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.
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.
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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
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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. Vertical Packaged Units
Vertical packaged units are typically designed for indoor or through-the-wall installation. These units are applied in hotels and apartments. Some designs have a water-cooled condenser, which can be fed from a cooling tower and/or city water. Many others use standard air-cooled condensers. Both style units have all other components mounted inside the package. Ductwork, if needed, can be connected to the unit to distribute the air.
FIGURE 4.2.20 Split system diagram (courtesy of the Trane Co.).
Split-System Packaged Units
Split-system packaged units can have the condenser mounted on an outdoor housekeeping pad or on a rooftop. Refrigerant piping connects the compressor section to an indoor air handling unit and evaporator coil. Unless they are heat pump type units, they cannot provide heat to the space. Heating coils can be installed in the air handling section,
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particularly if there is a central source of heat such as hot water or steam from a boiler. Alternatively, the indoor unit can be coupled to a gas-fired furnace section to provide heating.
Air Source Heat Pumps
Air source heat pump (ASHP) systems are typically rooftop units, either packaged complete or as split systems. Split-package heat pumps are designed with an air handling unit located inside the conditioned space, while the condenser and compressor are packaged in units for outdoor installation on a housekeeping pad or on the roof. During cooling mode, the heat pump operates an air conditioner. During heating mode, the system is reversed and extracts energy from the outside air and provides it to the space. Each of these cycles is shown schematically in Figures 4.2.21 and 4.2.22, respectively. The size of unitary heat pump systems ranges from approximately 5 to 70 kW (11/2 to 20 tons). In some cases, existing packaged cooling units with electric resistance heat can be upgraded to heat pumps for improved energy efficiency.
Heat pump applications are best suited to mild climates, such as the southeastern portion of the U.S., and to areas where natural gas for heating is less available. Space heating needs may exceed the capacity of the heat pump during extremely cold weather. This is because the units are most often sized to satisfy the cooling load requirements. As the outdoor temperature drops, the coefficient of performance (COP) of the heat pump decreases. A 26 kW (71/2 ton) rooftop heat pump unit that has a high temperature (8.3°C) COP of 3.0 can have a low temperature (–8.3°C) COP of 2.0 or less. Because the capacity also drops with outdoor temperature, heat pumps require supplemental electric resistance heat to maintain temperature in the building. Figure 4.2.23 shows typical trends in capacity and COP for an air source heat pump. Chapter 4.2 discusses the characteristics of heat pumps.
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FIGURE 4.2.21 Air or water source heat pump in cooling mode (courtesy of the Trane Co.).
FIGURE 4.2.22 Heat pump schematic showing heating cycle (courtesy of the Trane Co.).
FIGURE 4.2.23 System heating capacity as a function of outdoor air temperature.
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