Controlling room pressure is only one aspect of cleanroom facility design when creating segregated zones of different class. A broader view based on designed airflow rather than mere room pressure has many benefits, argues Frans Saurwalt of Kropman Contamination Control.
Specifying room overpressure in cleanroom design is a common contamination control concept. To achieve this the HVAC needs to be designed to control the room pressure by some means. Most commonly this is achieved using pressure controlled actuated dampers in the return ducting. These dampers have to be designed to modulate within a certain airflow range and with a specific accuracy and speed of reaction. Understanding the mechanisms that will create a room pressure, as well as those factors that challenge it, is essential for success. Cleanrooms designed for ‘containment’, i.e. intended to keep specific contaminants in, are not in the scope of this article.
To understand the principles governing room pressurisation, consider first a room being within the envelope of a general building. This room will have the same pressure as the surrounding environment as the construction is usually far from airtight.
Large volumes of air will be able to flow in and out without any significant resistance. In this case the pressure can be increased by two means: 1) by increasing the volume entering the room (and leaking away); or 2) by leakage reduction.
The first solution requires tremendous volumes of air, so the second solution is a more useful approach. This is based on the simple correlation that airflow over a ‘resistance’ will generate a pressure drop Dp (Pa). This can be formulated as:
where:
Ql = Leakage air volume (m3/s)
ρ = Specific density (kg/m3)
A1 = Area of leakage (m3)
µ1 = coefficient of contraction ( - )
For turbulent flow n = 2; for laminar flow n = 1
Common value for n=2 for large leaks (for small leaks n≈1.5) and µ≈0.75 (see Fig. 2).
The dynamic reaction of the room to pressure fluctuations can be considered as a capacity. According to Boyle Gay-Lussac law, the pressure, volume, temperature and the number of gas molecules are related.
Assuming a constant temperature, Boyle’s law can be applied for a given room volume. There is a distinct relation between the (room) pressure and the amount of gas in that given room volume. Boyle’s rule shows that when more air is supplied than exhausted, a large room will have a slower increase in pressure than a small room:
Equation 2
where:
P = absolute pressure (Pa)
V = Volume (m3)
C = Constant (at given temperature) (Pa m3)
For two situations:
Equation 3
As V2 could be considered the room volume V1 and the additional amount of air δQ added at pressure p1 in a given time to the room.
Equation 4
Where (dp) is the increase in pressure:
Equation 5
This will result in:
Equation 6
Over-pressurisation can be designed as the equilibrium of the differential pressure over the room leakage to the surroundings and the ‘offset’ in the air handling – the difference between the supply and the return air volume. Combining both effects (the degree of leakage and the volume of the room) will show that small rooms with limited leakage will require very accurate control of the offset in air handling. Larger leakage and/or larger rooms make controllability much easier in terms of the accuracy and reaction time (figure 5).
Figure 5: Controllability is related to the degree of leakage and the volume of a room
To achieve controllability for small and/or airtight rooms, mere use of a standard damper in a return duct to control the room pressure is insufficient. For specific situations high accuracy dampers with quick reaction controls have been developed, although even these require some leakage (= HVAC offset).
An affordable approach is designing facilities with a small standard damper in combination with ‘controlled leakage’ through a parallel return duct. This still requires precise dimensioning.
Segregation by pressure or flow
Having focused on room pressure up until now, it is essential to question its relevance and consider what will protect against the intrusion of contamination. Quite clearly, it is not the pressure but the air flow.
ISO14644-4 2001, Design-construction and start-up: section A.5: ‘Concepts to achieve segregation of clean rooms and clean zones’ illustrates this by referring to air flow or flow induced pressure difference. Eudralex GMP, Vol 4 Annex 1 §53 indicates: ‘A filtered air supply should maintain a positive pressure and an air flow relative to the surrounding areas of a lower Cleanroom grade under all operational conditions and should flush the area effectively’.
Figure 2: Differential pressure (Pa) in relation to the offset air volume (m3/h) per area of leakage (m2)
Figure 3: Rate of change in differential pressure (Pa) in relation to the rate of change in offset air volume (m3/h per sec) per room volume (m3)
The cascade design
As air flow is the protecting effect when no physical barrier is present and air flowing through a leak will encounter a resistance, this resistance over the aperture of the leak results in a pressure drop that can be exploited. Air flow will always be directed from higher pressure towards lower pressure and can be seen as a pressure flow cascade.
