The terms “low” pressure and “high” pressure in
duct design and selection have, unfortunately,
been given a rather wide latitude of meanings
in the HVAC industry over the years. The terms
have been applied to “pressure” and “velocity”
simultaneously because they are interdependant
in ductwork design. This section
of the catalog is devoted to the definition and
selection of “low” pressure equipment and
components.
The dividing line for “velocity” of air in ducts
has been defined in various applications as
anywhere from 1500 to 2500 fpm and nominally
as 2000 fpm. Empirical data has shown that
duct sections operate satisfactorily over the
above range of velocities at 1" water gage ("wg).
Low pressure systems are chosen where duct
space allows, where air noise is a consideration,
and where particle conveyance such as wood
chips or grain is not a requirement.
Space limitations in modern buildings have
restricted the size of air conditioning ducts
and equipment. Therefore, to convey the
necessary volumes of air, higher velocities must
be employed. Increased velocities produce
higher duct friction losses. In order to maintain
flow against the higher duct friction, it is
necessary to have greater pressures at the air
source. Therefore, the terms “high pressure”
and “high velocity” generally go hand in hand.
Conversely, this is true of “low pressure” and
“low velocity”.
The use of the terms “high velocity” and “high
and medium pressure” in this catalog refer to
any static pressure class of 3" wg or greater,
and “low pressure” refers to 2" wg or less.
SMACNA recommendations on pressure and
velocity are shown in Table 37-1. The listed
classifications pertain to ducts only. Casing
and plenum construction designs are provided
in the SMACNA “Low Pressure” manual and in |
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the “High Pressure” manual, but their respective
designs have been based on historical
acceptability.
| Former Duct Class |
Pressure Rating |
Pressure |
Seal Class² |
Velocity (fmp)³ |
| High Pres. |
10" |
Pos. |
A |
2000 Up |
| Medium Pres. |
6" |
Pos. |
A |
2000 Up |
| Medium Pres. |
4" |
Pos. |
A |
2000 Up |
| Medium Pres. |
3" |
Pos. or Neg. |
A |
4000 Dn |
| Low Pres. |
2" |
Pos. or Neg. |
|
2000 Dn |
| Low Pres. |
1" |
Pos. or Neg. |
|
2000 Dn |
| Low Pres. |
1/2" |
Pos. or Neg. |
|
1500 Dn |
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1 Reproduced in part by Permission From SMACNA High Pressure Duct Standards - 3rd Ed.
2 Seal Class A: All seams, joints, fastener penetrations and
connections sealed.
3 General velocity level through this pressure rated section of the system. Certain points may have higher or lower velocities, e.g., fan outlet or restricted passage, yet not require a different pressure class. The designer makes determinations of duct class after analyzing velocities and operating pressures.
Spiral Manufacturing stocks a complete line of low pressure fittings and installation accessories.
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