Danforth’s Fixed Cleanroom Design-Build process considers the following key factors for cleanroom installation:
Cleanroom Space Cleanliness Classification
Cleanroom space cleanliness classification is defined in terms of the ISO Standard 14644-1 establishing allowable airborne particle density per cleanliness classification. Various size micron particles ranging from 0.1 microns to 5.0 microns per cubic foot are allowable in the occupied space. The space cleanliness class is the single most important factor to be determined as it impacts room construction, maintenance and energy usage. Typically, processes that are more sensitive will have more stringent cleanliness requirements.
Personnel and Work Flow
Evaluation of personnel and work flow within the cleanroom are key factors to be considered as cleanroom workers are the largest source of contamination. Critical processes should be isolated from personnel, access doors, and pathways. Cross contamination pathways are to be minimized whenever possible. Air locks or buffer zones should be utilized to isolate critical spaces.
Space pressurization is to be maintained positive to adjacent spaces of lower cleanliness classification. Pressurization is essential in preventing contaminants from infiltrating a cleanroom. Room pressure differentials are to be maintained at a minimum of 0.02 in.wg. Higher pressure differentials cost more to operate and make doors harder to open. Space pressures are cascaded with pressure decreasing as you move toward the lower cleanliness levels. General construction considerations such as room tightness and door sweeps should be evaluated for maintaining pressure and establishing pressure gradients between rooms.
Air Changes per Hour (ACH)
Airflow and Cleanroom classification are directly related by Air Changes per Hour (ACH) flow rates. As you increase the ACH, all other things being equal, you will increase the amount of filtration that is being done. This, in turn, increases the Cleanliness level in the space. To illustrate how air changes per hour are considered in a design, consider the following example. A Cleanroom space with low occupancy; low particle generating process and positive pressure in relation to adjacent spaces may require 15 ACH, while the same room with a high occupancy, high traffic pattern and high particle generating processes will require 30 ACH.
Cleanroom exfiltration flow is the planned outflow of air necessary to maintain positive pressure to adjoining spaces having lower static pressure. Unplanned exfiltration is air that escapes through electrical outlets, fixtures, window frames, door frames, wall and floor interfaces, wall and ceiling interfaces and access doors. Unplanned exfiltration, or room leakage, can inhibit positive, cascading pressurization. Testing and corrective action is needed to maintain room pressure. Blower Door testing and Thermographic imaging are a couple methods of testing room pressure.
Additional variables need to be considered when designing a Cleanroom. Among those variables are Space Temperature, Relative Humidity and Airflow Laminarity. Acceptable Space Temperature ranges are typically 65 – 68 degrees F. Cleanroom workers are typically suited in a closed-cell garment that retains heat. Consequently, Space Temperatures are lowered to compensate for this and provide worker comfort. Relative Humidity is often maintained year round from 30 – 50% RH. This often requires Mechanical Humidification in the cooler months and Dehumidification in the warmer months. Laminarity of Supply Airflow in the occupied space is established by covering the ceiling with a high percentage of HEPA filter diffusers and paying close attention to the placement of these filters within the footprint of the space.
Heating, Ventilating, and Air Conditioning (HVAC) system design for Cleanrooms often includes systems that have substantially more supply air than that needed for cooling. In Cleanrooms heat load is typically less critical to design than the Cleanliness Level and Space Pressurization requirements. Low Sidewall return air intakes are used to control contamination by drawing the particles downward toward the floor. Higher airflow rates return smaller percentages of air to the Air Handling Unit and the remainder of the air is re-circulated through Fan Powered HEPA Filter Units. Cleanrooms designed to higher Cleanliness Levels, require more space for the mechanical systems and this must be considered in the design of the structure. It is important to communicate the required mechanical equipment space requirements to the project owner early in the design phase.
Simplify design as much as possible. Cleanrooms have many potential pitfalls. Supervision by an experienced design/build team is recommended to assure a successful project, meeting the client’s design and performance requirements.