Central cooling and/or heating plants generate cooling and/or heating in one location for distribution to multiple locations in one building or an entire campus or neighborhood. Central cooling and heating systems are used in almost all classes of buildings, but particularly in very large buildings and complexes or where there is a high density of energy use. They are especially suited to applications where maximizing equipment service life and using energy and operational workforce efficiently are important.
The following facility types are good candidates for central cooling and/or heating systems:
Campus environments with distribution to several buildings (described further in Chapter 12).
High-rise facilities
Large office buildings
Large public assembly facilities, entertainment complexes, stadiums, arenas, and convention and exhibition centers
Urban centers (e.g., city centers/districts)
Shopping malls
Large condominiums, hotels, and apartment complexes
Educational facilities
Hospitals and other health care facilities
Industrial facilities (e.g., pharmaceutical, manufacturing)
Large museums and similar institutions
Locations where waste heat is readily available (result of power generation or industrial processes)
Larger systems where higher efficiency offsets the higher first cost of a chilled-water system
This chapter addresses design alternatives that should be considered when centralizing a facility’s cooling and heating sources. Distribution system options and equipment are discussed when they relate to the central equipment, but more information on distribution systems can be found in Chapters 11 to 15.
1. SYSTEM CHARACTERISTICS
Central systems are characterized by large chilling and/or heating equipment located in one facility or multiple smaller installations interconnected to operate as one. Equipment configuration and ancillary equipment vary significantly, depending on the facility’s use. See Chapter 1 for information on selecting a central cooling or heating plant.
Equipment can be located adjacent to the facility, or in remote stand-alone plants. Also, different combinations of centralized and decentralized systems (e.g., a central cooling plant and decentralized heating and ventilating systems) can be used.
Primary equipment (i.e., chillers and boilers) is available in different sizes, capacities, and configurations to serve a variety of building applications. Operating a few pieces of primary equipment (often with back-up equipment) gives central plants different benefits from decentralized systems (see Chapter 2).
Multiple types of equipment and fuel sources may be combined in one plant, but typically only in large plants. The heating and cooling energy may be a combination of electricity, natural gas, oil, coal, solar, geothermal, waste heat, etc. This energy is converted into chilled water, hot water, or steam that is distributed through the facility for air conditioning, heating, and processes. The operating, maintenance, and first costs of all these options should be discussed with the owner before final selection. When combining heating generation systems, it is important to note the presence of direct-firing combustion systems and chilled-water production systems using refrigerants, because the safety requirements in ASHRAE Standard 15 must be met.
A central plant can be customized without sacrificing the standardization, flexibility, and performance required to support the primary cooling and heating equipment by carefully selecting ancillary equipment, automatic control, and facility management. Plant design varies widely based on building use, life-cycle costs, operating economies, and the need to maintain reliable building HVAC, process, and electrical systems. These systems can require more extensive engineering, equipment, and financial analysis than decentralized systems do.
In large buildings with interior areas that require cooling while perimeter areas require heating, one of several types of centralized heat reclaim units can meet both these requirements efficiently. Chapter 9 describes these combinations, and Chapters 13 to 15 give design details for central plants. Using recovered energy for reheat at the zone level is a common use for lower-grade heating. It can significantly reduce the total energy use of buildings that commonly have simultaneous heating and cooling loads (e.g., hospitals, large hotels).
Central plants can be designed to accommodate both occupied/unoccupied and constant, year-round operation. Maintenance can be performed with traditional one-shift operating crews, but may require 24 h coverage. Higher-pressure steam boiler plants (usually greater than 15 psig) or combined cogeneration and steam heating plants require multiple-operator, 24 h shift coverage.
Primary cooling and heating can be provided at all times, independent of the operation mode of equipment and systems outside the central plant.
Using larger but fewer pieces of equipment generally reduces the facility’s overall operation and maintenance cost. It also allows wider operating ranges and more flexible operating sequences.
A centralized location minimizes restrictions on servicing accessibility.
Energy-efficient design strategies, energy recovery, thermal storage, and energy management can be simpler and more cost-effective to implement.
Multiple energy sources can be applied to the central plant, providing flexibility and leverage when purchasing fuel.
Standardizing equipment can be beneficial for redundancy and stocking replacement parts. However, strategically selecting different-sized equipment for a central plant can provide better part-load capability and efficiency.
Standby capabilities (for firm capacity/redundancy) and back-up fuel sources can easily be added to equipment and plant when planned in advance.
Equipment operation can be staged to match load profile and taken offline for maintenance.
A central plant and its distribution can be economically expanded to accommodate future growth (e.g., adding new buildings to the service group).
Load diversity can substantially reduce the total equipment capacity requirement.
Submetering secondary distribution can allow individual billing of cooling and heating users outside the central plant.
Major vibration and sound-producing equipment can be grouped away from occupied spaces, making acoustic and vibration controls simpler. Acoustical treatment can be applied in a single location instead of many separate locations.
Issues such as cooling tower plume and plant emissions are centralized, allowing a more economic solution.
Equipment may not be readily available, resulting in long lead time for production and delivery.
Equipment may be more complicated than decentralized equipment, and thus require a more knowledgeable equipment operator.
A central location within or adjacent to the building is needed.
Additional equipment room height may be needed.
Depending on the fuel source, large underground or surface storage tanks may be required on site. If coal is used, space for storage bunker(s) will be needed.
Access may be needed for large deliveries of fuel (oil or coal).
Fossil-fuel heating plants require a chimney or flue and possibly emission permits, monitoring, and treatments.
Multiple equipment manufacturers are required when combining primary and ancillary equipment.
System control logic may be complex.
First costs can be higher compared to alternatives with rooftop units (RTUs), water-source heat pumps (WSHPs), self-contained equipment, and other systems.
Special permitting may be required.
Safety requirements are increased.
