Glossary of Key Technical Terms

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

A

Absorption chiller (Absorber): A refrigerating machine that uses heat energy (such as gas, steam, or hot water) and an absorption input to generate chilled water. Unlike vapor-compression chillers, it employs a thermal process rather than a mechanical one. These chillers can be susceptible to crystallization if the condensing water temperature is too low.
Adsorption: A process where fluid molecules concentrate on a surface due to chemical or physical forces.
Air-cooled chillers: Typically packaged-type chillers where the controls, compressors, evaporator, and air-cooled condenser are all on the same skid. Their capacity can reach up to 450 tons.
Air Eliminator (Air Vent): A device in a steam or water distribution system that opens to release air or noncondensables and closes when steam or water is present.
Air separator: A device used in a chilled-water system to remove air.
Approach temperature (Cooling Tower): The difference between the cooling tower’s leaving-water temperature and the ambient wet-bulb temperature. Cooling towers are typically not selected with an approach less than 5°F (2.8°C).
Arc flash hazard study: A type of electrical system study.
ASHRAE: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. A professional association that publishes standards and guides for HVAC&R.

B

Balancing valve: A valve used in a piping system specifically for controlling fluid flow, not typically for shutting off flow.
Blowdown: 1. The discharge of water from a steam boiler or open recirculating system (like a cooling tower) that contains high total dissolved solids, necessary to reduce their concentration. 2. In pressure-relief devices, it’s the difference between the actuation pressure of a pressure-relief valve and its reseating pressure.
Board (slab) thermal insulation: Semirigid insulation preformed into rectangular units that possess a degree of suppleness related to their dimensions.
Brazed PHEs: A type of Plate Heat Exchanger (PHE) featuring a close approach temperature (within 2°F [1.1°C]) and high heat transfer coefficient. They are constructed from stainless steel plates and frames brazed with copper or nickel and cannot be disassembled for cleaning.
Btu (British thermal unit): A unit of energy approximately equal to the heat required to raise one pound of water by one degree Fahrenheit.
Building Automation System (BAS): A system ideally suited for commercial building automation.
Building chilled-water pumps: Pumps located on the customer’s side of the district cooling connection.
Butterfly valve: A type of valve commonly used for isolation, consisting of a metal disc mounted on a rod.
Bypass valve: A valve that controls flow through a bypass pipe, usually positioned between the supply and return of a chilled-water system.

