Volume 6, Number 2, Spring 2006

The Application of Differential Pressure Analysis

In Determining the Extent of

Outdoor Air Infiltration into Buildings


Robert Choate

Department of Mechanical Engineering

Western Kentucky University



Rod Handy

Department of Mechanical Engineering Technology

Purdue University



Kevin Schmaltz

Department of Mechanical Engineering

Western Kentucky University






The infiltration of humid outdoor air is a major problem for many manufacturing facilities.  In particular, manufacturers that produce such items as absorbent hygiene products or humectants must control the humidification conditions in order to ensure product quality and high first-run yield.  This paper describes a technique of using differential pressure analysis in order to estimate the volume of air entering such a facility through its various cracks/crevices, windows, doors, and other openings.  A mathematical relationship, based upon a best-fit graphical estimation, was presented that related the pressures of the air imbalance with the rate of outdoor air infiltration.  With some specific conditional modifications, this approximation could also be applied to other comparable scenarios found in the manufacturing environment.




An air mass imbalance can result in a pressure difference between the inside and outside of the manufacturing space.  This pressure difference is readily noted when, after entering a particular space, the door slams shut immediately on manufacturing personnel.  In many cases significant infiltration airflow velocity is evident in the large passageways and doorways leading into a particular manufacturing space from adjacent areas such as raw materials storage, maintenance shops, and finished goods spaces.  A major issue with an imbalance air condition at many manufacturing sites during the summer months is the infiltration of outside air at unacceptable moisture content levels.


The affects of humidity on office and manufacturing personnel is widely documented in the literature 1-5.  However, research efforts or case studies targeting humidity concerns related to product performance or quality are somewhat limited.  Some production items by their very nature, such as absorbent hygiene products and various humectants, must be manufactured in controlled environments free from the potential detrimental effects of high humidity conditions.  Efforts have been made to cut down on the amount of outdoor air that infiltrates a building from various cracks, crevices, and other openings in buildings that house these types of operations as well as others 5-8.


Mathematical relationships can be used as a means of estimating the extent of a particular outdoor air infiltration.  The literature provides several accounts of mathematically estimating specific airflow balances/imbalances and pressure differentials 7-10.  The proper use of these relationships can assist in the development of an effective and efficient strategy for taking corrective action when the environmental conditions are less than optimal.


The purpose of this paper is to describe a technique used to conduct a differential air pressure analysis at a facility that experienced problems with moisture control and an excessive negative pressure differential plant wide.  In order to estimate the severity of the problem, differential pressure measurements were performed around the perimeter of the manufacturing space under a range of manufacturing space operational conditions.  These measurements were then used to estimate the extent of outside air infiltration.




Based on the differential pressure, the infiltration rate into a particular manufacturing space can be estimated by using one of two methods: the air change method and the crack method 10.  The crack method is generally considered to be the most accurate when the crack and pressure characteristics can be properly evaluated.  In this investigation, the pressure differential between the inside and outside of the manufacturing space was measured, and therefore, known under various operating conditions.  However, the accuracy of predicting air infiltration is restricted by the limited information on the air leakage characteristics of the many components: walls, doors, windows, and roofing systems, that makeup the structure.


The air change method requires an assumption of the number of air changes per hour (ACH) that the manufacturing space will experience.  For this analysis a common range of 0.5 ACH (very low) to 2.0 ACH (very high) was used 10.  The infiltration rate is related to ACH and space volume as follows:





                         = infiltration rate, CFM or m3/s

V = gross space volume, ft3 or m3

C = 60 for English units; 3600 for SI units

The second method, the crack method is generally more accurate than the air change method.  It is assumed that outdoor air infiltrates the indoor space through cracks around doors, windows, lighting fixtures, and joints between walls and floor and even through the building material itself.  The amount depends on the total area of the cracks, the type of crack and the pressure difference across the crack.

The volume flow rate of infiltration may be calculated by:



                        A = effective leakage area of the cracks

C = flow coefficient, which depends on the type of crack and the nature of the flow in the crack

                        DP = outside-inside pressure difference, Po - Pi

n = exponent depending on nature of the flow in the crack, 0.4 < n < 1.0.


The pressure difference in Equation (2) results from the effects due to the wind (dynamic), due to the stack effect (elevation) and due to building pressurization or depressurization.  For the purpose of this investigation, building pressurization (or depressurization) was only considered.  To include secondary effects would require measurements or estimates of external building wind velocity and an estimate of the stratification of the air density due to building height.



To use the ACH method, the manufacturing space gross volume must first be estimated.  Based on measurements performed and provided data, the estimated gross space volume for this particular manufacturing space was 2,440,000 ft3.  Applying Equation (1) with a 0.5 ACH, the estimated infiltration rate is 20,300 CFM.  Similarly, at the higher rate of 2.0 ACH, the infiltration rate is 81,300 CFM.  Therefore, it is reasonable to assume the actual volume is between these two bounding conditions.

To use the crack method, the building differential pressure was measured and compared under several different operating conditions of the building air conditioning units and of the production system air handling units.  The calculated air balance rates under these conditions were used to yield the following experimentally fit relationship, also shown in Figure 1.







Figure 1: Building differential pressure with respect to negative airflow volume


The exponent in Equation (3) of 1.88 is within the expected range of the exponent “n” used in Equation (2) (1/1.88 = 0.532).  The conditions of near-operation supply rate of the air conditioning units and exhaust airflow rate of the air handling units of the production systems also provided a check of the reasonableness of these results.

With a normal net inflow of 123,950 CFM, the differential pressure between the building and the outside based on Equation (3) is a negative 0.100 inches of water.  Using published experimental data for typical building surfaces 10, a differential pressure of 0.100 inches of water will generate estimated wall and roof infiltration of 0.30 CFM/ft2 for estimated infiltration of 8,400 CFM through the walls and 36,700 CFM through the roof for a total building infiltration of 45,100 CFM, which is in the range estimated by the ACH method.  The difference (78,850 CFM) between the wall and roof infiltration estimated by the crack method and the total infiltration estimate of 123,950 CFM can be attributed to window and door infiltration, which were not completely characterized in this study.

An artificially lower negative air balance condition was also created during the collection of differential pressure data.  For these conditions, a 62,150 CFM flow rate resulted with a differential pressure of 0.027” of water between the inside and outside of the manufacturing space per Equation (3).  At a differential pressure of 0.027 inches of water, the estimated wall and roof infiltration is 0.14 CFM/ft2.  Under this condition, the infiltration is estimated at .1960 CFM through the walls and 17,150 CFM through the roof for a total infiltration of 19,110 CFM or a reduction of 21,790 CFM (approximately 54%).

This reduction in the infiltration rate estimate is illustrative of the potential benefit for maintaining control of the conditioned air space pressure, which could be realized through the adjustment of the air balance within the manufacturing space.




In summary, a technique for estimating air infiltration in a manufacturing facility was presented.  A mathematical relationship was elucidated which provides the means for approximating the values for building infiltration rates of outdoor air.  With specific conditional modifications, this relationship could be used to predict air filtration estimates under numerous manufacturing conditions.




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