Components of a Typical HVAC System
An HVAC system is made up of a number of components.
- Air Handling Unit (AHU)
- Humidifiers and De-humidifiers
- Volume control and Fire/Smoke Dampers
- Air Distribution Diffusers
- Air return Grills
Air Handling Unit
The air handling unit contains heating and cooling coils, the air circulating fans, and the supply and return filters.
Depending on the requirement of hot air or air conditioning, hot water or chilled water can be supplied to the coils. Alternatively, a heat pump can be used with an evaporator/condenser supplying the hot or cold medium.
Excessive noise due to the air being circulated through the ducting is both annoying and distracting to the workforce. To mitigate the noise, attenuators (baffles or silencers) are fitted to the ducting.
The fans or blowers draw the air through heating/cooling coils, circulating it through the supply and return ducting
Suction and Discharge Filters
Suction filters are on the return ducting into the AHU; these remove any dust smoke or other undesirable particles or fumes before the air enters the heating/cooling coils and fan chambers. Discharge filters remove any remaining particles before the air is recirculated again.
Air conditioning can be a very dry method of keeping a building cold. In the seventies, I was flown out to Dubai for a final interview for an engineer’s job at the Dubai Aluminum Smelter and was offered the job, subject to a medical. However, the rheumatoid arthritis that was to bother me for the rest of my life was just kicking in and the smelter’s doctor thought the heat would exacerbate this.
The other medical problem was my daughter's asthma. She was only a few years old but the doc felt that the dryness of the essential air conditioning would bring on more frequent asthma attacks. I didn't take the job and flew back to Inverness the next day, where on arrival home my wife informed me that an offshore oil concrete platform construction yard, away up in the Western Highlands of Scotland wanted me to start as Project Engineer immediately; but that’s another story!
Back to the present and the use of humidifiers; these spray water into a dry hot or cold air stream using very fine sprays.
De-humidifiers on the other hand take the wetness out of the air stream by passing the wet air over cooling coils and storing the condensed water in a tray underneath.
The actual condition of the air supply to the rooms and offices of a building is very important in preventing sick building syndrome with high humidity promoting the growth of molds, mildew and mites, and with low humidity causing throat irritation and dry skin. It is very important to get the humidity of the workplace at the optimum level. Most building regulations recommend this to be maintained between 30% and 50%.
High and low humidity can cause problems not only to employees working in the environment, but also the building’s structure: steel, composites, and wood. However, in the HVAC system, the humidity in the air can be controlled by a humidistat or de-humidistat. The dehumidifier can have antibacterial/ultraviolet treatment against Legionnaires' Disease and the growth of other bacteria/organisms in the condensed water storage tray.
Diffusers and Grilles
Diffusers, being designed to slow the discharge velocity of the air, are normally located in the room’s ceilings. This is to ensure the air is distributed evenly and in the required direction into the rooms and offices of the buildings. These are set into the ceiling; the louvers are automatically adjustable or fixed, directing the air into the return ducting. The return ducting is run back to the discharge plenum, through the return filter, then back into the air handling unit, completing the system circuit. Several examples of efficient diffusers and grilles available from www.atlantasupplies.com are shown below; please click on images to enlarge.
1. Volume control dampers are incorporated into the ducting to control the quantity and velocity of the air flow. Fire and smoke dampers and can also be installed that shut automatically in the event of a fire. These dampers need to be reset manually.
2. The heating and cooling coils can be supplied by hot or chilled water; or a heat pump can also be used. This is explained in more detail in the next section.
3. Ambient air from outside the building can be added and mixed with the circulating air from time to time to freshen the hot or cold air supply to the rooms or offices.
A typical flow diagram for a building's HVAC system is shown in the sketch below. Please note that two types of heating/cooling coils are shown inside the AHU; normally only one type is used unless the system is being operated in very cold winter locations. In this case the heat pump can be accompanied by a hot water coil for winter use.