C

Casing: A thin-wall pipe that encloses the carrier pipe and its insulation, designed to prevent the insulation from getting wet.
Cathodic Protection: A technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell.
Cell (in a cooling tower): The smallest subdivision of a cooling tower that can operate as an independent heat exchange unit. It is enclosed by exterior walls or partitions and may contain one or more fans, stacks, or distribution systems.
Cellular elastomeric (cellular rubber) thermal insulation: Insulation primarily composed of natural or synthetic elastomers, processed to form a flexible, semirigid, or rigid foam, typically with a closed-cell structure.
Cellular glass insulation: An insulation material made from glass and carbon, forming a structure with millions of hermetically sealed cells.
Cellular polystyrene thermal insulation board: Insulation consisting of cellular polystyrene in board form, produced by heat and pressure from expanding foamable polystyrene beads or by in-situ foaming of molten polystyrene.
Cellular polyurethane thermal insulation: Insulation primarily made from the catalyzed reaction product of polyisicyanate and polyhydroxy compounds, usually processed with fluorocarbon gas to form a rigid foam with a predominantly closed-cell structure.
Central Plant: The facility where thermal energy (cooling) is generated for a District Cooling system.
Centrifugal chiller: A type of vapor-compression chiller that is common for large capacities in District Cooling Systems (DCSs).
CFC (Chlorofluorocarbon): A class of refrigerants whose production has been banned worldwide due to their destructive impact on the stratospheric ozone layer.
Chemical treatment: The process of adding chemicals to water to help control its chemistry and biological activity (e.g., to prevent scale, corrosion, or microbiological growth).
Chilled Water: Water cooled in the central plant and distributed through pipes to provide cooling to buildings.
Chiller: A refrigerating machine used to transfer heat between fluids, typically a complete indirect refrigerating system with a compressor, condenser, evaporator, and all operating and safety controls.
Chiller barrel: A shell-and-tube evaporator specifically used to cool water or a secondary coolant in water-chilling packages.
Chiller efficiency: A measure of a chiller’s performance, typically expressed as kW/ton refrigeration or heat input per ton.
Chiller Plant: A system that generates chilled water on-site for air-conditioning or other cooling needs within a building.
Chiller staging: The process of sequencing chillers on and off based on the cooling load requirements.
Chiller-aided storage: A thermal storage system that uses a chiller to supplement the cooling capacity provided by the storage. Also known as live load chilling.
Chilled water (CHW): The cooling medium normally delivered by a district cooling system.
Chilled-Water Distribution System: The network of pipes, pumps, and other equipment that transports chilled water from a central plant to customer buildings and returns the warmer water.
CHWST (Chilled-Water Supply Temperature): The temperature of the chilled water as it is supplied to the building or system.
CHWRT (Chilled-Water Return Temperature): The temperature of the chilled water as it returns from the building or system after absorbing heat.
Cogeneration: See **Combined heat and power (CHP).
Commissioning: A systematic process to ensure that systems are designed, installed, functionally tested, and capable of being operated and maintained in conformity with the design intent. It encompasses pre-design, design, construction, and occupancy/operations phases.
Commissioning Authority (CA): The entity responsible for ensuring that all elements of a district cooling system are designed, installed, and operated as an integrated whole, in accordance with the owner’s requirements.
Combined cycle: A type of power plant that employs more than one thermodynamic cycle, such as combining a combustion turbine to drive a generator with a steam turbine generator driven by steam produced from the hot exhaust gases.
Combined heat and power (CHP): A general term describing energy technology configurations that efficiently produce both electricity and thermal energy from a single fuel source, or a facility that recovers thermal energy normally wasted in power-only generating plants.
Compression Chillers: Chillers that produce cooling by mechanically compressing a refrigerant, typically powered by electricity. Common types include reciprocating, rotary, and centrifugal.
Compressor-aided storage: The operation of a compressor of an ice storage system during the discharging period.
Compressor surge: A condition in a centrifugal compressor where the momentum of the refrigerant gas is insufficient to overcome the thermal lift requirement, causing a temporary reversal of flow. This condition can lead to accelerated wear and damage.
Condenser water (CW): The water used to cool the chiller condensers, rejecting heat to the outside environment.
Condenser-water return temperature setpoint reset: A control strategy for condenser water that adjusts its return temperature setpoint.
Conductivity: A measure of a material’s ability to conduct an electric current, often used as an indicator of dissolved solids in water.
Connection charge: A one-time charge made by the DC provider to help offset the costs of serving the building, possibly associated with meters or other equipment the provider installs.
Consumer Connection (Energy Transfer Station – ETS): The equipment located within a customer’s building that transfers thermal energy from the DC system to the building’s internal heating, ventilation, and air conditioning (HVAC) systems.
Consumption charge: Represents the actual cooling used by the building over a billing period, varying with seasons, occupancy, etc., and typically measured in kWh, Btus, or ton-hrs.
Control valve: A device that controls flow in a system in response to a signal.
Cool storage: Technology or systems specifically used to store cooling capacity.
Cooling density (District Cooling System): A measure of cooling demand per unit area.
Cooling Tower (CT): Equipment that rejects heat from the chiller condensers to the outside environment. Key design considerations include sizing, basin design, and water treatment.
Cooling Tower Drift: Fine water droplets that are carried out of a cooling tower with the exhaust air.
Cooling Tower Makeup Water: Water added to a cooling tower system to replace water lost through evaporation, drift, and blowdown.
Cooling Tower Ton: A traditional unit representing the total heat rejection capacity of a cooling tower serving an electric centrifugal chiller, typically 15,000 Btu/h.
COP (Coefficient of Performance): A dimensionless measure of equipment performance and efficiency for a refrigeration cycle, defined as the heat input in the evaporator (cooling load) divided by the work input to the refrigeration cycle.
Corrosion: The deterioration of pipe materials due to chemical reactions with the thermal fluid or the external environment, often leading to leaks.
Counter-flow Tower: A type of cooling tower where air flows vertically upward against the downward path of the water.
Cross-flow Tower: A type of cooling tower where air flows horizontally across the downward path of the water.
Crossover bridge (or Decoupler): A branch pipe connection between supply and return lines, intended to hydraulically decouple two independently pumped water loops.
Cv (Flow Coefficient): A measure of the flow capacity of a valve, defined as the flow rate of water in US gallons per minute at a pressure drop of 1 psi. Used in selecting control valves.

