In 2013 ASHRAE estimated that the area of Cleanrooms built worldwide exceeded 12 million m2, with an annual growth rate of 5%, so it is likely that by now this figure has long exceeded 15 million m2.
Cleanrooms have application and use in an increasingly wide range of industrial and research activities; they are no longer linked only to high-tech and high value-added sectors. However, the criteria for the design, calculation, construction and operation of cleanrooms have remained virtually unchanged since the days when cleanrooms were designed for high value-added industries in times of low energy costs.
Nowadays, the situation has changed, Clean Room users belong to very different sectors, many of them with more modest market conditions and expectations of return on investment and, in addition, in a common situation of increasing energy costs.
There are still sectors of Clean Room users who, due to their technification and exceptionality, maintain a high added value of their products, with very high returns on investment; but even in these cases, they are subject to social, institutional and media pressure to reduce their energy consumption.
In this line, part 16 of the famous ISO 14644 "Clean Rooms and Controlled Annexed Premises", dedicated to Energy Efficiency in Clean Rooms, has been edited. This standard defines the process to reduce and optimise energy costs, acting globally on the Clean Room life cycle process, from conceptual design to dismantling and decommissioning. It applies not only to newly built or refurbished rooms, but also to existing and active cleanrooms.
ISO 14644-16 proposes a systematic approach for the assessment of energy savings based on seven steps:
1.- User Requirement Review (URS) and design review from an energy efficiency perspective
2.- comparative analysis of the performance of the Cleanroom
3.- Identifying energy reduction opportunities
4.- Impact assessment of energy reduction opportunities
5.- Selection of energy reduction opportunities
6.- Implementation
7 - Monitoring and feedback of experience
The User Requirements (URS) are the "first stone" in the construction of a clean room and the energy consumption of the future installation depends to a large extent on their correct definition.
ISO 14644-16 attaches paramount importance to the correct and balanced drafting of URS:
It is sometimes believed that the stricter and more restrictive the conditions required of the White Room, the higher the quality; in reality, over-specification only gives the "appearance" of quality and adds technical complexity, installation cost and operating cost.
For example, conditions such as temperature and humidity defined with stricter limits than necessary can have a significant economic impact without providing quality value. In many Clean Rooms the products or materials processed have thermo-hygrometric limits that are wider than the comfort limits of the workers. In this sense ISO 14644-16 reminds that the generally accepted comfort limits for relative humidity are in the range of 30-70%, however, it is very common to find indoor relative humidity specifications of 40-60% or 45-55% in installations that are eminently for comfort.
Another important factor where over-specification is self-defeating is in the "footprint" of the Cleanroom; the larger the Cleanroom or CleanZone, the higher the energy cost. Therefore, URS should help to define a rational layout, where spaces are adjusted to their real needs and coordinated according to the process flow in order to lose as little space as possible in corridors and connecting areas.
There are concepts where paradoxically over-specification can help energy rationalisation. Personnel are the major source of contamination in a Cleanroom and therefore most of the design effort goes into eliminating the contamination contributed by people; therefore, a tighter specification of clothing within the Cleanroom will reduce the contamination emitted and therefore the work of the facility to maintain classification conditions.
Air movement together with temperature and humidity compensation is one of the main sources of energy consumption in Clean Rooms, so reducing the flow of supply air has a direct impact on energy consumption. In general, the air volume is proportional to the cube of the ventilator power, so a halving of the air volume reduces the fan consumption by a factor of eight.
Possibly one of the most widespread concepts linked to cleanrooms is the concept of recirculations/hour or changes/hour (ACH Air Changes per Hour). It is an almost sacred and virtually untouchable concept. Many Cleanrooms are questioned and even rejected for not scrupulously complying with the specified ACHs, regardless of the fact that the particle counts at rest and activity are clearly within the limits.
ISO 14644-16 demystifies and questions the real usefulness of ACH as a calculation method for defining ventilation flow rates. The function of ventilation in a Clean Room is to remove the particles generated by the activity inside the room itself; therefore the standard proposes to calculate the ventilation flow rate from the particles emitted inside the room per unit of time using the formula:
C: Required concentration of particles of a given size (counts/m3);
D: Total particulate emission rate from personnel and equipment, in (counts/s);
Q: Supply air flow rate (m3/s);
Ɛ: Ventilation efficiency (dimensionless ).