A large pipe distribution system may be necessary (which may actually be an advantage for some applications).
Cooling and Heating Loads
Design cooling and heating loads are determined by considering individual and simultaneous loads. The simultaneous peak or instantaneous load for the entire portion or block of the building served by the HVAC and/or process load is less than the sum of the individual cooling and heating loads (e.g., buildings do not receive peak solar load on the east and west exposures at the same time). The difference between the sum of the space design loads and system peak load, called the diversity factor, can be as little as 5% less than the sum of individual loads (e.g., 95% diversity factor) or represent a more significant portion of the load (e.g., 45% diversity factor), as is possible in multiple-building applications. Computerized load calculation programs can be used to model different schedules (occupancy, lighting, etc.) with peak thermal loads, to determine applicable diversity factors. The peak central plant load can be based on this diversity factor, reducing the total installed equipment capacity needed to serve larger building cooling and heating loads. The design engineer should evaluate the full point-of-use load requirements of each facility served by the central system. However, it is important to review applicable codes and/or requirements of the local authority having jurisdiction (AHJ), which may limit the use of diversity factors.
Opportunities for improving energy efficiency include
Staging multiple chillers or boilers for part-load operation. Using correctly sized equipment is imperative to accurately provide the most flexible and economical sequencing of equipment. Modeling programs or spreadsheets are an excellent way to compare different staging sequences. Controls can handle more sequences, but controls for special staging will have a higher first cost.
For central chiller plants, consider incorporating variable frequency drives (VFDs) onto one base-loaded primary chiller. Multiple VFD installations on chillers allow more flexibility in energy control of chiller plant operation. Note that increased part-load efficiency can only be achieved if the required lift changes. That occurs when the condensing temperature is lowered (cooler condensing fluid) or the discharge chilled-water temperature is raised.
Staging cooling tower operation with the chillers can increase efficiency and reliability by using as much of the surface as possible. Coordinating system equipment operating characteristics is required.
Using heat exchange devices to transfer free or waste energy from one source to another, allowing mechanical equipment to be unloaded or turned off completely.
Discrete loads (e.g., server rooms) may be best served by an independent system, depending on the overall load profile of systems served by the plant; a small, independent system designed for the discrete load may be the most cost-effective approach (although it sacrifices redundancy). Central plants sized for minimum part-load operation may not be able to reliably serve a small, discrete load. For example, a 2000 ton chiller plant serving multiple facilities could be selected with four 500 ton chillers. In a remote facility connected to the central distribution system, an independent computer room server has a year-round 5 ton load. Operating one 500 ton chiller at less than 100 tons reliably may be extremely inefficient and possibly detrimental to the chiller plant operation if there are no other loads at that time. Serving the constant remote load independently allows the designer the flexibility to evaluate the chiller plant as a whole and individual subsystems independently, so as to not adversely affect both systems.
Peak cooling load time is affected by outdoor ventilation, outdoor dry- and wet-bulb temperatures, period of occupancy, interior equipment heat gain, and relative amounts of north, east, south, and west exposures. For typical office and/or classroom buildings with a balanced distribution of solar exposures, the peak usually occurs on a midsummer afternoon when the west solar load and outdoor dry-bulb temperature are at or near concurrent maximums. However, the peak cooling period can shift, depending on required ventilation rates and internal load profiles.
Diversity of building occupancies served can significantly affect the diversity factor. For example, in a system serving an entire college campus, the peak cooling period for a classroom is different from that for a dormitory or an administration building. Planning load profiles at academic facilities requires special consideration. Unlike office and residential applications, educational facilities typically have peak cooling loads during late summer and fall.
Peak heating load has less opportunity to accommodate a diversity factor, so equipment is most likely to be selected on the sum of individual heating loads. This load may occur when the building must be warmed back up to a higher occupied-space temperature after an unoccupied weekend setback period. Peak demand may also occur during unoccupied periods when the ambient environment is harshest and there is little internal heat gain to assist the heating system, or during occupied times if significant outdoor air must be preconditioned or some other process (e.g., process heating) requires significant heat. To accommodate part-load conditions and energy efficiency, variable-flow may be the best economical choice. It is important for the designer to evaluate plant operation and system use.
The configuration of a central system is based on use and application. Two types of energy-efficient designs used today are primary variable flow and primary/secondary variable flow.
Primary variable flow uses variable flow through the production energy equipment (chiller or heating-water generator) and directly pumps the medium, usually water, to the point of use. Variable flow can be achieved using two-way automatic control valves at terminal equipment and either variable-frequency drive (VFD) pumping (Figure 1) or distribution pressure control with bypass valve (Figure 2). Both concepts function based on maintaining system pressure, usually at the hydraulically most remote point (last control valve and terminal unit) in the water system. The design engineer should work with the equipment manufacturer to ensure maintenance of minimum equipment flow rates throughout the building load profile, and to determine whether any additional components or ancillary equipment is required. Chillers are typically selected with higher water velocities (6 to 8 fpm) at full load in the evaporator bundles to provide good turndown on the water flow. The chilled-water pumps are not dedicated, but run off of a common header to allow the chillers to operate over a wide range of flows (Figure 1). The valve in the bypass line is closed until system flow drops to the minimum flow of the largest chiller. Flow transitions while turning a chiller off and on are typically the most critical. Check each chiller’s minimum and maximum flow to ensure that the pumps can maintain the required flow rate range from minimum to 100% load. More chillers or those with higher turndown ratios make this easier. Avoid variable primary systems where there are poor controls or limited operator training.
Primary/secondary variable flow hydraulically decouples the primary production system (chilled- or heating-water source), which is commonly constant flow. A variable-flow secondary piping system distributes the chilled or heating medium to the point of use (Figures 3 and 4).
