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Homework Help: Relative humidity of the air entering a compressor

  1. Jan 23, 2016 #1
    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. jcsd
  3. Jan 24, 2016 #2
    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%
  4. Jan 24, 2016 #3


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    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
  5. Jan 24, 2016 #4


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    Staff Emeritus
    Science Advisor
    Homework Helper

    cubic meters per minute. Big difference.
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