Cleanroom Air Flow

Single Pass vs. Recirculating Air in Cleanrooms: Which Cleanroom HVAC Design Do I Need?

There are two common types of cleanroom HVAC systems: single-pass cleanrooms and recirculating cleanrooms.

The type of cleanroom you need depends on your isolation, temperature, and humidity control requirements. In a single-pass cleanroom HVAC design, air is filtered into the cleanroom and passed out to the surrounding environment. In a recirculating air design, the filtered and conditioned air is continuously recirculated back through the system, maintaining a steady temperature and humidity level for greater control of the controlled environment.

How Important are Cleanroom Airflow and Air Changes per Hour?

Air change rates and airflow HVAC designs are the heart of the cleanroom envelope and ensure compliance with ISO, FDA, cGMP, and USP 797 regulatory standards. “Air change rate” refers to how quickly the air inside a cleanroom is pumped through and replaced with newly filtered air from outside the cleanroom. Airflow and air change rates determine:

Particulate levels
Temperature and relative humidity
Heat load, static, and gradients
Operational and maintenance costs and requirements

A cleanroom's air change rate (ACR) also helps determine its classification. For ISO Class 9 through Class 6 cleanrooms, ACR is calculated by air changes per hour; for Class 5 through Class 1 cleanrooms, ACR requirements are measured in meters per second of airflow. For example, an ISO Class 8 cleanroom requires a minimum of 10 air changes per hour—the air inside the cleanroom will be entirely “new” every six minutes or less. In a Class 5 cleanroom, air must travel through the room (from the filter input to the exhaust ports) at no less than 40 feet per minute.

Single Pass Systems for Cleanrooms

Cleanrooms By United most commonly provides single pass cleanroom HVAC designs in modular hardwall and softwall cleanrooms. Using HEPA FFUs (fan filter units), air is introduced into the ceiling plenum using filtered attic air intake vents that pull in ambient or conditioned air. In a hardwall single-pass cleanroom, the air is exhausted through low wall exhaust grills and back into the ambient (outside) environment. In softwall single-pass cleanrooms, the air is typically exhausted under the vinyl curtain. Advantages of a single-pass cleanroom HVAC system include:

HEPA modules available with variable speed controllers from 200-850 CFM
Smaller in size, ranging up to 1,000 sq. ft.
Lower operating cost than recirculating systems

The cleanroom must also be conditioned low enough to offset the heat loads of the HEPA fan filter motors, light fixtures, process equipment, air change rates, and the number of personnel inside the work envelope.

Recirculating HVAC Systems for Cleanrooms

Recirculating cleanroom airflow systems filter air through a dedicated air handling system designed to maintain the cleanroom at the required operating environmental tolerances. Some key features and benefits of a recirculating cleanroom HVAC design include:

Typically paired with static HEPA filtration modules instead of fan-powered modules
Offer temperature, relative humidity, and pressurization controls through an air handling unit interface
Can achieve unidirectional airflow for airflow and heat load removal in high-airflow required areas
Reduced cost and maintenance requirements with supply air from the HVAC system

Automated Control Systems for Multiple Cleanrooms

Automated Control Systems (ACS) apply to all classes of cleanrooms, and all industries would benefit from ACS to maintain a precisely controlled environment. While ACS isn’t a standard included feature, they are highly recommended on any system with 20 or more HEPA units. ACS can also be used for a single cleanroom installation or across multiple rooms, zones, or addresses.

Automated Control Systems have multiple options for cleanrooms, including:

Available in 115v or 277v
HEPA or ULPA filters
NCR or RSR style fan filter units

Contact Us for Complete Cleanroom Airflow HVAC Solutions

Cleanrooms by United has been a provider of cleanrooms and controlled environments since 1983. As a trusted manufacturer of cleanrooms for commercial industries, we offer a variety of solutions designed to meet your requirements. We will work with you to determine whether a single-pass or recirculating cleanroom HVAC system suits your unique application.

Request a quote or contact us to get your cleanroom project started today.

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