General Ventilation Principles to Maximise Pig Productivity06 January 2017
Pig production around the globe has become very sophisticated with well-controlled environments that, if controlled properly, maximize animal well-being and production efficiency. The major thermal challenge in pig housing is economical mitigation of heat stress. Unlike cold weather periods where relatively inexpensive heating devices can be used to optimize thermal conditions, mechanical compression cooling methods are generally cost prohibitive, write Steven J. Hoff, PhD, PE and Brett C. Ramirez, GRA Department of Agricultural and Biosystems Engineering, Iowa State University.
Therefore, methods such as evaporative cooling, high pressure fogging, animal sprinkling, and elevated airspeed control are techniques used to combat the production losses associated with heat stress. This article will focus attention on animal housing principles in warm and hot environments.
NATURAL VENTILATION SYSTEMS
Natural ventilation systems are very common world-wide in both cold and hot climate regions. Economic considerations, electrical supply concerns, and ample labor availability are major drivers for naturally ventilated barns. Several key factors can be used to project the effectiveness of natural ventilation barn performance.
Local Meteorological Factors
Relying on natural ventilation (NV) systems for hot climate regions of the world demands effective wind potential. Each ventilation engineer has a comfort level with natural ventilation and the associated wind potential required. Wind potential for this discussion is defined as the percent time calm conditions persist in the heat of the summer. Local historical weather data can be used to make this assessment. The author of this article has a cut-off comfort level about 5% calm periods in the heat of the summer, with calm defined as wind speeds less than 1 m s-1. Take the NV barn shown in figure 1, representing an 18 m wide, 60 m long, and 3 m high average interior height barn representing a very typical NV barn found in the USA. For winds against the side wall curtain opening (can be opened to 1.2 m), the fresh air exchange rate is predicted to be as shown in figure 2.
As shown in figure 2, direct perpendicular winds (labeled 0o, or similarly 360o, and 180o) will ventilate this barn at 44, 88, 132, and 176 fresh air changes per hour (ACH) for perpendicular winds of 1, 2, 3, and 4 m s-1. These results assume that both side wall curtains are open at 1.2 m. In most regions of the world and for most pig barn situations, an exchange rate of 100 ACH is sufficient to provide reasonable temperature control in the heat of the summer.
To take advantage of wind ventilation, orientation relative to predominant hot weather winds is paramount implying that barn orientation is critical. Exposing the largest barn opening perpendicular to predominant hot weather winds is crucial. For example, the wind rose shown in figure 3 represents the historical average August wind patterns for Des Moines, Iowa USA. A naturally ventilated barn planned for this area must be oriented to capture these predominant hot weather winds, implying that the long axis of the barn needs to be along the east-west axis, thereby exposing the side wall curtains to these hot weather winds. The wind rose pattern shown in figure 3 is very useful for making this assessment. The rings provide an assessment of percent time winds originate from the compass locations shown and the colorations indicate percent time at various wind speeds. For example, for the wind rose map shown, 14.1% of the time winds originate from due south (180o) with 8.5% of the time at speeds greater than 10 miles per hour (4.47 m s-1). For this particular month, the average wind speed is 8.5 miles per hour (3.8 m s-1) and the percent calm (defined as wind speeds less than 1 m s-1) is 5.6%.
Clearly, impeding wind potential to a naturally ventilated facility is counterproductive. Many instances exist where separation from a naturally ventilated barn and nearby obstructions is insufficient to allow adequate barn wind exposure. The recommended separation distance (Ls, m) between a naturally ventilated barn and an obstruction with a total height (OH, m) and length (OL, m) is given in equation 1 (MWPS, 1989) and shown in figure 4. The minimum separation from any obstruction potentially impeding wind exposure is 15m.
For example, an 8.0 m tall (ground to roof peak) obstruction that is 46.0 m long, should not be any closer than 39.0 m to a naturally ventilated barn as depicted in figure 4.
Interior Barn Obstructions
Interior obstructions to airflow can have deleterious effects during hot weather periods. Metal gating provides the least obstruction to airflow while concrete-formed gating can obstruct the natural ventilation process, especially the gating along the long-axis of the barn as might be the case for walkways. The barn’s interior design needs to match the natural airflow paths of hot weather winds.
Naturally ventilated systems are economically attractive and, with ample labor support, can be ventilated without electricity. Complete manual control of vents however, is an admission that good climate control is not of concern. Wind potential changes and directions shift chaotically rendering manual control of vents, at optimal levels, impossible. Many modern control systems are designed with natural ventilation control in mind allowing for rapid responses to external weather disturbances, with climate control capabilities rivaling fan ventilated pig housing. Minimal electrical input is required for the controller itself and the curtain and ridge vent machines required for actuation.
