Figure 1. Base case schematic of the Boston Convention & Exhibition Center chiller plant.
Ever since Willis Carrier popularized air conditioning in the early 1920s, chiller plant designers have rarely deviated from the established design parameters of 2.4 gpm/ton and 3.0 gpm/ton for the chilled water flow and the condenser water flow, respectively. These figures are the consequence of maintaining a 10 degrees F temperature differential (DT) between the supply and return temperatures of the chilled water and condenser water and the chiller efficiencies and controls that were common at that time.

When air conditioning started gaining popularity in the early 1920s, the most "high-tech" portable tool available to the engineer was the slide rule. Consequently, the manufacturing processes at the time were limited in flexibility; these limitations are reflective of the technology and computing power of the period. In order to mass-produce, chiller manufacturers could only offer standard-size chillers at 10 degrees DT for both chilled water and condenser water flows.

Today, engineers entering the field of hvac in general and chiller plant design in particular are still being indoctrinated to the same concepts using manuals originally written in 1950s and revised in 1970s. These concepts are outdated and retrograde. It is the dawn of the 21st century and it is time to rewrite the book.

Advances in computing powers of the past 20 years, combined with the proliferation of the PC in the business world and at home, thanks to the microprocessor, have removed many constraints of the design, engineering, and manufacturing processes. The digital revolution of the past two decades has far-reaching consequences in the field of hvac. Chiller efficiencies have nearly doubled. Efficiencies of water-cooled chillers went from the 1.0-kW/ton range to 0.50-kW/ton nowadays.

These achievements are the consequences of improved and more complex compressor and heat exchanger design as well as tighter tolerances of the manufacturing processes. The use of electronic components in control system technology allows systems to operate safely and closer to the edge, including at low flow and with improved head pressure control. The increased availability of variable speed has a growing impact on the part-load operation of the chiller.

These technological advances allow the chiller manufacturers to produce and deliver any chiller size at a wide range of temperature parameters; however, chiller plant designs nowadays still rarely deviate from the standards established nearly a century ago. While these standards streamline the design process, the subsequent lack of innovation and initiative by the professionals in the field does not do justice to the owner who is footing the bill to operate the plant.

A Case Study

With a footprint of 1.6 million sq ft, the new Boston Convention & Exhibition Center (BCEC) will feature 516,000 sq ft of contiguous exhibition space on one level, 160,000 sq ft of meeting space, which includes 86 meeting rooms, and a 41,000 sq-ft-ballroom.

Figure 2. Proposed chiller plant configuration.

Conventional Design

The new BCEC has a peak summer load of 6,800 tons. The conventional design would be to adhere to the old, established standards. The chillers would be selected at 2.4 gpm/ton in the cooling mode. Chilled water would be produced and distributed at 44 degrees and returned at 54 degrees. In the heat rejection mode, the condenser water flow would be set at 3.0 gpm/ton with 85 degrees entering condenser water temperature.

There is not much engineering involved in this chiller plant configuration as long as the associated pipes and pumps are sized and connected correctly. A possible composition of the chiller plant would be four chillers at 1,500 tons each and two chillers at 400 tons each for a total plant capacity of 6,800 tons. The conventional chiller configuration is to connect all these chillers in parallel. Each of the 1,500-ton chillers would be rated at 0.538 kW/ton. A schematic of this plant configuration is illustrated in Figure 1.

The significant parameters of this conventional chiller plant design are presented in Table 1.

The two 400-ton chillers are double-bundle, heat recovery chillers; they are used to handle the winter loads and they are used as peaking machines in the summer.

Proposed Design

In the proposed design, the design parameters departed from the above-described conventional plant. Chilled water would be produced at 40 degrees and returned at 56 degrees. The condenser water design parameters would be at 90 degrees supply and 110 degrees return. The four 1,500-ton chillers would be combined into two pairs connected in a series. The two pairs are then connected in parallel with the two 400-ton chillers. A schematic of this proposed chiller plant configuration is illustrated in Figure 2.

Such a plant configuration is a major departure from the conventional chiller plant design, and it requires careful engineering to ensure the proper loading of the chillers and flows through the evaporators and condensers at low-load conditions. The combined efficiency of each pair would be 0.651 kW/ton. The characteristics of this proposed plant are presented in Table 2.

The sacrifice on the compressor efficiency is compensated by the greatly reduced chilled water flows and condenser water flows. The total chilled water flow has been reduced by 37.5%, and the total condenser water flow has been reduced by 52.2% when these parameters are compared to the conventional design approach.

Through the increase of the condenser water parameters, the number of cooling tower fans and the horsepower required is significantly reduced. Overall, this proposed chiller plant design, when compared to the conventional design, incurred an 848 kW penalty for the compressors and, on the other hand, benefited from a 1,126 bhp reduction total for pumps and cooling tower fan motors.