The most difficult part is the estimation of the parameters D and Ɛ
The emission rate of the equipment can be assumed to be really low, as one of the conditions for equipment and materials entering a Clean Room is that they do not generate particles. The highest rate of particulate matter comes from people and depends primarily on clothing and activity level.
The parameter Ɛ is also difficult to estimate, as it depends on the characteristics and position of the air inlets and outlets, the geometry of the room, position of equipment, etc... Possibly the use of CFD systems can help in this estimation.
The C parameter would be the particle level corresponding to the ISO class to be achieved, although the standard recommends taking a lower level of particles as the "alert or safety level". So for an ISO 7 which allows up to 352,000 particles/m3 of 0.5 µ, a much lower C parameter should be chosen (e.g. 1/3 which would be 117,330, or 1/4 which would be 88,000).
In any case the D and Ɛ values will always be theoretical and subject to controversy. ISO 14644-16 proposes to complement the theoretical calculation with an experimental alternative system, based on the actual measurement of particles in the room under operational conditions. The system is developed in three stages:
1.- Design: A flow rate Q1 is determined by an initial estimation of particulate emission and ventilation effectiveness. For this first design very conservative data can be taken since it is considered that the obtained flow rate, Q1, will be optimised in the following steps.
2.- Essay: The flow rate Q1 is tested in the room under operational conditions and the actual concentrations are measured for different particle sizes. With the data obtained, a new flow rate Q2 is calculated, generally lower than Q1, which would achieve the expected results.
3.- Operation: The operating Q2 flow rate is used and monitoring data is used to confirm that it is the appropriate flow rate or to optimise a new, more efficient Q3 flow rate.
In any case, the final flow rate, in addition to achieving the required cleanliness rate, must not compromise other cleanroom parameters such as temperature, humidity and pressure.
This new approach renders the concept of recirculations/hour or ACH of no real value. The standard itself gives a very clear example: Two Cleanrooms with the same sources of particulate emissions need the same airflow, but if one has more height (and therefore more volume) the ACH would be different, although the particulate result will be similar in both rooms.
During periods of inactivity, when personnel, who are the maximum contributor to particulate generation, are not in the room, the ventilation regime could be reduced without compromising the classification of the room. The reduction must be co-ordinated with an automatic adjustment of the return air compounds in order to maintain the relative pressure regime in the room, which could also be lower than required during the period of activity.
During downtime at reduced operation it is important to close the access points to the room to prevent contamination from entering.
During long periods of downtime it may be more cost-effective to shut down the ventilation system. The impact of the shutdown should be analysed by assessing the contamination caused by depressurisation, the migration of particles indoors, the cleaning requirements before restarting the installation and the recovery time required to return to an operational state.
Once ISO 14644-16 demystifies the recirculations/hour or ACH, the consequence is the demystification of another concept, also untouchable until now and linked to the ACH: the constant flow rate in discharge. Until now, the basic premises for ventilation in a Clean Room were constant flow in supply and variable flow (for pressure control) in return. But if it is accepted that the driving flow rate depends on the particulate emission, the variation in the particulate emission rate would imply the possibility of variation in the driving flow rate.
This introduces the concept of "adaptive control". There are sectors where continuous particulate monitoring is required in the Cleanroom, e.g. Annex 1 GMP for grades A and B or European aerospace ECSS for all classes up to ISO 8.
If there is a continuous monitoring system, it is possible to define a control procedure that adjusts the driving flow rate proportionally to the detected particulates in real time. It is not a simple procedure, involving filtering and averaging of the counts to have a stable control signal and considering other factors that must be maintained such as pressurisation and proper temperature and humidity control. However, if well developed, it can be one of the most efficient energy management systems for cleanrooms.
In unidirectional flow systems (UDAF), the determining factor is not the air flow rate but the velocity. The value of 0.45m/s ± 20% (0.36-0.54m/s) has remained unchanged since it was defined in the 1960s by the US Air Force and enshrined in the venerable US FED-STD 209.
