**Contents**

[Minienvironments 2](#minienvironments)

[Summary 2](#summary)

[Principles 2](#principles)

[Approach 2](#approach)

[Case Studies 3](#case-studies)

[Related Best Practices 6](#related-best-practices)

[References 7](#references)

[Resources 8](#resources)

#  

# 

## Minienvironments

### Summary

Cleanroom air‑recirculation systems typically account for a significant
portion of the HVAC energy use in cleanrooms. High electric power
density is normally required for fans to deliver large volume of
airflows that were designed, supplied, recirculated, and exhausted
within a given time. With the increasing demand for specific
contamination control, it is important to optimize design of clean
spaces. Best practice in cleanroom air system design includes
right‑sizing the systems in cleanrooms and adopting minienvironments.
Implementing and integrating minienvironments in cleanrooms can improve
contamination control and save significant energy .

### Principles

A minienvironment is a localized environment created by an enclosure to
isolate a product or process from the surrounding environment. The
advantages in using minienvironments include the following:

1)  Minienvironments may create better contamination control and process
    integration.

2)  Minienvironments may maintain better contamination control by better
    control of pressure difference or through use of unidirectional
    airflows, e.g., cleanliness‑class upgrade required for certain
    process.

3)  Minienvironments may potentially reduce energy costs.

The use of fan‑filter units (FFU) in minienvironments is common. The
energy efficiency of such air‑delivery systems can vary significantly
because of the difference in energy performance, airflow paths, and
operating conditions. Simply adding minienvironments with fan‑filter
units in an existing cleanroom will increase power density and energy
intensity for delivering airflow in the space served, if everything else
is unchanged. However, by considering contamination control requirements
in the various spaces minienvironments can be integrated with the
surrounding cleanroom to optimize the overall electric power demand for
the facility and to achieve specific cleanliness in each area. In
addition, selecting energy efficient minienvironment systems will
further improve the overall energy efficiency of the clean spaces.

### Approach

Although minienvironments are becoming more popular, their energy and
airflow performance can vary significantly. Owners, designers, suppliers
and operators need to best use resources to determine the adoption of
minienvironments, their integration with surrounding cleanroom spaces,
and optimize the design, control, and installation of the
minienvironments. Best practice with regard to improving energy
efficient cleanrooms and minienvironments includes the following:

-   Determine the cleanliness requirements for contamination control for
    both minienvironments and the surrounding cleanroom. For example,
    cleanliness levels do not need to be more stringent than the process
    occurring in the cleanroom requires.

-   Use computation‑fluid‑dynamics (CFD) modeling and particle‑count
    monitoring or experiment to assist in the design process.

-   Determine the airflow velocities as well as air‑change rates. Using
    optimal air‑change rates will allow designers to lower construction
    costs as well as to reduce energy costs while maintaining the high
    level of air cleanliness that is required in cleanroom facilities.
    Often in minievironements, what is needed is a positive pressure
    relative to the surrounding spaces and high airflows through the
    minienvironment are not necessarily required.

-   Improve efficiency of the minienvironment systems and design,
    including using energy efficient fan‑filter units and better
    controls.

-   Optimize airflow rates of the surrounding cleanroom areas and where
    possible, reduce airflow rates whenever feasible.

### Case Studies

Performance data from a case study on one minienvironment suggests that
within the range of airflow speeds measured, the electric power density
(in W/ft^2^) of a minienvironment generally increased with the increase
of airflow speeds (Figure 1). This trend was affected by the operating
performance of the controller and the airflow rates. For example, the
electric power density reached a maximal value when the airflow speed of
the FFU reached 95 fpm, and it decreased with the increase of airflow
speeds beyond 95 fpm while inside its operating range (Figure 1).
Performance data from a case study on five operating minienvironments
suggests that within the range of airflow speeds measured (50 fpm ‑ 100
fpm), the electric power density of these minienvironments typically
increased with the increase of airflow speeds within each
minienvironment (Figure 2).

Corresponding to the operating airflow speeds that ranged from 50 fpm to
100 fpm, and power density of the minienvironments ranged from 26
W/ft^2^ to 32 W/ft^2^ with an average of 28 W/ft^2^ (Figure 2). Lower
power density generally indicates better energy performance to achieve
the desired cleanliness within the minienvironments.

Best practice should consider implementing minienvironments in
cleanrooms during planning and design stages of a project, whenever such
an option presents a good solution to contamination control. Because of
the much smaller minienvironment volume compared to that of full-scale
cleanrooms (e.g., ballroom), the airflow rate and the electric power
required for a minienvironment can be significantly reduced.
Furthermore, lowering electric power density in cleanrooms, by
integrating minienvironments will significantly reduce energy use for
the whole facility. In the case study, power density of
air‑recirculation systems in the ISO Class 4 cleanroom was measured as
10 W/ft^2^, in which a number of minienvironments were located (Figure
2). This was lower compared to the group of ISO Cleanliness Class 4
cleanrooms with a range of 16 to 38 W/ft^2^in a previous study (Xu
2004).

