Northeast Medical Center (NMC), a 457-bed regional hospital located in Concord, NC, has shared the good fortune of a growing southern North Carolina. This growth has meant newly designed renovations and additions from the exterior aesthetics to the interior plant system - including an expansion to the central chiller plant in the spring of 2003. These operational improvements to the infrastructure of NMC were necessary and timely as expressed by Phil Stephens, director of Facilities Management Services, "We were losing our back-up capacities and we also needed to remain ready for future growth." Consequently, the plant gained an 800-ton chiller, a cooling tower, and a second plate-and-frame heat exchanger, which is the main protagonist in this story (Figure 1).
THE PLATE-AND-FRAME HEAT EXCHANGERBy definition, the plate-and-frame heat exchanger produces partial waterside economies in cooler temperatures by supplementing the chiller(s). Hence, the NMC facilities staff knew that focusing on the new plate-and-frame heat exchanger would be one way to gain efficiencies during the winter and shoulder seasons of the year.
The major reason a partial waterside economizer system is ever considered is to allow the cooling tower to function as economically as possible within certain temperature ranges which, in this case study, range from 37°F to 47° ambient wb. While the cooling tower can produce cold water on its own, to do so at certain cooler temperatures means it must precisely balance its energy among the various types of equipment including the towers, pumps, and chillers. Therefore, the plate-and-frame heat exchanger in concert with the chiller(s) can reduce overall plant energy consumption.
THE ADVANTAGES OF A SERIES (VS. A PARALLEL) WATERSIDE ECONOMIZERAccording to Carl Walker, HVAC supervisor for NMC, "Knowing that operating our original heat exchanger (installed in parallel) at or above 37° to gain efficiencies was not possible, we needed to analyze the new heat exchanger (installed in series) to see if we could gain efficiencies by operating it at or above 37°."
The original plate-and-frame heat exchanger was installed in parallel, which is common, and is often found in chilled water plants in the Midwestern and Northern regions of the United States. As with all plate-and-frame heat exchangers, they operate in conjunction with a cooling tower to produce cold water during winter and shoulder seasons. However, unlike series installed heat exchangers, their piping does not temper (cool off) the water prior to its entering the chiller(s). As a result, the operational sequence for the heat exchanger installed in parallel is much simpler, e.g., the CHWS temperature is reached or the chiller(s) take over. In other words, the tower and heat exchanger fans and pumps cycle if load fluctuates.
Before the plant was expanded, NMC had one plate-and-frame heat exchanger installed in parallel. With this heat exchanger, the highest temperature the plant could deliver from waterside economies would be 44° (or the heat exchanger leaving water temperature (HXLWT) at 37° wb. Any chilled water supply temperature of 44° or more would result in complaints from hospital staff within the critical departments such as radiology, oncology, and information services because their equipment was configured to accept chilled water at or below 44°. The calculation was as follows:
Tower leaving water temperature (TLWT)
Tower approach (5°) + wb temperature (37°) = 42°
Heat exchanger leaving water temperature (HXLWT)
TLWT (42°) + heat exchanger approach (2°) = 44°
The new plate-and-frame heat exchanger was installed in series. This new pumping arrangement allowed the heat exchanger to act alone when wb temperatures were at or below 37° (just like the plate-and-frame heat exchanger installed in parallel) and with a chiller when wb temperatures ranged above 37° to 47°, because its piping allows it to temper the chilled water return before it enters the chillers is connected upstream from the chillers. This operational sequence is used to minimize total plant energy and is referred to as a partial waterside economizer sequence.
MAKING SERIOUS/SERIES DECISIONS WITHIN THE SYSTEMDuring the process of accommodating the new plate-and-frame heat exchanger, it was also vital to assess how the other equipment within the plant might be affected. In the case of NMC, it meant making a few adjustments to their equipment safety sequences, which could have reduced the life expectancy of their machinery.
To ensure proper operation of the plant's modern centrifugal chillers required a +15° DT between condenser water return (CWR) and chilled water supply (CHWS) temperatures. For example, if the hospital needed to produce 42° CHWS, then the corresponding condenser water supply (CWS) would have to be at 57°. A centrifugal chiller can tolerate lower DTs for up to the first 20 minutes of operation, but each brand of chiller is slightly different in this regard. (Note: Consult the operation manuals for each type of chiller within your plant to determine the exact times and temperatures.)
For partial waterside economizer operation, the condenser water DT requirement necessitates separate condenser water temperatures between the chillers and the heat exchangers. This can be accomplished by either:
- Isolating the towers that serve the chillers from those towers that serve the heat exchangers (this was NMC's choice); or
- Decoupling the condenser loop, which means the water is separately blended and pumped through the chillers and heat exchangers.
THE NEXT LOGICAL QUESTION: EFFICIENCYAfter operating the newly acquired equipment for one year, the NMC facility staff wanted to know specifically when the plate-and-frame heat exchanger would make the system more efficient at producing cooler water than the chiller alone. To accomplish this comparison, the efficiencies for each alternative had to be determined.
Since efficiency for chilled water systems is typically measured in kW/ton, the first calculations determined the capacity (tons) and energy (kW) for each piece of equipment. Additionally, a number of other unique calculations were also required, as well as the ultimate outcome of determining comparative efficiencies. (Sidebar 1.)
The exact NMC comparative efficiency calculations can be found in the second sidebar and are simplified below for each alternative, namely, the heat exchanger/chiller alternative and the chiller only alternative given a set of distinctive conditions. In addition, see Table 4 for an efficiency comparison across various wb temperatures.