MECHANICAL VENTILATION SYSTEMS
Mechanically ventilated barns add complexity to climate control not required in naturally ventilated pig barns. Fans, combined with some level of planned fresh-air distribution system, all controlled with a well-tuned controller, can in many cases control a pig barn better than many homes, provided proper mechanical systems exist to combat heat and cold stress conditions. Many factors contribute to the success of a mechanically ventilated system. The listing below is a brief discussion of some of the more important factors.
Fresh-air Distribution System
Sizing barn ventilation systems for proper fresh-air exchange rates to control temperature, humidity, gases, and particulates is an obvious first need in any pig housing system. Besides this obvious need, the most important element in the success of a pig house ventilation system is the distribution of fresh-air. Most pig house ventilation systems are negative pressure-based where a suction is established across planned openings (i.e., fresh-air intakes) either placed at the ceiling, side, or end walls. Proper spacing of planned openings for ensuring adequate distribution is a necessary first step, followed by the selection of openings that have the characteristics necessary for accommodating opening spacing needs.
This is counter-intuitive to most ventilation systems installed where an opening system is selected first, followed by the quantity required to deliver the required fresh-air, followed finally by the placement of these openings. The following discussion will clarify proper design sequences.
Most all planned openings used for animal house ventilation systems, whether ceiling, side or end wall installed, use some form of a rectangular opening. This opening, operating at typical static pressures used for animal housing ventilation design, produce the air distribution pattern as shown in figure 5 (as viewed from above) for one-way and two-way openings. From the edge of the rectangular opening, an approximate 10 degree air-jet spread is realized with air-jet throws of approximately 4.5 m. This information can be used to start the process of positioning planned openings for proper fresh-air distribution. For example, an 18 m wide barn requires four “air-jet throws” to accommodate the 18 m barn width. This requirement demands two rows of bi-flow openings to properly encompass all regions laterally in the barn as shown in figure 6. Finally, along the long-axis of the barn, the air-jet spread information combined with the physical length of the planned opening can be used to select spacing needs. A 60 m long barn would require 23 openings along the long axis of the barn and with a two-row arrangement requires 46 total openings to satisfy the fresh-air distribution needs of the barn. This planned opening requirement has been determined independent of the quantity of air required, which is yet to be determined; counter-intuitive for sure but absolutely necessary for proper fresh-air distribution. Finally, if the barn interior ceiling height is 2.4 m, and it is determined that 60 ACH is required to be delivered by these planned openings, equating to 155,520 m3 hr-1, each planned opening selected requires a maximum capacity per two-way inlet of 3,381 m3 hr-1 inlet-1. Selection, balancing, and control of the planned opening system can now take place.
Fan systems available today are quite competitive resulting in a wide selection of reliable choices in the variety of airflow capacities allowing for tight staged control of barn airflow as a function of seasonal and pig density demands. The rated capacity of a fan is the capacity tested as new and in a controlled laboratory setting. Caution on using the rated airflow capacity for design is warranted to allow for future inefficiencies. The author uses a ventilation system design factor of 0.85 on all published and tested fan capacities to allow for future changes from tested conditions. Research on in situ fan testing has shown that this might be generous especially for belt-driven fans where the slightest release in belt tension can reduce the rated fan capacity to 60% of tested.
Like fan systems today, control systems have become quite competitive across manufacturers allowing for a wide selection of reliable systems capable of multiple zone control each with multiple fan staging capabilities. As with any control system, the feedback sensors used for control need to be reliable and enough sensors be placed as close to the animal occupied zone (AOZ) as possible. The climate felt to humans walking through barns can be very different than that experienced in the AOZ. Controller selection should be from the same manufacturer as that supplying the fans and planned opening system, and should be based on not only reliability and level of sophistication desired, but most importantly in many cases with “service after the sale” in mind.
The hardware that accommodates any ventilation system is crucial to the overall success of a pig house environmental control system. Zone and space heating placement and control is crucial to satisfy macro-climate barn needs and especially the micro-climate AOZ needs. For best performance, environmental control systems should be selected from manufacturers where complete systems are provided, allowing for the best match between planned openings, fans, controls, and heating/cooling equipment.