Another design is a straight constant-volume primary system. Hydronic pumps distribute water through the energy equipment straight to the point of use. Pumping energy is constant, as is the distribution flow, which generally requires a means to maintain the design flow rate through the system while in operation. Thus, these systems are generally more expensive to operate and less attractive in central system designs. Take particular care with this type of system design, because many current energy codes do not allow using pump motors without variable-frequency drives.
When using either primary/secondary or primary variable-flow designs, the engineer should understand the design differences between two- and three-way modulating valves (see Chapter 47). Variable-flow designs vary the flow of chilled or heating water through the distribution loop. As terminal units satisfy demand, the valve modulates toward the closed position. The operating pump(s), if at design operating conditions, increase system pressure above the design point. To compensate, a VFD is typically installed to vary the speed of the distribution pump. Pump speed is usually controlled by a pressure differential sensor or sensing control valve position. As the pressure differential increases or the valve closes, the sensor sends a signal to the VFD to reduce speed. The affinity laws then allow flow to reduce in direct proportion to the change in speed. As system demand requires increased flow, the control valve modulates open, reducing system differential pressure. The reduction in pressure difference measured causes a corresponding increase in pump speed (and therefore flow) to meet system demand.
With a primary/secondary design, minimum system flow for the chiller or heating plant is accomplished by using a decoupler bypass between the primary and secondary systems. Because the primary system is constant volume (i.e., the system maintains design flow during operation), water recirculates within the plant to maintain the minimum design flow required by the production plant, allowing the secondary (load) side to vary flow rate to match demand. Care should be taken during design to ensure minimum flow is achieved under part-load operation throughout the system. When selecting the distribution system pump, ensure distribution flow does not exceed production flow, which could cause a temperature drop if flow exceeds demand, leading to low ΔT syndrome [see, e.g., Taylor (2002)]. The engineer should evaluate operational conditions at part load to minimize the potential for this to occur.
With primary variable-flow designs, the primary chilled- or heating-water pumps are of a variable-flow design, again using a VFD to control pump speed. Primary variable-flow systems use modulating two-way control valves to reduce water flow across each heat transfer device (as in the primary/secondary system). Chillers and boilers require a minimum water flow rate to maintain proper operation. A bypass or some other method must be considered to maintain the minimum water flow through the equipment. When using multiple chillers or boilers, ensure that the minimum flow of the smallest unit is close to or less than the building system’s minimum flow.
With a straight constant-volume system, flow is constant at the design requirement. These systems commonly use three-way control valves, and throttle closed (as in a two-way valve) as terminal unit demand is satisfied. However, the three-way valve also has a bypass port to allow up to design flow to flow around the terminal heat transfer device and return to the central plant. With constant-volume designs, temperature swings typically occur across the plant and terminal heat exchange devices, because design flow does not change.
Energy Recovery and Thermal Storage
Depending on the operations schedule of the building(s) served, energy recovery and thermal storage strategies can be applied to a central cooling and heating plant. See ASHRAE Standard 90.1 for systems and conditions requiring energy recovery. Water-to-water energy recovery systems are common and readily available. Thermal water or ice storage also can be very adaptable to central plants. See Chapter 26 in this volume and Chapters 34 to 36 and 41 in the 2015 ASHRAE Handbook—HVAC Applications for more information on energy-related opportunities. Thermal energy storage (TES) systems may offer multiple strategies for both part-load and peak-load efficiency control. Examples such as base-loading plant operation for a more flat-line energy consumption profile of a large plant may help in negotiating energy costs with a utility supplier. Thermal energy storage of chilled water, ice, or heating water offers a medium for redundant capacity, with potential reductions in both heating and cooling infrastructure equipment sizing.
Primary Refrigeration Equipment
Chillers are the major cooling equipment used in central cooling plants. They remove heat from the working medium, which is then distributed throughout the building(s) and/or campus. Chillers vary in type and applications, and fall into two major categories: (1) vapor-compression refrigeration cycle (compressorized) chillers, and (2) absorption-cycle chillers. In some cases, the chiller plant may be a combination of these machines, all installed in the same plant. Cooling towers, air-cooled condensers, evaporative condensers, or some combination are also needed to reject heat from this equipment. Energy for the prime driver of cooling equipment may also come from waste heat from a combined heat and power (CHP) system. Chapters 38 and 43 discuss refrigeration equipment, including the size ranges of typical equipment.
Compressorized chillers feature reciprocating, scroll, helical rotary (screw), and centrifugal compressors, which may be driven by electric motors; natural gas-, diesel-, or oil-fired internal combustion engines; combustion turbines; or steam turbines. Compressor selection may change based on size, first cost, and life-cycle cost.
Compressors may be purchased as part of a refrigeration chiller that also includes the drive, evaporator, condenser, and necessary safety and operating controls. Reciprocating and helical rotary compressor units can be field-assembled and include air- or water-cooled (evaporative) condensers arranged for remote installation. Centrifugal compressors are usually included in packaged chillers, although they can be very large and sometimes require field erection. These types of chillers also require remote air- or water-cooled ancillary heat rejection equipment. Chapter 38 has information about compressors.
Absorption chillers may be single- or double-effect, fired by steam or direct-fired by gas, oil, or waste heat. Like centrifugal chillers, absorption chillers are built to perform with remote water-cooled ancillary heat rejection equipment (e.g., cooling towers). These absorption chillers use a lithium bromide/water cycle in which water is the refrigerant, and are generally available in the following configurations: (1) natural gas direct-fired, (2) indirect-generated by low-pressure steam or high-temperature water, (3) indirect-generated by high-pressure steam or hot water, and (4) indirect-generated by hot exhaust gas. Chapter 18 of the 2018 ASHRAE Handbook—Refrigeration discusses absorption air-conditioning and refrigeration equipment in more detail.
