Understanding Modular Chiller Plant Design: Insights from the Train Engineers Newsletter Live

Jeanne Harshaw welcomes participants to the Train Engineers Newsletter Live program, focusing on the design of modular chiller plants, emphasizing the growing popularity and acceptance of modular chillers and the importance of understanding their nuances for effective application.

The program introduces modular chillers, which are designed to produce chilled or hot water, highlighting key design features that differentiate them from package chillers. The discussion will cover equipment considerations, system considerations, and common applications for modular chillers.

Key terms are defined, starting with 'module,' which refers to a refrigeration unit that includes all necessary components for producing conditioned fluid, along with electrical connections and controls. The term 'modular chiller' is explained as a system consisting of one or more modules functioning as a single entity, with features like single-point water and power connections.

The modular chiller's design includes field-assembled modules shipped separately to the job site, featuring interconnecting valves, piping, and controls. A single-point power connection is essential for operation. The discussion includes details about water-cooled and air-cooled modules, highlighting their respective piping configurations and the use of dual-scroll compressors and heat exchangers.

The flow dynamics in modular chillers are explained, noting that each module operates in parallel, with each module typically having two refrigeration circuits. The example illustrates how flow is distributed among modules, with a design flow of 400 GPM resulting in 100 GPM per module, and how this changes with varying system flow rates.

Esty Tierney takes over to discuss the design of modular chillers, emphasizing the flexibility in using different module types and sizes. Key considerations for designing a modular chiller include the number of modules, compressor staging and operation, and delta T and temperature control, with a focus on how the number of modules affects the overall footprint of the chiller.

The discussion begins with the advantages of using multiple smaller modules in a chiller system for added redundancy. It is noted that while an 80-ton chiller can be constructed from four 20-ton modules, this configuration results in a footprint four times larger than a single 80-ton module, despite having the same nominal tonnage. The importance of considering the mechanical room's space allocation is emphasized, highlighting that footprint should not be the sole factor in determining the number of modules.

A key driver for adopting a modular design is the need for redundancy in chiller systems. Typically, a single module has two refrigerant circuits, which offers limited redundancy. The speaker explains that having more modules increases redundancy, particularly in critical cooling applications, which often require an additional module to satisfy N plus one requirements.

The discussion transitions to the importance of control steps in modular chiller design, specifically the total number of compressors. Most modules utilize on-off scroll compressors, resulting in only two control steps: one compressor running or two compressors running. This limited control can lead to inadequate temperature regulation, suggesting that more modules may be necessary to achieve tighter temperature control.

The speaker elaborates on compressor staging within a modular design, illustrating a module with two compressors on separate refrigeration circuits and a common dual-circuited brace plate evaporator. Assuming a 16-degree delta T with a 60-44 chilled water design, it is explained that each compressor contributes to an eight-degree delta T when both are operational. The system's ability to adjust to varying loads is highlighted, with the primary controller staging off compressors as entering temperatures decrease.

In part load scenarios, the system may experience delays in compressor activation, typically lasting several minutes, allowing for adjustments to dynamic system demands. The speaker illustrates how one compressor running results in a temperature output of 50 degrees, which may fluctuate as the entering water temperature decreases. The effectiveness of a modular chiller bank in meeting varying loads is attributed to the presence of multiple modules and control steps.

The discussion continues with an example of three modules operating in parallel under part load conditions. The lead controller effectively manages the system by utilizing the available steps of control. In this scenario, two modules operate at full capacity while the third operates at half capacity, resulting in a mixed leaving chilled water temperature of 44.7 degrees, which is much closer to the desired set point of 44 degrees. The speaker emphasizes that more modules lead to tighter overall temperature control.

The conversation shifts to the relevance of electrification in modern applications, noting that modular chillers are increasingly used for heating due to their capability to operate at high lift. The speaker poses a question regarding the control of leaving condenser water temperature instead of chilled water temperature, indicating a growing interest in this area.

The modular chiller heater stages compressors based on the leaving condenser water set point rather than chilled water temperature. A larger delta T, such as 30 degrees, is common for heating, with each compressor handling a 15-degree delta. The 120-degree set point is only achieved when 90-degree water enters and both compressors are operational. If the entering water is 95 degrees, indicating less heat requirement, only one compressor may run, resulting in a hot water output of 110 degrees, which is 10 degrees below the set point.

