ASHRAE Journal - December 2020 - 64

COLUMN IEQ APPLICATIONS

The sum of room leakages (∑Q, both air losses
and gains) is the flow offset value (∆V ) of the
room (Figure 1).

Estimate Leakage Airflow Through Doorways and
Cracks

TABLE 1 Examples of protected areas vs. contaminated areas. (Courtesy of Wei Sun, P.E.. 2020.

Presented in his ASHRAE Distinguished Lecturer Seminar/IEST Tutorial Course.)
PROTECTED AREA
(HIGHER PRESSURE)

Stairwells
Commercial
Spaces

BUFFER ZONE

Stairwells

Kitchen

Rest of Building

Restrooms

Rest of Building

CONTAMINATED AREA
(LOWER PRESSURE)

Rest of Building
N/A

Cooking Rooms

Toilet Rooms
To study room pressure control quantitaCleanrooms
Cleanroom
Airlock
Corridor
tively, one has to analyze leakage openings.
Labs
Corridor
Airlock
(BSL-3/4)
Lab
Room
Commonly either the " orifice " equation or
Controlled
Surgical Room
Anteroom
Corridor
Environments Operating Rooms
the " power " equation is used to approxiInfectious
mately describe air leakage through openIsolation Rooms
Corridor
Anteroom Airborne
Patient Room
ings, including irregular cracks or large
doorways. For simplicity,
TABLE 2 Control of contaminants' (particles, microbials or chemical gases/fumes) migration between rooms. (Courtesy of Wei
the orifice equation below
Sun, P.E.. 2020. Presented in his ASHRAE Distinguished Lecturer Seminar/IEST Tutorial Course.)
(2004 ASHRAE Handbook-
HVAC Applications, Chap. 52)
Minimize contaminants' migration from contaminated room into protected room
Goal
through " leakage paths " on the protected room's enclosure
is used in the remaining
discussion. This equation
Dynamic Leakages
When door is open, doorway is the main passage
shows the relationship
When door is closed, irregular cracks (minor openings) become the main
among leakage opening
Common Leakage leakage paths:
size (A), pressure differenStatic Leakages
Paths on Enclosure (1) Between the door and its frame
(2) Between ceiling grid and panels
tial (∆P) across the open(3) Not-well-sealed penetrations for ducts, pipes, conduits
(4) At joints between floor/wall, wall/ceiling, window/wall, etc.
ing, and the resulting leakage flow rate (Q) through
Driving Forces of (1) Pressure Differential (ΔP) across enclosure
the opening:
ΔP

Q = 2,610 A ∆P

(1)

Contaminants'
Movement Through
Leakages

(2) Contaminants' concentration differential (ΔC) across enclosure due to
mass diffusion

ΔC

(1) Make protected room in a higher pressure, create a pressure differential
where
(ΔP) across enclosure to reverse the leakage direction, e.g., to forcefully leak
To Reduce Static
Q = Volumetric leakage
out instead of leak in
Leakages Through
Approaches
Cracks
flow rate, cfm (L/s)
(2) Reduce contaminants' concentration differential (ΔC) across enclosure to
To Tackle
reduce
contaminants'
leakage
level
Contaminants'
A = Equivalent leakage
Migration
To Reduce Dynamic
(3) Or, add a staging/buffer zone (such as anteroom, airlock) between
area (ELA), ft2 (m2)
Leakages Through
contaminated and protected rooms to avoid direct-through leakage, where their
∆P = Pressure differential
Doorways
room-adjacency is necessary
across opening, in.
(1) Make protected enclosure structurally tighter to reduce air leakage, which
w.c. (Pa)
Enclosure's
Two Variables to
is to increase protected room's airtightness that is often quantified in equivalent
Airtightness
(ELA)
2,610= Unit conversion facDetermine a Pressure leakage area (ELA)
Differential (ΔP)
tor, dimensionless
Flow Offset
Between Protected and (2) Create a positive volumetric flow offset (ΔV) between incoming flow
ΔV = SA − RA − EA
Contaminated Rooms (supply air, SA) and departing flow (return and exhaust airs, RA + EA) in
(I-P units)
protected room
Substitute 2610 with 840
as the unit conversion factor
Air Leakage Rate vs. Pressure Differential
if SI unit is used.
Through Leakage Areas
For a pressure differential to be created, an flow
The orifice equation is further illustrated in Figure 3.1
offset is required to intentionally drive air to leak
If a room has an opening size A of 0.5 ft2 (0.0465 m2) or
between rooms; this allows air surplus in one room
is sealed to the same equivalent leakage area (ELA), and
and air deficit in the adjacent room(s), or vice versa
if a pressure differential (∆P) of 0.05 in. w.c. (12.5 Pa) is
(Figures 2a and 2b).
64

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