中央空调冷却塔英文文献翻译 第6页
baseline for future O&M decisions and is very important for a proper maintenance policy.
4.2.4 Packaged Equipment
Central HVAC systems are not always the best application for a particular cooling or heating load. Initial costs for central systems are usually much higher than unitary or packaged systems. There may also be physical constraints on the size of the mechanical components that can be installed in the building. Unitary or packaged systems come factory assembled and provide only cooling or combined heating and cooling. These systems are manufactured in a variety of configurations that allow the designer to meet almost any application. Cabinet or skid-mounted for easy installation, typical units generally consist of an evaporator, blower, compressor, condenser, and, if a combined system, a heating section. The capacities of the units ranges from approximately 5 kW to 460 kW (1.5 to 130 tons). Typical unitary systems are single-packaged units (window units, rooftop units), split-system packaged units, heat pump systems, and water source heat pump systems. Unitary systems do not last as long (only 8 to 15 years) as central HVAC equipment and are often less efficient.
Unitary systems find application in buildings up to eight stories in height, but they are more generally used in one-, two-, or three-story buildings that have smaller cooling loads. They are most often used for retail spaces, small office buildings, and classrooms. Unitary equipment is available only in preestablished capacity increments with set performance characteristics, such as total L/s (cfm) delivered by the unit’s air handler. Some designers combine central HVAC systems with packaged equipment used on perimeter building zones. This composite can solve humidity and space temperature requirements better than packaged units alone. This also works well in buildings where it is impractical for packaged units to serve interior spaces.
Table 4.2.7 lists some of the advantages and disadvantages of packaged and unitary HVAC equipment.
Table 4.2.8 lists energy efficiency ratings (EERs) for typical electric air- and water-cooled split and single package units with capacity greater than 19 kW (65,000 Btuh).
Typically, commercial buildings use unitary systems with cooling capacities greater than 18 kW (5 tons). In some cases, however, due to space requirements, physical limitations, or small additions, residential-sized unitary systems are used. If a unitary system is 10 years or older, energy savings can be achieved by replacing unitary systems with properly sized, energy-efficient models.
a Electric air- and water-cooled split system and single package units with capacity over 19 kW(65,000 Btuh) are covered here.
b EER, or energy efficiency ratio, is the cooling capacity in kW (Btu/h) of the unit divided by its electrical input (in watts) at standard (ARI) conditions of 35°C (95°F) for air-cooled equipment, and 29°C (85°F) entering water for water-cooled models.
c Based on ARI 210/240 test procedure.
d SEER (seasonal energy efficiency ratio) is the total cooling output kW (Btu) provided by the unitduring its normal annual usage period for cooling divided by the total energy input (in Wh)during the same period.
e Split system and single package units with total capacity under 19 kW (65,000 Btuh) are covered here. This analysis excludes window units and packaged terminal units.
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