Figure 614 Pressure pro le for out-and-back piping in Software

Printer QR Code ISO/IEC18004 in Software Figure 614 Pressure pro le for out-and-back piping

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water rst enters it, (4) the initial ll pressure (minimum operating pressure) at the tank, and (5) the maximum allowable operating pressure at the tank The initial ll pressure is determined by the system static head at the tank location plus an allowance to maintain a net positive head at the pump suction under all operating conditions Thus, if the tank is on the pump suction (Fig 621), a few feet of gauge pressure is suf cient If the tank is on the pump discharge (Fig 622), the minimum pressure must include the pump head plus a small margin For an attic tank (Fig 623), the extra static head (if the pump is in the basement or ground oor) will usually be suf cient The maximum pressure allowable is determined from the lowest-rated device in the system, most often the boiler or its pressure relief valve Because of this, it is preferable to pump away from the boiler

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The following ASME formula for determining the size of a closed system expansion tank is valid for temperatures between 160 and 280 F Vt where Vt Vs t Pa Pf Po (000047t Pa /Pf 00466)Vs Pa /Po (62)

minimum volume of expansion tank, gal system volume, gal maximum operating temperature, F pressure in expansion tank when water rst enters (usually atmospheric), ftH2O absolute initial ll pressure at tank, ftH2O absolute maximum operating pressure at tank, ftH2O absolute

For temperatures below 160 F, a simpler formula may be used:

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Figure 622 Tank at pump discharge, pumping into boiler (Not recommended)

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(63)

where E net expansion of the water in the system when heated from minimum to maximum temperature Expansion E equals the system volume times the percentage increase indicated in the graph of Fig 624 For chilled water, the minimum design temperature is used, combined with the maximum anticipated temperature during a summer shutdown Expansion tanks for high-temperature water systems are always provided with a cushion of inert gas (usually nitrogen) or highpressure steam, which is continuously maintained by an automatic control system with rapid response Maintenance of system pressure is critical to proper operation of the HTW system HTW systems are usually large and have a wide temperature difference from supply to return For details on handling expansion in HTW systems, see the relevant ASHRAE handbook chapter5

Some entrained or dissolved air is present in any piping system Air in excessive amounts can cause noise Air can also impede the ow of

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Figure 624 Expansion of water above 40 F (SOURCE: Copyright 1987, American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc, wwwashraeorg Reprinted by permission from ASHRAE Handbook, 1987 Systems and Applications, Chap 13, p 1314)

water in a closed piping system It is desirable to remove as much air as possible Air removal is based on two principles: (1) air will be entrained and carried along with the water stream at velocities in excess of 2 ft/s, and (2) air tends to migrate to high points in the system when ow is stopped The second principle is employed in the installation of air vents at high points in the piping system Air vents may be manual or automatic Automatic vents use oat valves or water-expansive materials to close the vent when water is present Under low-pressure conditions, automatic vents may allow air to enter the system Manual vents do not have this problem but do depend on regular operation by maintenance personnel Always provide a drain line from each air vent, to prevent damage if water is carried over The rst principle is utilized in air separation devices The most common is the centrifugal separator (Fig 625) This consists of a large vertical pipe section which the water enters tangentially near the bottom The combination of centrifugal force and decreased velocity separates the entrained air, which is removed through a vent at the top center of the separator The separator is usually vented to the expansion tank air cushion, although it can also be vented to atmosphere, manually or automatically

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