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.