Chillers and Their Pumps in Software

Drawing QR Code ISO/IEC18004 in Software Chillers and Their Pumps

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Primary pump location for high-rise buildings (From The Water Management Manual, SYSTECON, Inc, West Chester, Ohio, 1992, Fig 821C,D)

149 Connecting Multiple Chiller Plants A common and desirable situation encountered on campus-type installations and very large building complexes is to connect several chiller plants together to serve the chilled water requirements of all the buildings or zones This provides significant advantages over the arrangement of individual chillers for each building; these are (1) standby capacity is provided to all buildings, (2) operating chillers on very low loads is avoided because one chiller can handle the minimum loads for a number of buildings, and (3) by connecting the chillers together, the ability to reduce the chiller plant size is often realized due to the combined large load diversity of all the buildings This increases greatly the efficiency of chiller operation Figure 1410 describes such an arrangement This installation has four buildings with chillers and two without The buildings without chillers are new; there is no desire or need to install chillers in these buildings because there is more than enough excess chiller capacity in the other buildings to serve these two new buildings To utilize this excess capacity, it is necessary to install a campus supply and return loop The following are other design parameters that should be followed for such a facility: 1 No new central pump stations are to be used 2 A minimum of energy-wasting devices is to be used 3 Any building must be capable of being served by any of the chiller plants 4 The central control center operator must have the ability to select the most efficient chillers without changing any valving in the buildings 5 The flow in any chiller evaporator must not be restricted 6 If a chiller fails, any building load must be picked up automatically without any changes in coil valve position 7 Flowmeters must be installed in the loop bypass and in each building to determine individual building loads and to indicate the amount of excess capacity occurring in the bypass A temperature transmitter also must be located in the bypass to indicate reverse flow that should not occur under normal operation The building connections are described in Fig 1410a for buildings both with and without chillers The chiller pumps are constant-speed pumps, and the building pumps are variable-speed pumps The building pumps are controlled by differential pressure transmitters located

Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2006 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website

Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2006 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website

Multiple chiller plant connections (From The Water Management Manual, SYSTECON, Inc, West Chester, Ohio, 1992, Fig 69)

properly in each building The flow transmitters also provide the central control operator with data on each building s load and the flow from the supply loop that is occurring in each building The central bypass, as described in Fig 1410b, is necessary to ensure that the flow in any chiller evaporator is not reduced Sudden changes in campus load could drastically reduce the flow in a chiller evaporator and cause possible freezing in the tubes without this bypass Questions often arise on how to calculate the head for the chiller pumps and the building pumps as well as sizing the supply and return loops Both the chiller and building pumps must have the capacity to pump to and from the loop bypass under the most diverse load conditions that can occur on the entire installation Likewise, the design engineer must evaluate all the possible load conditions and select the one that requires the largest flows in the loops This will enable the system designer not only to select the head for both the chiller and building pumps but also to size the loop piping In actual operation, the normal load on the loops is much less than the maximum possible load This explains the need for the variable-speed building pumps If there is a great head variation on the chiller pumps, they also should be variable-speed pumps with differential pressure transmitters across the evaporators to maintain constant flow through each evaporator If the variation of this flow is expected to be moderate, the chiller pumps may be constant-speed pumps with an automatic flow control valve on each chiller The maximum head for the chiller pumps and that for the building pumps must be checked to ensure that the chiller or loop head never approaches that of the buildings Otherwise, the chiller pumps have the possibility of overpressuring the building pumps If there is a possibility of this, a pressure gradient should be developed for the condition to verify the pressure relationship If designer determines that significant energy can be saved by pumping a building with its chiller pumps, the building pumps should be equipped with a bypass This enables the building pumps to be shut down when adequate pressure is developed by the chiller pumps There are several advantages to this method of connecting multiple chiller plants over those systems which depend on computerization of flow and temperature transmitters to interconnect the chiller plants: (1) the central bypass ensures that adequate flow will be provided for all the chiller evaporators that have been placed in operation, (2) the system can operate with the central computer totally shut down, and (3) the central bypass method is much simpler, less expensive, and more reliable than the computerized control system

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