The discussion transitions to summarizing temperature control as a function of delta T and the number of compressors. As the number of compressors increases, each contributes a smaller percentage to the overall output. A 10-degree delta T with two compressors results in each managing five degrees, leading to potential temperature swings of five degrees. For processes requiring tighter control, a solution with five modules and ten compressors would be more suitable.

Flow is another critical component of temperature control, with the standard flow calculation equation presented: Tons multiplied by 24 divided by delta T equals GPM. Each module operates as an individual chiller, meaning flow variations affect delta T. For instance, a 50-ton module receiving 75 GPM results in a delta T of 60-44, but reducing flow to 50 GPM widens the delta T, potentially causing low temperature alarms due to the module's attempt to produce 36-degree water.

Flow issues can lead to low temperature alarms in modular systems, indicating a need for attention. Similar problems can occur on the condenser side, resulting in excessively high leaving condenser water temperatures. The importance of maintaining proper flow in modular chillers is emphasized, as they function by utilizing multiple chillers in parallel.

For those concerned about temperature swings with on-off scroll compressors, utilizing compressor technology with capacity control may be beneficial. Variable speed drives can enhance temperature and capacity control while improving part load efficiency. Modular designs allow flexibility in the number of variable speed compressors, where one or two can significantly impact a single module, though adding more may yield diminishing returns in larger chiller banks.

Centrifugal compressors with integral drives are highlighted as a viable option, offering modular efficiency and oil-free operation with shell and tube heat exchangers. Variable scrolls or screw compressors from various manufacturers are also considered. The key takeaway is to assess acceptable temperature fluctuations during system design, as comfort cooling applications may tolerate larger swings, while tighter control may necessitate smaller design delta Ts and compressors with capacity control.

The discussion highlights various strategies for temperature control in modular chiller systems. One approach involves utilizing a three-way thermostatically controlled mixing valve instead of relying solely on the modular chiller's temperature output. Additionally, the use of a mixing tank is mentioned as a common design to ensure a blended water supply. It is emphasized that selecting compatible control steps, delta T, and system design is crucial for effective operation.

The speaker transitions to the topic of installation, stressing the importance of customer education and preparation for modular chiller installation. A sample piping schematic is presented, illustrating the need for field assembly to connect the modules. Notably, the assembly process involves making 16 grooved couplings for a water-cooled chiller with five modules, which requires careful alignment and elevation adjustments. Control wiring and external temperature sensors are also highlighted as essential components that need to be field installed.

Final thoughts on equipment considerations focus on the significance of upfront planning for modular chillers. Understanding compressor staging between modules aids in determining the appropriate number and type of modules for chiller plant design. The speaker notes the potential need for incorporating hydronic components, such as valves and tanks, to enhance overall system effectiveness. Ensuring all necessary hydronic and control components are in place is crucial for a smooth installation process.

The discussion shifts to system considerations, particularly focusing on pumping strategies related to modular chillers. A basic primary-secondary system is reviewed, where two package chillers are piped in parallel, each with a dedicated pump. The introduction of additional pumps and chillers is explained as a method to meet increased system flow demands. The speaker outlines how modular chillers, consisting of three modules, operate in parallel, ensuring equal flow distribution and maintaining desired leaving fluid temperatures through compressor modulation.

The speaker elaborates on the advantages of variable flow systems over constant flow systems in modular chiller applications. A specific example is provided, detailing chilled water design conditions of 60-degree return and 44-degree supply at 300 GPM, with a current system load of approximately 135 tons. The importance of avoiding the mixing of unconditioned fluid is emphasized, showcasing the operational dynamics of modular chillers in maintaining efficiency and performance.

The system requires 200 GPM of chilled water, with 100 GPM passing through the decoupler. When 200 GPM of 59-degree chilled water mixes with 100 GPM of 43-degree chilled water, the return temperature to the chiller is recorded at 54 degrees. This lower return temperature indicates to the chiller controller that less than full load is sufficient to meet system demand.

In this scenario, two out of three modules are operational, each designed to produce a 16-degree delta T at 100 GPM. Consequently, the two active modules generate a fluid temperature of 38 degrees at their heat exchangers, while the inactive module does not contribute. The mixed fluid temperature exiting the chiller is 43.3 degrees, highlighting the variability in leaving fluid temperatures under part load conditions.

