Cleanroom HVAC systems may be one of the most complex design challenges for engineers because the standards required for implementation offer many options. The key factors in these types of systems are modeling actual particulate and contaminant generation, space requirements for the end-users, and the energy and filtration requirements for the given standard.
The current guidelines for particulate control provided by cleanroom HVAC systems are governed by the International Organization for Standardization, or ISO. It expanded the old U.S. Federal Standard 209E, which included six levels, by adding room air filtration as the lowest level of particulate control and adding two more stringent options.
|ISO class||Maximum concentration per m^3 of >=.1 micron particles||Max concentration for particles >=0.3 microns||Max concentration for particles >=0.5 microns|
|9 (room air)||N/A||N/A||35,200,000|
While the guidelines are relatively straightforward for particulate filtering, accomplishing that goal is anything but simple. Take, for example, guidelines for recommended air changes per hour (ACH) based on ISO level. Ranges vary, even for rough estimates, from 10 to 30 for an ISO 8 zone from one manufacturer to 15 to 25 from another. As the filtration requirements deepen, the range widens, with rough estimates for an ISO 5 zone ranging from 240 to more than 450 ACH.
As noted above, there are rough ranges for the recommended air changes per hour for a given ISO cleanliness level. However, the simple fact is that every cleanroom is different, as are the processes involved inside each one. Further, while the ISO standard focuses specifically on particulate control, the activity inside the cleanroom may require tight tolerances in both temperature and even humidity control.
Finally, add the question of whether the zones need to have positive pressure relative to the outside rooms, to prevent contamination, or negative pressure to prevent the release of dangerous diseases, and considerations quickly add up.
Adding even more complexity to the situation, building owners must meet ASHRAE’s 90.1 efficiency standard. Ecologically minded owners may also wish to meet the U.S. Green Building Council’s LEED certification requirements, although these standards are not mandatory.
In addition, HVAC systems engineers must account for the “step-down” rooms. As an example, you cannot go directly from room air to an ISO 6 cleanroom. Instead, you must pass through an ISO 8 zone and an ISO 7 zone to access the cleanroom.
There are two major options for cleanroom HVAC design, and both focus on how much makeup air or recirculation is possible given the regulations noted above. While all cleanrooms need some amount of outside air, the major decision is whether a large primary air handling unit is required in the situation.
When it is, the options range between a single air handling unit that may include cooling, heating, humidification, and dehumidification capabilities in a custom cabinet to a smaller air handling unit separate from that of the rest of the facility that is combined with secondary recirculation units. The latter is common where there are multiple suites with different requirements.
With the advent of more advanced control systems, it is also possible to increase the use of outdoor air economizers combined with recirculation systems. This can come in the form of a makeup air system with or without fan-filter modules.
While the use of options like fan-filter modules and recirculation units are certainly going to become a necessity for cleanroom HVAC systems that must meet high efficiency requirements, they are also subject to the most variability in terms of potential for HVAC misbalancing.
It has become even more crucial for HVAC specifying engineers to not just produce accurate models of airflow during actual cleanroom usage, but to ensure that the facility manager is completely conversant with ongoing testing, balancing, and retro-commissioning as necessary if annual testing requires it.
Windy City Representatives works with leading manufacturers of cleanroom HVAC systems; we understand how important it is to take multiple considerations into account when specifying these units. Contact us today for a quote or bid.