OPTIMIZING CONDENSER WATER FLOW RATES. W. A. Liegois, P.E. Stanley Consultants, Inc. Muscatine, Iowa

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1 OPTIMIZING CONDENSER WATER FLOW RATES W. A. Liegois, P.E. Stanley Consultants, Inc. Muscatine, Iowa T.A. Brown, P.E. Thermal Energy Corporation Houston, Texas ABSTRACT Most chillers are designed for a 10 degree temperature rise across the condenser unit. This paper will explore opportunities for the design engineer to reduce both operating costs and first costs by designing for higher temperature changes and their resultant lower flow rates. Other advantages include smaller footprints for cooling towers thus saving valuable space on the site or on the building roof. Real design condition examples will be presented. KEYWORDS Optimizing condenser water flow rates, reduced condenser water flows, increased condenser water temperature differential, reduced operating costs, reduced space requirements

2 OPTIMIZING CONDENSER WATER FLOW RATES W. A. Liegois, P.E. Stanley Consultants, Inc. Muscatine, Iowa T.A. Brown, P.E. Thermal Energy Corporation Houston, Texas INTRODUCTION A discussion of the optimum condenser water flow rate has been discussed for many years and there have been several articles published on this topic. This paper is not going to cover areas previously discussed nor are we going to try and prove or disprove their results and conclusions. The purpose of this paper is to identify additional factors which may influence the design engineer s decision on the best system for their specific installation. Previous studies have examined single chiller operations of approximately 500 tons. Others have looked at a variety of design issues that affect cooling tower performance and operation. These papers have not investigated any issues that may be unique to central chilled water facilities or where conditions at central chilled water plants may alter earlier conclusions. There are countless design parameters that influence this design issue. Each must be identified and defined in order to accurately make a comparison of 2 versus 3 gpm per ton condenser water flow. No matter how carefully these design parameters are selected, they would never all apply to every installation and any attempt to quantify the impact of each parameter would only confuse the issues. We believe that each system must be reviewed individually in order to make the correct design decisions. There is no right answer, only those that best fit a facility under the operating conditions planned for the installation. We will provide examples of how this process was applied to a specific installation and the conclusions that were drawn from that investigation. Our work will concentrate on new central chilled water facilities of 10,000 tons of refrigeration or more. These facilities normally have four or more chillers operating in parallel. These units may be identical or they could have different chiller sizes. Our discussion relates to new facilities only. Existing facilities that are considering changes to their condenser water pumping rates are faced with some unique conditions that the design engineer of a new facility can overcome.

3 We have assumed throughout this analysis that regardless of the condenser water flow rate, that all systems were optimized for their respective conditions. This means that condenser pressure drops for example was optimized for either 2 or 3 gpm per ton. We did not take a chiller designed for 3 gpm per ton and then look at its performance at 2 gpm per ton. We allowed the chiller supplier to run their optimization selection program and provide the best chiller fit for each condenser water flow rate. This flexibility is not available to the design engineer when examining existing facilities. The objective of any design is to optimize the entire facility design not any subsystem. This is also the objective of this paper, to present information that will allow designers to make optimum choices for their facilities. DESIGN BASICS Our discussion is centered on large central chilled water facilities of 10,000 tons or more. Electric motor-driven centrifugal chillers are common in facilities of this type. Other options such as absorbers or steam turbine-driven chillers are also available. We have chosen to only examine motor-driven chillers because they are probably the most common selection and mixing the prime mover selection will cloud the issue of optimum condenser water flow. We have used a traditional cooling tower ton, which is 15,000 Btu per hour per ton of refrigeration. This represents 12,000 Btu per ton from refrigeration and an additional 3,000 Btu per ton for heat of compression. For traditional systems designed for 3 gpm per ton this represents a 10 F temperature rise across the chiller condenser. This has been the standard rating condition as stated in ARI 550/ The design temperature rise for a system designed for 2 gpm per ton is; therefore, 15 F. Much of the design information for this paper was taken from a cooling tower design study conducted by Stanley Consultants, Inc., for the Thermal Energy Corporation (TECO) proposed facility expansion located in Houston, Texas. The facility was conceptually designed for 55,000 tons of all electric cooling using 11, nominal 5,000-ton chillers. The facility would be adjacent to the current TECO Facility on the Campus of the Texas Medical Center. PIPING ISSUES An evaluation of the applicable piping issues can be narrowed to a focus on the condenser water system piping, since the chilled water system piping configurations will be identical between the 2 and 3 gpm per ton scenarios. Assuming that the design of the condenser water system piping is based upon achieving a specified flow velocity or a specified headloss, the energy expended to pump water through the system will be fairly uniform between the 2 and 3 gpm per ton scenarios. However, the 2 gpm per ton case will produce a reduced nominal pipe size (NPS) for

