VENTILATION IN THE FACE OF HIGH ENERGY COST Dr. Steven …€¦ · N.C. Healthy Hogs Seminar, Page 1 VENTILATION IN THE FACE OF HIGH ENERGY COST Dr. Steven Matthis ... of ventilation,

N.C. Healthy Hogs Seminar, Page 1

VENTILATION IN THE FACE OF HIGH ENERGY COST

Dr. Steven Matthis Sampson Community College

Livestock and Poultry Technology Department Clinton, NC 28339-0318

Email: [email protected]

Introduction

The definition of ventilation, “ventilation is the proper movement of air through a building replacing the stale air inside with fresh outside air” seems so simple, but in actuality the management of the ventilation is an ongoing process in swine facilities. Ventilation management is hard enough to do on a daily basis but the anxiety of high energy cost makes the process even harder. The objective of this project moves beyond basic ventilation principles and encompasses all aspects of energy consumption related to ventilation principles, such as heaters, heat lamps and lighting. The objective of swine facilities ventilation is to replace the stale air in the buildings. The problem occurs when it comes to removing the air and replacing it economically without causing undue stress on the pigs. Ventilation is a never ending task on a swine farm especially on extreme hot and cold days. A review of the five basic reasons for ventilation will help us focus on daily ventilation problems in the face of high energy cost. 1. Remove excess heat. 2. Remove excess moisture. 3. Minimize dust. 4. Limit the build-up of harmful gases. 5. Provide oxygen for respiration.

The purpose of this presentation is to show how to accomplish all of the necessary ventilation procedures while lowering energy costs.

• Assessing The Environment

In this next segment, the pigs’ environment will be discussed. Many times, managers do not stop long enough to assess the pigs’ environment. The ventilation equipment cannot control the environment alone, it must be managed. Before you can manage the pigs’ environment you most ask yourself four questions: 1. What is the problem? 2. What caused the problem? 3. What can I do to manage or fix the problem? 4. What are the most economic solutions to this ventilation problem?

In this presentation, an emphasis will be placed on the economical aspect of facilities

ventilation and how management can address these on a daily basis.


Managers must consider all environmental factors before making changes in the pigs’ environment. There are many external conditions that affect the pigs’ production that can be readily measured and controlled. These elements include: ambient room temperature, fan cfm, heat stratification, minimum ventilation, humidity, drafts, unplanned openings and cost of operation. Management must constantly remember that the combined effects of two or more of these elements make the environment extremely difficult to evaluate and control. Farm managers must be empowered to make changes only if they understand the microenvironment of the pig. The microenvironment of the pig “at pig level” can change hourly, daily, and seasonally. When ventilation is not corrected, an uncomfortable micro-environment can

have a negative effect on the pigs’ performance. Environmental changes should be limited to 5°F or less. Extreme changes in ventilation create an economical burden on the farmer, as well as creating health risks for the pigs.

What is the Problem?

The statement, “It’s not how much you make that matters, its how much you can save that puts you ahead of the competition” is very true during times of high energy prices. High energy cost is a problem that needs to be addressed in all areas of pig production, but energy conservation at the farm level have potential for marked savings. LP gas prices have increased approximately 10% in 2006 while electric prices have only increased slightly. If future predictions hold true, gas prices will increase over the next few years. In order for a business to stay economically sound, methods of energy conservation must be utilized.

What Causes the Problem?

• Working Knowledge of Basic Ventilation Principles

One of the major roadblocks to controlling energy costs at the farm level is a lack of

knowledge of basic ventilation techniques. Maintaining proper humidity levels in the barn while properly ventilating each animal without increasing energy cost requires skill.

Need to Know Energy SavingsVentilation/Pig Basics

• Ventilating When Outside Air is Cooler Than Inside Air

Incoming air gains heat as it enters the building and expands to accept the moisture from the air in a swine facility. The moisture holding capacity of air nearly doubles for every 20-degree rise in temperature. The goal is to use the warm air like a sponge to soak up the excess moisture and transport it outside the building. We would like to maintain the relative humidity level between 50 and 70 percent.

Theoretically as cold air moves through the room along the ceiling, it is heated up. The cold air temperature is increased (doubled) approximately every four feet of entry in the building. Cold dry air enters the building and equal amounts of warm moist air exit the building. The temperature and moisture level of the air exiting the building is greater than it was when it entered the building.


