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Sizing Minimum Ventilation to Save Heating Energy in Swine Housing

by 5m Editor
30 November 2010, at 12:00am

Wasted heating energy due to over-ventilation is a costly problem for pig producers. Proper sizing of ventilation and use of variable speed fans can improve energy efficiency, according to the latest publication from Iowa State University Extension.

Wasted heating energy is a costly problem for producers, and ventilation accounts for 80 to 90 per cent of the heat lost in swine housing during the winter. Air exchange is critical to providing a healthy environment that fosters efficient pig growth by reducing humidity and noxious gases like ammonia and carbon dioxide. Since under-ventilation creates an unhealthy environment and over-ventilation wastes valuable heating energy, finding the right balance is the key to energy savings and efficiency.

Wean-to-finish buildings provide one of the greatest challenges to efficient heating. A good target for liquefied petroleum (LP) usage is two gallons per pig space per year. Actual usage will depend upon what time of year the weaned pigs are started in the building. Figure 1 shows that over-ventilating by as little as 10 per cent can increase annual LP consumption by 27 per cent. Likewise, over-ventilating by 40 per cent can double LP consumption. Over-ventilating is more common than expected since it is difficult to know just how much air is actually being exchanged.


Figure 1. Over-ventilation of a 1,000-head wean-to-finish facility significantly increases estimated LP usage.

To size minimum ventilation, refer to the rules of thumb that appear in Table 1. As an example, a 1,000-head wean-to-finish building with newly placed pigs should be ventilated at 1,500 cfm (1,000 pigs × 1.5cfm/pig; cfm = cubic feet per minute) during the coldest weather. This rate must be adjusted as the pigs get larger. It seems simple enough to pick a fan rated at 1,500cfm. However, to meet the changing needs of the pigs and to minimise the number of fans required, a controller is used to slow down the fan speed, which causes it to deliver less air. These fans, often called ‘variable speed’ fans, can be used to fine-tune the ventilation rate and save heating costs by reducing LP fuel consumption.

Table 1. Rules of thumb for swine ventilation (adapted from Midwest Plan Service).
Ventilation Rate (cfm/head)
Production Phase Weight (lbs) Winter Minimum Hot Weather
Sow and Litter 450 20 500
Nursery 12-30 1.5 to 2 25
Nursery 30-75 3 35
Finishing 75-150 7 75
Finishing 150 to Market 10 120
Gestating sow 400 14 250

Air Delivery

There are limits to how much a fan can be slowed down using variable speed and still be effective. Figure 2 shows how a 24-inch fan tested at BESS Labs performs with varied voltage. The important thing to notice is that half voltage does not yield half air- flow. For instance, full voltage at 0.10 inches of water will deliver 7,000cfm but at 120V only about 700cfm is delivered. Fans that receive less than half voltage are likely to require replacement more frequently because the electric motor will run hot. Fans operating at low speed also cannot operate against much pressure so it is important to protect fans facing prevailing winds. Figure 3 shows one method of protection.


Figure 2. Flow rate for a 24-inch fan for various voltages

Figure 3. Variable speed fan with wind protector

When selecting fans for variable speed usage, it is good practice to not expect them to deliver less than half their rated airflow at 0.10 inches of water. For instance, in the 1,000-head example above, 1,500cfm is needed and a selected fan might be rated at 3,000cfm. This fan can therefore be used with a variable speed controller to deliver half its rated amount. An additional fan would be required once the pigs grow beyond 75lbs and require more air.

The management of variable speed fans is a complex prospect. Controllers have settings for the minimum speed but the percentage of fan speed is not necessarily related to the percentage of the full air flow rate. However, using the percentage of fan speed as a guideline of full air flow is a good initial approximation. This percentage can then be adjusted based on room conditions. If relative humidity or gases are too high, the percentage can be increased. If the air quality is good, lowering the percentage may be appropriate. With experience, this becomes easier. Further usage of controllers will be addressed in another fact sheet.

Important Points to Remember

  • Size variable speed fans to run no lower than half of the full speed rated capacity.

  • Adjust the speed based on air quality. If the relative humidity is higher than 60 per cent or ammonia seems high, increase the speed. If the relatively humidity and gases are low, try reducing the fan speed slightly.

  • The energy cost of wasted heat exiting the building is far greater than the electricity required to operate variable speed fans.

  • Protect variable speed fans from prevailing winds.

  • Understand your controller and how it interacts with variable speed fans.

Summary

Ventilation is necessary for a healthy production environment but can also be costly in terms of demand for heating energy. Using variable speed fans can help to maintain good air quality while limiting heating fuel usage.

November 2010