FBE03: Building Construction & Science Lecture 8 ACTIVE Ventilation
Introduction • Active ventilation refers to HVAC (Heating, Ventilation, and Air Conditioning) systems that use energy to function. • active systems are usually needed to effectively maintain comfort in all situations
Active Ventilation • Purpose of HVAC systems: • adjust air temperature; • radiant temperatures; • relative humidity; and • air motion to put human inhabitants into the ‘comfort zone’
Active Ventilation • factors during system selection: • Initial cost, performance, and long-term expense of the system, • Fuel and power sources required to run the system, • Size and location of the equipment, • Heating or cooling medium used, • Distribution and return systems – particularly the size, • Outlet size, type, and locations, • Humidity, fresh air, and filtering, • Noise and vibration control.
HVAC systems • Three basic types: • All Air; • All Water; and • Direct Expansion or Refrigerant systems
HVAC systems • All Air Systems have the heating or refrigerating units control the quality of the air, from heat to cold and humidity to filtering. This air is then delivered through ducts to the final room destination. Common types are single and double duct systems. • Single duct systems force either warm or cold air at a constant temperature through low velocity ducts to the spaces. • Double duct systems deliver both warm and cold air simultaneously to mixing units that create the appropriate temperature to deliver to the space.
HVAC systems • All Water Systems deliver hot or chilled water to spaces, which runs in smaller piping than air ducts. This water runs into fan coil units that blow air over the hot or cold coils of fluid to heat or cool the space. Radiators are also used without the fans to heat spaces. Ventilation or air changes must be supplied separately from the system to get fresh air into the spaces.
HVAC systems • Direct Expansion Systems (Refrigerant Systems) are self-contained units used on rooftop or through wall applications. These systems can eliminate ductwork and can serve individual needs of spaces without providing unnecessary capacity – in other words you only run what you need. Small sections of ductwork can also be used to distribute air from a centrally located unit.
Air Distribution Systems • Air Distribution Systems can be centralized or de-centralized which affects the method of distribution. • De-centralized units often require little distribution runs or ductwork because they can blow air directly into the space. • Centralized systems require a more extensive distribution system, which means there is less equipment but more duct runs
Air duct systems • Air duct systems require more space than water systems and must be thought of early in the design process in order to work with structural systems, lights, wall layouts and other services. • These various systems can take large amounts of space above a ceiling and need to be considered with the structural system. • Ductwork distribution typically runs vertically in a chase to feed a floor and horizontally in the floor or ceiling to feed spaces within each floor. This system can be reversed with main feeds horizontally and individual feeds vertically
Air duct systems • Horizontal distribution patterns are typically defined as radial, perimeter or lateral. • Radial patterns use minimal duct runs, but rely on unobstructed space. • Perimeter loop systems work well to resist exterior heat/cold loads and are typically fed from the floor. • Lateral systems use the most duct run, but accommodate flexible distribution patterns, structural obstructions and segmented spaces on a floor
High vs. low supply and return • High vs. Low Supply and Return affects the efficiency of the system at different times of the year • The basic rule of thumb in cold climates is to supply low and return high, this puts warm into the space at the bottom and as it naturally rises it returns into the system • warm weather you are trying to supply cold air low and it wants to stay low rather than mix into the space – making the system inefficient during this cycle of the system
High vs. low supply and return • In cool weather the high supply keep the warm air at the ceiling. The typical way to deal with these problems are to increase the velocity of the air during the off cycle to blow past the natural tendency of the air temperature rise and fall, an inefficient means of solving this issue.
Registers and Diffusers • Registers and Diffusers are arranged to create the desired effects of airflow • Diffusers typically supply air at the ceiling and have curved fins to run air away from the source • Registers are air supply grilles with usually operable damper fins to control air direction. • Registers are typically wall or floor mounted • Return grilles are most often open grates or straight fins that simply pull air back into the system. • The returns are placed away from the supply to not short circuit the airflow through the space
Refrigeration & heat transfer • The Refrigeration Cycle is a process that moves heat from one place to another. • Kitchen refrigerators move heat from the storage area to the surrounding room. • Air conditioners move heat from the rooms to the outdoors • A large quantity of heat is required to change the state of liquid to steam
Cooling cycle • During the Cooling cycle an evaporator coil absorbs heat from its surroundings, heated refrigerant within the coil evaporates internally. • The refrigerant vapour is drawn into a compressor where pressure and, therefore, boiling (or condensing) temperature are increased. • The refrigerant vapour is then discharged into a condenser coil, where it gives up the latent heat absorbed in the evaporator and returns to a liquid state. • Finally, liquid refrigerant circulates through an expansion valve, where pressure and evaporation temperature are reduced; the cycle is then repeated
Heat Pump • A Heat Pump reverses the cooling cycle to extract heat from a low temperature source, such as outside air, to heat a building • The basic equipment is unchanged with the exception of a four-way reversing valve and controls that permit the condenser and evaporator to exchange functions