SELECTING FANS FOR LIVESTOCK BUILDINGS

Transcription

1 SELECTING FANS FOR LIVESTOCK BUILDINGS NEW 87:06 J.E. Turnbull, H.E. Huffman and N.A. Bird In Canada the range of weather conditions is so wide, from summer heat to winter cold, that the ventilation rate in livestock and poultry buildings can vary by a factor of at least 16 times. This plan explains alternative ways of obtaining this wide range of rates in fan-ventilated buildings. The principle of stepped ventilation using several different-sized fans is explained and compared with control schemes using two-speed, multispeed or variable-speed fans. This plan also dis cusses other important considerations in choosing fans and controls for ventilating modern livestock and poultry buildings. See Plan M-9700 for an explanation of ventilation principles and for recommended ventilation rates for livestock and poultry. FAN PERFORMANCE Fans provide the power to exchange air in mechanically ventilated livestock buildings. Consider five main factors when selecting ventilation fans: fan type (motor, drive, fan blade, enclosure) air-moving capacity (in relation to static pressure) energy efficiency of the combined components durability and maintenance noise level FAN TYPES Fans may be centrifugal (sometimes called 'squirrel cage') or propellor type (also called axial flow). Centrifugal fans make less noise and will work against higher pressures than propellor fans. They are used more for grain drying and for domestic hot-air heating systems. Propellorfans are generally preferred for livestock ventilation. They cost less and are better able to handle dirty air than centrifugal types. attached to a hub. The hub is either direct drive (keyed directly to the motor shaft) or belt drive. Direct drive is cheaper to manufacture and maintain, but belt drive offers more choice of fan speed (by changing the pulleys). In the past, large slow-turning belt-drive fans were popular. They tend to be quieter and more efficient for the big air flows needed for summer ventilation. Now manufacturers are tending to use more multispeed and variable-speed motors, with direct drive. Propellor fan blades are frequently stamped from sheet steel or aluminium and riveted to the hub at an angle that determines the 'pitch' of the fan. (Pitch is the theoretical 'length' of airflow that would pass through the fan in one revolution, at zero static pressure). More expensive blades may be cast aluminium; this offers the advantage of blades shaped to a better airfoil section, more like an airplane propellor. Molded plastic blades (a more recent development, particularly suited to smaller-diameter fans) also offer improved airfoil shape, better corrosion resistance and better selfcleaning. Fan motors should be totally enclosed (to keep out dust) and have sealed ball bearings for long, continuous service. Built-in thermal overload protection is required for CSA approval on automatic service. This assures that an overheated motor will switch off before it burns out, in case of being s talled by seized bearings, iced-up fan blades or shutters frozen shut. High-efficiency motors are worth the extra cost, considering that ventilating fans can run for thousands of hours each year. The popular permanent-split capacitor motor is quite energy efficient and has the advantage that it can be wired for single-speed, two-speed, multiple-speed orvariable-speed. Motors are available for 120- or 240-volt service. Use 240-volt connections wherever possible, to reduce unbalanced 120-volt loads and the stray 'tingle voltages' that these im balances can cause. Propellor fans generally have three or more curved blades The Canada Plan Service prepares detailed plans showing how to construct modern farm buildings, livestock housing systems, storages and equipment for Canadian Agriculture. To obtain another copy of this leaflet, contact your local provincial agricultural engineer or extension advisor.

