Ventilation Design Worksheet for Swine Housing1
This worksheet is intended to guide you through a simplified procedure of designing a ventilation
system. It will help you to decide on fans needed, inlets needed and how to set up the controls to create a prescribed environment. Ventilation design is an important factor in ensuring the
health and superior productivity of your animals. If information is needed beyond this simplified approach, seek agricultural engineer.guidance from an
Step 1. Winter Ventilation Rate
The winter ventilation rate is the rate used during cold weather to maintain air quality within the
building while minimizing the loss of heat. To find this, determine the number of animals in the ventilating air space. Find the "Cold Weather Rate" from Table 1. Multiply the number of animals
by the ventilation rate.
For Winter Ventilation Rate:
_____ Animals x _____cfm/head = _____cfm
(A)
Table 1. Recommended Ventilation Rates2
|
Housing
|
Weight
(lbs)
|
Cold Weather Rate
cfm/unit
|
Hot Weather Rate
cfm/unit
|
|
Sow and Litter
|
400
|
20
|
500
|
|
Nursery Pigs
|
12-30
|
2
|
25
|
|
|
30-75
|
3
|
35
|
|
Finishing Pigs
|
75-150
|
7
|
75
|
|
|
150-250
|
10
|
120
|
|
Gestating Sows
|
325
|
12
|
150
|
|
Boars & Breeding Sows
|
400-325
|
14
|
300
|
Example Step 1.
Consider a farrowing house (22' by 66') containing 24 crates. What is the winter ventilation rate?
24 Animals x 20 cfm/ unit = 480cfm
(A)
Step 2.Summer Ventilation Rate
The hot weather ventilation rate is the ventilation rate which is practical to provide during the
summer. At some point, the temperature will be higher than the ideal setting. Under this condition, the best that a system can do efficiently is to move enough air to maintain indoor
conditions only a few degrees warmer than the outside conditions. This is calculated using the same procedure as in step 1 except the rates from the last column of Table 1 are used to calculate this rate.
Summer Ventilation:
_____ Animals x _____cfm/head = _____ cfm
(B)
Example Step 2.
What is the hot weather ventilation rate for the farrowing house?
24 Animals x 500 cfm/ unit = 12,000 cfm
(B)
Step 3. Intermediate Ventilation Stages
In most mechanical ventilation systems there should be at least 4 stages, the cold weather rate,
the hot weather rate and 2 stages in between. In actuality the number of stages will depend on the total span of the ventilation rates and the availability of appropriate fans.
Cold Weather Rate = (A)
Hot Weather Rate =
(B)
Intermediate Stage 1 = {(B) - (A)}
+
(A) (C)
3
Intermediate Stage 2 = {(B) - (A)}
+
(C) (D)
3
Example Step 3.
Cold Weather Rate = 480 cfm (A)
Hot Weather Rate = 12,000 cfm (B)
2nd Stage = {12,000 - 480} + 480 = 4320 cfm (C)
3
3rd Stage = {12,000 - 480} + 4320 = 8160 cfm (D)
3
Step 4 Fan Capacity
The fan capacity for each stage should be determined. This capacity is then compared with fan
performance tests data to determine the availability of fans. The cold weather ventilation rate should be provided using continuous running fans. Use static pressure ratings of 0.125 to 0.1
inches of water for winter fans. Static pressures of 0.05 may be used for summer fans. Fan stages may be adjusted to accommodate specific fan sizes. For more information on fan
selection see "Choosing Fans for Livestock and Poultry Ventilation" which is ISU Extension publication Pm-1587.
Continuous Fan = (A)cfm
2nd Fan = (B) - (A)
= _____ cfm
3rd Fan = (C)
- (B) = _____ cfm
4th Fan = (D) -
(C) = _____ cfm
Example Step 4
Continuous Fan = 480cfm
2nd Fan = 4320 - 480 = 3840 cfm
3rd Fan = 8160 - 4320 = 3840cfm
4th Fan = 12,000 - 8160 = 3840cfm
Fan test data is examined to find adequate fans. It is found that an 8" fan was tested to deliver
521 cfm at 0.1 inches of water. This will be selected as our continuously running minimum ventilation fan. Likewise, an 18" fan was tested to deliver 3120 cfm at 0.1 inches of water. Two
of these will be used as the second and third ventilation stages. This means that at three stages, only 6761 cfm will be provided. To complete our system we will look for a fan to deliver
approximately 5239 cfm (12,000 minus 6761). A 24" fan is found which will provide 5620 cfm at 0.05 inches of water. To summarize, the actual fan stages appear below.
Table 2. Actual fan stages for the example.
|
Stage
|
Model
|
Rated CFM
|
Stage CFM
|
|
Continuous
|
8" fan Model X
|
521
|
521
|
|
2nd
|
18" fan Model Y
|
3120
|
3641
|
|
3rd
|
18" fan Model Y
|
3120
|
6761
|
|
4th
|
24" fan Model Z
|
5620
|
12,381
|
Step 5 Inlet Area Needed.
Inlets can be made with continuous slot inlets or commercially available, self-adjusting inlets.
These two cases will be handled separately below.
Continuous Slot Inlets
Continuous slot inlets, Figure 1, are used to bring air from the attic in winter and the eave in the
summer. They generally are adjusted by hand to different season conditions. These are generally used on one eave for buildings 20' wide or narrower and on both sides for buildings 20 to 40'
wide. Wider buildings use additional center-ceiling inlets. The inlet baffle adjustment is sized such that air will enter at a speed of 800 feet per minute. This will cause the air to be thrown
further into the building and cause proper air mixing. The throat of the inlet should be sized at least big enough to accommodate the hot weather ventilation rate. Continuous inlets should be
discontinued within 5 feet of sidewall fans.
