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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
Drop

(Total
Static Pressure)

Best Practice
Pressure Drop (Total Static Pressure)

Recirculation Air

1.5
to 3 inches

0.5
to 1 inch

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.


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%.

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<sup>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<sup>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

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 mainte