HVAC equipment selection is critical decision in cleanroom design
Breakthroughs in science and technology have increased demand in the pharmaceutical and electronics markets. With the need for clean production, design, construction, and assembly and packaging processes are under intense pressure to create clean environments. Schedules are shorter, design criteria are often uncertain, and the useful life of facilities can be unknown. All of these trends involve increased risk for design professionals, equipment manufacturers, and contractors.
With a focus on the heating, ventilation, and air conditioning (HVAC) system, this article addresses those issues and offers recommendations that can reduce risk. Because standards and design criteria differ significantly from one application to the next, this overview cannot include specific standards or design guidelines. Instead, the broad-scope ideas discussed below can be used as a guide when planning a cleanroom.
Most importantly, clarify project requirements before building a cleanroom. Whether planning on the bid-spec route or design-build, take time to establish and document the requirements; they will become the roadmap for all concerned. Changing requirements midstream can result in mistakes, delays, and cost overruns.
Project requirements should include the capabilities of the cleanroom (present and future), a schedule for completion, and a budget. Energy efficiency or operating cost may also be goals, but production reliability is often more important.
With careful consideration given to cleanroom requirements and HVAC equipment selection, risk can be reduced significantly while costs are controlled. Risk-mitigating factors include the following strategies:
- provide redundant components;
- slightly oversize the air handler casings and coils;
- configure equipment so it can be serviced without the need for a shut-down;
- design the system for ease of operation;
- design flexibility into the system to anticipate changes; and
- incorporate packaged chiller systems to shorten installation and start-up time.
When project requirements are not clear or are likely to change in the future, potential for risk increases. Three key design aspects can help minimize risk: simplicity, reliability, and flexibility. These are used in the selection of key components of the HVAC system. Common characteristics of cleanrooms are contaminant control, and temperature and humidity control. Downtime in most cleanrooms is costly, so system reliability is crucial.
Control of Contaminants
Contaminant control is typically much more stringent in cleanrooms than in comfort cooling HVAC applications. In addition to 30% efficient filters used to protect the air handler coils, HEPA filters are used to remove over 99% of particles 0.3 micron and larger.
The best location for the HEPA filters is at the very end of the air delivery system in the ceiling grid. This location ensures that the cleanest possible air is entering the room. The quantity and location of HEPA filters depend on the airflow rate required to achieve the desired air change rate in the room and on the equipment layout.
The goal is to provide a certain direction and airflow velocity at the point of production or packaging. The air coming into the room is clean, but the system also needs to control any particles that are generated inside the room by quickly capturing them and steering them away from the product.
Additional methods of reducing contaminants include:
- providing a tight room envelope to eliminate infiltration;
- providing a vapor barrier to eliminate vapor transmission;
- providing an air lock for passage of people and materials;
- maintaining a positive pressure in the room with respect to the surrounding spaces; and
- adhering to owner operational practices.
If the system uses a dedicated outside air handler, use HEPA filters in that unit as well. Once the room is in operation, the dirtiest source of air entering the system will be from the outside air intake. The room air handlers will be moving primarily recirculated air from the cleanroom, so introduced outside air should be brought to the same level of cleanliness before mixing it with the recirculated air.
Maintaining tight temperature and humidity conditions in the cleanroom through different seasons requires several environmental processes: air cooling, dehumidifying, heating, and humidifying. Sometimes there is also a low-temperature process cooling requirement.
The room itself is usually relatively stable. In this case, the outside air becomes the most variable part of the process because of temperature and humidity fluctuations. It is for this reason that a dedicated outside air handler is recommended. Pretreating the outside air ensures that the room air handlers see little variability. The air conditions entering the room air handlers are fairly constant, and temperature and humidity from these air handlers can be held to extremely tight ranges.
While it is important to set room conditions low enough to achieve the desired result, be careful not to arbitrarily set lower room conditions than needed to avoid drastic increases in equipment cost.
Room Supply Air Handlers
When outside air is preconditioned and filtered, the demands on room air handlers are reduced because the large, variable outside air load has already been taken care of. In this example, the room air handlers may have a 30% efficient prefilter, a four-row sensible cooling coil, and redundant fans.
The room air handlers are designed with four fans that are isolated from each other with back draft dampers. Each fan is equipped with a variable frequency drive. Each fan compartment is accessible from outside the air handler so that it does not require shutdown if a fan, motor, variable frequency drive, or belt fails. This design improves flexibility and makes the system more reliable. Operating sequences are simple, and maintenance is easier. Furthermore, if three fans are sized to handle the full airflow requirement, there is room for additional airflow if it is ever needed.