Heat, excessive moisture, pathogens, and gases are expelled from the building. The example below explains this concept.8

Proper static pressure rates will attach the incoming air stream to the ceiling and warm it up as it approaches the center of the room. Every .01 increase in static pressure will move the incoming cold air approximate 2 feet closer to the center of the room. An increase in static pressure increases air velocity. The table below illustrates the relationship between static pressure and air velocity. Setting the static pressure properly will decrease the wind chill on the pigs at floor level. The incoming air will have enough time to warm up and slow down before reaching the floor. The correct static pressure setting directs the air stream across the ceiling to heat up and absorb moisture. The table illustrates the relationship between static pressure and airflow in a 24-foot wide farrowing room.8

• Does This High Air Speed Create A Cold Draft On Pigs?

The following picture shows the cause and effect of proper static pressure. As air moves into the facility along the ceiling, it warms up and slows down. The picture illustrates that a 10 mph


intake will slow down to less than 1 mph before it reaches the pigs area when the static pressure is correct.8 The air speed needs to be around 30 ft/min at pig level.

• Moisture Removal and Ventilation

To avoid health problems in the pig, it is important to calculate the proper moisture removal rates for a swine facility. The picture on the previous page shows the accumulation of moisture in a facility and its removal.8

How moisture is removed:

1. Cold outside air carries .0003 lb/water in 11 cubic feet.

2. The outside air is -20°F with 100% RH. 3. Outside air is warmed and expanded after entering the building. 4. The expanded air picks up moisture from the animals and building and is now 60

degrees at 75% RH. 5. The air is now 13.3 cubic feet and holds .00831 lb/water.

The Math of Moisture Removal:

Inside air: .00831 lb/water Outside air: - .00030 lb/water Difference: .00801 lb/water

.00801 ÷ 13.3 cubic ft/air = .0006 lb .0006 lb of water removed for every cubic foot air exchanged.

Relation of Moisture Removal and the Rate of Ventilation:

Is moisture removal in your facilities acceptable? If your air exchange rates are not meeting minimal ventilation rate (MVR), then the answer is, “NO”.

A 120-lb pig produces approximately .19 lb of water per hour. If .0006 lb of water were removed per cubic foot air then, each pig would need 320 cubic feet of air per hour to remove the moisture he produces. (.19 lb/hr/pig/.0006 lb/cubic foot = 320 cubic feet per hour per pig)


This would be equivalent to 5.3 cubic feet per minute per pig. (320 cubic feet ÷ 60 min/hr = 5.3 cubic feet per min per pig) A 1000-pig unit with 120 lb pigs would need 5,300 cubic feet per minute for moisture removal.

1. Fan Operation: Output/Maintenance

Fans are one of the most overlooked pieces of machinery on the farm, but they are essential

for efficient pig production. Insufficient maintenance and cleaning results in higher energy costs because it reduces fan efficiency. Laboratory test have proven that three parts of the fan need routine maintenance or efficiency will be reduced as a result.

A. Dirty fan blades and louvers can reduce output by 40 percent5. A decrease in

minimum ventilation requirements by 40 percent could directly reduce production in mechanically ventilated barns. Extreme stratification could occur in naturally ventilated buildings where stir fans are utilized to mix heated air at floor level.

B. Worn belts and pulleys reduce the RPM output of belt driven fans. The

reduction in CFMs could be as much as 25% in some cases5. The wear takes place over an extended time and is not recognized during routine inspections. Proper belt and pulley maintenance will help decrease energy cost and improve fan ventilation performance.

C. Routine belt alignment is important. New fans should be checked for proper alignment. The alignment on many belt drive fans is never checked throughout the life of the fan. Check the alignment regularly especially if a motor is replaced. A pulley out of alignment could wear a belt and reduce efficiency in a few hours. Remember, pulley alignment and belt replacement are very important to long term fans performance.

The key points to remember on fan maintenance are (1) dirty fan blades could reduce

efficiency by 40 percent; (2) worn belts and pulleys could reduce efficiency by 25 percent. High energy costs are directly related to a decrease in efficiency. A decrease in efficiency could mean a decrease in minimum ventilation rates. Compare your minimum ventilation rates to the rates suggested below.