In summary, in order to create opportunities for a significant overall
energy savings, best practice should include measures to reduce fan
power for both minienvironments and the cleanrooms that contain them.
Reducing the fan power density as well as optimize floor area of the
minienvironments and cleanrooms will lead to overall energy savings.
Specifically, best practice to reduce electric power density of
minienvironments may include the following

-   Optimize (e.g., reduce) the airflow speeds and/or pressure inside
    the minienvironment.

-   Improve the energy efficiency of the air systems, e.g., choose an
    efficient FFU, for the minienvironment.

In addition, as part of integrating minienvironments to cleanroom
spaces, best practice to reduce overall electric power density is to
reduce power density of the primary cleanroom surrounding the
minienvironments. The following enlist some of the best practice
approaches:

-   Optimize airflow rates and air‑change rates, e.g., reduce airflow
    speeds in the cleanroom.

-   Select the right type and size of air‑handling unit for
    recirculation.

-   Optimize exhaust and make‑up air systems.

-   Adopt variable‑speed‑drive motors in air systems.

-   Minimize system resistance.

![](media/image1.wmf){width="5.995833333333334in" height="4.1in"}

Figure 1. Power Density and Airflow Speeds for a Minienvironments under
Different Operation

![](media/image2.wmf){width="5.996527777777778in" height="4.1in"}

Figure 2. Power Density and Airflow Speeds for Various Minienvironments
inside a Cleanroom

### Related Best Practices

-   Air change rate

-   Fan‑filter units

-   Demand Control Filtration

-   Fan Efficiency

-   Right Sizing

-   Filters

### References

1.  Tannous, G. 1996. Air Flow Simulation in a Minienvironment, Solid
    State Technology, July 1996, pp. 201-207.

2.  Tannous, G. 1997. Optimization of a minienvironment design using
    computational fluid dynamics. Journal of the IEST, Vol. 40 (1):
    29‑34.

3.  Rothman, L., R. Miller, R. Wang, T. Baechle, S. Silverman; and D.
    Cooper. 1995. SEMATECH minienvironment benchmarking project. Journal
    of the IEST, May/Jun 1995, Vol. 38 (3): 36‑41.

4.  Xu, T. 2005. Investigating the Performance of a Minienvironment
    System. Proceedings of 51^st^ ESTECH Conference, Schaumburg,
    Illinois, The Institute of Environmental Sciences and Technology
    (IEST). LBNL Report, LBNL‑56767.

5.  Xu, T. 2005. Opportunities for Sustainable Design and Operation of
    Clean Spaces: A Case Study on Minienvironment System Performance.
    Final Report to California Energy Commission, LBNL‑56312. May 2005.

6.  Xu, T. 2005. Energy Implications of Minienvironments in Clean
    Spaces: A Case Study on Minienvironment Energy End‑use and
    Performance. Final Report to California Energy Commission,
    LBNL‑57503 (draft).

7.  Xu, T. 2004. Efficient Airflow Design for Cleanrooms Improves
    Business Bottom Lines. Journal of the IEST, Vol. 47 (1): 24‑28,
    Institute of Environmental Sciences and Technology (IEST), Rolling
    Meadows, Illinois 60008-3841, USA.

8.  Xu, T., and M. Jeng. 2004. Laboratory Evaluation of Fan‑filter
    Units' Aerodynamic and Energy Performance. Journal of the IEST, Vol
    47(1): 66‑73. The Institute of Environmental Sciences and Technology
    (IEST), Rolling Meadows, Illinois 60008-3841, USA.

9.  Gale, S. 2004. FFUs: Setting a course for energy efficiency, Special
    Report, Cleanroom Magazine, September 2004,
    <http://cr.pennnet.com/Articles/Article_Display.cfm?Section=Articles&Subsection=Display&Article_ID=211810>

10. The Institute of Environmental Sciences and Technology (IEST). 1998.
    IEST‑RP‑CC012.1: Considerations for Cleanroom Design. Handbook of
    Recommended Practices, Contamination Control Division. Rolling
    Meadows, IL 60008-3841, USA.

11. The Institute of Environmental Sciences and Technology (IEST). 2002.
    IEST‑RP‑CC028.1: Minienvironments. Handbook of Recommended
    Practices, Contamination Control Division. Rolling Meadows, IL
    60008-3841, USA.

12. International Organization for Standardization (ISO). 2004. ISO
    14644‑7, Cleanrooms and associate controlled environments - Part 7:
    Separative Devices (clean air hoods, gloveboxes, isolators and
    minienvironments). The Institute of Environmental Sciences and
    Technology (IEST). Rolling Meadows, IL 60008-3841, USA.

### Resources