**Contents**

[Efficient HVAC Air Systems 2](#efficient-hvac-air-systems)

[Fan‑filter Units 2](#fanfilter-units)

[Summary 2](#summary)

[Principles 2](#principles)

[Approaches 4](#approaches)

[Case Studies 5](#case-studies)

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

[References 7](#references)

[Resources 7](#resources)

#  

# Efficient HVAC Air Systems

## Fan‑filter Units

### Summary

The HVAC systems in cleanrooms may use 50 percent or more of the total
cleanroom energy use. Fan energy use accounts for a significant portion
(e.g., over 50%) of the HVAC energy use in cleanrooms such as ISO
Classes 3, 4, or 5. Three types of air‑handling systems for
recirculating airflows are commonly used in cleanrooms: 1) fan‑tower
systems with pressurized plenum, 2) ducted HEPA systems with
distributed‑fans, and 3) systems with fan‑filter units. Because energy
efficiency of the recirculation systems could vary significantly from
system type to system type, optimizing aerodynamic performance in air
recirculation systems appears to be a useful approach to improve energy
efficiency in cleanrooms.

Providing optimal airflows through careful planning, design and
operation, including air change rate, airflow uniformity, and airflow
speed, is important for controlling particle contamination in
cleanrooms. In practice, the use of fan‑filter units (FFUs) in the
air‑handling system is becoming more and more popular because of this
type of system may offer a number of advantages. Often modular and
portable than traditional recirculation airflow systems, FFUs are easier
to install, and can be easily controlled and monitored to maintain
filtration performance. Energy efficiency of air handling systems using
fan‑filter units can, however, be lower than their counterparts and may
vary significantly from system to system because of the difference in
energy performance, airflow paths, and the operating conditions of FFUs.

### Principles

An FFU usually consists of a small fan, controller, and a HEPA/ULPA
filter enclosed in a box, which fits into common cleanroom ceiling
grids, typically 2x4 ft or 4x4 ft. The small fans force air through
filters and thereby consume considerable energy in providing cleanroom
air recirculation.

![](media/image1.png){width="2.6354166666666665in"
height="2.4569444444444444in"}Figure 1 Fan‑filter Unit

FFU systems are required for many specific applications; however, FFU
systems tend to be less energy efficient than pressurized plenum systems
for recirculating air in a cleanroom (Figure 2).

![](media/image2.wmf){width="5.995833333333334in" height="4.1in"}

Figure 2 Energy efficiency of air recirculation systems

Fan power is proportional to the cube of airflow rate or airflow speed.
A reduction in the air change rate by 10% may result in a power
reduction of approximately 27%. Providing the flexibility of speed
control for the unit may help to improve energy efficiency of the units
in operation.

Energy efficiency of FFU systems can vary significantly from system to
system. There are many factors contributing to the overall efficiency.
These factors include the size and layout of the overall recirculation
systems, the efficiency of individual fans and fan‑filter units, the
filter media, the controllability of the airflows, and pressures in the
air systems.

### Approaches

FFU applications in recirculation air systems are becoming more popular
in cleanrooms and minienvironments. The energy efficiency and airflow
performance of such systems can vary significantly.

The design and layout of air delivery systems for air‑cooling has
significant effect on the overall energy efficiency of the whole air
recirculation system using FFUS. In addition, an important step to
improve FFU system efficiency is to use and install energy efficient
FFUs in cleanrooms or minienvironments. Selecting energy efficient FFUs
is critical because they tend to be more efficient under a range of
typical operating conditions compared with less‑efficient ones do. At
the same time, it is critical that users need to understand and optimize
the operating conditions by monitoring and controlling the FFU systems
so that maximal energy efficiency can be achieved while maintaining
effective contamination control.

