Chilled-Water Systems for Cooling

After packaged equipment, chilled-water systems are the most common air-conditioning systems in U.S. commercial buildings, and they predominate in large buildings. Each system includes refrigerant, air, and water loops, and each loop has a circulation device (pump, fan, or compressor). The components of a chilled-water system include a chiller, air-handling units with chilled-water coils, chilled-water loop(s) with chilled-water pump(s), a condenser water loop, condenser water pump(s), and a cooling tower. Optimizing chilled-water systems requires careful integration of these components. The chiller packages a compressor with water-to-refrigerant heat exchangers (evaporator and condenser). It is the heart of the system (and generally the single largest energy user). However, simply selecting a high-efficiency chiller does not guarantee high performance.

Over the past two decades or so, the constraints and opportunities for chilled-water systems have changed dramatically, opening new opportunities for greatly increased efficiency.

• Mandatory phase-out of CFC refrigerants such as R-11 and R-12 has forced users to consider refrigerant or chiller replacement. As of early 2004, roughly half the CFC-using chillers had been converted or replaced with units using other refrigerants (Air-Conditioning and Refrigeration Institute. 2003. "Economy Affects CFC Chiller Phaseout." Press Release. Arlington, Va.: Air-Conditioning and Refrigeration Institute). If nothing else is done, this can have a negative impact on efficiency.
  
  
• Manufacturers have improved the efficiency of their top-end chillers by about 40 percent, offering dramatic reductions in energy use and peak demand.
  
  
• New design tools, such as DOE-2, EnergyPlus, and enhanced versions of Carrier's HAP and Trane's Trace, have improved the ability of designers to understand both peak loads and annual energy use, giving new emphasis to specifying equipment with high part-load efficiency.
  
  
• Cost-effective variable speed drives (VSDs) are being applied to pumps, fans, and the chillers themselves, greatly improving part-load efficiency and minimizing the inefficiencies associated with oversized equipment.
  
  
• Advanced controls allow closed-loop feedback to optimize many more variables than earlier systems could accommodate. Using new sensors and VSDs, controls can now optimize approach temperatures, air-side (and occasionally water-side) economizers, and air delivery.
  
  
• New air distribution systems are emerging. Under-floor air distribution (UFAD) may account for 10 percent of large office construction today, and dedicated outdoor air systems (DOAS) are gaining popularity, too. Both promise greater comfort and lower energy costs, and both may reduce capital costs with good designs.
  

These changes illustrate why chilled-water system design requires an experienced engineer. Traditionally, the engineer estimated or calculated the building load (peak demand), and selected equipment to serve that load. Today, the optimal process differs and should include the following steps amplified from unpublished EPA working papers).

First, peak demand remains very important since so many utilities impose demand charges or time-of-day rates. The demand portion of the electric bill is often roughly the same size as the energy portion. To limit demand charges, the design team should reduce loads wherever it can be done cost-effectively. For example, up-to-date lighting is generally very profitable for both new construction and retrofits. As noted in Chapter 2, modern lighting systems can save 50 to 80 percent of lighting energy (Advanced Lighting Guidelines. EPRI TR-101022s, Revision I. Palo Alto, Calif.: Electric Power Research Institute). Well-designed systems should satisfy needs while using 1 watt/ft2 or less.

Similarly, HVAC "parasitics," the energy and peak demand of system fans and pumps, can rival the chiller's demand. Thus, proper sizing of ducts, fans, piping, and pumps can substantially reduce the size of the motors required. This reduces component costs. Since the engineered design leads to smaller equipment at peak loads, this also means that variable speed drives (below) are more likely to operate where they are most efficient.

Second, part-load efficiency is just as critical for almost all buildings. Designing for part-load efficiency requires comprehensive energy analyses. Again, the building's "parasitics" (the energy use by pumps, fans, and other auxiliary equipment) are generally rich lodes of potential savings through proper pipe and duct sizing (to reduce pressure-related losses) and proper motor selection. For example, a heat recovery wheel with very high pressure drops may require more (fan) energy at part loads than it saves, and thus must be equipped with a bypass if it is to be used cost-effectively.

This leads to a third guideline: think about the system, not just the components.

• Variable speed drives improve part-load efficiency with low peak-load penalties (indeed, since pumps are almost always substantially oversized, variable speed drives almost never run at full speed, so very few ever see penalties from the drive electronics).
  
  
• Consider combining variable speed drives with primary-only water loops using two-way valves on cooling coils. Primary-only pump strategies deliver chilled water directly from the chilled-water pump to the terminal equipment, rather than using hierarchic secondary (or even tertiary) loops for pressure control. Modern chillers are much more tolerant of variable water flow rates and inlet temperatures.
  
  
• Select cooling coils for high temperature drops at design conditions to reduce pumping energy.
  