Ancillary Refrigeration Equipment
Ancillary equipment for central cooling plants consists primarily of heat-rejection equipment (air-cooled condensers, evaporative condensers, and cooling towers), pumps (primary, secondary, and tertiary), and heat exchangers (water-to-water). For more detailed information on this additional equipment, see Chapters 26, 39, 40, and 42 to 48.
Air-cooled condensers pass outdoor air over a dry coil to condense the refrigerant. This results in a higher condensing temperature and thus a larger power input at peak condition (although peak time may be relatively short over 24 h). Air-cooled condensers are popular in small reciprocating and helical rotary compressor units because of their low maintenance requirements. Air-cooled equipment may also be appropriate in high-wet-bulb environments, because the cooling tower supply water temperature is a function of the wet-bulb temperature. They are also more common in smaller systems and in cooler climates.
Evaporative condensers pass outdoor air over coils sprayed with water, thus taking advantage of adiabatic saturation to lower the condensing temperature. Freeze prevention and close control of water treatment are required for successful operation. The lower power consumption of the refrigeration system and much smaller footprint of the evaporative condenser are gained at the expense of the cost of water and water treatment used and increased maintenance cost.
Cooling towers provide the same means of heat rejection as evaporative condensers but pass outdoor air through an open condenser return water spray to achieve similar adiabatic cooling performance. Either natural or mechanical-draft cooling towers or spray ponds can be used; the mechanical-draft tower (forced draft, induced draft, or ejector) can be most easily designed for most conditions because it does not depend on wind. Cooling tower types and sizes range from packaged units to field-erected towers with multiple cells in unlimited sizes. Location of heat rejection equipment should consider issues such as reingestion or short-circuiting of discharge heat rejection air because of location in a confined area, tower plume, proximity to outdoor air intakes, and the effect of drift on adjacent roadways, buildings, and parking lots. As with evaporative condensers, freeze protection and water treatment may also be required for successful operation.
Makeup water subtraction meters should be evaluated for both evaporative condensers and cooling towers by the design engineer or owner. In many areas, sewage costs are part of the overall water bill, and most domestic water supplied to a typical facility goes into the sewage system. However, evaporated condenser water is not drained to sewer, and if a utility-grade meter is used, potential savings in plant operating costs can be made available to the owner. Metering should be evaluated with chilled-water plants, especially those in operation year-round. Consult with the water supplier to identify compliance requirements. Consider piping cooling tower water blowdown and drainage (e.g., to remove dissolved solids or allow maintenance when installing subtraction meters) to the storm drainage system, but note that environmental effects of the water chemistry must be evaluated. Where chemical treatment conditions do not meet environmental outfall requirements, drainage to the sanitary system may still be required unless prefiltration can be incorporated.
Water pumps move both chilled and condenser water to and from the refrigeration equipment and associated ancillary equipment. See Chapter 44 for additional information on centrifugal pumps, and Chapters 12 to 14 for system design.
Heat exchangers provide operational and energy recovery opportunities for central cooling plants. Operational opportunities generally involve heat transfer between building systems that must be kept separated because of different pressures, media, cleanliness, or other characteristics. For example, heat exchangers can thermally link a low-pressure, low-rise building system with a high-pressure, high-rise building system. Heat exchangers can also transfer heat between chemically treated or open, contaminated water systems and closed, clean water systems (e.g., between a central cooling plant chilled-water system and a high-purified, process cooling water system, or between potentially dirty pond water and a closed-loop condenser water system).
To conserve energy, water-to-water heat exchangers can provide water-side economizer opportunities. When outdoor conditions allow, condenser water can cool chilled water through a heat exchanger, using the cooling tower and pumps rather than a compressor. This approach should be considered when year-round chilled water is needed to satisfy a process load, or when an air-side economizer is not possible because of design requirements or limitations (e.g., space humidity requirements, lack of space for the required ductwork).
Plant Controls.
Direct digital control (DDC) systems should be considered for control accuracy and reliability. Temperature, flow, and energy use are best measured and controlled with modern DDC technology. Pneumatic actuation might be considered where torque or rapid response of power actuation is required; medium pressure
(typically 30 to 60 psig) is best, though electric actuation is more common. Consider using programmable logic controllers (PLCs) for larger central plants or where future expansion/growth is possible. Smaller plants can use the onboard chiller control panel, providing a simplified control system at a lower installed cost.
Codes and Standards.
Specific code requirements and standards apply when designing central cooling plants. For cooling equipment, refer to ASHRAE
Standard 15;
Chapter 52 of this volume provides a comprehensive list of codes and standards associated with cooling plant design, installation, and operation. Follow manufacturers’ recommendations and federal, state, and local codes and standards.
Primary Heating Equipment
Boilers are the major heating equipment used in central heating plants. They vary in type and application, and include combined heat and power (CHP) and waste heat boilers. Chapter 32 discusses boilers in detail, including the size ranges of typical equipment.
A boiler adds heat to the working medium, which is then distributed throughout the building(s) and/or campus. The working medium may be either water or steam, which can further be classified by its temperature and pressure range. Steam, often used to transport energy long distances, is converted to low-temperature hot water in a heat exchanger near the point of use. Although steam is an acceptable medium for heat transfer, low-temperature hot water is the most common and more uniform medium for providing heating and process heat (e.g., heating water to 200°F for domestic hot water). Elevated steam pressures and high-temperature hot-water boilers are also used. Both hot-water and steam boilers have the same type of construction criteria, based on temperature and pressure.
A boiler may be purchased as a package that includes the burner, fire chamber, heat exchanger section, flue gas passage, fuel train, and necessary safety and operating controls. Cast-iron and water-side boilers can be field-assembled, but fire-tube, scotch marine, and waste-heat boilers are usually packaged units.
Energy Sources.
The energy used by a boiler may be electricity, natural gas, oil, coal, or combustible waste material, though natural gas and fossil-fuel oil (No. 2, 4, or 6 grade) are most common, either alone or in combination. Selecting a fuel source requires detailed analysis of energy prices and availability (e.g., natural gas and primary electrical power). The availability of fuel oil or coal delivery affects road access and storage for deliveries.
