## Hine Design: Variable Speed Drive Control of Recirculation Fans for Class 100 Cleanroom

Project Benefits Summary

+-----------------------------------------+----------------------------+
| ### Annual Energy Savings               | 372 MWh/y                  |
+-----------------------------------------+----------------------------+
| *Annual Energy Cost Savings*            | \$36,000/y                 |
+-----------------------------------------+----------------------------+
| *Actual Project Cost*                   | \$55,000                   |
+-----------------------------------------+----------------------------+
| *Project Payback*                       | 1.5 years                  |
+-----------------------------------------+----------------------------+

**Facility Description**

![](media/image1.jpeg){width="3.0708333333333333in"
height="2.227777777777778in"}Hine Design, a subsidiary of Asyst
Technologies, operates a robotics manufacturing facility in Sunnyvale,
California. The 45,000-ft^2^ building includes 4,000-ft^2^ of class 100
cleanroom space, 6,000-ft^2^ of combined clean air return chases and
class 10,000 assembly areas, with the remaining building space serving
as their operations and engineering offices. The facility operates from
8am to 5pm, Monday through Friday, and is closed on weekends and
holidays.

![](media/image2.wmf){width="5.975in" height="4.2875in"}All of the clean
air provided to both the class 100 and class 10,000 spaces is filtered
by 99.99% efficient HEPA (high efficiency particulate air) filters
installed in fan powered HEPA units (FPHs). The class 100 space is
comprised of 6 individual bays surrounded by return chases and with the
large class 10,000 assembly area at the north side of the bays. As shown
in Figure 1, air is supplied to the bays by dedicated FPHs and exits
through low sidewall returns into the return chases. The FPHs
recirculate the air from the chases into mixing plenums where
conditioned air is also supplied from two package units located on the
roof. The mixed air in the plenum then passes through 30% filters into
the ceiling plenum above the FPHs. There is no process exhaust from the
bays, but exfiltration from the chases into the office areas requires a
small amount of makeup air to keep the cleanroom positively pressurized.
Therefore, the rooftop package units primarily condition return air from
the class 10,000 assembly area and intake only a small amount of makeup
air. Due to the nature of the manufacturing process and the naturally
mild Sunnyvale climate, there is no provision for humidity control in
the package units.

**Project Description**

In order to reduce energy use in their cleanrooms, Hine Design hired
Northern Pacific Mechanical to design and implement new control logic.
Two specific controls were retrofitted onto the system serving the class
100 bays to provide the energy saving benefits:

-   Variable speed drives (VSDs) on the FPHs serving the class 100 bays
    (shown in Figure 1)

-   A custom control system that schedules the speed of the VSDs based
    upon occupancy patterns

On normal operating days (M-F), the control system operates the VSDs in
the occupied mode from 5am to 5pm, and on weekend days, it operates the
VSDs in occupied mode from 6am to 10am. Based upon particle measurements
within the bays, it was determined that 60% fan speed is appropriate to
maintain cleanliness during operation. At all other times, the control
resets the VSDs to 15% speed to maintain positive flow through the HEPA
filters and the rooftop package units are shut down. As will be
discussed later, when 15% speed is commanded by the control system, the
VSDs actually run at 0 Hz (they turn the fans off).

The theory supporting the energy savings associated with this type of
system is the "cube law" for fans. This law states that the power
required by a fan changes as the cube of the flow induced by it (i.e.
power ∝ flow^3^). This indicates that as the flow through a fan is
reduced or increased by a known factor, the power required by the fan is
reduced or increased by the same factor cubed. Our measurements confirm
savings proportional to the cube law (see the calculations in Appendix
A): at 60% speed, fan power is predicted by the cube law to drop by 86%;
our measurements show an 82% reduction in fan power.

The energy analysis for this project, including formulas, can be found
in Appendix A. The energy cost savings, based upon our measurements, is
approximately \$36,000 per year. The incremental cost of installing the
VSDs and the control system was \$55,000, so the simple payback for this
project works out to 1.5 years.

