Packaged Rooftop Air Conditioners

 
Packaged Rooftop Air Conditioners

More than half of all U.S. commercial floor space is cooled by self-contained, packaged air-conditioning units, most of which sit on rooftops (Figure 1). Also called unitary air conditioners or simply "packaged units," these mass-produced machines include cooling equipment, air-handling fans, and sometimes gas or electric heating equipment. Rooftop units (RTUs) are available in sizes ranging from 1 ton to more than 100 tons of air-conditioning capacity.

Figure 1: Up on the roof
Rooftop units are the workhorses of commercial air conditioning and are used widely in industrial facilities as well.
Source: Platts

The three main power consumers in a rooftop unit°™compressor, supply fan, and condenser fan°™account for about 75, 15, and 10 percent, respectively, of the RTU's peak power (Figure 2).

Figure 2: Anatomy of a rooftop unit
The RTU shown contains electric cooling and gas heating components.
Source: Platts
What Are the Options?

Efficiency. RTUs of the same capacity are usually available with a wide range of efficiencies. The Air-Conditioning and Refrigeration Institute (ARI) defines efficiency in several different ways:

EER (energy efficiency ratio): The ratio of the rate of cooling (Btu per hour) to the power input (watts) at full-load conditions. The power input includes all inputs to compressors, fan motors, and controls.
SEER (seasonal energy efficiency ratio): A seasonally adjusted rating based on representative residential loads. SEER applies only to RTUs with a cooling capacity of less than 65,000 Btu per hour.
IPLV (integrated part-load value): A seasonal efficiency rating method based on representative annual commercial loads. It applies to RTUs with cooling capacities equal to or greater than 65,000 Btu per hour.

EER is the rating of choice when determining which RTU will operate most efficiently during full-load conditions. SEER and IPLV are better indicators of which RTU will use less energy over the course of an entire cooling season.

The cooling efficiencies of RTUs under 250,000 Btu per hour are certified according to standards published by the Air-Conditioning and Refrigeration Institute. (ARI standards also apply to RTUs of 250,000 Btu per hour and over, but ARI has no certification program and does not publish efficiency data for this size range.)

ARI is the main source of information about energy efficient RTU products. The organization maintains directories (available in both print and electronic formats) that include products from all ARI member-manufacturers. ARI also maintains a database on its Web site, but it only list units up to a capacity of 65,000 Btu per hour.

Compressor. Most RTUs use efficient reciprocating compressors, with several control options to consider. RTUs normally handle part-load conditions with simple on/off switches, operated by programmable timers, to stage compressors. As an alternative to completely shutting off the compressor, some units offer multiple valve-operated cylinders within the compressor that can be shut off individually. Effectively, shutting off cylinders creates a smaller cooling unit that is nevertheless operating with the original heat exchangers, and the result is a more efficient RTU. Another option is hot-gas bypass, which allows the compressor to provide reduced cooling at low loads. However, this option reduces capacity without reducing energy consumption.

Condenser. Nearly all RTUs under 20 tons have air-cooled condensers, which are about 20 percent less efficient than the evaporative condensers used in larger and more efficient models. Because evaporating water can remove more condenser heat than a stream of ambient air, lower condenser temperature and pressure are attained, and the compressors can therefore run at lower power. For smaller units, however (below about 20 tons), the energy required for pumping and spraying the water can outweigh the compressor energy savings gained by evaporative cooling. Other potential drawbacks are that the savings from water cooling decrease in humid climates and that evaporative condensers require more maintenance than air-cooled condensers.

Economizers. An economizer is an additional dampered cabinet opening that draws air from the outside when outside air is cooler than the temperature inside the building, thereby providing "free" cooling. Many codes, standards, and utility programs already require the use of economizers, and most RTUs have this option. Economizers can reduce energy use by anywhere from 15 to 80 percent depending on conditions, and they are usually cost-effective given their minimal additional cost.

Controls. Programmable digital controls offer flexible settings that can be tailored to the application and are increasingly available as standard equipment. A good example is a seven-day time clock that consistently operates the RTU according to occupancy schedules and nighttime temperature setbacks. Digital controls are also easily tied into a central energy management system for monitoring and control as part of an overall building control strategy.

Cooling coils. Smaller RTUs normally use direct-expansion evaporator coils, in which air is blown over a fin-and-tube heat exchanger that carries the evaporating refrigerant. Larger RTUs can use either direct-expansion or chilled-water coils. In the latter, the cooling water is piped to the RTU from a remote water-chilling unit. A key variable in coil design is the face area, which determines the air velocity over the coil. Most RTUs keep this face velocity below 600 feet per minute to prevent condensed water in the airstream from blowing off the coil and into the duct system.

