Direct Wood-Fired Heat Exchanger (often called an "Indirect Frying System") is used to heat edible oil without exposing it to a direct flame. This prevents oil carbonization and extends the "life" of the oil.

The terms 2-Pass and 3-Pass refer to the number of times the hot flue gases (smoke and heat from the wood fire) travel through the internal tube bundle before exiting the chimney.

1. Two-Pass Wood-Fired Heat Exchanger

The 2-pass system is a simpler, more compact design usually found in smaller batch frying setups.

• Pass 1: The wood burns in the furnace (combustion chamber), and the hot gases move forward through the primary tubes.
• Pass 2: The gases hit a reversal chamber at the back and travel through a second set of tubes back toward the front, where they exit into the chimney.
• Best For: Small-scale units 100 kg/hr production). It is easier to clean but slightly less fuel-efficient than a 3-pass system.

2. Three-Pass Wood-Fired Heat Exchanger

The 3-pass system is the industry standard for high-efficiency frying lines. It extracts the maximum possible heat from the wood before releasing it.

• Pass 1: Combustion in the furnace (Radiant zone).
• Pass 2: Flue gases travel back to the front through the first set of smoke tubes (Convection zone).
• Pass 3: Gases travel once more to the rear through a final set of tubes before exiting.
• Efficiency: Offers up to 75%–82% thermal efficiency, significantly reducing wood consumption.

Comparison: 2-Pass vs. 3-Pass

3. Key Component: The Coil (Oil Side)

While the "passes" describe the flue gas, the Edible Oil flows through a separate Seamless SS 304 Coil.

• Velocity: The oil is pumped at a specific velocity  to ensure it doesn't "crack" or burn inside the tubes.
• Surface Area: 3-pass systems provide more surface area for the oil to absorb heat, meaning you can heat more oil with less wood.