Energy for heating may also come from waste heat from a CHP system. Plant production efficiency is improved when waste heat can be reclaimed and transferred to a heat source. A heat recovery generator can be used to convert the heat byproduct of electric generation to steam or hot water to meet a facility’s heating needs when the thermal load meets or exceeds the heat rejection of the electric generation. This can help with cost-effective plant operation.
Additionally, attention to the design of an efficient and maintainable condensate return system for a central steam distribution plant is important. A well-designed condensate return system increases overall efficiencies and reduce both chemical and makeup water use.
Codes and Standards.
Specific code requirements and standards apply when designing central heating boiler plants. It is important to note that some applications (e.g., high-pressure boilers) require continuous attendance by licensed operators. Operating cost considerations should be included in determining such applications. Numerous codes, standards, and manufacturers’ recommendations need to be followed. Refer to
Chapter 52 for a comprehensive list of codes and standards associated with this equipment, plant design, installation, and operation.
Ancillary Heating Equipment
Steam Plants.
Ancillary equipment associated with central steam heating plants consists primarily of the boiler feed unit, deaerator unit (both with receiver and pumps), chemical feed, and possibly a surge tank and/or heat exchanger(s).
Boiler feed equipment, including the receiver(s) and associated pump(s), serve as a reservoir for condensate and makeup water waiting to be used by the steam boiler. The boiler feed pump provides system condensate and water makeup back to the boiler on an as-needed basis.
Deaerators help eliminate oxygen and carbon dioxide from the feed water (see Chapter 49 of the 2019 ASHRAE Handbook—HVAC Applications for more information).
Chemicals can be fed using several methods or a combination of methods, depending on the chemical(s) used (e.g., chelants, amines, oxygen scavengers).
Surge tanks are also applicable to steam boiler plants to accommodate large quantities of condenser water return and are primarily used where there is a rapid demand for steam (e.g., morning start-up of a central heating plant).
See Chapter 11 for more information on steam systems.
Hot-Water Plants.
Ancillary equipment associated with central hot-water heating plants consists primarily of pumps and possibly heat exchanger(s). Water pumps move boiler feed water and hot-water supply and return to and from the boiler equipment and associated ancillary equipment. See
Chapter 44 for additional information on centrifugal pumps, as well as
Chapters 11 to
13 and
Chapter 15 for system design.
Heat Exchangers.
Heat exchangers offer operational and energy recovery opportunities for central heating plants. In addition to the heat exchange and system separation opportunities described in the section on Refrigeration Equipment, the operational opportunities for heat exchangers involve combining steam heating capabilities with hot-water heating capabilities.
Air-to-water and water-to-water heat exchangers provide opportunities for economizing and heat recovery in a central heating plant (e.g., flue gas exhaust heat recovery and boiler blowdown heat recovery).
For more detailed information on ancillary equipment, see Chapters 31 to 33 and Chapter 35.
The major piping in a central cooling plant may include, but is not limited to, chilled-water, condenser water, city water, natural gas, fuel oil, refrigerant, vent, and drainpipe systems. For a central steam heating plant, the major piping includes steam supply, condensate return, pumped condensate, boiler feed, city water, natural gas, fuel oil, vent, and drainpipe systems. For a central hot-water heating plant, it includes hot-water supply and return, city water, natural gas, fuel oil, vent, and drainpipe systems. In the 2017 ASHRAE Handbook—Fundamentals, see Chapter 22 for information on sizing pipes, and Chapter 37 for identification, color-coding, abbreviations, and symbols for piping systems.
Design selection of cooling and heating temperature set points (supply and return water) can affect first and operating costs. Water systems with a large temperature difference between supply and return water have lower flow requirements and can allow smaller pipe sizing and smaller valves, fittings, and insulation, which can lower installation cost. However, these savings may be offset by the larger coils and heat exchangers at the point of use needed to accomplish the same heat transfer. A similar design strategy can be achieved with steam pressure differential.
Determining the optimum cooling and heating water supply and return temperatures requires design consideration of equipment performance, particularly the energy required to produce the supply water temperature. Although end users set water temperatures, the colder the cooling water, the more energy is needed by the chiller. Similar issues affect hot-water supply temperature and steam operating pressure. Supply water reset may be used when peak capacity is not needed, potentially reducing energy consumption. This reduces chiller power input, but those savings may be offset by increased pump power input because of a higher water flow rate with higher supply temperature. Also, raising chilled-water temperatures diminishes the latent removal capacity of cooling and dehumidification equipment, and may adversely affect control of interior humidity.
Energy implications for the whole system must be considered. The design engineer should consider using higher temperature differences between supply and return to reduce the pump energy required by the distribution system, for both heating and chilled-water systems. Additionally, central plant production systems (e.g., boilers, steam-to-heating-water converters, chillers) operate more efficiently with higher return water temperatures.
During conceptual planning and design, or as early in design as possible, the engineer should evaluate the type(s) of facilities (existing and planned future) to which the central system will be connected. When using variable-flow distribution with a constant design temperature split between the supply and return medium, it is critical to avoid low ΔT syndrome [see, e.g., Taylor (2002)]. The engineer should attempt to ensure the condition is minimized or avoided, or at minimum perform due diligence to make the owner aware of the potential shortfalls where it is allowed to occur. With existing constant-flow/variable-temperature systems, take measures to avoid losing the conceptual design strategy of a variable-flow/constant-temperature split. Examples of connection strategies less costly than full renovation of a connected facility are (1) return recirculation control, to maintain a design temperature across a facility connection, or (2) separation of primary distribution supply from the secondary facility connection by a plate-and-frame heat exchanger with a control valve on the primary return controlled by the secondary supply to the facility. If the temperature split is allowed to fall, increased flow of the medium through the distribution system will be required to accomplish the same capacity. This increase in flow increases pump power and chiller plant energy, compromising the available capacity and operation of the system.