2 Fan enclosures include the safety guard, motor support frame, orifice plate and mounting flange, and usually backdraft shutters. On small fans, the shutters may be an integral part of the enclosure (either inside or outside the fan and motor); on large fans the shutters are usually separate, mounted to the outside of the wall opening. Advantages claimed for inside-mounted shutters are less chance of frozen shutters and better accessibility for cleaning. Recent improvements to corrosion resistance of fan enclosures include superior enamel coatings on 'satincoat' galvanized steel, more components made from stainless steel, and enclosures made entirely out of fiberglass or other durable plastics. FAN TESTS Choose fans on the basis of air-moving capacity at comparable static pressures, not on blade diameter. Fan ratings by independent testing laboratories tend to be more conservative and realistic than ratings published by manufacturers. In the U.S., fans are rated by the Air Movement and Control Association, Inc. (AMCA), Chicago, Ill. In Canada, many are rated by the Prairie Agricultural Machinery Institute (PAMI), at Lethbridge, Alberta T1K IL6. Selecting fans on the basis of performance reported by an independent testing agency such as PAMI or AMCA is highly recommended. Test reports by PAMI include airflows measured at a range of speeds and static pressures, usually without the backdraft shutters in place. PAMI does report the effect of shutters on fan performance, but only in the single-speed 'direct' mode, or where the shutters are permanently housed in the fan enclosure. Tables 1 and 2 are typical fan performance tables taken from PAMI reports, for two fans by the same manufacturer. Note that when the test static pressure was increased from 0 to 62 Pa (0 to 1/4 in. water gauge), in single-speed direct mode, the airflows dropped, respectively, to 62% and 78% of flows at zero static. Ventilating fans almost never operate at zero static pressure, so it is more realistic to compare and select fans at a 'standard' static pressure such as 25 Pa (0.1 in. wg). Total efficiency performance is also included in the PAMI test reports. Total efficiency (air power output/ electric power input) is expressed in percent. As with airflows, always compare total efficiencies at a standard static pressure such as 25 Pa, not at zero static. Looking at Table 2, for example, it is interesting to compare the 39% efficiency of this fan at full speed and 25 Pa static pressure, with the same fan at its minimum variable speed and the same pressure (only 3% efficiency). This dramatically shows that it is not economical to use oversized fans throttled down to obtain the low ventilation rate required for cold weather. For low, controlled rates, use small fans running at their intended speeds. PAMI performance tables can also be used to compare fan efficiency in terms of airflow per unit of electric power (typically L/s per watt). These ratings, like total efficiency, can also be used for comparing fans as long as you are careful to compare them at the same static pressure, such as 25 Pa (0.1 in. wg). For example, the small fan reported in Table 1, at 24.9 Pa static pressure, single-speed direct, gives 603 L/s at kw (or 210 watts). Its performance can be calculated as 603/210 = 2.87 L/s per watt. In comparison, the bigger slowerfan reported in Table 2 gives 2400/401 = 5.6 L's per watt; thus, it gives 2.2 times as much ventilation per unit of electric power consumed. Noise level is also important. Noise represents wasted power-that is, noisy fans tend to be less efficient and more annoying than quiet ones. PAMI test reports also include a noise level, measured in decibels, or db(a), at a standard distance of 1.5 m downstream from the fan TABLE 1 DANOR PLEASANTAIRE MODEL SD12-EVX FAN PERFORMANCE AT TYPICAL LEVELS OF OPERATION TABLE 2 DANOR PLEASANTAIRE MODEL SD24-FVX FAN PERFORMANCE AT TYPICAL LEVELS OF OPERATION

3 discharge. While this is not a true measure of the noise you'll hear in a room upstream from the fan, the ratings are comparable between different fans. FAN HOODS Wall-mounted fans are vulnerable to the pressure effects of winds blowing about the building. Where an unprotected fan discharges straight into a 30 km/h headwind, the shutters will not start to open unless the fan develops at least 32 Pa (0.13 in. wg), a pressure that can keep the shutters virtually closed! A properly designed weatherhood can overcome most of these wind effects as well as protect against snow and freezing rain. Most fan hoods supplied by manufacturers do not extend down far enough to give good headwind protection, although they are generally adequate for stopping rain and snow. Figure 1 shows an improved weatherhood design, featured in many single-storey Canada Plan Service drawings. It has an opening at least twice the combined area of all the fan shutters grouped inside (to minimize discharge velocity and back pressure), it discharges downwards (not dead against the wind) and it dis charges into a region of lower wind pressure (below the midheight of the wall). Choose the width W to fit within a multiple of the building rafter spacing, and make it at least twice the width of the largest fan shutter assembly. The inside bottom edge of the hood opening is brought out from the wall by at least 100 mm (4 in.); this keeps the wall cleaner (no dirty drips) and moves the opening out from the zone of highest back-pressure due to stalled wind. The continuous roof slope protects the weatherhood from damage due to ice and snow, and the hood is big enough for you to stand inside when servicing the shutters. Figure 1 also shows the correct placement of the fan blade on the shaft - the blade depth should be 2/3 inside and 1/3 outside the edge of the hole. A removable insulated box seals and protects the summer fans when they aren't operating during the winter. With smaller fans where the motor does not protrude inside the wall surface, this winter cover can be a simple, insulated flat panel or hinged door, sealed with weatherstripping. 1 2 x 6 framing and rafter extensions 2 siding and trim to match wall 3 smooth interior finish (plywood, etc.) correct position of fan blade in the fan orifice 4 removable weathertight box to seal summer ventilation fan during cold weather; make from 25 mm (1 in.) Styrofoam, glued together with Foamstick adhesive Figure 1 Improved weatherhood for wall-mounted exhaust fans

