Static pressure calculation in HVAC design
What do you mean by Static Pressure?
Static pressure describes the resistance experienced by air as it
travels through an HVAC system. In other words, it is the pressure a fan must
overcome to move air through ducts, as needed for heating, ventilation and
cooling. Static pressure and airflow are the two main aspects that determine
the operating point of a fan, as well as power consumption.
Various types of HVAC equipment establish an airflow through ducts,
and two common examples are air-handling units (AHU) and packaged rooftop units
(RTU). These pieces of equipment are designed to deliver a specific performance
in terms of airflow and velocity.
For an HVAC system to operate normally, equipment capabilities must
match the needs of the air duct system. In particular, air handling systems must
be capable of overcoming the static pressure. Other parameters considered by
HVAC designers include the installed cost, operating expenses, maintenance,
noise and vibration.
Air Duct Design Process: A Brief Overview
To design duct systems and specify their air-handling equipment,
first you need to calculate heat load and airflow according to ASHRAE
standards. Then, optimal locations for air outlets, fans and air conditioning
equipment can be specified. Finally, duct routes are established according to
the available space.
During the design process, it is very important to avoid
location conflicts with other building systems such as electrical and plumbing
installations. However, this can be simplified with modern MEP BIM modelling
software such as Autodesk Revit.
HVAC engineers can achieve the best results if they consider the
following basic rules when specifying air ducts:
- Minimize ductwork losses to
reduce the fan horsepower required.
- Avoid sudden direction
changes in ducts and provide turning vanes to minimize pressure drop.
- Minimize noise and vibration, since they can be
disruptive for occupants. Vibration can also shorten the service life of
equipment.
- Cost-effective design:
Maximize performance, minimize the usage of power, space and materials.
- Aspect ratio as close
to 1 as possible, no higher than 4.
Duct systems can be classified into three types based on their
static pressure:
- Low-pressure systems,
with a static pressure up to 2 in. w.g.
- Medium-pressure
systems, with a static pressure from 2 to 6 in. w.g.
- High-pressure systems, with
a static pressure over 6 in. w.g.
A higher static pressure leads to increased noise levels. Thus, the
recommended practice is designing air duct systems to have the lowest static
pressure that is technically possible.
The Equal Friction Design Method
There are three main methods that can be used to design air duct
systems:
- Static regain method
- Velocity method
- Equal friction method
The equal friction method is the most common by far, given its ease
of calculation and reduced time consumption. The other two methods are rarely
used in the HVAC industry.
According to the ASHRAE Fundamentals Handbook Ch. 34, the equal
friction method consists on sizing air ducts for a constant pressure loss per
unit of length. The friction rate is described in terms of average pressure
drop per hundred feet of duct.
The ASHRAE Handbook provides charts with a suggested range of
friction rate and air velocity, to offer design flexibility. The most
cost-effective friction rate changes depending on project conditions:
- A low friction
rate is recommended when electricity is expensive and installed
ductwork is affordable.
- A high friction
rate is recommended when electricity is cheap and installed ductwork
is expensive.
After the initial sizing procedure, pressure loss is calculated individually
for all duct sections, and they are resized as needed to balance losses.
Fig 1: Rectangular to
Round Transition Duct fitting
How are Friction Losses Classified?
For design purposes, losses are classified by their source: those
caused by the ducts themselves, and those caused by fittings.
- Duct friction
losses depend on duct size, length and material roughness, as well as
air velocity. The path with the highest-pressure loss is called the
critical path - a major factor to consider during duct design.
- Fitting
losses account for the highest fraction of total losses. They occur
as air moves through filters, offsets, elbows, dampers, coils, and other
fittings and accessories. ASHRAE provides fitting loss coefficients to
simplify their selection - using the right fittings in the right locations
can lead to significant cost reductions and energy savings.
Once all the duct losses have been accounted for, HVAC designers can
easily select a fan that will provide the required static pressure and airflow.
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