THE HEAT EXCHANGER/CHILLER ALTERNATIVEFirstly, the efficiency of the heat exchanger:
Heat exchanger capacity (tons) = 160 tons
Heat exchanger energy (kW) = 105.6 kW
Heat exchanger efficiency (kW/ton) = 0.65 kW/ton
Secondly, the efficiency of the chillers:
Chilled water return temperature to the chiller = 53°
Chiller maximum capacity (tons) = 720 tons
Number of chillers operating = one chiller
Chiller, one load = 688 tons
Chiller energy (kW) (CHLR) = 406 kW (CHLR)
Chiller efficiency (kW/ton) = 0.59 kW/ton
And, lastly, the efficiency of the combined heat exchanger/chiller combination vs. the chiller only alternative:
The heat exchanger/chiller efficiency = 0.60 kW/ton
Chiller-only efficiency = 0.72 kW/ton
Thus, under the above set of conditions, using the heat exchanger/chiller combination was more efficient than using only a chiller at 0.60 kW/ton and 0.72 kW/ton, respectively. (See Table 4 for efficiency comparisons under varying conditions.)
IT TAKES TWO (HEAT EXCHANGERS AND CHILLERS) TO COORDINATE EFFICIENCIESAs previously demonstrated, NMC's facility staff found a way to determine when to use the different equipment combinations to obtain the plant's optimal efficiency during the coolest times of the year. The outcome was determined by calculating total plant kW at given increments of overall plant loading for the equipment choices and using these numbers as "a base of operational decisionmaking." But, where did these calculations begin?
As stated, the goal for NMC's facility staff was to design a tool to determine the combinations in which to operate the plate-and-frame heat exchanger and one or two chillers during ambient conditions that yielded wb temperatures ranging from 37° to 47°.
As a first step, data for each combination of the new plate-and-frame heat exchanger and chiller(s) had to be gathered. (Tables 1-3.) Then, a template could be designed and decision guidelines drawn to determine the optimal combinations of equipment for maximum plant efficiency at varying temperatures. This was a complex undertaking and required a number of parameters and many calculations. More specifically, to reach the bottom line involved knowing (and using) the following data:
- Main building return water temperatures
- Outdoor wb temperatures
- Overall efficiencies of the various combinations of operating equipment:
- Plate-and-frame heat exchanger
- One chiller
- Two chillers
- One chiller with the plate-and-frame heat exchanger
- Two chillers with the plate-and-frame heat exchanger
DASHBOARD DECISION-MAKING TOOL PROVIDES THE ANSWERS FOR NMCFor NMC's situation, the ideal operational decision-making tool would give staff members the data necessary to operate the plant in the most efficient manner, both day-to-day and from degree to degree. It would entail collecting and tracking data as well as calculating and clearly displaying the ongoing decision markers. The NMC facilities staff found that the dashboard decision-making tool (DDMT) for facility/staff was the tool that best fulfilled their unique needs (Figure 2).
DDMT provides a clear road map for the plant operators of NMC because it graphically presents key decision markers or operating indices through charts, graphs, gauges, and dials. And, as this article reveals, it also allowed facilities managers to run mock scenarios in order to test different combinations for efficiencies prior to actual implementation.
LOOKING AHEADWhen capital budget funds become available to replace an aging control system, NMC will purchase new controls, which will include kW monitoring of the chillers, tower fans, tower pumps, and chiller pumps. This system will contain a control algorithm that will automatically select the combination of equipment that produces the highest energy efficiency. In essence, it will provide the same information that the DDMT supplied (during this interim period for NMC) as well as providing an automated decision to alter the equipment mix for its highest and best equipment use. Nevertheless, the DDMT has "other lives."
Beyond its initial interim solution for monitoring new equipment and its impact on overall plant efficiencies and even when an upgraded control system is installed, the NMC staff intends to continue using the program for commissioning and seasonal checks. Having a dashboard tool allows staff to run test sequences of various equipment mixtures in the anticipation of new equipment or changes in the seasons. Its use extends beyond the largest hospitals like NMC. It may also prove beneficial to those facilities that don't have automated control systems or have disabled their controls.
And, as a side note, an automated system does not ensure efficiency, and that's one of the reasons NMC is going to continue using DDMT. Automated systems are only as good as the quantity and quality of their information. As a result, they are limited and cannot describe the full range of condition combinations. Additionally, arbitrary changes to control sequences can also have potentially disastrous effects on efficiencies. If operators configure equipment to run for either too short or too long a period of time, then the chillers and/or plate-and-frame heat exchangers will operate at extremely low loads and, as a result, efficiencies can plummet.
THE WRAP-UPThere are two key steps prior to initiating operational change to a hospital's chilled water plant system: perform a detailed energy analysis of the entire plant; and determine the most stringent requirements for equipment being served by the plant whether a data room air handler, MRI, and, in the case of NMC, a linear accelerator. This gives plant operators non-departure points in terms of chilled water supply temperature and flow. With this background information in hand, recommendations can be made relative to the optimum operational sequences of the plant's major energy consuming equipment.
With the assistance of a tool like DDMT, operators could run mock scenarios in order to test different combinations for efficiencies prior to actual implementation, and then make improvements without waiting for a control upgrade budget approval while preventing costly inefficiencies resulting from an operator trial and error process.
In their ongoing search for the most efficient plant utilization, NMC's facilities engineering department will continue to purchase new equipment and make adjustments in its use. And, if the reader has followed this story and its detailed analysis to determine the most efficient use of just one piece of equipment, it will also not come as a surprise to know that approaching the entrance to the facilities department at Northeast Medical Center, a placard reads, "Through these doors walk the finest hospital engineers in the world."