# HVAC Air Systems

## Low Pressure Drop Air Systems

### ![](media/image1.png){width="4.156944444444444in" height="3.1215277777777777in"}

> Construction of Pressurized Plenum Recirculation Air Handling System
> Utilizing Vane Axial Fans

### Summary

Fan energy can account for 20 to 40% of total cleanroom energy use. Fan
energy use is directly proportional to the pressure drop that the fan is
pushing air through. Thus, the more restrictive the supply system, the
higher the pressure drop, and the higher the fan energy use. Strategies
for lowering the pressure drop include lower face velocity air handling
units, low pressure drop filters, optimized design of ducting and air
paths, including open plenum and centralized air handler types of
configurations. Low pressure drop designs are applicable to all fan
systems from recirculation air handler systems to makeup air handlers.
Other benefits of low pressure drop systems are less noise, more
effective dehumidification, better filter effectiveness, and in some
cases lower total first cost (when avoided electrical and noise
abatement equipment is included in the cost analysis).

Table 1. Pressure Drop Design Targets

+---------------+--------------------------+--------------------------+
| **System**    | **Typical Pressure       | **Best Practice Pressure |
|               | Drop**                   | Drop** (Total Static     |
|               |                          | Pressure)                |
|               | (Total Static Pressure)  |                          |
+---------------+--------------------------+--------------------------+
| Recirculation | 1.5 to 3 inches          | 0.5 to 1 inch            |
| Air           |                          |                          |
+---------------+--------------------------+--------------------------+
| Makeup Air    | 6 to 10 inches           | 2 to 5 inches            |
+---------------+--------------------------+--------------------------+

### Principles

-   The air handler system power consumption can be estimated by the
    following equation. Note that the efficiency is the product of the
    fan, motor, belt and where equipped, variable speed drive
    efficiencies.

    $$\text{Fan}\text{Power}(\text{kW}) = \frac{\text{Airflow}(\text{cfm}) \times \text{Pressure}\text{Drop}(\text{in}\text{.}w\text{.}g\text{.})}{\text{6,345} \times \text{efficiency}(\text{\%})} \times 0\text{.}\text{746}$$

-   The pressure drop in a duct or air handler is approximately
    proportional to the face velocity squared.

-   The pressure drop in ductwork is inversely proportional to the fifth
    power of the duct diameter. For example, substituting a 16" duct for
    a 12" duct reduces the pressure drop by about 75%.

    $$\text{Duct}\Delta P(\text{in}\text{.}w\text{.}g\text{.}) \propto \left( \frac{1}{\varphi_{\text{duct}}\text{  }(\text{in}\text{.})} \right)^{5}$$

### Approach

The pressure drop of an air delivery system is the design parameter with
the largest impact on the power required by the system. Reducing
pressure drop does not necessarily require new or innovative equipment
or design techniques, it simply requires making lower pressure drop
design a priority and close coordination between the mechanical engineer
and the architect. Most engineers size air handlers with a "rule of
thumb" of 500 fpm. This saves time, but increases cost of ownership.
Below is a table illustrating the typical pressure drops found in
cleanroom recirculation air and makeup air handlers.

Table 2. Typical Recirculation Air Handler Design Pressure Drops^1^

+---------------------------------------+------------------------------+
| **Element**                           | **Recirculation Air Handler  |
|                                       | ∆P**                         |
|                                       |                              |
|                                       | **(in. w.g)^2^**             |
+---------------------------------------+------------------------------+
| Filters                               | 0.75                         |
+---------------------------------------+------------------------------+
| Coil                                  | 0.50                         |
+---------------------------------------+------------------------------+
| External Pressure Drop                | 1.0                          |
+---------------------------------------+------------------------------+
| System Effect                         | 0.30                         |
+---------------------------------------+------------------------------+
| Total                                 | 2.55                         |
+---------------------------------------+------------------------------+

1\. Assumes face velocity of 500 fpm.

2\. in. w.g. - inches of water gauge

^Table\ 3.\ Typical\ Makeup\ Air\ Handler\ Design\ Pressure\ Drops^1

+--------------------------------------+-------------------------------+
| **Element**                          | **Makeup Air Handler ∆P**     |
|                                      |                               |
|                                      | **(in. w.g)^2^**              |
+--------------------------------------+-------------------------------+
| Pre-filters                          | 1.0                           |
+--------------------------------------+-------------------------------+
| Pre-heat Coil                        | 0.50                          |
+--------------------------------------+-------------------------------+
| Cooling Coil                         | 1.0                           |
+--------------------------------------+-------------------------------+
| Dehumidifying Coil                   | 1.0                           |
+--------------------------------------+-------------------------------+
| Heating Coil                         | 0.50                          |
+--------------------------------------+-------------------------------+
| Final Filters                        | 1.0                           |
+--------------------------------------+-------------------------------+
| External Pressure Drop               | 2.5                           |
+--------------------------------------+-------------------------------+
| System Effect                        | 0.30                          |
+--------------------------------------+-------------------------------+
| Total                                | 7.80                          |
+--------------------------------------+-------------------------------+

1\. Assumes face velocity of 500 fpm.

