中央空调冷却塔英文文献翻译 第7页
FIGURE 4.2.18 Comparison between TXV and short-tube orifice systems capacity for a range of charging conditions and 95°F (35°C) outdoor temperature. (From Rodriquez et al., 1996).
As with any HVAC equipment, proper maintenance and operation will ensure optimum performance and life for a system. Split-system air conditioners and heat pumps are the most common units applied in residential and small commercial applications. These units are typically shipped to the construction site as separate components; after the condenser (outdoor unit) and the evaporator (indoor unit) are mounted, the refrigerant piping is connected between them. The air conditioning technician must ensure that the unit is properly charged with refrigerant and check for proper operation. If the system is under- or overcharged, performance can be adversely affected. Rodriquez et al. (1996) found that performance of an air conditioning system equipped with a short tube orifice was affected by improper charge (Figure 4.2.18).
The plot in Figure 4.2.18 clearly shows that for a 20% under-charge in refrigerant, a unit with a short tube orifice suffers a 30% decrease in cooling capacity. This same study also investigated the effects of return-air leakage. A common problem with new installations is improper sealing of duct connections at the diffusers and grills as well as around the return-air plenum. Leakage amounts as low as 5% in the return air ducts resulted in capacity and efficiency reductions of almost 20% for high humidity climates. These reductions dropped to about 7% for low humidity climates. The results of the charging and leakage studies suggest the need for the installation contractor, maintenance contractor, and system owner to ensure the proper installation of the air conditioning system.
FIGURE 4.2.19 Rooftop packaged heating and air conditioning unit. (Adapted from Carrier Corporation).
Packaged Units
Packaged units are complete HVAC units that are usually mounted on the exterior of a structure (roof or wall) freeing up valuable indoor floor space (Figure 4.2.19). They can also be installed on a concrete housekeeping pad at ground level. Because they are self-contained, complete manufactured units, installation costs are usually lower than for a site-built HVAC system.
Single-package units consist of a blower section, filter bank, evaporator coil, at least one compressor (larger units may have more than one), and an air-cooled condensing section. Units may also come equipped with a heating section. Heating is accomplished using either natural gas or electricity. Heat pump systems can be used in situations where electricity is the only source of energy. Unitary heat pumps are restricted in size to no more than 70 kW (20 tons).
As packaged units age and deteriorate, their efficiency often decreases while the need for maintenance increases. Upgrading existing packaged units to high-efficiency models will result in substantial longterm energy savings. In the last 10 to 15 years, manufacturers have made significant improvements in the efficiency of packaged units. The efficiency of energy transfer at both the evaporator and condenser coils has been improved, high-efficiency motors are now standard, and blower and compressor designs have improved in high-efficiency packaged units. Scroll compressors are now commonplace on mediumsized (70 to 210 kW; 20 to 60 ton) rooftop units. Energy efficiencies of newer units have a SEER in the range of 9.50 to 13.0. It is not uncommon to find older units operating at efficiencies as low as 6.0, and most operate at less than 9.0. Gas-fired heating sections typically have an annual fuel utilization efficiency
(AFUE) of about 80%. All newer packaged rooftop units are equipped with factory-installed microprocessor controls. These controls make maintaining equipment easier and improve energy efficiency of both the unit and the overall HVAC system. Control features include temperature setback and on/off scheduling. Larger systems can be delivered with variable air volume capability. Also, most units have an optional communication interface for connection to an energy management control system.
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中央空调冷却塔英文文献翻译 第7页下载如图片无法显示或论文不完整，请联系qq752018766
