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.
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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 design software.
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.
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.
HVAC design is a critical step of building projects, having a major impact on operation and maintenance expenses in the long run. Occupant comfort also depends strongly on a properly-specified HVAC system, and comfort can boost productivity in business settings. To achieve even higher performance, a ventilation system can be complemented with occupancy sensors and variable-frequency drives (VFD) for fan speed control.