Designing chilled water systems
Typically used for cooling and dehumidifying a building’s air, chilled water (CHW) systems circulate it throughout a building or campus complex. CHW systems also may be used for removing process or other heating loads.
3. Understand key equipment and its integration to improve energy efficiency.
Regardless of whether the design is for a new chilled water (CHW) system or a modification to an existing system, an early review of codes, standards, and regulations is necessary to allow for an expedient design and avoid conflicts that will cost time and money to resolve. Local, state, and federal codes and regulations will dictate permitting requirements that affect the location of buildings and equipment (central plants, cooling towers, buried piping systems), fuel handling and storage, environmental emissions and noise, water quality, and safety items.
Groups such as ASHRAE, Air Conditioning, Heating, and Refrigeration Institute (AHRI), the American Society of Mechanical Engineers (ASME), and NFPA all have standards to review for systems, equipment, and testing requirements.
A good primary resource for most engineers today is ASHRAE. ASHRAE’s various technical committees write standards and guidelines to establish consensus for such items as: methods of testing and classification, design, protocol, and ratings for systems and equipment components of those systems. These consensus standards and guidelines are developed by industry leaders with a wide variety of practical and technical/research experience, and published to define minimum values or to encourage acceptable and enhanced performance.
ASHRAE has numerous technical sources of information including a series of four handbooks that are updated every 4 years. Two of these handbooks, Fundamentals – 2013 and HVAC Systems and Equipment – 2012, contain several chapters filled with information and basic criteria needed to design CHW systems. Each handbook has an entire chapter dedicated to listing “Selected Codes and Standards Published by Various Societies and Associations” relevant to the topics covered within the handbooks.
All of the related building system codes—Building Officials Code Administrators International (BOCA) and International Building Code (IBC )—and system components such as piping (ASME B31), ductwork (SMACNA), motors and generators (IEEE, NEMA, UL), and other codes and standards are listed for reference. This is very valuable for any designer or engineer beginning a new project, as these resources are updated every 3 or 4 years.
There are several major components within a CHW system, but chillers are machines filled with refrigerants used in the exchange of heat to “create” and provide the cold water. When chillers are placed in rooms or confined spaces, the designer of the system must incorporate safety provisions to the equipment operator and/or the public. ANSI/ASHRAE Standard 15-2013: Safety Standard for Refrigeration Systems is the reference standard for “machinery rooms” that typically house the larger equipment (i.e., chillers, pumps) necessary for a CHW system. This standard should be used in conjunction with ANSI/ASHRAE Standard 34-2013, Designation and Safety Classification of Refrigerants.
ASHRAE Standard 90.1-2013: Energy Standard for Buildings Except Low-Rise Residential Buildings is the reference standard for energy efficiency. This standard illustrates minimum efficiency and control systems requirements along with commissioning for building envelope, HVAC, power, lighting, and other equipment, all of which is included in a CHW system design. In ASHRAE 90.1, Chapter 6 is where designers will find minimum energy efficiency requirements for HVAC and CHW system construction with listings for component items such as water- and air-cooled chillers, piping system design flow rates, insulation, and controls.
In addition, ASHRAE also published Guideline 22-2012: Instrumentation for Monitoring Central Chilled-Water Plant Efficiency, which helps designers better understand how to control CHW plants, and has recently developed a District Cooling Guide – 2013 under the auspices of ASHRAE Technical Committee 6.2, District Energy, which does an excellent job of covering items mentioned later in this article.
What is a CHW system?
From the early years of HVAC design, the use of CHW to transfer heat from areas of higher loads (e.g., building loads at air handler coils, or industrial equipment loads at heat exchangers) to a condensing water loop or a refrigeration system for heat rejection has been successful. In a very broad sense, a CHW system consists of the following components:
- A heat absorption component such as a chiller (or evaporator)
- A compressor in a refrigerant cycle
- A heat rejection component such as a cooling tower (or radiator)
- CHW piping
- Either condenser water (CW) piping (for a water-cooled system) or refrigerant based piping (for an air-cooled or evaporative-cooled distribution system) to move the separate fluid systems between the respective components.
Each of the CHW and CW/refrigerant distribution systems will include various additional components and devices such as a pump, a compressor, an expansion tank, air separators/air eliminators, water or refrigerant treatment and filtration devices, isolation and control valves, and a controls system consisting of numerous temperature, pressure, and flow rate metering and control devices. For chillers using air cooling on the condenser side, there is no need for a condenser water loop including piping, cooling tower, and pump. For this article, the fluid systems discussed will be water only.