When compared to a conventional chiller plant, the proposed chiller plant design would generate over 1 million kWh ($120,000) of electric savings/year, and at the same time, it is cheaper to build due to the reduced pipe sizes, pump sizes, and cooling tower fans. In some cases, 30 in. of water pipe can be reduced to 24 in. This pipe size reduction represents a significant saving on the structural support of the pipes as well, not only for the pipe itself, but for the weight of the water it would have carried under the conventional designs.

The energy savings estimated above were calculated using the Trace(r) (Trane Air Conditioning Economics) building modeling and simulation tool. In the electronic model of the convention center, the most optimistic booking schedule was assumed. The convention center would be booked year-round, under a rotation of 3 days of site preparation and 4 days of show, conference, and exhibition.

The air handlers used for this project are custom built taking into consideration the chilled water temperature parameters. The selection and design processes of the air handlers are not the subjects of this paper. However, it would appear that the cooling coils selected as designed (16 degrees DT at 40 degrees supply and 56 degrees return) would create less pressure drops both on the airside and waterside. These reductions in pressure are compared to the standard chilled water parameters of 10 degrees DT at 44 degrees supply and 54 degrees return. This observation is based on a coil selection using the TOPSS(r) (Trane Official Product Selection System) software tool using the two different sets of chilled water parameters.

It is assumed that the cooling coils of these custom-built air handlers would have resulted in similar pressure reduction as the standard off-the-shelf coils and air handlers. This reduction in the pressure drop in the AHUs would have resulted in additional fan energy savings. However the details of the fan energy savings are not presented in this paper.

Other Design Considerations

The same advances in the control technology have introduced two other chiller plant design innovations: variable-flow primary chilled water and variable-flow condenser water. These features were not considered in this case because they are not compatible with the proposed design temperature and flow parameters. At 1.50 gpm/ton of chilled water flow and 1.44 gpm/ton of condenser water flow, these chillers have reached the minimum allowable limits for a chiller to operate properly. Further reducing the flow through the evaporator and/or the condenser bundles at low-load conditions would jeopardize the safe operation of the chillers.

Variable condenser water flow and primary chilled water flow are effective energy savings features when using the conventional chiller design parameters (10 degrees DT).

High DT and low-flow design features require particular attentions to ensure that the chiller(s) selected would work properly under low-load conditions. The adverse impacts on chiller efficiency at part-load conditions can be enormous with low-flow chillers when all the components of the plant have not been engineered to work as a system. For this reason, it is necessary to conduct a study of the chiller plant to determine the anticipated loading conditions and profiles.

Conclusion

In addition to the electric energy savings, this chiller plant will have prevented the emissions of 1.1 million lb of CO2 per year, 8,800 lb of SO2 per year, and 3,100 lb of NOx per year. This is an overall win-win-win situation where the first cost is reduced 1, operating cost is minimized, plus significant environmental benefits are realized as an additional benefit.

Industrial standards are established, among other reasons, to streamline a repetitive process. Henry Ford took advantages of the standards that he established and introduced the concept of the assembly line with the Model T. The current Ford assembly lines have no doubt upgraded their standards since the Model T; however, the standards for the chiller plant designs appear to be stuck in the time when the Model T was introduced. It is time to adopt new engineering principles to catch up with the 21st century and be part of the digital revolution.

Computing powers are nowadays relatively affordable to the point that the personal computer has become a household appliance. Chiller plant designs should be studied, engineered, and built using the computing tools available today to provide the owner a plant that is economic to own and operate while at the same time, friendly to the environment. ES

Flow Tactics

As a reminder to the novice engineer, the 2.4 and 3.0 gpm/ton figures are derived in the following manners:

Formula: Btuh = gpm x range in degrees F x 500
Where: 500 = 60 min/hr x 8.33 lb/gal of water
Q = 500 x gpm x DT, therefore
gpm = Q/(500 x DT)
DT= 10 degrees F

In the cooling mode, Q = 12,000 Btuh/ton, therefore the chilled water flow = 12,000/(500 x 10) = 2.4 gpm/ton

In the heat rejection mode, Q = 15,000 Btuh/ton, therefore the condenser water flow = 15,000/(500 x 10) = 3.0 gpm/ton

The figure of 15,000 Btuh/ton assumes 3,000 Btuh/ton of heat of compression. This amount of heat of compression is equivalent to a 0.90 kW/ton chiller efficient. However, a water cooled chiller of that efficiency is no longer manufactured. A 0.55 kW/ton or better is most likely the current norm. To maintain the same 10 degrees F DT in the condenser water flow, the correct condenser water flow should be 2.78 gpm/ton or less. Yet, the chillers are still specified at 3 gpm/ton for no apparent reason other than to maintain the status quo "just because it's been the standard."

Occasionally a chiller is selected with 2 gpm/ton condenser water flow, and that seems to be a big deal. There are numerous papers published in defense of maintaining the 3.0 gpm/ton and some other in favor of the 2.0 gpm/ton. There seems to be a duel between an established standard and a newcomer.