle in each of the front, middle and rear pens. The additional data from these sensors is being used to develop models of heat transfer within the vehicle and will be reported separately. The automated data recording system was started prior to departure to the farm site and kept running continuously until after the pigs had been unloaded at the abattoir. 3.1. Experimental arrangements Data were collected during two commercial journeys moving pigs from farm sites to an abattoir. The liveweights of the pigs, veri"ed by weighing of the laden vehicle, ranged between 90 and 100 kg and pen numbers were adjusted accordingly to maintain a stocking density of about 235 kg m\ as recommended in the Welfare of Animals (Transport) Order 1997 (The Stationery O$ce,

1997). There were always 75 pigs each on the lower and middle decks. The additional instrumentation and electric generator increased the unladen weight of the vehicle precluding transporting pigs on the top deck. The two journeys were made from farms at comparable distances from the abattoir and involved signi"cant commonality in the routes. During the "rst journey a mid-way break was introduced where the fully laden vehicle was held stationary for a period of 2 h. The second journey was an uninterrupted run from farm to abattoir. By making these two similar journeys, it was possible to compare the performance of the ventilation system on both a moving and a stationary fully laden vehicle. The ventilation system was switched on prior to loading of the pigs and operated for the whole of the transit period including the stationary period of the interrupted journey. The system was only switched o! after all the pigs had been unloaded at the abattoir. One of the main concerns in formulating legislation for ventilation systems on board vehicles is to address the worst case scenario of a stationary fully laden vehicle. The initial evaluation of this ventilation system thus focussed not only on assessing the performance of the system per se, but also on quantifying the heat loads produced by the animals on both a moving and a stationary vehicle. A controlled ventilation system must be able to operate e$ciently in both these situations. Transport time details of the two exemplar journeys are presented in Table 1.

3.2. Data recording system The instrumentation system was designed to control the fans and record environmental conditions within the transport vehicle. Two modes of fan control were addressed. For experimental purposes, the fan throughput was "xed at a pre-determined level and the resultant changes in temperature and humidity of the ventilating air recorded. In the commercial situation however, it was recognized that the fan throughput would be adjusted for example to maintain a pre-determined temperature rise in the ventilating air and this option included in the control design.

Table 1 Details of journeys

Transport period, h Stationary period, h Journey distance, km

Interrupted journey

Continuous journey

10:00}17:00 13:00}15:00 176

09:30}15:30 * 189

434

P. J. K ET TL E W EL L E¹ A¸ .

and processed, this information was sent back to the control computer. 3.5. =ind speed and direction The pressure system also monitored the total and static pressure from a combined Pitot-static tube mounted above the cab. The Pitot-static tube was held into the #ow by a wind vane and the position of the vane was recorded by a continuous rotation potentiometer giving wind direction. Both the Pitot and static pressures were calibrated in situ by measuring wind speed upstream of the vehicle and making corrections for the interference e!ects of the cab. The calibration was conducted on a test track using a rig extending 5 m in front of the vehicle. 3.6. Fans Fig. 4. Schematic diagram of instrumentation system on vehicle

The instrumentation system was housed in a chassis tray mounted in the trailer lockers suspended under the main deck and communicated with the control computer in the vehicle cab. The control computer was a standard personal computer Pentium P90 running Microsoft Windows 95 which communicated via the RS232 ports. A schematic diagram of the instrumentation package is shown in Fig. 4.

3.3. ¹emperature and relative humidity A total of 20 inputs of both temperature and relative humidity were available using combined probes (Vaisala type HMP 31 UT). The manufacturer's quoted operational range for the temperature sensors was !40}#803C with an accuracy of $0)53C and for the humidity sensors the range was 0}100% with an accuracy of $3%. A measurement was made every minute.

A system was designed to house the fans mounted in two banks on each of the forward sides of the vehicle. The 300 mm diameter fans (Ziehl-EBM, 24 V DC) had a nominal maximum throughput of 0)5 m s\ per fan, depending on the pressure drop across the fan. The fans were "tted with pulse width modulation (PWM) feedback and were operated manually via an in-house motor controller. The fan control system was capable of operation in "ve pre-set stages. Fan speed was monitored by an integral tacho output built into each fan. Fan speed was used as a fail-safe measure to ensure that the system was operational and that animal welfare was not compromised. An alarm was programmed to sound if any fan speed dropped below 10% of its intended operational speed. During these studies the primary objective was to determine the heat production of the pigs during transport so the fans were always operated at a "xed stage (equivalent to 60% of their maximal throughput) throughout the whole transit period. 3.7. Fan calibration

3.4. Air pressure Measurements were made of external and internal pressure using individual di!erential pressure sensors (Scienti"c Electro Systems type 163 PC01036), each with a range of $1)25 kN m\, built to an in-house system interfaced with its own 386 personal computer running Microsoft DOS 6)22 via a 16 channel analogue to digital card (Amplicon type PC 74). The control computer was programmed to instigate a pressure recording sequence every 10 min and once data collection was completed

To calibrate the fan throughput prior to the initial experiments, one fan unit, complete with guards, was removed from the vehicle and attached to a fan calibration assembly. The volume #ow rates for the two fans in the housings were measured at "ve stages (speed settings) over a range of static pressure drops across the fans between !50 and #50 Pa. The calibration curves for the "xed speed used in these studies are presented in Fig. 5. The resultant volume #ow rate was calculated from the experimental measurements of the pressure drop across the fans.

V EN TI LA T I O N F O R L IV ES TO C K VE HI C L ES

Fig. 5. Calibration curve for a bank of two fans. Regression equation is y"!0)000001 x3!0)0001 x2!0)0094 x#0)6379 where x, static pressure in Pa; y, volume yow rate in m3 s\1

3.8. Data back-up recording During experimental journeys, with the exception of the pressure data, all the information was recorded not only on the control computer but also onto a microprocessor-based data logger (Data Electronics type 505 microprocessor and expansion unit) for real time monitoring and subsequent analysis.

4. Results 4.1.