A green approach to air conditioning systems

12/01/2008


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There's never been a better time for HVAC products, systems, and services that are environmentally responsible to businesses and their stakeholders.

The U.S. Environmental Protection Agency has always encouraged HVAC technologies that maximize efficiency with low emissions. Additionally, owners are encouraged to sustain and document the performance over the lifetime of the application. In some circumstances, it can be a challenge to meet the performance requirements in a cost-effective manner.

The use of low-flow/low-temperature, high-efficiency chilled water systems (LLH) is one way for building owners and engineers to deliver cost-effective, environmentally responsible comfort and added value without excessive added costs. When the systems are integrated with the entire HVAC system, an added benefit is improved IAQ and air-side acoustic characteristics.

LLH systems are not a new concept. They have been used by innovative system engineers for more than 20 years in new construction and renovation/retrofit projects. The fundamentals of an LLH system are similar to traditional systems design, with several notable differences. LLH systems use low supply temperatures and a larger temperature differential. This results in lower flow rates, which inherently can reduce the size of the supply fans, ductwork, pumps, piping, and other HVAC equipment. This provides an opportunity to reduce the first-cost investment in the system infrastructure with reduced operating costs.

LLH systems are not optimal for every building application. It is highly recommended that the system designer uses a comprehensive computerized system simulation program with lifecycle cost analysis to evaluate economic and environmental alternatives. The computer model must include all aspects of the building design to properly evaluate the interaction of individual component modifications.

Conventional VAV systems are typically designed for 55 F supply air temperature. In a larger system, this traditionally is accomplished using a water-cooled chiller with standard design conditions of 44 F leaving water temperature and a 10 F temperature differential for both chilled water and condenser water.

The following discussion will compare a traditional system design to an LLH system. The LLH system will use 41 F leaving chilled water temperature, and 16 F and 15 F, respectively, for the chilled water and condenser water temperature difference. A reduced supply air temperature also will be evaluated in conjunction with the lower chilled water temperatures. The lower air temperature will reduce humidity levels, helping to reduce IAQ by mitigating the potential for mold.

Chillers

When the leaving water temperature is reduced, the kW consumption on the chiller(s) will increase. First, in this example, two 400-ton chillers with screw compressors are used to deliver 800 tons. At ARI Standard 550/590 conditions2, these chillers will produce 44 F water, and will consume 464 kW at full load. When the leaving water temperature is reduced to 41 F, the energy consumption increases to 490 kW, a difference of 26 kW at full load. This is a single component of the system and must be balanced against other improvements in the performance of other equipment. While the chillers consumed 26 more kW at 16 F, the increased temperature difference reduces the water flow, which results in a smaller pump and motor.

The LLH design leverages components that will improve the most in overall system efficiency over the efficiencies of individual components. From a coefficient of performance perspective, chiller efficiencies have improved more than 70% in the past three decades, which allows unlimited flexibility to maximize the overall system performance.

Traditional building HVAC designs will have at least two chillers for redundancy. If these chillers are piped in a series configuration, there is an advantage from cascade cooling (see Figure 1). In this case, the full 26 kW increase can be recovered using lower leaving water temperatures. Cascade cooling simply means the first chiller will have a higher leaving water temperature and works with a higher suction temperature combined with the downstream machine. This configuration will produce the design 41 F leaving water temperature; this configuration can deliver first cost savings of 2% to 4% and energy savings typically in the range of 1% to 4%. Series chiller configurations are well suited for variable primary flow systems. The extra pressure drop, and thus greater pumping energy, of placing the two evaporators in series can be offset with variable speed pumps in a variable primary flow pumping system. This configuration allows for significant pump energy savings at partial loads.

Pumps

Often, the reason for reducing the water temperature is not due to coil performance; temperature might be lowered to reduce chilled water pumping energy consumption by reducing the water flow requirements.

In our example, a conventional 800-ton design would require 1,920 gpm (2.4 gpm/ton). Assume a system pressure drop of 110 ft with pump and motor efficiencies of 80% and 95%, respectively. The energy consumption of the motor would be 52 kW.

On this same system, if a variable frequency drive is applied to the pump, and the chillers producing a supply water temperature of 41 F, the required gpm would be reduced from 1,920 to 1,200 gpm (1.5 gpm/ton). The total pressure drop is reduced to 49 ft, which can be calculated using the pump affinity equation, which states that power varies by the cube of flow reduction. The energy requirement would have been reduced by 36 kW, representing a 70% reduction in pump energy consumption.