4 Figure 2 shows an exhaust fan that comes complete with its own weatherhood, also incorporating many of the good design principles shown in Figure 1. In this case the entire unit (fan, enclosure, weatherhood and butterfly shutters) can be caulked and screwed to the outside of the wall. A matching insulated door is available to seal the inside of the wall opening whenever the fan is out of use. The following examples explain the first two options. The third option (variable recirculation) is used by some ventilation equipment suppliers such as Aston, Danor, Fristamat, Axis-Air and Acme Fan-Jet. STEPPED FANS WITH THERMOSTAT CONTROLS THE DOUBLING RULE The upper part of Figure 3 shows the traditional way to fit fan stepped capacities to the humidity and temperature ventilation curves. In this example, the step 1 fan capacity is 1.3 Us per pig, that is 5/8 to 2/3 of the minimum rate for humidity control, so that this fan can run continuously. Subsequent steps 2, 3, 4, 5, etc. can then be about double each previous step rate (the 'doubling rule') -that is, 2.6, 5.2, 10.4 and so on. The final step may not be double the previous step in many cases. In addition, a step 1 ventilation rate based on 5/8 to 2/3 of the minimum moisture control ventilation for a room full of livestock can easily over-ventilate that room if it is only partly stocked during cold weather. Therefore, it is desirable to have some way to manually reduce the step 1 rate - more later on this. 1 molded plastic fan enclosure and smooth tapered orifice 2 continuous -duty totally enclosed motor with ball bearings 3 molded plastic fan blades and hub 4 butterfly-type backdraft damper weatherhood turns down 90 and discharges remote from wall Figure 2 Small exhaust fan with matching weatherhood attached (Photo courtesy of Del-Air Systems Ltd., Humboldt, Sask.) CHANGING VENTILATION RATES WITH WEATHER Plan M-9700 explains how ventilation rates are set for humidity control in cold weather and temperature control in warm to hot weather. It also gives, for different animals and poultry, the lower and upper limits of recommended ventilation rate, from the minimum 'step 1' to the 'maximum summer' rate. The maximum summer rate is typically about 16 times and may be up to 32 or more times the step 1 rate. When selecting ventilation system components you have several ways to get this wide variation in ventilation rate, such as by using: many fans, cycled on and off according to temperature; fewer fans, with variable speeds controlled according to temperature; variable recirculation of fan output back into the room. STEPPED SINGLE-SPEED FANS AND THERMOSTATS The lower part of Figure 3 shows how each of the stepped fans can be controlled by a thermostat, with each succeeding thermostat adjusted a little higher than the previous one so that all fans (or fan groups) will start and stop in the correct order. Starting at -30 C outside, the step 1 fan is ON, the step 2 fan is OFF, and because the ventilation rate is low, the room temperature slowly rises from 15 to 17 C. At this point the step 2 thermostat trips to ON and fan 2 starts. With both step 1 and step 2 fans now running, this doubles the ventilation rate, the temperature drops rapidly to 15 C and the cycle repeats. As the weather warms up outside, both building and ventilation heat losses diminish so that the step 2 ON periods get longer and the OFF periods shorter, until at about -5 C (in this example) the step 2 rate just balances the animal heat supply. As outside temperature continues to rise above -5 C, the step 2 fan stays ON, but the room temperature still rises slowly to 20 C and step 3 starts. Now step 1, 2 and 3 fans all combine to give the step 3 rate, which is four times that of step 1 (the doubling rule, above). The stepping process continues until, at about 27 C, all fans are running continuously, giving the maximum capacity (step 5 or 6). Mechanical thermostats all have some 'temperature differential' between START and STOP; for farm thermostats this is typically about 2 C, to ensure a good 'snap' action when the contacts open or close. This differential is approximately constant throughout the whole range of setpoint adjustment. Considering again the lower part of Figure 3, we see that there is a distinct temperature separation between successive thermostat settings. For example, the STOP temperature of the step 3 thermostat is above the START temperature of the step 2, otherwise temperature overlap between successive thermostats may confuse the fan control sequence. This is a fundamental limitation of stepped thermostat controls their temperature setpoints should be separated over a fairly broad temperature range.