D

DB (Dry Bulb): Dry bulb temperature. The temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture.
dBA (Decibel A-weighted): A unit for measuring sound pressure level, using the “A” weighting filter.
DBB (Design/Bid/Build): A project delivery process where a consulting engineer prepares a detailed design, including plans and specifications, that is then put out for bids to qualified contractors.
DC (District Cooling): The practice of meeting the cooling needs of buildings (i.e., air-conditioning) from a central plant located off the building site, typically delivering chilled water to the building’s air-conditioning system.
DCI&CS (District Cooling Instrumentation and Control System): A comprehensive system designed to maintain and monitor the complete operations and functionality of a District Cooling Plant (DCP), including interfaces with building services controls and Energy Transfer Stations (ETS).
DCP (District Cooling Plant): The central plant where chilled water is produced.
DDC (Direct Digital Controller): Controllers ideally suited for commercial building automation systems (BAS) but generally not ideal for the industrial nature of a modern DCI&CS.
Decoupled (hydraulically independent): Refers to two water loops that are hydraulically separated, often by a crossover bridge or decoupler.
Delta P (ΔP): The pressure difference between the supply and return lines in a system.
Delta T (ΔT): The temperature difference between chilled-water supply and return temperatures. Maintaining a high ΔT is generally more cost-effective as it allows for smaller distribution pipes and can reduce pumping energy.
Delta T Control Valve: A valve whose primary function is to provide the chilled-water supply temperature required by the building per contract, and whose secondary function is to ensure the customer’s return water temperature is per contract.
Demand charge: A charge that represents the commitment of chiller plant and distribution system capacity the district cooling provider has made to a building.
Demand-limited storage: A type of thermal storage system controlled specifically to limit electric power demand.
Desalination: Any of several processes that remove excess salt and other minerals from water.
Desorption: The liberation of a gas that was held in a substance by sorption.
Design/Bid/Build (DBB): See DBB.
Design specifications (DS): Documents that describe how a system (or modifications to it) will be built to meet the requirements outlined in the Functional Requirements Specification (FRS). These typically include Hardware Design Specifications (HDS) and Software Design Specifications (SDS).
Direct-Buried System: A DHC piping system where insulated pipes are installed directly in the ground, often within a protective casing.
Direct-contact storage (External melt): A type of ice storage system where ice is formed by direct refrigeration and then melted by immersion in circulating water or a secondary coolant. Also called static direct contact storage.
Direct current (DC): Refers to direct current electricity.
Direct digital controller (DDC): See DDC.
Distribution network: The piping system that transports chilled water from the central plant(s) to the consumer systems.
Distribution System: The network of insulated pipes that transports the thermal fluid (hot water, steam, or chilled water) from the central plant to individual buildings.
Distributed Control System (DCS): A type of controller considered acceptable for most DCI&CS applications.
Diversity Factor: A ratio used in DHC system design to account for the fact that the sum of individual building peak demands is typically higher than the actual simultaneous maximum demand on the central plant.
Drift: Small droplets of water that are entrained in the cooling tower’s discharge air.
Dry bulb (DB): See DB.

E

ECWT (Entering Condenser-Water Temperature): The temperature of water entering the chiller’s condenser.
EEMS (Expert Energy Management Systems): Systems that are typically Level 5 applications interacting with a DCI&CS.
Electrical-driven, water-cooled, centrifugal-type chillers: Globally common in the industry, favored for their high efficiencies, reliable operation, and lower unit pricing.
Encapsulated Ice TES: A type of ice thermal energy storage where water is frozen within containers (e.g., spheres) stacked inside a tank, using a heat transfer fluid to freeze and melt the ice.
Energy Transfer Station (ETS): Also referred to as energy substation, end user interface, or customer/consumer interface, its primary purpose is to transfer energy and custody of chilled water from the provider to the customer. It typically houses equipment for the building interconnection.
Engineer/Procure/Construct (EPC): A project delivery process where the design and construction services are contracted to a single entity.
Equalizer piping: Piping connecting the basins of multiple cooling towers or cells to maintain a common water level.
Equivalent full-load hours (EFLH): The ratio of total annual energy consumption to peak hourly demand.
Evaporator: The section of a chiller where heat is absorbed into the refrigerant from the chilled water circuit.
Evaporative Condenser: A heat rejection device that combines elements of a cooling tower and a condenser, where refrigerant is cooled by water spray and air.
Expansion bend: A bend, typically a loop, incorporated into a pipe run to alleviate stresses induced by expansion and contraction due to temperature changes.
Expansion joint: A device within a structure or pipe run designed to accommodate variations in length caused by thermal expansion or contraction.
Expansion Loop: A curved section of pipe designed to provide flexibility and absorb thermal expansion and contraction, preventing stress on the piping system.
Expansion tank: A tank used in a closed-water system to accommodate changes in water volume due to thermal expansion and contraction.

