Tube systems have a plastic tube with a fan blowing fresh clean outdoor air into the tube as shown in Figure 1. Tubes commonly are made of either polyethylene or PVC and have one, two or three rows of holes uniformly distributed along the tube's length. They are called positive pressure tubes because the fan blowing into the tube creates a positive pressure inside the tube.
Figure 1. Schematic of a mechanically ventilated barn with a tube system.
Tube systems can be used in naturally and mechanically ventilated calf, heifer and cow barns. In calf barns they are commonly used to:
- Supply fresh outdoor air to the calves,
- Distribute the fresh air fairly uniformly along the tube,
- Direct fresh air into individual calf pens,
- Enhance room air mixing, and
- Break up stagnant areas with ammonia and airborne bacteria that can be at concentrations that lead to respiratory stress in calves.
Mechanically ventilated calf barns with tube systems must have an exhaust fan running continuously to maintain a negative pressure in the barn and avoid pushing moisture into walls. The exhaust fan typically is the same make and model as the tube fan. Mechanically ventilated barns with tube systems must have wall or ceiling inlets that open as the ventilating rate increases when weather becomes warmer and additional exhaust fans turn on.
Tube systems should be used to distribute clean fresh air taken directly from outdoors through a wall or a clean well-ventilated attic. Tubes should not be used to recirculate air in a calf barn. A tube used to recirculate air will collect dust and microorganisms in the bottom of the tube. While tubes can be cleaned, they are seldom cleaned enough to prevent disease transmission to the next batch of calves. A University of Kentucky publication recommends replacing dirty polyethylene tubes in livestock barns every two years. Polyethylene tubes that have collected dirt at the bottom should be replaced immediately to avoid blowing the dirt and bacteria into the air and exposing newborn calves.
Tubes should not take air from areas housing older heifers or cows. Air from areas housing older heifers or cows usually contains airborne bacteria and gases that can cause disease or respiratory stress in pre-weaned and newly weaned calves.
Key tube system decisions include fan airflow rate, fan performance, tube diameter and length, hole diameter, number of holes, rows and position, and mounting height. University of Wisconsin faculty developed a spreadsheet called the Positive Pressure Tube Calculator (PPTC) to help design tube systems. The spreadsheet is very helpful to tube system designers. Alternative tube designs can be assessed by comparing the area ratio, air jet throw and height.
The area ratio or aperture ratio is the sum of all the individual tube hole areas over the tube cross-sectional area based on the tube diameter. The recommended area ratio range in the PPTC is between 0.8 and 1.2. Tube systems with very high area ratios can have poor airflow uniformity and discharge more than twice the airflow from holes at the closed end compared to holes near the fan. Tubes with large area ratios can also become unstable near the fan end and collapse and inflate (Saunders and Albright). Tubes with lower area ratios will operate at higher static pressures and have lower airflow rates but will have more uniform airflow distribution.
Air jet throw is the distance from the tube outlet hole to where the air jet velocity is 60 fpm (Figure 2). Sixty feet per minute (fpm) is considered to be an air velocity that does not feel drafty. It is difficult to measure and feel air velocities of 60 fpm or less. Tube throw distances can range from 3 to 10 feet or more depending on the tube area ratio, tube fan performance and hole diameter. Throw is the distance it takes for air shooting out tube holes at between 1,200 fpm and 1,800 fpm to drop to 60 fpm.
Figure 2. Throw is distance from tube outlet to where the air jet drops to 60 feet per minute (fpm). Air velocities at the hole are commonly between 1,200 and 2,000 fpm.
Tube mounting height, hole location (e.g. 3 o'clock or 5 o'clock) and throw impact the height above the floor when the air jet speed reaches 60 fpm. Drafts are a critical consideration when designing a tube system for calves in individual pens.
Airflow through tubes and out the holes is complex. Uniformly sized and distributed holes can provide fairly uniform air distribution. More uniform airflow is achieved using a smaller area ratio. Special analysis is needed to assess airflow uniformity.
Rated fans, with measured airflow rates at static pressures ranging from 0.0 to 0.3 inches of water, are recommended for use in tube systems. The flow rate and performance of unrated fans is unknown. Work with a reliable equipment supplier to find a fan.
Well designed, constructed and managed tube systems are an effective way to distribute fresh outdoor air uniformly and break up stale air pockets in calf barns without creating drafts.