A LL -W ATER S YSTEMS
8.7 PIPING DESIGN CONSIDERATIONS
Several issues are of special importance to the design of water distribution networks for all-water systems.
8.7.1 Flow Diversity
The system (block) peak cooling load is likely to be less than the sum of the space peak loads. If the cooling capacity of the ter-minal units is automatically controlled using two-way valves, it is improbable that all valves will be fully opened concurrently. Conse-quently, some reduction from the sum of the individual terminal flows is possible in selecting pumps and sizing pipe mains. In the-ory, the diversity factor may be viewed as the ratio of the block cooling load to the sum of the peak loads for each terminal. Such a theory is based upon the supposition that all space thermostats are set at the design space temperature and that flow is reduced in direct proportion to terminal load reductions. Neither is correct because other operating conditions also affect the degree of diversity. A gen-eral rule is to reduce the aggregate flow of all terminals by a diver-sity factor equal to the square root of the ratio of the block load to the sum of the peaks. Diversity should not be applied to risers or horizontal mains serving only a single building orientation.
8.7.2 Water Pressure and Flow Balance
Balancing the water flow through a large number of parallel-connected coils is at best difficult and laborious. With reasonable care in design, however, a water distribution network can be suffi-ciently self-balancing to make field adjustment of terminal coil flow generally unnecessary. Inclusion of automatic flow control valves can also move a system toward a self-balancing condition.
Chilled-water flow variations within 25% of design flow affect a fan-coil unit’s cooling capacity on the order of 10%. This provides considerable tolerance. Consequently, some difference in the flow resistance of the piping circuits conveying water to the many termi-nals is tolerable without significantly affecting the performance of terminals situated on the longer runs. Two conditions, however, must be met:
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1. The flow resistance in the parallel terminal subcircuits is at least equal to 40% of the variation of the pressure drops in the distribution piping circuits, i.e., the difference between the greatest and the least pressure drop in the circuits.
2. The pressure drops of the parallel subcircuits have reasonable consistency, i.e., where possible, avoid mixing high and low loss terminals on common circuits. A subcircuit includes the control and manual valves, the connecting piping, and the ter-minal unit coil.
A closed piping loop may be designed as either direct return or reverse return (see Figures 8-5 and 8-6). The direct-return system is popular because it requires less piping; however, balancing valves may be required on subcircuits. Since all water flow distances are vir-tually the same in a reverse-return system, balancing valves require less adjustment. It is possible, with careful piping layout, to provide many of the advantages of a reverse-return piping system while keep-ing pipkeep-ing lengths similar to a direct-return pipkeep-ing system.
Quantitative evaluation of the flow variations that a circuit imbalance creates is beyond the scope of normal system design pro-tocols. As a general guideline, an imbalance of 10 to 15 ft (30 to 45 kPa) of head is safe. This latitude allows the use of direct return risers; the imbalance can be controlled by judicious pipe sizing.
Figure 8-5. Direct-return piping system.
Figure 8-6. Reverse-return piping system.
Reverse-return design of horizontal mains looping within a building perimeter and feeding risers usually does not incur significant addi-tional piping expense over direct return and is good practice. Mains that serve major sections of a building and are parallel connected with each other should be provided with a means for balancing and flow measuring.
8.7.3 Control of System Pressure Differential
Where two-way valve control of water flow through terminal unit coils is employed, the system flow will be reduced at partial load. The valves reduce flow by adding resistance to the coil piping subcircuit. As flow is reduced, pipe friction also decreases and available pump head increases, requiring even further closure of the terminal valves. If the differential pressure imposed across these valves becomes great enough, the maximum shutoff pressure rating of the valves could be exceeded. The valves would not close, defeating space temperature control. Barring that, “wire drawing”
at the valve seat is a real possibility, which could lead to costly replacement of the terminal valve seats. Wire drawing involves ero-sion of the valve disk and seat by high fluid velocities in valves that operate for extended periods at minimal opening. Several preven-tive measures are available to a designer:
1. Control the system pressure differential with taps in the supply and return piping, usually located at the hydraulically most remote terminal device in the piping network, to vary the speed of the pump, reducing its capacity and developed head at part load, as described in Section 8.8.
2. Regulate a throttling valve in the supply mains (near the pump discharge) from a system pressure differential controller (in lieu of speed control). This shifts part of the task of restricting flow from the terminal valves (see Figure 8-4) by maintaining an artificial head on the pump.
3. Install three-way valves at the terminals in lieu of two-way valves. At reduced load, the valves bypass the coils. This incurs some disadvantage since these bypasses are, in effect, short cir-cuits unless a resistance equal to the terminal water coil is built into each bypass—which can be accomplished with automatic flow control or balancing valves located on the return line after the bypass valve. Without this, at partial system load, system flow will actually increase, as will pumping horsepower. Also, some terminals may be starved of water, since the pressure dif-ferential at those terminals may fall below that needed to
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deliver the water flow desired by the space thermostat. The installed cost of three-way valves is somewhat higher than for two-way valves. Some designers favor a single bypass valve, located near the extremities of the mains, that is controlled by system pressure differential. The effect is similar to a main throttling valve but can have the effect of increasing system flow and horsepower. A spring-loaded relief valve is unsuitable for this purpose; an industrial-quality control valve is required.
4. Install a combination of two- and three-way terminal valves.
This practice limits the reduction of flow and the buildup of pressure differential across the terminal valves without a need to control flow velocity, main throttling valves, or a bypass valve. A little study will indicate the percentage mix, but a lay-out that ensures a minimum of ablay-out 20% of the total system flow generally suffices. Locating the three-way valves in the greater-resistance paths ensures good flow throughout the extremities of a direct-return system.
5. Use automatic flow-limiting valves (also called pressure-inde-pendent control valves). These valves are designed to provide consistent coil flow conditions in the face of varying system pressures (caused by the opening and closing of valves throughout the water distribution system). Such valves can be especially beneficial in two-pipe distribution arrangements, where water flow rates differ from heating and cooling service at any given load. They also permit easier balancing of direct-return distribution systems.
If a system is equipped solely with two-way valves, a no-flow condition can occur at the pump. Serious damage to the pump could result when the motor energy heats noncirculating water within the pump impeller. Prevent this either by adopting a pressure control option that will ensure flow or equip the pump with a bypass.
Another potential hazard relating to the use of two-way valve con-trol is the freezing of sections of the distribution piping if exposed to very low temperatures (possible in poorly insulated exterior wall cavities under no-flow conditions).
8.7.4 System Cleanliness
The performance of an all-water system can be seriously impaired by dirt and corrosion products in the piping system. The following safeguards avoid this problem:
• Protect piping material on the job site prior to installation.
• Protect openings in the piping during installation.
• Flush and chemically clean the piping system prior to operation.
• Locate strainers upstream of pumps, chillers, and heat exchangers.
• Install scavenging (side-stream) filters in pump bypasses selected for 5% to 10% of system flow.
• Provide isolation valves and accessible drain connections for risers.