F

Factory acceptance testing (FAT): Testing procedures that are executed at the contractor’s factory before shipment to the district cooling provider’s site.
Field joint: A location in a piping system where carrier piping segments are joined together. In preinsulated piping, this includes insulating the joint and providing a leaktight jacket.
Field installation verification (FIV):.
Fill (Cooling Tower): The heat transfer media within a cooling tower that increases the surface area between airflow and water flow, enhancing cooling effectiveness.
Fill thermal insulation (Loose-fill): Insulation in granular, nodular, fibrous, powdery, or similar forms, designed for installation by pouring, blowing, or hand placement.
Flowmeter: A device used to measure the flow rate of chilled water, often for billing purposes.
Foamed-in-place thermal insulation (Foam-in-situ insulation): Insulation formed by introducing chemical components and a foaming agent into prepared cavities, which then react to fill the space.
Fouling Factor: A measure of the resistance to heat transfer due to buildup of deposits on heat exchanger surfaces.
FRP (Fiberglass Reinforced Plastic): A composite material used for piping, known for good corrosion resistance.
Functional requirements specification (FRS): A document that describes what the system (or its modifications) is intended to do, outlining the district cooling provider’s requirements.

G

Gasketed PHEs: Plate Heat Exchangers (PHEs) that use gaskets, allowing them to be disassembled in the field for cleaning or plate removal/addition for capacity adjustments.
Generator (Absorption Chiller): The component in an absorption chiller where a solution of refrigerant and absorbent (e.g., water/lithium bromide) is boiled by heat (gas, steam, or hot water), releasing refrigerant vapor and concentrating the absorbent solution.
GHG (Greenhouse Gas): Gases present in the Earth’s atmosphere that contribute to warming near-surface global temperatures through the greenhouse effect.
Globe valve: A type of valve used for regulating fluid flow in a pipeline, consisting of a movable disk-type element and a stationary ring seat within a generally spherical body.
GRP (Glass-Reinforced Plastic): See FRP.
GWP (Global Warming Potential): A relative scale that compares a given mass of a greenhouse gas to the same mass of carbon dioxide (whose GWP is by definition 1) in terms of its contribution to global warming over a specific time interval.

H

Heat gain: The heat absorbed by the chilled water from its surroundings (e.g., ground) as it is transported through the distribution system.
Heat exchanger (HEX): A device designed to transfer thermal energy between two hydraulically separated systems, typically preventing the mixing of fluids that may have different chemical treatments.
Heat rate: A measure of how efficiently a power generator uses fuel, expressed as the number of British thermal units of fuel required to produce a kilowatt-hour of electricity.
Heat rejection: The process of dissipating heat from the chiller condensers to the outside environment, either to the atmosphere or a body of water.
Heat Rejection Area: The surface area required by chillers or cooling towers to dissipate waste heat to the environment.
Heat storage: Technology or systems used to store heating capacity.
HCFC (Hydrochlorofluorocarbon): A commonly used class of refrigerants, many of which are in the process of being phased out.
HDPE (High-Density Polyethylene): A common material used for piping systems and jackets on insulated systems.
Hermetic drive: A chiller arrangement where the motor is contained within the same housing as the compressor and is in direct contact with the refrigerant.
HFC (Hydrofluorocarbon): A commonly used class of refrigerants, many of which are in the process of being phased out.
HHOT (Hand-Held Operator Terminal): A compact, portable device that can connect directly or wirelessly to a controller to interrogate, program, and control system parameters.
Hot tapping: An operation where a branch connection is made to a pipe main while the pipe remains in service or “hot” (though it’s a misnomer for chilled-water pipes).
HVAC (Heating, Ventilation, and Air Conditioning): The systems within a building that provide these functions.
Hydraulic analysis/modeling: The process of laying out and sizing a distribution system, using computer-aided design programs, based on required flow rates and assumed temperatures.
Hydraulic decoupler (Crossover bridge / Decoupler): A cross-connection between supply and return piping, typically at the chilled-water plant, used to hydraulically decouple the flow through the chillers from that in the distribution system.