The CHW portion of the system circulates and flows between the chiller and the building loads through pumping by the CHW pump (although dependent upon the system, usually referred to as the primary pump), and can be operated as constant flow or variable flow. For water-cooled chillers, a condenser water loop is necessary, and always operates when the chiller is energized to operate. This loop also requires a condenser water pump to circulate the CW through the piping between the chiller and the cooling tower or heat rejection device (radiator or closed circuit cooler). The CW system has traditionally been a constant flow (CF) system, but recently designs have included variable flow (VF) in this system as well. Any variable flow application (CHW or CW) increases the intricacy of the design, construction, and operation of a system, but at times of low load and corresponding reduced flow rate requirement, may offer significant pump energy savings. Decisions regarding constant and variable system flows dictate designs typically referred to as primary/secondary (PS) and variable primary (VP) system designs.
Selecting a CF versus a VF system requires many considerations during the design effort. As with any design, the designers of a CHW system should consider various options and equipment through discussions with the owner, and recommend one or more of these options to meet the project goals and performance requirements. Among the many important items to consider regarding these system designs are any system constructability and budgetary constraints, system operability, operations and maintenance costs, and energy consumption costs.
Depending on the size of the building and the related cooling loads necessary to cool and dehumidify the building’s airstreams or other processes where some form of cooling is needed, the CHW system may have more than one of the larger components mentioned (chillers, cooling towers, pumps), and may be independent from nearby surrounding buildings. Or the building may have some combination of CHW distribution piping systems connected to a larger thermal utility network that serves several buildings simultaneously from a large, remote central plant arrangement.
CHW system types
The first step in designing any efficient, effective HVAC system for a building is to perform an accurate building load calculation and energy model. The 2013 ASHRAE Handbook-Fundamentals Chapters 18 and 19, and ASHRAE 90.1 provide methods and guidelines for developing HVAC load calculations and building energy modeling. The type of CHW system designed and installed and the amount of the CHW required for these cooling loads will be a major component in the overall building energy usage. When designing new or retrofitting existing CHW systems, the interaction between all building loads as related to outdoor air (OA) ventilation requirements, and the energy needed to condition that amount of airflow, along with the internal building’s return air (RA) loads and any other process heat loads, should be part of the system considerations so all the equipment can be sized and controlled properly to account for all the energy impacts, including the energy transfer for hydronic system preheat or precool opportunities.
An independent, stand-alone single chiller system type is relatively easy to design and operate, but even though the first cost is less, this system is typically the least energy-efficient design for buildings. This is because chillers are normally selected within a small percentage range of the calculated design process loads of the building (or buildings) they serve. Based on a variety of research, and dependent on the building loads throughout the day, the majority of the time the CHW system operates at part load and is in the 45% to 60% range. The chiller operates at full capacity for only a small percentage of time.
Designers should select a chiller at a higher part load efficiency to maximize energy savings based on the larger run hours at part load. Review AHRI’s Standard 550/590: Performance Rating of Water-Chilling and Heat Pump Water-Heating Packages Using the Vapor Compression Cycle for more details. Additionally, if the building’s cooling requirements include any mission critical functions within its structure, provisions for redundancy (N+1) must be incorporated in the design. If a single chiller fails, or a related single pump or cooling tower associated with the chiller fails, the CHW system or all cooling capacity is lost. Thus, many CHW systems have two or three redundant equipment components installed. This provides some level of backup and allows for more efficient operation at low-load time periods. There are exceptions with some manufacturers who provide dual compressor chillers that can operate at a high-efficiency point at 50% capacity (one compressor), and also provide some redundancy for a chiller plant.
Figure 2 is a schematic that shows a building single-chiller CHW system. Figure 2 illustrates a similar independent system, but where multiple components would be installed because the building, and the cooling load, is larger or redundancy is required (N+1). In both single-chiller and multiple-chiller arrangements, the CHW loop can be either constant flow or variable flow (which must remain above manufacturer required minimum flows).
The use of two or more chillers with part load capacity will provide more opportunities to improve the CHW system part-load performance and help reduce energy consumption, and can greatly assist in providing redundancy in the design. These chillers can be designed to operate in series or parallel modes. (Figure 5 is a parallel chiller arrangement.)
Figure 6 shows a large 1450-ton chiller, which is one of three in a parallel arrangement. The parallel arrangement is more common with chillers that are typically the same type and size, but is not mandatory. The chillers do not need to be sized individually to meet the building capacity but can be operated together to do so. In this case, the CHW will flow in parallel paths through both chillers and will generally experience similar pressure drops. In a series chiller arrangement, the CHW flow will go through both chillers in series and the water pressure drop is additive. In both arrangements, one or both chillers may be on variable speed drives (VSDs) and the CHW, and even the condenser water, loop can be either constant flow or variable flow.
