Adobe Reader Touch A heat exchanger condenses a saturated vapor stream of ethyle

Question

Adobe Reader Touch A heat exchanger condenses a saturated vapor stream of ethylene glycol using cooling water. 310 kg/h of water enters at 25 °C and 1.0 bar and leaves as steamat 1.0 bar. The saturated ethylene glycol vapor enters at 120 kPa and exits as a liquid at 150 °C and 120 kPa. The heat duty of the exchanger (heat transferred from the hot to the cold stream) is 230 kW a) What is the temperature of the ethylene glycol vapor entering the heat exchanger? (25 pts) b) What is the temperature of the steam exiting the heat exchanger? (25 pts) Water | P= 1.0 bar T 25 C Saturated Vapor Ethylene Glycol Liquid Ethylene Glycol P 120 kPa P 120 kPa T 150 C Steam P- 1.0 bar 8:58 AM 4/22/2018

Explanation / Answer

(a) Antoine equation is given by

log10(Psat[mmHg])=A-B/(T[oC]+C)

P = 120 kPa = 900.074 mm Hg

Put P = 900.074 in the Antoine equation, we get

T = 202.96 C

This the saturation temperature at P = 120 kPa

Inlet temperature T = 202.96 C

(b) Heat duty = 230 kW or kJ/s

Heat gained by water = m(Hf - Hi)

At 1 bar and 25 C, it is liquid.

Initial specific enthalpy Hi = Cp(25-0) = 4.187*(25-0) = 104.675 kJ/kg

.......................(base temperature in the steam table is 0 C)

Hf = specific enthalpy of the exit stream of water.

m = 310 kg/h = (310 / 3600) kg/s

.: 230 = (310/3600) * (Hf - 104.675)

.: Hf = 2775.64 kJ/kg

From the saturated steam table, at P = 1 bar,

specific enthalpy of saturated steam = 2674.95 kJ/kg

As specific enthalpy of saturated steam at P = 1 bar is less than 2775.64 kJ/kg, steam is superheated.

Now, look for P = 1 bar and H = 2775.64 kJ/kg in the superheated steam table.

We get,

T = 150 C

A B C

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