Input and BudgetsAs a designer, it is important, at the start of a project, to get input from the owner. Even a modern, well-planned plant has the risk that the user may not buy into the design, if he/she is not involved in the process. Begin getting input by reviewing construction documents with clients, conducting review meetings, visiting existing facilities with similar equipment, and talking with other facility personnel. Develop a project team that includes the clients and work with them throughout the project. This should include new potential users (operators).
During the onset of planning, define a budget. The budget should be determined from a conceptual cost estimate. The conceptual cost estimate is determined during the master-planning phase. There are several ways to determine a conceptual cost estimate. Some rule-of-thumb numbers for chiller plants are featured in Table 1.
These numbers can vary by equipment size, amount of chilled water the distribution system requires, demolition requirements, constructed space for future equipment, new building type, and more. A more accurate way to determine a conceptual cost estimate is to get budget quotes from equipment vendors and estimate the other part of the plant from a cost estimating book or computer program. This is done after determining the specific equipment that you are considering and/or evaluating and having the experience to know what is required for a complete plant. When the project is in the design stage, the conceptual cost estimate should be based on material takeoffs from the plans and quotes for major equipment.
Space in campus buildings is money. Determine how much space is necessary for now, considering future demands. When making these decisions, use sound economics. Find the best buy for what is needed, not the cheapest. A central plant can free up space in the basement of a campus building that would be occupied by a chiller, pump, controls, and electrical equipment. The space may be used for storage, office space, or other purposes.
Location, Location, LocationOne of the first general arrangement concerns should be major equipment location. Consider the chiller location relative to the chilled water pump(s) and condensed water pump(s) early in the design stage. Locating the chiller on the first floor with the pumps located in the basement means that space above the chiller is free for maintenance.
This will be the case if the cooling towers are located on grade or if the cooling tower piping is routed through the operating floor along a wall, with cooling towers located on the roof. A problem with locating the chiller on the operating floor above the pumps is that the building superstructure is more costly.
Some existing chiller plants have the chiller located below the pumps. The piping between the chillers and the pumps makes it harder to perform maintenance. For larger chillers (1,000 ton and above), it is wise to leave clear space above the chiller and install a hoist beam, a bridge crane, or a portable-type crane for maintenance.
Plan for maintenance space around the chiller, including space to lay equipment next to the machine, such as compressors and tube bundle covers, while still leaving the maintenance crew room to work. Leave at least the width of a chiller between units, if possible. Also save room for tube pull space on one end of the chiller, if it is a chiller in which the tubes can be pulled from one end. There are some units that need tube pull space on both sides of the unit and this should be considered. Note this space as tube pull space on the construction documents.
If possible, allow for at least 4 ft of clear space around the nonmaintenance sides of the equipment. This may be difficult to maintain as the project progresses into the later stages of design, but it is good to start with this clear space.
Cooling tower location is critical. Locating the tower on grade may be the least costly, but this causes the tower to be highly visible and has the largest impact on the surrounding areas, with regard mostly to sound. If you locate the cooling tower on the roof, the cost of the building superstructure goes up due to the weight of the tower. To improve the aesthetics of a cooling plant with a tower located on the roof, a screen wall may be installed to hide the tower.
The location of major electrical equipment must also be considered. This includes transformers, switchgear, switchboards, and starters. Transformers and switchgear are heavy and the task of finding the appropriate locations is similar to chillers and cooling tower logistics.
Plan electrical space, considering the location of the electrical equipment relative to the equipment it serves, heat gain to the space with adequate ventilation, future equipment, replacing equipment, and size. Isolate electrical equipment in a separate room to avoid piping routed above electrical equipment and to isolate the heat gain of the electrical equipment from the plant. This is done so that the electrical equipment is provided adequate ventilation or cooling, if required. Locating transformers and/or switchgear outdoors solves some of these problems, but outdoor equipment may cause maintenance and aesthetic issues.
Choosing ChillersChiller types include electric-driven centrifugals; steam turbine-driven centrifugals; single-, double-, and triple-effect absorption; ammonia chillers; and gas-fired. With changing energy prices, it may be advantageous to have more than one type. A chiller plant with more than one type of chiller is called a hybrid chiller plant.