**Analysis Methodology**

![](media/image3.jpeg){width="2.6708333333333334in"
height="2.0083333333333333in"}To determine the energy savings associated
with the VSD control, power measurements were taken in cleanroom bay 6.
In order to measure both modes of operation, the system operated over a
period of one day. Implied in this measurement is the assumption that
the percentage of power saved in this bay is equivalent to the power
that is saved in all the bays. A PowerSight true RMS power meter (shown
at right measuring VSD power) collected the data at one minute intervals
for just over 24 hours. As shown in Figure 2, the power demand during
each time interval is essentially constant. Therefore, measurements were
taken for only one day, assuming that this data represented the power
demand during each mode of operation throughout the year. In order to
determine the savings associated with this system, we also measured
power demand with the VSD running at full speed for a 15 minute period
(the spike at the far right on the Figure 2 shows our measurements at
full speed). Without the VSDs and controls, all of the FPHs would run at
full speed 24 hours a day, even at night to maintain positive flow
through the HEPA filters. These measurements were then used to calculate
the annual energy cost savings based upon actual average utility rates
for Hine (see Appendix A).

![](media/image4.wmf){width="5.995138888888889in"
height="4.0993055555555555in"}

**Figure 2**

# Discussion

The measurements illustrated above show that the fans draw no power at
15% speed. Therefore, the assumption that 15% speed maintains positive
flow through the HEPA filters was incorrect. It is likely that the VSDs
have been setup with a minimum operating frequency, typically 20 Hz (33%
speed), below which they will shut their output to zero power. Our
investigation of the VSDs with the manufacturer found that the drives
have a low limit parameter that can be set to any frequency (for 15%
speed, this minimum needs to be 9 Hz). This discovery will lead to very
slightly increased energy use as Hine resets the minimum VSD speed to
allow operation at 15% speed and achieve their goal: positive flow
through the HEPA filters to prevent particle release. Extrapolating the
measured results for the system, we have determined that increasing the
fans to 15% speed will increase annual energy use by 1,540 kWh/year
\[(0.15)^2.66^ x 45.9 kW x 5,214 h/y\]. The net annual energy savings
would then be reduced from just over 372 MWh/y to about 371 MWh/y -- a
truly insignificant reduction of 0.4%! The cost impact of this "fix"
would be about a \$150 increase in annual energy bills.

Furthermore, if Hine does modify the VSDs to actually maintain positive
flow through the HEPA filters at all times, they may find that their
particle counts drop during normal operating conditions. Based upon this
information, the existing normal operating speed of 60% may no longer be
necessary to maintain their class 100 rating, at which point they can
further reduce their energy use by slowing the fans down even more. This
feedback effect should at least offset the meager energy use increase,
however it requires that Hine test their particle levels to determine an
appropriate fan speed under the potentially cleaner conditions.

One other discovery during our study of the facility was that the 99.99%
HEPA filters installed in the FPUs were *used* when they were installed;
i.e. they were already at least partly loaded (dirty). This actually
improves the efficiency of the filter because, during use, the particles
fill the pores in the filter media making it even harder for other
particles to pass through. However, loading of the filters also makes it
more difficult for the air to pass through them (higher filter pressure
drop), increasing the amount of energy needed by the fans to recirculate
the air. Another consequence of filter age is that they begin to degrade
(common problems are sagging, tears, loose framing, etc.) and release
particles from stress points. It may be worth investigating the
opportunity to replace the filters with new filters to see how particle
counts and fan energy are influenced. We suspect that fan energy and
particle levels will be reduced, allowing further reductions in fan
speed and related energy use. The flexibility of VSD controls makes all
of these options possible.

Many cleanroom operators, including projects we evaluated at Applied
Materials, Conductus, Exar, and Lam Research, have installed energy
saving controls on their recirculation fan systems that are similar to
the Hine system. Some have installed VSDs that run at constant speed
without scheduling, allowing them to minimize airflow based upon
particle counts, but without the need for independent fan control logic.
This type of system works especially well for facilities that operate
around the clock, where scheduling is not necessary. Still other
facilities, like Applied Materials, are taking the Hine scheduling idea
to another level by installing occupancy sensors that control VSD speed
based upon the activity in the individual clean areas. Rather than fixed
scheduling of fan speed, the occupancy sensors detect whether the space
is in use and modulate the fans up and down accordingly. In this way,
the fans can be reduced any time the cleanrooms are unoccupied,
including during normally occupied times. Another innovation for fan
speed control that also expands on the Hine system concept is that of
real-time particle counting and control of the fans. This system counts
particle levels continually and modulates fan speed to maintain whatever
cleanliness level is required for the space supplied by each fan. This
idea has the potential of tapping into energy savings that few
facilities have achieved[^1].

***Figure 3: Hine Design Case Study Data Analysis***

![](media/image5.wmf)

[^1]: For more about this, see "Energy Savings in Cleanrooms from
    Demand-Controled Ventilation" by David Faulkner, et. al. in the
    *Journal of the Institute of Environmental Sciences; Nov/Dec 1996,
    pages 21-27.*