HVAC Systems Design Handbook part 7

191 Chapter 7 Design Procedures: Part 5 Central Plants 7.1 Introduction The design and construction of central plants for heating and cooling is one of the most challenging and interesting aspects of the HVAC design profession. Central plants range in size from small to very large, from residential to industrial utility scale. There are many ar- eas of individual expertise and many levels of competence among de- signers. In this chapter we discuss several fundamental types of plants and aspects of plant design, still leaving much detail to literature and experience beyond the scope of this book. See Ref. 1 for additional discussion of the topics treated here. 7.2 General Plant Design Concepts Independent of the service being produced, some concerns are common to central plants. 1. Siting. Central plants preferably are located in the middle of or adjacent to the loads they serve. Distribution piping costs may loom large if primary piping runs long distances to get to the service point. On the other hand, the combining of multiple service units into one plant is the act which achieves the economy of scale and the conven- ience of operation, so distance is a tradeoff, but the central location is still a favored point to start. For large plants serving congested cam- puses, a remote or peripheral location may be preferred. This allows better access to the plant and removes plant, traffic, noise, and emis- sions from the more densely populated areas. Source: HVAC Systems Design Handbook Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 192 Chapter Seven For high-rise buildings, there is the question of the basement, roof, or in between. On-grade locations have the best access. Sometimes buildings are occupied from the ground up during extended construc- tion, suggesting a low-level site. Where water systems are involved, pressures may become very high at lower building levels. This is less of a problem with chillers than with boilers. Systems with boilers often take the equipment to the roof, partly for pressure considerations, partly to eliminate the problem of taking the flue up through the building, partly for emission dispersion. Cooling towers need to be near the chiller served if possible, to reduce the cost of piping, but the cooling-tower vapor plume can be a problem in cool weather if it im- pacts the building (window cleaning, condensation on structure, etc.). A vapor plume is a cold-weather visual problem in year-round opera- tion and may cause a local ‘‘snow’’ effect in cold climates. 2. Structure. The enclosure and support for major plant equip- ment should be strong enough to withstand vibration, to support equipment and piping, to contain yet accept expansion and contrac- tion, to enclose and subdue noise, and to support maintenance through access and hoist points. In some environments, plant structures are fully enclosed by heavy masonry. In the industrial environment, in mild climates, plant struc- tures may be open, offering only a roof and access, possibly a sound enclosure. Some well-designed plants may take on an aesthetic aspect including large expanses of glass and careful lighting. It is a fun ex- perience to sculpt in pipe and equipment for all to see. This can be accomplished with little premium construction cost, but it takes more design time and an artist’s inclination. Some feel that a plant that looks good may work better, since more time is given to function and layout than in the ‘‘quick and dirty’’ arrangements so often encoun- tered. Well-arranged plants usually are more easily maintained, given the space associated with form and symmetry. As a general note, reinforced-concrete floors and below-grade walls have proved to be durable. Steel-frame superstructures with inter- mediate floors of concrete and steel work very well. Steel members with grating for walkways are very popular. Plant enclosures should allow for future equipment replacement or addition, with wall openings and possibly roof sections which can be removed and replaced. 3. Electrical Service. Many plants, particularly those with chillers or electric boilers, comprise a major electrical load for the facility. Proximity to the primary electrical service is a cost concern. The elec- trical service should be well thought through, and should allow for any projected plant expansion, if not in present gear, at least in space and concept. Since the plant environment may be coarse (although Design Procedures: Part 5 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Design Procedures: Part 5 193 cleanliness is a virtue), electrical equipment is often housed in a sep- arate room with filtered, fan-forced ventilation. Some electronic gear needs to be in an air conditioned space. Where many motors are involved in a plant, motor control centers (MCCs) are preferred to individual combination starters. Large plants may have several MCCs to reduce the length of wiring runs. The electrical service should have a degree of redundancy. Hospitals and other critical-care facilities require access to at least two inde- pendent utility substations. This carries into the large plant in the form of multiple transformers and segmented switch groups with tie breakers. Standby power generation may be included in plant design in addition to backup power for life safety issues. 