Finally, the building or buildings may not have any chillers or cooling towers, but only CHW distribution piping systems connected to a larger thermal utility network from a remote CHW central plant (CP) arrangement. Typically these central CHW plants serve multiple buildings of various types ranging in function or use, size, construction materials, age, and cooling loads. Some buildings may have more than one CHW loop inside its walls. The building’s piping distribution systems may or may not have a pump (typically identified as a secondary or tertiary building pump depending on the system) within its structure. The larger CHW central plant that provides the buildings with CHW may have chillers in a parallel or series arrangement, and may have pumps (primary and/or secondary) located within the CP building.
Another term for this type of arrangement that is becoming more common is a district cooling plant (DCP) that also serves a localized campus, whether in a college or university setting, an industrial complex, or large urban mixed-use site. The design of these DCPs must take into account the diversity of all loads throughout the area they serve including when the different peak loads will occur. (See Figures 3 and 4 for schematic arrangements of a larger central plant.)
CHW pumping schemes
Regardless of the CHW plant location, an overall campus thermal utility master plan can provide the design options for consideration and evaluation of pumping schemes for circulating CHW. There are two common configurations for CHW plant pumping schemes that will work with the selected CHW equipment to deliver the CHW to a building or group of buildings:
- Primary-secondary (PS)
- Variable-primary (VP).
In the PS scheme, the primary CHW loop is typically constant volume flow while the secondary loop is variable volume flow. There are still some older systems where the secondary loop is also constant volume. This loop will have three-way valves located at some or all of the building loads to allow for required minimum flow rates. However, these systems are commonly being replaced because the technology and efficiencies of the chillers have increased, as have the energy costs associated with operating the distribution system. The VP scheme, sometimes called direct-primary, can be either a constant or a variable volume flow system. Again, because energy costs are so important, this loop is usually variable flow with variable frequency drives (VFDs) on the primary pumps.
The designer should become aware of the various advantages and disadvantages for either scheme, which include: central plant operators’ familiarity with their different operational modes, different size pump motor requirements, different capital investment requirements for infrastructure, and “low delta T syndrome.” See Figure 5 for a schematic that shows a proposed PS CHW system; Figure 6 is a schematic that shows a possible VP CHW system.
Along with the discussion of pumping schemes, it is important to understand the phenomenon known as low delta T syndrome, and its subsequent impact on chiller plant capacity and energy usage.
CHW system design considerations
CHW systems are all designed for a differential temperature or delta T between the CHW supply and return water between the chiller and the building loads. This delta T will affect building equipment (air handling or fan coil units) coil sizes, distribution system pumping costs, and chiller sizing and costs with associated energy costs required to produce the differential. A higher delta T usually means the costs will increase for the chiller as it will affect the chiller evaporator log mean temperature difference (LMTD) and require longer tubes or more chiller passes, which in turn increases chiller pressure drops that need to be overcome by the pumps. Table 2 illustrates a distribution pumping cost relationship.
For the planned system, designers need to vary their selections of CHW supply temperatures along with the CHW delta T ranges to determine the best balance for each. Selecting a chiller for a higher delta T may reduce other equipment cost and energy use when compared to the traditional 10 F delta T. At higher temperature differentials of 12 to 18 F delta T, low supply water temperatures (38 to 40 F), and variable flow with modulating valves, a design strategy could reduce pump energy (lower flow) and piping installation cost (smaller pipe sizes).
However, lower leaving water temperatures use more energy that may not be offset by perceived gains in pumping and fan energy savings. Colder supply water means higher compressor horsepower costs. And the selected delta T will also affect a building’s air handler coils regarding flow rates and supply air temperatures. The distribution loop’s supply temperature should be set for the building’s temperature and humidity control needs. The total annual system energy use must be considered for any of these options.
Low delta T syndrome
Low delta T syndrome occurs when a design CHW temperature range is not maintained. Every CHW plant will experience low delta T at some point during its continued operation. This phenomenon causes plant operators to run extra pumps and chillers to meet CHW load. This in turn reduces the plant’s cooling output capacity and wastes energy. A CHW plant’s output capacity can be defined by the following equation for a water-only system:
Q (Btu/hr) = 500 x gpm x delta T
Because load is directly proportional to flow rate and delta T, a change in delta T will require a change in flow rate for the same load. A change in flow rate implies a change in delta T for the same load. In a plant setting, if delta T is low, at least three problems can occur: increased pump energy usage, an increase in chiller energy usage, and an inability to meet some cooling loads. Designers should research and become familiar with this phenomenon but realize low delta T is a symptom of problems on the load side at the buildings and possible overpumping at the plant on the CHW loop (see Table 3).
