Basics about chiller

Back To The Basics

First, it's important to build a foundation of fact. Overall chiller efficiency continues to depend on four, and only four, elements:

• Water-to-refrigerant heat transfer
• Refrigerant-cycle thermodynamics
• Power conversion and transmission efficiency
• Centrifugal compressor efficiency

While the differences from one manufacturer's design to another aren't great in any of these four elements, they are significant enough to merit inspection. The Engineers Newsletter focused on this twice in the past; once in 1981 and again in 1994. Good news! The basics are unaltered and the laws of physics and thermodynamics remain unchanged, so the fundamentals we all learned in our physical science classes still hold true.
That being the case, let's examine the latest chiller performance claims in light of each of these elements.

Heat Transfer

Thanks to advanced technology, and the additional research and development activity prompted by the advent of new alternative refrigerants, heat transfer continues to improve in both evaporators and condensers. The fruits of this labor are evident, as we now observe refrigerant-to-water "approach" temperatures in the 2 to 4 F range. For evaporators, this means it's now economically feasible to obtain refrigerant gas temperatures roughly 3 F lower than the chilled water temperature produced there. Likewise, saturated condensed liquid in the condenser can often be within 3 F of the leaving condenser water temperature. Twenty years ago, normal approach temperatures were 7 to 10 F.

Refrigeration-Cycle Thermodynamics

"Simple cycle" efficiencies based on alternative refrigerants are a trifle lower than their CFC predecessors in HVAC centrifugal chiller applications. As Table 1 indicates, R-123 is about two percent less efficient than the R-11 it replaces, while R-134a is about three percent less efficient than R-12. Various alterations can be made to improve "simple-cycle" refrigeration performance. One of the most common compressor adaptations is the addition of an economizer, also known as an interstage flash chamber. Since this device is physically placed between stages of compression, its use is limited to multistage compressors. Of course, multiple economizers can be used if more than two stages of compression are involved. Table 2 compares the cycle efficiencies for common compressor/economizer arrangements with the "simple cycle" values presented in Table 1
Liquid subcooling benefits some refrigerant cycles. Unfortunately, its potential benefits are insignificant in water-cooled chiller applications because of the small temperature difference between the heat sink (condenser water) and saturated condensed liquid. Heat transfer improvements further diminish this temperature differential. Not surprisingly, subcooling cycles are seldom employed with this kind of equipment.
All commonly used refrigerants respond to changes in "thermodynamic head." As is the case for an ideal refrigerant, less theoretical power is required when the pressure differential between the evaporator and the condenser is reduced. Charts 2 and 3 show the effect of different "heads" on the theoretical power consumption of R-123 and R-134a cycles, respectively. Clearly, head reduction affects each of these refrigerants in a very similar manner.

Power Conversion And Transmission Efficiency

There is almost nothing new to report on these technologies. (We say "almost" because we'll examine the effects of frequency variation later in this article.) Induction motor efficiencies remain in the 93 to 96 percent range for three-phase, 60-Hz motors in the size ranges used by centrifugal compressors.
This performance holds true for both hermetic and open motors; apart from the cooling system, their designs are virtually identical. (The power sacrificed by open motors to drive cooling-air fans is minimal and unworthy of analysis here, although the effect on equipment room cooling and ventilation can be significant.) Both motor designs, for example, use two journal bearings to support the rotor. Motor performance ratings reflect the frictional losses associated with these bearings.
(Editor's Note: The analysis presented in this issue is based on a motor with an overall efficiency of 95 percent.)
Compressors operating at speeds higher than the motor's synchronous speed (3600 rpm for 60-Hz, two-pole motors) require some kind of "step-up" transmission. Various arrangements are used, each with additional gears and bearings. By themselves, the gears absorb about one percent of their power transmission output in heat-producing friction. Efficiency losses attributable to friction typically amount to about 1 hp (2,545 Btu/hr) per bearing.
Consider the performance impact on a common transmission design that employs two gears and five bearings: frictional bearing losses are roughly 5 hp (12,726 Btu/hr) or nearly 4 kW which, in turn, amounts to about one percent of the power output of a 540-kW motor. Thus, total gear and bearing losses in the transmission of an efficient 1,000-ton chiller are about two percent, leaving a transmission efficiency of 98 percent. Of course, chillers without transmissions suffer no such loss.

Compressor Efficiency

Incremental improvements in compressor efficiency continue to be achieved with the ongoing competition between single- and multiple-stage compressor technologies. Both designs are highly refined and well-suited to water chiller applications.
As evidenced by the typical centrifugal compressor "map" in Chart 4, all constant-speed centrifugal compressors lose efficiency at part-load conditions. Consequently, manufacturers generally select compressors at their "sweet spot" for the rated design conditions. Normal chilled water system unloading not only results in lower pressure differential demand, but proportionally less volume as well; i.e., fewer tons of capacity.
Pressure differential, on the other hand, basically follows saturated refrigerant pressure/temperature values between the condenser and evaporator. These values track closely with the leaving water temperature for each of these heat exchangers.
Therefore, as a system unloads, we can expect a lower temperature rise in the condenser water, as well as a potentially lower condenser water supply temperature. These two affects combine to reduce the pressure differential significantly, leading to a much more efficient refrigerant cycle ... but at the expense of reduced compressor efficiency. The net result is better overall chiller part-load efficiency ... to a point. Chart 5 shows this relationship. Points plotted on this chart originate from the data contained in Charts 4 and 2, and incorporate the ARI unloading factor for decreasing condenser water temperature.