General requirements for clean rooms
Let’s start with the basics. The general framework conditions for cleanrooms are defined in several standards and guidelines and contain various factors relevant to energy efficiency. Compliance with the respective regulations is not only a prerequisite for qualification in conformity with the standards. It also provides an initial framework for the energy-saving potential of the cleanroom. For example, DIN EN ISO 14644 Sheet 16 and VDI 2083 Sheet 4.1 provide specifications for the energy efficiency of cleanrooms and clean-air devices. EU GMP Guideline Appendix 1 describes requirements for outflow velocities and pressure stage concepts as well as time specifications for idle and operating states. In addition, there are DIN EN 1822 and DIN EN ISO 16890 (with requirements for HEPA filters and their classification) as well as EN 16798-3/TR EN 16798-4 (with general requirements for the energy efficiency of non-residential buildings) and the Ecodesign Directive (with framework parameters for the energy efficiency of fans and air velocities in the unit). ISO 5001 (with its regulations on energy management systems, the determination of energy performance indicators at the process level, and the success monitoring of optimisation implementations) should also not be forgotten. Based on these parameters, initial energy-saving measures can already be planned depending on the future area of application of the cleanroom.
User requirement specification
No two cleanrooms are alike; the requirements are often highly individual. The respective requirements provided for in the specifications of the cleanroom are therefore much more specific than any guidelines. In general, the requirements for the process determine the requirements for the air-conditioning technology. The energy-relevant influencing variables listed here offer different potentials for reducing energy consumption. For the room temperature, the decisive factor is which ambient temperatures are required for the process and which tolerances are permissible. If the room humidity is relevant for the product, the question of possible tolerances also arises. For the cleanliness class, the first decisive factor is which requirements the manufacturing process must fulfil or, in the GMP environment, which cleanliness class and which flow type (turbulent mixed ventilation or low-turbulence displacement flow) are required for the process. In addition, the loads in the room arising from the number of people, particle release, heat loss from machines, and process exhaust air are the basis for determining the required ventilation. In addition to the type of air flow and the room pressure, the process also influences the minimum fresh air requirement. This can considerably increase the energy demand of the cleanroom. If all these factors are taken into account during the planning phase, considerable savings could be made.
Technical approaches to energy saving – heat recovery
In addition to planning, the technology used is also crucial for saving energy. One way to achieve more energy efficiency in cleanroom operations is effective heat recovery in the ventilation system. There are different concepts here – each with its own advantages and disadvantages:
The cross-flow/counter-flow heat exchangers operate with pure heat transfer via an air/air plate heat exchanger and can achieve efficiencies of up to 90%. However, they run the risk of cross-contamination. The concept also necessitates anti-icing protection and additional reheaters. In addition, there is a large space requirement in the ventilation unit.
In the closed cycle system, which also works with pure heat transfer, the heat is recovered via water as an intermediate medium. Additional heat sources thus can be integrated into the system, thus leading to an efficiency of up to 80% for high-performance systems. There is also no risk of cross-contamination here. However, these advantages come with high investment costs and a large space requirement.
Rotary heat exchangers combine heat and moisture transfer and can thus achieve efficiencies of up to 85%. Although no frost protection is required here, cross-contamination can occur between the air flows, and the system causes a relatively high pressure loss.
With circulating air, high energy savings can be achieved with low investment costs – if this is possible from a process-technical point of view. However, there is also a risk of cross-contamination here.
In addition to heat recovery in the ventilation system, there are other possibilities for energy recovery. One approach is the use of waste heat from recoolers in refrigeration or the feed-in of process waste heat.
Which concepts and measures are most efficient depends on the respective cleanroom and the technical framework conditions and must always be considered on a case-by-case basis.
Technical approaches to energy saving – efficiencies of ventilation units
Another approach for more energy efficiency is to optimise the ventilation units. For example, by choosing the right fans and drive technology. In order to implement this, several influencing factors starting with the choice of ventilation concept must be taken into account. As already described, this also depends on the requirements in the specifications. Another decisive factor is the demand-oriented selection of components for the ventilation units in order to be able to combine efficiency and requirements here as well. Furthermore, the duct network offers potential for savings. On one hand, the tightness of the ducts should be ensured as best as possible in order to prevent air and power losses. On the other hand, short and streamlined duct network planning can contribute to greater efficiency. For example, by optimising the air velocity in the duct or placing the technical centre as close as possible to the area to be supplied.
The heating and cooling technology and the temperature level for both media are also decisive for the overall efficiency of the system. Higher cooling medium temperatures in the refrigeration circuit lead to a better degree of utilisation and lower heating medium temperatures. At suitable temperatures, part of the refrigeration technology can also be used in a ‘heat pump circuit’.
General recommendations
Considering the possibilities and influencing factors discussed above, there are some basic principles for more energy efficiency in cleanroom operations that you should always follow:
- Keep the outdoor air volume as low as necessary
- Install recirculation units depending on the available space
- Check the efficiencies of the systems and system components
- As a matter of principle, consider whether the system must run permanently at full power or whether different operating phases such as night setback or weekend operation are possible
1,084 words. Reprint free of charge. Please cite Weiss Technik GmbH as the source.