2\. in. w.g. - inches of water gauge

The air handler is the single greatest pressure drop item due to the
coils and filters it contains. To reduce the pressure drop, specify a
low face velocity unit in the 250 to 450 fpm range. The fan power
requirement decreases approximately as the square of the velocity
decrease. The standard arguments against reducing the face velocity are
usually refuted by a lifecycle cost analysis that includes the high
energy costs of a cleanroom system, the continuous operation, and the
additional first costs associated with supplying electrical, fans,
motors, drives, and silencers to higher pressure drop systems. The need
for additional floor space is a non-issue when rooftop units are used
and can be mitigated through close design coordination with the
architect in most cases.

Figure 1. Low Face Velocity Concept

![](media/image2.wmf)

> Drawing courtesy of Lee Eng Lock, E-Cube, Pte. LTD. (www.eco-web.com)

The first cost of the coil is typically only increased slightly, since
the coil requires fewer rows than in a standard air handler as
illustrated in the diagram. The amount of actual coil is not increased
so much as it is simply spread out. Additional considerations are that
the fan motor size can be reduced 25 to 50% or more, which means a
smaller VFD (variable frequency drive, also known as a variable speed
drive), electrical wiring and circuits; the larger filter surface area
can allow a longer change interval, reducing maintenance requirements
and cost. A full system cost analysis that looks beyond the simple air
handler box often finds the "cost premium" of a properly sized air
handler to be negligible or even negative compared to a typical 500 fpm
face velocity system. In one cleanroom cost analysis, a lower face
velocity rooftop system was found to have a lower *first cost* as well
as a lower operating cost due to the downsizing of the electrical supply
infrastructure that the lower power system allowed.

Lower face velocity reduces the pressure across the filters and the
chances of unfiltered air leaking past poor filter rack seals or tears
in the media. The use of filters with lower pressure drop, such as
extended surface minipleat media, is frequently a drop-in option to
reduce energy costs. These filters also have a larger surface area and
load up much more slowly.

Airflow path layout is an important factor in determining system
pressure drop. For example, a pressurized plenum configuration has lower
pressure drop than a ducted HEPA configuration. A ducted HEPA system has
multiple branches tapped from a main duct to connect individual HEPA
filters. Numerous taps and the amount of smaller sized ducting in ducted
HEPA systems result in consistently high pressure drops. A pressurized
plenum offers a much larger air path by eliminating the ducting. The
cost associated with a reduction in ducting can be used to offset .

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

![](media/image3.jpeg){width="6.375694444444444in"
height="3.1458333333333335in"}

Figure 2. Recirculation Air Handling System Design Schematic at Facility
C

The recirculation fan system shown above (referred to as Facility C in a
recent benchmarking study) uses VFD controlled vane axial fans to
pressurize a large plenum. 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 and
provide optimal efficiency at the low design pressure rise of 1" w.g.
The fans run at a low rpm, so no silencers are required to maintain a
low noise cleanroom environment. The only significant pressure drop in
the system is through the HEPA filters, resulting in low required static
pressure.

The design power consumption per airflow delivered from the system was
5,000 cfm/kW. However, the system was measured as performing at an
impressive 10,140 cfm/kW. The large difference is due to the numerous
design conservatisms inherent in air movement systems, such as
oversizing for future build out and the use of fully loaded filter
conditions. The use of VFD fans allowed the safety factors included to
account for unpredictable system to be converted into ongoing operating
savings after construction. The capability to convert design
conservatism and safety factors into operational savings is a hallmark
of good low pressure drop design. Oversizing ducts rather than fans
provides a safety factor on the fan size that lowers operating costs
and, in the case of makeup or ducted systems, provides far more
flexibility for future expansions.

![](media/image4.wmf){width="7.126388888888889in"
height="4.375694444444444in"}

Figure 3. LBNL Benchmarking Study - Measured Pressurized Plenum
Recirculation Air Handling Performance

The figure above shows the variation in performance between pressurized
plenum recirculation air systems as found in the LBNL Benchmarking
project. The worst performer was the cleanroom in Facility J at 2185
cfm/kW. The best performer was the cleanroom in Facility C. The best
performer saved 78% of the fan energy of the worst performer. The
significant difference is due to the air pressure drop of the
recirculation air handling system. The total operating pressure drop for
the recirculation fan system at Facility J was 1.9 inches w.g. as
compared to 0.45 inches w.g. for Facility C.

### Related Best Practices

Recirculation System Types Demand Controlled Filtration

Air Change Rates Right Sizing

Fan Filter Efficiency

### References

1)  

### Resources

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

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

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

-   <http://hightech.lbl.gov/>

-   Clean Spaces, Chapter 16, *ASHRAE HVAC Applications h*andbook, 2003.