Consider hydraulically modeling the cooling and heating media (chilled water, heating water, steam, domestic water, natural gas, etc.). With an emphasis on centralizing the source of cooling and heating, a performance template can be created for large plants by computerized profiling of cooling and heating delivery. Hydraulic modeling provides the economic benefits of predesigning the built-out complete system before installing the initial phase of the project. The model also helps troubleshoot existing systems, select pumps, project energy usage, and develop operation strategy.
Energy conservation and management can best be achieved with computerized design and facility management resources to simulate delivery and then monitor and measure the actual distribution performance.
5. SOUND, VIBRATION, SEISMIC, AND WIND CONSIDERATIONS
Proper space planning is key to sound and vibration control in central plant design. For example, central plants are frequently located at or below grade. This provides a very stable platform for vibration isolation and greatly reduces the likelihood of vibration being transmitted into the occupied structure, where it can be regenerated as sound. Also, locating the central ventilation louvers or other openings away from sound- and vibration-sensitive areas greatly reduces the potential for problems in those areas. As a guide, maintain free area around louvers as specified in ASHRAE Standard 15.
Vibration and sound transmitted both into the space served by the plant and to neighboring buildings and areas should be considered in determining how much acoustical treatment is appropriate for the design, especially if the plant is located near sensitive spaces such as conference rooms or sleeping quarters. See the section on Space Considerations for further discussion of space planning.
Acoustics must also be considered for equipment outside the central plant. For example, roof-mounted cooling tower fans sometimes transmit significant vibration to the building structure and generate ambient sound. Many communities limit machinery sound pressure levels at the property line, which affects the design and placement of equipment. See Chapter 48 of the 2019 ASHRAE Handbook—HVAC Applications for more detailed information on sound and vibration.
Depending on code requirements and the facility’s location with respect to seismic fault lines, seismic bracing may be required for the central plant equipment and distribution. For instance, a hospital located in a seismically active area must be able to remain open and operational to treat casualties in the aftermath of an earthquake. Additionally, outdoor equipment in some areas needs appropriate bracing for wind loads. This is particularly apparent in critical operations facilities in areas susceptible to hurricanes or tornadoes. Most building codes require that measures such as anchors and bracing be applied to the HVAC system. Refer to the local authority responsible for requirements, and to Chapter 55 of the 2019 ASHRAE Handbook—HVAC Applications for design guidance.
In the very early phases of building design, architects, owners, and space planners often ask the engineer to estimate how much building space will be needed for mechanical equipment. The type of mechanical system selected, building configuration, and other variables govern the space required, and many experienced engineers have developed rules of thumb to estimate the building space needed. Although few buildings are identical in design and concept, some basic criteria apply to most buildings and help approximate final space allocation requirements. These space requirements are often expressed as a percentage of the total building floor area; the combined mechanical and electrical space requirement of most buildings is 6 to 9%.
A central system reduces mechanical space requirements for each connected building. Where a central plant is remote or stand-alone, space requirements for equipment and operation are 100% for the plant.
Space for chillers, pumps, and towers should not only include installation footprints but should also account for adequate clearance to perform routine and major maintenance. Generally, 4 ft service clearance (or the equipment manufacturer’s minimum required clearance, whichever is greater) around equipment for operator maintenance and service is sufficient. For chillers and boilers, one end or side of the equipment should be provided with free space, to allow for tube pull clearance. In many cases, designers provide service bay roll-up doors or ventilation louvers (if winter conditions do not cause freeze damage issues) to allow tube access. Overhead service height is also required, especially where chillers are installed. Provision for 20 ft ceilings in a central plant is not uncommon, to accommodate piping and service clearance dimensions.
Plant designs incorporating steam supply from a separate central boiler plant may use steam-to-heating-water converters and the heating distribution equipment (e.g., pumps) along with the chilled-water production equipment and distribution infrastructure. Where a boiler installation as well as heating distribution equipment and appurtenances are required, the plant’s physical size increases to account for the type of boiler and required exhaust emissions treatment. Generally, for central heating plants, steam or heating water and chilled-water production systems are separated during design, which may further increase the overall footprint of the plant.
The arrangement and strategic location of the mechanical spaces during planning affects the percentage of space required. For example, the relationship between outdoor air intakes and loading docks, exhaust, and other contaminating sources should be considered during architectural planning. The final mechanical room size, orientation, and location are established after discussion with the architect and owner. The design engineer should keep the architect, owner, and facility engineer informed, whenever possible, about the HVAC analysis and system selection. Space criteria should satisfy both the architect and the owner or owner’s representative, though this often requires some compromise. The design engineer should strive to understand the owner’s needs and desires and the architect’s vision for the building, while fully explaining the advantages, disadvantages, risks, and rewards of various options for mechanical and electrical room size, orientation, and location. All systems should be coordinated during the space-planning stage to safely and effectively operate and maintain the central cooling and heating plant.
In addition, the mechanical engineer sometimes must represent other engineering disciplines in central plant space planning. If so, it is important for the engineer to understand the basics of electrical and plumbing central plant equipment. The main electrical transformer and switchgear rooms should be located as close to the incoming electrical service as practical. The main electric transformers and switchgear for the plant and the mechanical equipment switchgear panels should be in separate rooms that only authorized electricians can enter. If there is an emergency generator, it should be located considering (1) proximity to emergency electrical loads, (2) sources of combustion and cooling air, (3) fuel sources, (4) ease of properly venting exhaust gases outside, and (5) provisions for sound control.
The main plumbing equipment usually contains gas and domestic water meters, the domestic hot-water system, the fire protection system, and elements such as compressed air, special gases, and vacuum, ejector, and sump pumps. Some water and gas utilities require a remote outdoor meter location.
