Tube systems with variable speed fans
Positive pressure tube systems are used to distribute clean, fresh outside air in calf barns. Many tube systems are sized to provide the minimum ventilating rate for the calves. The recommended minimum ventilating rates are expected to manage humidity levels and condensation in cold weather and maintain acceptable air quality, and manage airborne bacteria and virus concentrations. These minimum ventilating rates are based primarily on animal size and age, and number of animals.
Sometimes owners and managers want to use two speed or variable speed fans in positive pressure tube systems so that they can adjust the airflow rate from the minimum rate to a higher rate as animal numbers increase, calves grow, or outside temperature increases. It is simple to install either a two speed fan or variable speed fan and the appropriate controller. However, fan performance changes as fan speed changes and this can impact tube system performance.
Figure 1. Fan performance curves for an inline centrifugal fan at two speeds, 1,900 revolutions per minute (RPM), and 3,800 RPM.
Figure 1 shows airflow rate and static pressure relations as the fan speed, in revolutions per minute (RPM), is changed from 3800 RPM to 1900 RPM. Both airflow rates and static pressures decrease as RPMs decrease. The fan shown would be able to provide the mild weather ventilating rate for 40 calves, 1,200 cubic feet per minute (CFM), and the minimum cold weather ventilating rate, 600 CFM at the two fan speed settings.
Positive pressure tube system performance depends on the fan airflow rate and static pressure relation, tube diameter, hole diameter, number of holes and their location. So how will a tube system perform when the fan speed changes? It depends.
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.
Performance characteristics that can be used to indicate whether a tube system is working well include air jet throw, airflow uniformity along the tube length, operating static pressure and tube air velocities. Air jet throw is the distance from the tube outlet to where the air jet drops to a speed of 60 feet per minute (fpm). Air at 60 fpm is not considered to be drafty even in cold weather (Figure 2). Air flow uniformity describes how the amount of airflow out of tube holes near the fan compare to the airflow out of holes at the closed end. Ideally airflow should be uniform but in most cases airflow out of holes near the fan is lower than out of holes at the closed tube end.
University of Wisconsin faculty developed a spreadsheet called the Positive Pressure Tube Calculator (PPTC) to help design tube systems. Table 1 gives PPTC results for a tube designed to supply the mild weather ventilating rate, 30 CFM per calf to a room with 40 calves using the high speed fan in Figure 1. Flow uniformity, assessed based on static regain, indicates that the tube system produces a uniformity ratio of 0.84, the operating static pressure is around 0.16 inches of water, and the air jet throw ranges from 13 to 15.5 feet. When the low fan speed is used, the fan supplies the minimum cold weather ventilating rate of 15 CFM per calf. The uniformity ratio is around 0.85, the operating static pressure drops to slightly below 0.05 inches of water, and the air jet throw ranges from 7.2 to 8.5 feet. The lower air jet throw will reduce the potential for drafts but it may not provide the fresh airflow desired at calf level. The low static pressure also raises concerns about how well the tube system might perform at the low fan speed.
Table 1. Positive Pressure Tube Calculator Results
|Tube length||39 feet|
|Height of tube (bottom of tube)||11 feet|
|Tube diameter||12 inches|
|Rows of holes||2|
|Hole diameter||1.5 inches|
|Hole interval (distance between holes)||11.5 inches|
|Total number of holes||82|
|Static pressure||0.18 inches of water|
|Hole positions||4 o'clock & 8 o'clock|
|Air jet throw||10.6 feet|
|Height at which air jet is 60 fpm||5.7 feet|
Tube systems that have fans operating at lower fan speeds will have lower airflow rates, tube air velocities, operating static pressures and air jet throw distances than the systems have at full fan speed. The lower air jet velocities at the lower fan speed will reduce the chances of air drafts at the lower ventilating rate and fan speed. Insufficient air jet throw could be a problem at the slower fan speed. Airflow uniformity along the tube length is similar as long as the operating static pressure is not too low that the fan becomes unstable near the fan end. Increasing fan speed will increase air jet speed and operating static pressure.
When planning to use two speed or variable speed fans, it is recommended to size the tube for the highest speed. Do not size a tube system for the low fan speed with plans to operate it at a higher fan speed. The increase in fan power requirements, static pressures and air jet throw make this an undesirable option.
Variable speed fans can be used with positive pressure tube systems when the fan speed change is not excessive. Each case needs to be analyzed to determine an acceptable fan speed change because of the relation between the fan used and the tube design selected.