More than half of all U.S. commercial floor space is cooled by self-contained, packaged air-conditioning (AC) units.

Most of those are rooftop units (RTUs). Also called unitary air conditioners, these mass-produced machines include cooling equipment, air-handling fans, and sometimes gas or electric heating elements.

Condensing units are also very common and the only difference is the cooling coil or evaporator is not contained within the unit. Condensing units often reside on the rooftop and the cooling coil or evaporator resides in some sort of air handling unit within the facility.

Rooftop air conditioning units can be found in a variety of sizes from one to 100+ tons, with multiple efficiencies at each size range. The three main energy consumers in RTUs are the compressor (75%), supply fan (15%), and condenser fan (10%). The efficiency of an RTU is a function of the size of the heat transfer surfaces, efficiency of the compressor and fan motors, operating parameters like loading, and other ancillary systems like ductwork and insulation. RTUs of the same capacity are usually available in a 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 typically applies to RTUs with a cooling capacity of less than 65,000 Btu per hour or residential AC units.

Most RTUs use reciprocating piston or scroll compressors, and RTUs may contain a single compressor or multiple compressors. RTUs normally handle part-load conditions with simple on\off controls to stage compressors and regulate cooling output.

As an alternative to completely shutting off the compressor, some units offer multiple valve-operated cylinders within the compressor that can unload the cylinders individually. Unloading cylinders effectively creates a smaller cooling unit with reduced capacity, which is operating with the original heat exchangers, and the result is a more efficient RTU.

Another output control option is hot-gas bypass, which bypasses hot refrigerant around the condenser to provide reduced cooling at low loads. However, this option reduces capacity without reducing energy consumption and wastes considerable energy.

Economizers

Most facilities require some cooling even when the outdoor temperature is lower than the indoor temperature. An economizer is a damper opening that draws up to 100% outside air when the outside air is cooler than the temperature inside the building, thereby providing “free cooling.”   Economizers can reduce energy use by 15 percent or more, and are normally required by energy codes for larger air conditioning units.

Thermostat controls

Programmable thermostats offer flexible settings that can be tailored to the application and are increasingly available as standard equipment.

A good example is a seven-day programmable thermostat that consistently operates the RTU according to occupancy schedules for ventilation air and a nighttime temperature set-up and setback.

Making the best choice

Consider size, efficiency and system maintenance when choosing a new air conditioning system.   Emphasis should be put on accurately calculating cooling loads so as to avoid inadequate cooling capacity or poor performance associated with an improperly oversized RTU.   A good design is one where the cooling capacity of the unit barely handles the cooling load. This will provide good moisture removal and humidity control.

A significantly undersized RTU won't be able to provide sufficient cooling, whereas an oversized unit not only costs more but also leads to higher costs for associated ductwork and other auxiliaries. Oversized equipment is also poor at controlling humidity as the units cycle on and off too frequently.

No matter what equipment you choose, it's important to make sure the overall system is designed to be efficient, and that it is properly maintained.  A low-static-pressure duct system will reduce control problems, noise and the fan power required. 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.

Contact Fox Service Company at 442 – 6782 for information on new equipment or maintenance & repairs of existing equipment.

Material Courtesy of Alliant Energy