Future-proofing a chilled-water system

Chilled-water system design often focuses on the proper initial operation of the system. However, understanding—and planning for—changes that occur as the system ages is critical to designing a chilled-water system that will operate properly for years to come.

06/14/2018


Learning objectives

  • Understand what will happen to chilled-water coil performance as a facility evolves and the chilled-water system gets older.
  • Be aware of issues that will occur if long-term changes to the chilled-water system performance are not accounted for in the initial design of chillers, pumps, and cooling coils.
  • Learn some recommended practices for chilled-water system component design to prevent long-term issues from occurring.  

A properly functioning chilled-water system is critical to the comfort of commercial facilities. Often, much design consideration is given to the initial operation of these systems. However, chilled-water systems are expected to operate effectively for decades, even as the facilities they serve constantly evolve and change.

Figure 1: A recently completed campus integration project provided a 320,000-sq-ft expansion and renovation of Genesis Medical Center’s east campus in Davenport, Iowa. IMEG Corp. provided mechanical, electrical, and plumbing (MEP); civil; and technology design (along with architectural lighting and medical equipment planning) for the $139 million project, which included a major renovation and addition to the central utility plant. A 2,400-ton primary-secondary chilled water plant optimization strategy was used to serve the project and the existing campus. Courtesy: IMEG Corp.Chilled-water systems are a balance between the chillers and the various chilled-water coils throughout the system. Much attention is given to how chiller performance impacts chilled-water system operation. However, just as important, cooling coil performance also impacts chilled-water system operation.

To evaluate best design practices for proper long-term operation of chilled-water systems, it's first necessary to understand what happens to the performance of chilled-water cooling coils as a system ages. To do this, we'll analyze some common changes that occur during the life of a chilled-water system and the impacts these changes have on chilled-water coil performance.

This analysis will be performed using a commercial coil-selection software. By modifying the coil-selection parameters, it's possible to simulate changing conditions at chilled-water coils. For simplicity, this analysis will focus on a single chilled-water coil in a central air handling unit (AHU). This will be used to represent what occurs at all coils throughout a system.

First, we'll start with a baseline cooling coil, which represents the coil's performance at system start-up. The required operating conditions of the baseline coil (summarized in Table 1) are used to select the baseline coil. (The waterside performance of this baseline coil selection is summarized in Table 2.) For the rest of this analysis, the physical properties of this baseline coil selection (number of rows, fin spacing, fin type, tube type, etc.) will be held constant. This will simulate changing conditions at this specific coil. 

Fouling

The first cooling coil change we'll analyze is fouling. As water and air flow through and around the coil, particulates and scale accumulate on the airside and waterside surfaces of the coil, a phenomenon known as fouling. Fouling impedes the heat transfer at the coil and affects coil performance (see 2016 ASHRAE Handbook—HVAC Systems and Equipment). Specifically, chilled-water coils can develop significant fouling (see 2017 ASHRAE Handbook—Fundamentals). Coil-selection software simulates the effects of fouling with the fouling-factor parameter. This factor represents an additional thermal resistance in the coil heat-transfer calculations.

Table 1: Several performance requirements are used to select baseline cooling coil. Courtesy: IMEG Corp.

From this author's experience, dirty cooling coils can experience fouling factors on the range of 0.001 sq ft-°F-hr/Btu. This fouling-factor value is at the top of the range considered by the Air-Conditioning, Heating, & Refrigeration Institute's (AHRI) Standard 550/590, so it represents a realistically dirty coil. Adding this fouling factor to the baseline coil results in the modified waterside performance summarized by Change A in Table 2.

Table 2: This highlights the changes to cooling coil waterside performance as the system ages and the facility evolves. Courtesy: IMEG Corp.

The next cooling coil change we'll analyze is increased airflow. Renovations are an inevitable reality as facilities evolve. Often, renovations result in additional airflow requirements due to newer codes, increased heat loads, or additional square footage. Typically, this additional airflow is provided by simply increasing the speed of the fans within the AHU serving the space. The cooling coils in the AHUs normally remain untouched, due to cost limitations. While increasing the fan speed may satisfy the needs of the renovation, it does negatively affect the chilled-water coil performance. Over time, numerous renovation projects can lead to significantly higher airflows through a cooling coil than originally designed.

It's common to come across AHUs in existing facilities that are being run harder than they were designed to run. Increasing the airflow through the baseline coil by 15% while leaving all other operating conditions the same (entering air conditions, leaving air temperature, entering water temperature) results in the modified waterside performance summarized by Change B in Table 2.

