• Cleanroom ISO classification
• The layout of the cleanroom
• The number of people working in the room
• The equipment in the room (heat gain)
• The use of a fume hood or biosafety cabinet (air extraction)
• The lighting system
• The pressure differential
• The outside temperature and humidity
• The precision level required

 1) THE LAYOUT OF THE CLEANROOM
The volume of air in the cleanroom will influence the amount of airflow needed. The bigger the room, the more air you need. The width, length, and height of the classified rooms and their layout must be used for HVAC calculations. People often forget that the height of the room directly impacts the CFM (airflow). One way to save on costs is to lower the ceiling. You can see the difference by comparing the airflow in the calculator using the same inputs but varying the size of the room or the height of the ceiling.

2) THE NUMBER OF PEOPLE WORKING IN THE ROOM
The airborne contamination level of a cleanroom depends largely upon the activities in the room and the personnel. People are responsible for most of the particles generated in a cleanroom. Airborne particles, such as skin flakes, cosmetics, perfume, spittle, clothing debris (lint, fibers) and hair, are the usual suspects. When designing the HVAC system, the number of people working in the room at the same time must be taken into account. The more people working in the cleanroom, the more airflow is needed to get rid of the contaminants. People generate airborne contaminants, but also heat. The number of operators is also used to calculate the level of conditioning to compensate for the heat that they produce. People in the cleanroom usually wear coveralls to limit contamination. Therefore, it is important to maintain a comfortable environment, usually between 66.5°F and 70°F (19°C and 21°C).

3) THE EQUIPMENT IN THE ROOM
Similar to people, equipment generates heat and dust. The heat gain produced by the equipment inside the cleanroom is used to determine the cooling required. The equipment in the room, along with the product manufacturing, generate dust that must be removed with the correct amount of air.

4) THE FUME HOOD OR BIOSAFETY CABINET (BSC)
A fume hood or laminar flow cabinet needs constant air supply—just like the cleanroom. This air supply must be accounted for in the cleanroom HVAC calculations. Moreover, if the fume hood exhausts air outside the building, like with a BSC, the exhausted air must be replaced with fresh air. This fresh air will need to be conditioned (temperature and relative humidity). This requires a larger air make-up or air handling unit. If the air extracted from the hood is not accounted for in the HVAC calculations, there might not be enough air pushed into the room to maintain positive pressurization. On the flip side, the pressurization may become too negative and suck in some dirty air from the exterior of the room.

5) THE LIGHTING SYSTEM
The required level of lighting will also affect the heat generated inside the cleanroom and therefore the cooling required. Regular office lighting of 300 lux versus high-precision lighting of 1200 lux will not generate the same quantity of heat.

6) THE PRESSURE DIFFERENTIAL
The pressure must be greater in more stringent classified rooms, so the air leaks towards the lesser clean rooms. Positive pressure prevents dirty air from entering into the cleanroom. In a negative pressurized cleanroom, the opposite occurs; the airflow must be greater in the adjacent room.

7) THE OUTSIDE TEMPERATURE AND HUMIDITY
If air can be re-circulated in the cleanroom, the outside weather only slightly impacts the HVAC system. However, for cleanrooms working with hazardous products, the air make-up can go as high as 100% of fresh air. In these types of cleanrooms, the HVAC systems are more complex. For example, in winter in some regions, the HVAC systems need to take the outside air at -22°F (-30°C) in winter, warm it up to 68°F (20 °C), remove the humidity, and bring it into the room, over and over.

8) THE PRECISION LEVEL REQUIRED
Last but certainly not least, the degree of precision you need will also influence the design of the HVAC system. High-precision temperature control systems can control to ± 0.25°F (± 0.15°C) and to ± 2% for humidity. Cleanrooms rarely need that high degree of precision. In most cases, ± 2°F (± 1°C) precision for temperature and ± 10% for humidity is sufficient. The level of precision depends on the operations in the cleanroom.

HOW DO YOU CALCULATE CFM FOR A CLEANROOM?
You can read this article for a simplified CFM calculation example or try this cleanroom design calculator to get an approximation of the CFM, ACH per classified room, and quantity of lighting fixtures. It also gives you an estimate on the number of HEPA filters that are needed and the number of low air return you need for your cleanroom. Remember though these are only estimations. Airflow must be calculated by an HVAC engineer. Many of the elements listed above are not taken into account in the calculator.