4. Valving. In central plants there is no substitute for isolation valves for every piece of equipment. Multiple high-pressure steam boilers require double valving with intermediate vent valves to protect workers inside a unit that is down for maintenance. Valves should be installed in accessible locations. 7.3 Central Steam Plants Some general concepts of steam distribution were presented in Chap. 6. Steam plants require considerations of siting, structure, and elec- trical service, as described in this chapter. Boilers are the primary component of steam plants and are supported by a host of auxiliary components such as boiler feed pumps, deaerating feedwater heaters, condensate holding tanks, water softeners, blowdown heat recovery systems, water treatment systems, flue gas economizers, fuel-handling equipment, etc. See Fig. 7.1. Each component of the steam system is available in a range of qual- ity and performance characteristics. Selection depends on duty and on the sophistication of the plant operation. Equipment for a smaller school will be of a different character than for a campus or an indus- trial plant. With all the subjective differences, the technical calcula- tions are similar. Because condensate originates in heat exchange devices as a fluid without pressure, it must drain by gravity to a collection point. If a steam plant can be located at the low point of the served system, the entire condensate return line may flow by gravity. Otherwise, inter- mediate collection points and booster pumps may be required. An important aspect of a steam plant is the condensate storage ves- sel. When a boiler fires up after a time of setback or at the onset of a peak heating load, a significant amount of feedwater will be evapo- rated and sent out into the system with a time lag before any of the condensate will get back to the plant. The storage tank must hold Design Procedures: Part 5 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 194 Chapter Seven Figure 7.1 Steam plant diagram. enough water to sustain the initial demand, and then it must have enough ‘‘freeboard’’ or residual capacity to accept the returning con- densate after an evening load shutdown. Failure to provide adequate storage is observed through storage tank overflow, with high makeup water rates and high treatment costs. Small plants often use the feedwater heating tank as a combination storage-and-preheat vessel. Most steam plants use a version of a feedwater heater to remove dissolved oxygen by bringing the feedwater to the boiling point. This also tempers the water to reduce the potential for damaging the boiler with a shot of cold water. Feedwater makeup to boilers is accomplished with feedwater pumps. If feedwater is heated to near the boiling point, the pumps must have a low net positive suction head (NPSH) to avoid cavitation. Small plants often have a dedicated pump for each boiler with a level control on the boiler drum which cycles the pump on a call for more water. Larger plants usually have a continuously running pump for several boilers with modulating valves and automatic level controls to maintain a constant level in the boiler steam drum. 7.3.1 Steam plant controls In a very small steam system, a space thermostat may cycle the boiler on and off, and the steam drum-level control will activate the feed- Design Procedures: Part 5 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Design Procedures: Part 5 195 water pump. In a more complex system, the boiler(s) will maintain a constant steam pressure in the main header, and a pressure control will modulate the fuel input to match the load. For multiple-boiler operation, there may be a plant master control which will apportion the load to the several boilers on a proportional or a programmed basis. 7.3.2 Flue gas economizers Flue gas economizers are often used on steam boilers to pick up an additional 3 to 7 percent of combustion efficiency. Reclaimed heat from the economizers may be used for combustion air preheating or feed- water preheating. In either case, care must be taken to keep the ex- iting flue gas above the water vapor condensation temperature, and for feedwater heating, there must be adequate flow to avoid steaming in the economizer. 