The last cooling coil change we'll analyze is the decreased leaving-air temperature setpoint. Again, due to the constant evolution of spaces within a facility, it is common for spaces to get too warm. If airflow cannot easily be increased in those spaces, a common tool employed by maintenance personnel is to lower the discharge-air temperature leaving the AHU. While this may satisfy the immediate needs of the warm spaces, it also negatively impacts the chilled-water coil performance. Decreasing the leaving-air temperature from the baseline coil by 2°F while leaving all other operating conditions the same results in the modified waterside performance summarized by Change C in Table 2. A

review of the performances in Table 2 shows how drastically these changes can affect cooling coil performance. Each change results in lower waterside delta T and increased chilled-water flow at the cooling coil. In practice, cooling coils often deal with a combination of the three changes explored. As the coil performances degrade, the chilled-water flow will ultimately have to increase to maintain setpoints, either by increased chilled-water pump differential pressure or by staff readjusting balancing valves.

It's no wonder that "low-delta T syndrome" is so common in chilled-water systems. As this analysis shows, there are several factors working in unison to decrease the waterside delta T at chilled-water coils throughout the system. Lower waterside delta T is almost a natural part of a chilled-water system's aging process.

The perils of short-sighted design

Figure 2: Chilled-water system arrangement and component-selection criteria are used in a shortsighted chilled-water system design. Courtesy: IMEG Corp.As a cautionary example, let's explore the long-term implications of a chilled-water system design approach that only focuses on the initial operation of the system. For simplicity's sake, this example will focus on a basic chilled-water system with two chillers and a primary/secondary pumping arrangement. (refer to Figure 2 for an overall schematic of the chilled-water system used in this example.)

Going back to the baseline cooling coil selection from Tables 1 and 2, assume that all cooling coils in the chilled-water system were selected in the same manner. As a result, the average cooling coil design's waterside delta T would be 12°F, and the design chilled-water supply temperature to the cooling coils would be 45°F. Chiller selection is generally dictated by the required capacity of the chiller.

A careful analysis of all the connected cooling equipment and building loads should be performed to determine the maximum cooling capacity required by the chillers. After completing this analysis, assume that the chillers are selected with this required capacity at a 12°F waterside delta T and 45°F chilled-water supply temperature, which matches the cooling coils. In this example, assume that each chiller is sized at 50% of the required cooling capacity.

Figure 3: A chilled-water system’s design is prepared for the inevitable degradation to come. The design changes may not appear significant, but the long-term implications of these changes are critical. Courtesy: IMEG Corp.For the primary and secondary pumps, assume these are also sized at the same 12°F delta T as the chillers and cooling coils. This results in the primary and secondary loops having the same design flow rate. (This system arrangement and the selection criteria used for the system components is summarized in Figure 2.)

Now let's analyze how this chilled-water system design will be controlled and operated throughout the life of the system. When the chilled-water system first begins operation, everything should perform as designed if properly installed and commissioned. First, the cooling coils will achieve their design performance including the design waterside flow rate and delta T. The secondary pump will modulate its speed to satisfy the needs of the coils. The chillers, and their associated primary pumps, will be staged as required to maintain the chilled-water supply temperature setpoint. At low-load conditions, only a single chiller and primary pump will operate.

Once the cooling load of the facility exceeds 50% of the design capacity of the system, the second chiller will turn on. When the system is at its design capacity, the flow through the primary loop will equal the flow in the secondary loop; there will be zero flow through the decoupler. Everything appears to operate as intended and designed.

However, let's now explore how this system will be controlled and operated several years in the future. From the cooling coil analysis, we know the delta T being achieved at the cooling coils will have decreased. This means the chilled-water flow rate required by the cooling coils with respect to cooling load will be higher than design. From the cooling coil simulation results, it's reasonable to envision the required cooling coil flow rate at 50% of design when the cooling load is only at 40% of design.

In this situation, the secondary pump will still be able to modulate its speed to satisfy the needs of the chilled-water coils. A single chiller will be operating at 80% of its capacity to maintain the chilled-water supply temperature setpoint. However, the flow in the secondary loop is at 50% of its design, which is equal to the flow in the primary loop with one chiller operating. As the cooling load increases, the secondary loop will now have a higher flow rate than the primary loop. When this happens, there is "reverse" flow in the decoupler. Warm secondary return water will mix with primary supply water.

Although the chiller is generating 45°F water, the secondary chilled-water supply temperature will be higher than 45°F, which won't satisfy the cooling coils in the system. As a result, the chilled-water system needs to turn on the second chiller, even though the first chiller is only operating at 80% of its capacity. This is not ideal, as two chillers are now running when only one chiller is required. 


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