ASHRAE Journal - September 2009 - 64

ASHRAE 19 drained from the piping so that it will not accumulate to cause noise and water hammer. This is why additional steam traps must be installed at condensate collection points along the length of a steam main. It is also a powerful argument for insulating steam pipes to reduce heat loss and condensate running load. The running condensate load is generally smaller than the warm-up load formed when the cold pipe is initially brought to operating temperature. 50 59–2009 YE A R S JOURNAL f i g . 5 Vacuum heating unit. Mechanical Vacuum Heating Systems The warm-up time required in a large steam heating system was a major inconvenience. For example, the engineer of a large school building had to arrive hours before the children to light fires in the boiler, raise steam pressure, warm up the piping, and ensure that the system was vented and drained so that the school would be warm at the start of class. The addition of a vacuum pump allowed the school engineer more time in bed because vacuum in the system promotes faster steam generation and faster, more even, steam distribution. A vacuum pump attached to the condensate piping removes much of the air from the condensate pipes, the radiators, the steam supply piping, and even the top of the boiler. When the boiler water starts to warm up, steam begins to form earlier because steam temperature and pressure are directly related. According to the steam tables, a drop in pressure from 0 psig (0 kPa) to 8 in. Hg (27 kPa) vacuum lowers the boiling temperature from 212°F to about 197°F (100°C to 92°C).1 64 ASHRAE Journal As steam forms, it can flow very quickly into the system because air no longer must be pushed out of the vents before steam can fill the radiators. Also, drainage of the heavy warm-up load is easier because vacuum in the condensate piping gives a larger pressure differential across the traps. Many vacuum pump designs were developed. However, one of the most reliable and quietest designs uses a standard centrifugal pump, venturi and special discharge valve so that a single pump can act as both a vacuum producer and condensate transfer pump. The vacuum unit in Figure 5 uses a cast iron receiver that is partitioned horizontally. The upper chamber is vented to the atmosphere, and always contains a supply of hurling water. The lower chamber is connected to the condensate returns by means of a basket strainer that catches any particles large enough to damage the pump. A vacuum switch (usually mounted on the strainer) senses rising pressure in the condensate returns as air leaks into the system and turns on the a s h r a e. o rg centrifugal pump. The pump takes suction on the hurling water, and discharges it through a nozzle and venturi back into the upper chamber. High velocity water creates a low pressure region (a vacuum) in the throat of the venturi, which is connected to the lower chamber. Air from the system is drawn into the lower chamber, then through a check valve, and discharged into the upper chamber where it is vented from the system. If condensate is present in the lower chamber, it too can be drawn into the upper chamber. A special discharge valve is held closed by pump discharge pressure to ensure that hurling water is always available in the upper chamber. As the water level rises in the upper chamber, this valve opens to divert condensate to the boiler. If vacuum in the system is within acceptable limits (usually 3 to 8 in. Hg; 10 to 27 kPa), then high levels of condensate in the lower chamber can also start the pump by means of a float switch. It is sometimes difficult to maintain vacuum tightness in large systems September 2009

ASHRAE Journal - September 2009

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