What is the Difference Between Negative Pressure and Positive Pressure Cleanrooms?

Cleanroom pressure refers to the relative pressure between the cleanroom and the external environment. Controlling and maintaining this pressure differential is crucial for ensuring the stable operation of the cleanroom. Negative pressure cleanrooms and positive pressure cleanrooms are two common types of cleanroom configurations. This article will delve into the differences between negative-pressure cleanrooms and positive-pressure cleanrooms, providing you with a deeper understanding to make informed decisions when choosing between the two types.

Cleanroom Pressure

 

The definition and principle of negative pressure cleanrooms

A negative pressure cleanroom is a type of cleanroom where the internal air pressure is lower than the external environment, allowing outside air to flow into the room while preventing indoor air from escaping to the outside.

The definition and principle of negative pressure cleanrooms

 

The definition and principle of positive pressure cleanrooms

A positive pressure cleanroom is a type of cleanroom where the internal air pressure is higher than the external environment, causing air to flow out of the room. In a positive-pressure cleanroom, filtered air is continuously supplied into the room through HEPA filtration and the cleanroom HVAC system. If the door or window of the cleanroom is opened, air will be expelled from the room, preventing contaminated or unfiltered air from entering the interior.

The definition and principle of positive pressure cleanrooms

 

Similarities between positive-pressure cleanrooms and negative-pressure cleanrooms

Similarities between positive-pressure cleanrooms and negative-pressure cleanrooms

Positive pressure cleanrooms and negative pressure cleanrooms both require:

  1. HEPA filters and HVAC systems
  2. Self-closing doors and appropriately sealed windows, walls, ceilings, and floors
  3. Multiple air changes per hour
  4. Online pressure monitoring systems

 

Differences between positive pressure cleanrooms and negative pressure cleanrooms

Differences between positive pressure cleanrooms and negative pressure cleanrooms

 

Pressure Control

Negative Pressure Cleanrooms: The internal pressure of a negative pressure cleanroom is lower than the ambient pressure outside, causing air to flow from the outside to the inside, preventing contaminants inside the cleanroom from spreading to the outside. The pressure control in negative pressure cleanrooms is typically between -5 to -15 pascals.

Positive Pressure Cleanrooms: The internal pressure of a positive pressure cleanroom is higher than the ambient pressure outside, causing air to flow from the inside to the outside, preventing unpurified air from contaminating the interior. Typically, the pressure control in positive pressure cleanrooms is between 10 to 15 pascals.

 

Airflow Direction

Airflow Direction

Negative Pressure Cleanrooms: The airflow direction in negative pressure cleanrooms is usually from the external environment to the interior of the cleanroom, and then the indoor air is expelled through the negative pressure control device to ensure that indoor air does not leak to the external environment.

Positive Pressure Cleanrooms: The airflow direction in positive pressure cleanrooms is typically unidirectional from the clean area to the non-clean area, either from top to bottom or from one side to another.

 

Calculation of Air Volume

The calculation of air volume in a positive-pressure cleanroom is determined by the cleanroom’s volume and the number of air changes. The formula is as follows: Q = V × N, where:

Q is the total required airflow (cubic meters per hour);

V is the volume of the cleanroom (cubic meters);

N is the required number of air changes (changes per hour).

When determining the air volume, factors such as the layout of the cleanroom, equipment, and personnel activities also need to be considered.

Therefore, when designing a positive-pressure cleanroom, the required number of air changes needs to be determined first, and then the total airflow required is calculated based on the volume of the cleanroom and the required number of air changes.

The calculation of air volume in a negative-pressure cleanroom differs from that in a positive-pressure cleanroom because it needs to consider the entry of air from outside. Typically, the formula for calculating the air volume in a negative-pressure cleanroom is as follows: Q = V × (N+1), where:

Q is the total required air flow (cubic meters per hour);

V is the volume of the cleanroom (cubic meters);

N is the required number of air changes (changes per hour);

1 represents additional air flow to supplement air entering from the outside.

