Homework Help: Relative humidity of the air entering a compressor

1. Jan 23, 2016

oxon88

1. The problem statement, all variables and given/known data
Air is drawn into a compressor at normal temperature and pressure (N.T.P.) and compressed to a pressure of 6 bar gauge. After compression the air is delivered at 1.2m3 min–1 and cooled to a temperature of 30°C, at which point condensate is collected at the rate of 2 litres per hour. Estimate the FAD (N.T.P.) of the compressor, and the relative humidity of the air entering the compressor.

2. Relevant equations
P1 * V1 / T1 = P2 * V2 / T2

3. The attempt at a solution

Inlet conditions
P1 = 1.013 Bar abs
T1 = 15°C = 288.15°K

Outlet conditions
P2 = 6 Bar + 1.013 bar = 7.013 bar abs
V2 = 1.2m3 min-1
T2 = 30°C = 303.15°K

P1*V1/T1 = P2*V2/T2

V1 = P2*T1V2 / P1*T2
V1 = 6 * 288.15 * 1.2 / 1.013 * 303.15

V1 = 7.9m3 min-1 FAD (NTP)

Can anyone help me with calculating the relative humidity of the air entering the compressor?

2. Jan 24, 2016

oxon88

Ive had a go at Calculating the Relative Humidity (RH). Have I used the correct method to calculate RH?

Inlet conditions
7.9m3 min-1 * 60 = 474m3 / hour

Using a Dew-point Chart (at 1.013 bar) to find the amount of moisture present in saturated air at 15°C = 12.5 g

Saturation Quantity = 12.5g * 474m3 / hour = 5925 g / hour

Output conditions
1.2m3 min-1 * 60 = 72m3 / hour

Using a pressure Dew-point Chart (at 7.013 bar abs) to find the amount of moisture present in saturated air at 30°C = 4 g

Saturation Quantity = 4g * 72m3 / hour = 288 g / hour

Absolute Humidity = 288g + 2000g = 2288 g / hour

Relative Humidity = (2288 / 5925) * 100 = 38.6%

3. Jan 24, 2016

Staff: Mentor

I read this as saying that hot air exits the compressor at 1.2 cu ft/min. CORRECTION: 1.2 m3/min. Then after this it is cooled to 30°C. Do you think this is what it's saying?

For an adiabatic process, I think you also have $pv^\gamma$=const
but whether you need to apply that here I can't say. (I have not dealt with thermodynamics of fluids since I was a student.)

Have you looked at the similar threads listed at the foot of this page? There's no guarantee they are automatically correct, of couse.

Last edited: Jan 24, 2016
4. Jan 24, 2016

SteamKing

Staff Emeritus
cubic meters per minute. Big difference.