Many thanks to both of you. It's amazing what you forget - fancy me forgetting steam tables. Oh well, it's been a long time since I used them...
Using skoutso's method, for 20°C, 100% RH air, humidity = 0.0146 kg/kg from a psychrometric chart I have. Converting this to mole fraction I get 0.02295 water, hence 0.02295 bara partial pressure. This compares OK with interpolating Perry's steam table which gives me 0.02337 bara. If I compress to 2 bara total pressure, if hot enough my water vapour pressure becomes 0.04674 bara. Looking down the steam table again I get 31.8°C dew point. Sounds good.
Assuming these are correct, trying Guidoo's correlations, I get:
1)
Pv = 0.0179 * EXP(0.0404*temperature)
Pv in bara, temperature in °C
So
Pv = 0.0179 * (EXP(0.0404*20) = 0.04016 which is a bit high
2)
temperature=24.8ln(55.7*Pv)
temperature in °C, Pv in bara
So
temperature=24.8ln(55.7*0.04674) = 23.7 which is a bit low
or, trying to work from the first correlation result,
temperature=24.8ln(55.7*2*0.04016) = 37.16 which is a bit high
So I'm not confident in these correlation unless you can spot my mistake? If we can correct this, they would be useful.
Either way, thanks for helpful answers.
Regards,
Stuart