4 the various lines in the condenser water system. The magnitude of this size reduction will be dependent upon the specific design standard selected. Table 1 provides a simplified comparison of the impact of arbitrarily selected design standards on the NPS of main system headers. Table 1 Installed Chilled Water Production Versus Condenser Water Header Size Header Size, Header Size, Header Size, Header Size, Installed Criteria 1 & Criteria 1 & Criteria 2 & Criteria 2 & Production 3 gpm per ton 2 gpm per ton 3 gpm per ton 2 gpm per ton (tons) (NPS) (NPS) (NPS) (NPS) 10, , , Criteria 1: Criteria 2: Design flow velocity less than or equal to 10 feet per second. Design headloss less than or equal to 2.50 feet of H 2 O per 100 feet of pipe. Note: This comparison assumes: 1. The flow of all cooling tower cells converges into a common header. 2. The flow is transported some distance and then dispersed to individual chillers. 3. The selected design standard is applied to the selection of the header size. Table 1 shows that for the lowest installed capacity (10,000 tons), using Criteria 1 the pipe size is reduced one pipe size, from 36 to 30 inches when the condenser water flow rate was reduced to 2 gpm per ton. Using Criteria 2 yields the same pipe size. As the size of the central chiller plant is increased to 15,000 and 20,000 tons the change in pipe size is even more pronounced. Criteria 1 results in a change of two pipe sizes for both plant sizes while Criteria 2 results in one pipe size for both the 15,000- and 20,000-ton plants. In a central plant application, where NPS and pipe run lengths can be significantly larger than in typical building applications, the reduction of the NPS by even a single line size can have an appreciable impact on the overall capital cost of an installation. Factors that would be expected to result in reduced capital costs for the condenser water system include: Reduced procurement cost for pipe, valves, instrumentation and fittings. Reduced shipping and field handling cost for pipe, valves and fittings. Reduced field fabrication cost for pipe and fittings. Reduced material, shipping and handling cost due to smaller pipe supports. Reduced material and labor costs due to smaller pipe support foundations. Reduced material and labor costs due to the reduction of pipe surface area requiring a coating.

