Understand HVAC Systems
What is a HVAC System? A HVAC system is a heating, ventilation, and air-conditioning system. A system that does any one (or more) of those tasks is called a HVAC system.
The objective of an HVAC system is to ensure that an indoor environment is both safe and comfortable for humans. Safety here mainly concerns the Indoor Air Quality or IAQ, meaning that the indoor air should have enough oxygen and be free of noxious gases. Comfort of course is based on human perception, which can vary within bounds. ASHRAE (the American Society of Heating, Refridgeration, and Air-conditioning Engineers), defines comfortable air quality as one “with which a substantial majority (80% or more) of the people exposed do not express dissatisfaction.”
Although there are many ways in which to make an indoor space in a hot climate comfortable, we shall focus on the most common type of air-conditioning (AC) system found in buildings today. Before we discuss that, a quick word. Most AC systems work on one principle: when a fluid is compressed, it “throws away” the heat it contains, and when it expands, it absorbs heat from its surroundings. Therefore by alternately compressing and releasing a fluid, one can make it “absorb” heat from inside a building and release it outside.
Such a system has 3 components: a central plant, a distribution system, and a rejection unit. The central plant, also called a chiller plant, contains the core of the system. This is the part that actually compresses the fluid (called a refridgerant) in order to make it give off its heat. The distribution system serves to distribute the “coolth” (the opposite of heat) generated by the chillers throughout the building, which must be transferred to the air inside the building. Since it is difficult to move all the air in the building to the central plant, an AC designer will usually transfer the coolth to another fluid, such as water, which is then circulated to a unit on every floor that in turn cools the air. Such a device is called an Air Handling Unit or AHU. It takes cold water from the chiller plant and cools the air coming into it. It essentially consists of a large fan and a heat exchanger, through which the cold water passes on its coolth to the air. Cold water is pumped to AHUs throughout the building through a well-insulated pipe.
The distribution system therefore either distributes coolth or collects heat from throughout the building—two different ways of phrasing the same thing—and passes the heat on to the chiller. The chiller must then reject, or pass on this heat to the external atmosphere. This work is done by the rejection unit. These are of two types, air-cooled and water-cooled systems. Air-cooled systems pass the heat directly to the air; the chillers must therefore be located on the terrace or in a place where a large quantity of air can be passed through to extract all the heat. In a water-cooled system, the heat from the chiller is transferred to a quantity of water, which then is taken to the roof of the building, where a cooling tower transfers the heat to the atmosphere. An air-cooled system consumes more energy but no water; a water-cooled consumes a fair quantity of water, but less energy, and is also quieter. Therefore use a water-cooled system wherever possible. It is also possible to use the water from a sewage treatment plant in a water-cooled system.
When designing an HVAC system, remember that...
When you design an HVAC system, remember that:
- All AHU rooms must have a floor drain, as condensation from the air will collect in the unit.
- One should provide fresh air to every AHU.
- Split Units do not provide any fresh air; they must be used only when one is certain that the doors to the space will be opened frequently.
- All AHU rooms and fan rooms will be at negative pressure, so the doors leading to them must open outwards and be airtight.
- Any rooms with foul air should be provided with extraction, so that they remain at negative pressure. This will prevent the foul air from drifting out to any other space. Kitchens and toilets are an example.
- In an office it is a good idea to provide standalone AC units for conference rooms and executive cabins. These can be used by people working late or on weekends, when the main AC system is off.