Chillers
Chillers are a liquid cooling device used for air-conditioning or refrigeration applications. They can vary in size from serving single residences to very large industrial facilities. For air-conditioning applications, chillers are generally used in medium to large buildings or campuses and typically are chilling water that is distributed throughout the building to air handling units, radiative cooling panels, chilled beams, or other types of terminals. Chillers can also be used to cool water and glycol mixtures for applications requiring lower temperatures, such as ice production for thermal storage.
The two most common types of chillers are based on a vapor compression cycle, which is the majority of the market, or on an absorption cycle, which is driven by heat or direct combustion.
Three types of condensers are most commonly used with chillers:
- Water-cooled, which typically uses separate cooling towers to reject heat
- Air-cooled, which usually are sold as part of the chiller
- Evaporative-cooled, which uses water to evaporatively cool the condenser coils
The configuration of chillers within a chilled water system can be simple or quite complex when multiple chillers and other components are involved.
Almost all BEM software includes models for both chillers and other components of a chilled water system, including configuration and control options.
How It Works
Chillers
Vapor compression cycle chillers
The most common type of chiller uses a vapor compression cooling cycle as shown below but cooling water instead of air.
The diagram below shows a pressure-volume diagram for a typical vapor compression system. The “heat in” comes from the water that is returning from the building through the piping system. The refrigerant in a liquid-vapor mixture expands in the evaporator to a vapor state and, in the process, cools the water by accepting heat. The compressor provides the work into the refrigeration cycle and is shown as “work in” by compressing the vapor. After the compressor, the condenser rejects the heat at a higher temperature, where the refrigerant returns to a liquid state. The condenser either rejects the heat directly to the air or to water that is then cooled by a cooling tower or fluid cooler. The refrigerant then goes through an expansion device that cools the refrigerant as some of it changes to a vapor to be used in the cycle again. The details of the process are not as important to understand as the fact that heat can be removed from the building at a lower temperature and rejected at a higher temperature by the use of a compressor, which takes energy.
Absorption chillers
An absorption cycle works differently and uses heat instead of a compressor as part of the cycle, but it also accomplishes the removal of heat from the building at a lower temperature and the rejection of that heat at a higher temperature. More about the absorption cycle can be found here:
Compressor types
For chillers that use the vapor compression cycle, several types of compressors are used, and they vary in performance and appropriate application:
- Reciprocating - generally 2 to 450 tons
- Scroll - generally 3 to 500 tons
- Screw - generally 30 to 1250 tons
- Centrifugal - generally 80 to 4000 tons
The type of compressor also impacts the types of refrigerants that are used, and these can have impacts on ozone depletion potential and global warming potential. The type of refrigerant also impacts the performance of the chiller.
Performance and efficiency ratings
The rating for the performance of chillers is described primarily using a metric for rated full load performance, such as kW/ton, COP, or EER, and a metric capturing part load performance, such as IPLV. IPLV, Integrated part load value, is an efficiency rating intended to be a better representation of actual operation vs. full load efficiency
IPLV = (0.01A) + (0.42B) + (0.45C) + (0.12D)
Where:
A = COP or EER @ 100% Load
B = COP or EER @ 75% Load
C = COP or EER @ 50% Load
D = COP or EER @ 25% Load
Chilled Water Systems
From a modeler perspective, chilled water systems can include the following components:
- Chillers
- Chilled water coils
- Pumps
- Valves
- Sensors
- Pipes
- Heat exchangers
- Thermal energy storage
But these can be arranged in a large variety of configurations. For chiller plants with water-cooled condensers, the condenser side of the plant topology contains a similar list, but cooling towers and fluid coolers are added while chilled water coils and thermal energy storage are not necessary.
Cooling tower types
The two types major types of cooling towers shown below are:
- counter-flow is when the water is flowing downward and the air flows upward through the same media
- cross-flow is when the water flows downward and the air flows across it perpendicular to the flow of the water.