5 In the case of large bore pipe, the cost of valves and flow meters in particular may vary considerably if the pipe changes one or two sizes. Additionally, reduced sized piping can result in a more compact system. This characteristic has the potential to reduce the overall floor space that is dedicated to a system, which would allow a facility owner to either dedicate additional floor space to revenue generating activities or reduce the overall size of a central plant building. Furthermore reduced pipe sizes are easier to route through a central chiller plant and also make equipment arrangements easier. PUMPING ISSUES Assuming the design of the condenser water system piping is based upon achieving a specified headloss per unit length of piping, an evaluation of the applicable pumping issues can be based upon a review of the difference between the pressure drop across the condenser vessels and the flow rate through the overall systems in the 2 and 3 gpm per ton configurations. Table 2 provides a comparison of the pressure drop across the condenser vessels of three different types of chiller units at both 2 and 3 gpm per ton. Table 2 Pressure Drop through Condenser Vessel Feet of H Unit 2 O, Feet of H 2 O, 3 gpm per ton 2 gpm per ton Chiller Chiller Chiller Average Note: Capacity of the chiller units ranges from 5,000 to 2,000 tons. For evaluation purposes, assume that the overall system headloss of a 3 gpm per ton condenser water system is 100 feet of H 2 O at its design operating point. Subtracting 28 feet of H 2 O (the approximate average pressure drop across the condenser vessels at 3 gpm per ton) from the overall system headloss yields a balance of system headloss of 72 feet of H 2 O. If a 2 gpm per ton system were designed to achieve the same headloss per unit length of piping as the aforementioned 3 gpm per ton system, then the balance of system headloss for the 2 gpm per ton system would ideally be 72 feet of H 2 O. Adding 16 feet of H 2 O (the approximate average pressure drop across the condenser vessels at 2 gpm per ton) to the balance of system headloss yields a design operating point of 88 feet of H 2 O for the equivalent 2 gpm per ton system. These results are depicted in Table 3.

6 Table 3 System Headloss Feet of H Unit 2 O, Feet of H 2 O, 3 gpm per ton 2 gpm per ton Overall System Condenser (avg) Balance of System From the differences between the overall system headlosses, mechanical and electrical power consumption can be determined. This data is shown in Table 4. Table 4 Condenser Water Power Consumption Unit BHP and kw, BHP and kw, 3 gpm per ton 2 gpm per ton Overall System 871 hp 511 hp 683 kw 401 kw kw / ton Note: This comparison is based upon a uniform chilled water production level of 10,000 tons with different multiples of chiller units, pump efficiencies of 0.87 and motor efficiencies of Therefore, the electrical power consumption of the condenser water system is understood to be approximately 70 percent higher in the 3 gpm per ton configuration than in the 2 gpm per ton configuration, on a per ton basis. CHILLER POWER REQUIREMENTS Chiller power requirements are set based on the compressor efficiency and lift requirements. The reduction in condenser water flow rate will increase the lift required by the compressor. This is the result of warmer cooling tower return temperatures. Our base assumption has been that the cooling tower supply temperatures are the same for both condenser water flow rates. This means the cooling tower return temperatures are approximately 5 F warmer with the reduced condenser water flow. Figures 1, 2, and 3 show the effect of increased condenser water temperature on three different chillers. Two curves have been plotted for each of three different chillers. The first indicates the compressor power required with a condenser water flow rate of 3 gpm per ton and the second is for 2 gpm per ton.

7 Chiller Power Requirements,kW per ton Part Load 2 GPM/ton 3 GPM/ton Figure 1 Chiller 1 Chiller 1 has relatively constant compressor power differential requirements at all load conditions. As the percent load decreases the differential power requirements between the two condenser water flow rates remains relatively constant. This means the compressor power penalty does not vary significantly with percent load. Chiller Power Requirements, kw per ton GPM/ton 3 GPM/ton Part Load Figure 2 Chiller 2 Chiller 2 has a different characteristic with the two curves converging at loads of 60 percent or less. The part load performance of this chiller is different from Chiller 1 because it uses hot gas bypass at loads of 60 percent or less. This results in very high power requirements at part load