ISO 14644-16 suggests applying the same reasoning used to reduce the flow rate in turbulent flow installations and to reduce the speed of UDAF units during periods of no activity inside. The standard indicates that in conditions of little or no activity, the speed could be reduced to around 0.2-0.3m/s.
The standard also suggests assessing the possibility of shutting down UDAF units installed inside Cleanrooms during periods of inactivity. In general, the air flow rate of a UDAF is usually much higher than the air flow rate of the Clean Room in which it is installed, so the energy savings achieved will have a considerable impact.
For a flow reduction to be transformed into energy savings, it is necessary to have ventilators that can translate flow reductions into energy reductions without mechanical or efficiency losses. When selecting fans for a Cleanroom, the following should be taken into account:
-High efficiency: to turn flow reduction into energy savings
-Speed variation: to allow adequate control of the required flow rate.
-Direct transmission: To avoid transmission losses between the engine and the turbine. (Classic belt and pulley drives, when in good condition, consume 10-15% of the engine's total energy, with poor or worn belts, the losses are much higher).
Air filters are an essential part of Clean Rooms, but they are also an important factor in energy consumption. High filtration efficiencies imply high pressure drop and pressure drop is directly linked to the energy required to overcome it. In general, pressure drop is proportional to the square of the speed and speed is proportional to the cube of power, so a 50% reduction in pressure drop reduces fan power by a factor of 2.8.
Oversizing of filters is a major cause of increased energy consumption in Clean Rooms. The addition of redundant filtration stages, in number and efficiency above requirements, exponentially increases consumption without adding real quality to the rated environment.
On the other hand, the over-utilisation of filters, maintaining filters that are still effective, but with a high clogging rate, also increases the energy cost of the installation. In this sense ISO 14644-16 recommends applying a "life cycle" policy to filters, replacing filters on the basis of energy efficiency criteria, i.e. when the cost increase due to energy consumption exceeds the amortisation cost of the new filter.
Thermal loads, both hot and cold, are another major factor in energy consumption in a cleanroom. ISO 14644-16 recommends approaching the energy efficiency of thermal loads from different angles:
-Reduction of thermal load: Increasing the efficiency of insulation with respect to the exterior and rationalising internal loads, studying how to minimise or insulate internal thermal loads.
-Rationalisation of set points: Selecting set points and variability ranges according to the actual needs of the room. ISO 14644-16 notes the possibility of allowing the humidity to fluctuate between 30 and 70% when the requirement for humidity control is solely for occupant comfort. It is also advisable to specify more flexible set points for temperature and humidity during periods of low occupancy or rest.
-Rationalisation of outside air: The outside air intake (OAI) is one of the major sources of heat load in the HVAC system of a Clean Room. The outdoor air rate must be calculated and justified in terms of overpressure, oxygenation and ventilation requirements. It is very common to determine the outdoor air flow rate as a percentage of the total supply air flow rate. According to ISO 14644-16 this practice has no rational basis of justification, does not bring quality to the Clean Room and is a source of energy inefficiency.
Unfortunately, in the Spanish version published by AENOR, "Fresh Air", i.e. outside air or make-up air, has been translated erroneously as "aire limpio" (clean air), which means that some reasoning such as the Air Renewal Rate (ACE) or the recommendations for reducing the flow of outside air are confusing and difficult to understand in the Spanish version of the text. Throughout the text two different concepts are mixed under the same definition, in some parts of the text "clean air" is associated with the concept of low particulate filtered air, while in other parts the concept of "clean air" is associated with fresh outdoor air.
ISO 14644-16 is a really useful document to rationalise and reduce energy consumption in cleanrooms. It offers a well-structured and well-founded methodology covering all factors that affect the performance of the White Rooms. Its methodology is applicable to new or refurbished installations as well as to installations in operation.
Industries related to Clean Rooms have in ISO 14644-16 a fundamental document to maintain viable operating costs, qualify for subsidies or exemptions linked to energy efficiency factors or comply with social responsibility policies in sustainability.
Miguel Ruiz
GMP Consultant