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The arrangement of the piping of the chilled water in any project is subject to alter-native approaches as a function of the experience of the designing HVAC engineer, local practices that have developed in the area within which the project is located, and the needs of the project. There are basically two alternative approaches that find applica-tion in tall commercial buildings. Either of these two basic alternatives is subject to variation by the design engineer, but any specific solution will be a modification of either of the basic concepts.

The first arrangement is one in which the pumps that are associated with the refrig-eration machines also distribute the chilled water to the cooling coils and other heat transfer equipment requiring chilled water that is installed in the project. A flow dia-gram of this arrangement is shown in Figure 7-1. This figure shows three chillers from three refrigeration machines. Each machine will handle one-third of the total load in the building. It is common on many projects that only two machines will be provided, each handling fifty percent of the total calculated load. It is also not unusual to include four

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machines. Two could be rated at one-third of the total calculated load and two rated at one-sixth of the load. This will provide machines for operation at light loads such as for the overtime needs of a limited data center within a large building. The number of machines and their relative capacity as a function of the total load for a project is a judg-ment that must be made by the design engineer as a function of the needs of the building and its usage in overtime and on weekends where there may well be partial occupancy.

It is not usual to provide spare refrigeration machines in most locations where service from the manufacturer or other service agencies and spare parts are readily available. In parts of the world with more limited access to service and parts, it is prudent practice to include a spare machine as well as an inventory at the job site of spare parts for the machines that would be provided by the refrigeration machine manufacturer as part of a response to the project specification.

In Figure 7-1, in addition to the three machines, there are four chilled water pumps.

Each of the chilled water pumps is selected for the rated flow through each of the chill-ers. If the control of the flow of chilled water at the heat transfer equipment in the project is effected by two-way control valves, which is usually the case, the pumped amount of chilled water will vary with the cooling loads in the building. The pumps, therefore, will be variable-flow pumps and will require variable-frequency drives. In addition, the pumps are piped in parallel, as are the chillers, so any machine can operate with any of the pumps. This provides pump redundancy in the event of a pump failure for any reason. The inclusion of the spare pump is relatively common, as pumps will be down for service or repair in a random fashion that cannot be coordinated with the needs of the project or the service requirements of the chillers.

While not shown, the refrigerant condensers on the refrigeration machines would be piped in a similar fashion where four pumps are provided with three machines and any pump can be used with any of the three machines. These condenser water pumps, Figure 7-1. Direct chilled

water pump distribution to cooling loads.

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however, will not have their flow change with load so they will not require variable-fre-quency drives.

The second common arrangement consists of primary and secondary pumps as shown in Figure 7-2. In this arrangement, contrary to the arrangement in Figure 7-1, each chiller is operated with a dedicated primary pump that will operate at constant speed and constant flow. It is possible to pipe both the chillers and the pumps in paral-lel, adding a spare as is the case in Figure 7-1.

The variable-speed secondary pumps shown in Figure 7-2 distribute the water to the chilled water coils installed in the air-conditioning equipment as well as the other heat transfer equipment that is required for the project. Proponents of this arrangement point to the fact that the flow through each chiller is constant and will not vary, as the control valves on the cooling coils and heat transfer equipment reduce the chilled water flow when the cooling load on the coil or on the equipment is reduced. Most chiller manufacturers will stipulate a maximum velocity through the cooler of the chiller, which is usually 10 fps (3m/s), but will also require that the flow not be reduced below a stated minimum velocity, which will be approximately 3 fps (1 m/s). The piping arrangement in Figure 7-2 will ensure that the flow is constant and eliminate any possi-ble flow propossi-blem.

In the arrangement of Figure 7-1, a bypass bridge could be required at the pumps when cooling capacity control at each piece of heat transfer equipment is being achieved by two-way throttling valves. Any bypass bridge that would be required in the arrangement shown in Figure 7-2 would be at the secondary pumps. In either case, Figure 7-2. Secondary

chilled water pump dis-tribution to cooling loads.

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the system under light load will pump more water than is needed at the heat transfer equipment, which will necessitate the inclusion of a bypass bridge to relieve the excess water being pumped. The designs being implemented today, however, usually include variable-speed pumps. With this design, the flow will tend to be proportion-ate with the load, eliminating the need for a bypass bridge. Accordingly, in both Fig-ure 7-1 and FigFig-ure 7-2, the bypass bridge is not shown and will not be required if variable-speed pumps are employed.