Reassessing the system with only one module operating under a 67-ton load instead of 135 tons reveals a mixed return temperature of 48.6 degrees. This situation increases mixing within the module, resulting in a leaving chilled water temperature of 43.3 degrees. However, the chiller controls must prevent freezing, which limits the module's leaving water temperature to 36 degrees, potentially leading to compressor cycling issues.

The discussion emphasizes the significance of design delta T, desired leaving fluid temperature, and the number of modules in operation, all of which affect system performance. Designers are more inclined to consider variable primary flow in modular chiller systems with more than three modules, although constant volume pumping is still feasible with careful design.

To mitigate freezing risks, several practical solutions are proposed. One option is to use a glycol mixture instead of pure water, which not only protects the heat exchanger from freezing but also allows for a lower freeze protection tripping point. Additionally, modular chiller manufacturers may recommend glycol in the evaporator loop when leaving fluid temperatures drop below 42 degrees.

Implementing variable speed compressors can enhance turndown capabilities, improve temperature control, and reduce cycling. This approach can be applied to one or more compressors within the system, providing flexibility in operation.

In a variable flow system, operational modules receive flow while inactive modules do not, preventing the mixing of conditioned and return fluid within the chiller. The upcoming section will delve deeper into the concept of variable primary flow.

Automatic isolation valves are crucial for systems utilizing variable primary flow. These valves ensure that when at least one compressor in a module is energized, the corresponding motorized isolation valve opens, while both compressors being off results in the valves closing. This setup is essential for the proper operation of modular chillers.

In water-cooled chillers operating year-round, maintaining a minimum entering condenser water temperature is vital, especially when the water is cold. Modular chillers equipped with motorized isolation valves on the condenser side can modulate to maintain the required leaving condenser fluid temperature, offering a solution to this challenge.

The discussion begins with the method of maintaining refrigerant head pressure control. When the entering fluid temperature drops below 55°F, the valve pinches down, reducing the flow rate through the condenser, which raises the leaving fluid temperature to an appropriate level. This approach simplifies the design, installation, and operation of the chiller plant by eliminating the need for field-installed and controlled devices. It is emphasized that pump speed should be controlled via differential pressure rather than a flow meter to avoid conflicts between the flow meter and chiller valves.

The speaker raises a critical question about the scenario when the chiller is off, all valves are closed, and the pump is still operational. This situation could lead to the pump deadheading, as there would be nowhere for the flow to go. Two solutions are proposed: installing a bypass line and valve external to the chiller, sized for the system's minimum flow, or keeping one valve open at all times to serve as a minimum flow bypass, thus eliminating the need for external solutions. However, it is noted that some applications may still require an external bypass based on minimum flow requirements.

The discussion transitions to the configuration of modular chillers, which are piped in parallel, allowing each module to experience the same flow. An example is provided with a three-module chiller designed for a flow rate of 300 GPM. As the system load decreases, the chiller can shut off compressors and close isolation valves, resulting in only two modules operating. Consequently, with the pump still moving 300 GPM but only two valves open, the operating modules experience a flow rate of 150 GPM, which is 50% above the design flow rate. This situation necessitates maintaining the design flow rate, leading to a discussion on pump control.

To maintain the desired flow rate for the chiller, the speaker emphasizes the importance of controlling pumps effectively. A comparison is drawn with traditional variable primary flow systems that utilize package chillers, highlighting features such as manifolded pumps with variable frequency drives (VFDs), automatic chiller isolation valves, and minimum flow bypass with modulating valves. The transition to modular chillers requires similar components for proper operation, including motorized automatic isolation valves for each module, which are crucial for effective variable primary flow operation.

The speaker outlines the essential system components required for the proper operation of modular chillers. Variable speed drives must be supplied with the system pumps, and motorized automatic isolation valves should be configured per module. It is recommended to limit the use of three-way valves in favor of two-way valves to minimize minimum flow, which is key to maximizing pump energy savings. Modular chillers can be selected to achieve very low flow turndown, allowing for significant energy savings when the bypass is matched to the chiller's minimum flow.

The discussion highlights the necessity of system differential pressure and chiller differential pressure, or a flow meter for the chiller, to ensure accurate flow control. Both modulating valves in the system and in the chiller require monitoring of differential pressure or flow rate. A flow meter is often preferred for the chiller due to its accuracy, especially when the chiller's differential pressure is low. Additionally, a minimum loop volume is required for stable system operation, which is considered a fundamental requirement for any chilled water system.