I

Ice-on-coil thermal storage: A container (tank) in which ice is formed on tubes or pipes.
Ice slurry: A pumpable mixture of ice and water, which may include additives.
IDEA: International District Energy Association.
In-Building Chiller Plant: A cooling system where the chiller and associated equipment are located within a single building, serving only that building.
Indirect Connection: A type of consumer connection where a heat exchanger separates the DHC thermal fluid from the building’s internal HVAC fluid, providing hydraulic and pressure isolation.
Indirect ice contact (Internal melt): An ice storage system using a method of heat exchange where ice in containers is formed and melted by a circulating secondary coolant enclosed in a pipe or tube.
Industrial heat pump: A device that can use the condenser water of a chilled-water plant as a heat source to provide higher heating temperatures (e.g., up to 220°F [104°C]).
Input/output (I/O): Refers to the signals and data exchanged between control systems and field devices.
Insulation: Material wrapped around pipes to reduce heat loss or gain, critical for maintaining thermal efficiency in DHC systems.
Integrated processes: Strategies that combine district cooling with other processes, such as turbine inlet air cooling.
Isolation valves: Valves used to isolate sections of a piping system.

J

Julian Date: The numerical day of the year (1 to 365), used in soil temperature calculations.

K

Kilopascal (kPa): A unit of pressure.
Kilowatt (kW): A unit of power.
Kilowatt-hour (kWh): A unit of energy.
KWe/kWt (Kilowatts electrical per kilowatt thermal): A measure of chiller efficiency, representing the electrical power consumed per unit of cooling energy produced.

L

Laminar Flow: A type of fluid flow characterized by smooth, parallel layers, which reduces heat transfer efficiency in cooling coils.
Latent storage: A method of thermal storage that utilizes a phase change of a medium (e.g., water to ice) for storing heating or cooling capacity.
LCWT (Leaving Condenser-Water Temperature): The temperature of water leaving the chiller’s condenser.
Legionella: A type of bacteria that, if cooling tower water is improperly treated, can lead to dangerous outbreaks, emphasizing the critical importance of proper water treatment.
Level 0 equipment: Field-installed equipment that directly monitors or controls the production, storage, distribution, or transfer of cooling energy and its media. These devices are connected via hard-wiring through a Level 1 device.
Level 1 equipment: Encompasses field instrumentation such as temperature, liquid flow, liquid pressure, outside-air humidity, and water quality sensors.
Level 2 controllers: Local plant controllers that provide the actual control and monitoring functions at the plant level by interfacing with Level 0, 1, 3, and 4 equipment. Programmable Logic Controllers (PLCs) and Distributed Control Systems (DCSs) are acceptable types.
Level 3 equipment: Refers to local Operator Interface Terminals (OITs) installed at the provider’s plants, often in control panels on the plant floor, allowing personnel to control and monitor equipment locally.
Level 4 equipment: Typically refers to Command Centers, which provide a centralized point of operation for the DCI&CS, allowing remote monitoring and control of plants.
Level 5 equipment: Refers to systems on the district cooling provider’s corporate network that typically interface with the DCI&CS, including billing and accounting systems, expert energy management systems, maintenance systems, optimization systems, and operations support systems.
Life-cycle cost (LCC) analysis: A method of economic evaluation that encompasses all major costs and charges associated with different options over the life of a project or contract, including the time value of money.
Load duration curve: A curve that sorts hourly cooling load data from highest to lowest to determine the number of hours that occur at or below a specific load, useful for evaluating plant options.
Load-leveling storage: A thermal storage system designed to smooth out the demand peaks by shifting cooling production to off-peak periods.
Low-ΔT syndrome: A common problem in the district cooling industry where chilled water is returned from a building at a temperature lower than optimal. This leads to inefficiencies in the central plant and distribution system, requiring more water flow for the same cooling capacity. It is preventable with proper design and maintenance.
LMTD (Log Mean Temperature Difference): Used in heat exchanger performance analysis.

M

Machinery rooms: Spaces where the volume of refrigerant exceeds the limits specified by ASHRAE Standard 15, requiring specific design criteria for safety and ventilation.
Makeup water: Water that is added to a system to compensate for losses due to evaporation, leaks, or other discharges.
Master planning: The process of developing a long-range development plan at the initial design stage of a major district cooling system, based on intelligence about potential customer bases.
Mechanical chiller: A refrigerating machine that uses mechanical energy input to generate chilled water.
Mechanical Joint: A general term for gas-tight joints obtained by mechanically joining metal parts, often using O-rings or lip seals, distinct from welded or cemented joints.
Meter: A device used to measure energy consumption, typically for billing purposes.
Mineral fiber thermal insulation: Insulation composed primarily of fibers manufactured from rock, slag, or glass, with or without binders.
MMBtu (Million British Thermal Units): A common unit for measuring energy, particularly in the context of natural gas and cooling.
Montreal Protocol: An international agreement established in 1987 to address the depletion of the ozone layer, initially banning Chlorofluorocarbons (CFCs) and later establishing phase-out plans for Hydrochlorofluorocarbons (HCFCs).
Multi-effect distillation (MED): A desalination process.

