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.
However, during the winter months when economizer mode is activated, the CHW supply temperature may rise from 42 to 52 F. As a result, the leaving supply air temperature from each fan coil is increased,and the capacities are reduced. Refer to Table 1, showing how CHW temperature affects performance of a fan coil unit.
During winter months when economizer mode has been enabled, the exterior perimeter offices are not affected. The cooling load in these spaces is significantly lower, and they are most likely in heating mode. However, the interior spaces are still in cooling mode and could be experiencing a cooling load very similar to the load during summer months. The only difference would be the cooler, outdoor ventilation air supplied to the fan coil unit, which would help reduce the mixed air temperature. However, if the reduction in the cooling load from the ventilation air is not greater than the capacity reduction in the fan coil unit due to the higher CHW temperature, the fan coil unit is now undersized for the space.
This situation is even more of a problem in the IT room, where ventilation air is extremely low and there is little chance that it will offset the capacity reduction in the fan coil unit. As a result, the IT room is not able to be properly cooled.
This is a common condition in buildings where all fan coil units are designed based on the lowest CHW supply temperature. For the IT room to operate correctly for the entire year, its fan coil unit would need to be sized based on highest entering water temperature allowed in the design sequence.
In another attempt to save energy during off-peak cooling conditions, the IECC and ASHRAE 90.1 both require CHW reset sequences to be implemented. If a CHW supply temperature reset is incorporated, it could be controlled by outside air temperature, zone-return water temperature, or building-return water temperature. The temperature shall be capable of being reset by at least 25% of the design supply-to-return water temperature difference.
Using a CHW temperature reset sequence can be very beneficial and result in significant energy savings during off-peak times of the year. A sample CHW reset sequence may be as follows:
The CHW system shall operate based on default conditions of 44 F supply/59 F return. The maximum system supply temperature shall be 52 F, and the minimum system supply temperature shall be 42 F.
CHW temperature shall be evaluated every 15 minutes, and reset based on the following criteria:
- If two or more air handling units/fan coil units are not meeting setpoint for 5 minutes, and both of their associated CHW valve positions are more than 90% open, then reset CHW supply temperature down 2 F.
- If all air handling units/fan coil units are meeting setpoint, and all air handling units have valve positions less than 90%, then reset CHW supply temperature up 2 F.
This control scheme is based on the number of coils in operation and their ability to meet space temperature setpoints. There are a number of different ways to accomplish this, but the reset sequence will often be dictated by a critical zone that requires the lowest CHW temperature. The challenge for the designer is to identify this zone, and ensure the cooling coil in this space is sized properly.
The sequence listed above is in compliance with ASHRAE requirements, but applying it can be problematic unless all factors are considered. Unless an air side economizer can be used for all spaces,most large buildings will still have a cooling load in the winter months due to electrical rooms, data rooms,or rooms that do not communicate with the building envelope. As a result, the same winter coolingproblems associated with water side economizer mode can occur with CHW reset. The building HVACequipment serving these spaces must be designed with cooling coils capable of operating at the highestreset temperature. Otherwise, the reset sequence will most likely never be implemented, especially inelectrical and data rooms where the cooling load is the same all year long.
CHW temperature design
The codes and standards require that water side economizer and CHW supply temperature reset be included in the design, but they do not dictate what the actual design temperatures should be. Overall, it is best practice to keep the CHW temperature as high as possible. This will provide the best energy efficiency for the central plant equipment, as chillers perform much better with the lowest amount of lift.(The term “lift” refers to the head pressure seen by the chiller, or the difference between leaving CHW temperature and the entering condenser water temperature.)
Another reason for using higher water temperatures is if the CHW system will be connected to water-cooled compressors used in food service equipment or water source heat pumps. These compressors may struggle with water temperatures below 44 F, and an appropriate review is necessary when CHW temperatures of 42 or 43 F are used. The equipment manufacturers can often make the systems operate at these lower temperatures, but additional components may be needed to accommodate the lower water temperatures.
While using a higher temperature CHW may result in higher chiller efficiency, it may also result in larger coils, larger pumps, and larger piping throughout the building. In addition, the climate and building air handling systems may dictate that a lower CHW supply temperature be used. If you are designing asystem in a hot and humid climate where dehumidification is required, then CHW supply temperatures will need to be low enough to drive down coil leaving air temperatures below the dew point temperature.
Because both higher and lower CHW supply temperatures provide benefits, using a CHW temperature reset sequence can capture the benefits of both. While there are problems associated with improperly implementing a CHW reset in a building (as mentioned above), significant energy savings are available when the system is coordinated.
When it comes to CHW systems, the mechanical code requirements mainly apply to installation of systems and not necessarily the design parameters. Both the Uniform Mechanical Code (UMC) and IMC have sections dedicated to “hydronic piping”; for this article, the requirements in the IMC will be discussed.
Chapter 12 of the IMC is titled “hydronic piping” and lists requirements for piping systems such as steam,hot water, CHW, steam condensate, and ground source heat pump loop systems. The section lists approved materials and methods of pipe installation, including requirements for fittings and joints between piping components. The overall intent is to ensure that piping components installed are rated for the temperature and pressure of the piping system they serve. System designers should verify their piping specification sections are in compliance with these requirements based on the project’s building type, sizeof piping, and operating pressure of the CHW system.
While most of the requirements in this section apply to piping material and installation, there are a few significant items listed in this code section that should be verified with the system design.
Section 1204 is titled “Pipe Insulation” and requires systems be installed with insulation per the IECC. The only insulation requirement listed in the IMC is to verify all CHW insulation has a flame spread index of 25 and a smoke-developed index of not more than 50 when installed inside a return air plenum. Not all pipe insulation is made with these properties, so it is important to note whenever pipes are routed through are turn air plenum.
Sections 1205 and 1206 list requirements that also should be considered for CHW system design and installation. They state that shutoff valves be installed on all equipment/appliances, and that the entire CHW system is capable of being drained. Section 1206.4 requires all penetrations through concrete elements to be sleeved. These items will need to be confirmed with the installing contractor during the construction process.
One other significant item is 1206.11, which states that “Provisions shall be made to prevent the formation of condensation on the exterior of the piping.” This requirement forces designers to think about the pipe insulation materials, as well as the thermal performance characteristics. In humid climates, the use of improper insulation materials on CHW piping systems could be problematic over the life of the building. Therefore, designers should work with insulation manufacturers to ensure materials are designed that will not become saturated by moisture. Once condensation occurs inside the piping insulation, the thermal performance of the insulation will be compromised.
Section 1208 requires the CHW system be tested hydrostatically at 1.5 times the maximum system design pressure, but not less than 100 psig. The duration of each test shall be not less than 15 minutes.Design specifications should ensure that testing requirements are at least equal to this requirement.
When it comes to designing CHW systems, the goal should always be to provide an efficient system that can properly cool the building it serves. Relative to other building systems, there are not many rules and requirements listed in the codes pertaining to CHW systems; however, the items listed need to be followed and can serve as a checklist for the designer. This checklist can help ensure the CHW system installed will serve its building well.
Brian Patalon is senior project engineer, mechanical, at JBA Consulting Engineers. He has been involved with chilled water system design in Las Vegas for 15 years.