To use this in a design the following steps need to be carried out:
- 1. Identify on the layout, the classification and preferred, allowed and prohibited airflow directions.
- 2. Establish the ‘supply’ air volume and the ‘return’ air volume.
- 3. Define the ‘overflow’ air volume and adjust the air balance accordingly.
Step 1 contains one additional element compared with a standard room pressure layout: the allowed overflow directions. Step 3 comes down to the final design. Here the beneficial effect of overflow can be exploited: as higher classified rooms have more air changes and allow for overflow into an adjacent room, this overflow can be designed. Depending on the size, class and use of the room, the required air supply to that room can be significantly reduced or even avoided.
A further benefit is that a cleanroom with all doors closed will normally be capable of providing the required protection against the infiltration of contaminated air. Normally, when a door is opened a significant exchange of air with that from the outside will take place. This challenges the cleanliness and that will need time to recover. When the design is for an abundant air flow towards the adjacent room with the door closed, the air will also flow through the door aperture when the door is opened, thus protecting against, or at least limiting, the exchange of less clean air. Temperature difference effects that increase air exchange are also reduced by the conditioning effect of the overflowing air.
Wind challenges
When utilising the relative pressure flow cascade, a challenge that has to be countered is the pressure around the building that envelops the cleanrooms. Depending on the weather conditions, building, configuration and position relative to surrounding structures, wind pressures on the building façade can be in the range up to 600 Pa.
A mere over-pressurisation in the normal order of magnitude of 50–60 Pa will not provide sufficient protection. Even a very airtight construction will have a level of infiltration, when exposed to these high external pressures, thus compromising the contamination control. The only way to protect cleanrooms against wind attack is by isolating the cleanroom from the building façade. This not only requires a spaced construction but also requires free internal expansion of infiltrating air. This configuration, which can be referred to as a box in box arrangement, is shown in Figure 4.
Monitoring the cascade design
As the concept of a cleanroom carries an element of protection against infiltration, this is a genuine item to verify and monitor. However, flow being the basic mechanism of protection cannot be practically measured as such. Pressure, however, can be very easily measured and monitored. So designing for flow but sizing for pressure drop along overflow devices provides the possibility of monitoring the designed functionality. Along the cascade, the differential pressure from room to room is the only relevant measure up to the surrounding ‘zero’ reference.
The ‘zero’ reference gives rise to a lot of confusion. One point of note is the use of a common reference pressure tube to which each individual room pressure can be related. Here, it should be noted that the pressure directly surrounding the cleanroom itself is the major factor to protect against. When the ‘zero’ reference is not identical to the direct surrounding static pressure, a false conclusion could be drawn that the cleanroom is in overpressure when it is not.
In line with the resolution of the wind attack challenge, a greater building envelope containing the cleanrooms will provide the optimal solution for the reference pressure. The reference pressure is all around within the building envelope. When a building is more confined and segregated, an enveloping reference is more difficult to achieve. Various examples of reference pressure lines with a non-zero reading compared with the surrounding envelop have been encountered in practice.
Misreading in an order of magnitude of a tenth of a Pascal can be the result.
Cost and efficiency gain
Utilising portions of high quality supply air twice or thrice in a pressure cascade with overflow has various additional benefits. It reduces the amount of recirculating air. In the example of the rooms in Figure 1 this added up to a 15% reduction in designed air volume. This saves substantial amounts of energy and associated running costs. Installation costs are reduced by smaller equipment, a reduction in control equipment and software as an overflow cascade is self-stabilising by nature when designed and sized properly. These cost-savings can add up to at least 5–10% of the installation costs. When, as has been designed in some projects, airlocks can be left out, using segregation based on ≥0.2m/s overflow, according to ISO14644-4 2001 A.5.2, both cleanroom construction costs and floor space can be reduced.
Figure 4: Box in box arrangement as well as differential pressure drop measurement and measurement against a reference line
In conclusion, conventional room pressurisation of modern airtight cleanrooms, especially small ones, becomes a significant design and commissioning challenge because of the required accuracy, resolution and reaction time of utilised controls. The pressure/flow cascade concept offers a stable energy and cost-efficient alternative that provides the additional benefit of better protection when doors are open. A box in box approach is useful to counter wind attack and as a relevant pressure reference.
This article is based on a presentation given at the ICCCS Zurich event held in Switzerland in September 2012.