In variable flow applications the designer should use a critical zone reset strategy, which allows the system pressure be driven by the most pressure-demanding valve. Motor efficiency should be analyzed over the range of modulation to optimize efficiency and power factor performance throughout the operating curve to meet projected performance.

Cooling towers

As a conventional system this 800 ton-load would require 2,400 gpm at 10 F temperature difference, or 3 gpm/ton. A typical cooling tower fan for this application would require about 40 BHP. In comparison, the LLH system design would require only 1,600 gpm at 15 F difference or 2 gpm/ton, and use a 30 BHP fan motor. This can result in a smaller tower with a smaller footprint.

However, the higher temperature difference will cause the chiller to work at a higher head pressure and consume more energy. System designers need the computer model and a lifecycle cost analysis program to establish the optimum balance between lower tower energy and the increased chiller energy consumption with respect to overall system optimization.

Cooling coils

This example of a traditional system uses a chilled water coil differential temperature of 10 F. The entering temperature would be 44 F, and the leaving temperature at 54 F. These conditions would produce 504 MBH (41 tons) of cooling and require 101 gpm of water. If the system designer selected a 16 F temperature differential, the same coil, now with 41 F entering water temperature, could produce the same 504 MBH with 63 gpm or 37.5% less water.

When the enter water temperature is reduced, the leaving water temperature does not go down, it goes up, typically 0.5 to 1 F for every 1 F reduction in entering water temperature. Actual performance depends on the type and circuitry of the coil. It is also important to note that the MBH delivered in this example remained the same, leaving the supply air temperature unchanged.

The same MBH can be delivered with substantially reduced water flows. If the water flow was held constant and the entering water temperature is reduced, additional capacity can be obtained. These alternatives can provide significant opportunities to either reduce pumping BHP or address cooling capacity problems.

Low air temperature applications

Typically, at least half of the HVAC system's energy consumption is from the air-side equipment of the system. This is due in part to the longer run time of fans and accessories in many chiller applications. Designers have been hesitant to extend LLH beyond the chilled water side of the system due to concerns over condensation. The LLH principles can be extended to the air side if the designer uses supply air temperatures in the 45 F to 48 F range for chillers and 50 F to 52 F for packaged equipment. This allows designers to optimize the energy consumption of both the air and refrigeration side of the system.

Historical data indicate that the combination of an efficient centrifugal chiller, and a supply air temperature (SAT) of approximately 45 F is a good balance point. If somewhat less efficient screw chillers are used, 48 F SAT offers the best balance. If scroll compressors are used, especially in air-cooled applications, like rooftop/VAV, the right SAT may be in the range of 52 F.

For example, a new office-warehouse building in Dallas required an efficient HVAC system for its 60,000-sq-ft two-story space. A 45 F low-temperature air distribution system, complete with ice storage, was installed. The system provided a comfortable indoor environment by controlling not only dry bulb temperatures, but indoor humidity levels, which were critical, based on the outdoor climate conditions. The owner saved approximately $17,000 per year, i.e., about 18%, of its annual operating costs compared to a traditional HVAC system. This was accomplished by leveraging the combination of reduced air-side and low chilled water off-peak utility rates.

Building automation system

An integrated control system is critical to the success of LLH systems. It allows the building owner to sustain maximum building performance. The control system also provides critical data necessary to proactively respond to performance fluctuations and report historical data. Five common strategies that enable building owners to achieve all the benefits associated with LLH are:

  1. Fan pressure optimization, which is a control strategy that will poll the VAV boxes to identify the most static-demanding box. Based on the box position, the controls will reset the system static pressure to the lowest possible pressure required to satisfy the air flow requirements. This can reduce fan energy consumption by as much as 20% to 40% and will diminish VAV box noise due to over-pressurization in the duct system. This also can aid the reduction of duct and VAV box leakage.

  2. Ventilation reset control strategies, such as demand control ventilation and the “Z factor,” will control the correct amount of ventilation on a real-time basis. This will minimize or eliminate costs associated with excess outside air.

  3. Chiller/tower optimization is achievable with BAS control strategies, which can calculate on a real-time basis the optimum balance point between the chiller, the tower, and the condenser water pumps. These programs can be used on new construction and are a reasonably priced upgrade on retrofitted installations, with cost-effective results.

  4. Auto-commissioning/auto-calibration can be done using BAS software to provide information from each zone, including the zone set-point and actual temperature, VAV box performance, and delivered CFM. Discrepancies can be identified with an alarm. This allows a more proactive response as opposed to creating a catastrophic failure.