The speaker clarifies the concept of the end-of-loop bypass, which is crucial for maintaining minimum flow back to the chiller when system valves are closing. This bypass should be located as close to the end of the loop as possible, ideally allowing for at least two minutes of volume to ensure sufficient loop time for the chiller to safely unload. A bypass positioned too close to the chiller may result in inadequate flow management, underscoring the importance of proper bypass placement in the system.

The discussion emphasizes the importance of fast-acting control for the bypass valve that directs chilled water back to the chiller, as delays can lead to nuisance trips due to the chiller needing time to unload. It is recommended to use a direct controller for this valve instead of a VAS to enhance response time. Additionally, the valve should be sized appropriately, typically around 1.5 times the design flow rate of a single module for common comfort cooling applications, although this may vary based on chilled water temperatures and glycol.

The speaker addresses the critical aspect of loop volume, noting that the industry standard for chilled water systems is a loop time of two to three minutes. For modular chillers used in comfort cooling, this rule still applies, and a simple calculation involves multiplying the chiller design flow by three to determine the required system volume. Adequate loop time is essential, especially if the bypass valve is positioned too close to the chiller, which may hinder compressor staging. In cases where the evaporator, system piping, and coils are inadequate, adding a buffer tank can help increase loop volume. Applications with tighter temperature requirements or those involving air-to-water heat pumps may necessitate longer loop times.

The conversation shifts to determining minimum flow for modular chillers, distinguishing between the minimum flow of a modular chiller and that of a single module. For a four-module chiller with a design flow rate of 400 GPM, each module has a minimum flow of 100 GPM. However, the minimum flow for a single module depends on design conditions, as different delta-T values (e.g., 8-degree vs. 18-degree) yield varying flow rates. The minimum flow must align with the design delta-T to maintain system efficiency.

The speaker describes a variable primary flow system featuring a single modular chiller, highlighting similarities and differences from previous systems. Key components include pumps fitted with variable frequency drives (VFDs), automatic isolation valves, and a preference for two-way valves over three-way valves to maximize flow turndown and energy savings. The discussion also mentions the importance of an end-of-loop modulating bypass and the need for a controller to coordinate these components effectively.

The speaker outlines various strategies for controlling variable speed pumps in a modular chiller system. The pump speed is primarily driven by signals from the chiller controller to a system controller, which adjusts based on the number of operating modules and the flow rate. For instance, if module strainers become clogged, the flow meter may prompt an increase in pump speed to maintain nominal flow. As more modules activate, the system controller incrementally raises the pump speed to ensure consistent design flow per module, ultimately achieving better temperature control with reduced mixing within the modules.

As the system reaches satisfaction, the two-way valves modulate closed, causing the system differential pressure to rise. The end-of-loop bypass valve opens to maintain desired system pressure. As the return temperature to the chiller decreases, the chiller controller stages modules off, closing their automatic isolation valves, which signals the system controller to reduce pump speed. This ensures the chiller remains satisfied while maintaining system pressure and minimum flow.

Two potential control strategies are discussed: one utilizing differential pressure maintained by the pump, and another controlling pumps to maintain system pressure. Both methods should be fast-acting and not reliant on a Building Automation System (BAS). High-quality components with high-accuracy capabilities are recommended for optimal performance.

Variable flow can also be applied to the condenser side, although it is less common. The flow rate can be controlled simply by adjusting the pumps to maintain desired flow at the condenser. Heat recovery can similarly be applied, using the same principles as the chilled water side.

The discussion shifts to the applications of modular chillers versus package chillers, particularly in the context of an expanding brewery. Modular chillers allow for incremental capacity increases, making it easier to add modules as needed without the upfront costs associated with package chillers. Proper sizing of headers and plant piping for maximum expected capacity is crucial during initial design.

In a 200-ton chiller plant scenario, using five 40-ton modules instead of two 100-ton package chillers allows for easier N plus 1 redundancy, requiring only a sixth module. Modular heat pumps offer flexibility for seasonal coverage and can also be used for heat recovery, recapturing heat for other building needs.

Modular chillers can be integrated into thermal energy storage systems to build ice, assuming adequate glycol is available. The concept of heat recovery is also applied to process loads, where dedicated chillers may be used for various temperature requirements, allowing for the reuse of heat removed from the building.

Chiller plants often feature equal-sized chillers, but an asymmetric design may be more efficient based on load analysis. A pony chiller, sized for low load conditions, can be utilized at night or during peak demand, while a swing chiller may be sized to provide additional capacity when needed.