7.3.3 Boiler testing It is often desirable or necessary to test steam boiler performance. To this end, a valve to open for discharge to atmosphere is included in the plant design. The test valve discharge line should include a sound silencer to minimize the noise. 7.4 Central Hot Water Plants Some general concepts of heating water distribution were discussed in Chap. 6. Chapter 10 discusses boilers and some other pieces of heat- ing plant equipment. Low-temperature water (LTW) heating systems (150 to 250ЊF) are simple in design. They include boiler(s), pump(s), and secondary com- ponents such as water treatment, air eliminators, and expansion tanks. See Fig. 7.2. The simplicity of these systems is compelling. They become so automatic and reliable that even in larger sizes they are often taken for granted. Most hot water plants serve loads of varying magnitude. If constant- flow systems were common in the past, variable-flow systems are be- coming more common because of the reduced pumping energy which can be obtained at lower loads. Water heating plants are usually designed for a heating differential of 20 to 40ЊF through the boilers. Return water temperatures below 140ЊF to the boiler should be avoided in most cases out of concern for flue gas vapor condensation and for ‘‘cold shock’’ of the boiler itself. Multiple hot water boilers are almost always piped in parallel. See Figs. 7.3 and 7.4. Where two boilers are selected, it is common to size Design Procedures: Part 5 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 196 Chapter Seven Figure 7.2 Elementary heating water system diagram. Figure 7.3 Central heating plant, multiple boilers / common pumps. each for 60 percent of the peak load, to allow one boiler to keep the system ‘‘alive’’ if the other boiler fails. For a three- or four-boiler or more system, boilers are usually sized so that the entire load can be carried even if the largest boiler fails. There is usually a smaller boiler sized to the summer load. Care must be taken not to underestimate Design Procedures: Part 5 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Design Procedures: Part 5 197 Figure 7.4 Central heating plant, multiple boilers / individual pumps. the peak summer demand. Undersizing the small boiler forces the use of a larger boiler, losing the benefit of the smaller selection. Water heating plants usually have a means of introducing an oxygen scavenging chemical with corrosion inhibitor to the system. Soft water is often used for fill water. Heating water systems should be quite tight, requiring little makeup water. Where glycol solutions are used for freeze protection, a means of introducing the glycol-water mixture must be included in the plant. This often takes the form of a holding tank with a feed pump. Glycol solutions require attention to materials in the system. Some elastomers are sensitive to some petroleum- derived glycols. Feedwater should be introduced to the system through a pressure- reducing valve, set for a pressure below the maximum operating pres- sure of the boiler. 7.5 High-Temperature Hot Water Plants High-temperature water (HTW) plants usually have supply water temperatures between 350 and 450ЊF. This discussion also includes plants with a supply temperature between 250 and 350ЊF because the principles are similar. These systems became popular in the post- World War II era, as an alternative to steam plants for large campus and military-base central heating systems. The advantage is related to the ability of water with high temperature differential to move large quantities of heat in smaller distribution pipe. Pumping and control may be simplified. System design pressures are similar as for high- Design Procedures: Part 5 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 198 Chapter Seven pressure steam, but must be handled carefully to avoid flashing re- lated to changes in elevation across the facility. The detailed design of HTW plants is a specialty beyond the scope of this book. There are few definitive works on the technology and only a few design offices across the country, with personnel having HTW experience. Chapter 14 of the 2000 ASHRAE Handbook, HVAC Sys- tems and Equipment, discusses the topic. A designer working in or with an HTW plant will find recognizable components. High pressure boilers are the heart of the plant. Almost any fuel can be accommodated. HTW boilers are almost always cir- culated with constant flow independent of the load, to avoid hot spots and steaming on the heat transfer surfaces. Some plants use a large drum with a steam cushion to accept the wide fluid expansion and contraction episodes encountered in large systems. An alternative and now more common practice is to use an expansion drum pressurized with nitrogen in a manner similar to a conventional lower-temper- ature heating plant. HTW plants usually serve variable-flow