Rooftop features for HVAC systems

Efficiency, control capabilities part of a wider range of system applications

12/02/2015


Packaged rooftop systems are commonly installed on curbs, which simplify roof drainage and acoustically isolate the unit from the floor below. Curbs can be either pre-fabricated or built on site. Photo courtesy Daikin AppliedToday's rooftop HVAC systems offer improved efficiency, better building ventilation capabilities, and advanced control capabilities. Rooftops are being used for an increasingly wide range of building applications. They mesh well with many current building designs for manufacturing, warehouses and offices. Unlike many applied systems, they are preassembled for rapid installation and startup, meeting needs of many owners for quick building access.

Range of Sizes and Styles

Most U.S. and Canadian rooftop units are rated in tons of air-conditioning capability (1 ton equals approximately 3.5 kW) and range in size from 6 to 150 tons, and of course can be installed in multiples. A prominent advantage of the rooftop approach is that it eliminates the need in the building interior for most mechanical plant equipment—chillers, air handlers, heating units and associated equipment. Interior building space can be dedicated to primary operations.

Today's new manufacturing plant is often a single-story structure. This is done to facilitate conveyor movement of materials and to simplify future plant expansion or process rearrangement. Horizontal design makes a wide expanse of rooftop available. Larger buildings commonly use multiple rooftop units to minimize interior ducting and to permit effective zoning, as well as to distribute the physical load. For example, a single building might include manufacturing, office, and warehouse areas. Each area has separate comfort, humidity and ventilation requirements, so multiple rooftop units make sense.

Units Combine Multiple Functions

Common elements of modern packaged rooftop units are electric compression cooling, air-handling, and solid state controls. In all but the warmest climates units also have heating components, which can either be resistance electric, heat pump or natural gas fired. Where the heat load is significant, gas is often preferred because of lower energy costs. Common additional features include building ventilation management, humidification/dehumidification, and exhaust air energy recovery. Unit types range from catalogued units with standardized optional elements to custom designs with specific features developed for individual applications.

As building designs and regulatory requirements have evolved, rooftop units for these application have changed as well. New units typically have higher-efficiency scroll or advanced reciprocating refrigerant compressors, often with variable outputs that give longer on-cycles for better dehumidification and smaller temperature swings. On the heating side, today's rooftops use heating modules with staged firing rates, again for higher seasonal efficiency and smaller temperature swings.

A total of 17 Daikin rooftop units were installed on this plant for ZF Transmissions. One attraction for the owner was the rapid installation cycle. Placing unit on the rooftop also frees up floor space for plant operations. Photo courtesy Daikin Applied

Efficiency Improvements Added

Trane, a division of Ingersoll Rand, offers a wide range of rooftop units for these applications. Al Fullerton, Product Business Leader - Unitary for Trane, points out various areas in which the product line has evolved to meet changing building needs. Fullerton notes, "We're adding variable speed compressors to our units. The IntelliPak® 20 to 75 ton line has direct-drive supply fans available, offering lower horsepower in most applications. We also have single-zone VAV available, which modulates supply air fans to save energy and improve humidity control at part-load conditions. These features, in addition to our variable-speed outdoor fans, offer large potential energy savings over previous generation rooftops."

On the gas heating side, Fullerton points out improvements made in newer systems. "We've being improving the turndown capability of our gas unit heater to deal with wider variation in outdoor air load on the rooftop units. On our large rooftops, we can provide up to 18 to 1 turndown. These increased turndown capabilities provide tighter discharge air temperature control when in the heating mode. This results in better occupant comfort." Higher turndown means longer operating cycles, for greater efficiency.

Economizers Reduce Operating Costs

A valuable function in rooftop units is the so-called economizer function. This is the capability of using filtered outdoor air for cooling in lieu of running the compressor, thus saving significant energy. Economizer systems test both the temperature and humidity of the outdoor air to ensure it is cool enough and dry enough to meet building comfort standards. The economizer function is used in many areas during the "shoulder" months when temperatures and humidity levels are both moderate. Use of economizer cooling is a priority in many new building efficiency standards.

According to Curran Ethridge, Senior Marketing Manager at Trane, this area has been a special priority in recent rooftop designs. "We've made great efforts to improve our products to meet and exceed current regulatory requirements, such as Title 24 in California. We have made numerous product enhancements with our high efficiency platforms, using compressor staging coupled with Human Interface controls to greatly increase integrated energy efficiency ratios (IEERs). With the addition of ultra-low leak economizers, we have found ways to meet and exceed these new energy standards."

Fan Improvements Improve Efficiency, Acoustics

Ethridge also mentions that the company has added direct drive plenum fans as an optional item in many of their large rooftop units. These fans provide energy savings as well as acoustical benefits. He adds, "We also have several options such as propeller exhaust fans that can save energy when applied properly."

