Properties of Humid Air

Question 1

To find mixing ratio for two air streams **

Answer 1

1. Mark two points for both streams using the given temperatures and relative humidities
2. draw a line between points
3. Mark a point as point M at the given temperature (remember that if it doesn’t fit on the curved lines, it will sit on the straight isotherms)
4. Draw a line up at each point to find the absolute humidity.
5. To find the mass of water, find the absolute humidity at saturation point (relative humidity=100%) and draw line up to x values

Question 2

To find temperature and x for mixture of air and steam

Answer 2

1. To mark your point 1 for air using values given in the question
2. Draw a line between given enthalpy value for steam and the bulb
3. Draw a parallel line from point 1 until it reachs above the bulb
4. Mark the point where the line means the saturation point (if question is asking temperature of saturated air)
5. Draw the line back to temperature axis and to x axis to find x
6. To find mass of water, find the difference between xs and x1

Question 3

To calculate the temperature if water is sprayed

Answer 3

1. Calculate enthalpy of water using Cp*T equation
2. Draw a line from enthalpy to bulb
3. Draw a parallel line from point 1
4. Mark point where line meets saturation point, find temperature on temperature axis

Question 4

To find G min:

Answer 4

1. Draw points for wet bulb temp, initial temp (bottom temp) and temp at top at the point where they meet the equilibrium line. (h for initial temp and wet bulb the same).
2. Draw a line between top and bottom temperatures, meeting the equilibrium line. This is the max operating line
3. Calculate the slope value.
4. Calculate Lcl value and plug these two into the expression for Gmin

Question 5

To find height of tower:

Answer 5

1. Look at the equation for height, you want to find the hgs and hg
2. Derive an expression for hg using operating line equation
3. Use sat enthalpy vs liquid chart to find hgs at even temperature intervals between top and bottom. To draw the operation line on the chart, find the enthalpy at inlet temp (same as wet bulb temp) and draw line until you hit max top temperature.
4. Find the hgs-hg for all values
5. Use trapezium rule to solve the integral (NTU)
6. Plug in values in HTU and then multiply by NTU

Question 6

Dehumidification

Answer 6

1. Mark point 1 on your chart
2. Bring to condensation (saturation point at the same moisture content). Mark the temperature where this occurs.
3. Mark point 4 on your chart (the cooled and condensed air)
4. Draw a line along the curve (remove the condensate) until it meets the extrapolated line of point 4.

Question 7

trapezium rule

Answer 7

(xn-x1)/n-1 (y1+yn/2 * sum y)