HVAC Systems Quiz

Applied Building Services - Lecture 2: Air Distribution Systems

• Air distribution systems are categorized into three main types: all-air, air-water, and all-water systems.
• All-air systems centrally produce conditioned air at an air handling unit (AHU) and distribute it through ductwork to spaces throughout the building.
• Air-water systems combine air-based and water-based cooling methods.
• All-water systems use hot or chilled water to transfer heat to or from a conditioned space through piping connected to a boiler, water heater, or chiller.

Air Handling Units (AHUs)

• Central heating and cooling systems use AHUs to distribute conditioned air.
• AHU functions include drawing outside and return air, mixing them, conditioning the mixed air, blowing the conditioned air into the space, and exhausting excess air.
• AHU components include inlet louver screen, parallel blade dampers, opposed blade dampers, relief air damper, actuator, mixed temperature sensor, heating coil, cooling coil, humidifier, supply fan, return fan, and filters.

Air Handling Unit (AHU) Diagram

• The diagram illustrates the internal components as  filter, preheat, cool, heat, and humidify section with air inlet and blow out/fine filter.

Single Zone Systems

• The goal is to supply treated air at a specific temperature.
• The heating or cooling effect depends on the temperature difference between supply and zone air and the flow rate.
• Controllable by changing air volume flow rate, supply air temperature or both.
• Systems have an inlet louver and screen to restrict rain, a return air damper, a relief air damper, a filter, a mixed temperature sensor, heating coil, cooling coil, humidifier, supply fan and return fan.

Mixed Temperature Sensor

• Used for temperature control in single zone systems.
• Normally installed downstream of the filter.
• Start-up: Mixed air temperature equals return air temperature.
• Operation: Outside air is brought into the system upstream of the sensor when dampers are open.
• Common setting: Mixed air temperature is between 13°C and 16°C.
• Adjusting dampers controls the mixed air temperature.

Heating Coil

• Raises the mixed air temperature when needed.
• Can be electric, gas, water or steam.
• Electric coils are common but costly.
• Gas and hot water choices have better cost efficiency but control can be an issue.

Cooling Coil

• Achieves cooling using cold water (5.5–9°C) or refrigerant.
• Condensation may occur due to the lower temperature of the coil.

Humidifier

• Adds moisture to the air by injecting water or steam.
• Controlled by a humidistat.

System Control

• Package rooftop, single-zone units produce treated air at a specific temperature.
• heating or cooling influence depends on the difference between supply and zone temperatures.
• Controlled by altering air volume, supply temperature, or both.

Multiple Zones Air System

• Units serve multiple zones with different varying loads.
• Each zone has an individual thermostat to regulate air volume and/or temperature.

Multiple Zones Air System - Single-Duct, Zoned-Reheat, Constant-Volume systems (Diagram)

• Permit zone control by reheating cool air to the needed temperature within each zone.
• The reheat coil is located near each zone and controlled by its own thermostat. (Diagram shows this system).

Multiple Zones Air System - Single-Duct, Variable-Air-Volume (VAV) Systems

• Used mainly for cooling in hot/warm climates.
• Each zone adjusts its supply air quantity through variable dampers controlled by room thermostats, and the supply fan provides the air.

VAV Advantages and Limitations

• VAV Advantages: Lower initial costs, low operating costs
• VAV Limitations: Poor air circulation at lower flows, leading to insufficient fresh air supply

Multiple Zones Air System- Constant-Volume, Dual-Duct, All-Air Systems

• Uses separate ducts for hot and cold air, and combines them through each zone as needed for temperature control, and maintains constant air flow.

Constant-Volume, Dual-Duct, All-Air Systems (multiple zones)

• Advantages: Serves highly variable loads like hospitals or laboratories
• No reheat coils near zones, avoids hot water coil leakage issue
• Disadvantages: High initial cost, requires two supply ducts, needs extra space above the ceiling

All-Water systems

• Hot or chilled water is used to transfer heat to or from a conditioned space.
• Water flows through pipes connected to a boiler, water heater, or chiller with suitable terminal units in each space.

Fan Coil Units (FCUs)

• Terminal units that condition air.
• Air passes a finned tube containing hot or cold fluid.
• Components include heating/cooling coils, circulation fan, and a control system.
• Typically connected to central boilers or water chillers for heating and cooling respectively.

Fan Coil Unit (FCU) Configurations

• Available in various configurations (e.g., horizontal, vertical).
• Can be used as ductless systems, but also connectable to ductwork. 

Air-Water Systems

• Combines air-based and water-based cooling.
• Applicable if the air system doesn't have enough cooling capacity.
• Used for building ventilation, pressurization and where FCU's are used in the water system design.

Air-Water Advantages and Disadvantages

• Advantages: Well-suited for retrofitting existing buildings with space restrictions, good with high sensible loads, works effectively with humidification.
• Disadvantages: Between-season operation may be difficult, mostly useable in exterior zones.