N

Naturally stratified storage: A type of thermal storage where temperature stratification is achieved and maintained solely by differences in water density, without mechanical separators.
Noncondensables: Gases (primarily air) that enter the refrigerant circuit of a chiller and do not condense at operating pressures and temperatures, leading to increased condensing pressure and reduced efficiency.

O

O&M (Operations and Maintenance): The ongoing activities required to keep a system or facility functioning efficiently and reliably.
ODP (Ozone Depletion Potential): A scale that compares the relative ability of chemicals to deplete the ozone layer, with CFC-11 fixed as the base (ODP of 1.0).
OIT (Operator Interface Terminal): A local terminal used by personnel to control and monitor equipment at the plant.
OLE (Object Linking Embedding): A technology that supports the linking and embedding of objects from one application seamlessly into another.
OPC (OLE for Process Control): A standard that specifies communication of real-time plant data between devices from different manufacturers.
Owner’s Project Requirements (OPR): Key requirements for a project defined in the pre-design phase of commissioning. Also known as the Owner’s Requirements Document (ORD).

P

Packaged plants: Modular plant components manufactured in a factory.
Parallel arrangement (Chiller): A configuration where chillers operate side-by-side to meet the cooling load.
Parasitic Loads: Energy consumed by auxiliary components within a system (e.g., pumps in absorption chillers) that do not directly contribute to the primary output.
Part load: The operation of equipment at less than 100% of its design load.
Partial storage system: A system where the load demand is met by a combination of stored thermal energy and an energy conversion device; it operates during both on-peak and off-peak periods.
Peer-to-peer: A communication structure where network-connected devices can be configured to communicate directly without needing an intermediary or arbiter.
Performance Metrics: Quantifiable measures used to evaluate the successful development and operation of a district cooling system, encompassing customer service, system operations, financial performance, and environmental impact.
Perlite thermal insulation: Insulation composed of natural perlite ore, a glassy volcanic rock that is expanded by heat to form a cellular structure.
PEX (Cross-Linked Polyethylene): A form of HDPE that has been cross-linked to provide higher strength at elevated temperatures.
Phase Change Materials (PCMs): Substances that absorb or release large amounts of latent heat at a constant temperature when they undergo a phase transition, used in some TES applications.
pH Adjuster: A chemical added to water to regulate its pH level, crucial for corrosion control and scale prevention.
PHEs (Plate Heat Exchangers): A specific type of heat exchanger commonly used in indirect district cooling connections, known for high heat transfer coefficients and the ability to achieve close approach temperatures.
Pigging: A mechanical cleaning method for pipelines using a device (a “pig”) pushed through the pipe to remove debris or clean internal surfaces.
Pipes: The conduits through which thermal fluids are transported in a DHC system. Common materials include steel, ductile iron, and plastic.
Pipe materials: A wide variety of materials used for district cooling distribution piping systems and jackets, including steel, copper, ductile iron, cement-based products, fiberglass-reinforced plastic (FRP), polyvinylchloride (PVC), and polyethylene (PE) or high-density polyethylene (HDPE).
PLC (Programmable Logic Controller): A type of controller considered acceptable for most DCI&CS applications.
Pneumatic control: Control devices that utilize compressed air signals to control inputs and outputs.
Point of delivery: The contractual boundary or physical point that divides responsibilities between the energy provider and the customer.
Point of No Pressure Change: A theoretical point in a DC system where static pressure remains constant, and around which expansion tanks and pressure control mechanisms are often designed to operate.
Polyethylene (PE): A material used for district cooling distribution piping and jackets.
Polyvinylchloride (PVC): A common material used for piping and jackets in district cooling systems.
ppm (parts per million): Denotes one part per 1,000,000 parts, a value of 1 x 10^-6.
PPE (Personal Protective Equipment): Equipment used to protect employees from workplace hazards.
Pre-insulated Piping: Piping systems where the insulation and often a protective jacket are applied to the pipe during manufacturing, designed for efficiency and durability, especially for buried applications.
Pressure dependent (PD): Describes a flow control device whose flow rate varies in response to changes in pressure.
Pressure Drop: The reduction in fluid pressure as it flows through a pipe, caused by friction with pipe walls and turbulence from fittings.
Pressure Independent Control Valves: Control valves that regulate flow based on demand, independent of pressure fluctuations in the distribution system.
Pressure transmitters: Devices that measure pressure and transmit a signal.
Primary side: Refers to the district energy provider’s side of the connection in a district cooling system.
Primary pumping: Pumping schemes specifically for the central plant.
Primary-secondary systems: A pumping scheme for hydronic systems, considered reliable, conservative, and easy to operate, typically using a hydraulic decoupler.
Programmable Logic Controller (PLC): See PLC.
Protective device coordination study: An electrical system study.
Provider: Refers to the district cooling provider, the entity offering district cooling services.
psi (pound per square inch): A unit of pressure, equal to 68.95 millibar.
psig (pounds per square inch gauge): Pressure measured above standard atmospheric pressure in psi.

