Designing hydraulic systems on high-rise, large buildings
Know how making correct decisions in the initial hydraulic plan helps the facility operate more efficiently and effectively.
- Understand how to recommend a hydraulic distribution system type in large complexes.
- Learn about standard design practices in central utility plants.
- Review secondary distribution practices in supertall buildings.
Hydraulic system components should be determined early in the design of a heating, ventilation or air conditioning system, regardless of capacity range or type of occupancy:
- Heating or cooling central utility plant.
- System type.
- System distribution.
- Design standardization.
While the components are similar, the integration of each into large projects can present special challenges. For any complex facility, a central utility plant acts as the cooling and heating generation source while the system distribution plan serves as the skeleton of the building.
Water distribution systems type
There are typically two major pumping configurations applied heating and chilled water distribution systems: primary-only and conventional primary-secondary. Over the past 20 years, there have been numerous industry publications examining constant primary flow and variable-secondary systems versus variable-primary systems.
ASHRAE and several manufacturers all provide extensive guides and case studies on those systems. The pros and cons of primary variable and primary (constant) and secondary (variable) are well documented. Modern chilled water distribution systems in large CUP applications have two main systems: variable-primary systems and primary (constant) secondary (variable) systems.
Modern CUPs are usually designed with centrifugal chillers with variable frequency drives. This allows the chillers to vary their load more easily from 10% to 100%. However, chillers need to maintain a minimum of 1.5 to 2 feet per second velocity to avoid laminar flow conditions in evaporator coils to maintain proper heat transfer factor.
In the case of centrifugal chillers with VFDs, a load can continue to drop to between 10% to 15% of design load. Flow needs to be maintained between 40% to 50% of the design flow to maintain a proper heat transfer rate. Comparing the load and flow values reveals the range gap where load and flow will not fully follow a linear relationship. Proper control of the distribution flow during the partial load condition between 10% and 40% should be carefully analyzed.
The industry has not yet widely recognized the practice of variable-primary and variable-secondary systems. A central plant with 7,000 to 10,000 refrigeration tons in commercial applications usually has complicated occupancy schedules and a large geometry complex. Based on design and operation trends in the commercial sector over the past 10 years, variable-primary and variable-secondary systems (as well as tertiary systems for some applications) are recommended to simplify operations and adjust for future system variations. Independent secondary systems can fully use the CUP’s built-in thermal mass before initiating the chillers from the primary loop. The fluctuated flow range in primary loop helps the facility avoids the low-return water syndrome.
As previously noted, variable-primary flow helps chillers fill in the gap between 10% to 30% load conditions. Variable-primary and variable-secondary systems can:
- Be fully independent.
- Enhance the partial load condition.
- Reduce the overall hours of chiller operation.
- Cut the amount of horsepower needed to operate the pump.
As illustrated in the Figure 1, the bypass valve in the variable-primary system is necessary to maintain minimum flow for either the chiller or the pump. The location of the pressure independent bypass valve should be either in the CUP or one of the remote distribution branches. After comparing the chiller and pump minimum flow in the case of the above plant capacity range, the pressure independent bypass valve size is likely to fall between the minimum flow of the chiller.
This critical component requires reliable performance with robust construction. During partial load conditions, it must constantly monitor CUP operation and track selected flow pressure at the distribution branch. While many reputable manufacturers are developing new and improved pressure independent valves, for an 8-inch valve size and above, the options are more limited.
In a real-world example running at the above-mentioned capacity (and where the CUP space and construction budget are met), a variable-primary and variable-secondary distribution system should increase the longevity of the chilled water distribution system (see Figure 2). For a CUP with half of the cited capacity range or smaller, variable-primary distribution with good perdition of load profile may be a better option. With no need for secondary distribution pumps, installation costs would be reduced and energy savings on chiller and pump operations could be realized.
Typical heating hot water system in large commercial plants has similar system design approach as above mentioned chilled water systems.
Central utility plant design
In commercial buildings, the initial design of the CUP includes not only chiller and boiler capacity but the total quantity of the source equipment as well. Planning for standardized systems, components and equipment is one of the most important design steps. It is especially key to designing large capacity CUP in complex commercial buildings. Identical major equipment will help the maintenance crew streamline routine upkeep, reduce spare parts storage and more easily troubleshoot problems.