secondary systems (the loads have control valves which meter the supply water to match the load) and therefore benefit from variable-speed control for the system pumps. To protect the plant from power outage, most HTW plants have standby power generation capability. To protect from sudden water loss due to rupture in the distribution system, quick-closing valves on the piping in and out of the plant are recommended. HTW plants usually look for return water temperatures ranging from 200 to 250ЊF. If the water comes back warmer than the design value, it becomes difficult to load the fixed-circulation-rate boilers. Building system designers working with HTW should recognize that steam generation at pressures above 15 lb/in 2 is not a good load for an HTW system. Since the HTW leaving the steam generator must be above the steam saturation temperature, it is impossible for a steam generator to get the return water temperature down to the plant de- sign inlet condition. Large HTW flows are required, and this wastes distribution system capacity. This problem can be relieved by cascad- ing the steam generator HTW return into a lower-grade heating ser- vice; but, in general, high-pressure steam requirements should be ac- commodated with an independent boiler. 7.6 Fuel Options and Alternative Fuels A nice feature of central heating plants is that if the load requirements are not extreme, almost any fuel source can be utilized to make steam or hot water. Coal, oil, and gas (natural, liquefied, manufactured) are Design Procedures: Part 5 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Design Procedures: Part 5 199 traditional fuels. But wood refuse, combustible by-product, and mu- nicipal and industrial waste and industrial process exhaust streams are all candidates for central plant heating sources. Under some con- ditions, electricity may be used as an energy input for a central plant. High-temperature geothermal waters or steam can be used through heat exchangers for central heating. Direct use of many geothermal resources incurs problems with corrosive and precipitate aspects of the waters. Solid fuels present design challenges related to delivery, handling and storage of the input fuel, and the collection, storage, and disposal of the residual matter. Still, there may be appropriate applications for coal, wood, bagasse, or refuse-derived fuels (RDF). 7.7 Chilled Water Plants Central chilled water plants for HVAC systems have evolved to a com- bination of factory-built chillers and pumps in a variety of piping and pumping configurations. Since the cooling effort may require a large amount of energy to drive the process, much attention is given to schemes which reduce energy use. In some office space cooling ser- vices, the cooling function may be considered noncritical and subject to a low initial and operating cost design concept. In other applications such as computer rooms and electronics manufacturing, the product may have such high value and the quality of product may be so sen- sitive to environmental conditions that no expense will be spared to provide reliable cooling. Interestingly, systems which have low operating cost may be quite reliable because it takes better equipment and better arrangements to operate with less energy input, assuming proven technology in the equipment design. There are several key factors in designing a quality chilled water plant: Ⅲ Well-configured chiller(s) Ⅲ Efficient pumps Ⅲ A good piping scheme with ample valving Ⅲ A good control concept Ⅲ Good access for maintenance and replacement Chillers as a piece of equipment are discussed in Chap. 9. Pumps are discussed in Chap. 6, as are several piping schemes. There is an old saying: ‘‘Pump out of a boiler and into a chiller.’’ While this is not a hard-and-fast rule, it has some basis in good prac- Design Procedures: Part 5 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 200 Chapter Seven Figure 7.5a Constant (or variable) volume system without control valve. tice. Pumping out of a boiler places the boiler at the pump suction, which is the lowest pressure point in the system. This allows any dissolved air to work its way out at that point. It lets the boiler be designed for and work at no more than the fill pressure of the system. Pumping through the chiller makes the chiller, which typically has a relatively high (10- to 20-ft) pressure drop, the first pressure-drop device in the system. This reduces the remaining pressure throughout the system. Chiller heat exchangers (tube bundles) are usually rated for 150 lb/in 2 gauge working pressure and are not threatened by the condition. 