Pumping system configurations
Primary-only systems can include one or more chillers, and the pumps usually appear before each chiller or can appear as a grouping of pumps. In the top of the image on Pumping System Arrangements, cooling coil output is controlled by varying the flow of chilled water through each coil using three-way valves while the flow through each chiller remains constant.
Generally, variable flow systems are less costly to operate compared to constant flow systems. Most buildings operate below their design loads most of the year, and variable flow systems allow lower pumping power during this time. Variable frequency drives used with pumps often do not add a significant cost compared to the savings. Primary/secondary systems provide a constant flow through each chiller, which is required for some chillers, but add additional pumps. Variable flow through the chiller and coils is a simpler system if the chiller is selected to be able to provide that, but more controls must be in place to ensure the efficient and safe operation of all the components.
Adding thermal storage such as chilled water storage or a heat exchanger to allow water side economizer increases the complexity further. A water-side economizer uses cooling tower water through a heat exchanger to cool the coils during cooler times of the year.
Information Needed for the Model
Information on modeling chillers starts with the capacity (tons or kW) and efficiency, usually expressed as coefficient of performance (COP). Since the performance of the chiller varies with the operating temperatures, for many models of chillers, the capacity and efficiency are represented by performance curves or tables that take into account the changes in performance based on the leaving chilled water temperature and either the entering or leaving condenser water temperature. This level of detailed data is often only available from chiller vendors. The common rating metric used for part load performance is IPLV (Integrated Part Load Value), but it is rarely used as an input to BEM software since it is an aggregation of multiple points of data. Instead, performance curves or tables of data points are used to represent the efficiency at varying levels of output. For some BEM software, the IPLV is produced as an output based on the detailed performance curves or tables.
The configuration of the chiller itself needs to be specified and often includes
- Types of chiller - such as reciprocating, scroll, screw, or centrifugal, if the electric motor is part of the chiller or separate or replaced by an engine or turbine, if it is vapor-compression or absorption, whether it can be used as a heat pump to also supply heating
- Type of condenser - such as air-cooled, evaporatively cooled, or water cooled and whether that is part of the chiller or remote from the chiller
- Rated and designed flow rate and water pressure drop through the evaporator and condenser side of the chiller
- Rated and designed chilled water supply temperature and condenser water temperature
- Limits on temperatures for the chilled water and condenser water
- Minimum and maximum part load ratio and sometimes the optimum part load ratio and part load ratio at which false loading (such as hot gas bypass) is necessary
- Whether the flow through the evaporator is fixed or modulated
- Power required for the condenser fan for air or evaporatively cooled condensers
- Power and setpoints for the basin heater, which prevents the cooling tower water from freezing,
- Heat recovery options, if appropriate
Off-rated condition performance varies considerably between chiller vendors and often between different product lines by the same vendor, so it is best to request data directly from the vendor when modeling a specific chiller. Many default chiller curves in simulation tools are based on older chillers that may be based on refrigerants that have already been phased out due to the Montreal protocol. One source of detailed chiller performance data is in Appendix J of ASHRAE Standard 90.1-2022. This new appendix provides default performance curves for the various baseline chillers in the standard.
An important resource for the future is Standard 205-2023 Representation of Performance Data for HVAC&R and Other Facility Equipment, which provides a standard schema for including liquid-cooled chillers (RS0001). Once manufacturers and BEM software embrace this standard format for data, it will be very easy to request a 205 file from a vendor and use it directly with software without any need to translate or interpret the data.
The chilled water plant specification is often closely related to how the BEM software describes the topology of the system but will generally include
- Pump type (fixed or variable flow), power and/or pump heat, and heat added to the circulating water
- Arrangement of pumps, chillers, coils, and bypasses
- Controls include resetting the chilled water setpoint temperature and how chillers are sequenced based on load
- Integration of water-side economizers
Common Measures
For chillers, some common energy-efficiency measures to consider are shown below:
- Higher nominal efficiency for the chillers
- Improved part load performance for the chillers, often with variable-speed compressors
- Substitution of a different type of chiller
- Multiple chillers that better match common load ranges
- Variable speed pumping
- For air-cooled chillers, substitution of a different condenser type, such as evaporatively cooled or water-cooled
- Substitution of a chiller with heat recovery to supply simultaneous hot water
- Substitution of chiller that can be used as a heat pump to also supply hot tower
For chilled water systems, some common energy-efficiency measures include:
- Changing the design of chilled water and condenser water temperatures; in general, chillers operate more efficiently with higher chilled water temperatures and lower condenser water temperatures.
