08 Jul Leakages Calculation
It is not necessary to explain here the importance of overpressure in pharmaceutical facilities. Is a GMP requirement to establish a differential pressure (normally 10-15 Pa) between cleanrooms and adjacent less clean spaces to avoid product contamination.
To achieve this, we will need in our design two premises:
- Airtight construction. As tighter is the envelope, less air we will need to achieve the desired overpressure. As more important are the leakages more is the airflow required and higher the operational costs.
- Leakage control. Creating a differential pressure difference is achieved by causing air movement through room leakages openings. This may be through wall openings or gaps around doors. We will not consider duct or pipe penetrations, cracks, etc, as these should be sealed according to best practices of airtight construction.
Leakages calculation is the first step we should know to calculate later the amount of required fresh air, within a complete air balance that we will explain in another post.
The Leakage Formula
Once agreed on the importance of knowing the leakages to successfully design an over-pressurized cleanroom, let us see how to calculate it.
The formula we will use is the following:
Q = volume air flow through the leakage (m3/s)
A = leakage area (m2)
840 = unit conversion factor, dimensionless
n = exponent which can vary between 0.5 and 1.0
Delta P = differential pressure between spaces, in Pascals
Exponent n we normally assume equal to 0.5 in our calculations. Nevertheless, this is dependant of the Reynolds number. As more turbulent is the airflow, n approaches 0.5. In a laminar flow, with low Reynolds, n approaches 1.
Brick walls 0.87 – 1.0
Leakage openings 0.77
Exterior walls 0.67
Door gaps 0.5
Air supply and Return openings 0.5
Shaft walls, floors 0.5
Preparing the Excel File. The Leakage Table
As usual, we will calculate with Excel. This could be another separate sheet from the others needed to complete the calculation.
First, we will prepare a sheet that will be the base for our calculations, covering all scenarios: type or leakages and differential pressures that we will have in our facility. I call this “Leakage Table”. A separate shell called “Leakage calculation” will retrieve the necessary data for calculation from the Leakage Table. The final aspect should be something like this:
In the following section put the door characteristics of your project. You can select the appropriate dimensions: length, width. Also, select an air gap for each type. Usually 2 to 4 mm. In this example, I selected 3 mm for a single and double door and 5 mm for roller doors:
Calculate the perimeter length in column D:
And the leakage area by multiplying the perimeter by the gap:
And now we can calculate the leakage for each element at different pressures by applying the above formula:
With all the table filled, we will assign the term “Dp” to the range of our available pressures, and “Type” for the available types of opening. This is only for my commodity when I use the match formula later.
Preparing the Excel File. The Calculation Sheet
This sheet will serve as a basis for any calculation. We only must adapt this to the characteristics of the project and the gap we want to consider. The next step is to create our calculation sheet, named “Leakage Calculation”.
Let’s calculate the leakages for the following example. Below is represented a couple of rooms, PAL In and PAL Out with their respective doors and pressures referred to adjacent spaces. The arrows show the air leakages:
With this layout our calculation sheet we write the room number, description, door size and reference, pressure:
And we find in the table we prepared earlier the corresponding leakage for each door and pressure, by combining the Excel functions Index & Match:
For each room, we will have leakage in and out by adding all of them to achieve the desired pressure.
Established the leakages for each room, we can go on with the air balance for each room to know the amount of fresh air we need. But we will explain this in another post.