If planning a large addition or new plant, an economic analysis should be performed to determine the lowest life cycle cost. A typical economic analysis model includes capital costs, maintenance costs, current electric and thermal energy utility rate structures, and escalation rates, keeping in mind future energy prices. It is also a good idea to perform a sensitivity analysis on the future energy prices, as well as inflation rates. Chiller plant costs and economic issues are extensive. They are better left to another article.
Distribution SystemsThere are several pumping schemes to consider in a central chilled water distribution system. Primary only and primary/secondary are the two most common. They are the only schemes that will be discussed in this article. A brief description of these, with some typical problems and solutions follows:
Primary. In this type of pumping system, the chilled water pump(s) is utilized to pump water through the chiller and out through the associated chilled water distribution system then through the connected building and back to the pump. Supply and return temperatures for the chillers and coils typically are the same. The water flow may vary with the temperature drop constant (variable pumping) or the water flow is constant with the temperature difference varying (constant-speed pumping). A typical primary system has constant flow with variable temperatures. It should be noted that a system with variable flow with constant temperature difference is one of the most efficient when considering energy savings.
There are some typical problems associated with a primary system under light load conditions. If the system has a lot of three-way valves, the valve bypasses cold water back directly to the central plant under light load conditions. This can cause the chiller to operate in an unloaded state. Some systems have had to operate additional pumps and chillers to meet system flow requirements with small temperature differences (2°F) resulting in higher energy consumption.
This problem can be corrected by reducing the number of three-way valves. This usually requires that the valves be replaced with two-way valves. Variable-speed pumping may also be considered for additional energy savings. Minimum allowable chiller flow always should be considered.
Primary/Secondary. This system has two pumping systems. One for pumping water through the chiller and the other for pumping water through the distribution system. The systems are decoupled so the pumps operate in parallel and not in series.
Chilled water distribution systems typically have two- and three-way temperature control valves to control building or process temperatures. If three-way valves are not used, a system pressure relief device should be installed to protect the pumps.
HydraulicsDesigners need to be aware of system operating pressures to have accurate temperature valve sizing. Buildings close to the central plant will have a higher pressure than buildings at the end of the distribution. This should be considered when sizing building control valves.
For larger campuses and campuses with pumping problems, a hydraulic analysis often is performed. There are many good computer programs that are used to make this analysis easier. For a typical hydraulic analysis, know required building chilled water peaks, chilled water flow, required central plant chilled water peaks, and design chilled water supply temperatures to the cooling coil and system pump data. This should be done after you have determined what type of plant you are going to install during the design outline or study phase.
SoundSound barriers are primary to the design of a new cooling tower, especially if the plant is near a residential or commercial area. Cooling tower features that inhibit sound include variable-speed drives, extending fan shrouds, installing screen walls with a dead air space, and proper sizing.
Screen walls, insulated ductwork, and soundproofing materials also may be implemented to reduce sound levels at the plant. Some soundproofing materials include sound block, sound doors, and acoustical sound covers. Mufflers and silencers are good to use on fans, air-handling units, and motor-driven equipment to reduce noise. If your project has strict noise requirements, have a qualified designer on the project team.
Other ConsiderationsIt is a good planning tool to develop a project schedule during the master plan phase. The schedule should have the project design milestones, major construction events, and major equipment delivery dates. To improve the schedule and to have more control over the type of equipment purchased, it is a good idea to procure the major equipment in separate contracts. If the equipment is procured early enough, the design of the plant can be based on the actual equipment shop drawings. This helps eliminate construction coordination issues later in the project. The schedule should be updated throughout the project. The schedule will also become more detailed.
Early in the project, define dust control and cleaning requirements. Make arrangements with vendors about equipment deliveries. If possible, plan construction during the nonpeak season.
The utility tie-ins should be planned and scheduled, if a facility requires almost continuous utility services. If planned properly, this minimizes outages. Temporary equipment may also be used. It is wise to get the city and code officials involved early in the project. This can reduce design changes and construction change orders.
Ensuring that institutions are ready for Generation Y is an important task. Addressing the planning and design of cooling plants and other system design issues helps to ensure that facilities will be able to meet campus utility expansions. ES