- Resetting supply water temperature based on overall load or outdoor temperature
- Optimizing control sequence for condenser water temperature
- Optimizing the staging of equipment
- Selection, sizing, and optimization of cooling tower operation
- Addition of waterside economizer
- Addition of thermal energy storage (especially if managing peak demand is important)
- Changing water distribution system (such as to primary only variable speed pumping)
- Higher efficiency pumping
- Pipe and valve sizing and insulation
Common Control Options
The control options for chillers themselves that are modeled in BEM software are limited and include when they are scheduled to be available and the control of the basin heater or heat recovery system. Most controls within BEM software are related to the chilled water system and include many of the items seen above in the common measures for chilled water systems.
Common Applications
Chilled water systems are commonly used for air-conditioning and for refrigeration applications. While very small units, primarily absorption, have been used in residential applications, the majority of the air-conditioning market for chillers is medium to large buildings where centralized control and maintenance are desired. Almost any type of commercial building can use chilled water systems, but they are more common in medium to large multi-floor buildings. Smaller buildings typically use decentralized packaged cooling systems. According to the 2020 ASHRAE Handbook on HVAC Systems and Equipment, central plants are commonly used in:
- Campus environments with distribution to several buildings
- High-rise facilities
- Large office buildings (typically over 150,000 ft2)
- Large public assembly facilities, entertainment complexes, stadiums, arenas, and convention and exhibition centers
- Urban centers (e.g., city centers/districts)
- Shopping malls
- Large condominiums, hotels, and apartment complexes
- Educational facilities
- Hospitals and other healthcare facilities
- Industrial facilities (e.g., pharmaceutical, manufacturing)
- Large museums and similar institutions
- Locations where waste heat is readily available (result of power generation or industrial processes)
- Larger systems where higher efficiency offsets the potentially higher first cost of a chilled-water system
Model Output Checks
There are three types of model output checks that are critical for chillers and chilled water systems:
- Confirmation of each chiller performance
- Confirmation of the chilled water system configuration
- Confirmation of the sequence of operation for the chilled water system
The first step is to confirm that each chiller is performing at rated conditions and at part load conditions, as expected by the performance data. Enable timestep reporting in the BEM software and check the results of the load, entering and leaving chilled water temperatures, entering and leaving condenser water temperatures for water-cooled or ambient air dry-bulb and wet-bulb temperatures (or humidity ratio or dew point) for air and evaporatively cooled, and the power consumption. Find points that mirror the rated, reference, or design conditions for the chiller and ensure the power consumption is as expected. Repeat the process for at least five other timesteps varying the temperatures in the range that data is available. In addition, confirm that operation does not exist for loading and temperature conditions that should not be possible for the chiller. This may include higher loads than design, much lower loads below the minimum part load ratio, or temperatures that the chiller cannot operate based on manufacturers' data. If your BEM software produces an estimate of the rated COP or kW/ton or for the IPLV, check those as well. This process needs to be repeated for each unique chiller in the model. If your model has multiple identical chillers, you can usually skip this process after the first chiller if all chillers reference the same data input.
Next, confirm that the chilled water system is configured as expected by using the design conditions on the chillers in the previous step and confirming the operation of each chiller, pump, and tower in the system. Confirm the power consumption for each pump and cooling tower, fluid cooler, air-cooled condenser, or evaporatively cooled condenser. For BEM software that tracks pressure drop, confirm the pressure at each point in the system on both the chilled water and condenser sides.