  
• Cooling tower capacity is much less expensive than chiller capacity. A larger tower will provide cooler water to the chiller at very low cost and thus improve its efficiency at "off-design" conditions. Propeller fans are much more efficient than centrifugal ones and should be selected when possible. Two-speed or "pony" motor systems give almost all of the benefits of variable speed drives for cooling tower fans, at much lower cost.
  
  
• Modern energy management systems are much more sophisticated and able to optimize performance. They allow simultaneous protection of equipment and maximum energy efficiency and are mandatory for complex chiller-based systems.
  
  
• In some cases, heat recovery ventilation or energy recovery (enthalpy) equipment will be cost-effective, typically by reducing the temperature and/or humidity of the outside air introduced for ventilation. It is important to evaluate this equipment both for its peak-load reduction capability and for the impact on part-load efficiencies: some designs have very high pressure losses, requiring large amounts of fan energy even at part load.
  

Thus, a healthy dose of system analysis is likely to pay huge demands. This starts with agreements among the design team on the loads to be met, after taking advantage of all appropriate ways to reduce them. It next requires a detailed load simulation to compute hourly loads for the design year. This has two virtues: (1) it allows comparison of the impacts of design changes, such as loads, approach temperatures, and the effects of adding variable speed drives or more efficient chillers; and (2) the resulting load profiles are also required as inputs to a bid package for chiller manufacturers to design or select the most appropriate chiller(s) for the application.

After following all these steps, you can generally choose a smaller, more efficient chiller with a larger than normal cooling tower to get an efficient, well-functioning system.

Selecting the right chiller. The type of chiller used in any given application is often determined by the desired cooling capacity (tons), product or manufacturer preference, and the viability of a chilled-water system for the application. Other considerations include footprint, weight, and availability. From an energy viewpoint there are three critical selection criteria: the type of condenser (air- or water-cooled), the type of compressor, and the unloading mechanism.

In the United States, built-up chiller systems are water-cooled. In addition to its core functions, the chilled-water loop can be quite useful for other loads such as water-cooled air conditioners for computer or electric equipment rooms. The condenser loop (serving the cooling tower) also provides a low-cost resource for heat-recovery water heating. The major reservations about chilled-water systems are higher first cost and cooling tower issues. In many projects, concerns about siting the cooling tower (physical appearance, biochemical control, water drift and risk of entraining condenser mist at air inlets, and noise) are the leading factors that discourage selection of water-cooled systems.

Four technologies dominate the electrical chiller market: reciprocating, scroll, screw, and centrifugal compressors. Each type has a size range where it is most cost-competitive. In general, centrifugal chillers are the most efficient both at full and part loads and reciprocating are the least efficient. In general, smaller chillers (less than 100 tons) are either reciprocating, screw, or scroll. Screw compressors generally range in size from 10 to 1,100 tons but are most common in the 100 to 300 ton range. In sizes above 300 tons, centrifugal compressors, which range from 70 to 9,000 tons, are generally more cost-effective.

Each chiller type has a unique mechanism for varying capacity with demand ("unloading"), which can substantially affect part-load performance. Reciprocating and scroll machines use multiple compressors to provide stages of cooling capacity. However, cycling compressors on and off results in relatively unstable chilled-water temperatures and is the least efficient of the unloading mechanisms. Screw machines use a slide valve for unloading, and centrifugal compressors use inlet vanes. Variable speed drives are the most effective means of varying capacity with demand for these types, and are increasingly specified.

Centrifugal chillers offer buyers the most flexibility in terms of full- and part-load efficiency. They come in a wide range of sizes and are typically engineered to the users' specifications from a range of shells, tube configurations, impellers, gears, and motors. Careful selection of a centrifugal machine can provide optimal performance as it can be designed to closely match the plant profile. Multiple-stage centrifugal chillers can also be appropriately applied for applications dominated by other compressor types (such as applications with low supply temperatures or high lifts encountered in thermal storage, low temperature air, or heat recovery systems).

Chiller performance ratings. Chillers are specified by their design capacity in tons (1 ton = 12,000 Btuh) and their design efficiency in kW/ton. ASHRAE Standard 90.1 establishes minimum energy efficiency levels. The New Buildings Institute has developed more stringent recommendations that provide increased energy and operating cost savings. These are contained in the table below.

• For motors above 5 hp, always use two-speed or pony motors (e.g., one large and one small motor); adjustable or variable speed drives (VSDs) will provide negligible savings over two-speed or pony motors. VSDs will provide tighter control of the condenser water temperature, which may be valuable with some control schemes, or required if the tower is serving sensitive equipment.
  
  
• Approach temperature optimization should be done by evaluating towers at several possible approaches including a 3ºF to 5ºF low-approach selection.
  
  
• Optimizing controls can save money (see section on HVAC Controls).