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Ventilating system with thermostatic on/off control

Kevin A. Janni

Thermostatic ventilating system controllers are electronic decision makers that adjust fans, heaters, sprinklers and mixing fans based on control settings and a measured temperature. Controller adjustments impact the temperature, relative humidity and airborne contaminate concentrations in the barn. They also impact energy costs if supplemental heat is used. Most dairy ventilating systems do not use heaters so this article assumes no supplemental heat is being added.

All mechanically ventilated dairy barns with exhaust fans, including tunnel and cross ventilated barns, need ventilating system controllers to turn fans on and off or adjust variable fan speeds. Thermostatic controllers use a measured temperature and the controller's logic to decide when to turn fans on and off. More sophisticated controllers can make decisions based on time and additional sensor inputs when relative humidity, ammonia or carbon dioxide sensors are used.

Common single stage thermostats turn one fan on or off based on the temperature measured. Multi stage thermostats control two or more devices and programmable controllers can adjust variable speed fans.

One key to successful ventilation management is to set controllers correctly so that they make appropriate ventilating system adjustments to maintain the desired environmental conditions as weather and barn conditions change. In dairy barns with lots of fans it is impractical to use and set single stage thermostats to manage the fans correctly as a system. Sophisticated controllers and computer systems are used in these barns. These programmable controllers require a good understanding of the ventilating and control system to make them perform the way you desire.

On/off thermostatic controllers use three pieces of information to decide whether or not to adjust a device being controlled. The three pieces of information are the set point temperature, the measured temperature and the switching differential. The thermostat's set point temperature is the desired temperature set by the operator. The measured temperature and the switching differential are used with the set point temperature to decide whether the controlled device should be turned on, off or left unchanged.

If thermostats turn fan motors on the instant the measured temperature rises slightly above or turn off when it falls below the set point temperature, it leads to premature fan motor failure. This is called rapid cycling. To avoid rapid cycling, agricultural controllers have a switching differential, typically 2 to 5°F, within which the controller does not make any adjustments. For example, if a fan controlled with a thermostat has a 50°F set point temperature and a 2°F switching differential, the thermostat would turn the fan on when the measured temperature was 52°F or above, the set point plus the switching differential. If the measured temperature decreases, the thermostat would turn the fan off when the measured temperature was 50°F or below the set point temperature. If the measured temperature remained below 50°F, the fan would remain off. The thermostat would not turn the fan on again until the measured temperature reached 52°F or above. When the measured barn temperature is between 50 and 52°F the controller does not change the fan operation.

Electronic controllers may have adjustable switching differentials. Smaller differentials will keep the temperature closer to the set point temperature and the equipment will cycle on and off more often. Larger differentials result in greater temperature variation around the set point temperature and less equipment cycling.

Fresh air tube system

Figure 1. Idealized ventilating rate for dairy barn as measured barn temperature goes from cold to hot

Figure 1 shows an idealized ventilating rate curve for an animal facility. In cold weather the system is operated at some minimum ventilating rate. As the measured barn temperature increases, the ventilating rate is increased slightly. As the measured barn temperature moves into the mild and warm range, the ventilating rate increases more and reaches the maximum ventilating system capacity as the barn temperature becomes very warm or hot.

Fresh air tube system

Figure 2. Ventilating system control using five single speed fans and a four-stage thermostat with set point temperatures of 33, 47, 62 and 72 ° F, and 3°F switching differentials

An ideal ventilating system would follow that ventilating rate curve in Figure 1. Practically this is not done. Figure 2 shows how a ventilating system with five single speed fans and a four-stage thermostatic controller with on/off control and 3°F switching differentials might perform and adjust ventilating rates as measured barn temperature changes. The four set point temperatures are 33, 47, 62 and 72°F. At cold temperatures, the constant minimum ventilating rate (1,200 CFM) is used. The first stage fan that moves 1,200 CFM turns on when the measured barn temperature reaches 36°F. When the measured barn temperature reaches 47°F or above, the second stage fan that moves 4,000 CFM turns on. At 65°F the third stage fan that moves 8,700 CFM turns on and at 75°F the fourth stage fan turns on. The stair step control system using single speed fans and on/off control can only approximate the idealize curve. Variable speed fan controllers will be a future topic.


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