The calculation of air volume in a negative pressure cleanroom needs to consider maintaining negative pressure inside the cleanroom to prevent harmful substances from leaking out. Therefore, in addition to meeting the required number of air changes, additional air flow is needed to supplement the air entering from the outside to maintain negative pressure. When designing a negative pressure cleanroom, first determine the required number of air changes and the required cleanliness level, and then calculate the total air flow based on the volume of the cleanroom and the additional air flow.

 

Air Change Rate

Air Change Rates

The air change rate refers to the number of times the indoor air is replaced with fresh air within a certain period of time.

Generally, the air change rate in a positive pressure cleanroom depends on factors such as the required cleanliness level, application requirements, and working environment. Typically, the air change rate in a positive pressure cleanroom is between 20 to 30 changes per hour. The selection of the air change rate needs to be determined based on specific application requirements and standards. For example, for applications requiring high cleanliness, such as pharmaceutical production or semiconductor manufacturing, a higher air change rate may be required to ensure the cleanliness of the indoor air. When determining the air change rate, factors such as the volume of the cleanroom, spatial layout, personnel activities, and equipment operation need to be considered.

Since the design purpose of a negative pressure cleanroom is to prevent the leakage of harmful substances from inside, it requires a higher air change rate than a positive pressure cleanroom.

Generally, the air change rate in a negative pressure cleanroom is usually between 30 to 40 changes per hour, or even higher, depending on application requirements and cleanliness levels. A higher air change rate can ensure timely renewal of indoor air, reduce the accumulation and leakage of harmful substances indoors. When determining the air change rate, factors such as the volume of the cleanroom, spatial layout, working environment, operation requirements, and the generation and emission of harmful substances need to be considered. It is recommended to comprehensively consider the actual situation and follow relevant industry standards and regulations to ensure the operational effectiveness and safety of the negative pressure cleanroom.

 

Material Selection

Positive Pressure Cleanroom: Typically, materials that are easy to clean and do not generate dust are preferred, such as stainless steel and smooth plastics.

Negative Pressure Cleanroom: Corrosion resistance and sealing performance also need to be considered to prevent harmful substances from leaking to the external environment.

 

Equipment Configuration

Equipment Configuration

Positive Pressure Cleanroom: Typically includes high-efficiency filters, positive pressure control devices, air shower devices, and air conditioning systems.

Negative Pressure Cleanroom: Typically includes high-efficiency filters, negative pressure control devices, and air exhaust devices.

 

Application Scenarios

Negative Pressure Cleanroom: Mainly used in places where it is necessary to prevent the spread of indoor pollutants, such as isolation wards for infectious diseases in hospitals and pathogen processing rooms in laboratories.

Positive Pressure Cleanroom: The application of positive pressure cleanrooms is more extensive, mainly used in places that require a clean environment, such as pharmaceutical factories, electronic manufacturing plants, and biological laboratories.

 

Conclusion

Because positive pressure cleanrooms and negative pressure cleanrooms have significant differences in design and application, we need to choose the appropriate and correct type of cleanroom based on specific needs and application scenarios.

 

FAQ:

 

What are the advantages of positive pressure cleanrooms and negative pressure cleanrooms?

What are the advantages of positive pressure cleanrooms and negative pressure cleanrooms

The advantages of positive pressure cleanrooms include maintaining indoor cleanliness higher than the external environment, providing a bright working space, and ensuring comfortable operations. The advantages of negative pressure cleanrooms include preventing the leakage of harmful substances, protecting the external environment, and ensuring the safety of workers.

 

What are the disadvantages of positive pressure cleanrooms and negative pressure cleanrooms?

The disadvantage of positive pressure cleanrooms is the risk of outdoor pollutants entering the space. The disadvantage of negative pressure cleanrooms is that the working space is relatively enclosed, and ventilation may not be as effective as in positive pressure cleanrooms.

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