# HVAC Air Systems

## ![](media/image1.png)Exhaust Optimization

### Summary

Exhaust airflow rates are typically dictated by process equipment
exhaust specifications. Equipment manufacturers' suggested exhaust
quantities have been found to be overstated and not science based. For
example, a recent study by International Sematech found that exhaust
airflows could be reduced in four devices typically found in
semiconductor cleanrooms: wet benches, gas cabinets, ion implanters and
vertical furnaces. The results of the study reported that a reduction of
total exhaust airflow by 28% exists among the four devices tested. The
same study, which measured fume capture and containment effectiveness,
found one piece of equipment where an increased exhaust rate was
required to maintain safe containment.

### Principles

-   All air exhausted from a cleanroom has to be replaced by conditioned
    and filtered makeup air.

-   For a cleanroom facility operating 24 hours a day, costs for exhaust
    air range from \$3 to \$5 per cfm (cubic feet per minute) annually.

-   Building and fire codes require minimum amounts of exhaust for some
    types of cleanrooms. For example, the Uniform Building Code's H6
    classification, which covers many common semiconductor cleanroom
    spaces, requires a minimum of 1 cfm/sf of outside air.

### Approach

Exhaust systems are provided for a variety of reasons. In most
industrial cleanrooms, exhaust design is driven by the need to protect
occupants from hazardous fumes generated by or in process equipment, or
to remove heat generated by equipment located in the workspace. The
first type of exhaust system usually involves the use of fume hoods, wet
benches, or equipment-integrated process equipment fume capture systems.
The fundamental approach to exhaust optimization must be to verify and
improve the safety of workers in the cleanroom.

Often, manufacturer recommendations for exhaust airflow rates are
significantly overstated and/or based on a crude face velocity approach
to estimating exhaust rates required for containment. Good practice
suggests using direct measurements of the containment to set the exhaust
rate. Methods such as tracer gas testing verify and document a safe
operating condition, resulting in safer use. Studies indicate that
proper optimization typically lowers overall facility exhaust flow
rates, resulting in energy savings in addition to the safety benefits.

Conditioning makeup air for a cleanroom is expensive. Makeup air goes
through several processes before it can be delivered to a cleanroom.
Dependent on the space setpoints and the outside climate, the air has to
be filtered, heated, cooled, pressurized by a fan, dehumidified and/or
humidified. Each CFM of makeup air also results in a CFM of exhaust,
which may require treatment before being released. The \$3 to \$5/cfm
energy cost estimate for exhaust air takes into consideration energy for
exhaust/scrubber fans and makeup air. Actual annual energy costs vary
depending on climate, utility costs, and the efficiency of the air
handling systems.

Following are examples of devices found in a cleanroom that can be
targeted to reduce the amount of energy-intensive make-up/exhaust air
required. In these examples, most of the recommendations require
operating the devices below the levels found in the Environmental,
Health, and Safety (ESH) Guidelines for Semiconductor Manufacturing
Equipment. In all cases, proper measurement of the equipment under its
actual operating conditions is required to ensure and enhance operator
safety. Case studies have shown that worker safety can typically be
verified at rates of exhaust below manufacturer's standard ratings. This
highlights that industry guidance and regulatory rules of thumb may be
able to be relaxed provided there is adequate alternative scientific
evaluation.

###### Wet Benches

Wet benches are stations for wet etching and cleaning of wafers and
devices. Products are automatically processed by being dipped and
agitated inside a bath. Exhaust air travels across the surface of the
bath to pull away toxic gases generated at the bench. Many wet bench
manufacturers use a general standard of 135--180 scfm (standard cfm) of
exhaust per linear foot of wet bench. The ESH guidelines recommend
maintaining a wet bench face velocity between 40--100 fpm (feet per
minute).

###### Gas Cabinets

Gas cabinets are designed to maintain a face velocity across the access
window, similar to that of a fume hood. A static pressure sensor
typically maintains the face velocity to ensure a safe working
environment. A baffled bypass that allows for a fixed amount of airflow
to be exhausted is also a component of a gas cabinet.

A reduction in energy consumption of a gas cabinet can be achieved by
eliminating the bypass airways and actively controlling the airflow via
a damper based on a static pressure sensor. Once the door is opened on a
gas cabinet, the exhaust flow would be increased to what would be
required to provide the adequate face velocity. Similarly, when the door
on the cabinet is closed the exhaust flow would be reduced to maintain a
fixed volume of airflow corresponding to a static pressure setpoint via
a volume damper.

###### Ion Implanters

Ion implanters typically consist of enclosures for gas delivery systems
and mechanical equipment. Exhaust for the gas delivery systems is
provided for safety. Exhaust is provided for mechanical equipment, such
as a vacuum pump, and electrical devices for heat removal.

###### Vertical Furnace

A vertical furnace has multiple locations where exhaust is required.
Typically, a vertical furnace consists of an oven chamber; gas
distribution panel (gas "jungle"); liquid chemical distribution system;
and a material handling chamber for automated wafer loading, processing,
and unloading.

###### Fume Hoods

Typically, fume hoods use a variable volume and exhaust system, although
low face velocity, constant volume hoods can offer the same benefits. A
25% reduction in average exhaust airflow (using a variable air volume
system) results in about a 58% reduction in the fan power required.
Significant additional energy savings are realized by a 25% reduction in
the air that is conditioned. Savings from VAV fume hoods are heavily
dependent on the fume hood operators understanding and respecting the
benefits of closing the sash when the hood is not in use.