R

Range (Cooling Tower): The difference between the entering and leaving water temperatures across the cooling tower.
Redundancy: The provision of additional equipment or systems beyond what is normally required, to ensure continued operation in case of failure.
Refrigerant: A substance used in the refrigeration cycle to absorb and reject heat. Common types have included HCFC-123 and HFC-134a, which are being phased out.
Refrigerant migration: A method of free cooling.
Refrigerant Management: Practices and technologies used to control the emissions and disposal of refrigerants, some of which can be harmful to the ozone layer or contribute to global warming.
Registered engineer: An appropriately qualified and licensed professional engineer.
Reflective thermal insulation: Insulation that reduces radiant heat transfer across spaces by using surfaces of high reflectance and low emittance, such as aluminum foil.
Reliability (DCS): A key performance metric for District Cooling Systems, generally expected to be very high (e.g., exceeding 99.94% availability) due to equipment redundancy and high operational supervision.
Reverse osmosis (RO): A desalination process that can be used to treat seawater or Treated Sewage Effluent (TSE) for cooling tower makeup.
RTD (Resistance Temperature Detector): A type of temperature sensor that measures temperature by correlating resistance with temperature.
RTTMS (Real-Time Thermal Modeling and Simulation): Systems capable of performing “virtual metering” as a backup to physical metering.

S

SCADA (Supervisory Control and Data Acquisition): A system that allows for monitoring and controlling the overall operations of a plant.
Scale Inhibitor: A chemical added to water to prevent the formation of mineral deposits (scale) on heat transfer surfaces.
Secondary side: Refers to the building or customer’s side of the connection in a district cooling system.
Secondary pumping: Pumping within the distribution loop or customer building.
Sensible Heat: Thermal energy associated with a change in temperature of a substance, without a change in its phase.
Sensible storage: A method of thermal storage that uses a change in temperature of a medium (e.g., water) for storing heating or cooling capacity.
Series-counterflow arrangement (Chiller): A chiller configuration where units are arranged in series, which can increase overall chiller-module efficiency, especially with large chilled-water ΔTs, because the compressor lift is split between two units. This can result in significant energy consumption savings.
Shadow prices: An assumption of CO2 emissions cost used for the purpose of comparing different options.
Shell-and-tube evaporator: A type of evaporator used to cool water or a secondary coolant, sometimes referred to as a chiller barrel.
Short-circuit study: An electrical system study.
S/cm (Siemens per centimeter): A unit of specific conductivity.
Sidestream filters: Filters installed to remove particulate matter from a portion of the circulating water in a system, often used in cooling towers or building connections.
Solenoid: A type of actuator that operates in a two-position (open/closed) mode.
SOP: Standard Operating Procedure.
Sorbate: A substance that is absorbed by or adsorbed on a sorbent.
Sorbent: A material that extracts one or more substances present in an atmosphere or mixture of gases or liquids due to an affinity for such substances.
Sorption: A general term covering both absorption and adsorption.
Standard atmosphere: A unit of pressure equal to 101.325 kPa (14.696 psi).
Steel: A common material used for district cooling distribution piping.
Strainer cycle: A method of free cooling.
Storage cycle (Thermal storage): A complete charge and discharge of a thermal storage device.
Stratification: The division of fluid into a series of layers, as with thermal gradients across a stream. This phenomenon is crucial for the efficient operation of Chilled Water Thermal Energy Storage (CHW TES) tanks.
Sub-Metering: The practice of metering individual customers within a building or facility served by district cooling, in addition to or instead of bulk metering for the entire building.
Supervisory Control and Data Acquisition (SCADA): See SCADA.
Suspended Solids: Small solid particles that remain in suspension in water, which can contribute to fouling and reduce heat transfer efficiency.