5 OUTSIDE TEMPERATURE, C Figure 3 Five-step or six-step ventilation control diagram based on growing and finishing pigs, kg liveweight, in a well-insulated swine barn with pen floors 30% slotted In this example (Figure 3), the control range is 15 to 27 C, an acceptable winter-to-summer range for growing and finishing pigs. However, brooding young animals (baby chicks, weanling pigs, etc.) requires a much closer temperature range. Some farm thermostats have a closer differential that helps solve this problem; for example, Gold Fan' and Multifan2 thermostats have 1 C differential. CHOOSING STEPPED FAN COMBINATIONS In practical terms, the ventilation system designer chooses fan sizes from one or more manufacturers' catalogs. To prevent icing of the cold weather fans (steps 1 and 2) they are ideally two identical units, both under the same weatherhood. However, the doubled and redoubled rates required for warmer weather (step 3 and above) call for two or more fans per step, usually distributed around the room perimeter. The principles of fan selection are best shown by example, in this case a 1 Gold Fan, by Euromac Imports Inc., Box 297, Port Williams N.S. BOP 1T0 2 Multifan, by A. Vostermans BV, FO. Box AJ, Venlo, Holland (Mention of specific manufacturers and trade names is for example only, and is not an endorsement by Agriculture Canada or Canada Plan Service distributing agencies)

6 growing/finishing barn for 500 pigs ranging kg liveweight. The stepped ventilation rates per pig (Figure 3) are multiplied by 500 pigs to obtain the required fan capacities at each step (Table 3). Table 3 suggests eight single-speed fans for this sixstep ventilation system. It is not possible to fit exactly the recommended stepped ventilation rates, but it is most important not to oversize the step 1 fan. TABLE 3 STEPPED VENTILATION SCHEDULE FOR 500 GROWING AND FINISHING PIGS, WITH SINGLE-SPEED FANS Ventilation Danor1 Total Venti- rate No. fan ventilation (L/(s.pig) of model Fan output2 lation step x 500 pigs) fans number 25 Pa) (L/s) EPD 14-VX EPD 14-VX EPD 18-VX EPD 18-VX SB 36J SB 36J By Canarm Ltd., 2157 Parkdale Ave., Brockville, Ont. K6V 5V6 (Mention of manufacturers is for example only and is not an endorsement by Agriculture Canada or agencies distributing Canada Plan Service publications) 2 Based on manufacturer's literature (or where available, on tests by Prairie Agricultural Machinery Institute, Lethbridge, Alta.) In Table 3 and corresponding Figure 4, ventilation steps 1 and 2 are handled by two similar fans. Step 1 runs 24 h per day, 365 days per year, a fact that underlines the importance of choosing top-quality fans. Using two similar fans for steps 1 and 2, either fan can be designated as the continuous -running step 1, either by switching the motor plugs or by reversing the thermostat setpoints. This also provides some backup in case of failure; one spare fan stored in reserve can replace either unit. Another good idea is to protect both step 1 and step 2 fans inside the same weatherhood. This way the step 2 (intermittent) fan is protected from freezing and icing by the warm air stream discharged continuously from step 1. This feature is less important for the steps 3, 4 and 5 fans because they never operate in the coldest weather. For a big animal population like this 500 pig finishing barn, eight fans and five thermostats can be justified for a total ventilation rate of L/s ( ft³/min), as given in Table 3 and Figure 4. However, smaller barn units for practical reasons must have fewer fans, and perhaps fewer stepped rates. This is where Multispeed and variable-speed fans can help. Read on. STEPPED VENTILATION WITH TWO-SPEED FANS Look again at Figure 3, but this time consider a smaller room for growing and finishing only 150 pigs. Table 4 gives another six-step ventilation schedule, but using two-speed fans to handle the whole ventilation range with fewer fans. Figure 5 shows suggested fan locations, using two weatherhoods to house the four fans. Step 1 ventilation (Table 4, column 3) is only 195 L/s. Few manufacturers can supply a single-speed fan having this low capacity combined with the durability necessary to survive such continuous service.(some fans in this capacity range use small, shaded-pole motors with sleeve bearings, suitable only for manually-controlled service such as in kitchens, milkrooms and lavatories; these fans were never intended for automatic service in barns). Note in Table 4 that the two-speed fans chosen for steps 1-2 and 3-4 each have low speeds about half of their high speeds (850 versus 1725 rpm). This 1/2 speed ratio gives fan capacities that usually fit the doubling rule better than fans having the more-typical 2/3 speed ratio. Nevertheless, the two-speed fan is still not an ideal solution to the problem of obtaining the small but predictable airflows required at step 1. Figure 4 Exhaust fan location plan for 500 growing and finishing pigs (CPS plan M-3428) with self-adjusting slot air inlets (CPS plan M-9715). Table 3 gives suggested fan capacities for steps 1 through 6