The discussion highlights the importance of sequencing chillers in a plant to match cooling loads effectively. For instance, a 200-ton chiller in a community center may struggle to meet demand during low occupancy events, such as Monday night meetings for local Cub Scout troops. In such cases, a 30-ton modular pony chiller with two modules can efficiently handle the load without triggering service calls, showcasing the benefits of judicious chiller sizing based on building models or historical data.

Modular chillers are particularly advantageous in retrofit scenarios, such as in historical buildings like museums where maintaining architectural integrity is crucial. The ease of installation is emphasized, as modules can fit through standard passenger elevators and doors, eliminating the need for disassembling larger package chillers. This feature simplifies the installation process significantly, making modular chillers a practical choice for such projects.

The discussion introduces air-cooled modular chillers equipped with hydronic free-cooling coils, also known as water-side economizers. These coils allow for cooling fluid without running the compressor when conditions are favorable. A free-cooling module can be integrated into a chiller bank, enhancing efficiency by cooling the fluid as much as possible before the remaining modules take on the load. This is particularly beneficial for data centers, which can utilize dedicated free-cooling modules as they expand, provided the initial piping and supports are sized for maximum capacity.

The impact of modular chillers on compliance with ASHRAE Standard 15 is discussed, particularly regarding the Refrigeration Concentration Limit (RCL). Modular chillers typically contain less refrigerant charge per circuit compared to conventional package chillers, reducing the likelihood of exceeding the RCL for the same net capacity. For example, a 120-ton water-cooled rotary chiller with two circuits contains 132 pounds of charge, while using four 30-ton modules would reduce the charge to 16 pounds per circuit, thereby decreasing potential charge release and ventilation requirements.

The conversation touches on ASHRAE Standard 90.1, addressing efficiency requirements for modular chillers. Similar to traditional package chillers, compliance can be achieved through either path A or path B, with both full and part load efficiency requirements needing to be met. Notably, each module in a modular chiller is certified as a single chiller, which differs from standard chiller plants where the capacity is based on the entire chiller. This distinction is crucial for understanding how modular systems are rated and their compliance with efficiency standards.

A 500-ton modular water-cooled scroll chiller consists of 10 50-ton modules, with efficiency requirements derived from the less than 75 tons row in the efficiency table. Modular chiller systems provide design flexibility, available in both water and air-cooled units, with options for heat recovery or free cooling. They can also function as heat pumps, supporting electrification initiatives, and can be configured with two, four, or six pipe units to meet specific design needs.

The capacity of modular chillers can be expanded by adding more modules, facilitating planned expansions. Their smaller size reduces redundant capacity and allows for easier service, as individual modules can be isolated while others continue to operate. This is particularly beneficial in retrofit projects where minimal construction is desired and space is limited, leading to faster and less intrusive installations while ensuring compliance with standards.

Each module in a modular chiller acts as an independent chiller, providing a single point of water connection and control to the Building Automation System (BAS), thus functioning like a single chiller externally. However, careful design considerations are necessary for the parallel flow arrangement and mixed water output to ensure appropriate redundancy and temperature control. On-off scroll compressor installations with few modules can lead to significant temperature fluctuations and nuisance alarms, necessitating additional control measures or variable speed compressors for stable temperature management.

Modular chillers behave differently in constant primary flow versus variable primary flow systems. In constant flow systems, each module experiences its design flow continuously, even when compressors are off, leading to mixed water temperatures at the output. Variable primary flow is now preferred for temperature control, requiring motorized valves that open and close based on module operation to prevent bypass flow. As more modules operate, the differential pressure decreases, necessitating increased flow to maintain design conditions.

Effective flow control can be achieved through various strategies, including controlling the pump or system bypass to maintain differential pressure. Control logic can also adjust flow based on the number of operating modules. The key takeaway is that flow management is a critical component of the initial control design, ensuring successful modular installations and maximizing the benefits of modular chiller systems.

The program concluded with a recap of modular chiller systems, emphasizing their advantages and operational insights. Participants were encouraged to refer to the bibliography for additional resources and to contact local Trane account managers for specific information on Trane systems. Continuing education credits were highlighted, along with a reminder to complete a survey regarding the program. Attendees were also informed about upcoming Engineers Newsletter Live programs and resources available on the Trane website.