7.7.1 Central plant piping configurations for water The design challenge in the central plant arrangement is to deliver service to the distribution system while operating the plant as effi- ciently as possible. Variations in load have more impact on chillers than on boilers, so the following discussion will concentrate on chiller plants. Most that is said also applies to heating plants. Ⅲ The simplest chilled water system (Fig. 7.5a) is that of a single chiller with a chilled water circulating pump connected to a single load. The system can operate without a control valve at the load, if the load control point sensor is used directly to control chiller load- ing; or, a variable-speed drive on the pump may be used with speed controlled by the load sensor. Chiller flow rate variations down to 50 percent or less of maximum are possible. Chillers are also avail- able with variable-speed drives for capacity control. Variable speed Design Procedures: Part 5 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. [...]... reserved Any use is subject to the Terms of Use as given at the website Design Procedures: Part 5 Design Procedures: Part 5 221 ‘‘Steam Systems ’; Chap 11, ‘‘District Heating and Cooling’’; Chap 12, ‘‘Hydronic Heating and Cooling System Design ’; and Chap 14, ‘‘Medium and High Temperature Water Heating Systems. ’’ 2 ASHRAE Handbook, 1999 HVAC Applications, Chap 33, ‘‘Thermal Storage.’’ Downloaded from Digital... McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Design Procedures: Part 5 Design Procedures: Part 5 219 Figure 7. 17 Engine driven cogeneration configured flow diagram (P&ID) for the proposed system with subsequent design for each component Detailed plant design is beyond the scope of this book Cogeneration plants require a substantial feasibility study,... the website Design Procedures: Part 5 Design Procedures: Part 5 205 pumps yields the signal to add or remove chillers from service Note that a one-to-one relationship between pump and chiller means that failure of either element makes the combination unusable The reliability of these systems can be increased by using an additional header between the pumps and the chillers (Figs 7. 7 and 7. 8) This allows... use is subject to the Terms of Use as given at the website Design Procedures: Part 5 Design Procedures: Part 5 211 Many water-based and some ice-based thermal storage systems wind up being ‘‘open,’’ or unpressurized This creates problems of water treatment, air venting, and possibly increased pumping head 7. 8.1 Sizing of thermal storage systems Thermal storage is typically sized on the basis of ton-hours... Chiller-based systems have been designed from 50 tons up to several thousand tons The technology for such systems is more than fifty years old See Figs 7. 13a and b for a simplified diagram of one variation of a chiller-based heat recovery system that uses deep wells as a heat source and as a heat sink 7. 10 Central Plant Distribution Arrangements The schematic plant arrangements in Figs 7. 1 through 7. 8 are... to the Terms of Use as given at the website Design Procedures: Part 5 Design Procedures: Part 5 203 Figure 7. 6 Multiple chiller plant with pressure bypass Ⅲ With the advent of low-cost, reliable, variable-speed pumping ca- pability, a chilled water plant scheme has developed which is becoming a favorite in the industry The concept is illustrated in Fig 7. 7 The plant is set up in a loop with one or... have energy implications, but are left to another discussion The HVAC systems designer will recognize plants as potential areas of specialty experience as assignments and interest allow References 1 ASHRAE Handbook, 2000 HVAC Systems and Equipment, Chap 7, ‘‘Cogeneration Systems ’; Chap 8, ‘‘Applied Heat Pump and Heat Recovery Systems ’; Chap 10, Downloaded from Digital Engineering Library @ McGraw-Hill... related to secondary system load Figure 7. 9a Secondary pumping with hydraulic isolation Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Design Procedures: Part 5 Design Procedures: Part 5 2 07 Figure 7. 9b shows a secondary pumping scheme... Figure 7. 14 Central plant serving a two-pipe distribution system Design Procedures: Part 5 216 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Figure 7. 15 Central plant serving a three-pipe distribution system Design Procedures: Part. .. (see Fig 7. 10) or, for greater reliability, can be headered and cross-connected so that each chiller can relate to two or more pumps and two or more cooling towers See Fig 7. 11 Some systems have been designed to maintain constant flow on even a large scale by putting chillers in series This allows chillers to stage on and off, but incurs the high cost of constant-flow pumping 7. 8 Thermal Storage Systems . 191 Chapter 7 Design Procedures: Part 5 Central Plants 7. 1 Introduction The design and construction of central plants for. given at the website. Design Procedures: Part 5 2 07 Figure 7. 9b Secondary pumping with three-way valve (not recommended). Figure 7. 9b shows a secondary