Minimum Ventilation Rates 1. Breeding, Gestation, Nursery, Finishing2

warm weather > 50°F, .09 CFM/lb of body wt.

cold weather < 50°F, .06 CFM/lb of body wt. 2. Farrowing2

warm weather > 65°F, 60-100 CFM/crate

cold weather < 65°F, 40-60 CFM/crate


These should be considered general guidelines you may need to make adjustments based on your particular animals and buildings.

2. Curtain Adjustment

Heat loss can be excessive through sagging or ragged curtains. Ropes and cables stretch and daily wear on curtain sided buildings allows for supplemental heat loss and an increase in gas bills. Sagging curtains or curtains with holes can account for drafts on pigs resulting in decreased production. In mechanically ventilated houses, fans have to run longer and harder to compensate for static pressure loss due to curtain maladjustment. It is possible to have a 20% reduction in air exchange due to leaky curtains. The incoming cold air does not project across the ceiling resulting in cold air falling on the pigs. See illustration below. Cold air drafts are produced when static pressure is compromised.

3. Evaporative Cool Cells Cooling pigs is essential in the summer months to maintain production and decrease pig

stress. Question, have you ever seen a one week pig old panting in the farrowing house with the

heat lamps off? This can happen if the cooling system is not maintained. A 15° reduction in


temperature is essential on a 100° day. Any one of the problems listed below can reduce efficiency of the evaporative cooling by 20%.

- Mineral deposits – white streaks develop on the pad due to high mineral deposits in

the water. - Clogged lines – filter clogged or algae growth in filter - Poor water distribution in lines, pads flow and saturation level reduced. - Holes in pad – air is not directed through the veins in the pad thus reducing

efficiency.

In winter months heat loss due to uncovered pads can greatly increase energy consumption. Cover the pads in winter months, but be aware of the rodent population on the farm. Covering the pad could lead to new pad installation in the spring, due to the pad being used as rodent beds. Poor rodent control could lead to high pad replacement cost.

4. Heat Lamps

One of the largest consumers of electricity in farrowing units is heat lamps. The illustration

below depicts an average farm situation using 175 watt heat lamps. Use your actual heat lamp numbers to determine your energy consumption.

Example6: 1) Cost of using a 175 watt bulb per day

175 watts x 24 hours per day = 4,200 watts

÷ 1000 = 4.2 kilowatts per day x .10 per kw = .42 per day per bulb 2) Cost of using 125 watt bulb per day 125 watts x 24 hours per day = 3,000 watts

÷ 1000 = 3. kilowatts per day x .10 per kw = .30 per day per bulb

Due to the large number of heat lamps burning continuously in a farrowing unit, a reduction in the amount of watts consumed could save a tremendous amount of money on electric bills during the summer. If this method of energy reduction is chosen, a close analysis should be given to the pigs’ micro environment. Proper bulb placement is essential at pig level.

Compare your farm: Example: 1) 175 watts bulbs Number of bulbs (300) x .42 = $126.00 per day Example: 2) 125 watts bulbs Number of bulbs (300) x .30 = $90.00 per day Amount saved (1) & (2): Example (1) $126.00 – Example (2) $90.00 = $36.00 per day savings using a smaller bulb This savings equals to $1, 080.00 per month savings on this farm.


There is a labor cost factor with bulb exchange in the spring and fall, but the energy savings compensate for the labor exchange factor. 5. Lights

Most farms have converted from the standard incandescent lights to a fluorescent light fixture. The energy savings is very beneficial on most swine facilities due to the number of light fixtures. The energy savings from the exchange can save up to seventy-five percent on the lighting of the facility.

Example: Light Savings An incandescent 100 watt light bulb can be replaced by a 24 watt fluorescent bulb. Consider a light burns an average of six hours per day and the farm has 100 bulbs.

Example 1: 100 watt bulb x 100 bulbs = 10,000 watts ÷ 1000 = 10 kw x 6 hr per day = 60 kilowatts x 30 days =

1800 kw x .10 per kw - $180.00 per month

Example 2: 24 watt bulb x 100 bulb = 2400 watts ÷ 1000 = 2.4 kw x 6 hr per day = 14.4 kilowatts x 30 days = 432 kw x .10 per kw = $43.00 per month The savings from this bulb exchange resulted in a $137.00 per month savings. Insert your farm numbers and compute the energy savings.

