In the pork industry, there are few “benchmarks” available for comparison of energy expenses by the swine enterprise. Most producers, facility managers and contract growers attempt to manage this expense by making ventilation system management decisions that impact not only the pig's environment, but the resultant consumption of electricity and propane.
The key to management of energy expenses lies in the selection and control of the ventilation equipment. In my work with pork production systems, the four mistakes I most commonly see are:
Incorrectly sized, minimum-ventilation fans;
Mistake # 1 - Incorrectly sized, minimum-ventilation fans
Incorrectly managed, variable-speed fans for minimum ventilation;
Incorrectly sized heaters; and
Incorrect ventilation controller settings for furnaces and variable-speed fans.
Heat loss of production units occurs via the ventilation system (air exchange) and via the building shell (insulation). A common obstacle to effective management of propane expense is a lack of knowledge about the ventilation system capacities in the facility vs. the estimated requirements for moisture control.
The Midwest Plan Service recommendations for minimum-ventilation rates in cold weather are listed in Table 1. These rates are designed to remove the moisture produced by pigs and their associated activities.
While many producers can recite the values in Table 1, they often cannot translate these values into capacities associated with their production facilities.
Table 2 lists approximate fan capacities for fan sizes often associated with minimum-ventilation fans. Note that Table 2 is an approximation, as specific fan performance is influenced by fan housing design, blade design and other items such as shutters, deflection hoods and discharge cones.
Specific fan performance details are available from the University of Illinois BESS Labs testing results (www.bess.uiuc.edu) and the performance estimates of specific fans from manufacturers.
Farrowing rooms are most commonly the victims of incorrectly sized, minimum-ventilation fans. This is because of the relatively low ventilation rate required for very cold weather.
At 20 cfm (cubic feet/minute) per crate, in cold weather, a 24-crate farrowing room only needs an estimated 480 cfm of fan capacity to maintain humidity levels. Many farrowing sites that I have visited have 14-in. or 16-in. diameter fans installed as the minimum-ventilation fan. Fans of this size often have estimated capacities of 2,000-2,500 cfm. To obtain 480-500 cfm, these fans must operate at 25% of their estimated capacity. Slowing variable-speed fans to this very low percentage of capacity almost always results in fans stalling, or at best, operating very poorly. As a consequence, managers often operate these fans at speeds — 1,000 cfm or higher — for minimum-ventilation rates.
Mistake # 2 - Incorrectly managed, variable-speed fans
The net consequence of this “small” error in fan sizing is that heat loss from the ventilation system goes from an estimated 540 Btu/hr/°F to 1,080 Btu/hr/°F. This change in rate of heat loss means the furnace must come on to provide supplemental heat when the outside air temperature is approximately 10°F warmer than when the ventilation rate is correct.
If the farrowing room is maintained at 70°F, and the outside air temperature is 20°F, this mistake in ventilation rate is the equivalent of 27,000 Btu of extra energy consumption. If unvented propane furnaces are used as the supplemental heat source in the farrowing facility, this amounts to almost 7.5 gal. of extra propane usage per day.
At one site I worked with this past summer, the over-sizing of farrowing room minimum-ventilation fans, and their associated inadequate minimum speed settings, resulted in propane expenses for the past three winters that were equal to facilities in the same production system having twice as many females in inventory.
Mistake # 2 - Incorrectly managed, variable-speed fans
This mistake happens in all types of facilities. I recently visited a breed-to-wean site where the manager was told by the builder that the 16-in. minimum-ventilation fan should operate at 100% speed for each of the 24-crate farrowing rooms. As installed, the fans were rated at 2,500 cfm (104 cfm/crate), which is more than four times the recommended minimum rate.
While the moisture levels in the farrowing rooms were very good, the complaint was a very high propane expense relative to other units.
Many producers assume that the minimum speed setting in their ventilation controllers correlates to the relative ventilation rate for the fans connected to the controller. This may or may not occur. Variable-speed fan output is controlled in most controllers by changing the amount of voltage sent to the fan motor. Fan motors respond to voltage by changing their rpms, which is directly related to their cfm rate.
Each brand and size of motor has a specific response function to voltage. As a general rule, small fan motors respond within a narrow range of voltages, while larger fan motors respond over a wider range of voltages. That is, small motors (common to 12-in. and 14-in. fans) often increase rpms in relatively large increments when voltage changes as little as 2-3 volts in a 230-volt system. These fans often go from minimum speed to almost maximum speed as voltage signals change from 100 volts to 140 volts. On the other hand, motors typically installed in 24-in. fans often go from minimum speed to almost maximum speed as voltage signals change from 120 volts to 210 volts.
To compensate for this dramatic difference in fan motor response, many controllers have user-selectable “motor curves” installed. The purpose of the curve is to allow the user to tell the controller how to “talk” to the variable-speed fan to maximize the response of the fan over the desired operating range.
For example, one curve might send a voltage signal of 154 volts, while another curve may send a signal of only 96 volts at the same minimum speed setting with the controller reading 50% minimum speed for both curves.
Users of variable-speed fans should also recognize that 50% motor speed does not translate into 50% of the estimated fan ventilation capacity. A good rule of thumb is that 65% motor speed equates to 50% of fan capacity.
Mistake # 3 - Incorrectly sized heaters
No one wants to have a facility with heaters that can't keep up on the coldest day of the year, stocked with the smallest pigs. In addition, large-capacity furnaces are often priced very similar to smaller furnaces. As a consequence, the furnaces installed in many facilities are over-sized. The net effect is very short run times, with rapid fluctuations in temperatures as the furnace(s) operate.
A correctly sized furnace is one that just turns off on the coldest day of the year with the ventilation system operating correctly. As long as the furnace shuts off occasionally, it is putting out sufficient heat.
A common mistake with furnaces is to install two, 250,000-Btu furnaces in a 1,000-head nursery room. In these rooms, it is not uncommon for the air temperature to fluctuate 5-8°F with every furnace on/off cycle. In addition, the very rapid rise in air temperature associated with the furnaces cycling often results in the ventilation system increasing the ventilation rate, which leads to Mistake #4.
Mistake # 4 - Controller settings
A sure way to increase propane expense is to have the variable-speed fan increase the ventilation rate every time the propane furnace cycles on/off. To illustrate how much this can cost, see Figure 1.
Figure 1 plots the furnace run time (minutes/hour) for a 1,200-head, wean-to-finish room a few days after the pigs were placed into the facility. In addition to three, 250,000-Btu direct-fired propane furnaces, the pens were equipped with propane-fired infrared brooders located in the sleeping zone of each pen.
In the morning, after every furnace run cycle, the variable-speed fans on Stage 1 minimum ventilation increased their speed, effectively increasing the ventilation rate to the room. During this period, the controller was set to turn the furnaces off when air temperature was 1°F below the set point, with the minimum-speed fans increasing speed whenever air temperature was 0.1°F above set point.
At noon, I reset the controller to have the furnaces turn off at 1.5°F below the set point. For the remainder of the day, the run time of the direct-fired propane furnaces was 0 minutes.
The net result of changing the furnace off temperature setting by 0.5°F in this facility was a savings of $4.50/furnace/day in propane when propane was priced at $1.20/gal.
In general, furnaces should be set to turn off at 2°F below the set point temperature of a facility. If the variable-speed fans still occasionally operate after the furnaces cycle on/off, this should be increased. Variable-speed fans should never increase speed following a furnace heat cycle, as variable-speed fan increases are designed to remove heat, causing the furnace to cycle on/off sooner.