MITIGATING HEAT STRESS
Heat stress mitigation is probably the single most critical factor defining the success or failure of a ventilation system. The production losses associated with heat stress are pronounced and must be addressed with strategies that focus on the need of the pig and the “Capacity of the Environment to Displace Heat (CDH)”. Our research group is focused on the development of measurement systems and control strategies to maximize the ability of the thermal environment to displace heat produced by the pig, thereby minimizing as much as possible the deleterious effects of heat stress. Using this viewpoint, heat stress mitigation strategies can be selected based on expected performance relative to the heat produced by the pig if allowed to consume feed as if in the thermal neutral zone (TNZ).
In order to fully appreciate CDH, the heat displacement capacity of the environment surrounding the pig must be measured accurately and related properly with the pig. Our group has developed a Thermal Environment Sensor Array (TESA; Ramirez et al., 2016) to fill this need. TESA (figure 7) can be used to inexpensively measure dry-bulb temperature, relative humidity, airspeed, and black-globe temperature. These variables in turn can be used to assess CDH and ultimately to provide feedback for heat stress mitigation. The CDH then in turn must be adjusted to accommodate the energy intake of the pig as if within the thermal neutral zone (TNZ). As an example, a lactating sow will produce about 430 W of total heat if allowed to ingest feed at levels common in TNZ conditions. The surrounding thermal environment needs to have a CDH that at least equals this level in order to stave off the effects of heat stress. Minimizing any negative consequences of the thermal environment on housed animals is paramount to realizing an animal’s genetic potential. As a ventilation engineer, all those factors that thermally affect the housed pig must be analysed and considered when evaluating the CDH. Animal well-being and production efficiency are tightly related to the thermal energy exchange occurring between the animal and its surroundings. Animal housing systems have evolved to take advantage of this linkage.
Realizing the full genetic potential of our housed animals requires tighter control over all variables that affect animal performance and in most parts of the world, is related to heat stress mitigation. With proper climate measurements, as provided by TESA, the influences of air temperature, surrounding walls/ceiling/floor temperatures, airspeed, and water vapour content can be accurately measured and used to assess CDH. Some example uses of TESA related to modern barn design and control are discussed in the following sections.
Convective Heat Transfer Influence on Modern Barn Design
Animal housing designs today have been greatly influenced by the action of convection heat transfer. Consider the situation shown in figure 8 (see Livestock Housing, 2013). This plot shows the convective heat transfer coefficient (h) for pigs modelled as horizontal cylinders with the airspeed perpendicular to the long axis of the cylinder (“cylinder in cross-flow”). Two pig sizes are given, one for a 15 cm diameter pig (?) and the other for a 30 cm diameter pig (¦). At any given airspeed, the convective heat transfer coefficient increases and is higher for the smaller pig at any airspeed (labelled U∞). The benefit of increased airspeed is not linear however, and above about 2 m s-1, the influence of increasing U∞ on h diminishes (dh/dU∞, o) and represents reasonable target airspeeds for tunnel designed ventilation systems. Most all USA pig fattening and gestation facilities are designed with target airspeeds near 2 m s-1 and in some way in their past were dictated by convective heat transfer limitations of increasing U∞ on h.
Tunnel ventilation systems are designed to provide a sustained airspeed through the barn to take advantage of convective cooling and to allow any water applied to the pig via sprinkling systems to evaporate more rapidly. The tunnel design airspeed, if used exclusively for barn ventilation rate design, can be detrimental as the tunnel length increases. Simply put, since the purpose of tunnel ventilated barns is to produce a wind effect on animals, and since the convective heat transfer benefit reduces substantially above about 2 m s-1, it makes sense to use this knowledge to determine the maximum tunnel barn length that can be effectively ventilated in harmony with convective heat transfer. As 2 m s-1 air flows through the barn, heat, moisture, particulates and gases increase through the barn down the length of the barn due to prior occupant inputs to the ventilation air. Most maximum ventilation rates used for design are based in some way on minimizing the maximum temperature increase that a barn experiences. One method, called the “2oC rule” (Albright, 1990) is intended to limit the maximum temperature increase of the barn to no more than 2oC above the entering air temperature. Figure 9 plots the predicted temperature profile along the length of a tunnel ventilated barn with 100 kg pigs at a density of 0.70m2 pig-1 (see Livestock Housing, 2013).
Figure 9 gives predicted barn temperature rise for tunnel airspeeds of U∞=1, 2 and 3 (m s-1). If one follows the 2oC rule, a tunnel barn under these conditions with design airspeeds of 1, 2 or 3 m s-1 should be no longer than about L=50, 75 and 95 m, respectively. At an effective airspeed of U∞=2 m s-1, a barn length greater than L=75 m for this pig fattening example, will require a higher airspeed to maintain temperature control at the 2oC rule level, but the added airspeed designed will not add much further benefit in the way of convective heat transfer (by itself, in the absence of water evaporation).