Some form of hydraulic modeling of a building or thermal utility distribution CHW system should be completed for any design because of the relationship of the volumetric flow rate to the pressures that will be experienced in the system. The system pressures will dictate the selection of the component equipment (chillers, pumps, etc.) as well as the pressure class of all the distribution piping, fittings, and valves within the system. These pressures will, in turn, be related to the selected pumping scheme. The typical system delivering CHW from a chiller or the entire central plant is a closed loop hydronic system, and this means that the starting point within the system is the same as the ending point within that system. For reference, the typical condenser water system is considered an open loop, but it can be closed dependent on the heat rejection equipment used.
Every component within the CHW system will affect the pressure of the CHW at any point and will: fix the pressure at a particular level, increase the pressure, or decrease the pressure. Expansion tanks within a closed loop system will act as the point of constant pressure and be considered the reference pressure for the system, and will also allow for the expansion or contraction of the CHW due to thermal and volumetric changes in the closed system. The CHW pumps will increase pressure by raising the suction pressure at the pump by the total dynamic head of the system.
The total dynamic head of the system is defined as “equal to the total discharge head minus the total suction head of the CHW pump typically expressed in feet of water.” All equipment within the system (chillers, heat exchangers), and all piping, fittings, isolation and/or control valves, and any other appurtenances will decrease the system pressure through the friction effects as the water passes through the system.
Control sequences are a key element in achieving any energy management and savings goals. Most chiller control sequences are straightforward and easy to use for the operation of one or more chillers within a plant. All chillers have an internal sequence they use to run, and a series of safety sequences to prevent inadvertent damage while starting or running. An overall control sequence can be simply manually enabling the chiller or chillers to run as needed, although this could lead to wasted energy as the chillers will run even when not needed, or automating the process through a BAS.
The control schemes for a CHW system usually vary with the size and complexity of the system, and especially with the type of pumping scheme chosen. The system’s CHW flow can be controlled from static pressure, which provides some reliability but has limited flexibility for operational changes, and can waste energy in over pumping. Or, CHW flow can be controlled from differential pressure using delta P at the CHW plant, in the distribution system, and/or at the hydraulically most remote location. In addition, there is typically some type of chiller staging sequence such as with load or amps (kilowatts) of the motors, or some other strategy such as Btu metering and metering secondary CHW flow rates.
As mentioned earlier, ASHRAE has developed numerous sources of information for CHW systems that can be used as resources for the designer. Furthermore, ASHRAE 90.1 requires various efforts such as pump pressure optimization where pump control setpoints are varied due to control valve positions in the system, and CHW temperature reset, which uses feedback from the building control valves and outside air temperatures to reset the CHW supply temperature upward when available to reduce chiller loads. In some cases, these efforts may be easy, particularly if the CHW system is relatively small and/or the chiller plant is part of the building.
It is not as easy if the chiller plant is part of a campus environment, although decoupling the central plant control from any building-level control would allow the plant to operate as it needs to while the buildings all operate separately. A complete optimization of a plant must evaluate the efficiency of the entire CHW system and operate all the individual components (chillers, cooling towers, pumps) at various levels to optimize the overall CHW system operation.
Randy Schrecengost is a project manager/senior mechanical engineer with Stanley Consultants. He has extensive experience in design and project and program management at all levels of engineering, energy consulting, and facilities engineering. He is a member of the Consulting-Specifying Engineer editorial advisory board.
- ASHRAE Handbook – HVAC Systems and Equipment 2012
- ASHRAE Handbook – Fundamentals 2013
- ANSI/ASHRAE Standard 15-2013: Safety Standard for Refrigeration Systems
- ANSI/ASHRAE Standard 34-2013: Designation and Safety Classification of Refrigerants.
- ASHRAE Standard 90.1-2013: Energy Standard for Buildings Except Low-Rise Residential Buildings
- ASHRAE Guideline 22-2012: Instrumentation for Monitoring Central Chilled-Water Plant Efficiency
- ASHRAE District Cooling Guide – 2013
- McQuay International Chiller Plant Design, Application Guide 2002
- ITT Fluid Technology Corporation, 1968, Primary Secondary Pumping Application Manual
- ITT Fluid Technology Corporation, 1996, Large Chilled Water Systems Design Workshop Manual
- “Chilled Water Plant Pumping Schemes,” James J. Nonnenmann, PE, Stanley Consultants Inc.
- “Chilled Water System Hydraulics,” James J. Nonnenmann, PE, Stanley Consultants Inc.