Exercise-2
Hot exhaust gases at  are flowing at  through a heat exchanger and are cooled to by water flowing initially at  . The specific heat of water is  and that of exhaust gas is  . The overall heat transfer coefficient is  and the flow rate of water is  . Calculate surface area required when
- Exhaust gas and water flow counter-current
- Exhaust gas and water flow co-current.
Solution:
| Figure 34.1: |
A) Counter-Current and B) Co-Current flow of water and gas |
We have to find temperature of water at exit.
Heat given by exhaust gas = heat taken by water
For counter current
For co – current, one can calculate
We note that in a counter current heat exchanger, area required for heat transfer is lower than that required for co-current heat exchanger. In this example heat exchange area for counter- current heat exchanger is  lower than that required for co-current one. It is due to the efficient contact between heat exchanging fluids, exchanging heat in counter-current.
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