T

Tariff Structure: The pricing model used by district cooling providers to bill customers, typically including consumption, demand, connection, and penalty charges
Temperature Degradation: The undesirable increase in chilled water temperature in the supply line due to heat gain from the surrounding environment, reducing cooling effectiveness.
Temperature Measurement: Instruments that measure temperature, typically using thermal transducers, resistance thermometer elements, or liquid expansion capillaries.
Temperature transmitters: Devices that measure temperature and transmit a signal to a control system.
TES (Thermal Energy Storage): Involves storing energy as heat (or cooling) in a storage medium to separate the generation of cooling from its use in time. The basic engineering unit of TES capacity is the refrigeration ton-hour (12,000 Btu) or thermal kWh (3413 Btu).
- CHW TES (Chilled Water Thermal Energy Storage): Stores energy as sensible heat (change in temperature) of chilled water. The most common form uses thermally stratified tanks.
- Ice TES: Stores energy as latent heat (a phase change, specifically water to ice).
- LTF TES (Low Temperature Fluid Thermal Energy Storage): Stores energy as sensible heat, similar to CHW TES, but uses additives to allow storage at lower temperatures (e.g., -1.1°C to 2.2°C / 30°F to 36°F).
TES Tanks: Insulated tanks used for thermal energy storage, which can be located above or below ground.
Thermal insulation: A material or assembly of materials specifically used to provide resistance to heat flow.
Thermocline: A layer of fluid in which the temperature and density gradient is greater than, and which separates, the cooler fluid below it and the warmer fluid above it.
Ton of Refrigeration (TR): A customary unit of refrigeration capacity, equivalent to 12,000 Btu per hour, historically representing the amount of refrigeration capacity required to make one ton of ice from unfrozen water in 24 hours.
Ton-day of Refrigeration: Heat removed by a ton of refrigeration operating for a day, equivalent to 288,000 Btu.
Ton-hr: A unit of refrigeration energy supplied or used, equivalent to one ton of cooling over a one-hour period.
Total Dissolved Solids (TDS): The total amount of mobile charged ions, including minerals, salts, or metals dissolved in a given volume of water, impacting water quality and system efficiency.
Treated Sewage Effluent (TSE): An alternative water source for cooling tower makeup, characterized by high Total Dissolved Solids (TDS) and suspended solids, often requiring polishing and disinfection.
Turbidity: The cloudiness or haziness of a fluid caused by large numbers of individual particles, typically invisible to the naked eye, similar to suspended solids.
Turbine meter: A device that measures the rate of flow in a pipe using a rotor that spins as the fluid passes through.
Turbulent Flow: A type of fluid flow characterized by chaotic, irregular motion, which enhances heat transfer in cooling coils.

U

UNEP: United Nations Environment Programme.
Uninterruptible power supply (UPS): A system that provides continuous power to equipment, even during outages.

V

Valve Authority: The effectiveness of a control valve in influencing flow in its circuit, ideally a high value (around 0.5 or 50%) for optimal control.
Valve Vaults: Underground structures that house valves and other components of a buried distribution system, allowing for isolation and maintenance without excavation.
Variable primary flow: A pumping scheme where the flow rate varies within the system.
VFD (Variable Frequency Drive): A device used to control the speed of motors, commonly employed in chillers and pumps to optimize efficiency.

W

Water-cooled chillers: Chillers that reject heat to water, which is then typically cooled by cooling towers.
Water Hammer: A phenomenon characterized by a sudden pressure surge (or shockwave) in a piping system, typically caused by an abrupt change in the flow velocity of the fluid.
Water treatment: See Chemical treatment.
Wet-bulb Temperature (WB): The lowest temperature to which air can be cooled by the evaporation of water, important for cooling tower performance.
Weighted Average Cost of Capital (WACC): Used in economic analyses to convert capital costs into an annual amortization cost, consistent with a developer’s actual weighted average cost of capital.

Z

Zero Liquid Discharge (ZLD): While not explicitly defined, the sources discuss the disposal of rejected water from processes like Reverse Osmosis (RO) that aim to minimize liquid waste.