That flexibility has shown that in some instances, active cooling into the slab may shift upward of 12 hours separation from the time of peak load in the space, while still keeping the space operative temperature with the comfort range expected by ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy. If the flow rate In the secondary side is low then some chilled water will flow into the secondary and some will recirculate back to the chillers. The distribution system piping ranges in size from 6 inch up to 36 inch. Although cooling tower cost goes up as the cooling tower approach decreases, each project team should evaluate the cost benefit analysis to select close approach towers in the 2°F to 3°F range. The components of closed and open hydronic systems. This is typically limited to inputting project specific equipment performance data into the control software, which will, in turn, sequence a specified number of chillers, cooling towers and pumps based on operational “sweet spots” to meet building load. Chilled-water distribution system Chilled water is circulated through fixed piping —most commonly steel, copper, or plastic —that connects the chiller with various load terminals. 0 Courtesy: SmithGroup. This may leave the system able to operate at 100% economizer mode only when wetbulb temperatures are at or below 36°F. This chilled-water is then circulated through the entire building by the use of a pump. Review how and when to deploy a heat recovery chiller. HRCs can be used to meet or supplement the local building heating requirements while utilizing the benefit of the simultaneous cooling to cool the building or export this by-product to the campus chilled water distribution loop. The greatest impact on the cost of a large central chilled water system is provided by the temperature spread between the supply and return, usually termed the Delta T The central system designer seeks to achieve a large Delta T 14 [degrees]F or 16 [degrees]F are common. Chilled water is distributed to campus buildings through a single networked system consisting of parallel supply and return piping laid out in a grid configuration. In a two-pipe system, there are only two pipes connected to the air handlers, chiller and boiler. Campus-Wide System:A campus-wide central chilled water system has been developed in recent years and is being expanded to serve additional cooling loads as need arises and opportunity affords. The other is distributing hot water to the units and return- ing the water to the boiler. Figure 3: In Oakland, Calif., a 100% waterside economizer system can produce 55°F chilled water without the chiller operating. For example, in a scenario where two chillers are operating, the software may sequence three chilled water pumps online where traditionally there may only be two. A heat recovery chiller is designed to provide both heating hot water and chilled water. Operating the variable primary pumps in parallel to match the optimum efficiency point on the chilled water distribution system curve is an effective way to minimize the system pumping energy. Strategies such as chiller plant optimization, water side economizer and heat recovery chillers can create positive results by improving overall plant efficiency and reducing energy costs. To truly understand the benefits of chiller plant optimization, it is recommended to complete a baseline analysis of the existing system or new installation to help validate the benefits to system performance. Hospitals typically have large variable air volume air handling units that provide cooling and dehumidification and deliver air at a temperature of approximately 55°F. In the summer, the two-pipe HVAC system is manually switched to cooling, and in the winter, the system is switched to heating. 2. For example, referencing strategies in ASHRAE 90.1, this could mean using pumps with integral VFDs for a variable flow system or using chilled water reset in a system with integrated waterside economizer as described in the section below. A standard water-cooled chiller operates to remove heat from a chilled water loop and transfers that heat into a condenser water loop. Instead of operating in the 42°F to 44°F range, these systems tend to operate around 54°F to 58°F and supply radiant cooling systems, chilled beams or sensible only DOAS fan coil boxes. Maximum and minimum chilled water supply temperatures. Traditional chilled water systems producing 42°F to 44°F chilled water will be limited in how many hours they can take advantage of 100% waterside economizer, especially when the engineer has specified a traditional cooling tower approach of 6°F to 7°F and required a plate and frame heat exchanger with its 1°F to 2°F approach. The chilled water distribution system includes more than 6 miles of are direct-buried, ductile iron pipes up to 30 inches in diameter. Designing chilled water systems Typically used for cooling and dehumidifying a building’s air, chilled water (CHW) systems circulate it throughout a building or campus complex. The next level of optimization is through standalone software packages, which operate in the background using proprietary algorithms and work in conjunction with the building management system. The University’s three energy systems, Chilled Water, Cogeneration (producing steam and electricity) and Electric Distribution, are closely interrelated and operate together for optimal efficiency. 1. After completing the course, you should know: 1. The chiller is the section of the system where an exchange of heat occurred between the water that goes to the building and the evaporator. While six-pipe, dual-condenser heat recovery chillers are available, this discussion focuses on four-pipe, single-condenser heat recovery chiller applications. Understanding the actual building load so that equipment can be right-sized is critical. However, because of the role the chilled water system plays in thermal comfort of the building occupants, those potential energy savings strategies are not always pursued in favor of traditional approaches. As soon as all chilled water valves are less than 100% open, the BMS will linearly reset the chilled water supply temperature upward until the first valve must open 100% to satisfy the local load. When the pumping capacity matches the thermal load, it increases the temperature difference between the chilled water supply temperature and chilled water return temperature. 6. 3300 0 obj <>stream The first strategy is a chilled water supply temperature reset control sequence (ASHRAE 90.1-2019 Part 22.214.171.124), which should be deployed on all waterside economizer systems. For example, a centrifugal chiller with multiple compressors having the ability to stage them on and off based on operating at the lowest kilowatts per ton possible. There are different levels of optimization currently being applied in the industry ranging from simple sequencing of the equipment to the installation of electrical usage metering to enable system adjustments in real time through software. The basic concepts of piping system design. The chilled water distribution system must be evaluated before a new chiller plant design or existing chiller plant upgrade can be finalized. temperature. Chilled water is centrally produced and distributed throughout the campus, and this district cooling system shall be utilized wherever possible. Terminal equipment, proposed or installed. Establishing a baseline is an important aspect of this process especially as it relates to return on investment as there is a premium associated with chilled water plant optimization. A heat recovery chiller that is sized to provide the terminal reheat load during summer operation can offset the reheat load entirely while also providing chilled water and reducing the demand on the main chiller plant. The system incorporates “closed loops” to allow feeding areas from two directions. Select chilled water temperatures, flow rate, and primary pipe sizes; 3. Courtesy: SmithGroup. Pressure drop is overcome by the use of a chilled-water pump. Bypass line … An important aspect to note is owner and plant operator buy-in to the software to allow it to operate as intended. 