7 Figure 5 Exhaust fan location plan for 150 growing and finishing pigs (CPS plan M-3428). Tables 4 and 5 give suggested fan capacities for steps 1 through 6 TABLE 4 STEPPED VENTILATION SCHEDULE FOR 150 GROWING AND FINISHING PIGS, WITH TWO-SPEED FANS Ventilation Danor1 Total Venti- rate No. fan ventilation (L/(s.pig) of model Fan output2 lation step x 150 pigs) fans number 25 Pa) (L/s) rpm S-12-E (2-speed) 1725 rpm rpm S-14-E (2-speed) 1725 rpm rpm SD24-FVX rpm By Canarm Ltd., 2157 Parkdale Ave., Brockville, Ont. K6V 5V6(Mention of trade-names and manufacturers is for example only and is not an endorsement by Agriculture Canada or the Canada Plan Service) 2 Based on manufacturer's literature (or where available, on tests by Prairie Agricultural Machinery Institute, Lethbridge, Alta.) When fan blades designed for high speed are run at low, they are more easily affected by anything that increases the static pressure (such as a headwind, badly designed weatherhood, dirty backdraft shutters orclogged ductwork). Figure 6 shows these speed versus pressure effects. At zero static pressure, variable minimum speed, this variable-speed fan moves about 40% of its high-speed capacity (275 versus 681 L/s). However, at 25 Pa (0.1 in. wg), the pressure at which fans are more realistically rated, the high-speed capacity dropped only 10%, whereas the low-speed capacity dropped 50%! In other words, its high-speed capacity is quite reliable but at low speed it is very vulnerable to static pressure effects. Figure 6 Variable speed fan capacity versus static pressure (Reconstructed from PAMI test report 481, Del-ai model F8 fan)