Heat Stress and Heat Transfer Influences on Modern Barn Design
The animal industry uses two basic forms of cooling; indirect and direct. Indirect cooling is that method of cooling which first cools the air, allowing the animal to use this cooler air to in turn cool itself via sensible (non-evaporative) means. Two common methods of indirect cooling are used in animal housing, namely evaporative pad cooling and highpressure fogging. In both cases, liquid not originating from an animal’s surface, is being used to cool and humidify the air. The “penalty” with this mode of cooling is that the water vapour content surrounding the animal must increase thereby negatively affecting the ability of the animal to release moisture either via respiration or skin evaporation. In direct cooling, water that resides on an animal’s surface, via natural sweating or sprinkling, is allowed to evaporate with animal derived energy supplying the latent heat of vaporization (hfg). Indirect cooling is probably the most common method for cooling housed animals, although not necessarily the most efficient. Both methods will be discussed for their potential in efficiently cooling the animal.
Direct cooling, as described here, involves the evaporation of water directly from an animal’s surface. In the process of evaporating this moisture, latent energy derived from the animal is released by the animal. Several models exist to predict this process, one of which is
As airspeed over the wetted surface increases and the wetted surface area increases, combined with a lowering of the surrounding humidity ratio, results in the best potential for direct cooling.
Indirect vs Direct Cooling
An analysis of the potential cooling benefit for the animal will help decide (along with ultimately field experience) which method, direct or indirect, is best suited for the housed animal. Table 1 provides three specific cases where the comparison between indirect and direct cooling is made. In the hypothetical cases evaluated in Table 1, it was assumed that tunnel ventilation was used, with a design airspeed near the animal of 2 m s-1. Further, it was assumed that the barn was well insulated and that all surfaces were at the air temperature of the room. Finally, the room was assumed to house 100 kg pigs, modelled as horizontal cylinders with a total body surface area Ab=1.8 m2 (Brody et al., 1928). Latent heat production via respiration was modelled as presented in ASABE (2006). The comparisons given in Table 1 are for three representative climates. For these three climates, the convective (Qcv), radiative (Qrad), respiratory (Qresp), and skin evaporation (Qskin) are being predicted where it has been assumed that 30% of the pig’s surface area is coated with water (see Livestock Housing, 2013 for complete model development). For predicting indirect cooling, a 70% efficient evaporative cooling system was assumed. In all three climates presented, the predicted cooling benefit of direct cooling far outweighs the more traditional indirect cooling via evaporative pad cooling. Even when indirect is used along with direct cooling methods, the added advantage over direct cooling by itself is minimal. In terms of the Capacity to Displace Heat (CDH), direct cooling yields by far the largest CDH and the added benefit of combined indirect with direct cooling is minimal and unwarranted.
In housing systems today where sprinkling and direct cooling are used, the control systems have not followed the physics of direct cooling. For example, sprinkler control systems for pigs today (and all others this author is aware of), have as user-input options the ability to increase water application time as barn temperature increases. The physics of direct cooling does not support this action. Why increase water application time when the animal surface area required taking advantage of direct cooling does not change with temperature? A clever direct cooling system would apply water for a fixed time that ensures sufficient area wetting (Awetted) and then monitors the environment (T, RH, U∞) to reapply this same amount of water after an appropriate water evaporation time, thereby maximizing the direct cooling benefit and potentially saving significant water in the meantime. Physics of animal heat exchange must be allowed to dictate animal housing design and practices. TESA (Ramirez et al., 2016) is currently being used to provide the climate inputs required to make these scenario assessments, in real time at the field-scale level, and in turn to develop control strategies to most effectively alleviate the negative production consequences associated with heat stress.
SUMMARY AND CONCLUSIONS
This paper has attempted to review some of the ventilation and environmental control strategies for maximizing production efficiency, focussing on pig production and hot weather scenarios. Animal well-being and production efficiency are intimately related to the thermal energy exchange occurring between the animal and its surroundings. Animal housing systems have evolved to take advantage of this linkage. Production efficiency demands tighter control over all variables that affect animal performance. Better climate assessment through tools such as TESA (Ramirez et al., 2016) can be used to provide the necessary climate feedback that allows for improved heat stress mitigation strategies. Concerns related to global warming have forced housing systems to rethink heat stress control and many housing modifications are related with this topic.