The principal factor affecting air speed in the ventilation system is air resistance. Airflow increases air resistance in a direct proportion. Pressure loss is the name given to this phenomenon. A fan’s static pressure forces air to move in the ventilation system against some resistance. The lower the airflow in the fan, the higher the ventilation resistance in the system.

Flow Types

The majority of HVAC applications operate between laminar and turbulent flow regimes.

Laminar Flow

• Follow the direction of a boundary layer. The plenum in an HVAC system is a duct.

Turbulent Flow

• Flow that is parallel near the duct’s outside edges and perpendicular to the duct’s core.

Different Pressure Losses or Flow Resistance Types

Total pressure loss in a duct or fitting is referred to as “pressure loss”. The advantages of employing total pressure for duct computation and testing rather than only static pressure can be summed up in three key observations.

1. The only pressure that consistently decreases in ductwork is total pressure. This principle does not apply to static or dynamic pressures alone.
2. The total pressure is the sole way to measure the amount of energy present in an air stream. The total pressure loss, or combined potential and kinetic energy transition, represents the pressure losses in a duct.
3. The pressure is increased both statically and dynamically by the fan energy. Fan ratings that are only based on static pressure are imperfect but frequently utilized.

The components of pressure loss in ductwork

The three components of pressure loss in ductwork are dynamic losses in fittings, component losses in duct-mounted equipment, and frictional losses along the duct walls.

Component Pressure

Because of physical components that cause known pressure drops, including hoods, filters, louvers, or dampers.

Dynamic Pressure

Changes in the direction and velocity of the airflow cause dynamic losses. Any time an air stream turns, diverges, converges, narrows, widens, enters, exits, or passes through dampers, gates, orifices, coils, filters, or sound attenuators, dynamic losses take place.

Frictional Pressure

The cause of frictional losses in duct sections is air viscosity and momentum transfer between particles flowing at various speeds. Unless there are very lengthy duct runs or sizable parts using flexible ducts, these losses also generate minimal gains or losses in air systems.

Ductwork and associated problems

Another vital part that directly influences the performance of the complete HVAC system is the ductwork that distributes the conditioned air. Serious issues relating to this can result from inadequate duct design, poor installation, or leaky or broken ducting.

• Temperature management: It can be challenging to maintain a constant, comfortable temperature within your house when cooled or heated air is lost due to ducting flaws.
• Air adequacy: A defective duct system’s leaks and pressure imbalances can introduce and spread allergens and impurities that degrade the quality of your indoor air.
• Use of energy: Your HVAC system uses more energy if it has to work harder to make up for duct system problems.
• Lifespan of the equipment: Additional work also shortens the lifespan of your heating and cooling systems by causing wear and tear.

Basic principles of good duct design

The duct system’s function is to distribute the optimum amount of heated or cooled air throughout your home at the appropriate temperature, then return stale air to the HVAC system for reconditioning. Such a system must be designed with knowledge of air distribution and thermal gain/loss principles, high-quality materials and manufacturing, and expert installation. These elements are also taken into account in an efficient duct design:

• Correct sizing. The capacity of the HVAC system must be matched by the size of all the ducts. Undersized ducts won’t carry enough air, while oversized ducts will reduce the system’s efficiency.
• A smooth airflow. The duct system must deliver and return an equal volume of air to keep the HVAC system and the entire house at neutral pressure. There must be a certain quantity of supply and return ducts for this.
• Duct positioning Ideal duct placement is within the conditioned envelope of the house, and any ducts that must pass through an uninsulated space, such as the attic, must be well sealed and insulated to prevent energy losses.