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HVAC System Description

Terminology

HVAC System

HVAC (pronounced either "H-V-A-C" or, occasionally, "aitch-vak") is an initialism/acronym that stands for "heating, ventilation, and air conditioning". HVAC is sometimes referred to as "climate control" and is particularly important in the design of medium to large industrial and office buildings such as sky scrapers and in marine environments such as aquariums, where humidity and temperature must all be closely regulated whilst maintaining safe and healthy conditions within.

DX system (Direct-expansion system)

Uses the refrigerant directly as the cooling media. The refrigerant inside the evaporator absorbs heat directly from the air used for space conditioning.

According to the type of condenser used, the DX system has air-cooled and water-cooled system.

DX system basic components

Air-cooled system: Refrigerant hot gas discharge from compressor is being cooled by fan driven by motor to induce outside air to condense the refrigerant within the condenser coil.

Depending on the system requirements, the basic components of DX system can be categorized into various types of equipment combinations for split system, built-up DX system and portable/window unit

Split system: in a typical split system, the condenser and compressor are located in an outdoor unit. The indoor unit consists of evaporator coil, fan motor and blower.

Built-up DX system: dependent upon cooling capacity requirements which would necessitate larger equipment combination similar to those in the split system.

Window or portable unit: is a one piece type of air-con equipment installed in wall opening or modified window casing.

Water-cooled system: Refrigerant hot gas in the condenser coil is being cooled by re-circulating condenser water from cooling tower.

Water-cooled system consists of water-cooled packaged unit, condenser water pump unit, and cooling tower, including inter-connecting condenser water pipework.

Central system

Central system is an air conditioning system which uses a series of equipements to distribute cooling media to exchange heat and supply conditioned air from one point (eg. plant room) to more than one rooms.

Central system normally uses water as the cooling media for larger buildings. The refrigerant inside the evaporator absorbs heat from the water and this water is pumped to coils in order to absorb heat from the air used for space conditioning. Systems that employ water chillers are commonly called chilled-water system.

Central air-conditioning system is a process of removing heat content from multiple sources in the building through a series of heat exchange equipments which ultimately remove the heat to the outdoor atmosphere. This process involves the application of chiller, water pump units and cooling tower as shown below.

According to the type of condenser used, the central air-conditioning system has air-cooled or water-cooled system. Air-cooled and water-cooled systems are categorized by capacity and efficiency.

Chilled Water System Schematic Layout

Air-cooled chilled water system

Air-cooled condenser: A type of condenser in which refrigerant flows through the tubes and rejects heat to air that is drawn across the tubes.

Water-cooled condenser: A type of condenser that rejects the heat of the refrigerant to water flowing through it.

AHU: An air handler, or air handling unit often abbreviated to AHU, is a device used to condition and circulate air as part of HVAC system. Usually, an air handler is a large metal box containing a blower, heating and/or cooling elements, filter racks or chambers, sound attenuators, and dampers. Air handlers usually connect to ductwork that distributes the conditioned air through the building, and returns it to the AHU.

Fan-coil unit (FCU): A small terminal unit that is often composed of only a blower and a heating and/or cooling coil (heat exchanger), as is often used in hotels, condominiums, or apartments.

Expansion tank: A component of a closed piping system that accommodates the expansion and contraction of the water as temperature and, therefore, density, changes.

Water-cooled chilled water system

Cooling tower: Cooling towers are heat rejection devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to reject process heat and cool the working fluid to near the wet-bulb air temperature or rely solely on air to cool the working fluid to near the dry-bulb air temperature.

Condenser water pump unit: is a set of device by using mechanical forces to move the condenser water.

Chiller: A device that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This cooled liquid flows through pipes in a building and passes through coils in air handlers, fan-coil units, or other systems, cooling and usually dehumidifying the air in the building. Chillers are of two types; air-cooled or water-cooled.

Air-cooled chillers are usually outside and consist of condenser coils cooled by fan-driven air.

Water-cooled chillers are usually inside a building, and heat from these chillers is carried by recirculating water to outdoor cooling towers.

Chiller Components

There are several types of water chillers. They differ from each other based on the refrigeration cycle or the type of compressor they use.

Absorption water chillers make use of the absorption refrigeration cycle and do not have a mechanical compressor involved in the refrigeration cycle.

Water chillers using the vapor-compression refrigeration cycle vary by the type of compressor used. Reciprocating and scroll compressors are typically used in small chillers. Helical-rotary (or screw) compressors are typically used in medium-sized chillers. Centrifugal compressors are typically used in large chillers.

 
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