CHW system design
Chilled water (CHW) systems are cooling systems that circulate CHW throughout a building for cooling and dehumidifying a building’s air. They come in all shapes, sizes, and configurations.
- Review the codes and standards that govern the specification of chilled water systems.
- Understand temperature design of a chilled water system.
- Recognize potential economizer mode and chilled water reset design issues.
Buildings are cooled using different types of HVAC systems and equipment, from ceiling fans to district chilled water (CHW) plants. While residential buildings and smaller commercial buildings are often cooled using air-cooled equipment, CHW systems are typically the engineer's preferred choice for larger buildings.
A building containing a central plant with chillers, pumps, and appropriate ancillaries will provide a system that has the capabilities to accurately control supply air temperature at any entering air condition. This can be critical when designing systems requiring 100% ventilation air, or providing dehumidification in humid climates. This article will discuss a few things to consider when designing CHW systems.
There are a number of different energy codes and standards that are adopted by jurisdictions throughout the world. While they have small differences, their intent is to ensure systems are designed to maximize efficiency. For this article, both ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings and the International Energy Conservation Code (IECC) are discussed.
While the International Mechanical Code (IMC) lists a general requirement for CHW piping insulation, it refers to the IECC for specifics. Section C403.2.8 of the 2012 IECC states that all piping systems comply with Table C403.2.8. ASHRAE 90.1-2010 requires insulation thicknesses identical to the values listed in the 2012 IECC, using the baseline thermal conductivity.
Building CHW systems are required to have some form of water-side economizer per current codes and design standards. Typically, this is accomplished using a plate and frame heat exchanger piped to the CHW and condenser water systems, although indirect evaporative cooling coils are also permitted.
Section C403.4 of the IECC requires a water side economizer system to be incorporated into all CHW systems greater than 300,000 Btu/h output capacity. The economizer shall be capable of satisfying 100% of the expected cooling load at outdoor air temperatures of 50 F dry bulb/45 F wet bulb, with an exception for systems requiring lower CHW temperatures for dehumidification. ASHRAE 90.1-2010 lists requirements similar to this, although it includes exceptions that allow computer rooms to activate the economizer at 35 F dry bulb instead of 50 F. It is important to understand that systems with an air side economizer will meet the economizer requirement, and water side economizers need only be sized for HVAC equipment still requiring CHW to meet cooling loads at the listed temperatures.
Because the purpose of the economizer mode is to save energy, requirements are listed in the codes to ensure they are designed with this in mind. IECC Section 403.4.1.2 limits the pressure drop across the heat exchanger, or precooling coils, used for the water side economizer. It states that if the pressure drop across the heat exchanger is 15 ft or higher, then a secondary loop and circulating pump shall be provided so that the pressure drop through the heat exchangers is not seen by the CHW system during normal (non-economizer) conditions.
One method to meet this requirement is to install motorized control valves at the heat exchanger connection, and connect the heat exchanger to the CHW system as another chiller in parallel. This meets the requirement because water will only flow through the heat exchanger when economizer mode is enabled. A secondary pump would not be needed since the building's CHW and condenser water pumps could be used to flow water through the heat exchanger, provided they are equipped with variable frequency drives and can be adjusted to match the winter cooling load requirement.
Most designers have no issues with incorporating the required components of a water side economizer into their designs; however, the most challenging part is how to control it. Both ASHRAE 90.1-2010 and IECC require water side economizer mode to operate at 50 F dry bulb/45 F web bulb, with possible exceptions if the building contains a computer room.
The design complexities exist during times when outdoor air temperatures are at the higher end of economizer mode, or approximately 40 to 45 F wetbulb. The reason for the issue is most air handling systems are designed for the default CHW supply temperature, whether it is 42 F, 45 F, or somewhere in between. However, the CHW temperature during economizer mode may not be capable of delivering this water temperature, even with the most efficient plate and frame heat exchanger.
Figure 1 shows offices around the perimeter, with additional offices, an electrical room, and an information technology (IT) room interior to the building. Assuming all of these spaces are conditioned using CHW/hot water fan coil units, served from a central plant delivering CHW at 42 F, the designer would select coils based on a CHW temperature of 42 F supply/58 F return. During the summer months,there shouldn't be any issues, as the fan coil units are properly sized to handle the peak cooling load with 42 F CHW and everyone is happy.