The Berkeley fume hood developed by LBNL also allows for a significant
reduction in exhaust air. Tracer gas testing comparing the LBNL hood to
a standard fume hood has shown that *improved* containment can be
achieve with a 50% reduction in exhaust airflow.

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

International Sematech evaluated exhaust flows for four semiconductor
process tools -- a gas cabinet, an ion implant tool, a wet bench, and a
vertical furnace -- at Hewlett Packard's Corvallis, Oregon site.^1^ The
Sematech study focused on optimizing exhaust airflows for the
semiconductor process tools while documenting via tracer gas testing no
change or an improvement in worker safety. The four tools combined
resulted in an exhaust airflow reduction of 28%, from 2,994 scfm to
2,146 scfm. At an estimated \$4/cfm of exhaust, over \$3,300 of savings
could be realized per year. Optimization of exhaust for these types of
tools at a typical semiconductor facility could amount to a savings of
more than \$33,000 per year.

The wet bench in the study had a width of 35 inches. The recommended
exhaust flow based on manufacturers' standards would be 394--525 scfm.
This particular bench was operating at 574 scfm and 111 fpm. During
optimization, the wet bench exhaust airflow was reduced by 54%. The
corresponding face velocity was 66 sfpm, which was well above the face
velocity recommended by the ESH guidelines. Most importantly, the wet
bench was able to maintain the concentration levels of the gases
exhausted.

Testing on a gas cabinet showed that the cabinet was already safely
operating at a closed access door flow rate 60% below the manufacturer's
recommend airflow. The airflow and face velocity of the gas cabinet were
only marginally above the limits required by the local codes, so
additional savings were deemed not worth pursuing. Local code and site
requirements for face velocity across an open access window do not
prohibit turndown during the most common closed-window operating
condition. The airflow quantity with the access window closed and the
bypass damper closed was expected to be significantly less than when the
bypass damper was opened, yet the position of the bypass damper had a
negligible impact on the exhaust flow. It was discovered that air was
being bypassed through gaps between the filter and door, and also in
holes used to route sensor cables and purge lines. With these gaps
properly sealed, an estimated 58% savings in exhaust flow could be
achieved by reducing flow further in the normal, closed operating
condition.

The ion implant tool in this test had three locations at where exhaust
was required. The manufacturer recommended 500 cfm for each exhaust
location. As a result of the tracer gas testing, the exhaust quantity
for the gas cabinet was *increased* to improve the capture efficiency of
the box in case of a gas leak. Overall, the exhaust airflows were
reduced from 1,612 scfm to 1,232 scfm.

Exhaust reductions were made on a vertical furnace to the oven chamber,
and gas distribution panel. The material handling chamber and liquid
chemical distribution system were operating at the correct exhaust
quantities; therefore, no changes were made.

Table 1. Summary of International Sematech Exhaust Optimization Study

+------------------+--------------------+--------------------+--------+
| Tool             | **Baseline Exhaust | **Optimized        | *      |
|                  | Flow scfm**        | Exhaust Flow       | *Reduc |
|                  |                    | scfm**             | tion** |
|                  |                    |                    |        |
|                  |                    |                    | **%**  |
+------------------+--------------------+--------------------+--------+
| ## Wet Bench     | 574                | 254                | 56     |
+------------------+--------------------+--------------------+--------+
| **Gas Cabinet**  | 237                | ####### no change  | 0      |
+------------------+--------------------+--------------------+--------+
| ##               | 1,612              | 1,232              | 24     |
| Ion Implant Tool |                    |                    |        |
+------------------+--------------------+--------------------+--------+
| > Right Cabinet  | 778                | 296                | 62     |
+------------------+--------------------+--------------------+--------+
| > Left Cabinet   | 561                | 331                | 41     |
+------------------+--------------------+--------------------+--------+
| > Gas Cabinet    | 273                | 605                | (1     |
|                  |                    |                    | 21)^1^ |
+------------------+--------------------+--------------------+--------+
| ##               | 629                | 474                | 25     |
| Vertical Furnace |                    |                    |        |
+------------------+--------------------+--------------------+--------+
| > Oven Chamber   | 459                | 347                | 24     |
+------------------+--------------------+--------------------+--------+
| > Gas            | 29                 | 15                 | 48     |
| > Distribution   |                    |                    |        |
| > Chamber        |                    |                    |        |
+------------------+--------------------+--------------------+--------+
| > Material       | 13                 | ####### no change  | ###    |
| > Handling       |                    |                    | #### 0 |
| > Chamber        |                    |                    |        |
+------------------+--------------------+--------------------+--------+
| > Chemical       | 64                 | ####### no change  | ###    |
| > Distribution   |                    |                    | #### 0 |
| > System         |                    |                    |        |
+------------------+--------------------+--------------------+--------+
| > Exhaust        | 65                 | ####### 35         | ####   |
| > (other)        |                    |                    | ### 45 |
+------------------+--------------------+--------------------+--------+

1\. (xx) denotes an increase in exhaust flow.

### Related Best Practices

Right Sizing Low Pressure Drop

Air Change Rates

### References

1)  "Exhaust Optimization Studies on Four Process Tools at Hewlett
    Packard's Corvallis Site," Technology Transfer #01034098A-TR,
    International Sematech, 2001.

### Resources

-   Lee, Eng Lock; Rumsey, Peter; Sartor, Dale and Weale, John,
    "Laboratory Low-Pressure Drop Design," *ASHRAE* Journal, August
    2002.