3289 0 obj <>/Filter/FlateDecode/ID[<830717ED5C163C4C835482A908264B81><137A40009229F2459A784D5A3B14A5AF>]/Index[3277 24]/Info 3276 0 R/Length 70/Prev 1030250/Root 3278 0 R/Size 3301/Type/XRef/W[1 2 1]>>stream The chilled water distribution system must be evaluated before a new chiller plant design or existing chiller plant upgrade can be finalized. As system load and flow increase, the excess flow rate (from supply to return) in the bypass line decreases. Minimum flow rates must also be maintained within the condenser section of the chiller. What this means for a chilled water plant, as dictated by ASHRAE Standard 90.1 and the International Energy Conservation Code, is controlling the associated equipment, whether new or existing, to operate as efficiently as possible and ultimately consume the least amount of energy, while meeting the building needs. Even with the potential concerns, variable flow in the condenser water system is still a viable option and can further reduce the overall kilowatt per ton of chiller water produced throughout the entire range of plant operation. How Chilled Water Air Conditioning Works. Alterations in the existing distribution system are required in many chiller plant upgrades and they should not be overlooked in the proper design of an upgraded plant. These increased supply water temperatures greatly increase the available hours for 100% waterside economizer, showing economizer hours with a traditional approach cooling tower (see Figure 3). This would happen because three pumps operating at a lower frequency may use less energy that two pumps operating at 60 hertz. How to match pumps to systems. 3277 0 obj <> endobj Select cooling tower type, speed control option, efficiency, approach . Typically, heat recovery chillers are designed to provide hot water for space heating at 105°F to 110°F. Another application such as service water preheating normally uses heat recovery water temperatures of 85°F to 95°F. Evaluating the existing chilled water coils at varying chilled water supply temperatures is required to determine if the coils must be replaced or what temperature differences can be achieved with the existing coils (see Figure 1). The real beauty of waterside economizers is on display when they are paired with mild temperature chilled water systems. Select chilled water distribution . For many buildings, the chilled water system provides tremendous potential for creating energy savings. Once the chilled water distribution parameters are understood, the chilled water pumping arrangement can be designed. Care must be taken when reducing the flow in a condenser water system to avoid suspended solids from settling out in the system. A variable primary flow pumping system is typically the most energy-efficient system and provides the benefit of fewer pumps in the system. Campus Chilled Water System:UNK, UNL and UNO campuses are served by central utility plants operated by the Utility and Energy Management (UEM) department. A holistic approach to optimizing chilled water and air distribution systems. Changing the three-way control valves to two-way control valves and evaluating the use of two-way pressure independent control valves will solve many of these distribution issues. There is the obvious reduction in energy usage, which directly translates to dollars saved with the utility company. So, the elevated chilled water temperatures are ideal. When specifying a heat recovery chiller, it is important to consider the baseline heating and cooling load profiles of the building to properly size the heat recovery chiller. Space heating systems are normally designed at 140°F supply water temperature. Waterside economizer uses the evaporative cooling capacity of the cooling tower to produce cold water that is exchanged through a heat exchanger to provide chilled water that offsets the need for mechanical cooling. In climate zones without significant year-round high relative humidity, integrated waterside economizers can provide significant energy savings by reducing the hours of operation of chillers and by reducing the chiller load during hours when 100% economizer isn’t possible. 16 Chilled-Water Distribution System. Learn about the impact of pumping schemes and plant optimization of chilled water systems. This 400-ton system includes two water-cooled chillers piped in parallel. The 4-pipe water distribution system is actually two, 2-pipe systems in parallel; each system consisting of its own supply and return main. Chilled Water Distribution*. The benefits of waterside economizers increase with warmer chilled water supply temperatures, so they pair especially well with hydronic systems such as radiant cooling, chilled beams and dedicated outdoor air system fan coil boxes, where air-side economizers are either not applicable or not feasible. Two-pipe HVAC chilled water systemsare reasonably energy efficient. Chilled-water pump; 17 Distribution piping; 18 Pumping arrangements; 19 Condenser-Water System ~ Cooling tower ~ 20 Condenser-water pumping arrangements; 21 Unit-Level Controls; 24 Application Considerations. These can be in many forms when they are connected to a chilled water distribution system. When you pair these systems with close approach towers, you can see dramatic increase in hours of full economizer mode. If the building has been designed to be a low-load, high-performance building, teams may be able to install sufficient thermal storage to remove the need for chillers altogether to meet the sensible building load. To counter this overcooling, terminal reheat is required. A traditional chilled water design approach in a building with high internal loads, such as an office building results in a low percentage of operating hours that can be used for 100% economizer mode. This typically involves the installation of electrical energy usage meters for real time data collection in determining equipment sequencing as well as implementing predictive actions based on the software algorithms. Chilled water can also flow directly to terminal units located in the occupied space. This allows the software to sequence the equipment so it can operate most efficiently for longer periods of time throughout the year, thus providing a greater overall percent reduction in energy usage. Significant pressure drop differences in the chilled water piping distribution loops. At the point that system flow rate exceeds chiller flow rate, deficit flow. Figure 2: In this waterside economizer system diagram, when the economizers are optimized alongside each of these influencing systems, then the potential benefits of waterside economizing increase.
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