The heating and air-conditioning equipment room houses the (1) boiler, pressure-reducing station, or both; (2) refrigeration machines, including chilled-water and condensing-water pumps; (3) converters for furnishing hot or cold water for air conditioning; (4) control air compressors, if any; (5) vacuum and condensate pumps; and (6) miscellaneous equipment. For both chillers and boilers, especially in a centralized application, full access is needed on all sides (including overhead) for extended operation, maintenance, annual inspections of tubes, and tube replacement and repair. Where appropriate, the designer should include overhead structural elements to allow safe rigging of equipment. Ideally, large central facilities should include overhead cranes or gantries. It is critical that local codes and ASHRAE Standard 15 are consulted for special equipment room requirements. Many jurisdictions require monitoring, alarms, evacuation procedures, separating refrigeration and fuel-fired equipment, and high rates of purge ventilation.
A proper operating environment for equipment and those maintaining it must also be provided. This may involve heating the space for freeze protection and/or cooling the space to prevent overheating motors and controls. HVAC equipment serving the central plant itself may be housed in its own equipment room and serve the chiller, boiler room, and adjacent rooms (e.g., switchgear room, office space, generator room, pump room, machine shop). Where refrigeration equipment is installed, follow design requirements in ASHRAE Standard 15.
Location of Central Plant and Equipment
Although large central plants are most often located at or below grade, it is often economical to locate the refrigeration plant at the top of the building, on the roof, or on intermediate floors. For central plants, on-grade should be the first choice, followed by below grade. The designer must always ensure access for maintenance and replacement. Locating major equipment in roof penthouses may pose significant access problems for repair and replacement. For single high-rise central plants, intermediate-floor locations that are closer to the load may allow pumping equipment to operate at a lower pressure. A life-cycle cost analysis (LCCA) should include differences in plant location to identify the most attractive plant sustainability options during the planning stage. The LCCA should consider equipment maintenance access, repair, and replacement during the life of the plant, as determined in the owner’s criteria. If not identified, an engineer should suggest to the owner or client that maintenance criteria be included as a component in developing the LCCA (see the section on Maintenance Management Systems). Electrical service and structural costs are greater when intermediate-floor instead of ground-level locations are used, but may be offset by reduced energy consumption and condenser and chilled-water piping costs. The boiler plant may also be placed on the roof, eliminating the need for a chimney through the building.
Benefits of locating the air-cooled or evaporative condenser and/or cooling tower on the ground versus the roof should be evaluated. Personnel safety, security, ambient sound, and contamination from hazardous water vapors are some of the considerations that help determine final equipment location. Also, structural requirements (e.g., steel to support roof-mounted equipment, or a concrete pad and structural steel needed to locate equipment at or near grade) require evaluation. When locating a cooling tower at or near grade, the net positive suction head and overflow of condenser water out of the cooling tower sump should be studied if the tower is below, at, or slightly above the level of the chiller.
Numerous variables should be considered when determining the optimum location of a central cooling or heating plant. When locating the plant, consider the following:
Operating weight of the equipment and its effect on structural costs
Vibration from primary and ancillary equipment and its effect on adjacent spaces in any direction
Sound levels from primary and ancillary equipment and their effect on adjacent spaces in any direction
Location of electrical utilities for the central plant room, including primary electric service and associated switchgear and motor control center, as well as electrical transformer location and its entrance into the building
Location of city water and fire pump room (it may be desirable to consolidate these systems near the central plant room)
Accessibility into the area and clearances around equipment for employee access, equipment and material delivery, and major equipment replacement, repair, scheduled teardown, and rigging
Location of cooling, refrigerant relief piping, heating, vents, and boiler flue and stack distribution out of the central plant and into the building, along with the flow path of possible vented hazardous chemical, steam, or combustion exhaust products
Need for shafts to provide vertical distribution of cooling and heating services in the building
Future expansion plans of the central plant (e.g., oversizing the central plant now for adding more primary equipment later, based on master planning of the facility)
Architectural effect on the site
Location of boiler chimney/flue
Loading dock for materials and supplies
Roadway and parking considerations
Storage of fuel
Electrical transformer location
Underground and/or overhead utility and central cooling and heating system distribution around the central plant and to the building(s)
Wind effect on cooling tower plume or other volatile discharges such as boiler emissions.
Security around a central plant must be considered in design, and should include standby electrical power generation and production of central cooling and heating for critical applications in times of outages or crisis.
Restricted access and proper location of exposed intakes and vents must be designed into the central plant layout to protect the facility from attack and protect people from injury. Use care in locating exposed equipment, vents, and intakes, especially at ground level. Above ground and at least 10 to 15 ft from access to intake face is preferred. When this is not possible, fencing around exposed equipment, such as cooling towers and central plant intakes, should be kept locked at all times to prevent unauthorized access. Ensure that fencing is open to airflow so it does not adversely affect equipment performance. Air intakes should be located above street level if possible, and vents should be directed so they cannot discharge directly on passing pedestrians or into an air intake of the same or an adjacent facility.
7. AUTOMATIC CONTROLS AND BUILDING MANAGEMENT SYSTEMS
One advantage of central cooling and heating plants is easier implementation of building automation because the major and ancillary equipment is consolidated in one location. Computerized automatic controls can significantly affect system performance. A facility management system to monitor system points and overall system performance should be considered for any large, complex air-conditioning system. This allows a single operator to monitor performance at many points in a building and make adjustments to increase occupant comfort and to free maintenance staff for other duties. Chapter 47 of the 2019 ASHRAE Handbook—HVAC Applications describes design and application of controls.
