The HEPA filtered air flow in cleanrooms can be either one pass or recirculating.
Figure 1. Air flow simulation: Recirculating Modular Cleanroom
In a recirculating cleanroom, the air is pushed by the HEPA fan filter units from the negative pressure plenum into the cleanroom. The clean air flows to the cleanroom floor then goes thru low wall air returns back to the plenum. The majority of the air goes back thru the HEPA fan filter units gets cleaned again and is pushed back into the cleanroom in a circle. The hot air can be returned to the AC unit for cooling after which it is injected back into the plenum. Without air conditioning, the cleanroom would get very hot due to heat from HEPA fan filter units, cleanroom lighting, humans in cleanroom and process heat load. Humidifiers and dehumidifiers can also be added to give the cleanroom RH control. The AC, humidifier or dehumidifier are ducted to send the conditioned air to the plenum and is mixed with the recirculating air.
Advantages of a Recirculating cleanrooms
- Can have cleanroom temperature control
- Can have cleanroom humidity control
- Will have lower particle counts as the clean air is filtered over and over again
- Longer HEPA fan filter life as the HEPA fan filter units are filtering already clean air
Disadvantage of Recirculating cleanrooms
Table 2: One Pass Cleanroom - Air Flow
A one pass cleanroom draws air from outside the cleanroom into the plenum where the HEPA fan filter units push the clean filtered air into the cleanroom. The air goes to the floor then exits the cleanroom thru exhaust grills located on walls. A one pass cleanroom does not have temperature control and relies on the cooling of the warehouse surrounding the cleanroom to provide cooling.
Advantages of one pass cleanrooms:
- Lower cost since you are not installing dedicated AC
- Can take up less space
Disadvantages of one pass cleanrooms:
- Since cleanroom personnel typically are wearing cleanroom hoods, jackets and shoes, covering it can get uncomfortably warm inside of a one pass cleanroom.
- A one pass cleanroom transfers the cleanroom heat load to the warehouse. If the AC system of the warehouse is not designed to handle this load, the temperature of the warehouse can be raised significantly.
- It is also not possible to control the humidity of a one pass cleanroom. The one pass cleanroom will simply have the humidity of the air from the warehouse.
- The HEPA fan filter units will not last as long in one pass cleanroom because dirty outside air is constantly being introduced.
The author Anthony Chien has worked at American Cleanroom Systems for the past 10 years. He has more than 40 years of cleanroom experience. Anthony has a BS and MS in Electrical Engineering specializing in semiconductor manufacturing from the University of Illinois. American Cleanroom Systems is a design-build modular cleanroom manufacturer based in Rancho Santa Margarita CA. ACS is an expert in ISO-5 thru ISO-8 (class 100 to class 100k) modular cleanrooms for pharmaceutical cleanrooms, medical device cleanrooms, and industrial cleanrooms.
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