# HVAC Air Systems

## Recirculation Air System Types

### ![](media/image1.png){width="4.677777777777778in" height="3.248611111111111in"}

### Summary

Recirculation air handler fan energy can account for 10 to 30% of total
cleanroom energy use. There are three fundamental recirculation system
configurations: fan filter unit (FFU), ducted HEPA, and pressurized
plenum. The selection of the system configuration is usually dictated by
building configuration, initial investment cost, and constructability.

![](media/image2.wmf)

![](media/image4.wmf)

Figure 1. Examples of Cleanroom Configurations (MUAH-Makeup Air Handler,
RCU-Recirculation Air Unit, OSA-Outside Air)

The LBNL Benchmarking project measured the performance of a number of
operating installations of each air handling configuration and found the
pressurized plenum design to be the most efficient for class 10 and 100
cleanrooms. Pressurized plenum cleanrooms performed the best due to the
low pressure drop of the supply and return air paths. Within all system
configurations, a large variation in efficiencies was seen; regardless
of the configuration selected, there are many opportunities to reduce
the energy usage by over 50% compared to the average for that system
configuration. When determining the efficiency of a recirculation air
system, the fan energy used to provide sensible cooling also must be
considered.

![](media/image5.wmf){width="5.341666666666667in"
height="3.6881944444444446in"}

Figure 2. Average Measured Efficiency of Recirculation System Types

### Principles

-   The recirculation system efficiency equation is:

    $$\frac{\text{cfm}}{\text{kW}} = \frac{\text{Total}\text{Ceiling}\text{Airflow}(\text{cfm})}{\text{Recirculation Fan kW } + \text{ Sensible Cooling Coil Fan kW}}$$

-   Low fan power consumption is predominantly influenced by low
    pressure drop (see *Low Pressure Drop Air Systems* best practice).
    All system configurations have been seen to realize large energy
    savings from low pressure drop design.

-   Fan and motor efficiency becomes important only when taken to the
    extreme of small motors and fans, such as used in 2' x 4' FFU
    modules.

### Approach

Initial investment cost, site limitations imposed by the building
layout, long term maintenance, and energy efficiency all must be
considered when evaluating the recirculation air configuration for a
cleanroom facility. Data shows that a pressurized plenum system design
can be the best energy performer of the three fundamental system types,
however care must be taken in the initial design effort to properly
implement it. In all systems, low pressure drop design can yield
significant energy savings.

Low pressure drop design is key to an efficient system. The pressure
drops in the return air path as well as through the air handler or fan
filter are the main determinants of performance. The use of deeper
filters in air handlers and the ceiling operating at lower face
velocities lower the total pressure drop regardless of the recirculation
system type. Deeper filters with lower pressure drops result in energy
savings and can allow the downsizing of fans. Increasing the surface
area of HEPA or ULPA filters in a ceiling also results in energy savings
if the face velocity is then reduced. Utilizing plenum returns typically
allows far lower velocities and pressure drops than ducted systems,
regardless of system configuration. A low pressure drop sensible cooling
solution also is critical to maximize system efficiency. Very low face
velocity coils with a low row count were implemented successfully to
keep coil pressure drops to 0.20 inches w.g. (water gauge) or lower in
both pressurized plenum and FFU configurations.

Fan and motor efficiency also play a notable secondary role in the total
system efficiency. FFU cleanrooms consist of many small individual fans.
Small fans and motors are inherently inefficient when compared to larger
fans, causing poor performance in even some low pressure drop FFU
cleanroom configurations. The use of larger FFU modules, such as 4' by
4', can allow the use of more efficient fans and result in significantly
more efficient systems.

Variable flow capability is highly recommended to allow for
implementation of active flow control and/or off hours setback if the
space is ever unoccupied (without people in the space, airflow rates can
often be reduced without impacting the cleanliness of the space). Speed
controlled recirculation fans or FFUs can operate the cleanroom at lower
HEPA face velocities, lowering pressure drop and therefore lowering the
power cost per cfm. (See *Demand Controlled Filtration* best practice).

Another key consideration is cleanroom flexibility. The ability to
target specific cleanroom areas for higher levels of HEPA flow is often
desired. For example, providing higher coverage near the access doors of
minienviroments or over aisles. FFUs tend to be favored for their
flexibility, but ducted HEPA systems and pressurized plenums can also
provide flexibility, requiring only the rearrangement of the non-powered
filter and blank off panels in the ceiling grid. FFUs do tend to be able
to operate with lower plenum height requirements than a pressurized
plenum or ducted HEPA system, although in bay and chase systems the
height requirements can be equivalent.