For best practice, users and designers should require certain efficiency
criteria for the FFUs during the planning, design, and construction
process. Owners should require that suppliers provide energy performance
information for the units, along with other performance data such as
particle filtration, vibration, and noise. The energy performance data
should be based upon a uniform laboratory testing method developed by
LBNL and IEST. Using performance test data obtained with this method,
users will be able to select the units that are more efficient and
functional. Figure 3 indicates that total efficiency of FFUs varied from
unit to unit and for various operating conditions when a consistent test
method was used in laboratory settings. Under certain conditions, some
FFUs may or may not be able to perform, i.e., to provide the pressure
rise that is needed to overcome system resistance. The relative
magnitude of performance variations is so significant that it is
important to select an efficient and functional unit based upon the
laboratory testing for the anticipated design and operating conditions.

Figure ![](media/image3.wmf){width="5.995833333333334in" height="4.1in"}
Total efficiency of FFUs tested in laboratory

The best practice for FFUs is to adopt FFUs with higher efficiency, and
optimize the operation and control of such systems so that maximum
efficiency can be achieved. In some industries, such as biotechnology
and pharmaceutical areas, cleanrooms are commonly designed to follow
current good manufacturing practice (cGMP). For example, higher airflow
speeds are typically accepted (e.g., 90 fpm) for cleanroom air
recirculation. In these applications, best practice to lower airflow or
air change rates could be feasible but would be challenging. In any
case, improving the efficiency of FFU systems operating at the accepted
higher airflow speeds can result in more energy savings, however.
Variable‑speed drives (VSDs) should be used with fan‑filter units to
provide flexibility and efficiency.

Effective best practice would be to specify performance data such as
power consumption and airflows for the expected conditions. The provided
information should be based upon a consistent testing method to allow
comparisons of performance claims. This will help designers and users to
make informed life-cycle‑cost comparisons.

### Case Studies

Energy efficiency of FFU systems could vary by a factor of three or more
depending on the design of cleanroom systems, operating conditions, and
the units (Figure 4). For example, the benchmarking data suggests that
the efficiency of FFU systems in ISO Class 5 facilities could range from
1276 cfm/kW to 4224 cfm/kW, with all recirculation air systems providing
the required cleanliness levels (Figure 4). In the case of 4224 cfm/kW,
sensible cooling coils were integrated with the FFU recirculation air
system, which didn't require additional fans to deliver the cooled air.
The integration of sensible cooling device with the air‑recirculation
system significantly improved the overall air‑delivery efficiency of the
FFU recirculation system, compared to separate sensible cooling device
that requires additional fans to deliver cooled airflows as part of air
recirculation.

On one hand, cleanroom operators may use a lower air change rate (or a
lower airflow speed) than those recommended without compromising either
production or cleanliness requirements; on the other hand, if the whole
recirculation air systems can be designed, installed, and operated with
efficient units and control, the energy use for air systems can be
significantly lowered while maintaining effective contamination control.

FFU systems can monitor and control individual fan operation and are
advantageous in efficiently achieving effective contamination control
when coupled with demand control filtration.

![](media/image4.wmf){width="5.995833333333334in" height="4.1in"}

Figure 4 FFU efficiency for ISO Class 5 cleanrooms (kW/cfm)

### Related Best Practices

-   Air‑change Rate

-   Minienvironments

-   Demand‑control Filtration

-   Fan Efficiency

-   Right Sizing

-   Filters

### References

1.  Xu, T. 2003. Performance Evaluation of Cleanroom Environmental
    Systems, Journal of the IEST, Vol 46: 66‑73. The Institute of
    Environmental Sciences and Technology (IEST), 5005 Newport Drive,
    Rolling Meadows, Illinois 60008-3841, USA.

2.  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.

3.  IEST. 1998. Considerations in Cleanroom Design (IEST RP-CC012.1),
    Institute of Environmental Sciences and Technology (IEST), Rolling
    Meadows, Illinois 60008-3841, USA.

4.  Xu, T. 2005. Laboratory Method of Testing Energy Performance of
    Fan‑Filter Units, Version 1.3 (2005), LBNL‑54626.

5.  IEST CC RP36.1 (Draft). Testing Fan‑Filter Units. Draft Recommended
    Practice. The Institute of Environmental Sciences and Technology
    (IEST), 5005 Newport Drive, Rolling Meadows, Illinois 60008-3841,
    USA.

6.  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), 5005 Newport Drive, Rolling Meadows, Illinois 60008-3841,
    USA.

7.  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>

### Resources