6. Unplanned Openings

Unplanned openings are serious ventilation/energy problems on older swine farms. Preventative maintenance is necessary to eliminate production problems and pig stress. Unplanned openings include:

- Doors that do not close properly or seal. - Curtains with holes due to age or rodents. - Fan louvers that are broken or hang in open position. - Missing louver blades - Unlevel curtains - Holes in concrete - Curtain pockets loose and flapping in the wind. - Inlet malfunction not adjusted properly. - Curtain machines not operating properly Test show severe shutter air leakage causes cold weather problems and an increase in gas

consumption between $5 and $6 per day.7 If maintenance and repair has not been a high priority at your farm, the increase in energy cost should boost your enthusiasm to make maintenance a priority. Unplanned openings can be prevented with a little preventative and continual maintenance.


7. Heater: Selection/Operation/Maintenance During the ventilation process, heat exchange is very critical. Energy is consumed and

sometimes wasted as we ventilate to meet oxygen requirements and animal heat requirements while removing excess moisture from the buildings. First determine the BTU requirements for the space and select the heater that matches the requirement. In many situations where the ventilation is not managed properly, the heater and fans work against each other causing an energy loss due to an unnecessary amount of warm air being expelled from the building. Control setup can be a solution to this energy problem. False readings by a control unit placed directly in the path of heated air. Heater drift can also be associated with this problem. Heater drift occurs when the heat blast from the heater affects the control sensor and engages an exhaust fan to expel heat in the room. Heater drift – the control sensor has some degree of thermal lag so as the room heats up the air temperature actually goes up faster than the control can read it. Typically the sensor is 1-2 degrees F behind the air temp. When the heat shuts off the control sensor has time to catch up with the room air temp. If the heater shut off point and the set temperature to bring the fan on are too close it will cause your fans to move more air than the desired minimum ventilation rate. More air = more heat cost.2

Maintenance of heater unit is critical for safe and efficient operation. Most agricultural

heaters are unvented which increase moisture accumulation in the buildings. This increased moisture adds to the existing humidity control measures. An unvented heater adds approximated .95 gallons of water vapor in the air for every gallon of LP gas burned. Minimum ventilated rates need to be adjusted slightly higher to compensate for the increased moisture production related to increased unvented heater usage. Example: Increased Moisture Production (LP Gas Heaters)

A standard 225, 000 BTU gas heaters will burn approximately 2.36 gallons of gas for each hour of operation. (1 gallon of LP Gas = 95,000 BTU’s) If the gas heater operates 10 hours per day it will burn approximately 23.6 gallons of gas. The amount of moisture will equal 22.42 lbs of water vapor. This equates to approximately 2 gallons of water per day. Ventilation may need to be increased to compensate for the higher moisture level. At today’s LP gas rate of $1.50 per gallon, the cost of operating one 225,000 BTU heater for 10 hours could be $35.40. If close management of the ambient temperature is maintained through controlled heat loss and reduced stratification and a 10% energy savings could be achieved, the farm could save $3.50 per day per heater. Multiply these savings by the number of heaters on the farm times 30 days per month to obtain a sizable amount of savings. Heater management is critical to improving energy cost. Maintain heater to produce maximum heat. Heater output should maintain a temperature of

approximately 300°F. This temperature is hard to measure due to the intense heat. A good rule of


thumb to test heat or output is to use an electronic temperature measuring gun to test the discharge temperature.

How Do You Fix The Problem?

1. Be diligent in improving ventilation and conserving energy. Due to the many different

concerns involved in swine ventilation, there is room for energy conservation in all swine facilities.

2. Assess the pigs’ environment as often as possible especially during changing weather.

Know your ventilation rates and calibrate your fan output.

Ventilation rates are determined by pig size and the season of the year use the suggested formula supplied in Appendix I to determine your ventilation rate. Once you know your essential ventilation rate you can determine the air flow of your existing fans. Use Appendix II to estimate fan output.

3. Maintain fans on a regular basis. Use the following checklist as a guideline to efficient fan operation to reduce energy.

Fan energy reduction checklist: A. Clean Motor:

• Use forced air

• Do not use pressure washer

• Dirty motor runs hot

• Check all bolts and clips

• Check for signs of damage

B. Clean Shutter/Louvers:

• Use a brush

• Check for smooth operation

• Replace bent louver sections

• Use graphite on light louver bushings

• Remember – 40% reduction in efficiency

• Properly operating shutters will stop cold outside air from coming in when not in operation

C. Grease Bearings:

• Grease every 4-6 months

• Use a small amount of grease

• Reduce friction on moving parts and increases efficiency

D. Check Pulley for Wear:


• A V pulley must have smooth sides

• Belt must ride slightly out of the top of the pulley

• Performance can be reduced 25% or more if belts sink down into the pulley only 1/8 to 3/16 inch

E. Belt Tension:

• Check new insulation after 3 days

• Follow up checks every 4-6 months

• Approximate rule of thumb. One half in deflection between pulleys.