  5. Virtual graphics help ensure building systems are easy to understand and operate. Realistic graphics speed problem diagnosis and solutions, by identifying the mechanical systems in a “picture-type format.”

System selection tools

Lifecycle cost analysis tools help system designers and building owners model operating cost savings by calculating the optimum supply air temperature and the right leaving water temperature. When combined with a program similar to E-Grid, today's advanced system design tools also can evaluate a project from both lifecycle cost and “carbon footprint” perspectives providing detailed information on levels of utility-generated CO2, SO2, and NOx.

Building models also give owners a baseline for performance comparison, which is crucial to identifying minor problems before they become major failures.

Figure 2 shows an example of the effectiveness of these tools in determining energy savings. Using a lifecycle cost program, the base “conventional VAV” system was modeled in six different sites with the energy consumption set at 100% for each city.

Designers should use lifecycle cost analysis tools to optimize the right water and air-side temperatures and flows. It is important they understand how to design an HVAC system that is efficient at both full- and part-load.

Building owners and designers are increasingly concerned with energy conservation and environmental stewardship, and are searching for cost-effective system options from the design community. LLH systems can deliver both low first cost and energy costs in new construction and retrofit chilled water applications. These systems meet the efficiency and sustainability recommendations of the EPA; when designed using advanced selection tools, installed with an integrated control system, and supported by trained operators, they allow building owners to document the building's predictive to actual performance and carbon footprint. In today's challenging energy economy, building owners need proven systems that will deliver the performance necessary to meet their increasingly integrated environmental sustainability and business goals.

Conclusion

Building owners and designers faced with increased concerns for energy conservation and environmental stewardship and are searching for cost effective system options for their projects. LLH systems can deliver both low first cost and reduced energy costs in new construction and retrofit chilled water applications. These systems not only meet the efficiency and sustainability recommendations of the EPA, but when designed using advanced selection tools, installed with an integrated control system, and supported by trained operators, allow building owners to compare predicted energy use to actual performance and carbon footprint. In today's challenging energy economy, building owners need proven systems that will deliver the performance necessary to meet their increasingly integrated environmental sustainability and business goals. More information on LLH systems is available in the ASHRAE Green Guide3.



Author Information

Smithart, director of systems and solutions at Trane, has more than 30 years of industry experience%%MDASSML%%29 years with Trane and four years with Danfoss Turbocor. He is a recipient of the U.S. EPA Climate Protection award, one of only five people in the world to have received this prestigious award at the time.


Reducing cold downdrafts and condensation

Cold downdrafts and condensation are two critical elements that must be properly managed in low air temperature applications. The following are suggestions to mitigate these issues.

Cold downdraft recommendations:

• The use of parallel, fan-powered VAV boxes at the perimeter is a cost-effective method to control drafts efficiently. An energy benefit of these devices is the ability to mix warm plenum/return air with the cold primary air to satisfy the space needs prior to engaging reheat. These units should be selected to meet acoustical requirements and properly controlled to provide required air flows during low loads, which will eliminate on/off sound fluctuations due to fan cycling.

• Cooling-only variable air volume boxes can be used within interior spaces. Using a supply air reset schedule is recommended to allow for additional hours of economizer usage in addition to minimizing reheat. The interior boxes will need to be designed to meet their loads using this higher reset temperature.

• To accomplish proper air distribution, use linear slot diffusers with aspirating characteristics, described as the “Coanda” effect. Properly selected diffusers will induce room air to mix with the supply air at the ceiling before it is dispersed into the room.

Condensation recommendations:

• Cold surfaces must be kept inside the humidity controlled envelope.

• Insulate supply air ductwork and piping.

• Consider the space temperature and humidity level. Compare this to the dew point of any cold service under suspicion. Insulate the surface or change the space parameters to avoid condensation.

• Night setback and morning pull-down can be controlled by an interior dew point sensor. In a pull-down mode the process should occur in gradual steps to avoid overcooling.

• Positive building pressure is critical to control the interior environment. Ensure the building automation system has the ability, ideally on a floor-by-floor basis, to maintain a slight positive building pressure.

• Design the “P” traps and pitch the condensate drains correctly because LLH systems produce a good deal of condensation. Collecting this condensation by piping the condenseate lines and running them to a tank is an excellent way to conserve water for use in irrigation or make-up water for cooling towers.



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