8 The above principle applies to all two-speed propellor fans, and even more so to the voltage-controlled variable-speed and multispeed fans where the low-end speeds are only 35 to 40% of full speed. Where reduced fan speeds are used to obtain the critical step 1 ventilation, good design of the weatherhood is absolutely essential to minimize back pressure. For reliable step 1 and step 2 ventilation in smaller rooms, the choice of suitable fans is limited. Here it is preferrable to choose small-diameter, high-speed fans that do not lose so much capacity when the static pressure goes up. Some new, small fans (250 mm diam. or less) use 3400 rpm motors to improve their performance at high static pressures. Refer again to Table 4. The more-predictable performance of some of these small-diameter high-speed fans (as compared with two-speed and multispeed fans running at reduced speed) suggests that steps 1 and 2 could be better handled by a matched pair of the high-speed fans. The two-speed fan shown for steps 1 and 2, although it appears to match the calculated ventilation rates, is actually quite unpredictable in the low-speed mode - its step 1 output could vary from 370 down to 60 L/s (zero pressure, to 62 Pa respectively) depending on the tailwind or headwind situation. Table 5 shows another ventilation schedule for the same example, 150 pigs, used in Table 4. This time, four variable-speed fans smooth the transition between the ventilation steps. Steps 1 and 2 are handled by one small variable-speed fan. For step 1, the operator adjusts the variable fan speed (by manual voltage control) until satisfied with a compromise between air quality (smell?) and heating economy (the fuel or electric bill). This low-end manual adjustment feature is especially handy for rooms that may not be fully stocked during cold weather. The 'nose test', although not infallible, is a better indicator of total air quality than a temperature sensor alone. A variable speed controller by Phason is one example that has this manual minimum speed setting as a standard feature. For range 3-4, a second variable-speed fan is temperature-controlled to add 443 to 1180 L/s of ventilation. There must be adequate temperature separation between this variable controller and the range 1-2 controller to prevent temperature overlap and resulting confusion of the two controls. For range 5-6 another variable speed fan and tem perature controller smoothly adjusts the ventilation for warm -to-hot weather. For small rooms, a small number of variable-speed fans provide smoother transitions from range to range, especially during the rapid temperature fluctuation of spring and fall. The disadvantages include extra costs for the speed-controllers. One cost-saving idea is to use one multispeed fan controller for either a small fan in winter or a larger fan in summer. Plug either fan into the controller, depending on the season. TABLE 5 VENTILATION SCHEDULE FOR 150 GROWING AND FINISHING PIGS, USING VARIABLE-SPEED FANS Ventilation Danor1 Total Venti- rate No. fan ventilation (L/(s.pig) of model step x 150 pigs) fans number Fan output2 (L/s (a 25 Pa) lation (L/s) (variable F12 manual) (full speed) (low, auto) F (high, auto) (low, auto) F (high, auto) By Del-Air Systems Limited, P0. Box 2500 Humboldt, Sask. SOK 2A0 (Mention of trade-names and manufacturers is for example only and is not an endorsement by the authors or by Agriculture Canada) 2 Based on tests by Prairie Agricultural Machinery Institute, Lethbridge, Alta. VENTILATING LARGE ROOMS Big, high-density housing units (for caged laying chickens, for example) require correspondingly more fans. To minimize equipment, installation and energy costs, choose the biggest, slowest-turning, most energy-efficient fans available. Avoid unnecessary options like variable speed or multispeed motors and controllers - the large number of fans can give you all the flexibility you need with ordinary thermostats. Also, choose a minimum number of fan sizes (preferably, two) to reduce the cost of stocking spare fans. For example, choose fans and a fan arrangement plan for caged laying chickens. The cage room is 11.4 x 90 m (38 x 300 ft), a very long narrow room to suit modern mechanized cages with frequent manure removal. From Plan M-9700, Table 1, calculate the minimum (step 1) to maximum stepped ventilation rates. Use the 'doubling rule' (explained earlier) to fill in the intermediate steps, then match the stepped ventilation rates with exhaust fans from the manufacturer's catalog as follows: Total Ventilation No. of Fan airflow ventilation Step (L/s) fans 25 Pa) (L/s) x = x = x = x = x = x = The above example shows that it is possible to match the stepped ventilation rates quite well using only two fan sizes, four Danor SD18-FVX fans and 10 Danor SB42J fans. All fans can run at full speed to obtain their highest efficiency at all times. The big summer fans are belt-driven, slow-turning fans with a very low noise level rating.

9 1 fan weatherhood 2.4 m (8 ft) wide for two big fans or two small fans plus one big fan (see Figure 1) 2 self-adjusting air inlet continuous along both walls (see Plan M-9715) Figure 7 Exhaust fan and air inlet arrangement plan for caged laying hens

10 The next step is to plan the ventilation layout. It is convenient to put two big fans in each weatherhood, or one big fan plus two small ones. This indicates six fan groups spaced equally along one or both walls. Figure 7 shows all six along one wall, an arrangement that is quite satisfactory provided that air inlets are along both walls. Alternatively, the fan groups could be rearranged with three along each wall, spaced at 28.8 m (96 ft). As previously stated, step 1 and 2 fans should be paired to prevent freezing of the step 2 fans during their off periods. From this point, the stepped fans are arranged by thermostat setpoints to make the ventilation at all steps as uniformly spaced as possible. It is important to place all the ventilation thermostats (six, in this case) at one location so that they all sense the same temperature. One of two locations is preferred; either at eye-level in the center of the room, or in the airstream approaching one of the continuous running step 1 fans. some warm spring day the maximum ventilation may be needed. This situation may call for a maximum/minimum ventilation ratio of 32:1 or even 100:1, instead of the 16:1 that is typical for continuous housing of mixed-age or adult livestock. This extreme difference between minimum and maximum ventilation requirements magnifies the difficulties of obtaining precise step 1 and step 2 ventilation rates with fans supplied by a particular manufacturer. An associated problem is the very low winter ventilation rate required for starting young animals. The incoming cold air doesn't have enough momentum to mix and blend with the warm air inside. Instead, the dense cold air settles to the floor and the warm air is displaced to the ceiling. Various ways of solving these temperature stratification and draft problems are discussed in Plans M-9710, Fresh Air Inlets and M-9750, Ventilating and Heating Small Rooms. MINIMUM TO MAXIMUM VENTILATION FOR ALL IN/ ALL-OUT HOUSING Consider now the problem of steps 1-2 ventilation when a room is refilled in winter with a group of young veal calves, weanling pigs or broiler chickens. For the next month or two this uniform group of young animals will grow rapidly so that on