# Isentropic compression and humidity

1. Jul 26, 2008

### killerbee

Trying to assess how relative humidity impacts COT, compressor outlet temperature, on a turbo charger compressor.

It appears that humdity reduces gamma in the temp prediction formula, thus reducing COT. Dry air results in increased COT, and humdity reduces it.

Any opinions on this?

2. Jul 31, 2008

### nucleus

Humid air is lighter than dry air and has the same effect as being at a higher altitude, in that it reduces P1 pressure; therefore, the compressor has a higher-pressure ratio (P2/P1). An increase in pressure ratio increases the COT.

As for the question of Gamma changing-- In a perfect gas it is assumed to have a constant specific heat, the specific heat ratio k=Cp/Cv is constant, therefore, it does not change.

The amount of water vapor air can hold depends on the temperature, with the higher the temperature, the more water vapor it can hold. The effect can be determined by Dalton's law of partial pressures or by using online calculators.
http://en.wikipedia.org/wiki/Density_of_air
http://en.wikipedia.org/wiki/Partial_pressure

3. Jul 31, 2008

### killerbee

On a psychrometric chart, the process of humidification through evaporative cooling, increases dry air concentration about 7%, unless I ran the problem incorrectly.

Also, I found studies that claim Cp/Cv = 1.32 for saturated air (steam) at 1 atm, and 1.40 is what we typically use for air. This is what i was referring to. The lower gamma results in lower COT...all in theory.

Thanks for the reply...this is what I am trying to nail down. The psychrometric chart says density increases.

4. Jul 31, 2008

### nucleus

On a psychrometric chart, the process of humidification through evaporative cooling, increases dry air concentration about 7%, unless I ran the problem incorrectly.Also, I found studies that claim Cp/Cv = 1.32 for saturated air (steam) at 1 atm, and 1.40 is what we typically use for air. This is what i was referring to. The lower gamma results in lower COT...all in theory.Thanks for the reply...this is what I am trying to nail down. The psychrometric chart says density increases.

You lost me with that! In your first post you are talking about dry and humid air, now you are talking about steam.

You will have to come back more information as to the type of compressor you are referring to, the temperature(s) you are operating at etc.

5. Aug 1, 2008

### killerbee

Just take 20% humid air, at 120 degrees, and run it to saturation on the psychrometric chart.

The end result is higher density air at a lower temp

6. Aug 4, 2008

### my8950

What are you working on? I was interested/excited to find this forums and see a question like yours...

7. Aug 4, 2008

### killerbee

more effective/efficient pre-turbo evaporative cooling...and "wet compression", similar.

Also, assessing the combustion quality of humid vs dry air.