If you have fans that are old and need replacing, consider replacing them with a more energy efficient model. Use ventilation laboratory results to choose fans with high cfm’s per watt output. Below is an example of fan CFMs per watt. Compare your fan output. Example: 14” Direct Drive Fan (Minimum Ventilation) Fan 1 produces 8.5 cfm/watt at 2020 total cfm air flow Fan 2 produces 13.8 cfm/watt at 2030 total cfm air flow Energy Saving Per Year (Actual Figures from Bess Laboratory)

Fan 1 2020 ÷ 8.5 ÷ 1000 x 8760 hr/yr x $.10 per kw hr = $208.00 per year

Fan 2 2030 ÷ 13.8 ÷ 1000 x 8760 hr/year x $.10 per kw hr = $128.00 per year Energy Savings = $80.00 per year Savings similar to this could pay for the difference in cost of a more efficient fan in 2 years. 4. Inlet adjustment is critical to maintain proper static pressure and control drafts in the barn.

In mechanically ventilated houses use the following information as a guide. Air coming in a properly maintained inlet will get approximately 2 feet of throw across the ceiling for every .01 inches of static pressure in the room.2,5 If the air current does not cling to the ceiling until the incoming air is warmed recalculate your inlet opening and fan output. Calculate air inlet opening for proper static pressure using Appendix III. Curtain sided houses need to stir

the air to prevent stratification. A goal of a 2° difference in the ceiling and floor temperature should be set. A greater difference than this could increase heating cost due to the warm supplemental heat being flushed from the room at the top of the curtain due to the rise of the heated air. Curtains need to be checked for snags and holes on a regular basis. Holes can be patched or a plastic liner can be draped over the existing curtain using special chips to hold it in place. If an extra plastic layer is added, check cables for wear. Curtains could fall due to the excess weight. Another method of controlling heat loss is adding a thin layer of roll insulation. This could be used on one or both sides of the building as long as proper


ventilation can be maintained. The roll insulation can be easily installed and can be reused. During high energy prices this is a viable alternative to reduce energy cost on curtain sided houses.

5. Prevent evaporative cooling problems by:2 1. Cleaning the reservoir on a regular basis. 2. Change the filter regularly and cover to prevent algae growth. 3. Clean discharge holes to prevent dry areas on the pad. Dry areas do not cool. 4. Us the proper algae agent. Clorox is not recommended. Pads can deteriorate and be

ineffective. 5. Mineral deposits clog pads. Pads become white due to build up on the pad. Us a

bleed off technique if this problem exists.

6. Use the proper heat lamp for the season of the year. If possible, install temperature control devices to regulate the temperature. Make sure the temperature sensor for the control is located at pig level, but not contained in a small plastic tube. (Example: ¾ PVC pipe) This could cause an inaccurate reading due to heat being trapped in the pipe on pig, lying against the sensor.

Industry research suggests that 70% of the energy used in farrowing houses is used by heat lamps.9 Heat pads can reduce the energy cost by 50% while maintaining the comfort zone of the pig. Heat pads can be more easily regulated to fit the heat requirements of the pigs. Heat pads focus the heat on the pig, not on the sow, thus helping to maintain the cooler ambient temperature of the room and improving the comfort level on the sow while reducing energy costs. Sows perform better while you save money on heat costs.

7. The best suggestion for energy conservative with lights is to change to fluorescent light bulbs. They burn 75% less electricity, are cooler to operate and last 10 times longer than regular light bulbs. Turn lights off when not in use. Make it company policy to turn lights off. Electric companies suggest turning incandescent lights off every time you leave the room and fluorescents if you will be gone more than 15 minutes.

8. Unplanned openings are unaccepted during high energy cost. The industry can’t afford them. Patch all holes using a spray insulation material. Prepare an energy savings list to be checked on a regular basis. Most of the direct energy loss is in the winter months, but energy conservation needs to be a major part of production practices on a continuous daily basis.