To confirm that the chilled water sequence of operations is working properly, it is first important to make a table identifying all the different unique operating modes that the chilled water system can have and modes of operation that should not occur. For a multiple-chiller system with identically sized chillers, each chiller is likely to be sequenced on and off, so that is one mode for each chiller. If the chiller plant has different-sized chillers, the sequencing can be even more complicated. If there is a chilled water supply temperature reset control, that may only occur when the last chiller is operating, or it may occur at any load condition. If pumps are sequenced or are linked to chiller operation, those should also be identified. Once the table of operating modes is constructed, confirm using timestep data from the BEM software that each mode occurs when expected, the operating temperatures, and that each component is loaded and uses power as expected. It is also important to confirm that switching between modes is occurring correctly; for this, identify a key indicator for each mode and scan timestep results for when that transition occurs. Review the other data prior to and after the transition between modes to confirm that it occurred as expected.
Related Energy Code Requirements
Building energy codes and standards such as the IECC, ASHRAE Standard 90.1, Title 24, and other local energy codes contain many specific requirements for chillers and chilled water systems. The following list is an example based on ASHRAE 90.1-2022:
- 6.4.1.2.1 Liquid-Cooled Centrifugal Chilling Package Cooling Efficiency Adjustment - describes how to adjust minimum efficiency requirements if the design temperatures are beyond the range of AHRI 550/590
- 6.4.1.2.2 Chilling Packages Employing Freeze-Protection Liquids
- Table 6.4.3.10.1 DDC Applications and Qualifications - describes exceptions for some chillers from the DDC requirements
- 6.4.3.11 Chilled-Water Plant Monitoring
- 6.4.3.11.2 Electric-Motor-Driven Chiller System Recording and Reporting
- 6.5.1.2 Fluid Economizers
- 6.5.4.2 Hydronic Variable Flow Systems.
- 6.5.4.3 Chiller and Boiler Isolation
- 6.5.4.4 Chilled- and Hot-Water Temperature Reset Controls.
- 6.5.4.7 Chilled-Water Coil Selection
- 6.5.2.2 Hydronic System Controls.
- 6.5.5.4 Tower Flow Turndown
- 6.5.6.3 Heat Recovery for Space Conditioning
- 6.5.11.2 Compressor Systems - for refrigeration
- Table 6.8.1-3 Liquid-Chilling Packages—Minimum Efficiency Requirements
- Table 6.8.1-16 Heat Pump and Heat Recovery Water-Chilling Packages—Minimum Efficiency Requirements
- 11.5.2.2.3 H03: HVAC Cooling Performance Improvement
- 11.5.2.2.7 H07: Improved HVAC Sequence of Operations
- 11.5.2.8.5 G05: HVAC Cooling Energy Storage.
Related to a performance method include:
- 12.5.2 HVAC Systems part b
- Table 12.5.1 Modeling Requirements for Calculating Design Energy Cost and Energy Cost Budget Part 10
- Table 12.5.2-1 Budget System Descriptions footnote e
- Table 12.5.2-2 Number of Chillers
- Table 12.5.2-3 Water Chiller Types
- Table 12.5.2-4 Cooling Tower Leaving Water Temperature
- Table G3.1 Modeling Requirements for Calculating Proposed Building Performance and
- Baseline Building Performance Part 10
- G3.2.1.5 Purchased Chilled Water
- G3.2.2.1 Equipment Efficiencies
- G3.2.3.6 Piping Losses
- G3.2.3.7 Type and Number of Chillers
- G3.2.3.8 Chilled-Water Design Supply Temperature
- G3.2.3.9 Chilled-Water Supply Temperature Reset
- G3.2.3.10 Chilled-Water Pumps
- G3.2.3.11 Heat Rejection
- G3.3.2.8 HVAC Systems part b
- Table G3.2.3.7 Type and Number of Chillers
- Table G3.5.3 Performance Rating Method Water Chilling Packages
- Normative Appendix J Sets of Performance Curves
One of the most important is Table 6.8.1-3 Liquid-Chilling Packages—Minimum Efficiency Requirements, which is reproduced below:
Rating methods for chillers primarily follow AHRI 550/590. The latest version from 2023 is titled: “Performance Rating of Water-chilling and Heat Pump Water-heating Packages Using the Vapor Compression Cycle.” For absorption chillers, AHRI 560-2000 “Standard for Absorption Water Chilling and Water Heating Packages” is used.
