Example

In a desalination plant, drinking water is produced by condensing steam in a heat exchanger. Steam at 0.1 MPa and 150⁰C enters the heat exchanger. Shown in fig at the rate of 10000 kg/h and emerges as a liquid at 40⁰C. Cooling water enters the heat exchanger at 25⁰C and leaves at 35⁰C. Estimate the loss in availability in the heat exchange process. The ambient temperature is 300K

Figure 26.1

First law:

= 2775.8 kJ/ kg --------------- = 7.5984 kJ/ kg K

= 167.45 kJ/ kg --------------- = 0.5721 kJ/ kg K

= 104.77 kJ/ kg --------------- = 0.3670 kJ/ kg K

= 146.56 kJ/ kg --------------- = 0.5049 kJ/ kg K

10⁴ (2775.8) + (104.77) = 10 4 (167.45) + (146.56)

or

Change in availability of system

Change in availability of cooling water

Net change in availability

But

Hence net change in the availability

kJ

Therefore the loss in availability kJ