Take Home Message

• There is room for energy conservation on most farms.

• Prepare an energy audit/awareness program. This can be accomplished during a routine management service person farm walk through.

• Train all employees on energy conservation measures. Make employees aware of the farm budget related to energy cost. Equip employees with the tools they need to stop energy loss due to unplanned openings.

• Set up a preventative farm maintenance program to address heat lamps, heaters, evaporated coolers, curtains and fans.

• Measure the output of mechanical equipment. Make sure that you are receiving maximum efficiency from every device. Example: fan, heaters, inlets, stir fans, evaporative cool systems.

For More Information On Ventilation and Troubleshooting

• Energy Conservation Ventilating and Heating Swine Building. Pork Industry Handbook.

• Agricultural Ventilation Fans, Performance Efficiencies. Bioenvironmental and Structural Systems Laboratory. www.bess.uiuc.edu

• Troubleshooting Swine Ventilation Systems. Pork Industry Handbook.

• Swine Facility Ventilation: Theory and Application. NC Health Hog Seminars.

• Monitoring Air Quality Instruments. NC Healthy Hog Seminars.


References

1. Minimum ventilation: how to keep your heating bills from going 50% higher. 2. Aerotech Inc. Isaac Singletary

3. Midwest Planning Service

Swine Housing and Equipment Handbook MWP5-8 Mechanical Ventilating Systems for Livestock Housing MWP5-32 Natural Ventilating Systems for Livestock Housing MWP5-33 Heating, Cooling and Tempering Air for Livestock Housing MWP5-34

4. Environmental Control for Confinement Livestock Housing Purdue University, Don D. Jones

5. The Basic of Poultry House Ventilation and Electrical Wiring

Robert Botcher, NCSU

6. Agricultural Ventilation Farms: Selection and Maintenance National Food and Energy Council 7. Test Show Fan Shutter Air Leakage Causes Cold Weather Problems, Jim Donald, Auburn University

8. The University of Georgia Cooperative Extension Service College of Agricultural and Environmental

Sciences

9. Alternative Heating Systems Inc. Norm Loewen

____________________________________________ 1 Kohler, EM. Protection of pigs against neonatal enteric colibacillosis with colostrum and milk from orally vaccinated sows. American Journal of Veterinary Research. 1974. 35(3), 331-338. 1 Damm BI, Friggens NC, Nielsen J, Ingvartsen KL, Pedersen LJ. Factors affecting the transfer of porcine parvovirus antibodies from sow to piglets. Journal of Veterinary Medicine. 2002 A49. 487-495. 1 Alexander TJL, Thornton K, Noon G, Lysons RJ, Gush AF. Medicated early weaning to obtain pigs free from pathogens endemic in the herd of origin. Vet Rec. 1980. 106, 114-119. 1 Connor J. Elimination of specific agents from herds: Elimination by medication. Proc MN Swine Conf. Vet. 1992. 151-157. 1 Harris DL. Isolated weaning. Large An Vet. 1990. May/June. 10-12. 1 Wiseman BS, Morrison RB, Dial GD, Molitor TW, Pijoan C, Bergland M. Influence of weaning age on pathogen elimination and gowth performance of commingled pigs derived by medicated early weaning (MEW). Proc 12th IPVS Congress. 1992. 500. 1 Harris DL, Armbrecht PJ, Wiseman BS, Platt KB, Hill HT, Anderson LA. Producing Pseudorabies –free swine breeding stock from an infecteid herd. Vet. Med. 1992. 87(2). 166-170. 1 Gramer ML, Christianson WT, Harris DL. Producing PRRS-negative pigs from PRRS-positive sows. Proc Al D. Leman Swine Conference. 1998. 190-193. 1 Vincent AL, Richt JA, Wesley RD, Lager KM. Influenza vaccine research ath the National Animal Disease Center. Proc Al D. Leman Swine Conference. Sept 23-26, 2006. 70-74. 1 Oliveira S, Pijoan C, Morrison R. Evaluation of Haemophilus parasuis control in the nursery using vaccination and controlled exposure. Journal of Swine Health and Production. May-Jun 2004. (3): 123-128.

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VENTILATION IN THE FACE OF HIGH ENERGY COST Dr. Steven …€¦ · N.C. Healthy Hogs Seminar, Page 1 VENTILATION IN THE FACE OF HIGH ENERGY COST Dr. Steven Matthis ... of ventilation,