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Homework Help: Psychometry question- need help on where to begin?

  1. Dec 30, 2015 #1
    1. The problem statement, all variables and given/known data
    Moist air leaves the humidifying unit of a “winter” air-conditioning plant at a rate of 0.6 m3 /s. The air has a temperature of 30°C and is 60% saturated. Dry saturated steam at 100°C is supplied to the unit at a rate of 7.015 g/s. Determine the specific humidity and temperature of the air entering the unit, assuming that the pressure is 1.01325 bar throughout.

    2. Relevant equations
    steady flow equations e/g -Qin = ma(H2-H1) etc..
    psychometric chart analysis

    3. The attempt at a solution

    I usually am comfortable solving such questions however do not know how to set it up in order to begin. I am confused by the use of the word unit and have never seen a question on an air conditioning system that gives me the value of temperature and mass flow rate of steam entering the humidifier. Isn't it the steam that humidifies the air and usually in these types of questions the temperature and relative humidity of the air/water entering the system and leaving the system are given however in this case theyre only given for the water leaving and for the steam that humidifies the air into water. I have no idea how to incorporate or make use of the steam values, nor can i find any similar examples on youtube or google, and would greatly appreciate anyones input. My problem here is where to begin rather on how to solve the whole question.

  2. jcsd
  3. Dec 30, 2015 #2
  4. Dec 30, 2015 #3
    I know psychometry has to be used but I don't understand what to do with the steam values as I have never seen a question that uses them instead of the values of temperature/relative humidity and mass flow rate of the air entering the system
  5. Dec 30, 2015 #4
    Psychometry is a psychic phenomena, and psychometrics is the discipline of psychological measurement. Might just be splitting straws here but it could turn out to be relevant.

    Aha! "Not to be confused with psychrometrics, the measurement of the heat and water vapor properties of air."
  6. Dec 30, 2015 #5


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    Homework Helper

    Humidity is the amount of water vapor present in a sample of air. The incoming air has a certain percentage of water vapor already contained in it, and additional water vapor is being added in the form of 'dry' saturated steam, which means that this steam is composed of 100% water vapor. If you thoroughly mix the two streams, that is, the incoming moist air mixed with the steam, what is the specific humidity and temperature of the final product? You do have to understand what the term 'specific humidity' means in this context. You'll also need to know the enthalpies of the air and the steam in order to find the final temperature of the mixture.
  7. Dec 30, 2015 #6
    "Unit" means the humidifier. The word "unit" is used for any arbitrary piece of processing equipment.
    You have two streams entering the humidifier, and one stream exiting. The first step is to characterize the exit stream. I assume your tables give you the specific volume and specific enthalpy of 30 C air at 1 atm and 60% saturation, correct? If you know the specific enthalpy, what is the mass flow rate? What mass flow rate of bone dry air and of water does this imply? What is the rate of enthalpy exiting the unit?

    Next, characterize the inlet stream of steam.

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