Software to consider when designing, managing, and improving central plant performance should include the following:
Automatic controls that can interface with other control software (e.g., equipment manufacturers’ unit-mounted controls)
Energy management system (EMS) control
Hydraulic modeling, as well as metering and monitoring of distribution systems
Using VFDs on equipment (e.g., chillers, pumps, cooling tower fans) to improve system control and to control energy to consume only the energy required to meet the design parameter (e.g., temperature, flow, pressure).
Computer-aided facility management (CAFM) for integrating other software (e.g., record drawings, operation and maintenance manuals, asset database)
Computerized maintenance management software (CMMS)
Automation from other trades (e.g., fire alarm, life safety, medical gases, etc.)
Regulatory functions (e.g., refrigerant management, federal, state and local agencies, etc.)
Automatic controls for central cooling and/or heating plants may include standard equipment manufacturer’s control logic along with optional, enhanced energy-efficiency control logic. These specialized control systems can be based on different architectures such as distributed controls, programmable logic controllers, or microprocessor-based systems. Beyond standard control technology, the following control points and strategies may be needed for primary equipment, ancillary equipment, and the overall system:
Discharge temperature and/or pressure
Return distribution medium temperature
Head pressure for refrigerant and/or distribution medium
Stack temperature
Carbon monoxide and/or carbon dioxide level
Differential pressures
Flow rate of distribution medium
Peak and hourly refrigeration output
Peak and hourly heat energy output
Peak and hourly steam output
Flow rate of fuel(s)
Reset control of temperature and/or pressure
Night setback
Economizer cycle
Variable flow through equipment and/or system control
Variable-frequency drive control
Thermal storage control
Heat recovery cycle
See Chapter 42 of the 2019 ASHRAE Handbook—HVAC Applications for more information on control strategies and optimization.
The coefficient of performance (COP) for the entire chilled-water plant can be monitored and allows the plant operator to determine the overall operating efficiency of a plant. Central plant COP can be expressed in the following terms:
Annual heating or refrigeration per unit of building area (Btu per hour per square foot per year)
Energy used per unit of refrigeration (kilowatt-hours per ton)
Annual power required per unit of refrigeration per unit of building area (kilowatts per ton per square foot per year)
All instrument operations where cooling or heating output are measured should have instrumentation calibration that is traceable to the National Institute of Standards and Technology (NIST).
The importance of local gages and indicating devices, with or without a facility management system, should not be overlooked. All equipment must have adequate pressure gages, thermometers, flow meters, balancing devices, and dampers for effective performance, monitoring, and commissioning. In addition, capped thermometer wells, gage cocks, capped duct openings, and volume dampers should be installed at strategic points for system balancing. Chapter 38 of the 2019 ASHRAE Handbook—HVAC Applications indicates the locations and types of fittings required. Chapter 36 of the 2017 ASHRAE Handbook—Fundamentals has more information on measurement and instruments.
8. MAINTENANCE MANAGEMENT SYSTEMS
A review with the end user (owner) should be done, to understand the owner’s requirements for operation (e.g., if an owner has an in-house staff, more frequent access may be required, which may affect the extent to which a designer incorporates access). Reviewing ASHRAE Guideline 4 and Standard 15 with the owner can provide insight to the development of a maintenance plan. If maintenance is outsourced as required, extensive access may not be as high a priority. In some cases, regulatory and code access may be the only determining factors.
Operations and maintenance considerations include the following:
Accessibility around equipment, as well as above and below when applicable, with minimum clearances per manufacturers’ recommendations and applicable codes
Clearances for equipment removal
Minimizing tripping hazards (e.g., drain piping extending along the floor)
Adequate headroom to avoid injuries
Trenching in floor, if necessary
Cable trays, if applicable
Adequate lighting levels
Task lighting, when needed
Eyewash stations for safety
Exterior access for outdoor air supply and for exhaust
Storage of mechanical and electrical parts and materials
Documentation storage and administrative support rooms
Proper drainage for system maintenance
Outlets for service maintenance utilities (e.g., water, electricity) in locations reasonably accessible to equipment operators
Adequate lines of sight to view thermometers, pressure gages, etc.
Structural steel elements for major maintenance rigging of equipment
Typical operator maintenance functions include cleaning of condensers, evaporators, and boiler tubes. Cooling tower catwalk safety railing and ladders should be provided to comply with U.S. Occupational Safety and Health Administration (OSHA) requirements.
9. BUILDING SYSTEM COMMISSIONING
Because a central plant consumes a major portion of the annual energy operating budget, building system commissioning is imperative for new construction and expansion of existing installations. During the warranty phase, central-plant performance should be measured, benchmarked, and course-corrected to ensure design intent is achieved. If an energy analysis study is performed as part of the comparison between decentralized and centralized concepts, the resulting month-to-month energy data should be a good electronic document to benchmark actual energy consumption using the measurement and verification plan implementation.
Ongoing commissioning or periodic recommissioning further ensures that the design intent is met, and that cooling and heating are reliably delivered after the warranty phase. Many central systems are designed and built with the intent to connect to facilities in a phased program, which also requires commissioning. Consider retro- or recommissioning whenever the plant is expanded or an additional connection made to the existing systems, to ensure the original design intent is met.
Initial testing, adjusting, and balancing (TAB) also contribute to sustainable operation and maintenance. The TAB process should be repeated periodically to ensure levels are maintained.
When completing TAB and commissioning, consider posting laminated system flow diagrams at or adjacent to the central cooling and heating equipment, indicating operating instructions, TAB performance, commissioning functional performance tests, and emergency shutoff procedures. These documents also should be filed electronically in the central plant computer server for quick reference.
Original basis of design and design criteria should be posted as a constant reminder of design intent, and to be readily available in case troubleshooting, expansion, or modernization is needed.
As with all HVAC applications, for maintainable, long-term design success, building commissioning should include the system training requirements necessary for building management staff to efficiently take ownership and operate and maintain the HVAC systems over the useful service life of the installation.