Introduction

If you arrive here for the first time, you should know that this article takes part in a series of design considerations for cleanroom HVAC calculations. Previous considerations you should have clear to proceed with the cleanroom air balance are the following:

• Air supply calculation. You can get an idea of this by reading this article: About Air Changes
• Leakages calculation. This is important to know the minimum Fresh Air required to get the desired room pressure. This is explained in this article: Leakages Calculation

The Air Balance

The air balance for a cleanroom or a set of them for an individual Air Handling Unit (AHU) system is quite simple.

First of all, we can calculate the airflow supply to the room. This is obtained normally from the number of air changes required, or the heat gains of the room.

When we supply a determined amount of air into a cleanroom, some part of it will not return to the system due to the leakages. In the opposite way, it could be possible that a certain amount of air of any next overpressurized room can be infiltrated into our room. This air will be added to the air return ducting system.

Even, our room can have an outdoor exhaust, as, for example, a dedicated extraction for a washing machine. In this case, we will consider this amount of air in the same category. That means we will not add this air to the air return ducting system.

Through the leakage calculation, and the pressure requirement we can determine the minimum amount of required fresh air.

An Example

Let’s calculate an example. Imagine we have the following specifications:

• Room Area: 50 m² 
• Height: 3 m
• Grade: C
• Minimum Air Changes: 30
• Required Pressure: 30 Pa
• Extraction hood: 900 m³/h

With these data, we can calculate the air supply by multiplying the ACH (30) by the room volume (150 m³), obtaining 4500 m³/h.

So, our system and the remaining unknown data is as follows:

The next step is to know the leakages and infiltrations. For this, we have to know the surroundings of our room in order to calculate them. If we have a layout as shown below, and we follow the procedure described in the leakages calculation post (remember it depends on the pressure!), we get the following values:

So we can now introduce the new findings into our system (infiltrations and leakages):

Then, let’s see how much airflow is returning to the AHU by solving this simple balance (sum of inputs equals the sum of outputs):

Air Supply + Infiltrations = Air Return + Leakages + Exhaust

4500 + 107 = Air Return + 236 + 900

Air Return = 3471

The Fresh Air needed is straightforward to get:

Fresh Air + Air Return = Air Supply

Fresh Air + 3471 = 4500

Fresh Air = 1029

By the last, you should consider that the ductwork is not 100% tight. Remember the post Pharmaceutical HVAC Ductwork!

To include 1-2% of air supply leakages and air return infiltrations could be a good idea for the air balancing and the thermo-hygrometric of the system.