How is vacuum affected by atmospheric pressure in a steam condenser?
Posted on : 15-08-2010 | By : cherexposed | In : steam vacuum reviews
Also i need information about vaccum trouble and it’s solve
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Actual vacuum in the condenser is not affected by atmospheric pressure. However, the instrument used to measure the vacuum maybe affected by (a change) in atmospheric pressure. In the case of a bourdon tube type of gage the vacuum exerts its force on the inside of the tube and the atmosphere exerts its force on the out side of the tube. As atmospheric pressure increases the condensers vacuum would also seem to increase (become more negative or closer to perfect = 30 inches Hg, or 0 psia). Of course the reverse is true if atmospheric pressure decreases.
in steam condensers absolute pressure is still positive, however indicated gage pressure will vary depending on atmophearic pressure. The higher the atmosphearic pressure for a given "vacuum" the higher the indicated vacuum is. Normally bourdon tube type gages are used for vacuum indication, however bellows type pressure indications such as transducers still are affected by ATM pressure unless designed to be unaffected and contain sealed constant pressure chambers, but those are expensive and no one uses them.
When steam is condensed in a closed system, the pressure automatically comes down to vacuum.
e.g. If steam is condensed at 66°C by cooling water, the pressure will automatically come down to about 27"Hg vacuum (3"Hg Absolute), due to the huge change in volume from steam to water.
However, when steam is condensing, some dissolved air is present which of course, does not condense.
This will build up and destroy the vacuum.
A barometric (surface) condenser has a double (primary & secondary) ejector system that ‘pulls out’ the uncondensables and maintains the vacuum. The primary ejector condenser, has a ’seal-loop’ (about 34 feet of water) connected to the water outlet and connects to the surface condenser , the gases will build up above the water in the primary condenser and these are ‘pulled out’ by the secondary ejector set into its condenser that again has a seal-loop.
The gases again, build up above the water and, as their pressure builds up to just above atmospheric, a check valve opens to vent the gases to atmosphere.
Any mal-function of the ejectors and/or their condensers, or any air getting into the surface condenser by way of the turbine shaft seals, shaft leak steam system, condensate pump gland, control and other valve glands, poor gaskets in any associated pipe-work, depending on the rate of air ingress, will destroy the vacuum.