ASHRAE 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings Appendix G provides a baseline for building compliance with the performance rating method (G.184.108.40.206 lists coil and plant sizing factor and capacity). For a CUP with a cooling capacity between 7,000 to 10,000 refrigeration tons, the total quantity of chillers is determined based on plant coincident load and a full study of various partial load conditions for the facility. One chiller is recommended to handle a maximum of 25% of the calculated peak coincident load.
As previously noted, modern CUPs generally opt for centrifugal chillers with VFDs. Chiller load varies from 10% to 100% capacity while chilled water minimum flow varies from 40% to 45% (value slightly varies by different manufacturer). In straight math, the single chiller can be operated at 10% to 12.5% of total facility coincident design flow and a minimum of 2.5% of plant coincident load. Verifying the two parameters is determined during the total quantity of plant machines analysis.
Project budget also plays an important role for most commercial buildings. After a load profile is completed, it’s typical that the same capacity of chillers and boilers in modern CUPs can be designed in parallel. With this approach, the CUP does not need to be oversized.
Mechanical system analysis
It is worth analyzing the total numbers of chillers, boilers, heat exchangers (if district cooling or heating source is being used) and primary distribution pumps when designing for a large-capacity CUP. The best option is to integrate uniformly sized equipment with built-in resilience. The only exception could be free cooling if the 24/7 load is too small to have identical cooling tower partial load capacity. In a practice with automatic bypass valve and cooling tower cold water basin heaters found in most commercial facilities, it is possible to use identical cooling towers that match the heat exchanger/building 24/7 cooling load’s performance on a one-to-one basis.
To offer the most flexible combination of operation patterns, chillers, cooling towers, boilers and primary distribution pumps are recommended to be one-to-one. When CUP is set up with a modular operation and control type, plant equipment will have equal operation time and ultimately result in no limitation of interchange among the same type of equipment. Under this configuration, chillers and boilers typically connect into a common-sized header instead of a telescope-type header arrangement. Plant equipment piped into a common size header acts to divert or mix temperatures with low local pressure drop. A common header sized for the designed peak flow of a plant results in little or no increase in the cost of the pipe header. It creates a scenario for the plant to operate with much less friction loss for most days of operation.
The common pipe, also called a decoupler, maintains low pressure drop by keeping the same pipe size as the main pipe header. The same common pipe header concept is recommended for distribution pumps. A prefabricated header applied in the central plant is possible if grooved pipe connections are an option.
Chillers and pumps can be paired to work with any equipment within the same group (see Figure 7). Without oversized equipment, the CUP has built-in redundancy. Uniformed arrangement of valves, check valves, motorized valves, flow switches and similar components with interchangeable features help CUPs operate more flexibly and limit the amount of necessary parts storage. With a common size header, the application will further simplify to CUP maintenance routine.
Tall and specialty buildings
Large, complex facilities typically fall into two categories to describe their physical grandness: vertical and horizontal. These are either supertall buildings or structures such as convention centers that occupy a large footprint. A distribution system in a large-footprint complex requires multiple areas of differential pressure monitoring sensors. These sensors are either located at remote branches or critical zones. Secondary pump VFDs will constantly adjust the frequency in response to those values. This overview, however, focuses on the distribution system of supertall buildings.
The Council on Tall Buildings and Urban Habitat defines a supertall building as being more than 984 feet in height. Buildings fitting this widely accepted definition depend on a system of vertical zone level occupancy functions.
Multiple levels of building service system floors are needed to maintain delivery of HVAC to typical floors. These mechanical, electrical and plumbing equipment floors provide hydrostatic pressurebreak, house central air handling or makeup air units or provide a tertiary pumping system to distribute hydraulic systems to each local floor.
One common feature found in high-rise buildings, especially supertall buildings, is the core that contains many vertical transportation routes to building occupancy and building service systems. Because of the central location of the core, it also defined the unique construction process of high-rise buildings. Standardized design in high-rise buildings magnifies its benefits.
For supertall buildings, a strategically located pressure break heat exchanger is essential. Depending on the type of manufacture, heat exchangers and pumps have pressure limitations of 400 and 300 pounds per square inch, respectively.
For Air-Conditioning, Heating and Refrigeration Institute certified heat exchangers, a 2ºF approach benefits both pressure drop and its footprints and maintains desired chilled water temperature at the upper zone.