Another way to improve flexibility and hedge against future requirement changes is to upsize air handlers to the next cabinet size. With a little extra cost, this change provides numerous benefits. The fan variable frequency drive allows the fan to run at current airflow needs, with extra capacity available if needed. In the meantime, operating costs are lower due to lower pressure drop through the air handlers.
Because of a cleanroom’s tight temperature and humidity requirements, outside air is not used for cooling; instead, mechanical cooling is needed year round. In this case, an atomizing-type humidifier can be used. In addition to humidifying, this type of humidifier provides cooling as water evaporates into the airstream.
This “free” cooling brings significant energy savings and avoids cooling capacity. Even after considering the cost of providing compressed air and the additional preheat required when the outside air is cool, savings are substantial. As an added measure of safety, do not take this cooling credit when sizing the cooling coil. This type of humidifier is very reliable, requiring almost no maintenance.
Conditioning the Outside Air
The dedicated outside air handler improves reliability and flexibility. Several components that accomplish this are included in the make-up air handler:
- 30% efficient prefilters;
- preheat coil;
- deep, 12-row cooling coil;
- high efficiency particulate air final filters; and
- reheat coil.
As with the room air handlers, the outside air handler can usually be oversized at minimal extra cost while greatly improving flexibility. If future program changes are anticipated and the outside air handler is found to be too small, it is much easier to add a second air handler or replace the outside air handler than to replace all of the room air handlers. Assuming the room exchange rate remains the same, the room air handlers simply take a larger percentage of preconditioned outside air and a smaller percentage of recirculated air. The load on the room air handlers does not change.
The conditioned outside air is delivered to numerous room air handlers. The pressure loss in each duct run is different, and the pressures within the ducts may vary. The use of a linear air flow valve in each duct run ensures constant outside air flow even when pressures vary in the ducts. As long as the outside air flow to each room air handler is constant, the air valves can be set and locked in place. They do not require an actuator (see Figure 1).
A novel design can be implemented that involves matching the outside air handler water flow to the room air handler water flow. This results in an outside air coil with a greater water flow rate and low water temperature rise. The lower leaving water temperature provides greater dehumidification, and the water can then be used in the room air handler cooling coils to provide additional sensible cooling required due to space and fan motor heat gains.
If the coils are equipped with their own pumps, there is a slight pumping energy penalty at the outside air handler coil pump. The total water temperature increase is greater than industry norms, and energy savings are realized at the chiller (see Table 1).
The high return water temperature is acceptable in this application because the high airflows passing through the cleanrooms allow a 65°F air temperature supply. Water temperature returning to the chiller is 4°F warmer than a conventional design, which improves chiller efficiency.
Reheat coils provide any heat required to neutralize the ventilation air temperature after subcooling the air for dehumidification purposes.
Sometimes there is a low temperature process-cooling requirement. All cooling loads could be handled from one chiller plant; all the cooling needs would be handled by 30°F glycol. Because only a portion of the total cooling load needs to be at 30°F, this would be terribly inefficient.
Two chiller plants should be designed, one for the 42°F air conditioning system and one for the 30°F process system. This will save energy and also limit the amount of glycol to the 30°F process system. Separating the process load may introduce a problem if the load is very intermittent. Large surges in load make chiller control difficult and quick response impossible. If this is the case, couple the process chiller with an ice storage bank. The ice storage bank will smooth out the load to the chiller. Because the chiller is decoupled from the process load, it can run longer and the chiller capacity can be reduced. (See a graph of load versus cooling capacity in Figure 4.)
A water chiller package is shown in Figure 2, and the low temperature glycol chiller package is shown in Figure 3. The packages are useful in saving construction costs and in shortening the project schedule. Schedule reduction is achieved by reducing field labor and by testing the package at the factory. Redundant compressors and pumps provide an extra measure of safety.
In conclusion, when project conditions introduce uncertainty, take steps to lessen the risk. Providing a dedicated outdoor air handler offers many benefits, including tighter control over temperature and humidity, simplicity of operation, and easier and less costly changes if additional outside air is needed.
Slightly oversizing air handlers and chillers offers a hedge against future changes. Designing equipment with multiple fans, pumps, and compressors provides system reliability. Using atomizing humidifiers and pressure independent airflow valves reduces maintenance and minimizes down time. Designing packaged chiller systems saves money and shortens the schedule.