2 replies to this topic

[jimbobble]

I am trying to solve a dewpoint problem with a 600MW Generator. It uses hydrogen as a coolant. The dewpoint temperature of the incoming hydrogen is -65 degrees Fahrenheit. The machine dewpoint is at 23 degrees Fahrenheit. I would like to lower the dewpoint in the generator. The pressure is at 42psi. We have a hydrogen coolers in the machine. Question is If I increase the pressure to 50psi will it help to drop the dewpoint? Is it possible to lower the dewpoint at this pressure? Is there a chart out there to aid me in this process?

Edited by jimbobble, 29 June 2010 - 10:40 AM.

[Art Montemayor]

Jim:

Firstly, I have a problem with this thread’s title. I don’t think you meant to type in your job title, and really meant to call it “Hydrogen dewpoint reduction”. If you agree to this, let me know so I can change the title of the thread and thereby have it within our database retrieval system.

Secondly, I think we are dealing with a misconception of terminology with respect to the term “dewpoint”. This is common with electrical engineers who have to contend with chemical engineering terms and processes. That’s OK; all we need is to revisit the meaning of the term.

The dew point of hydrogen is the temperature to which a given volume of hydrogen must be cooled, at constant barometric pressure in order for any water vapor present in the hydrogen to condense into liquid water. The condensed water is called dew, and hence the term. The dew point, therefore, is a saturation point – i.e., it is the temperature at atmospheric pressure at which the hydrogen gas can hold no more water vapor. Therefore, the dewpoint is a measure of the water content of any gas and it is read in degrees of temperature (but always at a base pressure – atmospheric in this case). The dewpoint measurement in temperature can be converted to other units of measure, such as parts per million (mass), parts per million (volume), or lbs/cubic feet.

Within the power industry it is common practice to use hydrogen as a direct coolant for the generator stator windings. Hydrogen is used because it has a high heat transfer capacity and is more efficient at transferring heat than other mediums.

The re-circulation of hydrogen within an electrical enclosure removes heat from the generator, transferring it via a heat exchanger into a secondary cooling circuit which usually uses demineralized water. Often this de-mineralized water is then cooled either by sea water or river water. As it is not possible to hermetically seal the generator set castings, there is a potential for moisture to ingress from the surrounding atmospheric air through packings and gaskets. Similarly, as the heat exchanger becomes more porous with age, it will allow moisture to get into the hydrogen. It is, therefore, essential that the dew point of the hydrogen be monitored and maintained at a safe margin, as a build-up of moisture in hydrogen can lead to a risk of what is termed “flashover”.

An increase in the dewpoint simply means that additional water vapor has been picked up by the hydrogen – usually through atmospheric leaks or exposure to water. Therefore, simply increasing the pressure of the system does not alleviate or reduce the quantity of water vapor in the hydrogen. You simply must take steps to remove the excess water vapor in the closed circuit.

Yes, it is possible to lower the dewpoint at the same relative pressure. There are manufacturers of equipment that does just that. Usually, for low dewpoints (as in your case) one usually employs an adsorption process unit to “dry” the hydrogen.

There are charts and calculators for determining the mass quantity of water vapor contained in the circulating hydrogen. You can find these through the Internet. Go to: http://www.phymetrix.com/Software.htm and you can download a dewpoint calculator.

I hope this helps and I await your reply.

[jimbobble]

You may go ahead and change whatever you need to make it clearer and more concise.