HVAC EQUIPMENT: COOLING SOURCES (see Chapter 16) General The process of cooling is actually removal of heat. Comfort cooling is almost always accomplished by cooling air and then distributing the air into the space, where it mixes with the room air and cools down the entire volume.

HVAC EQUIPMENT: COOLING SOURCES Processes of cooling (see section 16.4 for details) There are four principal processes of cooling: a) vapor compression refrigeration, b) absorption refrigeration, c) evaporative cooling, and d) natural ventilation. Cooling processes almost always involve circulating air through a machine that cools air down and blows it with a fan back into the space to be conditioned. The fluid that imparts the cooling effect to air is either a refrigerant--which changes from a liquid to a gas--or water. The central cooling generation equipment is referred to as DX (direct expansion) when the fluid used is a refrigerant. If the fluid is water, then the equipment is called water chiller.

HVAC EQUIPMENT: COOLING SOURCES Vapor compression refrigeration (see sections 16.4, 16.6, 16.7 for details) The vapor-compression cycle is a method of transferring heat from one location to another. The cycle consists of four basic components: a) evaporator, b) compressor, c) condenser, and d) expansion valve. The evaporator transfers heat from the space to the refrigerant, causing the liquid refrigerant to evaporate into a gaseous state. The compressor compresses the refrigerant gas, causing it to become much warmer than the outside air. The refrigerant enters the compressor on the "suction side." After it leaves the compressor, the refrigerant is referred to as "hot gas." It uses electricity as energy.

HVAC EQUIPMENT: COOLING SOURCES The condenser transfers heat to the outdoors, and thus condenses the compressed refrigerant back into a liquid for use in the next cycle. Piping that transfers refrigerant away from the condenser is called the "liquid line." The final step in the cycle is the expansion of the refrigerant in an expansion valve. This relieves the pressure built up by the compressor. Temperature is thus reduced even further allowing the refrigerant to absorb more heat from the interior space when it re-enters the evaporator. A number of chemical compounds, commonly known as freon, are used as refrigerants. R-22, R-134, R404. Until the mid-1990's, dominant refrigerants were chloroflorocarbons (CFC). Hydroflorocarbons (HFC) and hydro-chloroflorocarbons (HCFC) are gradually replacing the CFCs in existing air-conditioning equipment.

HVAC EQUIPMENT: COOLING SOURCES Direct expansion (DX) When the interior air is cooled directly by passing it over an evaporator in which the refrigerant is expanding from a liquid to a gas, the process is known as direct expansion. A remote condenser cannot be located more than 200' away from the evaporator in a DX system. Suitable only for small systems. Multiple DX units to be used for larger buildings.

HVAC EQUIPMENT: COOLING SOURCES Chillers (see sections 16.6,16.7) A water chilled system has to be used for larger buildings. In such a system, the entire refrigeration cycle occurs within a single piece of equipment known as a chiller. An electrically driven water chiller uses the same vapor-compression refrigeration as a DX system. But instead of cooling air, it chills water which is pumped to the air handling units. An air handling unit is an equipment which mixes fresh and return air, cools and dehumidifies the mixture, and distributes it to different areas within a building.

HVAC EQUIPMENT: COOLING SOURCES Condenser water in a water-cooled system is normally re-circulated through a cooling tower, through which heat is rejected into the atmosphere. In a cooling tower, warm condenser water (95ºF) is sprayed into the air and allowed to trickle down over wood, steel, or plastic slats. Outdoor air is drawn through the tower over the trickling water and is then discharged into the atmosphere. This results in evaporation some water. This process decreases water temperature about 10ºF.

HVAC EQUIPMENT: COOLING SOURCES Absorption refrigeration (see section 16.4 for details) This system uses thermal energy instead of electrical energy. Compressor is replaced here by a) an absorber with salt solution that absorbs water vapor from the evaporator, b) a pump that circulates a diluted salt solution, and c) a generator that boils water to reconstitute the salt solution. Because of this reason, an absorption equipment may be very large.

HVAC EQUIPMENT: COOLING SOURCES Cooling efficiency The efficiency of a cooling equipment is measured by an index known as SEER (seasonal energy efficiency ratio). It is the number of BTU removed by 1 watt of electrical energy input. SEER ratings may range from less than 5 to more than 14. Higher the load on the equipment, less the efficiency. Heat pumps (see sections 16.0, 16.5) Capacity of a refrigeration machine: It is measured in several units including BTUH and kW. A common unit in the US is the ton of refrigeration. One ton = 12,000 BTUH.

HVAC EQUIPMENT: COOLING SOURCES Evaporative cooling It is the process of removal of sensible heat and converting it into latent. Heat energy is required to transform a liquid into a vapor state. Amount of heat required is the latent heat of vaporization. As water is evaporated into vapor, sensible heat is drawn from air, reducing the dry-bulb temperature accordingly. The system uses a blower that draws outside air in, passing it through pads kept moist by re-circulated water. The cooled air is delivered directly to the indoor space. The system works only in hot-arid climatic conditions.