Another industry leader in rooftop comfort system technology is Daikin, which offers a range of rooftop solutions ranging in size from 3 to 150 tons. According to Julie Carver, Director of Marketing Strategy, a strong emphasis is placed on part-load efficiencies. She explains, "Part-load efficiencies—98% of equipment operating time—are the best measure of efficiency. This is where our Rebel™ [3 to 28 ton] rooftop units excel, providing superior IEER rooftop ratings with up to 84% more total energy savings than ASHRAE's 90.1 efficiency standard."

Carver adds, "This is also the first unit to meet and exceed DOE's Rooftop Unit Challenge specification for energy savings and performance, and one of the industry's most energy efficient rooftops." She also notes that the unit features high-efficiency variable speed compressor and fan motors, advanced electric motor technologies, and indoor fans with no belt losses.

Use of a helicopter for rooftop unit placement is becoming increasingly popular. It allows very short installation cycles. Photo courtesy Daikin Applied

High Gas Heat Turndown Ratios

On the gas heating side, Carver points out that current models feature high turndowns to 5-10% of full capacity and modulating gas furnaces, which are provided on about 10% of today's units. Another energy saving feature for both gas and electricity is 2 inch foam panels with R-13 insulation to reduce thermal losses through the casing.

A major challenge facing building designers and operators today is meeting heightened requirements for building ventilation while still maintaining high energy efficiency levels. This is especially important for manufacturing buildings. In northern climates, the winter energy challenge is bringing in large volumes of frigid outdoor air for ventilation, while exhausting heated building air. In many areas, the even greater summer or year-round challenge is the introduction of hot, humid outdoor ventilation air. New rooftop systems are often equipped with or supplemented by heat exchangers, energy wheels, or similar devices for recovering the energy from exhaust air.

Dehumidification Approaches

Several solutions are used for dealing with moist outdoor ventilation air. One is the improved capabilities of cooling systems for part-load cooling with variable speed compressors and fans. With these, the normal dehumidification function extends over longer cycles. Many newer rooftop units, including those from Trane, Daikin, Carrier and Lennox, also offer hot gas reheat dehumidification functions for ventilation air. These can continue to operate when building cooling needs are met, but continued dry ventilation air is needed.

Energy wheels in newer units make a further contribution to dehumidification when dew points are high and cooling loads are moderate. Finally, dedicated gas-fired desiccant dehumidification systems can do an excellent job of pretreating ventilation air for supply to the rooftop units. This solution is especially valuable in industries such as pharmaceuticals, snack foods, or printing, where required humidity levels are lower than can be achieved by air cooling only.

Advanced Remote Control Capabilities

Control technology for rooftop units has also evolved significantly in the past decade. Fullerton from Trane remarks, "Advanced control systems allow operators to see what is happening with their rooftop units without going to the roof. Alarms can be sent by text or email to facility management personnel when problems occur." He notes that today's control systems can also be accessed by smart phones or tablets. "We also offer 24/7 monitoring service agreements if a customer wants to outsource some of their maintenance." They also offer wireless control systems, which are especially helpful in plant operations where internal operations are frequently changed and comfort or ventilation requirements change accordingly.

Daikin Applied's Intelligent Equipment™ capability offers customers powerful remote diagnostics and the ability to monitor and control their HVAC system 24/7 from anywhere in the world via mobile phone or computer. Daikin Applied also offers the ability to tap into real-time and historical data for planning preventive maintenance and to monitor equipment performance before a failure occurs.

An excellent example of the application of rooftop systems is an installation on a building expansion for ZF Transmissions in Grays Court, South Carolina. The facility manufactures automotive transmissions. The 450,000 square foot building expansion uses 12 Daikin RoofPaK™ and 5 Daikin Maverick™ units for a total capacity of over 1,000 tons. Because of tight schedules, helicopter lifts were used to put the units in place. According to Mike Bledsoe, local Daikin representative, all 17 units were placed on their rooftop locations in 90 minutes, a clear demonstration of the benefits of unit packaging.

Maintenance - Key to Efficiency and Long Life

Rooftop HVAC systems can have a long operating life—15 to 20 years or more is not unusual. Fullerton from Trane points out, "There is no substitute for a strong preventive maintenance program. This can help avoid emergency replacement situations." Even if your rooftops are well-maintained and still operating, it might be useful to have an energy analysis performed by a qualified contractor. As Fullerton points out, "Then a proactive decision can be made whether to replace the unit based on energy savings, or to wait until a major component fails. Fixing on failure often limits the potential savings of a replacement unit, since availability of a new unit tends to drive the decision process."

Today's Units Have Advantages

Rooftops that are being installed today have clear advantages: higher operating efficiency, powerful ventilation capabilities, greater turndown ratios, and better controls. Whether owners are contemplating replacing older systems or are involved in new plant construction, the recommendation is to give the new rooftop units serious consideration.

This article originally appeared in the Gas Technology Winter 2015 issue.



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