Author Topic: Talking Thermodynamics  (Read 127331 times)

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1290 on: February 15, 2020, 10:26:57 AM »
Hi Willy, I must have been tired when I posted last night, I found post no 1283 this morning, where we were discussing running the engine on air, and it looks like you found it too.

I covered what happens in the compressor and piping to the engine inlet, basically the moisture in the air due to humidity is compressed along with the rest of the gas mixture we call air.  In the compressor, the water stays in vapour form so does not condense at the compressor discharge temperature.  However, after the compressor, in the after cooler and accumulator and piping, the air and contained moisture cool, and will generally reach the dew point before it gets back to room temperature, so water condenses in the system.  Hence it is important to have drain points on the accumulator and at the ends of branches in the piping.  You donít want that water in the piping running down into the engine.  The air is now at 100% humidity at the pressure in the system.

Next we need to consider what happens as this air passes through a throttle valve and the engine.  Because of the condensation in the system there is less moisture in the air than there was in the original inlet air, so depending on the thermodynamic path through which the air passes from high pressure in the piping to atmospheric pressure in the exhaust, it will not necessarily reach its dew point, so moisture will not necessarily condense. 

We can assume adiabatic expansion as the air does work during the expansion, and it will cool in the process.   Part of the expansion will also be throttling so constant enthalpy.  You can see the complexity of the calculation increasing.  By the time the cooler air is exhausted to atmospheric pressure, whether there is any condensation will depend on the final temperature and the initial air humidity.  I am not convinced there will be much condensation, but not keen to try and work through all the calculations by hand.  In the end the answer still relies on many assumptions.

Remember also that when the air in the pipe cools, if the temperature is lower than the atmospheric dew point, there will be condensation on the outside of the piping, but that does not mean condensation inside the pipe.  It just means the inside temperature is lower than the dew point of the outside air. 

It would be very helpful to know the temperature inside the pipe and the atmospheric air humidity and temperature.  But where the engine air finally exhausts to atmosphere, I would expect some sign of moisture if it is condensing.  But if it is merely cool, and all the moisture is on the outside of the pipe, then there is probably not a major worry inside the engine.  In the end it gets back to observation and inspection. 

I think a procedure to warm up the engine with dry air, while you drive it with that car tyre might be adequate.  Along with adequate lubrication while the engine is running.

 Your comment that there was moisture on the outside of the inlet piping needs further thought.  If so, it would indicate that the air inside the pipe was cooler than the atmospheric air dew point.  How did that come about.  Or was it just near the valve chest, where the metal might all be cooled a little by the exhaust.

I hope that helps a little with understanding.  Understanding will always help in interpreting observations.  I really hope that someone else can come in with some experience on this issue.

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1291 on: February 16, 2020, 01:43:48 AM »
Hi MJM , thanks for the info ..I shall talk to the engine crew soon. The pic of the steam chest shows spots of rust around the steel bolts, and there does not seem to be any around the holes that do not hold the steam chest on.? I don't think the moisture has crept through the gasket so is this because the steel reacts differently with brass as far as the thermodynamics is concerned ? of course the brass will not rust and the tube was firmly attached with the jubilee clip so the coldness to create the rust must have come from the ambient atmosphere . the bolts on the other side of the steam chest cover do not seem to be affected ?................................ I have not seen the compressor set up at Beeleigh so don't really know how it is all connected up. I shall make enquires soon.

Willy