Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
271
WIND PRESSURE EFFECTS
Wind effects generate surface pressures which can change
supply and exhaust fan capacities, infiltration and exfiltration of
air, and interior building pressure. Wind can affect environmental
factors (e.g., temperature, humidity, and air motion), dilution
ventilation, and control of contaminants from exhausts.
The pressure exerted by wind on a building surface can be
calculated from the following equation:
Where:
Pw = Wind pressure in pascals (Pa)
Cw = Dimensionless pressure coefficient ranging
from –0.8 for leeward walls through 0.8 for
windward walls
Da = density of air in kilograms per cubic meter
(kg/m 3 )
V = Wind velocity in kilometers per hour (km/h)
EXAMPLE:
For a wind velocity of 11 km/h and a pressure coefficient of
–0.8 (leeward side), the wind pressure on the building is as
follows:
Pw = –58 Pa
STACK EFFECT
Stack effect or thermal buoyancy is the difference between
indoor and outdoor temperature which causes a pressure
difference that affects air movement in a building. Whenever
outdoor air is colder than indoor air, the building draws in air
near the base (infiltration) and expels air near the top
(exfiltration). Conversely, when outdoor air is warmer than
indoor air, the building draws in air near the top (infiltration)
and expels air near the base (exfiltration).
The level of the building at which the differentials of indoor
and outdoor static pressures are equal (with zero wind speed)
is called the neutral pressure level or neutral plane (Fig. 3).
Location of the neutral plane depends on the distribution of
outdoor openings. If one large opening dominates building
leakage, the neutral plane is found close to that opening. For
vertical openings uniformly distributed, the neutral plane is at
midheight. In general, the neutral plane for tall buildings varies
from 0.3 through 0.7 of total building height.
Stack effect can be calculated from the following equation:
C5153
NEUTRAL
PLANE
REVERSE STACK EFFECT NORMAL STACK EFFECT
NOTE: ARROWS INDICATE DIRECTION OF AIR MOVEMENT
Where:
DP = Pressure difference in pascals (Pa)
T
o
= Outdoor absolute temperature in degrees
kelvin(˚F) (273 + ˚K)
T
i
= Indoor absolute temperature in degrees
kelvin(˚F) (273 + ˚K)
h = Height of building in meters (m)
K
t
= Thermal draft coefficient (dimensionless,
varies from 0.63 to 0.82 depending upon
tightness of floor separation relative to that
of the exterior walls)
Fig. 3. Stack Effect in a Building.
CHARACTERISTICS OF FANS
AND FAN LAWS
NOTE: This text provides an overview of fan characteristics and
fan laws. For more information, see the Trane Air
Conditioning Manual listed in REFERENCE.
GENERAL
In airflow systems, a fan converts mechanical, rotative energy
into fluid energy. This is basically accomplished by a wheel or
propeller which imparts a forward motion to the air. For HVAC
applications, fans rarely exceed a total pressure of 3.0 kPa.
Fans must be properly installed to achieve smooth control
and correct performance. In general, manufacturer recom-
mendations should be followed and the following noted (from
Engineering Fundamentals of Fans and Roof Ventilators, Plant
Engineering, Copyright 1982):
— Fans should be located so the discharge of one does not
enter the intake of another fan.
— Intake area should be at least 20 percent greater than
the fan wheel discharge area.
— Fans located opposite from each other should be
separated by at least six fan diameters.
— Elbows or other abrupt duct transformations on the
discharge side of the fan should not be closer than one
diameter from the fan wheel.
— Direction of fan discharge and rotation should be
selected to match duct or mounting requirements.
Pw = Cw
Da x V 2
2
Pw = –0.8
1.2 x 11 2
2
∆
P = 4200K
t x
( )
1
To
–
1
Ti
x h