Design of a tubular heat exchanger for heating crude oil with diesel

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Based on my calculations, I think a floating-head heat exchanger with four tubes, a heat transfer tube length of 6 meters, and tubes of 25×2.5 mm in diameter is the most suitable option. However, no matter how I make adjustments, neither the area margin nor the pressure drop on the shell side meets the required standards. Could someone please advise me on what the problem might be. The problem is as follows: A refinery uses diesel at 175°C to preheat crude oil from 68°C to 112°C. The flow rate of the diesel is 28,000 kg/h; its density is 715 kg/m³, specific heat is 2.48 kJ/kg·K, thermal conductivity is 0.133 W/(m·K), and viscosity is 0.64×10⁻³ N·s/m². The flow rate of the crude oil is 36,000 kg/h; its density is 815 kg/m³, specific heat is 2.2 kJ/kg·K, thermal conductivity is 0.128 W/(m·K), and viscosity is 3×10⁻³ N·s/m². The fouling thermal resistance on both sides of the heat transfer tubes can be taken as 0.000172 m²·K/W, and the pressure loss on each side should not exceed 0.3×10⁵ N/m². Design a suitable shell-and-tube heat exchanger.