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Posted by Steamer on June 11, 2005 at 07:12:27:
In Reply to: steam pressure posted by Tom Looft on May 18, 2005 at 20:03:17:
It's always good practice to run the boiler at the design pressure. Reducing pressure inside the boiler increases the size of steam bubbles and leads to carry-over of water highly contaminated with dissolved salts, with the consequent deposition on heat exchanger surfaces, steam traps and so on. Also if your system has batch productions, the boiler cannot cope with a sudden steam consumption increase due to his lower pressure, originating a tide with capacity for destruction of the plant with water hammer.
So in conclusion and in general lowering boiler pressure is not advisable.
But it's true that in the point of use the pressure should be as lower as possible to get energy savings. Let's have an example in round numbers:
Suppose you are heating 1000lb/min of water on a heat exchanger from 50ºF to 176ºF
To heat up this water you will need to provide from 9 672 371 Btu/hr.
As in the heat exchanger is normally used the specific enthalpy of evaporation (condensation) and we release the condensate at steam temperature, from steam tables you can see that at 150 psi the enthalpy is 857,582 Btu/lb so we will need 11225 lb/hr of steam (9 672 371 / 857,582).
If instead you can use the heat exchanger or a new one at 75 psi the enthalpy will be 895,16 btu/lb and you will only need 10754 lb/hr.
This will have a saving of 4.2 % on steam consumption and with a boiler running at 90% efficiency on combustion the savings will be of 4.67% on fuel. If you do not have a flash steam recovery system this will also provide an extra saving of 4.2% on compensating water.
Please note that this savings are only related with the costs of heating the flow of water in this example.
So in conclusion using the steam at the lowest possible pressure will save money.
The steam table values are from www.spiraxsarco.com/us. In Europe they provide a video called "Boilers- The inside information" were among several things you can see what happens inside the boiler when you lower the pressure.
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