Question

Exhaust gas from the power plant boiler is used to heat water. The exhaust gases flow at the rate of 0.25 kg/s at 150⁰C, with a specific heat capacity of 1000 J/kgK. The water entering the HEX at the rate of 0.05 kg/s at 15⁰C is to be heated to 90⁰C. The HEX is to be of shell and tube type with one shell pass and four tube passes. The water flows inside the tubes, which are made of copper (2.5-cm ID, 3.0-cm OD), (k=392 W/mK). The heat transfer coefficient at the gas side is 115 W/m² K, while the heat transfer coefficient on the waterside is 1150 W/m² K. A scale on the waterside offers a thermal resistance of 0.002 m² K/W. Calculate:

(a) The overall heat transfer coefficient is based on the outer tube diameter.
(b) The mean temperature difference for the heat exchanger.
(c) The required tube length for each tube pass.
(d) Determine the water exit temperature if the water flow rate is doubled, giving a new water heat transfer coefficient of 1820 W/m2 K ? Assume: water specific heat is 4180 J/kgK

Answer 1

This solution pertains to the calculation of heat exchange parameters such as the overall heat transfer coefficient, mean temperature difference, tube length, and exit water temperature for a shell and tube heat exchanger.

Step-by-step explanation:

The task involves calculating various parameters for a heat exchanger (HEX) used in a power plant. The key concepts here include the overall heat transfer coefficient, mean temperature difference, tube length required for the HEX, and the effect of a change in water flow rate on the water exit temperature.

To calculate the overall heat transfer coefficient based on the outer tube diameter, you will need to account for the individual heat transfer coefficients on the gas and water sides, thermal resistance of the scale, and the conductivity of the tube material. The mean temperature difference can be calculated using the inlet and outlet temperatures of both the water and the gas. The tube length for each pass would require using the calculated overall heat transfer coefficient and the heat exchanged to determine the required surface area and then the length given the number of passes and tube dimensions.

If the water flow rate is doubled, this influences the heat transfer coefficient on the waterside due to increased turbulence, and as a result, the exit temperature of the water will change. The new temperature can be found using a modified version of the heat exchange equation that includes the new flow rate and heat transfer coefficient.