8 conditions below 60 percent. This chiller also shows that at part load, the difference in power requirements between the two different condenser water flow rates is reduced. That is, at reduced load, the chiller compressor power requirements are closer at 80 percent load and are virtually identical at 60 percent load or less. Chiller Power Requirements, kw per ton GPM/ton 3 GPM/ton Part Load Figure 3 Chiller 3 Chiller 3 shows that the curves get markedly closer together as the load decreases from 100 percent. This chiller does not employ hot gas bypass at part load conditions. The power penalty for the chiller compressor alone; however, is reduced at part load conditions. These curves show that the part operation of the chillers does affect the differential power requirements between the 2 and 3 gpm per ton conditions. Therefore, it is essential that any economic analysis must be done using the actual anticipated chiller load. There are not many chillers that actually operate at full load for any significant number of hours per year. In fact, plants with multiple units often attempt to keep chillers operating at there most efficient point which is normally below full load as is the case for Chillers 1, 2, and 3. This is done by bringing units on-line and taking them off-line as building loads change. In large central facilities this can be done relatively easily and the chillers are allowed to operate at their optimum condition for more hours per year. These curves also clearly show that all chillers perform differently. Therefore, chiller information that is site specific must be obtained in order to perform an accurate economic evaluation. The selection of chiller type, refrigerant, size, and prime mover may all affect this analysis of optimum condenser water flow rate. It is clear; however, that the chiller power consumption is always higher with reduced condenser water flow rates. The central issue is whether the increased power consumption of the chiller is offset by other power savings from the condenser water pump and cooling tower.

9 Changes in the condenser water flow rate do not have any substantial impact on the capital cost of the chiller itself. There will normally be differences in the compressor selection and the condenser designs may change but these are not normally significant enough to change the selling price for the individual chiller unit. COOLING TOWER DESIGN Changes to the condenser water flow rate have the greatest impact on the cooling tower design. In simplistic terms, the cooling tower footprint (length x width) is primarily a function of the condenser water flow rate. Therefore, when the flow is reduced by 33 percent, so is the footprint of the tower. This is because the cooling tower fill is normally designed for a fixed flow rate per square foot of fill. Therefore, as the flow rate is decreased, the area required for fill is also decreased linearly. This smaller footprint can have some significant advantages at project sights where space is limited. Many central chiller plants designed for 3 gpm per ton have the cooling towers located on the roof of the chiller building or some neighboring structure. In many cases the footprint for the cooling tower is larger than that required for the chiller building itself. This means that the space required for a central chilled water facility could be set by the space required for the cooling tower. Even if the cooling towers are to be located at grade, the space required for a system designed for 2 gpm per ton is 33 percent smaller. This could have significant advantages for any site where space is a premium. The tower thermal load; however, remains the same for both condenser water flow rates. The temperature change of the condenser water in the tower primarily affects the tower fill height. This means that the cooling tower designed for 2 gpm per ton will normally have a footprint which is 33 percent smaller than one designed for 3 gpm per ton but will be taller. This increase in packing height is not linear with temperature and does not mean the tower will be 33 percent taller. Normally, for central chiller plant installations, multiple cooling tower cells are employed to achieve the required degree of cooling. By reducing the condenser water flow rate the number of cooling tower cells could be reduced. This means that only 2/3 as many cells would be required for condenser water flow rates of 2 gpm per ton. This approach of reducing the number of cooling tower cells may be more advantageous than reducing the footprint of each individual cooling tower cell. Cooling tower fan power is more difficult to predict. It is a function of several design parameters but in general, towers designed for 2 gpm per ton with a 15 F temperature differential require 15 to 20 percent less fan power than those designed for 3 gpm per ton.

