Chilled Water Loop
Understand how the chilled water loop works. Explore component overviews, measurements, and additional resources.
Overview
A chilled water (CHW) loop consists of two main components: pumps and motors, and a piping network. The pumps circulate chilled water from the chiller’s evaporator through the piping network to terminal units, such as water-to-air heat exchangers in air handling units or radiant equipment like chilled beams, before returning it to the chiller.
Components
The main components associated with the CHW loop are primary pumps for primary-flow systems and primary and secondary pumps for primary-secondary flow systems.
Primary CHW Pump and Motor
A primary CHW pump and motor circulates chilled water from the evaporator section of the chiller to the building in a primary-only system or to the secondary pump and motor through the piping network.
Primary CHW pumps can operate at:
Secondary CHW Pump and Motor
A secondary CHW pump and motor circulates chilled water from the primary piping network to the building. A secondary CHW pump and motor are found in primary-secondary flow systems and can be at constant speed or can be equipped with variable frequency drives that are controlled by the differential pressure in the secondary piping network.
Secondary CHW pumps can operate at:
Primary Piping Network
The primary CHW piping network is connected to the evaporator section of the chiller and extends through the building to deliver chilled water to that facility in a primary-flow system. In a primary-secondary system, the primary piping network is dedicated to circulating chilled water through the evaporator of the chiller.
Secondary Piping Network
The secondary CHW piping network is connected to the primary piping network to deliver chilled water to the facility.
Typical CHW Loop Configurations
A CHW loop can be designed as either a primary flow system or a primary-secondary flow system. Below are the common configurations, how they work, and how to quantify the entire system.
Primary Flow System
A primary flow chilled water system consists of CHW pumps that circulate chilled water from the air-conditioned zone to the chillers. There are usually multiple pumps in the system to allow for staging and redundancy. These pumps often pump water into a single common pipe called a header, which allows them to run in parallel and service multiple chillers; however, some facilities have a single pump dedicated to each chiller.
Primary flow systems can be either constant-primary-flow or variable-primary-flow.
A constant-primary-flow system usually has a staged cooling system and constant speed CHW pumps.
A variable-primary-flow system has primary CHW pumps with variable speed drives that regulate the flow of the system based on the cooling load of the building.
How Do I...
Assess System Energy Usage
The primary energy consumption in a chilled water loop is the electricity used to run the pump motors. The heat loss across the piping network is a characteristic of the chilled water loop, but the energy consumption impact of this heat loss is measured at the chilled water plant.
Quantify Electricity Usage (kWh) of a Constant Primary-Flow System
Determine power draw of pump motor and total operating time.
Equipment to Measure
What to Measure:
- Average of spot measurements of true RMS power draw (kW) of the pump motor.
- Operating time (hours) of the pump.
Measurement Locations
Quantify Electricity Usage (kWh) of a Variable Primary-Flow System
Determine power draw of pump motor, total operating time and outdoor air temperature.
Equipment to Measure
What to Measure:
- Average hourly true RMS power draw (kW) of the pump motor.
- Total operating time (hours) of the pump.
- Average hourly outdoor air temperature (°F).
Measurement Locations
Assess System Performance
The amount of cooling provided by the chilled water loop can be determined by measuring the supply and return water temperatures at the evaporator side of the chiller system, and the water flow rate through the evaporator. Additionally, the electricity usage or operating time of the chiller system should be used to estimate how much cooling is provided when the system is online.
Quantify Cooling Load (Btu)
Determine power draw of pump motor, chilled water supply and return temperatures, and flow rate across the evaporator.
Equipment to Measure
Chilled Water Loop Piping
What to Measure:
- Average hourly chilled water supply temperature (°F).
- Average hourly chilled water return temperature (°F).
Chiller Evaporator
What to Measure:
- Average hourly true RMS power draw (kW) of the chiller system or Chiller operating schedule.
- Average hourly water flow rate across the evaporator (CFM).
Measurement Locations
Primary-secondary Flow System
A primary-secondary chilled water system consists of a primary CHW loop with primary pumps as well as a secondary CHW loop with secondary pumps to distribute the cooling load to the building.
The following designs can be found in a primary-secondary flow system:
A constant-primary, variable-secondary flow system uses constant-speed pumps to maintain flow through the primary loop, while separate pumps in the secondary loop distribute flow and cooling load to the facility. A primary-secondary chilled water system consists of a primary CHW loop with primary pumps, which circulates water through the chillers, and secondary CHW loop with secondary pumps which takes a portion of water from the primary loop and distributes it to the building.
A variable-primary, variable-secondary flow system has variable frequency drives (VFDs) installed on primary and secondary pump motors controlled by the differential pressure in the piping network.
How Do I...
Assess System Energy Usage
The primary energy consumption in a chilled water loop is the electricity used to run the pump motors. The heat loss across the piping network is a characteristic of the chilled water loop, but the energy consumption impact of this heat loss is measured at the chilled water plant.
Quantify Electricity Usage (kWh) of a Constant-Primary / Variable-Secondary Flow System
Determine power draw of pump motor and total operating time.
Equipment to Measure
What to Measure:
- Average of spot measurements of true RMS power draw (kW) of the pump motor.
- Operating time (hours) of the pump.
What to Measure:
- Average hourly true RMS power draw (kW) of the pump motor.
- Total operating time (hours) of the pump.
- Average hourly outdoor air temperature (°F).
Measurement Locations
Quantify Electricity Usage (kWh) of a Variable-Primary / Variable-Secondary Flow System
Determine power draw of pump motor, total operating time and outdoor air temperature.
Equipment to Measure
What to Measure:
- Average hourly true RMS power draw (kW) of the pump motor.
- Total operating time (hours) of the pump.
- Average hourly outdoor air temperature (°F).
Measurement Locations
Assess System Performance
The amount of cooling provided by the chilled water loop can be determined by measuring the supply and return water temperatures at the evaporator side of the chiller system, and the water flow rate through the evaporator. Additionally, the electricity usage or operating time of the chiller system should be used to estimate how much cooling is provided when the system is online.
Quantify Cooling Load (Btu)
Determine power draw of pump motor, chilled water supply and return temperatures, and flow rate across the evaporator.
Equipment to Measure
Chilled Water Loop Piping
What to Measure:
- Average hourly chilled water supply temperature (°F).
- Average hourly chilled water return temperature (°F).
Chiller
What to Measure:
- Average hourly true RMS power draw (kW) of the chiller system or Chiller operating schedule.
- Average hourly water flow rate across the evaporator (CFM).
Measurement Locations
Further Reading
ASHRAE (2019). “ASHRAE Handbook: HVAC Applications,” Chapter 43. SUPERVISORY CONTROL STRATEGIES AND OPTIMIZATION. I-P Edition.
Taylor, S (2012). “Optimizing Design & Control of Chilled Water Plants Part 5: Optimized Control Sequences”. ASHRAE Journal, Vol. 54, No 6. American Society of Heating, Refrigerating and Air Conditioning Engineers; pp: 56-74.