Similar or Related Systems
Other cooling systems like packaged rooftop air conditioners and heat pumps, variable refrigerant flow systems also use a similar refrigerant cycle but are not centralized plants like chilled water systems.
Additional Resources
- Chilled Water Plant Design Guide. Energy Design Resources. December 2009.
- Cooling Plant Optimization Application Guide. Siemens. March 2004.
- CoolTools Chilled Water Plan Design and Specification Guide. Steve Taylor, Paul DuPont, Bruce Jones, Tom Hartman, Mark Hydeman. PG&E. May 2000.
- Design Guide for Cool Thermal Storage, Second Edition. Jason Glazer. ASHRAE. 2019.
- Development of Maximum Technically Achievable Energy Targets for Commercial Buildings Ultra-Low Energy Use Building Set. Jason Glazer, ASHRAE. 1651-RP. December, 2015.
- Designing Chilled Water Plants with DOE-2. Charles Eley. PG&E.1997.
- HVAC Simulation Guidebook. Volume I. Second Edition. Energy Design Resources. July 2012.
- Fundamentals of Design and Control of Central Chilled-Water Plants. Steve T. Taylor. ASHRAE. 2017
- Guideline 36-2021 - High-Performance Sequences of Operation for HVAC Systems. ASHRAE.
- HVAC Water Chillers and Cooling Towers. Herbert W. Stanford III. Marcel Dekker. New York. 2003.
- Multiple-Chiller-System Design and Control. Mick Schwedler, Ann Yates. Trane Application Engineering Manual. March 2001
- Optimizing Design & Control of Chilled Water Plants, Part 1: Chilled Water Distribution System Selection. Steven T. Taylor. ASHRAE Journal. July 2011.
- Optimizing Design & Control of Chilled Water Plants, Part 2: Condenser Water System Design. Steven T. Taylor. ASHRAE Journal. September 2011.
- Optimizing Design & Control of Chilled Water Plants, Part 3: Pipe Sizing and Optimizing Delta-T Steven T. Taylor. ASHRAE Journal. December 2011.
- Optimizing Design & Control of Chilled Water Plants, Part 4: Chiller and Cooling Tower Selection. Steven T. Taylor. ASHRAE Journal. March 2012.
- Optimizing Design & Control of Chilled Water Plants, Part 5: Optimized Control Sequences. Steven T. Taylor. ASHRAE Journal. June 2012.
- Overall chilled water system energy consumption modeling and optimization. N. Trautman, A. Razban and J. Chen, Applied Energy
- Variable Speed Chilled Water System Modeling and Optimization. Neal L. Trautman. Purdue University. 2020.
References
2020 ASHRAE Handbook - Heating, Ventilating, and Air Conditioning Systems and Equipment. Chapter 43 - “Liquid Chilling Systems,” Chapter 3 “Central Cooling and Heating Plants”
Chiller - https://en.wikipedia.org/wiki/Chiller
ANSI/ASHRAE/IES 90.1-2022 Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings
AHRI 550/590-2023 “Performance Rating of Water-chilling and Heat Pump Water-heating Packages Using the Vapor Compression Cycle.”
AHRI 560-2000 “Standard for Absorption Water Chilling and Water Heating Packages” is used.
ASHRAE Journal July 2011, Optimizing Design & Control of Chilled Water Plants, Part 1: Chilled Water Distribution System Selection, Steven T. Taylor.
ANSI/ASHRAE Standard 205-2023 Representation of Performance Data for HVAC&R and Other Facility Equipment
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