HVAC Fan Types and Functions

Introduction to Air Conditioning Systems

• Air systems provide complete sensible and latent cooling, heating, and dehumidification capacity in the air supply.
• Heating can be accomplished by the same airstream or by a separate heater.
• Two basic mechanisms to vary energy removed/supplied by the supply air:
  • Vary the temperature of the supply air (Constant Air Volume - CAV).
  • Vary the amount of the supply air (Variable Air Volume - VAV).

Initial Considerations

• All-air systems operate by maintaining a difference between supply air temperature and desired room temperature (T).
• All fans convert shaft power into heat.
• Inefficiencies add load if the motor is in the airstream.
• The fan type (blow-through or draw-through) affects load.
• Supply ducts can gain or lose heat from the surroundings.
• Uninsulated cooling ducts are subject to condensation, leading to water damage, mold growth, etc.
• Controlling humidity in a space can affect airflow and become the critical factor.

Air Temperature Versus Air Quantity

• Designers have flexibility in selecting supply air temperature and corresponding air quantity.
• The relationship between temperature and air volume is approximately linear and inverse.
• Traditional all-air systems are often designed for 13°C supply air, with a desired indoor temperature of around 24°C.
• Such systems are appropriate for modest latent heat loads and low air absolute moisture.
• Lower supply air temperatures may be required in spaces with high latent heats.
• Initial costs of lower airflow and low air temperature should be considered against potential operational problems including distribution issues, condensation, air movement, and decreased odor and contaminant removal.
• Advantages of cold-air systems include: lower humidity levels, and reduced fan energy consumption.

Zoning

• Exterior zones are affected by weather conditions and may require both heating and cooling at different times.
• The need for separate perimeter zone heating is determined by:
  • Severity of heating load
  • Nature and orientation of the building envelope
  • Effects of downdraft at windows and radiant effect of cold glass surfaces
  • Type of occupancy
  • Operating costs
• Separate perimeter heating is most often used with variable air volume (VAV) systems for cooling.
• Interior zones generally have relatively constant conditions because they are isolated from external influences.
• Interior zones usually require cooling throughout the year, with variable air volume (VAV) systems offering good temperature control.
• Interior spaces with a roof exposure may need treatment similar to perimeter spaces.

Open Plan and Perimeter Zones

• Open plan spaces receive heat gains from lighting, equipment, and people, with no heat loss to the conditioned space.
• Perimeter zones receive heat gains from lighting, equipment, people, and sun, with heat loss from ventilation, conduction, and infiltration (no heat loss to conditioned space).

Air Distribution System

• The air distribution system in all-air systems comprises two major subsystems: Air-handling units (AHUs) and distribution systems.
• Air-handling units generate conditioned air at sufficient positive pressure to circulate air to and from conditioned spaces.
• Distribution systems carry air from the AHU to the conditioned spaces.

Air Handling Units (AHU)

• AHUs are used in large installations to control air conditions like temperature, humidity, and air quality within desired ranges.
• AHUs comprise a series of elements to perform various treatments on air before emission to conditioned zones.
• AHUs do not produce thermal energy; they receive it from heating and/or cooling generators.
• AHUs incorporate components like exhaust dampers, exhaust-return fans, return air dampers, final filters, heating coils, reheat coils, moisture eliminators, supply fans, humidifiers, cooling coils, pre-filters, and sound attenuators.

AHU Functions

• Fans supply air.
• Cooling or heating coils treat air.
• Filters remove impurities from the air.
• Humidity controls set air humidity.
• Mixing sections control the amount of exhaust and outside air.
• Recovery systems recover energy from return air.

AHU Fan Types

• Centrifugal fans produce pressure through centrifugal force and kinetic energy imparted to the air.
• Axial-flow fans produce pressure through a change in the air velocity as the air passes through blades.
• Fan types consist of propeller, tubeaxial, vaneaxial, and airfoil fans.
• Blade curvature (backward curve) increases fan efficiency.

AHU Components

• AHU sections include fans, cooling or heating coils, filters, and mixing sections.
• System recovery components can also be used, such as heat recovery systems.

AHU Psychrometric Processes

• Basic methods for heating include using the latent heat of the fluid, using temperature differences between fluid (water) and air, and using electric heat.
• Other processes, like direct and indirect gas or oil-fired heat exchangers, add sensible heat to the airstream.
• Basic humidification methods include direct spray of recirculated water, compressed air forced through a nozzle into the airstream, and steam injection.
• Dehumidification can use adiabatic mixing of two or more airstreams into a common airstream, or using a desiccant or spraying air with a solution of desiccant and water.

Air-Handling Unit Components and Their Features

• Descriptions of various types of filters, including pre-filters, HEPA filters, filters for larger particles, and activated carbon filters, are presented.
• The methods used to control moisture in a space are also presented.
• The function of mixing boxes and how they can be helpful for space temperature control are also described.

Air Handling Units (AHU)

• Systems that use outdoor air for economizers need return air fans to handle the additional volume.
• Components will include dampers for controlling outside air and return air, as well as fans when a system is designed for mixed air operation.

Terminal Units

• Air terminal units (ATUs) are the devices located between the air systems and the conditioned space.
• ATUs can be passive devices, which deliver and extract air without creating a draft.
• Active ATUs control the quantity and/or temperature of supply air.
• ATUs function in various ways, such as in low velocity to high velocity systems, as well as constant volume or variable air volume. Different types include constant volume with reheat, variable air volume (VAV), throttling without reheat, throttling with reheat, induction VAV, and fan-powered.
• Terminal units such as grills, diffusers, and grilles deliver conditioned air to the required spaces.
  • Grilles are typically made of metal and are installed on walls or ceilings.
  • Diffusers also are made of metal and are used for a variety of applications, and provide air in many directions and planes.

Condensing Loops

• Condensing loops are a method for exchanging heat in buildings with a high volume-to-roof relationship, since there is not enough room for chillers.
• Condenser water systems may be recirculating or cooling tower systems and/or once-through systems.
  • Once-through systems utilize water from city, well, or lake sources.
  • Recirculating systems use cooling towers to exchange heat.
• Cooling towers are a type of condenser water system that cools water by transferring heat to the surrounding air. The method is appropriate when the volume of water is relatively small compared to the heat load.

Control of Air Conditioning Systems

• Controls should be automatic and simple for efficient operation.
• Outdoor air mixing dampers or valves control the amount of outdoor air admitted to the space.
• Controls may include systems such as constant volume reheat systems, and variable air volume (VAV) systems.
• Proper damper selection is critical in ensuring that the system has the ability to control the flow of different types of air.
• Advanced systems use direct digital controllers (DDC), and building management systems (BMS).