(a) Explanation Express the specific humidity. ω 1 = c p ( T 2 − T 1 ) + ω 2 h f g 2 h g 1 − h f 2 = c p ( T 2 − T 1 ) + ω 2 h f g @17°C h g @ 25 °C − h f @17°C (I) Here, specific heat at constant pressure of air is c p , initial and final temperature is T 1 and T 2 respectively, specific humidity at exit is ω 2 , initial specific enthalpy at saturated vapor is h g 1 , finalspecific enthalpy at saturated liquid is h f 2 respectively and final specific enthalpy at evaporation is h f g 2 . Express specific humidity at exit. ω 2 = 0.622 P g 2 P 2 − P g 2 = 0.622 P sat@17°C P 2 − P sat@17°C (II) Here, final pressure is P 2 , final vapor pressure is P g 2 and saturation pressure at temperature of 17°C is P sat@17°C . Conclusion: Refer Table A-4, “saturated water-temperature table”, and write the saturation pressure at temperature of 17°C . Write the formula of interpolation method of two variables. y 2 = ( x 2 − x 1 ) ( y 3 − y 1 ) ( x 3 − x 1 ) + y 1 (III) Here, the variables denote by x and y is temperature and saturation pressure respectively. Show thesaturation pressure corresponding to temperature as in Table (1). Temperature T ( °C ) Saturation pressure P sat ( kPa ) 15 ( x 1 ) 1.7057 ( y 1 ) 17 ( x 2 ) ( y 2 = ? ) 20 ( x 3 ) 2.3392 ( y 3 ) Substitute 15°C,17°C, and 20°C for x 1 , x 2 and x 3 respectively, and 1 .7057 kPa for y 1 and 2 .3392 kPa for y 3 in Equation (III). y 2 = ( 17°C − 15 °C ) ( 2.3392 kPa − 1 .7057 kPa ) ( 20°C − 15 °C ) + 1 .7057 kPa = 1 .938 kPa = P sat@17°C Thus, the saturation pressure at temperature of 17°C is, P sat@17°C = 1 .938 kPa Substitute 1 .938 kPa for P sat@17 ° C and 95 kPa for P 2 in Equation (II). ω 2 = 0.622 ( 1 .938 kPa ) 95 kPa − 1 (b) To determine The relative humidity. (c) To determine The enthalpy per unit mass of dry air.

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Author: CENGEL

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Chapter 14.7, Problem 32P

(a)

To determine

The specific humidity.

(b)

To determine

The relative humidity.

(c)

To determine

The enthalpy per unit mass of dry air.

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Chapter 14.7, Problem 31P

Chapter 14.7, Problem 33P

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The specific humidity.

Solution Summary: The author explains the specific humidity, the initial and final temperature, and the final specific enthalpy at evaporation.

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The specific humidity.

The relative humidity.

The enthalpy per unit mass of dry air.

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Chapter 14 Solutions