Livestock House Fans and Orchard Frost Fans: How to Size and Compare Them

Two fan jobs, one buying habit

A pig-house exhaust fan and an orchard frost fan look like very different machines, yet both are bought the same way: you start from the air the site needs and the conditions the fan will work against, then check each quotation against that duty. The free-air number printed in a catalogue is only the starting point.

On the site, house fans appear under livestock equipment, while frost fans sit with orchard and garden machinery. Below are the questions to settle before asking for a price.

Read airflow at static pressure, not free air

Every fan delivers less air as the resistance it pushes against rises. That resistance is the static pressure across the wall, measured in pascals or inches of water. A rated fan comes with a table or curve of airflow against static pressure, and the better designs have a flat curve, so airflow holds up as pressure climbs [1].

Penn State Extension puts the total static pressure of a typical livestock or greenhouse ventilation system at roughly 0.08 to 0.20 inch of water, which is about 20 to 50 Pa. For a first comparison it suggests reading each fan at 0.10 inch (about 25 Pa) or 0.125 inch, then moving to your own calculated pressure once the design firms up [1]. Inlets, dirty shutters, guards and wind blowing against the exhaust side all add to that pressure; a 10 mph headwind alone adds about 0.05 inch [1].

The Xinda 710 axial-flow fan page lists airflow at several pressure points for exactly this reason. Pick the row that matches your building, and ask any other maker for the same kind of table.

Sizing exhaust fans for a pig house

Fan sizing begins with the animals, not the fan. Purdue Extension's worksheet gives per-head rates in three bands: a cold-weather minimum for moisture control, a mild-weather rate, and a hot-weather rate. For a finishing pig of about 70 to 100 kg it lists 5 to 10 cfm in cold weather depending on floor type, 35 cfm in mild weather and 120 cfm in hot weather; a sow and litter needs up to 20, 80 and 325 cfm [2]. One cfm is about 1.7 cubic metres per hour, so 120 cfm per finisher is roughly 204 m³/h.

Multiply each rate by the number of animals to get three totals, then build fan stages to cover them: a small continuous stage for the winter minimum, and further fans that switch on in steps of 2 to 4 °F as the room warms [2]. For a 1,000-head finisher room the hot-weather total is near 204,000 m³/h. Divide that by each fan's airflow at your design pressure, not at 0 Pa, to get the fan count.

Whole-farm suppliers such as Xinda Farm & Livestock Machinery and Zhenggu Livestock Equipment quote ventilation as part of a house package, which helps when you want one party to answer for inlets, controls and fans together. A stand-alone fan price is still useful as a benchmark against the package.

Comparing efficiency on tested data

Independent laboratories such as the BESS lab at the University of Illinois run fans through standard tests and publish airflow and efficiency in cfm per watt at set pressures. Penn State's comparison of thirty-five tested 36-inch fans shows why this matters: at 0.10 inch, airflow ranged from about 6,200 to 13,000 cfm, and the best fans were close to twice as efficient as the worst [1]. A rule of thumb of 10,000 cfm for a 36-inch fan is fine for a rough plan, but final design should use rated data [1].

Accessories change the result. Shutters cut airflow and efficiency by 10 to 25 percent, with discharge-side shutters costing more than inlet-side ones, and guards take off a further few percent [1]. Ask for test data with the shutter and guard you will actually install, and ask which test standard and laboratory produced it.

How a frost fan works: the inversion

On a clear, calm night the ground loses heat fast, so the air near the crop becomes colder than the air a few metres higher. This layering is a temperature inversion. A frost fan, or wind machine, blows that warmer upper air down and mixes it into the cold layer at crop height [3][4].

UC Cooperative Extension in San Luis Obispo gives a simple worked case: if the air is 34 °F at 35 feet and 28 °F at 5 feet, mixing brings the 5-foot reading to about the average, 31 °F [4]. FAO's frost-protection manual expresses this as a rise of about one third of the inversion strength at 1.5 m within the area the machine influences, with better results close to the tower and under stronger inversions [3].

The corollary is that a frost fan needs an inversion to work with. FAO advises against wind machines where inversions are weak, and in winds above about 2.5 m/s, when an inversion is unlikely to form [3]. UC Extension recommends logging temperature at two heights through one or more frost seasons before buying [4].

Coverage, siting and what goes with a frost fan

FAO describes a typical conventional machine as a fan of 3 to 6 m diameter on a tower about 10 to 11 m high, tilted down by about 7 degrees and rotating around the tower once every 4 to 5 minutes. It puts the need at roughly 75 kW for each 4 to 5 ha, with protection fading towards the edge and often drifting into an oval downwind [3]. Machines are usually started near 0 °C, or at the crop's critical temperature [3].

Smaller units such as the Sanxin SX-FSD15-360 are sold with a stated area. Ask Sanxin which inversion and wind conditions that area assumes, and compare its motor power per hectare with the FAO figure.

The orchard floor matters too. Grass taller than about 5 cm kept soil 1 to 3 °C cooler than bare soil in the FAO data, and firm, moist soil stores more daytime heat [3]. Keeping the sward short with a tracked remote mower such as those from Changxu or a remote-controlled mower from other makers supports the fan. Wetting the soil a day or two ahead with a clean-water pump and lay-flat hose works the same way, and the wider irrigation range covers both.

FAO reports that under-tree sprinklers combined with a wind machine can double the benefit of either alone, while over-tree sprinkling with a fan running is harmful because the moving air increases evaporation [3]. If you add an impact sprinkler system, keep it below the canopy.

What to put in the enquiry

For house fans, send the house plan, animal numbers and stage, your three ventilation totals, and the design static pressure. Ask for airflow and power at that pressure with shutter and guard fitted, the test laboratory, the motor's electrical data, and what the control does at reduced speed. Plan a standby generator and an alarm with the electrical contractor, because a sealed house cannot go long without fans.

For frost fans, send a site plan, crop, block size, and at least a few nights of two-height temperature logs. Ask the maker for tower height, fan diameter and speed, rotation time, drive power, the coverage claimed and the conditions behind it, the automatic start temperature, and how faults are signalled. Sanxin lists both house and orchard fans, so one enquiry can cover both if your farm needs them.

Source notes

[1] Penn State Cooperative Extension, Agricultural and Biological Engineering — Selecting Rated Ventilation Fans (G85), E. F. Wheeler (1996 (PSU 5/96; PDF dated 2001)) [2] Purdue University Cooperative Extension Service — Worksheet for Sizing Livestock Housing Environmental Control Systems (AE-109), D. D. Jones and W. H. Friday (undated extension worksheet) [3] FAO (Food and Agriculture Organization of the United Nations) — Frost Protection: Fundamentals, Practice and Economics, Vol. 1 (Snyder and de Melo-Abreu), chapters 6 and 7 (2005) [4] University of California Cooperative Extension, San Luis Obispo County (M. Battany) — Measuring temperature inversion conditions to predict the benefits of wind machines for frost protection (2019 (upload date))