Question

In a double pipe cocurrent heat exchanger, operating at steady state, oil (specific heat capacity = 2100 J $kg⁻¹ °C⁻¹)$ entering at 7 kg $s⁻¹$ at $100°C$ is used to heat water (specific heat capacity = 4200 J $kg⁻¹ °C⁻¹)$ flowing at 3.5 kg $s⁻¹$ from $20°C$ to $50°C.$ If the overall heat transfer coefficient is 291 W $m⁻² °C⁻¹,$ the required heat transfer area (in $m²)$ is _____ (rounded off to one decimal place).

Official answer: 34.5 to 35.5