Cold Weather Rate
Continuous Rate/ 800 cfm/sq ft = _____ sq feet
2nd Stage
2nd Stage Rate / 800 cfm/sq ft = _____ sq feet
3rd Stage
3rd Stage Rate / 800 cfm/sq ft = _____ sq feet
Hot Weather Rate
4th Stage Rate / 800 cfm/sq ft = _____ sq feet
To determine the width of the opening at any one rate use the follow formula.
Required Area of Inlet (sq feet) x 12 in/ft = width of continuous inlet
Total Length of Continuous Inlet (ft)
Figure 1 A Continues Slot Eave Inlet.3
Example Step 5.
In the farrowing house example, the building was 66 feet long. Because it is relatively narrow
building, only one eave will have an inlet and the opposite sidewall will have the fans mounted in it. For this example, there is 66 feet of sidewall. Use actual stage rates to calculate inlet opening.
Continuous Stage
521cfm / 800 cfm/sq ft = 0.65 sq feet
0.65 sq feet / 66 ft x 12 in/ft = 0.12 in opening
Stage 2
3641cfm / 800 cfm/sq ft = 4.6 sq feet
4.6 sq feet / 66 ft x 12 in/ft = 0.83 in opening
Stage 3
6761 cfm/ 800 cfm/sq ft = 8.5 sq feet
8.5 sq feet / 66 ft x 12 in/ft = 1.5 in opening
Stage 4
12,381 cfm / 800 cfm/sq ft = 15.5 sq feet
15.5 sq feet / 66 ft x 12 in/ft = 2.8 in opening
The throat for this inlet should be larger than the largest opening (2.8 inches). This opening in
the ceiling would probably be at least 3 inches in this case.
Small openings with a continuous slot inlet are hard to manage because boards tend to warp
creating gaps that are bigger and smaller than desired. This will cause drafts in some places and stagnant air in others. An alternative is the use of commercially available, self-adjusting inlets.
Commercial Self-Adjusting Inlets
Commercially available inlets, Figures 2 & 3, generally come with a rating of maximum air flow
delivery. In the case of our example, 12,381 cfm is required. Many inlets are rated at 1200 cfm. This means that only 10 are required (one every 6 feet). In winter, every other inlet would
probably be closed to force more air through half the inlets. This would promote a better air jet and, therefore, better mixing. Inlets should be adjusted so that they deliver a jet of air at 800
fpm during any conditions.
For this example, air distribution would be adequate. In some cases the number of inlets should
be chosen based on distribution needs rather than rate. Locate inlets so they are no more than 10 to 12 feet apart in the direction that they do not deliver air. Locate inlets so they are no
more than 24 feet apart in the direction that they deliver air. Some inlets may be closed during the winter to permit easier adjustment of inlets.
Step 6 Required Attic Opening.
One thing that is often overlooked in the use of inlets that take air from the attic is the opening
that allows air from the outside into the attic. If this opening is too small it will restrict the air flow and will not allow the inlets to properly regulate the air flow. To prevent restricted airflow,
one square foot of opening should be allowed for every 400 cfm. This opening should be big enough to accommodate hot weather ventilation rates. A portion of the openings may be closed during winter.
Air flow rate cfm / 400 fpm = ______ square feet of attic opening
Example Step 6.
As calculated below, the opening into the attic should be 31 square feet during the summer. This
area may be provided through gable louvers, eave vents or ridge vents. Square footage should be based on the areas of the actual openings and not the overall louver size. If small openings
are used, the actual useable opening is only 66 percent of the entire opening. During winter, only 9 square feet may be provided so that north-side openings may be closed to prevent snow
from drifting into the attic. This is based on the needs for the 2nd stage to prevent choking of the system during intermittent weather.
Winter
3641 cfm / 400 fpm = 9 square feet of attic opening
Summer
12,381 cfm / 400 fpm = 31 square feet of attic opening
Figure 2 A System Using Ceiling Inlets.
Figure 3 Two Sided Ceiling Inlet.
Step 7 Thermostat Settings
Thermostats should be set so that no fan stage operates except the cold weather rate while
the furnace is on. Temperatures should be set at a comfortable level for pigs. Use "Thermal Environmental Guidelines for Swine", ISU Extension publication Pm-1586 for the initial settings.
Watch pigs to determine their comfort level and make appropriate adjustments as needed. Other stages should be set 3 to 5 degrees apart, depending on the precision of the controller.
Example Step 7
Assume that we are trying to maintain 65oF in the farrowing house. The ventilation stages would be as follows:
|
Stage
|
Stage Capacity
|
Total Capacity
|
Thermostat Setting
|
|
Continuous Fan
|
521 cfm
|
521 cfm
|
on continuously
|
|
Heaters
|
3000 Btu/hr-sow
|
72,000 Btu/hr
|
on at 64¡F
|
|
2nd Stage
|
3120 cfm
|
3641 cfm
|
on at 68¡F
|
|
3rd Stage
|
3120 cfm
|
6761 cfm
|
on at 71 ¡F
|
|
4th Stage
|
5620 cfm
|
12,301 cfm
|
on at 74 ¡F
|
Step 8 Sketch the System.
Sketch the ventilation system including fans, inlets, heaters and thermostat locations.
_________________________
1Developed by Jay D. Harmon, Ph.D., P.E., Assistant Professor and Extension Agricultural
Engineer, Agricultural & Biosystems Engineering Department, Iowa State University, Ames, IA.
2From MWPS-8 "Swine Housing and Equipment Handbook", MidWest Plan Service, Ames, IA.
3Figure from "Ventilation Worksheet for Dairy and Swine Buildings" by Brian J. Holmes, University
of Wisconsin - Extension, Madison, WI.
Iowa State University
University Extension
Ames, IA January 1997
Back to Livestock
|