FFU and pressurized plenum cleanrooms require sensible cooling to be
either provided by cooling coils located in the recirculation air path,
or by air handlers. In many benchmarked sites, the FFUs were found to be
efficient, but the sensible air handler was inefficient, thus
compromising the efficiency of the whole system. The most efficient FFU
systems were in cleanrooms where the coils were large and were in the
return air path, providing low pressure drop cooling that utilized only
the FFUs' built in fans.

### Real World Experiences (Benchmarking Findings / Case Studies)

![](media/image6.wmf){width="6.000694444444444in"
height="4.111805555555556in"}

Figure 3. Measured Recirculation Air Handling Performance for ISO Class
4 through ISO Class 7 Cleanrooms (LBNL Benchmarking Study)

The figure above shows the variation in performance between the three
fundamental recirculation system types in the LBNL Benchmarking project.
The FFU cleanrooms were measured to have similar performance as the
ducted HEPA cleanrooms. The most energy efficient systems were the
pressurized plenum configured cleanrooms, which had an average
efficiency more than double of the FFU and ducted HEPA cleanrooms.
However, the best FFU arrangement, which utilized plenum return with
integrated low face velocity sensible cooling coils, performed better
than some of the pressurized plenum cleanrooms. If a FFU configuration
is dictated by other design factors, careful design can result in a
system that is two to four times more efficient than a typical FFU
system.

The layout of the best FFU cleanroom (referred to as facility G)
measured in the LBNL Benchmarking project is shown in Figure 4. The
recirculation air handling efficiency for this 4,200 sf cleanroom was
4,224 cfm/kW.

![](media/image8.jpeg){width="1.9694444444444446in"
height="1.4666666666666666in"}

![](media/image9.jpeg){width="1.9694444444444446in"
height="1.4666666666666666in"}

Figure 4. Facility G FFU Cleanroom Layout and Cooling Coil in Cleanroom
and Interstitial Space

The recirculation fan system measured in a cleanroom referred to as
Facility C uses VFD (variable frequency drive also known as variable
speed drive) controlled axial fans to pressurize a plenum with a raised
floor and through-chase return. This design is inherently low in
pressure drop. Air return and supply are both via large plenum chambers,
under the floor and above the ceiling. The large airflow paths mean
negligible pressure drops compared to a ducted supply and/or return
system. Multiple large diameter axial vane fans are controlled by VFDs
with a design pressure rise of 1" w.g. At low pressure drops, axial fans
are the most efficient fan type. The only significant pressure drop in
the recirculation system is through the HEPA filters, resulting in low
required static pressure. Sensible cooling was done via an air handler;
in this application low loads minimized the fan power penalty incurred
by the relatively high pressure drop sensible cooling air path. Beyond
the significant energy savings of a low pressure drop system, the low
pressure drop allowed for a slower, quieter fan selection.

![](media/image10.wmf){width="7.225in" height="2.8152777777777778in"}

Figure 5. Recirculation Air Handling System Design Schematic at Facility
C

The design power consumption per airflow delivered from the system was
5,000 cfm/kW. However, the system was actually performing at an
impressive 10,140 cfm/kW. The large difference is due to the numerous
design conservatism's inherent in cleanroom systems, such as oversizing
for future build out, the use of fully loaded filter conditions, and
estimating poorly-defined system effects. The use of variable speed
drives on all the fans in the system allowed it to be tuned to the
actual operating conditions at balance, realizing significant efficiency
benefits from the built-in surplus capacity.

### Related Best Practices

Recirculation System Types Demand Controlled Filtration

Air Change Rates Right Sizing

Fan Filter Efficiency Low Pressure Drop Air Systems

### References

1)  http://ateam.lbl.gov/cleanroom/benchmarking/index.htm.

2)  Xu, Tengfang, "Considerations for Efficient Airflow Design in
    Cleanrooms," *Journal of the IEST*, Volume 47, 2004.

3)  Xu, Tengfang, "Performance Evaluation of Cleanroom Environmental
    Systems," *Journal of the IEST,* Volume 46, August 2003.

### Resources

-   Jaisinghani, Raj, "New ways of thinking about air handling,"
    *Cleanrooms* magazine, January 2001.

-   IEST-RP-CC012.1, "Considerations in Cleanroom Design," The Institute
    of Environmental Sciences and Technology (IEST), 1993.