How to Make the Best Choice

Select the right size. An undersized unit won't be able to provide sufficient cooling, whereas an oversized unit (the more frequent occurrence) not only costs more but will lead to higher costs for associated ductwork and other auxiliaries. Operating costs increase too, because oversized equipment spends more time at less-efficient part-load conditions. Specifiers and designers commonly overestimate loads because they fail to take into account the reduced air conditioning loads that result from energy efficient lighting, and they overestimate plug loads by using nameplate ratings of office equipment in the building.

It is also critical to use diversity factors when calculating internal loads. For example, consider a school: Peak load for the classrooms occurs when the classrooms are full; peak for the auditorium is during an assembly; and peak for a gym occurs during a basketball game with the stands full. However, peak load for the school is not the sum of all these loads, because they do not all occur simultaneously.

Consider high efficiency levels recommended by CEE. The Consortium for Energy Efficiency (CEE) offers a program known as the High-Efficiency Commercial Air Conditioning & Heat Pumps Initiative. The initiative's goal is to encourage the use of high-efficiency unitary (single-packaged and split-system) central air conditioning and heat pump equipment in commercial buildings. CEE currently suggests two efficiency tiers. Tier I specifies levels of high efficiency for commercial equipment that are at least 13 percent greater than the current federal standard. Tier II specifies equipment efficiency levels that are 10 percent higher than Tier I. The use of these tiers is promoted by participating utilities through education and rebate programs.

In June 1999, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) approved an efficiency specification for commercial air conditioning and heat pumps that is equivalent to CEE's Tier I (except that it is for smaller equipment, 65,000 Btu or less). ASHRAE sets professional standards for the HVAC industry, and this specification is part of its energy standard for commercial buildings (ASHRAE 90.1R). This new ASHRAE standard (CEE's Tier I) is likely to become the federal standard in 2003, and therefore CEE is starting to push harder for manufacturers to meet its Tier II requirements. CEE also maintains a database of equipment efficiency data.

Evaluate high-efficiency models by performing a cost-effectiveness calculation. The cost-effectiveness of a high-efficiency RTU depends on several factors, including cooling loads, operating hours, and the local cost of electricity. Use the calculation tool below for preliminary screening of high-efficiency options. For more accurate predictions of performance, an analysis that accounts for local climate conditions and part-load equipment performance is necessary.
Packaged Rooftop Unit Efficiency Screening Tool

No matter what equipment you choose, it's also important to make sure that the overall system is designed to be efficient (see Figure 3), that it's commissioned to operate as planned, and that it is properly maintained. A low-static-pressure duct system will reduce control problems, noise, and the fan power required. Comprehensive testing, adjusting, and balancing of the installed unit and its controls will maximize installed efficiency and comfort. Conducting regular tune-ups, correcting refrigerant charge, cleaning and adjusting the system to correct airflow and improve heat transfer, and repairing major duct leaks can yield surprising energy savings at low cost. CEE offers installation guidelines for commercial air-conditioning equipment.

Figure 3: Rooftop unit components, designed for energy efficiency
We designed this ideal packaged rooftop unit to maximize energy efficiency. Many available systems are built just like this, but many are not, or they have some but not all of these energy features.The RTU shown contains electric cooling and gas heating components.
Source: Platts

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Manufacturers of RTUs continue to offer increasing efficiency and decreasing prices. Currently, for sizes from 65,000 to 135,000 Btu per hour, the highest-efficiency RTUs on the market have EER values as high as 13.5; units from 135,000 to 240,000 Btu per hour have EER values as high as 13.1. Federal government energy-efficiency standards for this equipment are currently being revised, and the final rulemaking should occur by the end of 2004, with standards taking effect in 2008. At a minimum, the new efficiency standards will be 10.3 EER for units from 65,000 to 135,000 Btu per hour and 9.7 for units from 135,000 to 240,000 Btu per hour, but they will likely be higher°™even up to 11.5 EER in each category. Energy Star introduced an efficiency specification for commercial air-conditioning equipment in January 2002 that follows CEE's Tier II specifications. In addition, the Unitary Air Conditioner Technology Procurement Program, jointly sponsored by the U.S. Department of Energy and the Federal Energy Management Program, resulted in the selection of two manufacturers offering high-efficiency models to the public at discounted prices. Information on this program is available at .