Supertall buildings with multiple system levels require the use of “express risers.” An example of MEP floors chilled water distribution system is illustrated in Figures 5 and 6. It illustrates the express riser with higher value of pressure break heat exchanger. It represents a local zone riser with a local pressure break heat exchanger. Without a significant footprint for equipment, identical heating exchangers and distribution pumps offer built-in redundancy. The identical capacity of heat exchangers and pumps in express risers is even more important as the upper zone’s operation relies on more resilient and robust capabilities.
This example shows local risers designed with a 150 psi system. Although coils in terminal units and control valves have 300 psi ratings, this setup saves construction time, reduces product storage, contractor training time and avoids human error. All terminal units, like fan coil units, control valves, strainers, and shut valves are each 150 psi rated. Maintaining local risers serving typical floor units with a uniform 150 psi system helps optimize operation maintenance and spare parts inventory.
The same pressure rating concept can be applied to hydraulic accessories. In Figures 5 and 6, each pump is maintained with a 300 psi rating while prefilled diaphragmed expansion tanks have a rating of 125 psi. Common diaphragmed expansion tanks have two pressure rating options: 125 psi or 250 psi. Locating expansion tanks at the top of each pressure zone avoids struggling with the proper location, especially when applied to express risers.
Pump seals are often the most easily broken parts during high-pressure operation. Although there are 400 psi pump options, position pumps maintaining 300 psi extends the unit’s operation life and offers more manufacturer choices. Air separators are typically located at high temperature and low-pressure locations within the hydraulic loop where dissolved air has a higher chance of escaping from liquid media. The pump suction side is a good location for air separators and protecting the pump’s impellor.
There are also common types of air separators American Society of Mechanical Engineers rated at 125 or 300 psi. Installing the same type of pressure rating of air separator at the suction side of the pump while maintaining a 300 psi rating simplifies the supertall building system. Standardized design concepts dissolve the conflicted system into several standard system solutions.
Due to generic structural requirements, horizontal pipe distribution on typical floors is always a challenge. To accommodate service, risers distributed from building system floors are often the solution. ASHRAE Standard 111: Testing, Adjusting and Balancing of Building HVAC Systems indicates the hydraulic system balance and verification. For a large building, especially when applied to a supertall building, post-construction testing and balancing can be time-consuming. To balance each individual local riser in every pressure zone in a supertall building could be troublesome, especially if it relies on dynamic balance. Any change after a prebalanced system requires a rebalance.
Per ASHRAE terminology, a reverse return is a two-pipe system in which the heat transfer medium supplied to the first load is the last returned to the heat transfer equipment. This system includes water return piping from terminal units sized to provide equal lengths for balanced flow rates. Compared to the direct return system, the reverse return design has a generic self-balanced feature.
For large-footprint complex buildings, the reverse return is difficult to accomplish. However, it could be more feasible to establish in high-rise applications because the typical distribution is closer to the core area. This potentially solves the testing and balance issue and should not need to be rebalanced after system alteration.
In Figures 5 and 6, vertical risers are used for the reverse return in the hotel occupancy section of a mixed-use supertall building. This vertical application of reverse return solves the challenge of limited horizontal piping in limited ceiling cavity below structural beam. The vertical riser size is small and is located within 150 psi local zones, which leads to a small increase in piping cost.
Avoiding engineering conflict
Complex facilities with large capacity central plants present a complicated set of variables that could easily evolve into a set of conflicting systems. Engineers can apply a standardized concept to translate a difficult system into a simplified, uniform solution. To accomplish this, however, extensive engineering work is required early in the design stage.
Ideally, design and planning include a comprehensive review of all complex factors, providing the output with the following characteristics:
- A simple system designed with reduced construction team size.
- A robust system with built-in redundancies.
- A resilient system with interchangeable ways to operate.
- A standardized system that reduces building storage area and spare parts requirements.
- A user-friendly system that streamlines troubleshooting and reduces maintenance time.
Applying a standardized design practice can be key to developing better engineering practices and better facilities.
Suzan Sun-Yuan is a technical authority|mechnical with ESD. Sun-Yuan has extensive experience in the design of commercial, institutional and educational facilities. ESD is a CFE Media and Technology content partner.