10 In addition to the fan power savings, the capital cost of the cooling towers is significantly reduced when the design is changed to accommodate condenser water flow rates of 2 gpm per ton. In general, you can reduce the number of cooling tower cells by 33 percent. Savings in the neighborhood of 25 percent on the capital cost of the installation should be anticipated. This is less than the linear 33 percent because of the increase in tower packing height. ECONOMIC ANALYSIS The TECO Facility in Houston, Texas, is located on the Campus of the Texas Medical Center. They supply chilled water and steam to 21 institutions at 40 different customer sites. The Texas Medical Center is expanding and; therefore, the TECO Facility must also expand to meet the increased system loads. Land in the medical center area is a premium. Therefore, when investigating designs to expand the chilled water capacity, options which reduced the space required for the cooling towers were considered. The following presents the analysis of reduced condenser water flow for that facility. Tables 5 and 6 present the final operating cost analysis for two different chillers considered for that installation. In the case of Chiller 1, the net power required to operate the chiller motor, condenser pump and cooling tower fan were less for the 2 gpm per ton condenser flow rate condition at both full load and under all part load conditions. The additional chiller motor power requirements were overcome by the savings in condenser water pump and cooling tower fan power. Table 5 Chiller 1 Load in % F Delta Temp (3 gpm per ton) Compressor kw/ton Condenser Pump kw/ton Cooling Tower Fan kw/ton Total kw/ton F Delta Temp (2 gpm per ton) Compressor kw/ton Condenser Pump kw/ton Cooling Tower Fan kw/ton Total kw/ton Delta Electricity kw/ton

11 Table 6 Chiller 2 Load in % F Delta Temp (3 gpm per ton) Compressor kw/ton Condenser Pump kw/ton Cooling Tower Fan kw/ton Total kw/ton F Delta Temp (2 gpm per ton) Compressor kw/ton Condenser Pump kw/ton Cooling Tower Fan kw/ton Total kw/ton Delta Electricity kw/ton At full load the power requirements are almost identical but slightly less operating with condenser water flow rates of 2 gpm per ton. As load decreases, the advantage for the lower condenser water flow rate is increased. The condenser water pump and cooling tower fan power requirements remain constant at all part load conditions. The penalty in compressor power; however, is reduced at part load as was shown in Figure 1. The result is maximum savings at around 60 percent load Chiller 2 had slightly different results. At full load the power requirements were higher for the 2 gpm per condenser water flow rate. At all other load conditions; however, the lower condenser water flow rate resulted in operational savings. This facility expansion will include multiple chillers. In addition, the existing TECO Facility already has 23 installed chillers of various sizes and prime movers at two different locations. This makes matching production capacity to load demand very easy. Chillers are routinely brought on and off line to keep the production of chilled water at optimum performance. Chillers are normally operated at their lowest compressor power per ton of refrigeration point. This is not at full load but normally occurs around percent load. This means that even if Chiller 2 is selected for the facility expansion, it would not operate at full load for many hours per year. Therefore, there would be a net operational savings if the condenser water flow rate was selected at 2 gpm per ton.

12 Some increase in the operating costs may be acceptable. The capital cost reductions possible with reduced condenser water flow rates are not insignificant. Therefore, a payback analysis should be performed if the operating costs increase. This can be done by estimating the number of hours the units will operate and at what loads. This information along with the cost of electricity will allow the annual increase in operating cost to be determined. A payback time or return on investment may now be determined for the incremental capital expenditure. This analysis may show that even though the operating costs are higher with reduced condenser water flow rated, that the increased capital required for the higher condenser water flow rate is not justified. In addition, space will also be conserved. At the TECO Facility, by reducing the cooling tower space requirements, the existing land could accommodate approximately 15,000 to 20,000 tons of additional refrigeration capacity while maintaining the same basic design. This increased space utilization is worth millions of dollars in reduced capital costs to purchase more land. CONCLUSIONS Reduced condenser water flow rates can result in lower operating costs. At part load operation for many chillers. At full load operation for some chillers. Central chiller plants can operate chillers closer to their optimum condition for many hours per year because they have multiple units. Therefore, economic comparisons should be done based on expected performance conditions. Space savings for the cooling tower with reduced condenser water flow rates can be substantial. This can either reduce land requirements and costs or increase the potential chilled water capacity of a facility. For facilities with cooling towers on the roof of the chiller plant, this may optimize the size of the chiller building. Capital cost savings with reduced condenser water flow rates are real. Reduced condenser water pump costs. Reduced condenser water piping costs. Reduced Cooling Tower costs, approximately 25 percent. Small increases in operating costs may be justified based on reduced capital expenditures.