5. If you were able to measure the air inlet and outlet temperature you can esti

Question

5. If you were able to measure the air inlet and outlet temperature you can estimate the heat duty of the exchanger from these temperatures and the fact that the air flow rate through this exchanger is approximately 0.006 kg/s (0.0013 lbn/sec). Determine the heat duty using an energy balance on the air side of the exchanger using the data at a time of 60 sec. Heat duty is Hot side t Cole side ave unt time t amaunt ot Hert needet ts Transtr fom 6. The heat duty can also be measured by an energy balance on the water side of the heat exchanger. Determine the heat duty using the flow rate and temperature data for the water at 60 sec. How does this compare to what you obtained in question 5? Why might this measurement not be very accurate?

Explanation / Answer

5) heat duty is nothing but the amount of energy to be transfered.

Q = mCp (dT)

where m = mass flow rate = 0.006 kg/sec.

C = heat capacity = 1005 Jouls

dT = temperature difference.

Here time is give as 60 sec.

Heat rate = Q = 60* 0.006*1005*dT = 361.8 dT Jouls.

6)

let the water flow rate = m.

Water specific heat= 4200 J/ kg-k.

Time = 60 sec.

dT = water inlet and outlet temperatures.

Heat duty = Q = m*4200 * 60 * dT = 252000 m dT Jouls.

If we compare the above data with question 5, the specific heats of air and water are different, that makes different energy rate transfers. Water has got specific heat of 4200 J/ kg-k, which is higher than air.

this measurements are not accurate because there is always energy lost to the surroudings owing to conductions convection and radiation. Foulig factors and impurities also affect accurate measurement of heat transfer.

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